A low-carbon emission treatment method for aquaculture tail water

Abstract

This invention discloses a method for low-carbon emission treatment of aquaculture wastewater, comprising: electrochemical oxidation and flocculation treatment of the wastewater in an electrochemical reactor, converting macromolecular organic carbon into small-molecule volatile fatty acids, simultaneously forming composite flocs and treated effluent; the treated effluent enters a biofloc reactor, using volatile fatty acids as a carbon source, and through microbial assimilation, converts dissolved nitrogen and phosphorus into microbial protein and polyphosphates, forming bioflocs and biochemically treated effluent; the biochemically treated effluent is introduced into a photobioreactor, utilizing microalgae photosynthesis to absorb residual inorganic nitrogen and phosphorus and fix carbon dioxide, producing microalgae biomass and purified effluent; the bioflocs and microalgae biomass are mixed to prepare resource-based products. This method addresses the technical problem of existing technologies that treat organic carbon and nitrogen and phosphorus resources in wastewater as waste for high-energy-consuming degradation, without utilizing them as recyclable resources, thus neglecting their recovery value as carbon sinks and nutrient sources.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a method for treating aquaculture wastewater with low carbon emissions. Background Technology

[0002] Currently, with the development of aquaculture towards high-density and intensive farming, the wastewater from aquaculture is rich in uneaten feed, feces, nitrogen and phosphorus nutrients, and pathogenic microorganisms. Existing aquaculture wastewater treatment technologies mainly include physical filtration, biological filters, and chemical sedimentation.

[0003] However, to remove chemical oxygen demand (COD) and ammonia nitrogen from effluent, existing processes rely on energy-intensive aerobic biological treatment. During this process, organic carbon is converted into carbon dioxide by microbial respiration and emitted as part of the emissions. Incomplete denitrification also produces nitrous oxide. Simultaneously, nutrients such as nitrogen and phosphorus are converted into excess sludge for disposal rather than recycling. Therefore, current technologies treat organic carbon and nitrogen / phosphorus resources in effluent as waste through energy-intensive degradation, failing to utilize them as recyclable resources and neglecting their value as carbon sinks and nutrient sources. Summary of the Invention

[0004] The purpose of this invention is to provide a low-carbon emission treatment method for aquaculture wastewater, in order to solve the technical problem that existing technologies treat organic carbon and nitrogen and phosphorus resources in wastewater as waste for high-energy-consuming degradation, without utilizing them as recyclable resources, and neglecting their recycling value as carbon sinks and nutrient sources.

[0005] The technical solution of this invention is implemented as follows:

[0006] A method for treating aquaculture wastewater with low carbon emissions includes:

[0007] Step S1: The aquaculture wastewater is treated by electrochemical oxidation and flocculation in an electrochemical reactor, where large molecular organic carbon is converted into small molecular volatile fatty acids, and composite flocs and treated effluent are formed at the same time.

[0008] Step S2: The treated effluent enters the biofloc reactor, where the volatile fatty acids serve as the carbon source. Through microbial assimilation, dissolved nitrogen and phosphorus are converted into bacterial proteins and polyphosphates, forming bioflocs and biochemically treated effluent.

[0009] Step S3: The biochemically treated effluent is introduced into a photobioreactor, where microalgae absorb residual inorganic nitrogen and inorganic phosphorus and fix carbon dioxide through photosynthesis, producing microalgae biomass and purified effluent.

[0010] Step S4: Mix the bioflocs with the microalgae biomass to prepare a resource-based product.

[0011] A further technical solution is that step S1 includes:

[0012] Step S11: Introduce the aquaculture wastewater into the electrochemical reactor, wherein the anode of the electrochemical reactor includes a high oxygen overpotential electrode and the cathode includes an iron electrode or an aluminum electrode.

[0013] Step S12: Apply a DC electric field, control the current density to be 5-20 mA / cm², and the hydraulic residence time to be 15-60 min;

[0014] Step S13: Hydroxyl radicals are generated at the anode, which break the chain and open the ring of the large organic carbon molecules, converting them into small volatile fatty acids.

[0015] Step S14: The iron or aluminum ions dissolved from the cathode react with the phosphate in the tailwater to form composite flocs, and the treated effluent is obtained.

[0016] A further technical solution is that, in step S12, the current density is controlled to be 10-15 mA / cm², and the hydraulic retention time is controlled to be 30-45 min, so that 30%-50% of the organic carbon in the effluent is converted into the volatile fatty acids.

[0017] A further technical solution is that step S2 includes:

[0018] Step S21: The treated effluent is introduced into a biofloc reactor, and suspended packing is added to the biofloc reactor and inoculated with heterotrophic nitrifying-aerobic denitrifying bacteria and photosynthetic bacteria.

[0019] Step S22: Control the dissolved oxygen concentration of the biofloc reactor to be 0.5-1.5 mg / L, the hydraulic retention time to be 4-8 h, and the sludge age to be 5-10 d;

[0020] Step S23: Using the volatile fatty acids as a carbon source, dissolved nitrogen and phosphorus in the tailwater are converted into bacterial protein and polyphosphate through the assimilation of microorganisms, while the microorganisms proliferate to form a biofloc mixture.

[0021] Step S24: The biofloc mixture is subjected to gravity sedimentation to separate the bioflocs enriched with carbon, nitrogen and phosphorus, and the biochemically treated effluent.

[0022] A further technical solution is that step S23 includes:

[0023] Step S231: The biofloc reactor has reached a stable state, and the microbial community inside it has entered the logarithmic growth phase.

[0024] Step S232: The volatile fatty acids are taken up by microorganisms and metabolized through the tricarboxylic acid cycle to produce adenosine triphosphate and a first reducing power.

[0025] Step S233: The microorganisms use the adenosine triphosphate and the first reducing power to assimilate ammonia nitrogen and nitrate nitrogen in the tailwater into amino acids and then synthesize bacterial proteins. At the same time, they absorb dissolved phosphorus and convert it into polyphosphates for storage in the cells.

[0026] Step S234: The microorganisms multiply during the process of absorbing carbon sources and nutrients, causing the number of cells to increase exponentially.

[0027] Step S235: The proliferated microorganisms aggregate with suspended particulate matter and extracellular polymers in the tailwater to form the biofloc mixture with sedimentation properties.

[0028] A further technical solution is that step S234 includes:

[0029] Step S2341: The microorganisms actively take up the volatile fatty acids and dissolved nitrogen and phosphorus nutrients from the water into their cells through transport proteins on their cell membranes.

[0030] Step S2342: The volatile fatty acids are converted into carbon skeletons and precursor substances synthesized by cells through metabolic pathways;

[0031] Step S2343: The nitrogen and phosphorus nutrients and the carbon skeleton precursor are assembled in the cell to synthesize bacterial protein and polyphosphate.

[0032] Step S2343: As cell components are continuously synthesized, the microbial cell volume increases and divides into at least two daughter cells. The process of steps S2341-S2343 is repeated, causing the number of bacteria to increase exponentially.

[0033] A further technical solution is that step S3 includes:

[0034] Step S31: Introduce the biochemically treated effluent into a photobioreactor, inoculate the photobioreactor with domesticated microalgae, and set up a light system;

[0035] Step S32: Under light conditions, the microalgae absorb light energy through the light system, photolyze water molecules to produce oxygen and a second reducing force, and at the same time fix the residual carbon dioxide in the biochemically treated water.

[0036] Step S33: The microalgae utilize the second reducing force to actively transport the inorganic nitrogen remaining in the biochemically treated effluent into the cells, where it is converted into proteins through assimilation and metabolism.

[0037] Step S34: Actively transport the inorganic phosphorus into the cell, where it is converted into phospholipids through assimilation and metabolism;

[0038] Step S35: The microalgae continuously proliferate during the process of absorbing nitrogen and phosphorus and fixing carbon dioxide, forming microalgal biomass, while simultaneously purifying the effluent.

[0039] A further technical solution is that step S32 includes:

[0040] Step S321: The lighting system emits a combined spectrum of red and blue light, and the chlorophyll and carotenoids in the microalgae selectively absorb light energy of the corresponding wavelengths;

[0041] Step S322: The absorbed light energy excites the photosynthetic pigment molecules in the microalgae, causing them to transition from the ground state to the excited state, thus converting the light energy into chemical potential energy;

[0042] Step S323: The chemical potential drives the splitting of water molecules to produce oxygen and protons, while simultaneously generating the second reducing force;

[0043] Step S324: The second reducing power and the chemical potential work together in the carbon dioxide fixation reaction to convert the residual carbon dioxide in the biochemically treated effluent into organic carbon and store it in the microalgae.

[0044] A further technical solution is that step S4 includes:

[0045] Step S41: The bioflocs and the microalgal biomass are mixed at a mass ratio of 2:1 to 4:1, and the moisture content of the mixture is adjusted to 60%-80%.

[0046] Step S42: Add a binder to the mixture and mechanically stir to evenly disperse the bioflocs and microalgae biomass to form a paste;

[0047] Step S43: The paste is extruded and molded using a granulation device to form a granular matrix with a diameter of 2-5 mm;

[0048] Step S44: Dry the particulate matrix at a low temperature of 40-60°C to reduce the moisture content to 1% to 9% to obtain the resource-based product.

[0049] A further technical solution is that step S41 includes:

[0050] Step S411: Collect the bioflocs and determine their water content and carbon, nitrogen and phosphorus content; collect the microalgal biomass and determine its water content and carbon, nitrogen and phosphorus content.

[0051] Step S412: Based on the moisture content measurement results of the bioflocs and the microalgal biomass, calculate the amount of water to be added or removed to achieve the target moisture content of 60%-80%.

[0052] Step S413: Weigh the bioflocs and the microalgae biomass at a mass ratio of 2:1 to 4:1, and use a step-by-step amplification mixing method to first premix a small portion of the microalgae biomass with all the bioflocs, and then add the remaining microalgae biomass in batches to make the two evenly dispersed.

[0053] Step S414: During the mixing process, the moisture content is adjusted synchronously according to the calculation results of step S412, so that the moisture content of the mixture is controlled within the range of 60%-80%.

[0054] The beneficial effects of this invention are as follows:

[0055] 1. In step S1 of this invention, electrochemical oxidation is employed. By controlling the current density and residence time, macromolecular organic carbon is directionally converted into small-molecule volatile fatty acids, avoiding over-mineralization. In step S2, microorganisms use volatile fatty acids as a carbon source for assimilation metabolism, converting carbon into microbial proteins stored in bioflocs. In step S3, microalgae fix residual carbon dioxide through photosynthesis. Thus, organic carbon is transferred from the aqueous phase to bioflocs and microalgal biomass, achieving carbon sequestration rather than emission, and the overall process can achieve carbon negative emissions.

[0056] 2. In step S2 of this invention, microorganisms, under low-oxygen conditions, use volatile fatty acids as a carbon source to directly convert dissolved nitrogen and phosphorus into microbial proteins and polyphosphates through assimilation, achieving efficient enrichment of nitrogen and phosphorus from the aqueous phase to the solid phase. In step S3, microalgae further absorb residual nitrogen and phosphorus and convert them into proteins and phospholipids. The two biomass are mixed in step S4 to prepare a slow-release fertilizer or feed additive, transforming nitrogen and phosphorus resources from an environmental burden into economic products.

[0057] 3. This invention utilizes electrochemically generated iron / aluminum ions to replace external flocculants, saving on reagent costs; it uses volatile fatty acids as the carbon source for bioflocs, avoiding the need for external carbon sources; it controls operation under low dissolved oxygen conditions, reducing aeration energy consumption by more than 40% compared to traditional aerobic processes; and the oxygen produced by microalgae photosynthesis can be partially recycled back to the biofloc reactor, forming a gas cycle and further reducing energy consumption. Attached Figure Description

[0058] Figure 1 This is a flowchart of the steps for a method of treating low-carbon emissions from aquaculture wastewater according to the present invention.

[0059] Figure 2 This is a flowchart of step S1 of a method for treating low-carbon emissions from aquaculture wastewater according to the present invention.

[0060] Figure 3 This is a flowchart of step S2 of a method for treating low-carbon emissions from aquaculture wastewater according to the present invention;

[0061] Figure 4This is a flowchart of step S3 in a method for treating low-carbon emissions from aquaculture wastewater according to the present invention.

[0062] Figure 5 This is a flowchart of step S4 of a method for treating low-carbon emissions from aquaculture wastewater according to the present invention. Detailed Implementation

[0063] To better understand the technical content of this invention, specific embodiments are provided below, and the invention will be further described in conjunction with the accompanying drawings.

[0064] Example 1

[0065] This embodiment describes the treatment of wastewater from a shrimp farm. The farm is located in a southern coastal area with a stocking density of 500 shrimp / m³, a water exchange rate of approximately 10% / day, and a daily wastewater discharge of 50 m³. Wastewater quality parameters were monitored continuously for one week, with the following average values: pH 7.2-7.8, chemical oxygen demand (COD) concentration 180-220 mg / L, ammonia nitrogen concentration 12-15 mg / L, total nitrogen concentration 18-22 mg / L, total phosphorus concentration 3.5-4.5 mg / L, and water temperature 25-28℃.

[0066] Please see Figures 1 to 5 This invention provides a method for treating low-carbon emissions from aquaculture wastewater. The treatment system consists of an electrochemical reactor, a biofloc reactor, a photobioreactor, and a granulation and drying device connected in series.

[0067] Electrochemical reactor: A continuous-flow flat-plate electrochemical reactor with an effective volume of 2 m³, an electrode spacing of 20 mm, and an effective electrode area of ​​10 m². The anode is a titanium-based lead dioxide electrode, and the cathode is an iron plate electrode. It is equipped with a DC regulated power supply with a voltage adjustment range of 0-50 V and a current adjustment range of 0-500 A. An aeration device is installed at the bottom of the reactor for stirring and mass transfer.

[0068] Biofloc reactor: A cylindrical reactor with an effective volume of 5 m³, containing K3 suspended packing material at a filling ratio of 30%. The reactor bottom is equipped with microporous aeration discs and a variable frequency aeration fan; the top features an adjustable-speed stirring paddle; and a gravity sedimentation tank with an effective volume of 1 m³ is located on the side.

[0069] Photobioreactor: A tubular photobioreactor with a total effective volume of 2m³ is used. It consists of transparent glass tubes with a diameter of 50mm and a total length of 800m. It is equipped with an LED supplemental lighting system, with LEDs having a red-to-blue light ratio of 6:1 and a total power of 2kW. The light intensity is adjustable.

[0070] Granulation and drying equipment: an extrusion granulator with a granulation die aperture of 2.5 mm is used; a belt dryer is used with an adjustable drying temperature range of 30-80℃ and an adjustable conveyor belt speed.

[0071] In this embodiment, the electrochemical oxidation and flocculation treatment includes:

[0072] Step S1: The aquaculture wastewater undergoes electrochemical oxidation and flocculation treatment in an electrochemical reactor, where large molecular organic carbon is converted into small molecular volatile fatty acids, and composite flocs and treated effluent are formed simultaneously.

[0073] First, the aquaculture wastewater is continuously introduced into the electrochemical reactor at a flow rate of 1.5 m³ / h.

[0074] Specifically, in step S11, the anode of the electrochemical reactor is a titanium-based lead dioxide electrode, and the cathode is an iron plate electrode. The anode has a high oxygen overpotential characteristic, which can preferentially generate hydroxyl radicals rather than oxygen evolution under high current density; the iron cathode slowly dissolves Fe²⁺ under the action of an electric field.

[0075] Specifically, step S12 involves applying a DC electric field with a current density controlled at 12 mA / cm² and a hydraulic residence time controlled at 40 min. Actual operation monitoring showed that under these parameters, the concentration of hydroxyl radicals generated at the anode remained stable at 0.5-0.8 mmol / L, sufficient to achieve chain breaking and ring opening of macromolecular organic compounds.

[0076] Specifically, in step S13, the hydroxyl radicals (·OH) generated at the anode possess extremely strong oxidizing properties (oxidation potential 2.80V), breaking the chains and opening the rings of large molecular organic carbon (mainly starch, protein, cellulose, etc. from uneaten feed) in the effluent. Gas chromatography-mass spectrometry analysis showed that the molecular weight of large organic molecules in the influent was mainly distributed between 10-100 kDa, while the concentration of small molecule volatile fatty acids (mainly acetic acid, propionic acid, and butyric acid) in the effluent increased from the initial 10 mg / L to 85 mg / L, and the proportion of volatile fatty acids in the chemical oxygen demand increased from 5% to 42%. Simultaneously, approximately 35% of the organic carbon was converted into volatile fatty acids, preventing excessive mineralization into carbon dioxide.

[0077] Specifically, in step S14, Fe²⁺ dissolved from the iron cathode is partially oxidized to Fe³⁺ under the influence of an electric field. Fe²⁺ and Fe³⁺ react with phosphates in the effluent to form ferrous phosphate and ferric phosphate precipitates. Simultaneously, iron ions act as a flocculant, bridging suspended particles, colloids, and organic carbon in the effluent to form composite flocs with organic carbon as the framework and phosphates encapsulated. After gravity sedimentation, the composite flocs (which can be used as a soil conditioner) and the treated effluent are obtained. This step achieves a chemical oxygen demand (COD) removal rate of 28% and a total phosphorus removal rate of 32%.

[0078] In this embodiment, the biofloc conversion treatment includes:

[0079] Step S2: The treated effluent enters the biofloc reactor, where volatile fatty acids serve as the carbon source. Through microbial assimilation, dissolved nitrogen and phosphorus are converted into bacterial proteins and polyphosphates, forming bioflocs and biochemically treated effluent.

[0080] The treated effluent obtained in step S1 is introduced into the biofloc reactor at a flow rate of 1.5 m³ / h.

[0081] Specifically, step S21 involves adding K3 suspended packing material to the biofloc reactor at a filling ratio of 30% to provide a carrier for microbial attachment and growth. Bacillus subtilis and Rhodopseudomonas palustris are inoculated at an initial concentration of 1×10⁻⁶. 6 CFU / mL. Bacillus subtilis has strong heterotrophic nitrification and aerobic denitrification capabilities, while Rhodopseudomonas palustris is a photosynthetic bacterium that can utilize light energy for metabolism under low-oxygen conditions.

[0082] Specifically, step S22 involves controlling the reactor operating parameters: dissolved oxygen concentration is controlled at 0.8-1.2 mg / L via variable frequency aeration, hydraulic retention time is 6 hours, and sludge age is controlled at 7 days via daily sludge removal. Dissolved oxygen control is crucial in this step. A micro-aerobic environment of 0.8-1.2 mg / L provides sufficient electron acceptors for microorganisms to obtain energy while inhibiting the complete oxidation of organic carbon to carbon dioxide, thus promoting carbon flow towards assimilation and synthesis.

[0083] Specifically, step S23 involves microorganisms using the volatile fatty acids produced in step S1 as the main carbon source for metabolism.

[0084] Furthermore, including step S231, the reactor is controlled to run for 72 hours to reach a stable state, the microbial community enters the logarithmic growth phase, and the concentration of suspended solids in the mixed liquor increases from the initial 500 mg / L to 2500 mg / L;

[0085] In step S232, volatile fatty acids are taken up by microorganisms and metabolized through the tricarboxylic acid cycle to produce adenosine triphosphate and reduced coenzyme.

[0086] In step S233, microorganisms use adenosine triphosphate and reduced coenzymes to assimilate ammonia nitrogen and nitrate nitrogen in the tailwater into amino acids and then synthesize bacterial proteins. At the same time, they absorb dissolved phosphorus and convert it into polyphosphates for storage in the cells.

[0087] In step S234, the microorganisms continuously multiply during the process of absorbing carbon sources and nutrients, and the number of cells increases exponentially.

[0088] In step S235, the proliferating microorganisms aggregate with suspended particulate matter and extracellular polymers in the effluent to form a biofloc mixture with good settling properties.

[0089] Further refinement of step S234 specifically includes:

[0090] Step S2341: Microorganisms actively take up volatile fatty acids and dissolved nitrogen and phosphorus nutrients into the cell through transport proteins on the cell membrane;

[0091] Step S2342: Volatile fatty acids are converted into carbon skeletons and precursors for cell synthesis via metabolic pathways;

[0092] Step S2343: Nitrogen and phosphorus nutrients and carbon skeleton precursors are assembled in the cell to synthesize bacterial proteins and polyphosphates.

[0093] Step S2344: As cell components are continuously synthesized, the microbial cell volume increases and divides into two daughter cells. The above process is repeated, causing the number of bacteria to increase exponentially.

[0094] Step S24: The biofloc mixture is introduced into a gravity sedimentation tank for 30 minutes. The supernatant overflows and enters the next process, while the precipitated bioflocs enriched with carbon, nitrogen, and phosphorus are discharged from the bottom of the tank. The daily discharge of bioflocs is 5% of the effective reactor volume, i.e., 0.25 m³ / d. Analysis shows that the dry matter of the bioflocs contains 45% crude protein, 8% crude fat, 35% total carbon (dry basis), and 2.5% total phosphorus. Material balance calculations indicate that this step transfers 65% of the total nitrogen and 72% of the total phosphorus in the effluent to the solid phase of the bioflocs.

[0095] In this embodiment, the deep purification treatment of microalgae includes:

[0096] Step S3: The biochemically treated effluent is introduced into a photobioreactor, where microalgae absorb residual inorganic nitrogen and inorganic phosphorus and fix carbon dioxide through photosynthesis, producing microalgae biomass and purified effluent.

[0097] The biochemically treated effluent obtained in step S2 is introduced into the photobioreactor at a flow rate of 1.5 m³ / h.

[0098] Specifically, step S31 involves inoculating the photobioreactor with domesticated Chlorella proteoglycans, initially inoculating OD... 680The value is 0.2. Domesticated microalgae refers to algal species that have been adapted and cultured in effluent with the same water quality as this process for more than 7 days, exhibiting good tolerance and growth activity. The lighting system is set to a red-blue light combination (red light wavelength 660nm, blue light wavelength 450nm, ratio 6:1), with a total light intensity of 150μmol / m² / s and a light-dark cycle of 14 hours of light and 10 hours of darkness.

[0099] Specifically, step S32 involves microalgae performing photosynthesis under light conditions.

[0100] Further, including step S321, the red and blue light emitted by the illumination system is selectively absorbed by chlorophyll a, chlorophyll b and carotenoids in the microalgae;

[0101] Step S322: The absorbed light energy excites photosynthetic pigment molecules to transition from the ground state to the excited state, converting light energy into chemical potential energy;

[0102] Step S323: Chemical potential drives the splitting of water molecules to produce oxygen and protons, while generating the first reducing force.

[0103] In step S324, the first reducing force and chemical potential work together in the Calvin cycle to convert the residual carbon dioxide in the biochemically treated water into organic carbon stored in the microalgal cells.

[0104] It should be noted that the first reducing power refers to the reduced nicotinamide adenine dinucleotide produced by microorganisms in step S232 during the tricarboxylic acid cycle metabolism using volatile fatty acids as substrates. Its main function is to provide reducing equivalents and chemical energy for microorganisms to assimilate nitrogen and phosphorus and synthesize cell proteins. The second reducing power refers to the reduced nicotinamide adenine dinucleotide phosphate produced by microalgae in step S323 after receiving high-energy electrons through the light system during the photosynthetic light reaction stage. Its main function is to provide reducing capacity for microalgae to fix carbon dioxide and assimilate nitrogen and phosphorus. Although both originate from different metabolic pathways, they essentially serve as intracellular energy currencies, driving the resource conversion process of carbon, nitrogen, and phosphorus from the aqueous phase to the solid phase in this invention.

[0105] Specifically, in step S33, the microalgae utilize the first reducing force generated in step S32 to actively transport residual inorganic nitrogen from the biochemically treated effluent into their cells via nitrate transport proteins, where it is converted into protein through assimilation and metabolism. Analysis showed that the protein content within the microalgae cells accounted for 52% of their dry weight.

[0106] Specifically, in step S34, microalgae actively transport residual inorganic phosphorus into the cell via phosphate transporters, where it is converted into phospholipids through assimilation and metabolism. Analysis showed that the phospholipid content within microalgal cells accounted for 8% of their dry weight.

[0107] Specifically, in step S35, the microalgae continuously proliferate during the absorption of nitrogen and phosphorus and the fixation of carbon dioxide. The hydraulic retention time in the photobioreactor is 24 hours, and the concentration of microalgae biomass increases from an initial 0.2 g / L (dry weight) to 1.2 g / L (dry weight). The microalgae biomass is harvested by centrifugation, with a daily harvest of approximately 1.5 kg (dry weight). The effluent is tested and found to have the following concentrations: ammonia nitrogen less than 0.5 mg / L, total nitrogen less than 2.0 mg / L, total phosphorus less than 0.2 mg / L, chemical oxygen demand less than 30 mg / L, pH 7.0-7.5, and dissolved oxygen 5-6 mg / L. The water quality meets the Class I standard of the "Freshwater Pond Aquaculture Water Discharge Requirements" (SC / T 9101-2007).

[0108] In this embodiment, the preparation of resource-based products includes:

[0109] Step S4: Mix bioflocs with microalgal biomass to prepare resource-based products;

[0110] The bioflocs collected in step S2 are mixed with the microalgal biomass harvested in step S3 to prepare slow-release fertilizer.

[0111] Specifically, step S41 involves mixing bioflocs and microalgal biomass at a mass ratio of 3:1, adjusting the moisture content of the mixture to 70%. Specifically, in step S411, the moisture content of the bioflocs (85%) and the moisture content of the microalgal biomass (80%) are measured separately, and their carbon, nitrogen, and phosphorus contents are also determined.

[0112] Further, step S41 includes:

[0113] Step S412: Based on the measurement results, when mixed at a ratio of 3:1, the theoretical moisture content of the mixture is 83.75%, requiring additional dehydration to reduce it to the target moisture content of 70%.

[0114] Step S413: Weigh 120 kg (wet weight) of biofloc and 40 kg (wet weight) of microalgae biomass at a mass ratio of 3:1. Use a step-by-step mixing method: first, premix 5 kg of microalgae biomass with all 120 kg of biofloc, stir evenly, and then add the remaining 35 kg of microalgae biomass in three portions, stirring for 5 minutes after each addition.

[0115] Step S414: During the mixing process, excess water is removed by pressure filtration to keep the moisture content of the mixture within the range of 68%-72%.

[0116] Specifically, step S42 involves adding sodium alginate as a binder to the mixture at a concentration of 5% of the total mass. The mixture is then mechanically stirred for 15 minutes to uniformly disperse the bioflocs and microalgal biomass, forming a plastic paste. The extracellular polymers abundant in the bioflocs synergistically enhance the adhesiveness of the paste with sodium alginate.

[0117] Specifically, step S43 involves feeding the paste into an extrusion granulator, extruding it through a die with a 2.5mm aperture, and cutting it into cylindrical granule matrix with a length of 3-5mm.

[0118] Specifically, step S44 involves uniformly spreading the granular matrix on a belt dryer, setting the drying temperature to 50℃, and drying for 4 hours to reduce the moisture content to 5%-8%. Testing revealed that the resulting granules had a moisture content of 6.5%, a total nitrogen content (as N) of 6.8%, a total phosphorus content (as P2O5) of 4.2%, and a total organic carbon content of 28%. Soaking the granules in clean water for 72 hours resulted in nitrogen and phosphorus release rates of 18% and 15%, respectively, demonstrating good slow-release properties. This granular base fertilizer can be directly used as a base fertilizer for landscaping or as a base fertilizer for farmland.

[0119] This embodiment ran continuously for 30 days, and all indicators remained stable. The processing results are summarized as follows:

[0120] index Water ingress Out of water Removal rate Resource utilization rate Chemical oxygen demand (mg / L) 200 28 86% Approximately 45% is converted into biomass carbon. Ammonia nitrogen (mg / L) 13.5 0.4 97% Approximately 70% is converted into bacterial protein. Total nitrogen (mg / L) 20 1.8 91% Approximately 65% ​​is converted into bacterial and microalgal proteins. Total phosphorus (mg / L) 4.0 0.15 96% Approximately 72% is converted into polyphosphates and phospholipids.

[0121] Example 2

[0122] This embodiment treats the effluent from a marine industrialized recirculating aquaculture system to verify the applicability of the invention under different water quality conditions. The daily effluent discharge from this aquaculture system is 20 m³, the salinity of the seawater is 25‰, the pH value is 7.8-8.2, the chemical oxygen demand (COD) concentration is 150-180 mg / L, the ammonia nitrogen concentration is 8-12 mg / L, the total nitrogen concentration is 15-18 mg / L, the total phosphorus concentration is 2.0-2.5 mg / L, and the water temperature is 26-30℃.

[0123] The main difference between this embodiment and Embodiment 1 is:

[0124] Electrochemical reactor (step S1): Considering the high chloride ion concentration in seawater (approximately 15000 mg / L), a boron-doped diamond electrode was used at the anode to avoid the generation of toxic chlorine gas. The boron-doped diamond electrode has an extremely high oxygen evolution overpotential (approximately 2.5 V vs SCE), enabling the preferential generation of hydroxyl radicals rather than chlorine gas in a high-chlorine environment. The current density was controlled at 10 mA / cm², and the hydraulic retention time was 30 min. Testing showed that the concentration of volatile fatty acids in the effluent increased from the initial 8 mg / L to 65 mg / L, and the proportion of chemical oxygen demand (COD) increased from 4% to 38%.

[0125] Biofloc Reactor (Step S2): Due to the high salinity of seawater, salt-tolerant marine photosynthetic bacteria and salt-tolerant Bacillus subtilis are selected. No suspended packing is used in the biofloc reactor; circulating hydraulic shear force is employed to control the floc particle size between 100-200 μm to avoid salinity affecting floc settling performance. Dissolved oxygen is controlled at 0.6-1.0 mg / L, and the sludge age is controlled at 6 days. Testing revealed that the crude protein content in the dry matter of the bioflocs is 42%, and the total phosphorus content is 2.2%. This step transfers 60% of the total nitrogen and 68% of the total phosphorus in the effluent to the bioflocs.

[0126] Advanced microalgae treatment (step S3): Salt-tolerant Dunaliella salina was selected as the microalgae. This species grows well at a salinity of 25‰ and can accumulate β-carotene in high-salt environments. The hydraulic retention time in the photobioreactor was 36 h, and the dry weight yield of microalgae biomass was 0.8 g / L. Testing showed that the β-carotene content in the microalgae cells reached 3% of the dry weight, increasing the value of the by-products. Effluent quality: ammonia nitrogen <0.5 mg / L, total nitrogen <2.5 mg / L, total phosphorus <0.2 mg / L, chemical oxygen demand <35 mg / L.

[0127] Preparation of resource-based products (step S4): Bioflocs and microalgal biomass are mixed at a mass ratio of 2.5:1, and the moisture content is adjusted to 65%. Using the same granulation and drying process, the resulting granular slow-release fertilizer has a total nitrogen content of 5.8%, a total phosphorus content of 3.5%, and a total organic carbon content of 25%. This granular fertilizer can be used for soil improvement in coastal saline-alkali land, providing both nutrients and improving soil structure.

[0128] This embodiment operated continuously for 30 days, achieving stable treatment results. The comprehensive energy consumption for treating 1 m³ of effluent was 0.22 kWh (due to the high conductivity of seawater, the energy consumption of the electrochemical reaction was reduced), and the carbon emission intensity was -0.38 kg CO₂ eq / m³. The results indicate that this invention is also applicable to the treatment of high-salinity seawater aquaculture effluent, demonstrating good versatility.

[0129] Comparative Example

[0130] To verify the technical effect of the present invention, a comparative example was set up. The comparative example used the traditional "A² / O (anaerobic-anoxic-aerobic) process + chemical phosphorus removal" to treat aquaculture wastewater with the same water quality as in Example 1.

[0131] Comparative parameters: hydraulic retention time in the anaerobic tank was 2 h, in the anoxic tank it was 4 h, in the aerobic tank it was 8 h, dissolved oxygen concentration in the aerobic tank was 2.5 mg / L, sludge age was 15 days, mixed liquor recirculation ratio was 200%, and sludge recirculation ratio was 100%. Polyaluminum chloride (PAC) was used for chemical phosphorus removal at a dosage of 30 mg / L. The effluent from the secondary sedimentation tank was discharged after sand filtration.

[0132] The following is a data table comparing the discharge limits and treatment effects of aquaculture wastewater:

[0133] Table of Limits for Aquaculture Wastewater Discharge

[0134] Serial Number project Limit 1 Suspended solids (mg / L) ≤90 2 PH 6.5～9.0 3 Chemical oxygen demand (mg / L) ≤20 4 Total nitrogen (as N) (mg / L) ≤7.0 5 Total nitrogen (as phosphorus) (mg / L) ≤1.0

[0135] According to the emission limit standards for aquaculture wastewater, ensuring that the treated wastewater consistently meets the secondary seawater emission standards in the "Aquaculture Wastewater Discharge Standard" (DB46 / 475—2023) and simultaneously realizing the resource utilization of manure can promote the sustainable development of the aquaculture industry.

[0136] Comparison of treatment effects (for treating 1 m³ of effluent)

[0137] index Example 1 Comparative Example Chemical oxygen demand (mg / L) of effluent 28 35 Ammonia nitrogen in effluent (mg / L) 0.4 1.2 Total nitrogen in effluent (mg / L) 1.8 8.5 Total phosphorus in effluent (mg / L) 0.15 0.3 Energy consumption (kWh) 0.28 0.52 <![CDATA[Carbon emission intensity (kg CO2 eq)]]> -0.45 +0.75 Nitrogen resource destination 65% converted into microbial protein / fertilizer <![CDATA[65% is converted into N2 and escapes, 20% enters the excess sludge]]> Where do phosphorus resources go? 72% converted into slow-release fertilizer raw materials 55% ends up in chemical sludge (difficult to utilize). Sludge production (kg dry weight) 0.28 0.75 Operating costs (RMB) 0.55 1.10

[0138] in conclusion:

[0139] Specific embodiments of this invention demonstrate that a coupled process of electrochemical directional conversion, biological assimilation and enrichment, microalgal photosynthetic carbon fixation, and resource recycling can effectively treat freshwater and marine aquaculture wastewater, with effluent quality meeting discharge standards. This process converts organic carbon in the wastewater into bioflocs and microalgal biomass, achieving carbon sequestration; it also converts dissolved nitrogen and phosphorus into microbial protein and polyphosphates, which are then used to prepare slow-release fertilizers, realizing a closed-loop recycling of nitrogen and phosphorus resources. Compared to traditional processes, this invention reduces overall energy consumption by 40%-50%, lowers carbon emission intensity to negative values, and reduces operating costs by approximately 50%, demonstrating significant environmental and economic benefits and aligning with the technological direction of green, low-carbon, and circular development.

[0140] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for treating aquaculture wastewater with low carbon emissions, characterized in that, include: Step S1: The aquaculture wastewater is treated by electrochemical oxidation and flocculation in an electrochemical reactor, where large molecular organic carbon is converted into small molecular volatile fatty acids, and composite flocs and treated effluent are formed at the same time. Step S2: The treated effluent enters the biofloc reactor, where the volatile fatty acids serve as the carbon source. Through microbial assimilation, dissolved nitrogen and phosphorus are converted into bacterial proteins and polyphosphates, forming bioflocs and biochemically treated effluent. Step S3: The biochemically treated effluent is introduced into a photobioreactor, where microalgae absorb residual inorganic nitrogen and inorganic phosphorus and fix carbon dioxide through photosynthesis, producing microalgae biomass and purified effluent. Step S4: Mix the bioflocs with the microalgae biomass to prepare a resource-based product.

2. The method for low-carbon emission treatment of aquaculture wastewater according to claim 1, characterized in that, Step S1 includes: Step S11: Introduce the aquaculture wastewater into the electrochemical reactor, wherein the anode of the electrochemical reactor includes a high oxygen overpotential electrode and the cathode includes an iron electrode or an aluminum electrode. Step S12: Apply a DC electric field, control the current density to be 5-20 mA / cm², and the hydraulic residence time to be 15-60 min; Step S13: Hydroxyl radicals are generated at the anode, which break the chain and open the ring of the large organic carbon molecules, converting them into small volatile fatty acids. Step S14: The iron or aluminum ions dissolved from the cathode react with the phosphate in the tailwater to form composite flocs, and the treated effluent is obtained.

3. The method for treating low-carbon emissions of aquaculture wastewater according to claim 2, characterized in that, In step S12, the current density is controlled at 10-15 mA / cm², and the hydraulic retention time is controlled at 30-45 min, so that 30%-50% of the organic carbon in the effluent is converted into the volatile fatty acids.

4. The method for low-carbon emission treatment of aquaculture wastewater according to claim 1, characterized in that, Step S2 includes: Step S21: The treated effluent is introduced into a biofloc reactor, and suspended packing is added to the biofloc reactor and inoculated with heterotrophic nitrifying-aerobic denitrifying bacteria and photosynthetic bacteria. Step S22: Control the dissolved oxygen concentration of the biofloc reactor to be 0.5-1.5 mg / L, the hydraulic retention time to be 4-8 h, and the sludge age to be 5-10 d; Step S23: Using the volatile fatty acids as a carbon source, dissolved nitrogen and phosphorus in the tailwater are converted into bacterial protein and polyphosphate through the assimilation of microorganisms, while the microorganisms proliferate to form a biofloc mixture. Step S24: The biofloc mixture is subjected to gravity sedimentation to separate the bioflocs enriched with carbon, nitrogen and phosphorus, and the biochemically treated effluent.

5. The method for treating low-carbon emissions of aquaculture wastewater according to claim 4, characterized in that, Step S23 includes: Step S231: The biofloc reactor has reached a stable state, and the microbial community inside it has entered the logarithmic growth phase. Step S232: The volatile fatty acids are taken up by microorganisms and metabolized through the tricarboxylic acid cycle to produce adenosine triphosphate and a first reducing power. Step S233: The microorganisms use the adenosine triphosphate and the first reducing power to assimilate ammonia nitrogen and nitrate nitrogen in the tailwater into amino acids and then synthesize bacterial proteins. At the same time, they absorb dissolved phosphorus and convert it into polyphosphates for storage in the cells. Step S234: The microorganisms multiply during the process of absorbing carbon sources and nutrients, causing the number of cells to increase exponentially. Step S235: The proliferated microorganisms aggregate with suspended particulate matter and extracellular polymers in the tailwater to form the biofloc mixture with sedimentation properties.

6. The method for treating low-carbon emissions of aquaculture wastewater according to claim 5, characterized in that, Step S234 includes: Step S2341: The microorganisms actively take up the volatile fatty acids and dissolved nitrogen and phosphorus nutrients from the water into their cells through transport proteins on their cell membranes. Step S2342: The volatile fatty acids are converted into carbon skeletons and precursor substances synthesized by cells through metabolic pathways; Step S2343: The nitrogen and phosphorus nutrients and the carbon skeleton precursor are assembled in the cell to synthesize bacterial protein and polyphosphate. Step S2343: As cell components are continuously synthesized, the microbial cell volume increases and divides into at least two daughter cells. The process of steps S2341-S2343 is repeated, causing the number of bacteria to increase exponentially.

7. The method for treating low-carbon emissions of aquaculture wastewater according to claim 1, characterized in that, Step S3 includes: Step S31: Introduce the biochemically treated effluent into a photobioreactor, inoculate the photobioreactor with domesticated microalgae, and set up a light system; Step S32: Under light conditions, the microalgae absorb light energy through the light system, photolyze water molecules to produce oxygen and a second reducing force, and at the same time fix the residual carbon dioxide in the biochemically treated water. Step S33: The microalgae utilize the second reducing force to actively transport the inorganic nitrogen remaining in the biochemically treated effluent into the cells, where it is converted into proteins through assimilation and metabolism. Step S34: Actively transport the inorganic phosphorus into the cell, where it is converted into phospholipids through assimilation and metabolism; Step S35: The microalgae continuously proliferate during the process of absorbing nitrogen and phosphorus and fixing carbon dioxide, forming microalgal biomass, while simultaneously purifying the effluent.

8. The method for treating low-carbon emissions of aquaculture wastewater according to claim 7, characterized in that, Step S32 includes: Step S321: The lighting system emits a combined spectrum of red and blue light, and the chlorophyll and carotenoids in the microalgae selectively absorb light energy of the corresponding wavelengths; Step S322: The absorbed light energy excites the photosynthetic pigment molecules in the microalgae, causing them to transition from the ground state to the excited state, thus converting the light energy into chemical potential energy; Step S323: The chemical potential drives the splitting of water molecules to produce oxygen and protons, while simultaneously generating the second reducing force; Step S324: The second reducing power and the chemical potential work together in the carbon dioxide fixation reaction to convert the residual carbon dioxide in the biochemically treated effluent into organic carbon and store it in the microalgae.

9. The method for treating low-carbon emissions of aquaculture wastewater according to claim 1, characterized in that, Step S4 includes: Step S41: The bioflocs and the microalgal biomass are mixed at a mass ratio of 2:1 to 4:1, and the moisture content of the mixture is adjusted to 60%-80%. Step S42: Add a binder to the mixture and mechanically stir to evenly disperse the bioflocs and microalgae biomass to form a paste; Step S43: The paste is extruded and molded using a granulation device to form a granular matrix with a diameter of 2-5 mm; Step S44: Dry the particulate matrix at a low temperature of 40-60°C to reduce the moisture content to 1% to 9% to obtain the resource-based product.

10. A method for treating low-carbon emissions of aquaculture wastewater according to claim 9, characterized in that, Step S41 includes: Step S411: Collect the bioflocs and determine their water content and carbon, nitrogen and phosphorus content; collect the microalgal biomass and determine its water content and carbon, nitrogen and phosphorus content. Step S412: Based on the moisture content measurement results of the bioflocs and the microalgal biomass, calculate the amount of water to be added or removed to achieve the target moisture content of 60%-80%. Step S413: Weigh the bioflocs and the microalgae biomass at a mass ratio of 2:1 to 4:1, and use a step-by-step amplification mixing method to first premix a small portion of the microalgae biomass with all the bioflocs, and then add the remaining microalgae biomass in batches to make the two evenly dispersed. Step S414: During the mixing process, adjust the moisture content synchronously according to the calculation results of step S412, so that the moisture content of the mixture is controlled within the range of 60%-80%.

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