A sludge-based water retention agent and its preparation method

Abstract

The present invention discloses a sludge-based water retaining agent and a preparation method thereof, belonging to the technical field of environmental engineering. The surplus sludge is washed with the effluent from the sewage treatment plant, the washed sludge is cracked by cavitation treatment, and is subjected to flocculation treatment with an organic flocculant. After drying treatment, the sludge-based water retaining agent is prepared. The economic cost of preparing the sludge-based water retaining agent in the present invention is relatively low, and the obtained by-products can all be incorporated into the sewage or sludge treatment system in the sewage treatment plant, avoiding possible secondary pollution problems, realizing the sustainable reuse of the surplus sludge in the urban sewage treatment plant, and having good environmental and economic benefits.

Description

Technical Field

[0001] The present invention belongs to the technical field of environmental engineering, and particularly relates to a sludge-based water retaining agent and a preparation method thereof. Background Art

[0002] In order to solve the problem of water resource shortage and improve the plant growth conditions, water retaining materials have been widely used in soil improvement, agricultural water conservation, horticultural planting and other aspects. Water retaining materials such as high molecular polymers like sodium polyacrylate, polyacrylamide, polyethylene oxide, etc. can enable the soil to fully absorb and retain more water, reduce the evaporation and leakage of water, thereby significantly reducing the irrigation frequency, improving the soil moisture conditions, promoting crop growth, and increasing the yield and quality. However, water retaining agents synthesized chemically such as sodium polyacrylate, polyacrylamide and polyethylene oxide may cause soil and water pollution. For example, unpolymerized polyacrylamide monomers may be toxic, posing risks to the environment and human health.

[0003] Patent CN 110776925 B provides a water retaining agent prepared from surplus sludge, a preparation method and a usage method thereof. By using magnetic magnesium-aluminum hydrotalcite, ultrasonic action and electron beam irradiation treatment technology, organic substances such as proteins and polysaccharides contained in the surplus sludge can be degraded. Through the processes of freezing, drying, heat treatment and dissolution, the degraded polysaccharide molecules and the degraded protein molecules form an amphiphilic complex through hydrogen bonding, hydrophobic and electrostatic interactions, etc. Then, through a self-assembly reaction, the amphiphilic complex reacts in an aqueous phase to obtain a stable nanogel, and this nanogel can be used as a water retaining agent. This prior art not only realizes the resource utilization of surplus sludge, but also can improve the water-deficient and oxygen-deficient conditions of sandy soil and repair the ecological environment. The prepared water retaining agent will not cause heavy metal and microbial pollution to the soil during long-term use, nor will it cause soil compaction or deterioration of physical and chemical properties. However, its preparation process is cumbersome, requires the assistance of ultrasonic and electron irradiation technologies, and consumes a large amount of energy. Summary of the Invention

[0004] The object of the present invention is to provide a sludge-based water retaining agent and a preparation method thereof to solve the problems existing in the above-mentioned prior art. The organic components of the excess sludge are mainly composed of microorganisms and their extracellular polymeric substances (EPS). EPS is a high molecular polymer secreted outside the cells by microorganisms during growth and storage, including polysaccharides, proteins, nucleic acids, humic acids, etc. The EPS of the sludge can be divided into dissolved EPS, loose EPS, and compact EPS. Among them, dissolved EPS and loose EPS are polymers with simple structures and low molecular weights, which are easily degraded and not suitable as water retaining materials; compact EPS is mainly composed of substances with large molecular weights such as microbial capsules and humic acids, and has a stable network structure and rich hydrophilic oxygen-containing functional groups. It is a natural polymer with a high molecular weight. Therefore, the compact EPS in the sludge can be regarded as an ideal raw material for preparing water retaining materials, with the advantages of strong water absorption, green natural, and environmentally friendly. In order to efficiently crack the microbial cells in the excess sludge and at the same time efficiently separate and extract EPS to prepare a green bio-based water retaining material with a high water absorption rate and strong water retention ability, the present invention uses physical elutriation combined with a hydrodynamic cavitation reactor to perform efficient pretreatment on the excess sludge, and then separates and extracts EPS to obtain a sludge-based water retaining agent, realizing the sustainable reuse of the excess sludge.

[0005] One of the technical solutions provided by the present invention:

[0006] A preparation method of a sludge-based water retaining agent, comprising the following steps: using the effluent from a sewage treatment plant to elutriate the excess sludge, cracking the elutriated sludge through cavitation treatment, performing flocculation treatment with an organic flocculant, and after drying treatment, preparing the sludge-based water retaining agent.

[0007] The purpose of cracking the elutriated sludge through cavitation treatment is to promote the release of the contents of microbial cells that are easily biodegradable.

[0008] Preferably, a preparation method of a sludge-based water retaining agent, comprising the following steps: using the effluent from a sewage treatment plant to perform primary elutriation on the concentrated excess sludge until the sludge viscosity drops below 5.0 mPa·s when it reaches the same volume as the initial volume, adjusting the sludge pH to below 4.0 to obtain pretreated sludge; using a hydrodynamic cavitation reactor to continuously treat the pretreated sludge to obtain re-treated sludge; performing secondary elutriation on the re-treated sludge to obtain liquid phase a and precipitated sludge a, and incorporating the precipitated sludge a into the sludge system for treatment; adding an agent flocculant to the liquid phase a, then centrifuging, adjusting the pH of the centrifuged supernatant to neutral and then returning it to the sewage treatment plant for treatment as wastewater, and the solid phase part obtained by centrifugation is the precursor of the sludge-based water retaining agent, and drying the precursor of the sludge-based water retaining agent to obtain the sludge-based water retaining agent.

[0009] The greater the viscosity of the sludge, the more soluble and loose extracellular polymeric substances (EPS) in the sludge. The purpose of the primary elutriation in the present invention is to remove the above substances. By primary elutriation, the viscosity of the supernatant sludge is reduced to below 5.0 mPa·s, which means that the soluble EPS and loose EPS are basically removed.

[0010] Preferably, the water content of the concentrated excess sludge is 97%-98%.

[0011] When the sludge is discharged from the sedimentation tank, the water content is 99%-99.5% (the solid concentration is generally 0.5%-1.0%). After simple gravity concentration (such as sedimentation) treatment, the water content drops to 98%-99% (at the same time, the solid content rate increases to 2%-3%), and the volume of the sludge to be treated is reduced to 50% or even lower of the original volume. Treating such sludge at this time requires a smaller treatment container volume and higher treatment efficiency; but if the water content is to be reduced to below 97% (at this time, the solid concentration exceeds 3%), chemical agents and treatment equipment need to be added to achieve it stably, which increases costs and the economy is not ideal; in addition, when the water content is too low (accompanied by too high solid concentration), the sludge will become too viscous, which will increase the treatment difficulty. Therefore, the present invention controls the water content of the concentrated excess sludge at 97%-98%.

[0012] Preferably, the continuous treatment time is 20-30 min.

[0013] In the present invention, by continuously treating the same pretreated sludge repeatedly, the tightly bound extracellular polymeric substances on the sludge are peeled off. When the continuous treatment time is 20-30 min, the peeling of the tightly bound extracellular polymeric substances can be achieved, and the operating cost and effective cracking of sludge cells can be taken into account.

[0014] Preferably, the parameters of the hydrodynamic cavitation reactor are: the minimum diameter of the Venturi tube is 1 / 20 of the diameters of the sludge conveying pipes before and after it, and at the outlet of the sludge conveying pipe, the spraying velocity of the sludge is not less than 5 m / s.

[0015] Preferably, the re-elutriation means using the effluent of the sewage treatment plant to elutriate the re-treated sludge so that the viscosity of the re-treated sludge when it is consistent with the initial volume is reduced to below 5.0 mPa·s.

[0016] Preferably, the organic flocculant is polyvinyl alcohol, and 400-1000 mg of polyvinyl alcohol is added to each liter of liquid phase a.

[0017] More preferably, the polyvinyl alcohol is used as a polyvinyl alcohol solution with a mass concentration of 2%.

[0018] When too little organic flocculant is added, the flocculation effect is poor and the recovery efficiency of tightly bound extracellular polymers is low. However, the excessive addition of organic flocculant does not lead to a significant further improvement in the effect.

[0019] Preferably, the drying treatment is drying at 80 - 100 °C for 6 - 12 hours.

[0020] The second technical solution provided by the present invention:

[0021] A sludge-based water retention agent is prepared by the above preparation method.

[0022] The technical principle of the present invention is as follows: The effluent from the sewage treatment plant is used to initially wash the concentrated excess sludge, which can remove fine colloidal and primary colloidal substances in the excess sludge. These substances are easily degraded by soil microorganisms and are not suitable for preparing water retention agents; in the hydrodynamic cavitation reactor, due to the installation of a Venturi tube, when the sludge passes through the Venturi tube, the jet velocity of the sludge will suddenly increase. When the cavitation bubbles generated by the high-speed jet burst instantaneously, extremely high temperatures and pressures will be generated in an extremely small space and in an extremely short time, that is, the cavitation effect. Since the water molecules in the bubbles are extremely compressed and heated and then rapidly expand, this process will promote the generation of water molecules including hydroxyl radicals (·OH) and hydrogen radicals (·H). These radicals have high chemical activity. When the pH is lower than 4.0, they can carry out advanced oxidation on the sludge through a chain reaction. At the same time, the low pH can also cause the protein on the surface of the sludge microbial cell membrane to denature. Therefore, under the synergistic action of the denaturation of the protein on the cell membrane surface, advanced oxidation, and the high-speed frictional force between sludge particles, the sludge microbial cells will rupture rapidly, so that the cell contents are released.

[0023] After washing the sludge treated by hydrodynamic cavitation, the precipitated part is the sludge microbial skeleton, which can be incorporated into the sewage treatment plant sludge system for treatment. After washing the sludge treated by hydrodynamic cavitation, the obtained liquid phase part contains dissolved organic matter and insoluble tightly bound EPS.

[0024] Polyvinyl alcohol is a water-soluble synthetic polymer prepared by the alcoholysis reaction of polyvinyl acetate. When polyvinyl alcohol is added and dissolved in the liquid phase a, the hydroxyl groups on its molecular chain will form hydrogen bonds with water molecules, making the polymer chain carry a certain charge. These charges can neutralize the charges on the surface of suspended particles, reduce the mutual repulsion between particles, so that the fine insoluble tightly bound EPS is more likely to aggregate into larger flocs. After centrifugation, the supernatant and the solid phase part are obtained. The supernatant is refluxed to the sewage treatment plant, and the dissolved organic matter in it can be used as a carbon source for the sewage treatment plant. The solid phase part is the precursor of the sludge-based water retention agent, which can be made into a sludge-based water retention agent after drying treatment.

[0025] The sludge-based water retention agent made from sludge is mainly composed of compact EPS, which forms a stable network structure. This structural characteristic endows the water retention agent with a certain resistance to biodegradation. At the same time, compact EPS is not easily degraded and contains a large number of hydrophilic functional groups, making the prepared water retention agent contain a large number of hydrophilic functional groups. These functional groups give the water retention agent significant hydrophilicity, so it can effectively adsorb and retain a large amount of water, thereby reducing water evaporation and loss.

[0026] Compared with the prior art, the present invention has the following advantages and technical effects:

[0027] The present invention uses physical methods, namely effluent washing and hydrodynamic cavitation treatment in the sewage treatment plant, to efficiently break down sludge microbial cells, and no other chemicals are introduced except for acid, which has the advantages of being green and low-cost.

[0028] The present invention uses a small amount of organic flocculant and physical centrifugation to effectively separate and extract compact EPS. The prepared sludge-based water retention agent is a natural bio-based material, which has the advantages of being environmentally friendly, having a stable structure, a high water absorption ratio, and a strong water retention ability.

[0029] The economic cost of preparing the sludge-based water retention agent in the present invention is relatively low, and all the by-products obtained can be incorporated into the sewage or sludge treatment system in the sewage treatment plant, avoiding possible secondary pollution problems, realizing the sustainable reuse of the surplus sludge in the urban sewage treatment plant, and having good environmental and economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0031] Figure 1 It is the process flow chart of the preparation method of the sludge-based water retention agent in the embodiment of the present invention;

[0032] Figure 2 It is the structural schematic diagram of the hydrodynamic cavitation reactor used in the embodiment of the present invention, where 1 is the first sludge storage tank, 2 is the stainless steel self-priming vortex pump, 3 is the control valve, 4 is the pressure gauge, 5 is the second sludge storage tank, and 6 is the cavitation tube. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The various exemplary embodiments of the present invention will be described in detail below. This detailed description should not be construed as a limitation of the present invention, but should be understood as a more detailed description of certain aspects, characteristics, and implementation schemes of the present invention.

[0034] It should be understood that the terms described in the present invention are only for describing particular embodiments and are not intended to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0035] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0036] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of this application are merely exemplary.

[0037] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0038] The embodiment of the present invention provides a preparation method of a sludge-based water retaining agent. Physical elutriation combined with a hydrodynamic cavitation reactor is used to pretreat the excess sludge to remove fine colloidal and primary colloidal substances and efficiently crack the sludge microbial cells. Subsequently, an organic flocculant is used to separate the fine EPS to extract and prepare a high-performance water retaining agent, which well realizes the sustainable reuse of sludge. By efficiently cracking the microbial cells of the excess sludge to improve the physical and chemical properties of the sludge, the compact EPS can be effectively separated and extracted to prepare a high-performance water retaining material, thereby providing a new technical means for the efficient reduction and sustainable reuse of the excess sludge.

[0039] Figure 1 is the process flow chart of the preparation method of the sludge-based water retaining agent provided by the present invention. As Figure 1As shown in the figure, first, the effluent from the sewage treatment plant is used to initially wash the concentrated excess sludge to remove fine colloidal and primary colloidal substances until the sludge viscosity of the supernatant drops below 5.0 mPa·s. Subsequently, sulfuric acid is added to adjust the pH of the sludge to below 4.0 to obtain pretreated sludge; the pretreated sludge is continuously treated in a hydrodynamic cavitation reactor for 20 - 30 min to obtain re-treated sludge. After re-washing it, liquid phase a and precipitated sludge a are obtained. The precipitated sludge a can be incorporated into the sewage treatment plant sludge system for treatment; after adding 400 - 1000 mg of polyvinyl alcohol per liter of liquid phase a for flocculation treatment, the supernatant and the sludge-based water-retaining agent precursor can be obtained by centrifugal separation. The supernatant can be refluxed to the sewage treatment plant for treatment, and the sludge-based water-retaining material precursor can be prepared into a water-retaining material after drying treatment.

[0040] The structural schematic diagram of the hydrodynamic cavitation reactor used in the embodiment of the present invention is as Figure 2 shown, including a first sludge storage tank 1 and a second sludge storage tank 5; the first sludge storage tank 1 and the second sludge storage tank 5 are connected through a cavitation pipe; and a stainless steel self-priming vortex pump 2 is arranged on the cavitation pipeline between the first sludge storage tank 1 and the second sludge storage tank 5, which can pump the material in the first sludge storage tank 1 to the second sludge storage tank 5. A control valve 3 and a pressure gauge 4 are arranged between the stainless steel self-priming vortex pump 2 and the second sludge storage tank 5, which are respectively used to control the opening and closing of the cavitation pipe 6 and monitor the pipeline pressure in real time. By opening the control valve 3, cavitation is carried out in the second sludge storage tank 5 to form re-treated sludge.

[0041] Example 1 A preparation method of a sludge-based water-retaining agent

[0042] S1. The effluent from the sewage treatment plant is used to initially wash the concentrated excess sludge with a water content of 97% until the sludge viscosity of the supernatant drops to 4.9 mPa·s. The sludge at this time is adjusted to pH 3.5 with sulfuric acid to form pretreated sludge;

[0043] S2. The pretreated sludge is passed through a hydrodynamic cavitation reactor, where the minimum diameter of the venturi tube is 1 / 20 of the diameters of the sludge delivery pipes before and after it, and at the outlet of the sludge delivery pipe, the ejection velocity of the sludge is 6 m / s, and it is continuously treated for 30 min to obtain re-treated sludge;

[0044] S3. The effluent from the sewage treatment plant is used to re-wash the re-treated sludge to reduce the viscosity of the re-treated sludge to below 5.0 mPa·s, obtaining precipitated sludge a and liquid phase a. The precipitated sludge a is incorporated into the sludge system for treatment. After adding 800 mg of polyvinyl alcohol per liter of liquid phase a and then centrifuging, the pH of the centrifuged supernatant is adjusted back to neutral and refluxed to the sewage treatment plant for treatment as wastewater. The solid phase part obtained is the sludge-based water-retaining agent precursor. The sludge-based water-retaining agent precursor is dried at 100°C for 6 hours to obtain the sludge-based water-retaining agent.

[0045] Add 1 g of the sludge-based water retainer prepared in Example 1 to 1 L of water, stir and mix. After absorbing water for 10 min, remove the supernatant; Water retention capacity test: After the water-absorbed sludge-based water retainer is left standing naturally for 12, 24, 48, 72, and 96 hours, remove the supernatant and test the moisture content of the precipitate.

[0046] The measurement results show that: The sludge-based water retainer prepared in this example has a high water absorption rate. After natural water absorption for 10 min, the water absorption amount reaches 670 g / g, and the brine absorption ratio is 30 g / g; The sludge-based water retainer prepared in this example has a low water release rate and strong water retention capacity. The water retention rate at 12 h is 97.1%, the water retention rate at 24 h is 85%, the water retention rate at 48 h is 54%, the water retention rate at 72 h is 18%, and the water retention rate at 96 h is 4.3%.

[0047] Example 2 Preparation method of a sludge-based water retainer

[0048] S1. Use the effluent from the sewage treatment plant to conduct a primary elutriation of the concentrated excess sludge with a moisture content of 97% to reduce the sludge viscosity of the supernatant to 4.8 mPa·s. After adjusting the pH of the sludge to 3.8 with sulfuric acid at this time, form the pretreated sludge;

[0049] S2. Pass the pretreated sludge through a hydrodynamic cavitation reactor, where the minimum diameter of the Venturi tube is 1 / 20 of the diameters of the sludge conveying pipes before and after it. At the outlet of the sludge conveying pipe, the flow rate of the ejected sludge is 6.5 m / s, and continuously treat for 25 min to obtain the re-treated sludge;

[0050] S3. Use the effluent from the sewage treatment plant to conduct a secondary elutriation of the re-treated sludge to reduce the viscosity of the re-treated sludge to below 5.0 mPa·s, obtain the precipitated sludge a and the liquid phase a. The precipitated sludge a is incorporated into the sludge system for treatment. After adding 600 mg of polyvinyl alcohol to each liter of the liquid phase a, centrifuge. After adjusting the pH of the centrifuged supernatant to neutral, it is refluxed to the sewage treatment plant for treatment as wastewater. The solid phase part obtained is the precursor of the sludge-based water retainer. Dry the precursor of the sludge-based water retainer at 80 °C for 12 hours to obtain the sludge-based water retainer.

[0051] The measurement method is the same as that in Example 1. The results show that: The sludge-based water retainer prepared in this example has a high water absorption rate. The water absorption amount reaches 650 g / g after natural water absorption for 10 min, and the brine absorption ratio is 26 g / g; The sludge-based water retainer prepared in this example has a low water release rate and strong water retention capacity. The water retention rate at 12 h is 95.3%, the water retention rate at 24 h is 82%, the water retention rate at 48 h is 51%, the water retention rate at 72 h is 16%, and the water retention rate at 96 h is 3.7%.

[0052] Comparative Example 1

[0053] Same as Example 1, except that sulfuric acid was not added in S1 to adjust the pH.

[0054] The measurement results show that: for the sludge-based water retaining agent prepared in this comparative example, the water absorption amount in 10 min of natural water absorption reaches 430 g / g, the brine absorption ratio is 18 g / g, the water retention rate at 12 h is 90.5%, the water retention rate at 24 h is 72.4%, the water retention rate at 48 h is 40.7%, the water retention rate at 72 h is 10.1%, and the water retention rate at 96 h is 1.5%.

[0055] Comparative Example 2

[0056] Same as Example 1, except that primary elutriation was not carried out in S1.

[0057] The measurement results show that: for the sludge-based water retaining agent prepared in this comparative example, the water absorption amount in 10 min of natural water absorption reaches 540 g / g, the brine absorption ratio is 21 g / g, the water retention rate at 12 h is 94.8%, the water retention rate at 24 h is 81.2%, the water retention rate at 48 h is 47.1%, the water retention rate at 72 h is 10.5%, and the water retention rate at 96 h is 2.3%.

[0058] It can be seen from this that the sludge-based water retaining agent prepared by the present invention has excellent water retention ability, and since no other substances are introduced, it is green and environmentally friendly. It can be used as a soil water retaining agent in arid areas, and due to its high brine absorption ratio, it can also be used in the improvement of saline-alkali soil, having good application prospects.

[0059] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for preparing a sludge-based water-retaining agent, characterized in that: The following steps are involved: The concentrated excess sludge is initially washed with the effluent from the sewage treatment plant until the viscosity of the supernatant sludge drops below 5.0 mPa·s, and the pH of the sludge is adjusted to below 4.0 to obtain pretreated sludge; The pretreated sludge is continuously treated with a hydraulic cavitation reactor to obtain retreated sludge, the retreated sludge is again elutriated to obtain a liquid phase a and a precipitated sludge a, the precipitated sludge a is incorporated into a sludge system for treatment, an organic flocculant is added to the liquid phase a, and then centrifuged, the pH value of the centrifuged supernatant is adjusted back to neutral and then returned to a sewage treatment plant for treatment as wastewater, the solid phase obtained by centrifugation is a sludge-based water-retaining agent precursor, and the sludge-based water-retaining agent precursor is dried to obtain the sludge-based water-retaining agent; The organic flocculant is polyvinyl alcohol; Add 400-1000 mg of polyvinyl alcohol per liter of liquid phase a; The re-elution refers to the use of effluent from a sewage treatment plant to elute the re-treated sludge.

2. The method for preparing the sludge-based water-retaining agent according to claim 1, characterized in that: The moisture content of the concentrated excess sludge is 97%-98%.

3. The method for preparing the sludge-based water-retaining agent according to claim 1, characterized in that: The continuous treatment time is 20-30 minutes.

4. The method for preparing the sludge-based water-retaining agent according to claim 1, characterized in that: The parameters of the hydraulic cavitation reactor are as follows: the minimum diameter of the venturi tube is 1 / 20 of the diameter of the sludge conveying pipes before and after it, and the sludge ejection velocity at the outlet of the sludge conveying pipe is not less than 5m / s.

5. The method for preparing the sludge-based water-retaining agent according to claim 1, characterized in that: The drying treatment is performed at 80-100° C. for 6-12 hours.

6. A sludge-based water-retaining agent, characterized in that: It is prepared by the preparation method according to any one of claims 1 to 5.

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