A denitrification and phosphorus removal method based on sand filtration in-situ optimization

Abstract

The application discloses a denitrification and dephosphorization method based on sand filtration in-situ optimization, which comprises the following steps: (1) mixing green algae obtained by centrifugal treatment of green algae bacteria liquid with sodium alginate solution to obtain a sodium alginate green algae mixed solution; and preparing a metal cation solution; (2) pouring the sodium alginate green algae mixed solution into a sand filter tank filled with sand filter fillers in 1-2 times; then pouring the metal cation solution into the sand filter tank in 1-2 times, and solidifying to obtain modified sand filter fillers wrapped with microbial hydrogel; (3) feeding sewage to be treated into the sand filter tank for denitrification and dephosphorization purification; and (4) after the sand filter tank is operated for 15-30 days, discharging sewage in the sand filter tank, and repeating steps (1)-(3). The cross-linking reaction of sodium alginate and metal cations forms hydrogel, the green algae is fixed on the outer layer of quartz sand, and microbial hydrogel is formed, so that the erosion and wear of quartz sand in the filtration process can be protected, and the denitrification and dephosphorization effect is improved.

Description

Technical Field

[0001] This application relates to the field of water treatment technology, specifically to a nitrogen and phosphorus removal method based on in-situ optimization of sand filtration. Background Technology

[0002] Nitrogen and phosphorus are the main factors causing algal blooms and eutrophication. In nature, eutrophication is a phenomenon of ecosystem imbalance that can lead to the extinction of species within the aquatic ecosystem. When natural water bodies receive nitrogen- and phosphorus-laden wastewater, it promotes the rapid reproduction of autotrophic microorganisms, whose rapid growth rate leads to a shortened lifespan. After algae die, nitrogen and phosphorus remain in the water and are absorbed by other algae. Eutrophic water environments promote the proliferation of various algae, especially those with strong reproductive capabilities. This increase in algae not only poses a threat to human health, seriously affecting water quality and producing unpleasant odors, but also threatens the safety of drinking water. Studies have reported that chlorination of algae may generate trace amounts of potentially carcinogenic substances. When aquatic animals ingest toxins produced by cyanobacteria, the consequences can be fatal. Furthermore, toxins produced by red tide bacteria pose a potential threat to human health. Furthermore, eutrophication of water bodies negatively impacts water supply systems, increasing operating costs and consequently adversely affecting the fisheries industry, causing significant losses to aquaculture. Therefore, nitrogen and phosphorus removal are crucial in water treatment processes.

[0003] While my country's municipal wastewater treatment infrastructure is gradually improving and water treatment efficiency is increasing, residual nitrogen and phosphorus nutrients still exist in reclaimed water, further increasing the risk of eutrophication in rivers, lakes, and landscape water bodies. Traditional nitrogen and phosphorus removal processes are independent; both polyphosphate-accumulating bacteria and denitrifying bacteria are heterotrophic, leading to competition for carbon sources. Furthermore, biological nitrogen and phosphorus removal suffers from inherent contradictions in their processes: nitrifying bacteria require long sludge ages for growth, while polyphosphate-accumulating bacteria require short sludge ages for phosphorus removal, making simultaneous nitrogen and phosphorus removal in biological systems extremely difficult to control. Conventional municipal wastewater treatment processes often combine flocculation and adsorption technologies for advanced treatment, achieving a total phosphorus level below 0.3 mg / L. However, these methods struggle to quantitatively and target organic phosphorus removal, and the inefficient use of excessive reagents significantly increases chemical consumption and costs at wastewater treatment plants, contradicting the development direction of pollution reduction, carbon reduction, and green environmental protection in wastewater treatment. Therefore, developing advanced and cost-effective high-efficiency nitrogen and phosphorus removal technologies is becoming increasingly important.

[0004] Sand filtration is a rapid method for separating insoluble substances in water and wastewater treatment. To obtain high-quality effluent, filtration technology is widely used in the advanced treatment of wastewater. Typically used after chemical coagulation and biological treatment, filtration involves passing water through a bed of porous media such as sand, coal particles, or diatomaceous earth to separate suspended solids. Its main function is to remove dispersed suspended inorganic and organic particles, including various plankton, bacteria, filterable viruses, floating oil, and emulsified oil. However, sand filtration has limited effectiveness in removing some tiny particles and dissolved substances. To maintain filtration efficiency, the quartz sand filter media needs to be replaced regularly, increasing operating costs and maintenance workload. Furthermore, sand filtration is highly sensitive to water quality; poor water quality may lead to premature clogging or decreased filtration efficiency. Therefore, optimization of the sand filtration process is necessary to improve its wastewater treatment efficiency and reduce maintenance costs.

[0005] Patent CN118529889A establishes a resource-based treatment system for lithium iron phosphate production wastewater. It adds a sand filter and sand filter backwashing system after the clear water tank to filter out fine particles, suspended solids, colloids, organic matter, and other impurities brought in by the influent. The filtration mechanism involves a dense filter membrane forming on the upper part of the filter layer, which traps impurities in the water, ensuring excellent effluent quality. However, the addition of the sand filter backwashing system increases the complexity of the process, as well as costs and daily maintenance expenses.

[0006] Patent CN118420173A designs a method for deep water treatment using potassium permanganate catalytic oxidation and biological activated carbon. This method can replace ozone oxidation and biological activated carbon in a mid-stage activated carbon tank process, achieving the target water quality for deep treatment. It effectively ensures the required effluent turbidity and eliminates the water quality safety risks posed by microorganisms carried in the effluent from biological activated carbon. The purified water after activated carbon layer treatment undergoes sand filtration, forming a mid-stage activated carbon tank process: "raw water → pre-oxidation → coagulation and sedimentation → potassium permanganate → catalytic membrane-supported floating ceramic mesh cage → biological activated carbon → sand filtration." While this method further eliminates the risk of microorganisms carried in the effluent from biological activated carbon, it does not consider the saturation of the filter media in the sand filter, requiring regular replacement or backwashing. Summary of the Invention

[0007] This application provides a nitrogen and phosphorus removal method based on in-situ optimization of sand filtration. It utilizes the cross-linking reaction of sodium alginate and metal cations to form a hydrogel, fixing green algae onto the outer layer of quartz sand to form a microbial hydrogel. On one hand, this protects the quartz sand from erosion and wear during filtration; on the other hand, the hydrogel contains green algae capable of photosynthesis, resulting in excellent nitrogen and phosphorus removal from the water. Furthermore, this technology optimizes sand filtration in situ, simultaneously achieving nitrogen and phosphorus removal from wastewater.

[0008] A nitrogen and phosphorus removal method based on in-situ optimization of sand filtration, characterized by comprising:

[0009] (1) The green algae obtained by centrifuging the green algae culture was mixed with sodium alginate solution to obtain sodium alginate-green algae mixture; and a metal cation solution was prepared.

[0010] (2) The sodium alginate-green algae mixture is poured into a sand filter filled with sand filter media in 1 to 2 times; then the metal cation solution is poured into the sand filter in 1 to 2 times to solidify, and the surface of the sand filter media is coated with microbial hydrogel after solidification.

[0011] (3) The wastewater to be treated is fed into the sand filter tank for denitrification and phosphorus removal purification;

[0012] (4) After the sand filter has been running for 15 to 30 days, the wastewater in the sand filter should be drained and steps (1) to (3) should be repeated.

[0013] This application addresses the problems existing in the prior art by designing a nitrogen and phosphorus removal method based on in-situ optimization of sand filtration. Sodium alginate containing green algae and a metal cation solution are sequentially added to the sand filter, directly covering the surface of the quartz sand with a layer of microbial hydrogel. The microbial hydrogel not only reduces the wear and tear on the quartz sand packing material, but the green algae within it can also further remove nitrogen and phosphorus from the wastewater. Furthermore, the hydrogel has high mechanical strength and wear resistance, thus allowing for a long interval between the next application of sodium alginate and metal cation solution, resulting in less maintenance time, a simple process, and low cost.

[0014] Several alternative methods are provided below, but they are not intended as additional limitations on the overall solution above. They are merely further additions or optimizations. Provided there are no technical or logical contradictions, each alternative method can be combined individually with respect to the overall solution above, or multiple alternative methods can be combined with each other.

[0015] In step (1):

[0016] Optionally, the mass percentage concentration of sodium alginate in the sodium alginate-green algae mixture is 0.1-0.5%. Too low a concentration makes it difficult to form a hydrogel with the metal cation solution, while too high a concentration easily leads to agglomeration on the surface of the quartz sand, affecting the removal of suspended solids from the water.

[0017] Optionally, the mass percentage concentration of green algae in the sodium alginate-green algae mixture is 5% to 15%. Too low a proportion of green algae will result in unsatisfactory denitrification and phosphorus removal, while too high a proportion will waste the bacterial solution and inhibit bacterial growth and metabolism.

[0018] Optionally, the metal cation in the metal cation solution is a copper, calcium, barium, or strontium cation. A CaCl2 solution is preferred; because Ca... 2+ It has a better cross-linking effect with sodium alginate, which is more conducive to the formation of hydrogels.

[0019] Optionally, the mass percentage concentration of the metal cation solution is 2%-3%. If the cation solution concentration is too low, the hydrogel formation will be incomplete; if the concentration is too high, it can easily enter water bodies and cause pollution.

[0020] Optionally, the green algae bacterial solution is one or more of the genera *Chlorella*, *Chlamydomonas*, *Hydrocotyle*, *Volvox*, *Spirogyra*, *Abelmoschus*, or *Pteridium*, and the OD of the green algae bacterial solution... 600 The value is 1-2. Green algae can perform photosynthesis, removing nutrients such as nitrogen and phosphorus from the water.

[0021] Optionally, the green algae undergoes the following pretreatment:

[0022] Centrifuge the green algae culture at a specific speed and time, resuspend it in distilled water to the original mark, centrifuge again, and repeat 1-2 times. Preferably, the centrifugation speed is 4000-8000 rpm. -1 Centrifugation time is 5-15 min.

[0023] In step (2):

[0024] Optionally, the sand filter media is quartz sand.

[0025] Optionally, the volume ratio of the sodium alginate-green algae mixture to the sand filter media is 1:10 to 1:25; the volume ratio of the metal cation solution to the sand filter media is 2:10 to 4:10; and the curing time is 10-15 hours. If the curing time is too short, the resulting hydrogel will lack sufficient mechanical strength and be easily damaged; if the time is too long, the activity of the green algae will decrease.

[0026] Optionally, the thickness of the microbial hydrogel coating on the surface of the sand filter media ranges from 0.5 to 2 mm. Under the preferred combination of conditions described above, the thickness of the microbial hydrogel formed on the surface of the quartz sand is within the range of 0.5 to 2 mm. If the thickness is too thin, it is easy to fall off, and if the thickness is too thick, it will affect the transferability.

[0027] As the sand filter operates, the microbial hydrogel formed on the surface of the quartz sand will be degraded. Therefore, the quartz sand needs to be re-coated after a certain period of in-situ operation in the sand filter. The timing of re-coating depends on the state of the microbial hydrogel (such as the integrity of the coating, the thickness of the coating, and the elasticity of the hydrogel). Generally, after 15-30 days of operation, the state of the microbial hydrogel deteriorates significantly, requiring re-coating. When re-coating, the concentration of sodium alginate and metal ions can be appropriately adjusted within the previously mentioned ratio range according to the state of the hydrogel on the quartz sand surface.

[0028] Optionally, the amount of sand filter media added is 20-40% of the volume of the sand filter tank.

[0029] In step (3):

[0030] Optionally, the hydraulic retention time in the sand filter is 10 to 15 hours.

[0031] Optionally, the wastewater to be treated is wastewater that has undergone secondary treatment in a wastewater treatment plant. The sand filter is used for further nitrogen and phosphorus removal from the secondary treated wastewater.

[0032] Compared with the prior art, this application has at least one of the following beneficial effects:

[0033] (1) The method of this application is simple and low-cost: optimization is performed in situ, which is simple to operate and low-cost.

[0034] (2) The method of this application can reduce the wear and consumption of quartz sand: microbial hydrogel is formed on the surface of quartz sand, which can resist external impact and wear, and reduce the replacement and consumption of sand filter media.

[0035] (3) The method of this application can enhance the activity of green algae: by using the cross-linking reaction of sodium alginate and metal cations, green algae are fixed on the outside of quartz sand to form a microbial hydrogel, which maintains the biological activity of the bacteria and at the same time reduces the toxic effects of recalcitrant organic matter on the bacteria.

[0036] (4) The method of this application has the effect of removing nitrogen and phosphorus: it can further remove nutrients such as nitrogen and phosphorus from wastewater. Attached Figure Description

[0037] Figure 1 This is a flowchart illustrating the operation of the method described in this application. Detailed Implementation

[0038] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0039] Unless otherwise defined, 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 application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0040] Example 1

[0041] The sand filter media used in this embodiment is quartz sand with a particle size of 1 mm and a layer thickness of 0.7 m.

[0042] The specific preparation method is as follows:

[0043] (1) The purchased Chlorella (OD600=1, Huachu, compound Chlorella species) was heated at 4000 r / min -1 Centrifuge for 10 minutes, resuspend in distilled water to the original mark, centrifuge again, and repeat twice for later use;

[0044] (2) Preparation of sodium alginate solution: Dissolve a certain amount of sodium alginate in distilled water, heat to boiling and then cool.

[0045] (3) Then add the Chlorella after centrifugation in step (1) into the sodium alginate solution and stir evenly to obtain a Chlorella-sodium alginate mixture. The mass percentage concentration of sodium alginate in the mixture is 0.1% and the mass percentage concentration of Chlorella is 10%.

[0046] (4) Prepare a 2wt% CaCl2 solution;

[0047] (5) The Chlorella-sodium alginate mixture obtained in step (3) is evenly poured into a sand filter containing quartz sand (quartz sand volume ratio is 30%) twice, and then the CaCl2 solution prepared in step (4) is poured in, and allowed to stand for 12 hours to solidify. The volume ratio of sodium alginate-green algae mixture to sand filter media is 1:15; the volume ratio of metal cation solution to sand filter media is 3:10.

[0048] (6) Applications in wastewater treatment:

[0049] The sand filter treated in step (5) was directly applied to the secondary treatment of the wastewater in the wastewater treatment plant for further purification of the water quality, with a hydraulic retention time of 12 hours. The treatment results are shown in Table 1.

[0050] (7) After running for a certain period of time, observe the state of the hydrogel wrapped on the surface of the quartz sand, and repeat steps (1) to (6). In this embodiment, the wrapping is repeated every 20 days. The wrapping is repeated three times. The results of the last two treatments are close to the results of the first treatment.

[0051] Example 2

[0052] The sand filter media used in this embodiment is quartz sand with a particle size of 1 mm and a layer thickness of 0.7 m.

[0053] The specific preparation method is as follows:

[0054] (1) The purchased Chlorella (OD600=1) was heated at 6000 r / min -1 Centrifuge for 10 minutes, resuspend in distilled water to the original mark, centrifuge again, and repeat twice for later use.

[0055] (2) Preparation of sodium alginate solution: Dissolve a certain amount of sodium alginate in distilled water, heat to boiling and then cool.

[0056] (3) Then add the centrifuged Chlorella from step (1) into the sodium alginate solution and stir evenly to obtain a Chlorella-sodium alginate mixture. The mass percentage concentration of sodium alginate in the mixture is 0.5% and the mass percentage concentration of Chlorella is 15%.

[0057] (4) Prepare a 3wt% CaCl2 solution;

[0058] (5) The Chlorella-sodium alginate obtained in step (3) is uniformly poured into a sand filter containing quartz sand (quartz sand volume ratio is 35%) twice, and then the CaCl2 solution prepared in step (4) is poured in, and allowed to stand for 12 hours to solidify. The volume ratio of sodium alginate-green algae mixture to sand filter media is 1:10; the volume ratio of metal cation solution to sand filter media is 4:10.

[0059] (6) Applications in wastewater treatment:

[0060] The sand filter treated in step (5) was directly applied to the secondary treatment of the wastewater treatment plant for further purification of the water, with a hydraulic retention time of 12 hours. The treatment results are shown in Table 1.

[0061] (7) In this embodiment, after every 20 days of operation, the packaging is repeated, and steps (1) to (6) are repeated three times. The results of the last two processing steps are similar to the results of the first processing step.

[0062] Comparative Example 1

[0063] The process involves a single sand filter, without the addition of any other substances, and is otherwise identical to Example 1.

[0064] Comparative Example 2

[0065] The sand filter media used in this comparative example is quartz sand with a particle size of 1 mm and a layer thickness of 0.7 m.

[0066] The specific preparation method is as follows:

[0067] (1) Preparation of sodium alginate solution: To prepare a sodium alginate solution with a mass percentage concentration of 0.5%, add a certain amount of sodium alginate to distilled water to dissolve, heat to boiling and then let cool.

[0068] (2) Prepare a 3wt% CaCl2 solution;

[0069] (3) The sodium alginate obtained in step (1) is uniformly poured into a sand filter containing quartz sand (the volume ratio of quartz sand is 35%) twice, and then the CaCl2 solution prepared in step (2) is poured in, and allowed to stand for 12 hours to solidify. The volume ratio of sodium alginate-green algae mixture to sand filter media is 1:15; the volume ratio of metal cation solution to sand filter media is 3:10.

[0070] (4) Applications in wastewater treatment:

[0071] The sand filter treated in step (3) was directly applied to the secondary treatment of the sewage treatment plant for further purification of the water, with a hydraulic retention time of 12 hours. The treatment results are shown in Table 1.

[0072] (5) Repeat steps (1) to (3) after every 20 days of operation.

[0073] Comparative Example 3

[0074] Chlorella vulgaris at a concentration of 10% of the wastewater mass was directly added to the sand filter based on the wastewater volume, with a hydraulic retention time of 12 hours. The treatment results are shown in Table 1.

[0075] Table 1. Removal of nitrogen, phosphorus, and suspended solids in Examples 1-2 and Comparative Examples 1-3

[0076] Example of effect Total nitrogen removal Total phosphorus removal Example 1 14.7 mg / L → 11.5 mg / L 0.49 mg / L → 0.3 mg / L Example 2 14.9 mg / L → 12 mg / L 0.48 mg / L → 0.25 mg / L Comparative Example 1 14.8 mg / L → 14.5 mg / L 0.47 mg / L → 0.45 mg / L Comparative Example 2 14.5 mg / L → 13.3 mg / L 0.48 mg / L → 0.41 mg / L Comparative Example 3 14.6 mg / L → 13.1 mg / L 0.46 mg / L → 0.37 mg / L

[0077] As shown in Table 1, the sand filtration method optimized by this invention can significantly remove total nitrogen, total phosphorus and suspended solids while protecting the sand filter media, and the effect is significantly better than that of the comparative example.

[0078] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A nitrogen and phosphorus removal method based on in-situ optimization of sand filtration, characterized in that, include: (1) The green algae obtained by centrifugation of the green algae bacterial solution is mixed with sodium alginate solution to obtain a sodium alginate-green algae mixture. The green algae bacterial solution is one or more of the genera Chlorella, Chlamydomonas, Hydrophora, Volvox, Spirogyra, Abelmoschus, or Pteridophyta. The OD of the green algae bacterial solution is... 600 1-2; prepare a metal cation solution, wherein the mass percentage concentration of the metal cation solution is 2%-3%; the metal cation in the metal cation solution is a copper, calcium, barium, or strontium cation; (2) The sodium alginate-green algae mixture is poured into a sand filter filled with sand filter media in 1-2 batches; then the metal cation solution is poured into the sand filter in 1-2 batches for solidification. After solidification, the surface of the sand filter media is coated with microbial hydrogel. The sand filter media is quartz sand. The volume ratio of the sodium alginate-green algae mixture to the sand filter media is 1:10-1:

25. The volume ratio of the metal cation solution to the sand filter media is 2:10-4:

10. The solidification time is 10-15 hours. (3) The wastewater to be treated is fed into the sand filter tank for denitrification and phosphorus removal purification; the wastewater to be treated is the wastewater after secondary treatment in the wastewater treatment plant; (4) After the sand filter has been running for 15 to 30 days, drain the wastewater from the sand filter and repeat steps (1) to (3). Microbial hydrogels can not only reduce the consumption of quartz sand fillers, but the green algae inside can also further remove nitrogen and phosphorus from wastewater.

2. The nitrogen and phosphorus removal method according to claim 1, characterized in that, In step (1), the sodium alginate-green algae mixture contains: sodium alginate at a mass percentage concentration of 0.1-0.5%; and green algae at a mass percentage concentration of 5-15%.

3. The nitrogen and phosphorus removal method according to claim 1, characterized in that, In step (2), the thickness of the microbial hydrogel coating on the surface of the sand filter media ranges from 0.5 to 2 mm.

4. The nitrogen and phosphorus removal method according to claim 1, characterized in that, In step (2), the amount of sand filter media added is 20-40% of the volume of the sand filter tank.

5. The nitrogen and phosphorus removal method according to claim 1, characterized in that, In step (3), the hydraulic retention time in the sand filter is 10-15 hours.

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