Biofilm carrier for moving bed biofilm reactor

Abstract

The present invention relates to a carrier for supporting a biofilm in a moving bed biofilm reactor (MBBR) that has improved stability against environmental stresses. The carrier comprises a carrier material that includes at least one high-density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a bimodal or multimodal molecular weight distribution. Use of the carrier in a moving bed biofilm reactor (MBBR) process is also provided.

Description

Technical Field

[0001] The present invention generally relates to the field of moving bed biofilm reactors using carriers. More specifically, the present invention relates to a carrier element for growing a biofilm, which is designed to freely flow in a liquid purified from biologically mixed substances by the growing biofilm. Furthermore, the carrier element is further designed to have a significantly improved lifespan in the purification process compared to the carrier elements used in the prior art.

Background Art

[0002] In the biological treatment of water or wastewater, the water is passed through a certain type of reactor or several reactors (vessels or other spaces) where microorganisms are utilized to convert the pollutants in the water into harmless end products such as carbon dioxide and water. This treatment can be carried out under the supply of air (aerobically), without the supply of air (anaerobically), or without the supply of air but in the presence of a significant amount of nitrate (ions) (anoxically).

[0003] In order to enhance the efficiency of the treatment process, active microorganisms are suspended and grown in the reactor, and the microorganisms are separated from the water in the separation stage after the reactor and returned to the reactor (for example, the activated sludge process), or some kind of support material having a surface on which active microorganisms can grow as a biofilm and thus be retained during the process is introduced into the process reactor (biofilm process). Generally, the aim is to increase the content of active microorganisms in the process by preventing the above-mentioned organisms from escaping with the treated water.

[0004] There is also a mixed form of the above two process types called a hybrid process, in which a support material is introduced into the activated sludge process so that suspended microorganisms and microorganisms growing in biofilms can be utilized in the process. The biofilm process has several advantages compared to the activated sludge process. For example, a higher load can be applied, and the biofilm process is substantially less sensitive to variations and disturbances. Many conventional biofilm processes are based on filling the treatment reactor with a carrier material, which includes packings or blocks that are fixed and maintained immobile during the process. These embodiments of the process carry the risk of clogging of the biofilm bed by biomass or other particulate matter, and the risk of forming dead zones where the contact between water and active microorganisms is insufficient in the process.

[0005] A type of biofilm process that has been very successful for wastewater treatment over the past 25 years is the MBBR process, i.e., the "Moving Bed Biofilm Reactor", in which a carrier material that is kept suspended and mobile within the process reactor volume is utilized. The carrier material on which the microorganisms grow is maintained during the process by passing the effluent water through a strainer (sieve or grid) having an opening diameter or slot width small enough that the carrier material cannot pass through while the treated water does. The advantage of this type of process is that the risk of clogging the biofilm bed and the risk of forming dead zones are eliminated. The use of a carrier material that is kept suspended and mobile in the process has originally been reported for different hybrid process applications, i.e., suspended carriers have been fed into the activated sludge process to improve its function.

[0006] In a moving bed reactor, the biofilm grows on carriers that freely float within the reactor. The carrier material is generally pieces of foamed rubber or small pieces of plastic. The processes using pieces of foamed rubber are known by the names Captor and Linpor. The drawback of pieces of foamed rubber is that the growth on the outside of the pieces of foamed rubber clogs the pores, preventing the intrusion of substrates and oxygen into the interior of the pieces of foamed rubber, resulting in a small effective biofilm area. Furthermore, a sieve must be used to prevent the pieces of foamed rubber from exiting the reactor, and a system must be available to periodically pump the pieces of foamed rubber from the sieve to prevent them from clogging. Therefore, there were very few plants constructed using foamed rubber as the carrier material.

[0007] However, today, many purification plants are constructed using a moving bed process in which the carrier material is small pieces of plastic. The plastic pieces are usually evenly distributed throughout the water volume and are actually operated to the extent of filling up to approximately 90% of the maximum reactor volume with the biofilm carrier medium. The sieve holds the plastic pieces in place within the reactor. The reactor is operated continuously without the need for backwashing. This process is very flexible with respect to the shape of the bioreactor. The specific biofilm surface area is higher than that of a trickling filter bed but considerably smaller than that of a biological aerated filter (BAF) process. However, on a total volume basis, the moving bed process using small plastic pieces as the carrier material has been found to be as efficient as the BAF process, considering the expansion of the filter bed and the extra volume required for the flushing water reservoir in the BAF process. Examples of suppliers of moving bed processes using small plastic pieces as the carrier material are the systems of Veolia Water Technologies, Infilco, Degremont, Biowater Technology, and Aqwise.

[0008] In the MBBR process, the carriers are exposed to repeated collisions with each other and with other surfaces within the reactor, such as the reactor walls, immersion mixers, screens, and other equipment within the reactor. As a result, the surfaces exposed to other carriers or other surfaces within the reactor are kept clean from biofilm growth. Therefore, the efficiency of the process depends greatly on the areas protected from collisions, such as the internal passages or internal compartments of the carriers. In fact, this protected surface area of the carrier medium, along with the ability of the plastic pieces to clean each other from excessive biofilm formation due to collisions with each other and the other reactor inner surfaces, thereby eliminating clogging and dead volume, makes the MBBR process using plastic media quite efficient and effective.

[0009] Plastic carrier media for MBBR applications need to withstand harsh environments over long periods. When used in MBBR plants around the world for wastewater treatment, these plastic carrier media need to withstand, among other things, concentrated constant mixing shear forces, shear forces due to wall scraping, seasonal temperature variations, and all types of chemicals present in the wastewater and used in the wastewater treatment process in an aqueous medium over long periods. If the condition of the reactor walls is poor (e.g., cracked walls and rough walls), or due to the use of high-speed mixers, high aeration rates (resulting in higher shear forces and more collisions), or a high degree of carrier filling compared to the reactor volume (resulting in more collisions), the environment can become even more severe. Continued exposure to these environments will ultimately break the plastic carrier pieces and reach their end-of-life. When the plastic carrier is broken, it becomes impossible for the plastic carrier to function as a biofilm carrier in MBBR. Naturally, every wastewater treatment plant (WWTP) will desire the longest possible carrier media life. This is because it is costly to replace broken plastic carriers with new plastic carrier pieces required for the biological wastewater treatment process to treat the wastewater. In addition, if the carrier is not replaced within the time before it breaks, the damaged media pieces can pass through the screens and cause significant problems downstream of the WWTP and may ultimately leave the plant and spread into the natural environment.

[0010] The lifespan of the plastic carrier media of the prior art employed can widely vary in view of the different environmental conditions in different wastewater treatment plants as described above. When the conditions are rougher, the lifespan becomes shorter and, depending on the conditions, can vary between, but not limited to, 5 to 25 years. At the end of the lifespan of the prior art carriers, the MBBR tank walls and older worn-out equipment may have to be modified or replaced at high cost to reduce shear and friction when suspending new prior art plastic carrier media in the reactor. This is because otherwise, the lifespan of the new media might become significantly shorter than when the plant was first constructed.

[0011] Therefore, there is a need for carriers with a longer lifespan, especially carriers that can function even in older or worn-out equipment.

Summary of the Invention

Means for Solving the Problems

[0012] Accordingly, the present invention preferably seeks to alleviate, mitigate or eliminate one or more of the above-identified drawbacks and disadvantages in the art, alone or in any combination, and solves at least the above problems by providing a carrier for supporting a biofilm within a moving bed biofilm reactor (MBBR). This carrier has improved stability against environmental stress and comprises a carrier material that includes at least one high-density polyethylene having a bimodal molecular weight distribution, whereby the carrier material is characterized by having a bimodal or multimodal molecular weight distribution.

[0013] Also provided is the use of the above carrier in a moving bed biofilm reactor (MBBR) process.

[0014] The present invention has an advantage over the prior art in that a carrier for supporting a biofilm in a moving bed biofilm reactor (MBBR) has improved stability against environmental stress.

Brief Description of the Drawings

[0015] These and other aspects, features, and advantages of the present invention will become apparent and more readily understood from the following description of embodiments of the invention with reference to the accompanying drawings.

[0016]

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Modes for Carrying Out the Invention

[0017] The following description focuses on embodiments of the present invention applicable to plastic carriers according to the present invention, which have significantly increased resistance to withstand the harsh conditions commonly employed in different wastewater treatment plants, thereby significantly extending the carrier lifespan.

[0018] Accordingly, not only is the carrier lifespan extended for general MBBR applications, but the increased resistance may also facilitate delaying or eliminating the retrofit of older or worn-out equipment.

[0019] The plastic carrier according to the present invention may also facilitate better use of immersible mixers.

[0020] Similarly, if necessary, it may facilitate the utilization of higher mixing energy than general mixing energy.

[0021] Furthermore, it may facilitate the treatment of wastewater containing more severe plastic pollutants.

[0022] The improved properties of the carrier medium according to the present invention can cope with harsher conditions over much longer periods compared to prior art plastic carrier media, and it is believed that the generation of microplastics from the plastic carrier media can be significantly reduced or eliminated. Therefore, the plastic carrier according to the present invention may also easily utilize MBBR technology as part of the treatment line for the production of drinking water.

[0023] MBBR carriers are generally designed to have a structure that allows for a high surface area suitable for biofilm growth (see examples of carrier designs in Figures 30 - 36). Usually, this carrier is manufactured by extrusion or injection molding from one or more plastic raw materials, which become the carrier material when melted or melt - mixed in an extrusion facility. Then, this is formed into the desired carrier design and shape at the melting stage, and subsequently, the melt of the carrier material is cooled into solid carrier pieces. However, the carrier may be colored by the addition of colorants in the extrusion process, co - extruded with material inserts such as metal strips for additional functionality, and various inorganic substances can be added in the extrusion melting process to vary the density, in which case these also become part of the carrier material.

[0024] The carrier manufacturing process is generally continuous, and carrier pieces can be formed, for example, individually in the case of injection molding or by cutting a long cooled and solidified strand, as in the case of extrusion, into individual pieces.

[0025] Therefore, the MBBR carrier consists essentially of the carrier material.

[0026] Plastic carriers for MBBR are generally made from polymeric plastics.

[0027] One group of polymeric plastics is plastic polyolefins, which are high - molecular - weight hydrocarbons including high - density polyethylene, low - density polyethylene, polypropylene copolymer, polymethylpentene, and polypropylene.

[0028] These alone represent plastics with a specific gravity lower than water, or in other words, these alone are common plastics that are lighter than water. In order to easily keep the plastic medium for MBBR suspended and flowing in water within the reactor volume without the need to use excessive energy, the specific gravity of the plastic medium for MBBR must be less than that of water. Therefore, polyolefin is a good option for non-foamed plastic carrier media (foaming naturally reduces the specific gravity of the material).

[0029] Among polyolefins, the option for manufacturing the carrier medium for MBBR is polyethylene, because polyethylene is a general-purpose polymer with very high versatility and suitable properties for this type of application, and it can be easily molded at high speed according to the desired design of the carrier medium. Polyethylene can also be made to have a higher density (still less than the density of water) than, for example, polypropylene, which makes it easier to keep it in a suspended state due to lower buoyancy so that the carrier does not exactly float on the surface.

[0030] General description of polyethylene Like many other types of polymers, polyethylene has diverse material behaviors with various properties depending on its structure, and different types of polyethylene are readily commercially available. Its diversity can be explained by its molecular structure.

[0031] Polyethylene is a thermoplastic material composed of carbon atoms and hydrogen atoms that bond together to form high-molecular-weight products. Generally speaking, ethylene is converted into polyethylene by the application of heat and pressure. The polymer chains can be tens of thousands to millions of carbon units (i.e., methylene groups) in length. Short and / or long side-chain molecules (branched chains) exist together with the long main-chain molecules of the polymer. The longer the main chain, the more atoms there are, and as a result, the higher the molecular weight. The molecular weight, molecular weight distribution, and amount of branching determine many of the physical properties of the final product.

[0032] Molecular weight, its distribution, and crystallinity (density) have the greatest influence on the properties of PE. This crystallinity depends on the molecular weight and degree of branching. The less the branching of the polymer chain and the lower the molecular weight, the higher the crystallinity of polyethylene.

[0033] The most common types of polyethylene are low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). LDPE has a high degree of short-chain and long-chain branching along the main chain, while LLDPE has only a high degree of short-chain branching, and HDPE has a small amount of short-chain branching (Figure 1). Therefore, LDPE is more restricted (due to the lower linearity of the molecules) to form crystalline regions when cooled from the melt, resulting in a lower density. This is because the polymer chains cannot be easily folded into a highly ordered crystalline folded structure. On the other hand, LLDPE and HDPE have more linear polymer chains (especially HDPE), increasing their crystallinity, thereby providing the excellent mechanical properties and higher rigidity required for use as an MBBR. However, LLDPE has a lower density (0.91 - 0.93 g / cm 3 ) than HDPE (0.93 - 0.97 g / cm 3 ) due to more branching and is less suitable than HDPE as a material for the production of carrier media used in MBBR.

[0034] According to one embodiment, HDPE, as used herein, relates to polyethylene having a density of 0.93 - 0.97 g / cm 3 . Typically, its density exceeds 0.93 g / cm 3 .

[0035] According to one embodiment, HDPE, as used herein, relates to either a homopolymer or a copolymer. The homopolymer is produced from monomer ethylene, while the copolymer further contains at least one other monomer alkene, such as 1-butene, 1-hexene, or 1-octene.

[0036] General Routes of Destruction of MBBR Plastic Carriers Plastic carriers for MBBR applications (although not limited to, examples of different types of carrier media can be seen in Figures 30 - 36) need to withstand harsh environments over long periods. When used in MBBR plants worldwide for wastewater treatment, they need to withstand, inter alia, a certain shear force concentrated from mechanical mixers, a certain shear force from wall impacts, a certain shear force from impacts between carriers, seasonal temperature changes, and all types of chemicals present in wastewater and used in the wastewater treatment process in an aqueous medium over long periods. If the state of the reactor walls is poor (e.g., cracked and rough walls), or due to the use of high - speed mixers, high aeration rates (resulting in higher shear forces and more collisions), or a high degree of carrier filling compared to the reactor volume (resulting in more collisions), the environment can become even more severe. Continued exposure to these environments will ultimately break the plastic carrier pieces and reach their end - of - life. Naturally, since it is costly to replace them with new plastic carrier pieces, every WWTP will desire the longest possible carrier media life, and since new biofilms have to grow on the new carrier media, the replacement of existing carriers will temporarily disrupt the biological treatment process. Additionally, if the carriers are not replaced within the time before breakage, the broken media pieces may pass through the screens and ultimately leave the WWTP and spread into the natural environment.

[0037] In a wastewater treatment plant adopting the MBBR technology, when the plastic carrier medium reaches its end-of-life, the plastic carrier medium must be replaced sooner or later. The lifespan of the adopted carrier medium can vary widely in view of different environmental conditions in different wastewater treatment plants as described above. When the conditions are rougher, the lifespan will be shorter, but depending on the conditions, it can vary between 5 and 25 years, although not limited to this range. However, the type of polymer material used in the manufacture of the plastic carrier medium can also play a significant role in determining the carrier lifespan. Naturally, different types of materials have different properties, and some of these different properties may be very important for improving the lifespan of the carrier medium utilized in the MBBR process. If the property(ies) of HDPE (which is selected in the present invention for its properties as a raw material for carrier medium manufacture as described above) that affect the carrier lifespan in the MBBR process can be identified, in many cases, an attempt can be made to modify the above HDPE material without impairing other properties of HDPE that are required for having a functional MBBR process, and thereby efforts can also be made to improve its specific property(ies).

[0038] To identify the most important properties that the raw material should have for manufacturing a long-life plastic carrier medium, first, it is necessary to understand why and how the carrier breaks. The breakage of the carrier can be caused by many reasons, but generally, it can be divided into three categories. · For example, molecular decomposition of the HDPE material structure due to UV decomposition or chemicals attacking the polymer backbone results in chain scission and deterioration of physical properties, which, combined with continuous shear stress (high or low) in the reactor, destroys the carrier. If the molecular structure does not change, this means that if the material is melted and remolded into a carrier again, its mechanical properties may be restored. If the molecular structure deteriorates, the material will have permanently lost its mechanical properties even if remelted. · The aging physical deterioration of HDPE materials due to environmental factors such as continuous shear stress, temperature changes, and chemicals causes stress cracking in the materials. This is different from molecular degradation as it does not break the polymer bonds in the main polymer chain but instead breaks the secondary linkages between polymer chains, such as van der Waals bonds. These are broken when low mechanical stress causes elongation, and the resulting time-dependent chain slippage in the amorphous phase of the polymer propagates cracks and ultimately destroys the plastic. Stress cracking is accelerated, for example, by higher temperatures, increased stress concentration, and chemical concentration, ultimately leading to brittle fracture. Stress cracking can be significantly suppressed over a much longer time if the shear stress within the system is kept to a minimum. Physical degradation can be reversed by remelting the material, but of course, this is not practically achievable for MBBR carriers. · Plastic deformation - Excessively high shear forces within the MBBR reactor that are higher than the yield stress of the material as virgin material lead to the material yielding and beginning to stretch until sudden ductile fracture. This ductile fracture could be the mode of failure of the MBBR carrier when suddenly significantly high shear forces are applied in the reactor, for example, when the carrier medium becomes stuck in a mechanical mixer or elsewhere and significant forces are applied to the stuck carrier.

[0039] If molecular degradation of the plastic material can be eliminated (which can be measured), it can be said that it is either the plastic's plastic deformation or physical degradation over time that will ultimately damage the carrier medium and lead to the end of its life.

[0040] Physical degradation over time and plastic deformation each result in two types of mechanical failure of plastic materials, ductile failure (due to plastic deformation) and brittle failure (due to physical degradation over time). Ductile failure is generally the type of failure that occurs at high stress levels over a short period of time. At the macroscopic level, ductile tensile failure results in the observation of visible deformation (necking) in the polymer sample. Compared to ductile failure, polymers subject to brittle failure have a clean (smooth fracture surface) failure with little material deformation due to being stressed at lower stress levels over a long period of time.

[0041] Tensile ductility behavior is affected by the semi-crystallinity of the material. In Figure 2, the stress-strain curve for tensile ductile deformation is accompanied by a diagram of what occurs within the semi-crystalline polymer matrix at the microscale. First, before the yield point, no visible deformation of the material is observed, and the load is mainly borne by the hard crystalline lamellae. As the strain increases, the stress also increases and yield occurs. During the period between the yield point and the start of strain hardening, the load on the test sample remains at a relatively constant level. The deformation in this region is due to a combination of the rearrangement of the amorphous phase itself and the sliding of the crystalline lamellae past each other, however, the individual crystals themselves remain intact.

[0042] Between strain values of 0.5 and 1.0 in Figure 2, an increase in the orientation in the stretching direction of the crystalline and amorphous phases is seen with the increase in the stress-strain value. After a strain of 1.5, the sharp increase in the stress value with the increase in strain indicates the occurrence of strain hardening. During strain hardening, the amorphous phase reaches its full elongation, and further deformation of the polymer at this stage is due to the breaking and unfolding of the crystalline lamellae. The breaking of the crystalline lamellae into smaller chunks results in the characteristic rough fibrous surface of ductile failure, as can be observed under a scanning electron microscope (SEM). As the stress continues to increase with the increase in strain, ultimate failure occurs and the material breaks.

[0043] Compared with ductile fracture, polymers that undergo brittle fracture have a clean fracture with little material deformation. Visually, the fracture surface appears smooth. Under SEM, it can be seen that the surface actually consists of short random pull-outs. Brittle-type fracture occurs when low stress is applied over a long period of time.

[0044] As shown in Figures 3a - b, in the initial stage of brittle fracture, the amorphous phase begins to elongate under stress. Due to longer time, the interlamellar connections in the amorphous phase under stress relax and start to disengage from each other until the number of remaining connections becomes very small. When the few remaining interlamellar connections are stretched to their limits, they cannot pull apart the crystalline lamellae, resulting in the brittle fracture of the polymer as shown in Figure 3c.

[0045] Stress crack growth (SCG) is a phenomenon in semi-crystalline materials such as HDPE, where cracks can occur and grow slowly due to the presence of stress in the material. The long-term durability of the material can depend on its resistance to crack initiation and slow growth. Research has shown that SCG is one of the three main fracture modes (Figure 4) of semi-crystalline polymers.

[0046] Ductile fracture mode I results in yielding and reflects the tendency of materials to undergo large-scale irreversible "plastic" deformation under stress. This mechanism leads to the local expansion (swelling) of the deformed zone and final rupture (fracture, crack).

[0047] Fracture mode II is related to creep, creep rupture, and SCG. Creep is a time-dependent irreversible deformation when subjected to a constant tensile stress. Creep rupture is the final event of creep and is a measure of the time it takes for a material under a constant applied tensile load to fail.

[0048] Creep rupture is · temperature · stress concentration · fatigue · chemical environment can be accelerated by

[0049] Figure 5 represents a creep rupture curve in which the ductile - brittle transition indicates the onset of SCG.

[0050] After crack initiation, voids (voids) are generated in front of the crack. These voids gradually merge into larger voids where the highly oriented load - bearing fibrils spread. This process, known as crazing, continues until the most highly stretched fibrils break and lead to failure.

[0051] Here, it is important to examine MBBR carrier media that have operated for a significant period of time and have reached the end of their life or are very close to it. By doing so, it will be possible to determine which failure path caused the end of its life. Then, it will be possible to determine what can be done to the plastic raw material to block that failure path as much as possible.

[0052] Therefore, the first thing to examine would be to confirm whether molecular degradation of the plastic raw material occurs in two different MBBR processes with two different carrier designs (regarding size, morphology, and shape), and whether the carrier media in both plants have operated for a long enough time to reach the end of their life. This was done by comparing the molecular structure of the HDPE raw material in virgin unused carrier media (which was the same for both types of media) with the molecular structure of the raw material in two different carrier media that had reached the end of their life.

[0053] To compare the molecular structures of virgin HDPE raw material and HDPE in carrier media that had reached the end of their life, melt rheology measurements of those different materials were performed. Thermoplastic polymers such as HDPE become viscoelastic fluids when heated above their melting temperature. This means that they behave viscously or elastically in the molten state depending on how fast they flow or deform during their melt processing.

[0054] The polymer structure - rheology relationship is the key to the development of polymers with the correct rheology for processing. The ultimate goal of design engineers is to change (adjust) the material structure to provide better melt processing performance without sacrificing the performance of the final product (e.g., mechanical properties). Due to its sensitivity to polymer structure changes, melt rheology is a desirable technique for very accurately comparing the molecular structure differences between different HDPE raw materials. The metamorphic process responds very sensitively to small changes in the material, i.e., the rheology of the polymer melt is very sensitive to small changes in the polymer structure.

[0055] Determination of the failure path of MBBR plastic carriers Due to the sensitivity of rheology, rheology is the most convenient way to characterize polymers. A small amount of high molecular weight polymer can dramatically change the processing behavior and thus also the melt rheology. The important structural parameters that define the rheology of the polymer melt are the molecular weight (MW), the molecular weight distribution (MWD), and branching. An increase in molecular weight causes an increase in viscosity, while changes in molecular weight distribution and branching mainly affect the elasticity of the melt. The time dependence is affected by both.

[0056] Therefore, rheology measurements provide accurate and reliable data for determining whether the specific HDPE raw material used to manufacture the carriers that have reached the end of their life has been molecularly degraded during its use in a particular MBBR process.

[0057] The loss modulus characterizes the viscous properties of the material, i.e., the energy dissipation. The storage modulus is a measure of the elastic properties and identifies the energy stored in the material. In the range of Newtonian Plateau (Newtonian flat region), the loss modulus curve has a constant slope at low angular frequencies, and the storage modulus has another constant slope at low frequencies when the data is presented on a double logarithmic scale. In this region, the loss modulus G” exceeds the storage modulus G’, which means that the behavior of the sample is similar to that of a fluid. By increasing the angular frequency, the so-called crossover point (COP) between the storage modulus and the loss modulus indicates the transition from a more viscous-like deformation behavior to a more elastic deformation behavior. Therefore, the COP can be used as a criterion for qualitative evaluation of material properties.

[0058] The information obtained from the position of the COP is qualitative information regarding the average molar mass and its molar mass distribution (MMD) of the material. An increasing average molar mass is represented by a COP shifted to lower angular frequencies. At higher angular frequencies, shorter molecules remain mobile, while longer molecules already become immobile at lower angular frequencies. A vertical shift of the COP towards lower moduli indicates a broader MMD. This can be seen in Figure 6. At angular frequencies above the COP, there is not enough time left for the molecules to unravel. For a given strain or shear rate, the material has a rubber-like response. The elastic properties are dominant over the viscous or fluid properties. This region is the so-called rubbery region.

[0059] Since the frequency sweep according to Figure 6 provides detailed information about the molecular weight, molecular weight distribution, and branching, two different end-of-life carriers taken from two different MBBR reactors were compared and analyzed with virgin HDPE material before use in the plant, and the respective G x and ω x at the crossover point according to Figure 6 were determined. The aim was to establish these values for each sample in order to determine whether they had changed. If they had not changed, the molecular structure of the end-of-life HDPE sample would be intact, and in such a case, it would not be the cause of the destruction of the MBBR carrier.

[0060] Figure 7 shows a comparison between two carriers of different end-of-life after use in two different MBBR processes (medium 1 shown in Figure 30 and medium 2 shown in Figure 31), and the virgin HDPE material before use. As can be seen from Figure 7, the frequency sweep melting curves basically overlap each other and have very little difference. The values of the crossover points for all analyzed samples can be seen in Table 1. These results clearly show that there was no molecular decomposition of the HDPE raw material during operation in the MBBR process, which means that the damage and end-of-life of the carrier must be due to physical degradation either by physical degradation over time (formation of stress cracks) or plastic deformation.

[0061]

Table 1

[0062] Since no molecular decomposition occurred, it was necessary to investigate the physical degradation pathway. This was done using optical microscopy. Micrographs of two different types of damaged carrier media pieces samples from two different MBBR processes are shown in Figures 8 - 14 for process number 1 (medium 1) and Figures 15 - 21 for process number 2 (medium 2).

[0063] Figures 8 and 9 show on the left the virgin medium and on the right the non-damaged part of the carrier (medium 1, Figure 8) and the broken part of the carrier (medium 1, Figure 9). From these images, it is clear that the wall thickness was not particularly affected during the operation time of the MBBR reactor. Therefore, wall scraping / erosion was not seen very much. This was confirmed in all small pieces tested (but not shown here), including the broken pieces. This means that erosion of the medium by scraping due to wall thinning has little or no relation to the damage of the medium. However, in Figure 9, on the right, it can be seen that the outer wall part of the carrier is neatly cut off by a clean break by the vertical inner walls on each side.

[0064] Figure 10 shows the virgin carrier medium on the left and the damaged carrier medium piece (medium 1) standing on its side. Figures 11 - 14 show the damaged carrier medium (medium 1) standing on its side. From the standing-up angle, the formed stress cracks can be easily seen in all of Figures 10 - 14. Both smaller initiation cracks that have just started to grow and large stress cracks that have grown larger and are on the verge of destroying the outer wall caused by stress cracks that always end in brittle fracture as shown in Figure 10 are seen.

[0065] Figure 15 shows the virgin medium on the left and the damaged carrier medium from a different MBBR process (medium 2 with a different design shape compared to medium 1). From this image, it is clear that the wall thickness has not been particularly affected during the operation time of the MBBR reactor, similar to the case of medium 1. Therefore, little wall scraping / erosion was seen. This was confirmed in all the small pieces tested (but not shown here). This means that the erosion of the medium due to scraping caused by wall thinning has little or no relation to the damage of the medium. In Figure 15, it can be seen that the outer fins are shorter, but this may be due to scraping / erosion. However, Figure 16 points out that this is due to another cause.

[0066] In Figure 16, it can be seen that stress cracks have been initiated and started to grow in the vertical direction of the outer fins of medium 2, and have started to separate the fins from the outer wall. The propagation of this stress crack ends in brittle fracture, and the fins fall off from the outer wall, so it is considered that the fins are shorter in Figure 15 compared to the virgin medium. It is not due to erosion. This can be seen more clearly in Figure 17, where the stress crack is growing along the vertical axis of the fin within the notch of the outer wall. This is the clear behavior of the initiation, propagation, and final failure of stress cracks due to long-term continuous low-stress endurance.

[0067] Typically, the outer portion of the carrier (the outer wall in the case of medium 1 and the outer fins and walls in the case of medium 2) absorbs most of the shear stress within the reactor, and when they are partially or completely scaled off, continuous shear stress is applied at the 90-degree corners of the outer wall. At that point, stress cracks will instead start and grow from the detached portions within the outer wall as seen in FIGS. 18 - 20.

[0068] Finally, the outer wall ruptures as seen in FIG. 21, where a complete brittle fracture can be seen at one specific location on the outer wall. Of course, the cracks start at different times and propagate at different speeds, but ultimately, as seen in FIG. 22, two cracks propagate to the point of causing failure, thereby cutting through the entire portion of the outer wall. Generally, the first rupture occurs at the corner between the inner wall and the outer wall because the shear stress absorbed by this "heavier" region is greater. When all portions of the outer wall are finally cut, the shear stress within the reactor is absorbed by the inner wall, and then the inner wall will likewise begin to degrade.

[0069] Therefore, surprisingly, it has been found that the failure mechanism of the carrier medium in the MBBR process is brittle fracture due to the formation and propagation of stress cracks. This was unexpected because the tested media 1 and 2 were different carrier types that had been used in very different processes for different types of wastewater. Medium 1 was used in an aerobic process while medium 2 was used in an anaerobic process using submerged mechanical mixing, so different paths of carrier damage were expected for media 1 and 2.

[0070] Therefore, it has become clear that the HDPE raw material structure utilized in the manufacture of MBBR carrier media should be strengthened to have better properties regarding resistance to stress crack formation.

[0071] Methods to Extend the Life of MBBR Plastic Carriers The toughness of the polymer and its resistance to brittle fracture strongly depend on the molecular structure, particularly the molecular weight, molecular weight distribution, branching, crystallinity, and tie molecules. Tie molecules are embedded in the microcrystalline regions and transverse amorphous regions (see Figure 3), act as mechanical links between the crystal domains, and thus play a decisive role in the resistance to brittle fracture and the overall mechanical properties when subjected to stress.

[0072] It was previously explained that brittle fracture is thought to be caused by unraveling the entanglement of the interlamellar connections. The number and type of these tie molecules play an important role in the stress crack resistance of polyethylene. There are two types of interlamellar connections.

[0073] The first type is called the cross-linked tie molecule. The two ends of these molecules are embedded in two different crystal lamellae and thus connect them. Cross-linked tie molecules have strength by covalent bonds. The other type of interlamellar connection is thought to be made up of the entanglement of loose loops and fibrils and held together by van der Waals forces. Cross-linked tie molecules are called tie molecules. All other types of interlamellar associations are called entanglements.

[0074] The concept of tie molecules was first proposed by Brown and Ward in the study of brittle fracture of polyethylene. Brown and Ward theorized that there are two types of load-bearing molecular bonds in the amorphous phase of polyethylene. The first type consists of the covalent bonds of cross-linked tie molecules, and the second type includes van der Waals bonds between amorphous chains. Therefore, the brittle fracture stress σ F is the sum of the stresses sustained by both types of bonds

[0075] Based on the research by Brown and Ward, Huang and Brown theorized that the polymer chains must have an end-to-end distance (r, radius of gyration) greater than the thickness of the two crystal lamellar layers in order to crystallize in the two lamellae and thus become tie molecules (see Figure 2 again).

[0076] Huang and Brown considered that there were three types of amorphous phase materials, namely fibrils, loose loops, and tie molecules only, and any chain with an end-to-end distance greater than 2L had an equal chance of adopting any one of the three amorphous phase configurations. Therefore, the chance for a chain to become a tie molecule was only 1 / 3.

[0077] The radius of gyration of a molecule is a function of its molecular weight. Based on probability theory and (empirical) experimental observations, Huang and Brown developed a theory to explain the proportion of the area of the amorphous region occupied by cross-linked tie molecules as a function of the weight-average molecular weight (Mw) of the polymer.

[0078] This study showed that tie molecules could not be found below a certain molecular weight. Other studies also found that as the weight-average molecular weight increased, the number of tie molecules formed also increased. This means that as the weight-average molecular weight increases, the tie molecule concentration increases, and thus the stress crack resistance of polyethylene increases.

[0079] Huang and Brown's model gave a good explanation for the molecular weight effect on the stress crack resistance of polyethylene. Therefore, in the past 20 years, most studies on the stress crack resistance of polyethylene have focused only on the effect of tie molecules. However, Huang and Brown's theory could not explain the higher stress crack resistance of polyethylene with a higher comonomer (e.g., small inclusions such as 1-butene or 1-hexane in the polyethylene backbone) content at the high molecular weight end of the molecular weight distribution. In addition, although van der Waals bonds are much weaker than covalent bonds, Brown and Ward felt that they should not be ignored. Other studies in the past 10 - 20 years also speculated that in addition to tie molecules, other interlamellar connections (i.e., chain entanglements) could contribute to the overall environmental stress crack resistance of polyethylene.

[0080] Short-chain branches (SCBs) affect polymer properties by promoting chain entanglement and simultaneously reducing the material density. Studies have shown that as the SCB content increases from 0 to 4.6 butyls per 1000 carbon atoms, the observed rate of slow crack growth decreases by a factor of 104. Janimak and Stevens clarified the relationship between short-chain branching and tie molecule density. They charted their results along with data from Huang and Brown on a plot of tie molecule fraction versus branching density. Both datasets showed an increase in tie molecule fraction with increasing SCB. In addition to the number of SCBs, the length of the SCBs also affects the stress crack resistance of polyethylene. A study conducted by Yeh et al. found that as the SCB length increases from 2 to 6 carbon atoms, the stress crack resistance of polyethylene increases dramatically. This is thought to be due to an increase in the sliding resistance of chains with longer SCB branches.

[0081] The type of catalyst used in polymerization affects the short-chain branching distribution (SCBD) in polyethylene. Polyethylene produced using a Ziegler-Natta catalyst is known to have a higher SCB content at the low molecular weight end of the molecular weight distribution (MWD), as shown in Figure 23a. On the other hand, the use of a Ziegler-Natta catalyst in a tandem polymerization reactor can produce PE with shorter chain branches at the high molecular weight end of the MWD (Figure 22b), but these polyethylene materials also tend to have a bimodal MWD. Examination of metallocene catalysts has shown that, as shown in Figure 23c, short-chain branches are evenly distributed across the MWD. In the case of PE by metallocene catalysts and tandem polymerization PE, the presence of SCBs in higher MW chains results in a higher tie molecule density and thus a greater disruption of the regular chain folding mechanism for lamella formation. Therefore, these two types of polyethylene generally have higher stress crack resistance than polyethylene produced using a Ziegler-Natta catalyst in a standard process.

[0082] SCB facilitates the formation of tie molecules. However, SCB also disrupts the regularity of the microcrystals and impairs the strength of the microcrystals. The decrease in crystallinity means that the material density is low. Density is directly related to the stiffness and tensile yield strength of the polymer. By incorporating different amounts of model tie molecules into the linear polymer, it has been found that when the tie molecule density increases beyond a certain point, the polymer crystallinity and tensile strength are lost. For polyethylene used in applications such as MBBR wastewater treatment plants, both high environmental stress crack resistance and high mechanical stiffness and strength are desirable qualities. Therefore, it is important to balance these two properties. Therefore, due to the nature of the damage found, utilizing HDPE with a bimodal manufacturing technique (tandem process) that includes chain branching at the high molecular weight end of the MWD will generate a density high enough for stiffness and tensile yield strength while promoting much improved stress crack resistance for use as a raw material for the manufacture of plastic carrier media for MBBR with a much extended lifespan, it is hypothesized.

[0083] In summary, stress crack resistance is affected as described by the parameters shown in Table 2.

[0084]

Table 2

[0085] Raw materials for improving the lifespan of MBBR plastic carriers Generally speaking, the three different main types of HDPE are classified into different categories depending on how they are manufactured. These categories are the unimodal homopolymer of HDPE, the unimodal copolymer of HDPE, and the bimodal homopolymer or copolymer of HDPE.

[0086] The molecular structure of HDPE homopolymers is a linear backbone of repeating units (-CH2-CH2-), which means that since HDPE homopolymers are homopolymers produced from a single monomer type, only ethylene is used as the monomer for production. This type of HDPE is produced using one catalyst in one reactor. The result of this process is a polymer with a moderately broad molecular weight distribution (broad MWD), as seen in Figure 24. For example, for a sample of polyethylene, let's say some chains have 50,000 carbon atoms in them and others have 50,002 carbon atoms in them. This small difference doesn't amount to much. However, samples of synthetic polymers where all chains have the same molecular weight are rarely found. Instead, usually, a bell curve or distribution of molecular weights is seen. Some polymer chains are much larger than all the others and are at the high molecular weight end of the curve. Some are much smaller and are at the low molecular weight end of the curve. The largest number is usually accumulated around the central point, which is the highest point on the curve. For a unimodal polyethylene homopolymer, the broad MWD in Figure 24 is given.

[0087] With the evolution of increasingly sophisticated polyethylene technology, alpha olefins 4, 6, and 8 have become beneficial comonomers in the production of a wide range of unimodal copolymer HDPE resins. This also means that not only is ethylene still used as the monomer for HDPE production, but small inclusions such as 1-butene, 1-hexene, or 1-octene can be incorporated into the polymer backbone. By including these monomers, chain branching can be increased.

[0088] In one embodiment, the HDPE includes ethylene and at least one other alkene, such as 1-butene, 1-hexene, or 1-octene, whereby the HDPE becomes an HDPE copolymer.

[0089] Unimodal homopolymers of HDPE have high flexural rigidity (flexural modulus), but low stress crack resistance (ESCR). Reducing the melt index (which means increasing the melt viscosity and increasing the average molecular weight) can help reduce low stress cracking, but the processability of the polymer is impaired by the lower flow rate. The addition of an α-olefin 4, 6, or 8 as a comonomer increases the short chain branching of the resulting polymer, thereby improving the flow characteristics while increasing the stress crack resistance.

[0090] The emergence of new applications with more stringent operating requirements has pushed the performance requirements for HDPE resins to new levels. Today's class of unimodal HDPE resins generally functions very well in terms of physical properties and stress crack resistance (SCR), but bimodal HDPE resins exhibit even more significant improvements in all of these properties. Unimodal HDPE resins are produced using one catalyst in one reactor. The result of this process is a polymer with a moderately broad molecular weight distribution, as shown in Figure 24. This wide range of polymer chain sizes includes both smaller molecules that affect processability (e.g., extrusion flow rate) and much larger molecules that affect physical properties such as mechanical properties and stress crack resistance. Density is an important attribute of PE resins. For a given polyethylene material, reducing the density improves many important physical properties related to ductility (or lack of brittleness), such as SCR and many mechanical properties.

[0091] Density is controlled by incorporating comonomers into the polymer at relatively low levels during polymerization. These act to disrupt the crystal structure, creating short side chain branches and producing a lower density. However, this process is not completely efficient, as comonomers preferentially enter smaller, lower molecular weight chains that are less effective (compared to longer polymer chains) in affecting physical properties (see Figure 23a). This tendency for comonomer incorporation into shorter polymer chains limits the stress crack resistance and certain physical properties of unimodal resins at a given density.

[0092] Bimodal resins are essentially based on a combination of two polymers, a high molecular weight (HMW) polymer and a low molecular weight (LMW) polymer.

[0093] Typically, these resins are produced using two polymerization reactors (LMW and HMW) in series that are operated under separate process conditions. This process allows all of the comonomer to be incorporated into the high molecular weight fraction, and having all of the comonomer incorporated into the high molecular weight fraction is most desired in order to affect the properties (Figure 25). The result of this technique is a substantial jump (dramatic improvement) in the physical properties at a given resin density. Ultimately, this range of improved performance corresponds to a longer service life and potentially increased confidence that the carrier will maintain its integrity through more demanding types of environments.

[0094] Also, for example, during the manufacture of the carrier medium in the molten state, a unimodal molecular weight distribution (Figure 29) can be mixed with one of the bimodal molecular weight distributions of Figure 29, or two of the bimodal molecular weight distributions of Figure 29 can be mixed, or all of the molecular weight distributions of Figure 29 can be mixed to form a carrier medium for MBBR having a multimodal molecular weight distribution. Such compositions may also increase the confidence that the carrier will maintain its integrity through more demanding types of environments. Therefore, carrier media having a multimodal distribution can be utilized and determined in accordance with ASTM D6474.

[0095] Therefore, as described in the present invention, a unimodal molecular weight distribution is described as having one peak in Figure 29, a bimodal molecular weight distribution is described as having two peaks in Figure 29, and a multimodal molecular weight distribution is described as having more than two peaks over the entire molecular weight distribution.

[0096] In one embodiment, a carrier for supporting a biofilm within a moving bed biofilm reactor (MBBR), the carrier comprising a carrier material, the carrier material comprising at least one high density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a bimodal or multimodal molecular weight distribution, is provided.

[0097] The carrier of the present invention maintains its integrity through more demanding types of environments, and thus, in one embodiment, the carrier for supporting a biofilm within a moving bed biofilm reactor (MBBR) has improved stability against environmental stress.

[0098] In one embodiment, a carrier for supporting a biofilm within a moving bed biofilm reactor (MBBR) having improved stability against environmental stress, the carrier comprising a carrier material, the carrier material comprising at least one high density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a bimodal or multimodal molecular weight distribution, is provided.

[0099] A carrier comprising HDPE having a bimodal or multimodal weight distribution has improved stability against environmental stress. This means that the carrier has increased resistance to the long-term damage characterized above, i.e., brittle fracture resulting from the formation and propagation of stress cracks that an MBBR carrier undergoes during use.

[0100] This not only extends the carrier life for general MBBR applications, but the increased resistance may also facilitate, delay, or eliminate the retrofit of older or worn equipment having higher abrasion on the carrier.

[0101] It also means that the carrier may be more resistant to mixing energy, facilitating better use of an immersible mixer or the use of higher energy than typical mixing energy if required.

[0102] The improved carrier properties are also an advantage in wastewater containing more serious plastic harmful substances or in the treatment line for the production of drinking water, and in these scenarios, the generation of microplastics from the plastic carrier medium can be significantly reduced.

[0103] Four bimodal HDPE raw materials were utilized for the production of the first carrier media type for MBBR shown in FIG. 30. These were compared with carrier materials (FIG. 30) manufactured from unimodal HDPE homopolymer and unimodal HDPE copolymer raw materials with respect to their lifespan in the first heavy mixing test in the reactor volume containing water and the carrier media. The test was set to induce an increased shear stress on the carrier media that avoided plastic deformation but induced stress crack formation and brittle fracture, which was of course higher than the real-world situation due to time limitations but still lower than the yield stress of the raw materials.

[0104] For example, a carrier media for MBBR with a multimodal molecular weight distribution could be formed by mixing HDPE with a unimodal molecular weight distribution with HDPE with a bimodal molecular weight distribution during the production of the carrier media, or for example, by mixing two HDPEs with bimodal molecular weight distributions. According to the present invention, the multimodal distribution could be utilized and determined in accordance with ASTM D6474. Utilizing the carrier media for MBBR with the above multimodal molecular weight distribution could be interesting according to the present invention as it could lower the manufacturing price of the carrier media without overly reducing the stress crack resistance of the carrier media. Four batches (three according to the present invention and one control) of the second carrier media type for MBBR shown in FIG. 32, including 5 wt% bimodal HDPE copolymer and 95 wt% unimodal HDPE copolymer, 20 wt% bimodal HDPE copolymer and 80 wt% unimodal HDPE copolymer, 100% bimodal HDPE copolymer, and 100% unimodal HDPE copolymer (control), were also tested for lifespan in the second heavy mixing test.

[0105] Test on the Improved Lifespan of a Modified MBBR Plastic Carrier against Conventional MBBR Carriers The test was carried out in a cylindrical reactor volume with a diameter of 80 cm and a length of 150 cm, using a mixer blade according to Fig. 26 with a total blade length of 35 cm. In addition, a baffle plate is firmly fixed inside the reactor.

[0106] This reactor is filled with 340 L of water and a bulk volume of 170 L of the selected carrier type to obtain a filling degree of 50% in terms of the bulk volume of the carrier medium with respect to the water volume. The carriers are first mixed at a low speed for 24 hours to wet the carriers and make them easily suspend in the water volume during mixing. Then, the frequency is increased to have a tip speed of 60 m / s and a power density of 1700 - 2200 W / m 3 When the efficiency is 80%, the power density is 1700 W / m 3 and becomes so.

[0107] This heavy mixing is carried out until visible damage to the carriers (daily visual inspection of the carriers). The results of the test comparing the time to breakage among the first type of carrier medium (as depicted in Fig. 30) containing different types of bimodal HDPE copolymers, unimodal HDPE copolymers or homopolymers can be seen in Table 3. In addition, the same heavy mixing test was carried out for the second type of carrier medium (as shown in Fig. 32) containing blends (mixtures) of bimodal and unimodal HDPE copolymers with different amounts of bimodal HDPE incorporated into the blend. The results of these tests can be seen in Table 4.

[0108] Unexpectedly, it was found that even carriers with a low 5 wt% of the first high-density polyethylene having a bimodal molecular weight distribution showed a lifespan extended by more than 100% when compared with 100% unimodal copolymer carriers.

[0109] Thus, in one embodiment, the carrier material comprises high density polyethylene having a bimodal molecular weight distribution of at least 5 wt%, such as at least 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt% or 45 wt%, Thereby, this carrier material has a bimodal or multimodal molecular weight distribution.

[0110] In one embodiment, the carrier material comprises a first high density polyethylene having a bimodal molecular weight distribution of at least 5 wt%, such as at least 10 wt%, 15 wt%, 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt% or 45 wt%, and a second high density polyethylene, the second high density polyethylene having a unimodal, bimodal or multimodal molecular weight distribution, preferably having a unimodal molecular weight distribution, whereby the carrier material has a multimodal molecular weight distribution.

[0111] Table 4 also shows the results for a carrier containing a first high density polyethylene having a 20 wt% bimodal molecular weight distribution. This carrier showed a test life of 28 days compared to 7 days for a 100% unimodal copolymer carrier.

[0112] In one embodiment, the carrier material comprises high density polyethylene having a bimodal molecular weight distribution of at least 50 wt%, more preferably at least 80 wt%, most preferably at least 95 wt%, whereby the carrier material has a bimodal or multimodal molecular weight distribution.

[0113] In one embodiment, the carrier material comprises a first high density polyethylene having a bimodal molecular weight distribution of at least 50 wt% and a second high density polyethylene having either a unimodal, bimodal or multimodal molecular weight distribution, preferably having a unimodal molecular weight distribution, whereby the carrier material has a multimodal molecular weight distribution.

[0114] The 100% bimodal copolymer showed a surprising 51-day lifespan compared to the 7-day lifespan of a standard 100% unimodal copolymer carrier. Such a significantly increased resistance to harsh conditions (mimicking those commonly employed in different wastewater treatment plants) thereby significantly extends the carrier lifespan.

[0115] In one embodiment, the carrier material comprises a first high-density polyethylene having a first bimodal molecular weight distribution and a second high-density polyethylene having a second bimodal molecular weight distribution, whereby the carrier material has a multimodal molecular weight distribution.

[0116] Thus, in one embodiment, the carrier material comprises a first high-density polyethylene having a bimodal molecular weight distribution of at least 5 wt%, such as at least 10 wt%, 15 wt%, 20 wt%, 25 wt%, 50 wt%, 95 wt%, and a second high-density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a multimodal molecular weight distribution.

[0117] In one embodiment, the carrier material comprises a first high-density polyethylene having a first bimodal molecular weight distribution and at least one or more high-density polyethylenes having a unimodal or bimodal or multimodal molecular weight distribution, whereby the carrier material has a multimodal molecular weight distribution.

[0118] In one embodiment, the carrier comprises a plurality of high-density polyethylenes having a bimodal molecular weight distribution.

[0119] In one embodiment, the carrier material consists of a single high-density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a bimodal molecular weight distribution.

[0120] In one embodiment, the carrier comprises a high-density polyethylene having a 100% bimodal molecular weight distribution.

[0121] In one embodiment, the carrier material comprising high-density polyethylene having a bimodal molecular weight distribution comprises a low molecular weight fraction (LMW) and a high molecular weight fraction (HMW), and the peak ratio of the LMW fraction to the HMW fraction is from 10:1 to 1:10, preferably from 5:1 to 1:5, more preferably from 2.5:1 to 1:2.5, even more preferably from 2:1 to 1:2, and most preferably from 1.5:1 to 1:1.5.

[0122] In one embodiment, this peak ratio is determined according to method ASTM D6474.

[0123] The improved properties of the carrier medium according to the present invention can cope with coarser conditions over much longer times compared to prior art plastic carrier media, so the significant increase in resistance as described above may mean that renovation of older or worn equipment in an MBBR plant may not be necessary. This would result in savings in both resources and costs. In addition, the improved plastic carrier will increase the range of possible MBBR applications such as extending the use of immersible mixers, treating wastewaters containing substances harmful to HDPE properties, using higher mixing energies if required, and eliminating and / or minimizing the generation of microplastics from the plastic carrier in the treatment line for the production of drinking water.

[0124] It is clear from Table 3 that the bimodal-co-1 to 4 carrier medium is superior to the carrier containing bimodal-co-5 to 6, which in turn is superior to the unimodal copolymer carrier medium, and the unimodal homopolymer carrier medium has the shortest lifespan.

[0125] It is clear from Table 4 that increasing the amount of bimodal HDPE in the blend with unimodal HDPE significantly increases the lifespan. However, it remains interesting to have an MBBR carrier medium with a lower amount of bimodal HDPE. This is because an MBBR carrier medium with a lower amount of bimodal HDPE is manufactured at a lower cost, which means a lower initial cost for the end user, although they will pay for the lower lifespan. The much longer lifespan of the carrier medium used in Table 4 (Figure 32) compared to Table 3 (Figure 30) is due to the lighter type of carrier medium used in the heavy mixing test in Table 4, which results in less shear force generated in the reactor and a longer lifespan.

[0126] Bimodal - 1 to Bimodal - 6 are various commercially available HDPE copolymers with a bimodal molecular weight distribution. Unimodal - co - 1 to Unimodal - co - 4 are various commercially available HDPE copolymers with a unimodal molecular weight distribution. Unimodal - homo is a commercially available HDPE homopolymer with a unimodal molecular weight distribution.

[0127] 100% Bimodal copolymer is a commercially available HDPE copolymer with a bimodal molecular weight distribution. 100% Unimodal copolymer (control) is a commercially available HDPE copolymer with a unimodal molecular weight distribution. 5% Bimodal - co - 95% Unimodal - co, and 20% Bimodal - co - 80% Unimodal - co are 5:95 wt% and 20:80 wt% mixtures of the above - mentioned bimodal copolymer and unimodal copolymer, respectively.

[0128]

Table 3

[0129]

Table 4

[0130] Characteristic Evaluation of Improved MBBR Plastic Carrier As seen in Table 3, some of the bimodal carrier media had a longer lifespan in the heavy mixing test compared to others. Also, some of the unimodal copolymer-based carrier media had a longer lifespan than other unimodal copolymer-based carrier media. Therefore, it was important to characterize these differences in order to understand the preferred molecular structure of the HDPE carrier media according to the present invention. This was done according to the following method of measuring the specific characteristics required for an extended lifespan as a carrier media in different moving bed bioreactor applications.

[0131] The storage modulus (G’, in units of Pa) represents the elastic part of the viscoelastic behavior that describes the solid-state behavior of the sample. The loss modulus (G”, in units of Pa) characterizes the viscous part of the viscoelastic behavior, which can be regarded as the liquid-state behavior of the sample.

[0132] The storage modulus G’ represents the stored deformation energy, and the loss modulus G” characterizes the deformation energy lost (dissipated) by internal friction during flow. A viscoelastic solid with G’ > G” has a storage modulus higher than the loss modulus. This is due to connections within the material, such as by chemical bonds or physico-chemical interactions. On the other hand, a viscoelastic liquid with G” > G’ has a loss modulus higher than the storage modulus. The reason for this is that in most of these materials, there are no such strong bonds between individual molecules.

[0133] Therefore, for further property evaluation, apart from determining the crossover point for characterizing materials as described in Figure 6, the tanδ value could be determined as a function of frequency for all materials. Then, the tanδ values can be compared to examine the differences in internal bond strength between the materials. A higher internal bond strength is considered highly preferable for MBBR applications.

[0134] The tanδ value represents the ratio of the two parts of the viscoelastic behavior according to the following and is calculated by G” / G’. Then, the δ value could be calculated from the tanδ value. 1. For ideal elastic behavior, δ = 0°. There is no viscous part. Therefore, G” = 0 and tanδ = G” / G’ = 0. 2. Ideally, for viscous behavior, δ = 90°. There is no elastic part. Therefore, G’ = 0, and thus the value of tanδ = G” / G’ approaches infinity due to the attempt to divide by zero.

[0135] Therefore, the δ value will vary between 0 and 90°. The lower the δ value, the more elastic the behavior of the material, thereby making the internal chemical bonds or physicochemical interactions stronger, and thus making the material more suitable for MBBR applications.

[0136] Therefore, the carrier media tested in the heavy mixing test were characterized using a vibration rheology test, and the crossover point and δ value were determined for all the materials in Tables 3 and 4 according to Method 1 below.

[0137] Method 1 A TA instruments Discovery HR-2 hybrid rheometer with a plate-plate configuration was used. A 0.4736 g solid plastic carrier sample was placed on the bottom (lower) plate, and the gap between the plates in the rheometer was set to 25.0 mm. Then, the oven chamber was closed, and the sample was completely melted at 180°C. Next, the chamber was opened, and the molten polymer sample was smeared and evenly distributed on the bottom plate. The oven was closed again and brought to 180°C again. The gap between the plates was reduced to 1.0 mm, and vibration measurements were performed as follows. 1. Vibration time sweep at 180°C for 600 s at 2.0% strain and 10 Hz frequency 2. Immediately after the above vibration time sweep, a vibration amplitude sweep was performed at 180°C, 1.0 Hz frequency, and 0.01 - 100% logarithmic strain sweep, and 5 points / decade were recorded. 3. Immediately after the above vibration amplitude sweep, a vibration frequency sweep at 180°C at 5.0% strain and a logarithmic frequency sweep from 0.1 to 64.0 Hz followed, and 10 points / decade were recorded.

[0138] The crossover point was determined as the intersection of the loss modulus and the storage modulus plotted as a function of the angular frequency (from step 3 of method 1).

[0139] The oscillatory melt frequency sweep curve data generated after step 3 of method 1 can be seen in Figure 27 where the storage modulus and the loss modulus are plotted as a function of the angular frequency. Then, the crossover point data was determined from Figure 27 to characterize the tested material, which can be seen in Table 5.

[0140] In addition, δ values were calculated as a function of frequency for all materials. The data is shown in Figure 28. The range of δ values for the frequencies tested for each material can also be seen in Table 6.

[0141] Density values were measured according to ASTM 1505 method to confirm that the modified HDPE properties do not dramatically change the density of the carrier, which needs to be within the same range as the prior art carriers in order to still be usable in MBBR applications. These results can be seen in Table 7.

[0142] The modality (aspect) of the molecular weight distribution (unimodal or bimodal) of the polyethylene was determined according to ASTM D6474, and the unimodal and bimodal molecular weight distributions could be defined as examples according to Figure 29.

[0143] Including a small amount of comonomer branching using α-olefins while still maintaining a density high enough (>0.94 g / cm 3 ) defines the HDPE copolymer in the present invention, while the HDPE homopolymer was defined as not containing such comonomer branching.

[0144] In one embodiment, the carrier material has a density of 0.9 - 1.1 g / cm 3 , more preferably 0.92 - 0.98 g / cm 3a density, more preferably 0.94 to 0.96 g / cm 3 having a density of.

[0145] The density is determined according to the method of ASTM D1505.

[0146]

Table 5

[0147] It is clear from Figure 27 that the MBBR carriers containing the bimodal carrier medium (bimodal - co - 1 to 4 in Table 4) have a significantly lower (about 1 to 1.5) angular frequency at the crossover point than the MBBR carriers containing unimodal HDPE (about 6 to 40). In addition, it can be seen from Table 5 that two additional bimodal carrier media (bimodal - co - 5 and 6) can have a higher angular frequency (about 5 to 13) at the crossover point, but this is accompanied by the result that the time to breakage in the heavy mixing test is shorter, as seen in Table 3.

[0148] Thus, in one embodiment, the carrier material has a crossover point at an angular frequency of < 15 rad / s, preferably less than 6 rad / s, and even more preferably < 3 rad / s. The angular frequency and / or modulus of elasticity at the crossover point may be determined according to Method 1.

[0149] In one embodiment, the carrier material has a crossover point at a modulus of elasticity of 25000 to 45000 Pa. The angular frequency and / or modulus of elasticity at the crossover point may be determined according to Method 1.

[0150] In one embodiment, Method 1 includes determining the crossover point angular frequency, elastic modulus, and / or δ end value using a TA instruments Discovery HR-2 hybrid rheometer with a plate-plate configuration. In this method, a 0.4736 g solid plastic carrier sample is placed on the bottom plate, and the gap between the plates in the rheometer is set to 25.0 mm. Then, the oven chamber is closed, and the sample is completely melted at 180 °C. Next, the chamber is opened, and the molten polymer sample is smeared and evenly distributed on the bottom plate. The oven is closed again and heated to 180 °C again. The gap between the plates is reduced to 1.0 mm, and oscillatory measurements are performed according to the following steps: performing an oscillatory time sweep at 180 °C for 600 seconds at a strain of 2.0% and a frequency of 10 Hz; immediately thereafter, performing an oscillatory amplitude sweep and a logarithmic strain sweep from 0.01 to 100% at 180 °C and a frequency of 1.0 Hz, recording 5 points per decade; immediately thereafter, performing an oscillatory frequency sweep at 180 °C at a strain of 5.0% and a logarithmic frequency sweep from 0.1 to 64.0 Hz, recording 10 points per decade.

[0151] At the same time, the carrier media unimodal - co - 3 and 4, which had lower angular frequencies at the crossover point compared to the carrier media bimodal - co - 5, still had lower lifetimes in the heavy mixing test. This shows an important advantage of the bimodality where the comonomer is incorporated into the higher molecular weight fraction. In Table 5, it can also be seen that the carrier media unimodal - co - 1 and 2 have significantly higher angular frequencies (lower average molecular weights) at the crossover point compared to the carrier media unimodal - co - 3 and 4, and therefore, as seen in Table 3, have shorter lifetimes (lower molecular weights) in the heavy mixing test. Finally, the unimodal homopolymer, which showed a much higher angular frequency at the crossover point and contained no comonomer, had the shortest lifetime in the heavy mixing test (Table 3).

[0152] This clearly demonstrates the importance of bimodality over unimodality and also demonstrates the important advantages of higher molecular weight for different bimodal carrier media as well as for unimodal carrier media. Bimodality with the copolymer only in the higher molecular weight portion has the highest effect on higher stress crack resistance and is followed by a higher angular frequency (higher average molecular weight) at the crossover point. Higher stress crack resistance gives the carrier medium a significantly longer process life or the ability to withstand more severe conditions for a significantly longer time in MBBR applications.

[0153] In one embodiment, the carrier material comprises high density polyethylene having a bimodal molecular weight distribution comprising a low molecular weight fraction (LMW) and a high molecular weight fraction (HMW), where the LMW is a homopolymer or copolymer and the HMW is a homopolymer or copolymer, preferably the LMW is a homopolymer and the HMW is a copolymer.

[0154] Furthermore, the crossover points of the carrier media (depicted in Figure 32) tested in the heavy mixing tests of Table 4, which included blends of unimodal HDPE and different amounts of bimodal HDPE, were determined. The values of the crossover points for these blends are shown in Table 6.

[0155]

Table 6

[0156] The more bimodal HDPE the carrier medium contains, the lower the angular frequency at the crossover point, which is clear from Table 6 and indicates that the average molecular weight of the blend increases when the amount of bimodality in the carrier medium is increased. This is in good agreement with the increase in the time to failure of these carrier media as shown in Table 4. Thus, while it is preferred in terms of performance to utilize 100% pure bimodal carrier medium, however, the carrier medium can also be improved with smaller inclusions of bimodal HDPE. Since bimodal HDPE is generally more expensive to produce than unimodal HDPE, using a blend of bimodal HDPE and unimodal HDPE, or even by blending different bimodal HDPE materials, utilizing less than 100% of pure bimodal HDPE can be an interesting possibility for certain applications.

[0157] In one embodiment, the carrier material has a δ final value < 35 degrees, more preferably < 25 degrees, more preferably < 20 degrees, and even more preferably < 15 degrees. The δ final value may be determined according to Method 1.

[0158] From the data in Figure 28 and Table 7, it is clear that the MBBR carriers containing the bimodal - co - 1 to 4 carrier media have significantly lower δ values across the entire frequency range compared to the carriers containing the bimodal - co - 5 and 6 carrier media, which have δ values similar to those of the unimodal - co - 3 and 4 carrier media. In particular, when higher shear is induced on the sample, they have significantly lower δ values in the higher frequency range. The unimodal - co - 1 and 2 carrier media have higher δ values across the entire frequency range compared to those described above, and the unimodal - homo carrier media have the lowest δ values. This indicates that the carriers containing the bimodal - co - 1 to 4 carrier media, which have the lowest angular frequency at the crossover point, are more elastic in the melt at higher frequencies compared to the bimodal - co - 5 and 6 and unimodal samples. The bimodal - co - 5 carrier media have a higher angular frequency (lower average molecular weight) at the crossover point compared to the unimodal - co - 3 and 4 samples, but have similar δ values in the high - frequency range, again indicating that the bimodality with the comonomer incorporated into the high - molecular - weight portion can offset the lower average molecular weight compared to the unimodal copolymers. The lower δ values at higher frequencies mean that the internal connections within the material, such as chemical bonds or physico - chemical interactions, are stronger. This also emphasizes the significant improvement in the properties of the bimodal carrier media compared to the unimodal carrier media, and the further improvement of increasing the molecular weight of the bimodal carrier media, thereby significantly improving its lifespan in the MBBR process.

[0159]

Table 7

[0160] As can be seen in Table 8, the δ end - value decreases with the increase in the bimodal content in the bimodal - unimodal blend, which indicates stronger internal connections in the blend compared to the pure unimodal carrier media.

[0161]

Table 8

[0162] According to Table 9, most of the carriers containing bimodal HDPE have a slightly lower density than the carriers containing unimodal HDPE, but still fall well within the range suitable for MBBR applications.

[0163]

Table 9

[0164] According to Figure 29, the molecular weight distributions of different HDPEs can be either unimodal or bimodal. The unimodal molecular weight distribution shows one peak across the entire distribution, and the bimodal distribution shows two peaks across the entire molecular weight distribution. According to the present invention, the bimodal distribution is generally utilized and determined according to ASTM D6474.

[0165] Use of plastic carrier media with improved stability against mechanical stress in different MBBR applications The carrier media for MBBR having improved stability against environmental stresses claimed in the present invention not only significantly extends the lifespan of plastic carrier media in general MBBR applications, improves the end-user value in terms of cost efficiency and treatment performance, but also, in addition, eliminates microplastic contamination from worn carrier media over time and is important for other environmental benefits such as reducing the CO2 footprint for the production of carrier media due to their longer lifespan.

[0166] One embodiment is the use of a carrier in a moving bed biofilm reactor (MBBR) process.

[0167] The MBBR process is well-known in the art and is described in several publications, such as H. Odegaard et al. (1994). Briefly, MBBR is a water purification method in which wastewater is fed into a reactor containing carriers with biofilms that promote the desired impurity conversion. The carriers are traditionally made from extruded or injection-molded plastics and usually have a larger surface area than smooth elements of the same size (in many cases having a growth surface up to 100 times larger than the corresponding smooth object), with a density in the range of 0.90 - 1.20, usually 0.92 - 0.98, and particularly 0.94 - 0.96 kg / dm 3 and share other properties such as having a density in the range of 0.90 - 1.20, usually 0.92 - 0.98, and particularly 0.94 - 0.96 kg / dm. Recently, MBBR carriers have become larger and have a more complex substructure so as to increase their specific protected surface area, often having a protected surface area of 500 m 2 / m 3 or more, which can improve the process performance and reduce the required reactor tank volume.

[0168] The MBBR carriers with biofilms are left suspended in the water in the reactor for aerobic, anoxic or anaerobic water purification. Such reactors typically include an inlet pipe and an outlet pipe, and a sieve strainer that holds the carriers and optionally mixing means within the reactor and contains a number of carriers with biofilm formation on the protected surface of the carriers. Thus, when the carriers repeatedly collide with each other and are suspended within the reactor volume by the use of air and / or mechanical mixers, they repeatedly collide with the reactor walls and other equipment inside the reactor, continuously applying a significant shear stress to the carriers.

[0169] In the present invention, it has been found that the MBBR carrier is particularly sensitive to stress damage (as shown in FIGS. 8 to 22), and that an MBBR carrier having a carrier material with a bimodal or multimodal molecular weight distribution achieves improved stability against environmental stress in certain MBBR applications. This becomes particularly evident when using processes with larger carriers and smaller reactor tank volumes, since larger sized carriers and smaller tank volumes will increase the shear forces acting on each specific carrier. Similarly, carriers with complex sub-structures are likely to be weaker in terms of coping with these higher shear forces without breaking.

[0170] In one embodiment, the carrier has a protected surface area of at least 200 m 2 / m 3 , preferably at least 300 m 2 / m 3 , more preferably at least 400 m 2 / m 3 , for example at least 500 m 2 / m 3 , for example at least 600 m 2 / m 3 , for example at least 800 m 2 / m 3 , or has a protected surface area of 200 to 1200 m 2 / m 3 , for example 300 to 1100 m 2 / m 3 , for example 300 to 1000 m 2 / m 3 .

[0171] In one embodiment, the use of the carrier is in an MBBR process for purifying a liquid from contaminants, the MBBR process utilizing an MBBR system comprising at least one bioreactor, the bioreactor being aerated and / or mixed continuously or intermittently, the carrier being retained within at least one bioreactor, the carrier being kept in a suspended and mobile state either continuously or intermittently by aeration and / or by mixing of the liquid to be purified, and the carrier providing a protected surface for biofilm growth.

[0172] In one embodiment, a method of purifying a liquid from contaminants using an MBBR system, continuously or intermittently adding the liquid to be purified to a bioreactor containing and holding an MBBR carrier having improved resistance; continuously or intermittently aerating and / or mixing the liquid, thereby keeping the carrier in a suspended and mobile state; continuously or intermittently removing the purified liquid from the at least one bioreactor and the carrier providing a protected surface for biofilm growth of microorganisms that feed on the contaminants in the liquid to be purified.

[0173] In one embodiment, the carrier has a structure that enables a protected surface for biofilm growth, the protected surface being facilitated by the presence of holes, wells, protrusions, honeycomb structures or lattice structures (made from thin materials), and the carrier being more susceptible to stress damage.

[0174] However, some specific MBBR applications will gain even more benefits from using the carrier medium according to the present invention than general MBBR applications. These specific applications are listed below.

[0175] High-energy MBBR applications In MBBR applications that require high mixing energy or aeration energy, the shear forces within the bioreactor, and thus the shear forces applied to the carrier medium, are significantly higher than in MBBR applications where lower mixing energy or aeration energy is utilized. These higher shear forces acting on the medium shorten its lifespan until failure and force the plant operator to replace the carrier medium at a much higher cost and in a much shorter time than normal. Additionally, if the carrier medium is not replaced in a timely manner, broken plastic pieces can pass through the screen, deteriorating the biological treatment performance and potentially releasing plastic into the natural environment in terms of microplastic and macroplastic pollution. By utilizing the carrier medium according to the present invention, the operator of a wastewater treatment plant utilizing an MBBR application with high mixing energy and / or aeration energy can avoid unnecessary costs, avoid potential releases of microplastics and macroplastics, and reduce the CO2 footprint due to the significantly improved properties and lifespan of the carrier medium.

[0176] Shear force depends on several factors such as the reactor layout, etc., but is essentially related to the reactor size and the kinetic energy of the carrier. In the case of a mechanically mixed reactor, at least 15 W / m 3 The supplied stirring energy per reactor volume is generally regarded as a high-energy MBBR application. Similarly, at least 40 W / m 3 An aeration reactor with the supply effect of aeration energy per reactor volume is generally regarded as a high-energy application.

[0177] In one embodiment, the moving bed biofilm reactor (MBBR) process is a high-energy MBBR process. In a further embodiment, this high-energy process has at least 15 W / m 3 Stirring energy per reactor volume or at least 40 W / m 3 Aeration energy per reactor volume.

[0178] Submersible mixer applications MBBR plants that utilize submerged (in water) mixers generally present higher shear stress to the MBBR carriers when in motion compared to mixers mounted on top. Therefore, the use of such mixers, which is necessary in some wastewater treatment plants due to design and site limitations, can have an adverse effect on the carrier media lifespan. Therefore, utilizing the carrier media according to the present invention in such plants will significantly improve the lifespan of the carrier media despite the use of immersible mixers.

[0179] In one embodiment, the MBBR process includes mixing carriers in a bioreactor that utilizes a submerged mechanical mixer.

[0180] Retrofitted MBBRs, or existing MBBR plants with poor reactor wall conditions Retrofitting existing wastewater treatment plants and their infrastructure to match the average concentration of municipal sewage is generally necessary when the organic matter loading rate to the plant increases. In order to still be able to meet the effluent limit, the plant needs to be upgraded or retrofitted. The retrofit may include adding MBBR carrier media to the existing reactor volume to increase the treatment capacity. Generally, newly constructed and unused MBBR bioreactors will have smooth inner walls as depicted in Figure 38. Existing volumes that have already been utilized may have poor internal reactor walls that have deteriorated over time due to wear, which can cause, for example, but not limited to, wall cavities (Figure 39), protruding sharp objects (Figure 40), rough joints (Figures 41 and 42), incomplete gradients (grades) (Figure 43), or more severe wall deterioration (Figure 44). All of these types of defects or deteriorations will cause an increase in the shear stress on the carrier media maintained during aeration when the carrier media hits the reactor wall, thereby shortening their lifespan.

[0181] Generally, the reactor walls should be smooth so as not to cause enhanced wear of the carrier medium. The inner wall of a concrete tank should have the smoothness normally achieved using a steel formwork, and the joint ridges should be smoothly ground. Repairing a worn and rough wall to a smooth inner wall structure is costly and also means that the reactor has to be taken out of service during the repair. Instead, by using the carrier medium of the present invention, which has significantly improved stability against environmental stresses, it may be possible to avoid wall repair by using a carrier medium that can still withstand a rough wall structure during operation.

[0182] In one embodiment, the MBBR process utilizes a bioreactor having a non-smooth inner wall with defects or high roughness, for example, in an MBBR retrofit or an existing MBBR plant with poor reactor wall conditions.

[0183] MBBR applications where the wastewater contains harmful substances harmful to the carrier medium The stress cracking behavior of the carrier medium under stress and the time to brittle fracture can be further shortened when exposed to a stress cracking agent. For example, a stress cracking agent such as a solvent or a cleaning agent acts to reduce the cohesive forces that hold the tie molecules within the microcrystals, thus promoting faster (unwinding of entanglements) disentanglement from the crystalline lamellar regions. As a result, stress cracking is initiated even at lower stress values. Since wastewater and especially more concentrated industrial wastewater streams may contain some of these stress cracking agents, either at lower or higher concentrations, using the carrier medium according to the present invention will significantly increase the resistance to such stress cracking agents, thereby significantly extending the lifespan of the carrier medium. In some industrial applications where the wastewater is highly contaminated with substances that have a harmful effect on the molecular and / or physical stability of the prior art plastic MBBR carrier medium, the improved stability of the plastic MBBR carrier medium according to the present invention will mitigate such harmful effects.

[0184] In one embodiment, the wastewater purified by the MBBR process contains harmful substances that are harmful to the carrier.

[0185] MBBR applications with external chemical addition In many biological wastewater treatment plants, an external carbon source such as methanol or ethanol is added to a specific biological reactor to promote denitrification in the wastewater. Such carbon sources can act as stress crackers as described above, shortening the time to stress crack formation in the carrier medium and thereby potentially shortening its lifespan. In such applications, by utilizing the carrier medium according to the present invention, such harmful effects can be reduced and the lifespan of the carrier medium in the environments and applications where such external carbon source addition is carried out can be significantly increased.

[0186] In one embodiment, the MBBR process function depends on the addition of external chemicals such as, for example, but not limited to, an external carbon source.

[0187] MBBR as part of a treatment line for potable water production applications Using MBBR as part of a treatment line for the production of drinking water is an emerging technology. When using MBBR treatment to produce drinking water, a significant amount of excess suspended sludge is not generated in the reactor. This is because the water being treated for the production of drinking water is very dilute and does not contain as many pollutants as wastewater, so almost all of the biomass is formed as a biofilm on the carrier medium. Therefore, any possible degradation of the carrier medium due to shear stress in the MBBR reactor that generates microplastics can be transferred to the effluent from the treatment process and further enter the treatment line and the produced drinking water. In general MBBR applications for wastewater treatment, this possible microplastic contamination gets trapped in the suspended sludge, and then this suspended sludge is separated from the treated effluent, thereby preventing any possible microplastic contamination in the effluent. However, this is not necessarily the case in a treatment line for the production of microplastics. This is because very little suspended solids are generated due to the diluted water, and any possible microplastic contamination from the carrier medium can proceed further downstream in the treatment line. Using the carrier medium of the present invention, microplastic formation does not occur due to the significantly improved properties of the carrier medium, and thus the carrier medium of the present invention is well-suited for use as part of a treatment line in the production of drinking water in MBBR applications.

[0188] In one embodiment, the MBBR process is utilized as a treatment for the production of drinking water.

[0189] The present invention has been described above with reference to specific (and in some cases multiple) embodiments, but the present invention is not intended to be limited to the specific forms described herein. Rather, the present invention is limited only by the appended claims, and embodiments other than the specific above-described embodiments are equally possible within the scope of these appended claims, for example, different from those described above.

[0190] In the claims, the term "comprises / comprising" does not exclude the presence of other elements or steps. Further, even if individually listed, a plurality of means, elements or method steps may be implemented by, for example, a single unit or processor. Additionally, although individual features may be included in different claims, these may advantageously be combined if possible, and being included in different claims does not imply that a combination of features is not feasible and / or not advantageous. In addition, reference to the singular does not exclude the plural. The terms "a", "an", "first", "second", etc. do not exclude a plurality. The reference signs in the claims are provided merely as illustrative examples and shall in no way be construed as limiting the scope of the claims.

[0191] References Brown, N. and Ward, I.M. (1983), "The influence of morphology and molecular weight on ductile-brittle transitions in linear polyethylene", Journal of Materials Science, Vol. 18, pp. 1405 - 1420. Huang, Y. and Brown, N. (1988), "The effect of molecular weight on slow crack growth in linear polyethylene homopolymers", Journal of Materials Science, Vol. 23, pp. 3648 - 3655. Odegaard H. et al., "A new moving bed biofilm reactor - applications and results", Water Science and Technology, October 1, 1994 (1994 - 10 - 01). Janimak, J.J. and Stevens, G.C. (2001), "Inter-relationships between tie-molecule concentration, molecular characteristics and mechanical properties in metallocene catalysed medium density polyethylenes", Journal of Materials Science, Vol. 36, No. 8, pp. 1879-1884. Yeh, J.T. and Runt, J. (1991), "Fatigue crack propagation in high-density polyethylene", Journal of Polymer Science, Part B: Polymer Physics, Vol. 29, pp. 371-388.

Claims

1. A carrier for supporting a biofilm in a moving bed biofilm reactor (MBBR), the carrier comprising a carrier material, the carrier material comprising at least one high density polyethylene having a bimodal molecular weight distribution and not comprising polypropylene, whereby the carrier material has a bimodal or multimodal molecular weight distribution characterized carrier.

2. The carrier material has a density of 0.9 to 1.1 g / cm ３ The carrier according to claim 1 having such density.

3. The carrier according to claim 1 or claim 2, wherein the carrier material has a crossover point at an angular frequency of < 15 rad / s.

4. The carrier according to any one of claims 1 to 3, wherein the carrier material has a crossover point at an elastic modulus of 25,000 to 45,000 Pa.

5. The carrier according to any one of claims 1 to 4, wherein the carrier material has a δ end value of less than 35 degrees.

6. The carrier material is comprising at least 5% by weight of high density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a bimodal or multimodal molecular weight distribution, according to any one of claims 1 to 5.

7. The carrier material is comprising at least 5% by weight of a first high density polyethylene having a bimodal molecular weight distribution, and a second high density polyethylene, the second high density polyethylene having a unimodal, bimodal or multimodal molecular weight distribution whereby the carrier material has a multimodal molecular weight distribution, according to any one of claims 1 to 6.

8. The carrier material is comprising at least 50% by weight of high density polyethylene having a bimodal molecular weight distribution whereby the carrier material has a bimodal or multimodal molecular weight distribution, according to any one of claims 1 to 7.

9. The carrier material is comprising at least 50% by weight of a first high density polyethylene having a bimodal molecular weight distribution, and a second high density polyethylene having any of a unimodal, bimodal or multimodal molecular weight distribution whereby the carrier material has a multimodal molecular weight distribution, according to any one of claims 1 to 8.

10. The carrier according to any one of claims 1 to 6 and claim 8, wherein the carrier material consists of a single high density polyethylene having a bimodal molecular weight distribution, whereby the carrier material has a bimodal molecular weight distribution.

11. The carrier material is a first high-density polyethylene having a first bimodal molecular weight distribution and a second high-density polyethylene having a second bimodal molecular weight distribution The carrier according to any one of claims 1 to 9, comprising the same, whereby the carrier material has a multimodal molecular weight distribution.

12. The carrier material is a first high-density polyethylene having a bimodal molecular weight distribution of at least 5% by weight, and a second high-density polyethylene having a bimodal molecular weight distribution The carrier according to claim 11, comprising the same, whereby the carrier material has a multimodal molecular weight distribution.

13. The carrier material is a first high-density polyethylene having a first bimodal molecular weight distribution, and at least one or more high-density polyethylenes having a unimodal or bimodal or multimodal molecular weight distribution The carrier according to any one of claims 1 to 9, comprising the same, whereby the carrier material has a multimodal molecular weight distribution.

14. The carrier material containing high-density polyethylene having a bimodal molecular weight distribution is a low molecular weight fraction (LMW), and a high molecular weight fraction (HMW) The carrier according to any one of claims 1 to 13, comprising the same, wherein the peak ratio of the LMW fraction to the HMW fraction is from 10:1 to 1:

10.

15. The carrier material containing high-density polyethylene having a bimodal molecular weight distribution is a low molecular weight fraction (LMW), and a high molecular weight fraction (HMW) The carrier according to any one of claims 1 to 14, comprising the same, wherein the LMW is a homopolymer or copolymer and the HMW is a homopolymer or copolymer.

16. The carrier according to any one of claims 1 to 15, wherein the carrier is disc-shaped or saddle-shaped.

17. The carrier according to claim 16, wherein the carrier has holes, wells, protrusions, honeycomb structures or raster structures.

18. Use of the carrier according to any one of claims 1 to 17 in a moving bed biofilm reactor (MBBR) process.

19. The use according to claim 18, wherein the MBBR process comprises mixing the carrier in a bioreactor using an immersed mechanical mixer.

20. The use according to claim 18 or 19, wherein the MBBR process is a treatment for the production of drinking water.

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