A forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system and treatment method

Abstract

A forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system and treatment method, belonging to the technical field of aquaculture wastewater treatment. This forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system includes a filtration unit, a compound fertilizer forward osmosis unit, a single fertilizer forward osmosis unit, a struvite crystallization separation unit and a regulating control unit. The system and treatment method use a forward osmosis process driven by a secondary fertilizer extraction liquid to treat freshwater aquaculture wastewater, eliminating the energy consumption of forward osmosis extraction liquid regeneration, and are suitable for the characteristics of large water volume and low pollutant concentration of freshwater aquaculture wastewater and the current situation of no heat source at freshwater aquaculture sites. Through the adjustable two-stage fertilizer extraction liquid forward osmosis process, the actual needs of both the clean water recovery rate of aquaculture wastewater and the water quality of the treated aquaculture wastewater meeting the standards are taken into account.

Description

Technical Field

[0001] The invention relates to a forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system and a treatment method, belonging to the technical field of aquaculture wastewater treatment. Background Art

[0002] Freshwater aquaculture is the use of inland waters such as ponds, rivers, and lakes to artificially raise aquatic economic animals. While it brings economic benefits, it also brings problems such as aquaculture wastewater discharge and ecological environment deterioration. Freshwater aquaculture is mostly open and semi-static. Its high-density intensive aquaculture model makes the water quality susceptible to pollution by organic matter, agricultural chemicals, etc. The pollutants in aquaculture wastewater mainly come from feed residues and aquatic product excrement. The components of the pollutants are mainly nutrients such as nitrogen and phosphorus. Pollutants such as nitrogen and phosphorus produced by freshwater aquaculture will deteriorate the aquaculture water quality, thereby threatening the safety of freshwater aquaculture. The discharge of freshwater aquaculture wastewater will cause damage to the ecological environment. Therefore, the efficient treatment and reuse of aquaculture wastewater has become the only way for the development of the freshwater aquaculture industry.

[0003] At present, the treatment technologies for freshwater aquaculture wastewater at home and abroad are mainly divided into three technical methods: physical, chemical and biological. However, the low concentration of pollutants and large discharge volume of aquaculture wastewater make it impossible for existing wastewater treatment technologies to treat wastewater efficiently and economically. Physical methods such as filtration, adsorption, aeration, and stripping, and chemical methods such as flocculation sedimentation, chemical neutralization, and ozone disinfection have the characteristics of quick results, but they are costly, energy-intensive, and easily cause secondary pollution, thus limiting their application in actual projects. The biological treatment method occupies a large area and has a long treatment cycle. In the later stage of aquaculture, the aquaculture pond requires water to be changed about every 7 days, so the frequency of water changes does not match the biological treatment cycle. Therefore, it is necessary to develop aquaculture wastewater treatment technology with low cost and high treatment efficiency.

[0004] Forward osmosis technology has the advantages of low energy consumption, high pollutant retention rate, and strong anti-pollution ability. It is gradually replacing pressure-driven membrane technologies such as fine filtration, nanofiltration, and reverse osmosis for the treatment of aquaculture wastewater. The regeneration of the draw solution in forward osmosis consumes additional electricity or heat energy. There is no heat source at the freshwater aquaculture site and only electricity can be consumed. However, the low concentration of pollutants and large emissions of aquaculture wastewater make the cost of regenerating the draw solution that consumes electricity too high. Therefore, how to further reduce the energy consumption of forward osmosis wastewater treatment has become a technical problem that needs to be solved urgently.

[0005] Fertilizer draw liquid forward osmosis technology solves the problem of draw liquid regeneration and can effectively reduce the cost of aquaculture wastewater treatment. Concentrated fertilizer is used as draw liquid to recycle clean water from aquaculture wastewater. The diluted fertilizer draw liquid can be directly used for farmland irrigation without the need for draw liquid regeneration process.

[0006] A Chinese patent with publication number CN109133523A discloses a livestock and poultry breeding wastewater treatment device based on forward osmosis, which uses the coupling of forward osmosis and reverse osmosis technology to achieve that the treated wastewater meets the sewage discharge standard. However, the high-pressure pump of reverse osmosis has high power consumption and is not suitable for treating breeding wastewater with a large discharge volume. The treated wastewater only meets the discharge standard and is discharged, and the recycling of the breeding wastewater is not achieved.

[0007] The master's thesis "Research on Forward Osmosis Technology to Draw Clean Water from Unconventional Water Sources for Agricultural Planting" studied the forward osmosis water flux, reverse salt flux and other properties of a freshwater / wastewater-solid fertilizer system with concentrated fertilizer as the forward osmosis driving solution, but did not involve the recovery of nitrogen and phosphorus pollution in the wastewater and the recycling of wastewater.

[0008] The use of fertilizer extraction liquid forward osmosis technology in aquaculture wastewater treatment also requires solving the following technical problems:

[0009] (1) PO4 in aquaculture wastewater 3- and NH4 + The concentration is low, and the efficiency of recovering nitrogen and phosphorus nutrients through phosphate precipitation (struvite solid fertilizer) is low. It is necessary to use forward osmosis technology to pre-concentrate the aquaculture wastewater first to increase the concentration of nitrogen and phosphorus nutrients in the aquaculture wastewater. The existing pre-concentration processes mostly use membrane distillation technology to concentrate the draw liquid or heat and decompose the NH4HCO3 draw liquid to complete the regeneration of the draw liquid. Membrane distillation and heating and decomposition of NH4HCO3 solution require the draw liquid to be heated to 40-60°C, but freshwater aquaculture sites usually have no heat source, and the consumption of electricity for draw liquid regeneration makes the cost of aquaculture wastewater pre-concentration too high.

[0010] (2) During the forward osmosis process, the fertilizer extract must be diluted to a concentration suitable for farmland fertilizer. At the same time, in order to improve the economic efficiency of forward osmosis wastewater treatment, the extract concentration must be high enough to ensure a large enough water flux through the osmotic membrane. It is difficult to take into account both the concentration of the diluted extract and the economic efficiency of forward osmosis by relying solely on single-stage fertilizer extract forward osmosis.

[0011] (3) An important defect of forward osmosis is that the cations in the fertilizer extraction solution will reversely permeate the osmosis membrane into the aquaculture wastewater side, causing fertilizer loss. + When single element fertilizer is used as the extraction liquid for pre-concentration, NH4 + The reverse flux is greater than that of compound fertilizers, and the loss of fertilizers in the extraction liquid of single fertilizers will increase the NH4 in the aquaculture wastewater. + concentration, the phosphate precipitation chemical reaction on the aquaculture wastewater side cannot consume the excess NH4 + , resulting in NH4 + Exceeding the standard.

[0012] Therefore, there is an urgent need to develop a process suitable for the treatment and reuse of freshwater aquaculture wastewater, so as to achieve the repeated recycling of freshwater aquaculture wastewater by efficiently and economically recovering clean water and nitrogen and phosphorus pollutants. Summary of the invention

[0013] In order to overcome the deficiencies of the prior art, the present invention proposes a forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system and treatment method. The system and treatment method adopt a secondary forward osmosis technology with compound fertilizer and Mg(NO3)2 single fertilizer as the extraction liquid, and pre-concentrate the aquaculture wastewater through the forward osmosis process of the primary compound fertilizer extraction liquid, thereby reducing the regeneration cost of the forward osmosis extraction liquid and improving the recovery efficiency of nitrogen and phosphorus nutrients in the forward osmosis of the secondary single fertilizer extraction liquid. By adjusting the extraction liquid concentration in the forward osmosis of the two-stage fertilizer extraction liquid, the actual needs of both the clean water recovery rate of the aquaculture wastewater and the water quality of the aquaculture wastewater after treatment are taken into account. It not only realizes the recovery of nitrogen and phosphorus pollutants in the aquaculture wastewater, but also realizes the low-cost recycling of freshwater aquaculture wastewater.

[0014] The present invention adopts the following technical scheme: a forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system comprises a filtration unit, a compound fertilizer forward osmosis unit, a single element fertilizer forward osmosis unit, a struvite crystallization separation unit and a regulating control unit, wherein the filtration unit comprises a filtration tank and a sedimentation tank, a freshwater aquaculture wastewater pipeline is connected to the water inlet of the filtration tank, and the water outlet of the filtration tank is connected to the water inlet of the sedimentation tank.

[0015] The compound fertilizer forward osmosis unit comprises a compound fertilizer forward osmosis membrane device, a first wastewater pump, a first drawn liquid pump, a first regulating valve and a first drawn liquid mixing tank. The water outlet of the sedimentation tank is connected to the feed inlet on the upper left part of the compound fertilizer forward osmosis membrane device through the first wastewater pump, the drawn liquid outlet on the left side of the first drawn liquid mixing tank is connected to the drawn liquid inlet on the upper right part of the compound fertilizer forward osmosis membrane device through the first drawn liquid pump, and the drawn liquid outlet on the lower right part of the compound fertilizer forward osmosis membrane device is connected to the drawn liquid inlet at the bottom of the first drawn liquid mixing tank through the first mixing branch pipe and the first regulating valve and is connected to the farmland through the first drainage branch pipe.

[0016] The simple fertilizer forward osmosis unit comprises a simple fertilizer forward osmosis membrane device, a second draw liquid pump, a second regulating valve and a second draw liquid slurry preparation tank. The discharge port on the left side of the second draw liquid slurry preparation tank is connected to the draw liquid inlet on the upper right part of the simple fertilizer forward osmosis membrane device through the second draw liquid pump. The draw liquid outlet on the lower right part of the simple fertilizer forward osmosis membrane device is connected to the draw liquid inlet at the bottom of the second draw liquid slurry preparation tank through the second slurry preparation branch pipe and the second regulating valve and is connected to the farmland through the second drainage branch pipe. The discharge port on the lower left part of the compound fertilizer forward osmosis membrane device is connected to the feed port on the upper left part of the simple fertilizer forward osmosis membrane device.

[0017] The struvite crystallization and separation unit comprises a crystallization reactor, a second wastewater pump and a solid fertilizer tank, the discharge port at the lower left of the elemental fertilizer forward osmosis membrane device is connected to the feed port at the upper left of the crystallization reactor through the second wastewater pump and a circulation pipe, the water outlet at the upper left of the crystallization reactor is connected to the freshwater breeding pond through the water outlet pipe, the discharge port at the lower right is connected to the feed port of the solid fertilizer tank, and the water outlet at the bottom is connected to the circulation pipe;

[0018] The regulating and controlling unit comprises an ammonium ion signal detector and a PLC controller. The ammonium ion signal detector is arranged on the water outlet pipe of the crystallization reactor. The PLC controller is electrically connected to the ammonium ion signal detector, the first regulating valve and the second regulating valve.

[0019] A zeolite layer, a sand layer and an activated carbon layer are arranged in the filter tank, and guide plates arranged in staggered intervals are arranged in the sedimentation tank. The compound fertilizer forward osmosis membrane device and the simple fertilizer forward osmosis membrane device are divided into a raw material liquid chamber and an extraction liquid chamber by the forward osmosis membrane.

[0020] The treatment method of the forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system comprises the following steps:

[0021] a) Freshwater aquaculture wastewater is filtered through the zeolite layer, sand layer and activated carbon layer in the filter tank and then enters the sedimentation tank. The suspended particles in the aquaculture wastewater are removed through the guide plates arranged at intervals in the sedimentation tank. After pretreatment, the freshwater aquaculture wastewater with a suspended solid concentration lower than 100 mg / L is pumped into the compound fertilizer forward osmosis unit through the first wastewater pump for further treatment.

[0022] b) In the compound fertilizer forward osmosis unit, the freshwater aquaculture wastewater enters the raw liquid side of the compound fertilizer forward osmosis membrane device, and the compound fertilizer draw liquid with a concentration of 10-20% in the first draw liquid preparation tank is pumped into the draw liquid side of the compound fertilizer forward osmosis membrane device by the first draw liquid pump. Driven by the osmotic pressure on both sides of the forward osmosis membrane, the water in the freshwater aquaculture wastewater passes through the osmotic membrane and enters the draw liquid side. The pre-concentrated aquaculture wastewater enters the single fertilizer forward osmosis unit for further treatment. The diluted compound fertilizer draw liquid is regulated by the first regulating valve. A part of it enters the first draw liquid preparation tank through the first preparation branch pipe for configuring the compound fertilizer draw liquid for recycling, and the remaining part directly enters the farmland as fertilizer water through the first drainage branch pipe.

[0023] c) In the elemental fertilizer forward osmosis unit, the magnesium nitrate solution with a concentration of 12-20% in the second draw liquid mixing tank is pumped into the draw liquid side of the elemental fertilizer forward osmosis membrane device by the second draw liquid pump (9) as the elemental fertilizer draw liquid, and the aquaculture wastewater pre-concentrated by the compound fertilizer forward osmosis unit enters the raw liquid side of the elemental fertilizer forward osmosis membrane device. Driven by the osmotic pressure on both sides of the forward osmosis membrane, water molecules and part of NH4 +Ions pass through the osmotic membrane into the draw liquid side, and the secondary concentrated aquaculture wastewater enters the struvite crystallization separation unit for further treatment. The diluted elemental fertilizer draw liquid is adjusted in flow by the second regulating valve, and a part of it enters the second draw liquid mixing tank for the preparation of recycled elemental fertilizer draw liquid, and the remaining part directly enters the farmland as fertilizer water through the second drainage branch pipe.

[0024] d) In the struvite crystallization separation unit, the secondary concentrated aquaculture wastewater and the circulating aquaculture wastewater flowing out from the bottom water inlet of the crystallization reactor merge and then enter the crystallization reactor through the circulation pipe and the feed inlet. The Mg in the secondary concentrated aquaculture wastewater is 2+ PO4 3- and NH4 + A chemical precipitation reaction occurs in the crystallization reactor to generate struvite crystal solid fertilizer which enters the solid fertilizer tank. The aquaculture wastewater after precipitation treatment enters the freshwater aquaculture pond through the outlet pipe of the crystallization reactor for repeated recycling. The ammonium ion signal detector is used to detect the ammonium ion concentration in the aquaculture wastewater discharged from the outlet pipe.

[0025] e) The measured value of ammonium ions detected by the ammonium ion signal detector is used as the input signal of the PLC controller. According to the difference between the measured value of ammonium ions and the designed value, the PLC controller adjusts the opening of the first regulating valve and the second regulating valve to respectively adjust the flow rate of the drawn liquid from the outlet of the compound fertilizer forward osmosis membrane device entering the first drawn liquid preparation tank and the outlet of the single fertilizer forward osmosis membrane device entering the second drawn liquid preparation tank, until the measured value of ammonium ions reaches the designed value to meet the water quality requirements for freshwater aquaculture wastewater treatment.

[0026] The 10-20% compound fertilizer extraction liquid in step b is prepared by using solid compound fertilizer with TN concentration of 20%, TP concentration of 20% and TK concentration of 20%.

[0027] The above technical solution is characterized by: using a two-stage fertilizer extraction liquid driven forward osmosis process to treat freshwater aquaculture wastewater, eliminating the energy consumption of forward osmosis extraction liquid regeneration, and is suitable for the characteristics of large freshwater aquaculture wastewater volume and low pollutant concentration, as well as the current situation of no heat source at freshwater aquaculture sites. Through the adjustable two-stage fertilizer extraction liquid forward osmosis process, the actual needs of both the clean water recovery rate of aquaculture wastewater and the water quality of the treated aquaculture wastewater meeting the standards are taken into account.

[0028] The beneficial effects of the present invention are: a forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system and treatment method, the forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system comprises a filtration unit, a compound fertilizer forward osmosis unit, a single fertilizer forward osmosis unit, a struvite crystallization separation unit and a regulating control unit. The system adopts a secondary fertilizer extraction liquid forward osmosis technology, and the diluted extraction liquid can be directly used as fertilizer for farmland, eliminating the energy consumption cost of forward osmosis extraction liquid regeneration, which is in line with the energy consumption status of freshwater aquaculture sites without heat sources. The forward osmosis method coupling of compound fertilizer and Mg(NO3)2 single fertilizer as the draw liquid can adjust the concentration of the forward osmosis draw liquid through the regulating device according to the water quality of the treated freshwater aquaculture wastewater, taking into account the actual needs of both the clean water recovery rate of the aquaculture wastewater and the water quality of the treated aquaculture wastewater. When the ammonium ion concentration in the freshwater aquaculture wastewater after the secondary forward osmosis treatment is greater than the design value, the opening of the first regulating valve and the second regulating valve is reduced to keep the inlet flow rate of the compound fertilizer and single fertilizer solid fertilizer unchanged, thereby increasing the inlet draw liquid concentration of the compound fertilizer and single fertilizer draw liquid forward osmosis membrane assembly, and increasing the water flux of the compound fertilizer draw liquid forward osmosis membrane assembly, thereby increasing the ammonium ion concentration at the outlet of the freshwater aquaculture wastewater of the compound fertilizer draw liquid forward osmosis membrane assembly, thereby increasing the NH4 + The flow of ions passing through the forward osmosis membrane into the elemental fertilizer extraction liquid reduces the amount of ammonium ions at the outlet of the freshwater aquaculture wastewater of the elemental fertilizer extraction liquid forward osmosis membrane assembly, thereby reducing the concentration of ammonium ions in the freshwater aquaculture wastewater after secondary forward osmosis treatment, and offsetting the NH4 + The effect of increased ions on the reduction of water flux through forward osmosis membranes. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural diagram of a freshwater aquaculture wastewater treatment system integrating forward osmosis water and fertilizer.

[0030] In the figure: 1, filter tank, 2, sedimentation tank, 3, first wastewater pump, 4, compound fertilizer forward osmosis membrane device, 5, first extraction liquid pump, 6, first extraction liquid slurry tank, 6a, first slurry distribution branch pipe, 7, first regulating valve, 8, farmland, 8a, first drainage branch pipe, 8b, second drainage branch pipe, 9, second extraction liquid pump, 10, single element fertilizer forward osmosis membrane device, 11, second extraction liquid slurry tank, 11a, second slurry distribution branch pipe, 12, second wastewater pump, 13, second regulating valve, 14, crystallization reactor, 14a, outlet pipe, 14b, circulation pipe, 15, struvite solid fertilizer tank, 16, ammonium ion signal detector, 17, PLC controller. DETAILED DESCRIPTION

[0031] The technical solution of the present invention will be described clearly and completely below in conjunction with the embodiments and drawings of the present invention. In the description of the present invention, the connection between devices refers to the connection by pipelines, and the concentration is expressed in mass percentage unless otherwise specified.

[0032] Figure 1 The schematic diagram of the structure of a forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system is shown. In the figure, the forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system includes a filtration unit, a compound fertilizer forward osmosis unit, a single fertilizer forward osmosis unit, a struvite crystallization separation unit and a regulation control unit. The filtration unit includes a filtration tank 1 and a sedimentation tank 2. The freshwater aquaculture wastewater pipeline is connected to the water inlet of the filtration tank 1, and the water outlet of the filtration tank 1 is connected to the water inlet of the sedimentation tank 2.

[0033] The compound fertilizer forward osmosis unit comprises a compound fertilizer forward osmosis membrane device 4, a first wastewater pump 3, a first drawn liquid pump 5, a first regulating valve 7 and a first drawn liquid slurry preparation tank 6. The outlet of the sedimentation tank 2 is connected to the feed port at the upper left of the compound fertilizer forward osmosis membrane device 4 through the first wastewater pump 3. The drawn liquid outlet on the left side of the first drawn liquid slurry preparation tank 6 is connected to the drawn liquid inlet at the upper right of the compound fertilizer forward osmosis membrane device 4 through the first drawn liquid pump 5. The drawn liquid outlet at the lower right of the compound fertilizer forward osmosis membrane device 4 is connected to the drawn liquid inlet at the bottom of the first drawn liquid slurry preparation tank 6 through the first slurry preparation branch pipe 6a and the first regulating valve 7 and is connected to the farmland 8 through the first drainage branch pipe 8a.

[0034] The single-element fertilizer forward osmosis unit comprises a single-element fertilizer forward osmosis membrane device 10, a second draw liquid pump 9, a second regulating valve 13 and a second draw liquid slurry preparation tank 11. The discharge port on the left side of the second draw liquid slurry preparation tank 11 is connected to the draw liquid inlet at the upper right part of the single-element fertilizer forward osmosis membrane device 10 through the second draw liquid pump 9. The draw liquid outlet at the lower right part of the single-element fertilizer forward osmosis membrane device 10 is connected to the draw liquid inlet at the bottom of the second draw liquid slurry preparation tank 11 through the second slurry preparation branch pipe 11a and the second regulating valve 13 and is connected to the farmland 8 through the second drainage branch pipe 8b. The discharge port at the lower left part of the compound fertilizer forward osmosis membrane device 4 is connected to the feed port at the upper left part of the single-element fertilizer forward osmosis membrane device 10.

[0035] The struvite crystallization separation unit comprises a crystallization reactor 14, a second wastewater pump 12 and a solid fertilizer tank 15, the discharge port at the lower left of the elemental fertilizer forward osmosis membrane device 10 is connected to the feed port at the upper left of the crystallization reactor 14 through the second wastewater pump 12 and the circulation pipe 14b, the water outlet at the upper left of the crystallization reactor 14 is connected to the freshwater breeding pond through the water outlet pipe 14a, the discharge port at the lower right is connected to the feed port of the solid fertilizer tank 15, and the water outlet at the bottom is connected to the circulation pipe 14b;

[0036] The regulating control unit includes an ammonium ion signal detector 16 and a PLC controller 17 . The ammonium ion signal detector 16 is arranged on the outlet pipe 14 a of the crystallization reactor 14 . The PLC controller 17 is electrically connected to the ammonium ion signal detector 16 , the first regulating valve 7 and the second regulating valve 13 .

[0037] A zeolite layer, a sand layer and an activated carbon layer are arranged in the filter tank 1, and guide plates arranged in an interlaced manner are arranged in the sedimentation tank 2. The compound fertilizer forward osmosis membrane device 4 and the single fertilizer forward osmosis membrane device 10 are separated into a raw liquid chamber and an extraction liquid chamber by a forward osmosis membrane.

[0038] The treatment method of this forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system adopts the following steps:

[0039] a) Freshwater aquaculture wastewater is filtered through the zeolite layer, sand layer and activated carbon layer in the filter tank 1 and then enters the sedimentation tank 2. The freshwater aquaculture wastewater flows through the guide plates arranged alternately in the sedimentation tank 2 to remove suspended particles in the aquaculture wastewater. After pretreatment, the freshwater aquaculture wastewater with a suspended solid concentration lower than 100 mg / L is pumped into the raw liquid side of the compound fertilizer forward osmosis unit through the first wastewater pump 3 for further treatment.

[0040] b) In the compound fertilizer forward osmosis unit, freshwater aquaculture wastewater with a TN concentration of 4-8 mg / L and a TP concentration of 4-7 mg / L enters the raw liquid side of the compound fertilizer forward osmosis membrane device 4, and solid compound fertilizers with a TN concentration of 20%, a TP concentration of 20%, and a TK concentration of 20% are configured into a compound fertilizer draw liquid with a concentration of 10-20% in the first draw liquid preparation tank 6, which is pumped into the draw liquid side of the compound fertilizer forward osmosis membrane device 4 by the first draw liquid pump 5. Driven by the osmotic pressure on both sides of the forward osmosis membrane, the water in the freshwater aquaculture wastewater permeates the raw liquid side of the compound fertilizer forward osmosis membrane device 4. The osmotic membrane enters the draw liquid side, the pre-concentrated aquaculture wastewater is concentrated to a TP concentration between 15-28 mg / L and a TN concentration between 17-30 mg / L, and the concentrated freshwater aquaculture wastewater is used as the feed liquid of the single element fertilizer forward osmosis unit for further treatment, and the compound fertilizer draw liquid concentration is diluted to between 6-12%. The diluted compound fertilizer draw liquid is distributed by the first regulating valve 7, a part of which enters the first draw liquid mixing tank 6 for preparing the compound fertilizer draw liquid for recycling, and the remaining part directly enters the farmland 8 as fertilizer water;

[0041] c) In the single element fertilizer forward osmosis unit, the magnesium nitrate fertilizer solution with a concentration of 12-20% in the second draw liquid preparation tank 11 is pumped into the draw liquid side of the single element fertilizer forward osmosis membrane device 10 by the second draw liquid pump 9, and the water molecules in the aquaculture wastewater pre-concentrated by the compound fertilizer forward osmosis unit and the NH4 with a concentration of 30-60% +Ions penetrate the permeable membrane and enter the elemental fertilizer extraction liquid side, and the concentration of the magnesium nitrate extraction liquid is diluted to 8-10%. The diluted elemental fertilizer extraction liquid is distributed to the second extraction liquid mixing tank 11 for the configuration of the elemental fertilizer extraction liquid for recycling and to the farmland 8 for the fertilizer water for the farmland under the regulation of the second regulating valve 13. A small amount of Mg in the magnesium nitrate extraction liquid 2+ Reverse permeation membrane enters the aquaculture wastewater, and the Mg in the aquaculture wastewater after secondary forward osmosis concentration 2+ The concentration is 1.4-1.6%, the concentration of TP is 30-60mg / L, and the concentration of TN is 17-30mg / L.

[0042] d) In the struvite crystallization separation unit, the secondary concentrated aquaculture wastewater is combined with the circulating aquaculture wastewater in the crystallization reactor 14 and then enters the crystallization reactor 14 through the circulation pipe 14b from the feed port at the upper left of the crystallization reactor 14. 2+ PO4 3- and NH4 + A chemical precipitation reaction occurs in the crystallization reactor 14 to generate struvite crystalline solid fertilizer, which enters the struvite solid fertilizer tank 15 through the discharge port. The aquaculture wastewater after precipitation treatment flows out from the outlet at the upper left part of the crystallization reactor 14 through the outlet pipe 14a and enters the freshwater aquaculture pond for repeated recycling. An ammonium ion signal detector 16 for detecting the ammonium ion concentration in the treated aquaculture wastewater is provided on the outlet pipe 14a of the crystallization reactor 14.

[0043] e) The measured value of ammonium ions detected by the ammonium ion signal detector 16 is used as an input signal of the PLC controller 17. According to the difference between the measured value of ammonium ions and the designed value, the PLC controller calculates the adjusted drawn liquid concentration at the drawn liquid inlet of the compound fertilizer forward osmosis membrane device 4 and the drawn liquid concentration at the drawn liquid inlet of the elemental fertilizer forward osmosis membrane device 10, thereby respectively adjusting the openings of the first regulating valve 7 and the second regulating valve 13, respectively adjusting the flow rate of the drawn liquid outlet at the lower right portion of the compound fertilizer forward osmosis membrane device 4 entering the first drawn liquid slurry preparation tank 6 and the flow rate of the drawn liquid outlet at the lower right portion of the elemental fertilizer forward osmosis membrane device 10 entering the second drawn liquid slurry preparation tank 11, until the measured value of ammonium ions is reduced to the designed value to meet the water quality requirements for freshwater aquaculture wastewater treatment. The specific adjustment method is: when the ammonium ion concentration in the aquaculture wastewater after the secondary forward osmosis treatment increases, by reducing the opening of the first regulating valve 7 and the second regulating valve 13, the inlet flow rate of the compound fertilizer and the single fertilizer solid fertilizer in the first draw liquid preparation tank 6 and the second draw liquid preparation tank 11 is kept unchanged, thereby increasing the concentration of the draw liquid entering the compound fertilizer forward osmosis membrane device 4 and the single fertilizer forward osmosis membrane device 10, and increasing the water flux of the forward osmosis membrane, thus increasing the ammonium ion concentration at the outlet of the compound fertilizer forward osmosis membrane device 4, thereby increasing the NH4 +The flow rate of ions passing through the forward osmosis membrane in the elemental fertilizer forward osmosis membrane device 10 and entering the elemental fertilizer extraction liquid reduces the ammonium ion concentration at the outlet of the elemental fertilizer forward osmosis membrane device 10 and reduces the ammonium ion concentration in the aquaculture wastewater after the secondary forward osmosis treatment.

[0044] By adopting the above technical scheme, the freshwater aquaculture wastewater after filtration and sedimentation pretreatment enters the compound fertilizer forward osmosis unit for pre-concentration. Under the action of osmotic pressure, the clean water in the freshwater aquaculture wastewater with a TN concentration of 4-8 mg / L and a TP concentration of 4-7 mg / L passes through the osmotic membrane and enters the compound fertilizer extract liquid with a concentration of 10-20% configured by solid compound fertilizers with a TN concentration of 20%, a TP concentration of 20% and a TK concentration of 20%. The diluted extract liquid can be directly used as fertilizer for farmland, eliminating the energy consumption cost of regenerating the forward osmosis extract liquid, which is in line with the current energy consumption status of freshwater aquaculture sites without heat sources; the aquaculture wastewater is pre-concentrated by forward osmosis of the compound fertilizer extract liquid, thereby improving the recovery efficiency of nitrogen and phosphorus nutrients in the forward osmosis of the secondary single fertilizer extract liquid;

[0045] A 12-20% magnesium nitrate fertilizer solution was used as the draw liquid for the single element fertilizer forward osmosis unit, and the draw liquid recovered 30-60% of the NH4 in the aquaculture wastewater. + The extracted liquid with a concentration of 8-10% after dilution can be directly used as fertilizer for farmland, and the pollutant ammonia in the aquaculture wastewater can be recovered as fertilizer. 3- and NH4 + A chemical reaction occurs in the crystallization reactor to generate struvite solid fertilizer, which recovers nitrogen and phosphorus nutrients in the aquaculture wastewater while purifying the aquaculture wastewater.

[0046] When the measured value of ammonium ions detected by the ammonium ion signal detector is greater than the design value, the flow of compound fertilizer draw liquid and single fertilizer draw liquid entering the first draw liquid preparation tank and the second draw liquid preparation tank is reduced by closing the opening of the first regulating valve and the second regulating valve, thereby increasing the inlet draw liquid concentration of the compound fertilizer forward osmosis membrane device and the single fertilizer forward osmosis membrane device, thereby increasing the water flux of the secondary forward osmosis membrane. The increase in the water flux of the forward osmosis membrane increases the ammonium ion concentration at the outlet of the aquaculture wastewater in the compound fertilizer forward osmosis membrane device, thereby increasing the NH4 + The flow rate of ions entering the elemental fertilizer extract through the elemental fertilizer extract forward osmosis membrane reduces the ammonium ion concentration at the outlet of the freshwater aquaculture wastewater in the elemental fertilizer forward osmosis membrane device, reduces the ammonium ion concentration in the freshwater aquaculture wastewater after secondary forward osmosis treatment, and offsets the NH4 + The effect of increased ions on the reduction of water flux through forward osmosis membranes.

[0047] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, a person skilled in the art can still modify the technical solutions recorded in the above embodiments, or replace some or all of the technical features therein by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system, which includes a filtration unit, Features: It also includes a compound fertilizer forward osmosis unit, a simple fertilizer forward osmosis unit, a struvite crystallization separation unit and a regulation control unit. The filtration unit includes a filtration tank (1) and a sedimentation tank (2). The freshwater aquaculture wastewater pipeline is connected to the water inlet of the filtration tank (1), and the water outlet of the filtration tank (1) is connected to the water inlet of the sedimentation tank (2). The compound fertilizer forward osmosis unit comprises a compound fertilizer forward osmosis membrane device (4), a first wastewater pump (3), a first drawn liquid pump (5), a first regulating valve (7) and a first drawn liquid preparation tank (6); the water outlet of the sedimentation tank (2) is connected to the feed port at the upper left of the compound fertilizer forward osmosis membrane device (4) through the first wastewater pump (3); the drawn liquid outlet at the left side of the first drawn liquid preparation tank (6) is connected to the drawn liquid inlet at the upper right of the compound fertilizer forward osmosis membrane device (4) through the first drawn liquid pump (5); the drawn liquid outlet at the lower right of the compound fertilizer forward osmosis membrane device (4) is connected to the drawn liquid inlet at the bottom of the first drawn liquid preparation tank (6) through the first preparation branch pipe (6a) and the first regulating valve (7) and is connected to the farmland (8) through the first drainage branch pipe (8a); The elemental fertilizer forward osmosis unit comprises an elemental fertilizer forward osmosis membrane device (10), a second drawn liquid pump (9), a second regulating valve (13) and a second drawn liquid preparation tank (11); the discharge port on the left side of the second drawn liquid preparation tank (11) is connected to the drawn liquid inlet at the upper right part of the elemental fertilizer forward osmosis membrane device (10) via the second drawn liquid pump (9); the drawn liquid outlet at the lower right part of the elemental fertilizer forward osmosis membrane device (10) is connected to the drawn liquid inlet at the bottom of the second drawn liquid preparation tank (11) via the second preparation branch pipe (11a) and the second regulating valve (13) and is connected to the farmland (8) via the second drainage branch pipe (8b); and the discharge port at the lower left part of the compound fertilizer forward osmosis membrane device (4) is connected to the feed port at the upper left part of the elemental fertilizer forward osmosis membrane device (10); The struvite crystallization separation unit comprises a crystallization reactor (14), a second wastewater pump (12) and a solid fertilizer tank (15); the discharge port at the lower left of the elemental fertilizer forward osmosis membrane device (10) is connected to the feed port at the upper left of the crystallization reactor (14) via the second wastewater pump (12) and a circulation pipe (14b); the water outlet at the upper left of the crystallization reactor (14) is connected to the freshwater breeding pond via the water outlet pipe (14a); the discharge port at the lower right is connected to the feed port of the solid fertilizer tank (15); and the water outlet at the bottom is connected to the circulation pipe (14b); The regulating and controlling unit comprises an ammonium ion signal detector (16) and a PLC controller (17); the ammonium ion signal detector (16) is arranged on a water outlet pipe (14a) of a crystallization reactor (14); and the PLC controller (17) is electrically connected to the ammonium ion signal detector (16), the first regulating valve (7) and the second regulating valve (13).

2. A forward osmosis water-fertilizer integrated freshwater aquaculture wastewater treatment system according to claim 1, It is characterized in that A zeolite layer, a sand layer and an activated carbon layer are arranged in the filtering pool (1); guide plates arranged in an interlaced manner are arranged in the sedimentation pool (2); and the compound fertilizer forward osmosis membrane device (4) and the elemental fertilizer forward osmosis membrane device (10) are separated into a raw material liquid chamber and an extraction liquid chamber by the forward osmosis membrane.

3. The method for treating freshwater aquaculture wastewater by a forward osmosis water-fertilizer integrated system according to claim 1, It is characterized in that The following steps are involved: a) The freshwater aquaculture wastewater is filtered through the zeolite layer, sand layer and activated carbon layer in the filter tank (1) and then enters the sedimentation tank (2). The suspended particles in the aquaculture wastewater are removed by the guide plates arranged in a staggered manner in the sedimentation tank (2). After pretreatment, the freshwater aquaculture wastewater with a suspended solid concentration lower than 100 mg / L is pumped into the compound fertilizer forward osmosis unit through the first wastewater pump (3) for further treatment; b) In the compound fertilizer forward osmosis unit, the freshwater aquaculture wastewater enters the raw liquid side of the compound fertilizer forward osmosis membrane device (4), the compound fertilizer draw liquid with a concentration of 10-20% in the first draw liquid preparation tank (6) is pumped into the draw liquid side of the compound fertilizer forward osmosis membrane device (4) by the first draw liquid pump (5), and driven by the osmotic pressure on both sides of the forward osmosis membrane, the water in the freshwater aquaculture wastewater passes through the osmotic membrane and enters the draw liquid side, the pre-concentrated aquaculture wastewater enters the single element fertilizer forward osmosis unit for further treatment, the diluted compound fertilizer draw liquid is regulated by the first regulating valve (7), a part of it enters the first draw liquid preparation tank (6) through the first slurry preparation branch pipe (6a) for preparing the compound fertilizer draw liquid for recycling, and the remaining part directly enters the farmland (8) as fertilizer water through the first drainage branch pipe (8a); c) In the elemental fertilizer forward osmosis unit, the magnesium nitrate solution with a concentration of 12-20% in the second draw liquid mixing tank (11) is pumped into the draw liquid side of the elemental fertilizer forward osmosis membrane device (10) by the second draw liquid pump (9) as the elemental fertilizer draw liquid, and the aquaculture wastewater pre-concentrated by the compound fertilizer forward osmosis unit enters the raw liquid side of the elemental fertilizer forward osmosis membrane device (10). Driven by the osmotic pressure on both sides of the forward osmosis membrane, water molecules and part of NH 4 + The ions pass through the osmotic membrane and enter the draw liquid side. The secondary concentrated aquaculture wastewater enters the struvite crystallization separation unit for further treatment. The diluted single element fertilizer draw liquid is adjusted in flow rate by the second regulating valve (13). A portion of the single element fertilizer draw liquid enters the second draw liquid mixing tank (11) for preparing the single element fertilizer draw liquid for recycling. The remaining portion directly enters the farmland (8) as fertilizer water through the second drainage branch pipe (8b); d) In the struvite crystallization separation unit, the secondary concentrated aquaculture wastewater and the circulating aquaculture wastewater flowing out from the bottom water inlet of the crystallization reactor (14) are combined and then enter the crystallization reactor (14) through the circulation pipe (14b) and the feed inlet. The Mg in the secondary concentrated aquaculture wastewater is 2+ ,PO 4 3- and NH 4 + A chemical precipitation reaction occurs in the crystallization reactor (14) to generate struvite crystal solid fertilizer which enters the solid fertilizer tank (15); the aquaculture wastewater after precipitation treatment enters the freshwater aquaculture pond through the outlet pipe (14a) of the crystallization reactor (14) for repeated recycling; and the ammonium ion signal detector (16) is used to detect the ammonium ion concentration in the aquaculture wastewater discharged from the outlet pipe (14a); e) The measured value of ammonium ions detected by the ammonium ion signal detector (16) is used as an input signal of the PLC controller (17). According to the difference between the measured value of ammonium ions and the designed value, the PLC controller adjusts the opening of the first regulating valve (7) and the second regulating valve (13), respectively adjusting the flow rate of the drawn liquid from the outlet of the compound fertilizer forward osmosis membrane device (4) entering the first drawn liquid preparation tank (6) and the flow rate of the drawn liquid from the outlet of the elemental fertilizer forward osmosis membrane device (10) entering the second drawn liquid preparation tank (11), until the measured value of ammonium ions reaches the designed value and meets the water quality requirements for freshwater aquaculture wastewater treatment.

4. The method for treating freshwater aquaculture wastewater by a forward osmosis water-fertilizer integrated system according to claim 3, It is characterized in that The 10-20% compound fertilizer extraction liquid in step b is prepared by using solid compound fertilizer with TN concentration of 20%, TP concentration of 20% and TK concentration of 20%.

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