WO2024251121A1 - Method for purification and cyclic utilization of eel farming wastewater and use thereof - Google Patents

Abstract

The present invention belongs to the technical fields of cyclic utilization of water resources and environmental protection. Disclosed are a method for purification and cyclic utilization of eel farming wastewater and the use thereof. The method for purification and cyclic utilization of eel farming wastewater specifically comprises: subjecting eel farming wastewater to precipitation, an anaerobic treatment and a filtration and flow-splitting treatment, so as to obtain clear water and concentrated water; dividing the clear water into a first portion and a second portion, subjecting the first portion to catalytic electrolysis for denitrification, mixing same with the second portion, and subjecting the resulting mixture to aeration and biological activation to obtain recyclable effluent; and adding a flocculating agent and a coagulant aid to the concentrated water, and subjecting same to air flotation deslagging, microfiltration, and catalytic electrolysis for denitrification, so as to obtain up-to-standard effluent. The method provided in the present invention is applicable to eel farming wastewater under different pollution conditions, can obtain effluent which has good water quality and meets the effluent standard, and can achieve cyclic utilization and pollution-free discharge of eel farming wastewater; and the method is simple in terms of process and low in terms of treatment cost, and has good practical application prospects.

Description

A method for purifying and recycling eel breeding wastewater and its application Technical Field

The invention belongs to the technical field of water resource recycling and environmental protection, and in particular relates to a purification and recycling method for eel breeding wastewater and its application.

Background Art

Eel (Anguillidae) belongs to the order Anguilliformes, which is also known as eel, white eel, white eel, river eel, etc., including Asian eel, Japanese eel, bicolor eel, American eel (South American eel and North American eel) and European eel. Eel is rich in high-quality protein and various amino acids and vitamins required by the human body, and contains high content of fish oil. It has high nutritional value and tastes fresh, so it has extremely high economic value for breeding.

Since the eel farming process requires regular removal of stale water and replenishment of new water, a large amount of eel farming wastewater is generated. Eel farming wastewater contains high concentrations of pollutants such as ammonia nitrogen, total nitrogen, total phosphorus and COD _Cr . If it is not treated and directly discharged into the environment, it will lead to excessive concentrations of COD _Cr , BOD ₅ , nitrogen and phosphorus in natural water bodies such as rivers and lakes, as well as eutrophication of water bodies. Therefore, eel farming wastewater must be purified.

Since different species of eels grow at different speeds and eat different amounts of bait, the concentrations of major pollutants in the discharged wastewater also vary greatly. For example, during the growth process, the bicolor eel eats less bait every day, and the feces it discharges are also less. The ammonia nitrogen, total nitrogen, total phosphorus and COD _Cr in the discharged wastewater are all low. The ammonia nitrogen in the discharged wastewater is ≤2mg/L, the total nitrogen is ≤5mg/L, the total phosphorus is ≤3.0mg/L and the COD is ≤30mg/L; the American eel grows faster, and during the growth process, it eats more bait every day, and the feces it discharges are also more. The ammonia nitrogen, total nitrogen, total phosphorus and COD _Cr in the discharged wastewater are all high. The ammonia nitrogen in the wastewater is ≤7mg/L, the total nitrogen is ≤20mg/L, the total phosphorus is ≤3.5mg/L and the COD _Cr is ≤50mg/L. In addition to the above, the concentration of major pollutants in eel farming wastewater is also related to the growth period of eels. In the young and middle-aged period, due to fast growth, eating more and discharging more, the concentrations of major pollutants such as ammonia nitrogen, total nitrogen, total phosphorus and COD _Cr in the wastewater are also higher. At the same time, even for the same type of eels and the same pond, the concentrations of major pollutants such as ammonia nitrogen, total nitrogen, total phosphorus and COD _Cr in the wastewater in winter and summer also show a situation of low concentration in winter and high concentration in summer. In addition, the concentration of major pollutants in eel farming wastewater is also closely related to the breeding mode and drainage method.

At present, the treatment of eel farming wastewater mainly focuses on the treatment of COD _Cr , BOD ₅ , ammonia nitrogen and total phosphorus in the wastewater. The main methods include multiple oxidation ponds in series with phosphorus removal agent precipitation, biochemical method, artificial wetland method, and a combination of oxidation ponds and precipitation methods. However, the COD _Cr , BOD ₅ , ammonia nitrogen and total nitrogen in the effluent of the method of multiple oxidation ponds in series plus phosphorus removal agent precipitation are mostly difficult to meet the discharge standards, and will still cause great pollution to the surrounding environment; when the eel breeding wastewater is treated by biochemical method or artificial wetland, due to the low COD _Cr and BOD ₅ in the eel breeding wastewater and the serious imbalance of carbon, nitrogen and phosphorus ratio, it is necessary to continuously add carbon source during operation, the treatment cost is high and the operation effect is poor; when the oxidation pond and precipitation method are combined to remove the total phosphorus and total nitrogen in the eel breeding wastewater, it is necessary to continuously add coagulant, and the addition amount during the treatment process is as high as 120-300g/m ³ , which will produce a large amount of sludge, not only the phosphorus removal effect is poor, but also the cost is as high as 0.6 yuan/m ³ or more. Therefore, with the rapid development of eel breeding industry, there is an urgent need for an economical and practical treatment method that can achieve the purification and recycling of eel breeding wastewater.

Summary of the invention

The purpose of the present invention is to adapt to eel breeding wastewater with different pollution conditions, realize the purification and recycling of eel breeding wastewater, and provide a purification and recycling method and application of eel breeding wastewater.

In the first aspect, the method for purifying and recycling eel breeding wastewater provided by the present invention adopts the following technical scheme:

A method for purifying and recycling eel breeding wastewater, specifically comprising the following steps:

S1. Pretreatment of eel breeding wastewater: performing sedimentation and anaerobic treatment on the eel breeding wastewater to remove solid particles such as feces and residual feed in the water, and oxidizing carbon-containing organic compounds in the eel breeding wastewater into _CO2 and converting organic phosphorus into phosphate to obtain a crude treated water sample; filtering and diverting the crude treated water sample to obtain clear water and concentrated water;

S2. Treatment of clean water: the clean water is divided into a first part and a second part, the first part is subjected to catalytic electrolysis for denitrification, and then mixed with the second part for aeration and biological activation to obtain recyclable effluent;

S3. Treatment of concentrated water: The concentrated water is subjected to flotation, microfiltration and catalytic electrolysis denitrification in sequence to obtain effluent that meets the standards.

In some specific embodiments, in S1, the temperature of precipitation and anaerobic treatment is 15-35° C., and the time is 3-5 hours.

In some specific embodiments, in S1, the filtration and diversion is: using a fiber rotary filter with a filtration pore size of 5-20µm to filter the coarsely treated water sample to obtain clear water and concentrated water.

In some specific implementations, the volume proportion of the clean water is 80-90%, and the volume proportion of the concentrated water is 10-20%.

In some specific embodiments, in S2, the biological activation is: taking the bacterial and algal culture solution and mixing it with aerated clean water, and culturing it at 20-35° C. for 18-36 hours.

In some specific embodiments, the bacteria and algae culture solution includes at least two of photosynthetic bacteria, Chlorella, diatoms, Spirulina and Bacillus subtilis.

In some specific embodiments, the density of bacteria and algae in the bacteria and algae culture solution is 10 ⁷ to 10 ¹⁰ /L.

In some specific embodiments, the amount of the bacteria and algae culture solution added is 1~5‰ (v/v).

In some specific embodiments, the method further comprises step S4, wherein the sludge produced in the sedimentation and anaerobic treatment and flotation deslagging is subjected to gravity separation and filter pressing to obtain dewatered sludge.

In a second aspect, the present invention also provides application of the above method for purifying and recycling eel breeding wastewater in the treatment of eel breeding wastewater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a method for treating eel breeding wastewater provided in Example 1 of the present invention.

DETAILED DESCRIPTION

In order to achieve the purification and recycling of eel farming wastewater with different pollution levels, the inventors have made the following solutions after extensive and in-depth research.

The method for purifying and recycling eel breeding wastewater provided in the present invention comprises pre-treating the eel breeding wastewater to obtain clean water and concentrated water; on the one hand, when the clean water is purified, the clean water is divided into two parts, one part is subjected to catalytic electrolytic denitrification, and the other part is directly aerated and bioactivated after being mixed with the clean water subjected to catalytic electrolytic denitrification, and the obtained effluent can be used for eel breeding, thereby realizing the recycling of eel breeding wastewater; on the other hand, the concentrated water is subjected to flotation, microfiltration and catalytic electrolytic denitrification, and the effluent meets the emission standards.

In the present invention, firstly, the eel breeding wastewater generated in the eel breeding process is precipitated and anaerobically treated, specifically: the eel breeding wastewater is discharged into a sedimentation tank under anaerobic conditions, and solids such as feces and residual feed in the eel breeding wastewater are settled under the action of gravity. At the same time, under the action of anaerobic microorganisms in the sedimentation tank, organic carbon-containing compounds in the sewage are oxidized into carbon dioxide, organic nitrogen compounds are hydrolyzed into ammonia nitrogen, nitrates are denitrified into nitrogen gas, and organic phosphorus compounds are oxidized and decomposed into phosphates, so that the obtained crude treated water sample is subjected to subsequent treatment.

In the present invention, there is no clear limitation on the conditions for precipitation and anaerobic treatment, that is, precipitation and anaerobic treatment can be carried out under conventional conditions; in some specific embodiments, the temperature of precipitation and anaerobic treatment is 15-35°C, more specifically 15°C, 18°C, 20°C, 22°C, 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, 31°C, 32°C, 33°C, 34°C, 35°C or any value therebetween; the time is 3-5h, more specifically 3h, 3.5h, 4h, 4.5h, 5h or any value therebetween.

In the present invention, the roughly treated water sample overflows into the filtering and diverting device for filtering and diverting to obtain clean water and concentrated water with different pollution degrees. The device used in the filtering and diverting is not limited, and any device that can achieve the filtering and diverting effect in the prior art can be used. In some specific embodiments, the filtering and diverting device is selected as a fiber rotary filter, and the main fish pests such as Dactylopius and its eggs, Trichodina and its eggs, and Ichthyophthirius and its eggs in the water body are filtered by the fiber rotary filter to obtain clean water, and a small amount of clean water is used for backwashing to obtain concentrated water.

Among them, a fiber rotary disc machine with a corresponding filter aperture can be selected for filtration according to the turbidity degree of the eel breeding wastewater. The conventional filter aperture of the fiber rotary disc machine is 5~20µm, and specifically can be 5µm, 8µm, 9µm, 10µm, 12µm, 15µm, 16µm, 17µm, 18µm, 19µm, 20µm or any value therebetween.

In some specific embodiments, the volume ratio of clean water and concentrated water obtained in the filtration diversion is regulated to achieve preliminary regulation of the pollution degree of clean water and concentrated water, which is beneficial to subsequent treatment. The volume ratio of the clean water obtained can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90% or any value therebetween, and the volume ratio of the concentrated water obtained can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20% or any value therebetween.

In the present invention, the clean water is treated in a split and combined manner, and the clean water of the first part is first subjected to catalytic electrolysis denitrification, so that the ammonia nitrogen in the water is oxidized to nitrate by the hydroxyl radicals generated by electrolysis during the electrolysis process, and the nitrate is reduced to nitrogen gas by the hydrogen atoms generated by electrolysis, and at the same time, the hydrogen ions, hydroxyl radicals and sodium hypochlorite in the water can react with the ammonia nitrogen and nitrate in the second part to oxidize the ammonia nitrogen in the water to nitrogen gas and reduce the nitrate nitrogen to nitrogen gas. Preferably, the volume ratio of the first part and the second part can be adaptively adjusted according to the pollution condition of the clean water, which has better adaptability.

In the present invention, the following specific reactions are involved in the catalytic electrolytic denitrification in both the treatment of clean water and the treatment of anhydrous water:

1. One of the main reactions of catalytic electrolytic denitrification: H ₂ O→H·+OH·,

Because of the presence of ammonia in wastewater, the standard potential of NH ₃ /N ₂ is -3.1, which is much lower than the standard potential of H ⁺ /H ₂ , which is 0. Therefore, the hydroxyl radicals produced by electrolysis first react with ammonia to form nitrates. At the same time, nitrates react with hydrogen atoms produced by electrolysis to form nitrogen.

NH ₃ +9OH·→NO ₃ ^- +6H ₂ O, 6H·+2NO ₃ ^- →N ₂ ↑+3H ₂ O.

2. The second main reaction of catalytic electrolytic denitrification:

NH ₄ ⁺ +NaClO→NH ₂ Cl+H ₂ O+Na ⁺ (monochloramine),

NH ₂ Cl + NaClO → NHCl ₂ + H ₂ O + Na ⁺ (dichloramine),

2NH ₂ Cl+NaClO→N ₂ ↑+3HCl+H ₂ O+Na ⁺ (main denitrification reaction 1),

Overall reaction formula: 2NH ₄ ⁺ +3NaClO→N ₂ ↑+3HCl+H ₂ O+3Na ⁺ .

While removing ammonia nitrogen, catalytic electrolysis oxidizes organic phosphorus and polyphosphorus in the wastewater into phosphate.

In the present invention, clean water treated by catalytic electrolytic denitrification and aeration is discharged into a biological activation pool, and the water body is mixed with a bacterial and algae culture solution for biological activation, and cultured at 20-35°C for 18-36 hours to rebuild the balance of the water body's microecology and restore the activity of the water body.

The temperature and time required for bioactivation can be determined according to the conditions of the water body. In some specific embodiments, the temperature of bioactivation can be 20°C, 21°C, 23°C, 25°C, 26°C, 27°C, 28°C, 29°C, 30°C, 32°C, 34°C, 35°C or any value therebetween, and the time can be 18h, 20h, 21h, 23h, 25h, 27h, 28h, 30h, 32h, 34h, 36h or any value therebetween.

In the present invention, the microorganisms contained in the bacteria and algae culture solution are selected as microorganisms that are beneficial to the growth of eels, and specifically can be but not limited to photosynthetic bacteria, chlorella, diatoms, spirulina and Bacillus subtilis, used alone or in combination, and the water body is restored to activity by the large-scale reproduction of photosynthetic bacteria, chlorella, diatoms, spirulina and/or Bacillus subtilis. Among them, the active substances such as subtilisin, polymyxin, nystatin, gramicidin, etc. produced during the proliferation of Bacillus subtilis have a significant inhibitory effect on pathogenic bacteria, and can stimulate the growth and development of eel immune organs, activate T and B lymphocytes, increase the level of immunoglobulins and antibodies, enhance cellular immunity and humoral immunity, and improve group immunity. Bacillus subtilis can also synthesize _VB1 , _VB2 , _VB6 and nicotinic acid and other B vitamins, and improve the activity of interferon and macrophages in eels; more specifically, the bacterial cell density in the bacteria and algae culture solution is ¹⁰⁷ ~ ¹⁰¹⁰ /L.

In some specific embodiments, the amount of bacteria and algae culture solution added is 1~5‰ (v/v), that is, 1~5mL of bacteria and algae culture solution is added to 1L of the water sample to be bioactivated. At this time, the concentration of bacteria in the water sample is appropriate, which is conducive to subsequent cultivation and achieves an ideal bioactivation effect. The amount of bacteria and algae culture solution added is 1‰ (v/v), 2‰ (v/v), 3‰ (v/v), 4‰ (v/v), 5‰ (v/v) or any value therebetween.

In the present invention, in the treatment of concentrated water in step S3, flocculants and coagulants are first added to the concentrated water for flotation to remove 90-98% of phosphorus and 40-75% of COD _Cr in the concentrated water.

In some specific embodiments, the flocculant can be, but is not limited to, one or more of ferrous sulfate, ferrous chloride, basic aluminum chloride, calcium chloride, sodium polyacrylate, polyacrylamide, polystyrene sulfonate and polyethylene oxide; the addition amount is 5-200 mg/L, specifically 5 mg/L, 10 mg/L, 25 mg/L, 50 mg/L, 60 mg/L, 75 mg/L, 80 mg/L, 85 mg/L, 90 mg/L, 95 mg/L, 100 mg/L, 110 mg/L, 115 mg/L, 128 mg/L, 140 mg/L, 150 mg/L, 157 mg/L, 180 mg/L, 190 mg/L, 200 mg/L or any value therebetween.

In some specific embodiments, the coagulant aid may be, but is not limited to, polyacrylamide; its addition amount is 0.1-20 mg/L, specifically 0.1 mg/L, 0.5 mg/L, 0.8 mg/L, 1.0 mg/L, 1.5 mg/L, 2.0 mg/L, 5.0 mg/L, 8 mg/L, 10 mg/L, 15 mg/L, 20 mg/L or any value therebetween.

A certain amount of sludge will be generated during sedimentation, anaerobic treatment and flotation slag removal. Therefore, the treatment method provided in the present invention also includes the treatment of sludge: collecting the sludge from sedimentation, anaerobic treatment and flotation sludge removal, performing gravity separation and filter pressing to obtain dewatered sludge and filtered water, and the filtered water can enter the concentrated water treatment process and be discharged after purification.

The present invention also provides the application of the above eel breeding wastewater purification and recycling method in the treatment of eel breeding wastewater, which can achieve ideal treatment effects on breeding wastewater with different pollution conditions, and the effluent water quality is high, which can be recycled in eel breeding, and the treatment cost is low and the operation is simple, and it has good application prospects.

Embodiments of the present invention are described in detail below, and the examples of the embodiments are intended to be used to explain the present invention, but should not be construed as limiting the present invention. In the embodiments, if specific techniques or conditions are not specified, the techniques or conditions described in the literature in this area or the product specifications are used. If the manufacturer is not specified for reagents or instruments used, they are all conventional products that can be obtained commercially.

Example

This embodiment is used to illustrate the purification and recycling method of eel breeding wastewater with a production volume of 3,000 tons/day. Referring to Figure 1, the method specifically includes the following steps:

1. Sedimentation and anaerobic treatment of eel farming wastewater

The eel farming wastewater is discharged into the sedimentation tank for sedimentation and anaerobic reaction at 25-35°C for 8 hours. On the one hand, solid particles such as feces and residual feed in the eel farming wastewater are removed by sedimentation. On the other hand, the carbon-containing organic matter in the eel farming wastewater is oxidized into _CO2 and the organic phosphorus is converted into phosphate through the action of anaerobic microorganisms in the sedimentation tank.

2. Filtration and diversion

The eel farming wastewater that has undergone sedimentation and anaerobic treatment overflows from the sedimentation tank to a fiber rotary disc filter with a filtration pore size of 15µm for filtration and diversion. The resulting clean water accounts for 80% of the total effluent volume, and the concentrated water accounts for 20% of the total effluent volume.

3. Treatment of clean water

(1) Catalytic electrolytic denitrification: A portion of the clean water obtained by filtering the fiber rotary disc enters the catalytic electrolyzer at a flow rate of ^100m3 /h, and catalytic electrolytic denitrification is carried out under the conditions of voltage of 62V and current of 850A;

(2) Aeration: The clean water after catalytic electrolysis denitrification enters the gas separation tower, and after the nitrogen is discharged from the gas separation tower, it enters the aeration tank. At the same time, another part of the clean water obtained by fiber disc filtration is directly pumped into the aeration tank. In the aeration tank, the clean water after catalytic electrolysis denitrification is mixed with another part of the clean water obtained by filtration and diversion, and then aerated for 12 hours;

(3) Biological activation: The aerated clean water flows into the biological activation pool, and a bacterial culture solution with a cell density of ¹⁰⁷ /L is added at a dosage of 5‰ (v/v). The bacteria are cultured at 25°C for 18 hours, so that the photosynthetic bacteria, Chlorella, diatoms, Spirulina and Bacillus subtilis in the bacterial culture solution can multiply in large quantities in the activation pool, thereby restoring the activity of the clean water and continuing to be recycled in eel farming.

3. Treatment of concentrated water

(1) Flotation: The concentrated water obtained by fiber disc filtration enters the flotation device, and 50 mg/L of ferrous sulfate and 10 mg/L of polyacrylamide are added to remove phosphorus and organic compounds in the concentrated water by flotation.

(2) Microfiltration: The solution obtained after flotation enters the microfiltration machine, which removes tiny solid particles in the water;

(3) Catalytic electrolysis denitrification: The solution obtained through microfiltration enters the catalytic electrolyzer at a flow rate of ^25m3 /h, and catalytic electrolysis denitrification is carried out under the conditions of voltage of 62V and current of 350A. The solution after catalytic electrolysis denitrification enters the gas separation tower, and the nitrogen is discharged from the gas separation tower and meets the emission standards.

4. Sludge treatment

The sludge from the sedimentation tank and the flotation device enters the plate and frame filter press for filtration and dehydration to obtain dehydrated sludge and sludge filtered water, which enters the flotation device for treatment together with concentrated water.

The water quality indicators of the inlet and outlet water in each treatment step of this embodiment are shown in Table 1. After the eel breeding wastewater is precipitated in the sedimentation tank and filtered by the fiber turntable, the wastewater is separated into two paths, one is clean water and the other is concentrated water; the clean water is treated with catalytic electrolytic denitrification, aeration and biological activation, and the effluent COD _Cr ≤30mg/L, ammonia nitrogen ≤0.5mg/L, total nitrogen ≤3mg/L and total phosphorus nitrogen ≤0.2mg/L, which can be recycled to the breeding pond for reuse; and the concentrated water is treated with flotation, microfiltration and catalytic electrolytic denitrification, and the effluent COD _Cr ≤40mg/L, ammonia nitrogen ≤1.0mg/L, total nitrogen ≤3mg/L and total phosphorus ≤0.5mg/L, which meets the discharge standard.

Table 1. Water quality indicators of inlet and outlet water (unit: mg/L)

Example 2.

This embodiment is used to illustrate the purification and recycling method of eel breeding wastewater with a production volume of 10,000 tons/day, which specifically includes the following steps:

1. Sedimentation and anaerobic treatment of eel farming wastewater

The eel farming wastewater is discharged into the sedimentation tank for sedimentation and anaerobic reaction at 25-35°C for 8 hours. On the one hand, solid particles such as feces and residual feed in the eel farming wastewater are removed by sedimentation. On the other hand, the carbon-containing organic matter in the eel farming wastewater is oxidized into _CO2 and the organic phosphorus is converted into phosphate through the action of anaerobic microorganisms in the sedimentation tank.

2. Filtration and diversion

The eel farming wastewater that has undergone sedimentation and anaerobic treatment overflows from the sedimentation tank into a fiber rotary disc filter with a filtration pore size of 10µm. The clean water obtained by filtration accounts for 90% of the total effluent volume, and the concentrated water accounts for 10% of the total effluent volume.

3. Treatment of clean water

(1) Catalytic electrolytic denitrification: A portion of the clean water obtained by filtering the fiber rotary disc enters the catalytic electrolyzer at a flow rate of ^375m3 /h, and catalytic electrolytic denitrification is carried out under the conditions of voltage of 62V and current of 5600A;

(2) Aeration: The clean water after catalytic electrolysis denitrification enters the gas separation tower, and after the nitrogen is discharged from the gas separation tower, it enters the aeration tank. At the same time, another part of the clean water obtained by fiber disc filtration is directly pumped into the aeration tank. In the aeration tank, the clean water after catalytic electrolysis denitrification is mixed with another part of the clean water obtained by filtration and diversion, and then aerated for 12 hours;

(3) Biological activation: The aerated clean water flows into the biological activation pool, and a bacterial culture solution with a cell density of ¹⁰¹⁰ /L is added at a dosage of 3‰ (v/v). The culture is incubated at 30°C for 28 hours to allow the photosynthetic bacteria, Chlorella, diatoms, Spirulina and Bacillus subtilis in the bacterial culture solution to multiply in large quantities in the activation pool, thereby restoring the activity of the clean water and continuing to be recycled for eel farming.

3. Treatment of concentrated water

(1) Flotation: The concentrated water obtained by fiber disc filtration enters the flotation device, and 50 mg/L of ferrous sulfate, 50 mg/L of polyethylene oxide and 20 mg/L of polyacrylamide are added to remove phosphorus and organic compounds in the concentrated water by flotation.

(2) Microfiltration: The solution obtained after flotation enters the microfiltration machine, which removes tiny solid particles in the water;

(3) Catalytic electrolysis denitrification: The solution obtained through microfiltration enters the catalytic electrolyzer at a flow rate of ^25m3 /h, and catalytic electrolysis denitrification is carried out under the conditions of voltage of 62V and current of 350A. The solution after catalytic electrolysis denitrification enters the gas separation tower, and the nitrogen is discharged from the gas separation tower and meets the emission standards.

4. Sludge treatment

The sludge from the sedimentation tank and the flotation device enters the plate and frame filter press for filtration and dehydration to obtain dehydrated sludge and sludge filtered water, which enters the flotation device for treatment together with concentrated water.

The water quality indicators of the inlet and outlet water in each treatment step of this embodiment are shown in Table 2. After the eel breeding wastewater is precipitated in the sedimentation tank and filtered by the fiber turntable, the wastewater is separated into two paths, one is clean water and the other is concentrated water; the clean water is treated with catalytic electrolytic denitrification, aeration and biological activation, and the effluent COD _Cr ≤40mg/L, ammonia nitrogen ≤0.5mg/L, total nitrogen ≤3mg/L and total phosphorus ≤0.2mg/L, which can be recycled to the breeding pond for reuse; and the concentrated water is treated with flotation, microfiltration and catalytic electrolytic denitrification, and the effluent COD _Cr ≤40mg/L, ammonia nitrogen ≤1.0mg/L, total nitrogen ≤3mg/L and total phosphorus ≤0.5mg/L, which meets the discharge standard.

Table 2. Water quality indicators of inlet and outlet water (unit: mg/L)

Example 3.

This embodiment is used to illustrate the purification and recycling method of eel breeding wastewater with a production volume of 1000 tons/day, which specifically includes the following steps:

1. Sedimentation and anaerobic treatment of eel farming wastewater

The eel farming wastewater is discharged into the sedimentation tank for sedimentation and anaerobic reaction at 25-35°C for 8 hours. On the one hand, solid particles such as feces and residual feed in the eel farming wastewater are removed by sedimentation. On the other hand, the carbon-containing organic matter in the eel farming wastewater is oxidized into _CO2 and the organic phosphorus is converted into phosphate through the action of anaerobic microorganisms in the sedimentation tank.

2. Filtration and diversion

The eel farming wastewater that has undergone sedimentation and anaerobic treatment is overflowed from the sedimentation tank into a fiber rotary disc filter with a filtration pore size of 20µm. The clean water obtained by filtration accounts for 95% of the total effluent volume, and the concentrated water accounts for 5% of the total effluent volume.

3. Treatment of clean water

(1) Catalytic electrolytic denitrification: A portion of the clean water obtained by filtering the fiber rotary disc enters the catalytic electrolyzer at a flow rate of ^42m3 /h, and catalytic electrolytic denitrification is carried out under the conditions of voltage of 670V and current of 1000A;

(2) Aeration: The clean water after catalytic electrolysis denitrification enters the gas separation tower, and after the nitrogen is discharged from the gas separation tower, it enters the aeration tank. At the same time, another part of the clean water obtained by fiber disc filtration is directly pumped into the aeration tank. In the aeration tank, the clean water after catalytic electrolysis denitrification is mixed with another part of the clean water obtained by filtration and diversion, and then aerated for 12 hours;

(3) Biological activation: The aerated clean water flows into the biological activation pool, and a bacterial culture solution with a cell density of ¹⁰⁸ /L is added at a dosage of 1‰ (v/v). The culture is incubated at 35°C for 18 hours to allow the photosynthetic bacteria, Chlorella, diatoms, Spirulina and Bacillus subtilis in the bacterial culture solution to multiply in large quantities in the activation pool, thereby restoring the activity of the clean water and continuing to be recycled for eel farming.

3. Treatment of concentrated water

(1) Flotation: The concentrated water obtained by fiber disc filtration enters the flotation device, and 50 mg/L of ferrous sulfate and 10 mg/L of polyacrylamide are added to remove phosphorus and organic compounds in the concentrated water by flotation.

(2) Microfiltration: The solution obtained after flotation enters the microfiltration machine, which removes tiny solid particles in the water;

(3) Catalytic electrolysis denitrification: The solution obtained through microfiltration enters the catalytic electrolyzer at a flow rate of ^10m3 /h, and catalytic electrolysis denitrification is carried out under the conditions of voltage of 65V and current of 320A. The solution after catalytic electrolysis denitrification enters the gas separation tower, and the nitrogen is discharged from the gas separation tower and meets the emission standards.

4. Sludge treatment

The sludge from the sedimentation tank and the flotation device enters the plate and frame filter press for filtration and dehydration to obtain dehydrated sludge and sludge filtered water, which enters the flotation device for treatment together with concentrated water.

The water quality indicators of the inlet and outlet water in each treatment step of this embodiment are shown in Table 3. After the eel breeding wastewater is precipitated in the sedimentation tank and filtered by the fiber turntable, the wastewater is separated into two paths, one is clean water and the other is concentrated water; the clean water is treated with catalytic electrolytic denitrification, aeration and biological activation, and the effluent COD _Cr ≤40mg/L, ammonia nitrogen ≤0.5mg/L, total nitrogen ≤3mg/L and total phosphorus ≤0.2mg/L, which can be recycled to the breeding pond for reuse; and the concentrated water is treated with flotation, microfiltration and catalytic electrolytic denitrification, and the effluent COD _Cr ≤40mg/L, ammonia nitrogen ≤1.0mg/L, total nitrogen ≤3mg/L and total phosphorus ≤0.5mg/L, which meets the discharge standard.

Table 3. Water quality indicators of inlet and outlet water (unit: mg/L)

Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention without departing from the principles and intent of the present invention.

Claims (10)

A method for purifying and recycling eel breeding wastewater, characterized in that the method comprises the following steps: S1. Pretreatment of eel breeding wastewater: performing sedimentation and anaerobic treatment on the eel breeding wastewater to remove solid particles such as feces and residual feed in the water, and oxidizing carbon-containing organic compounds in the eel breeding wastewater into _CO2 and converting organic phosphorus into phosphate to obtain a crude treated water sample; filtering and diverting the crude treated water sample to obtain clear water and concentrated water; S2. Treatment of clean water: the clean water is divided into a first part and a second part, the first part is subjected to catalytic electrolysis for denitrification, and then mixed with the second part for aeration and biological activation to obtain recyclable effluent; S3. Treatment of concentrated water: The concentrated water is subjected to flotation, microfiltration and catalytic electrolysis denitrification in sequence to obtain effluent that meets the standards. The method for purifying and recycling eel breeding wastewater according to claim 1 is characterized in that, in S1, the temperature of precipitation and anaerobic treatment is 15~35°C and the time is 3~5h. The method for purifying and recycling eel breeding wastewater according to claim 1 is characterized in that, in S1, the filtration and diversion are: using a fiber rotary filter with a filtration pore size of 5~20µm to filter the coarsely treated water sample to obtain clear water and concentrated water. The method for purifying and recycling eel breeding wastewater according to claim 3 is characterized in that the volume proportion of the clean water is 80~90%, and the volume proportion of the concentrated water is 10~20%. The method for purifying and recycling eel breeding wastewater according to claim 1 is characterized in that, in S2, the biological activation is: taking the bacterial and algal culture solution and mixing it with aerated clean water, and culturing it at 20~35°C for 18~36h. The method for purifying and recycling eel breeding wastewater according to claim 5 is characterized in that the bacteria and algae culture solution includes at least two of photosynthetic bacteria, Chlorella, diatoms, Spirulina and Bacillus subtilis. The method for purifying and recycling eel breeding wastewater according to claim 5, characterized in that the density of bacteria and algae in the bacteria and algae culture solution is 10 ⁷ ~10 ¹⁰ /mL. The method for purifying and recycling eel breeding wastewater according to claim 5 is characterized in that the addition amount of the bacteria and algae culture solution is 1~5‰ (v/v). The method for purifying and recycling eel breeding wastewater according to claim 1 is characterized in that the method further comprises step S4, in which the sludge produced in the sedimentation and anaerobic treatment and flotation deslagging is subjected to gravity separation and filter pressing to obtain dewatered sludge. Application of the method for purifying and recycling eel breeding wastewater according to any one of claims 1 to 9 in the treatment of eel breeding wastewater.