Food processing wastewater recycling and purification treatment method

Abstract

The application discloses a food processing wastewater recycling and purification treatment method and belongs to the technical field of wastewater resource treatment. The method is characterized by a specific combination mode and parameter limitation of multistage physical screening, mineral adsorption coupling micro-nano ozonation catalysis, membrane biological degradation, nano composite adsorption decolorization and odor removal, and selective nanofiltration gradient reuse, solves the problems of low treatment and reuse rate of existing food processing wastewater, fast membrane pollution, large fluctuation of water color and organic matter, high production of residual sludge and difficult disposal, and realizes the effects of high proportion of wastewater recycling, stable operation, relatively controllable reagents and energy consumption, and obvious improvement of resource benefit.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment and resource reuse technology, and in particular to a method for recycling and purifying food processing wastewater. Background Technology

[0002] The food processing industry encompasses numerous sub-sectors, including fruit and vegetable products, meat, poultry and aquatic product processing, deep processing of grains, oils and starches, brewing and fermentation products, dairy products, and the manufacture of snack and puffed foods. The wastewater discharged during its production processes exhibits the following typical water quality characteristics: It contains large amounts of suspended and colloidal animal and plant proteins, starch, cellulose, fats, and pectin; the chemical oxygen demand (CODcr) typically ranges from 800 mg / L to 4500 mg / L, with daily and intraday concentration differences exceeding five times; and the five-day biochemical oxygen demand (BOD) is also high. The concentration is generally between 300 mg / L and 2200 mg / L. The biodegradable BOD5 / CODcr ratio is mostly between 0.4 and 0.65, showing a certain degree of biodegradability, but it fluctuates drastically due to high oil content and seasonal changes in production formula. The wastewater often contains emulsified oils, food colorings, flavorings, preservatives, and a small amount of alkaline agents used for cleaning, resulting in effluent color as high as 80 to 300 times, pH value fluctuating widely between 4.5 and 10.0, and suspended solids (SS) usually reaching 200 mg / L to 1200 mg / L, and containing a certain amount of large-sized fruit peel fragments, meat and bone fragments, grain husks, and other coarse impurities. Some aquatic or pickled food processing wastewater also contains high concentrations of sodium chloride, i.e., total dissolved solids (TDS) can reach 5000 mg / L to 15000 mg / L. If this type of wastewater is discharged directly, it will severely deplete the dissolved oxygen in the receiving water body and cause it to turn black and smelly. If it enters a municipal sewage treatment plant, the high nutrient content and water quality impact will easily cause the activated sludge to expand or even collapse.

[0003] Current technologies commonly employ a classic process combination for food processing wastewater treatment: "bar screen—equalization tank—primary sedimentation—hydrolysis acidification—aerobic biological contact oxidation or activated sludge process—secondary sedimentation—coagulation sedimentation or filtration—disinfection and discharge." Some improved solutions add biological treatment units such as biological filters (BAF), rotating biological discs, upflow anaerobic sludge blankets (UASB), or anaerobic-anoxic-aerobic (A² / O) variants to enhance the removal of organic matter and nitrogen and phosphorus. For advanced treatment and reuse, ultrafiltration (UF) or reverse osmosis (RO) membrane separation modules may be connected in series. However, the above-mentioned existing treatment methods have several shortcomings: First, traditional bar screens only have two fixed mesh sizes (coarse and fine) for interception. Fine fibers and colloidal proteins can easily pass through the screen and enter the subsequent equalization tank, causing long-term accumulation and clogging of pipelines and aeration heads, as well as increasing the load on the subsequent sedimentation tank. On the other hand, using simple mechanical filtration often results in filter screen clogging, requiring manual cleaning and affecting continuous operation. Second, the equalization tanks mostly use simple air stirring and mixing without implementing online precise pH control. After the food production line uses acid and alkali cleaning and alternates drainage to neutralize, the pH may still deviate from the suitable range for microorganisms, resulting in slow start-up and poor shock resistance of subsequent biological treatment. Third, conventional gravity sand filtration or single-stage sedimentation has limited ability to remove colloidal proteins, starches and some soluble macromolecules from wastewater, and the filter media surface is prone to biofilm coating, which leads to periodic caking and requires frequent replacement or acid washing. The specific adsorption efficiency of mineral modified adsorption materials for specific food components has not been fully considered. Fourth, biologically treated effluent still contains some non-biodegradable natural food pigments, Maillard reaction products, and a small amount of dissolved organic matter, resulting in high effluent color and difficulty in further reducing TOC. Direct reuse, especially in contact with product cleaning, can easily lead to sensory odors and hygiene hazards. Simply adding activated carbon adsorption tanks is too costly to operate and waste carbon regeneration is troublesome. Fifth, existing reuse schemes often reuse all the effluent after deep treatment by reverse osmosis desalination and overall softening and desalination as pure water. However, the reverse osmosis recovery rate is generally only 50% to 65%, and the concentrate needs to be treated separately. The failure to implement tiered diversion and reuse according to the different water quality requirements of each water point in food processing results in unnecessary high energy consumption and high chemical consumption. Blindly desalinating all food wastewater with medium salt content also wastes the usable moderately saline water for non-product contact initial washing. Sixth, the excess activated sludge production is large and has a high water content. The cost of traditional thickening-dewatering and off-site disposal is increasing year by year. The biomass energy contained in the sludge is not effectively recovered, and the waste organic matter is not utilized as a resource. Seventh, hollow fiber membranes in membrane bioreactors have a significantly faster fouling rate than those used in urban wastewater. Conventional constant flux or constant negative pressure operation lacks a combination of intermittent suction and chemical cleaning strategies tailored to the characteristics of food wastewater components, which can easily lead to irreversible decay of membrane flux and shorten membrane replacement cycles.

[0004] In summary, there is currently a lack of a method for the high-proportion recycling and purification of food processing wastewater that integrates multiple technologies in a specific order and with parameters, including spiral stepped fine screening, pH adaptive precise control, modified mineral multi-stage deep bed adsorption, micro-nano ozonation catalytic specific degradation, membrane bioreactor and nanocomposite adsorption for deep decolorization and odor removal, water quality graded diversion with partial nanofiltration purification and reuse, and sludge-residue ultrasonic-enhanced anaerobic biogas production. This is the technical problem that this invention aims to solve. Summary of the Invention

[0005] In view of this, the present invention provides a method for recycling and purifying food processing wastewater to solve or alleviate one of the technical problems existing in the prior art, and at least provides a beneficial option.

[0006] The technical solution of this invention is implemented as follows: A method for recycling and purifying food processing wastewater includes the following steps: S1. Solid-liquid pre-separation step: The raw mixed wastewater generated from food processing is introduced into a stepped screen with a rotating spiral scraper. The stepped screen is arranged in sequence along the water flow direction, with a first-stage stainless steel wedge screen with a screen width of 8mm, a second-stage stainless steel wedge screen with a screen width of 3mm, and a third-stage polypropylene precision screen with a screen width of 0.5mm. An inclined screw conveyor is set below each screen to transport the intercepted solid residue to the slag tank. During the screening process, the influent flow rate is maintained at a fluctuation range of no more than ±5% per cubic meter per hour and the liquid level in the screening zone is constant. After three-stage screening, the median particle size of the suspended solids in the liquid wastewater is reduced to less than or equal to 80μm and flows into the collection and equalization tank. S2, pH adaptive adjustment step: The liquid wastewater obtained in step S1 is pumped into the pH adaptive adjustment tank. The pH adaptive adjustment tank is equipped with three sets of variable frequency stirring paddles and an online pH-ORP-temperature integrated probe. The probe feeds back the pH signal to the PLC controller in real time. The PLC controller automatically adjusts the amount of dilute sulfuric acid or sodium hydroxide solution added by the metering pump according to the pH value of the influent to stabilize the pH value of the effluent in the adjustment tank between 6.8 and 7.5. The hydraulic retention time of the adjustment tank is 2.5h to 4.0h, and the bottom of the tank is equipped with perforated aeration pipes for intermittent micro-aeration to prevent sedimentation and anaerobic odor release. After adjustment, the wastewater is sent to the subsequent treatment unit by a lift pump. S3. Three-stage variable velocity deep bed adsorption filtration steps: After adjustment, the wastewater passes sequentially from top to bottom through a three-stage variable velocity deep bed adsorption filtration unit consisting of a composite mineral ceramsite primary filter layer, a modified zeolite intermediate filter layer, and a gradient-graded quartz sand fine filter layer. The initial empty tower flow rate is 8 m / h. When the filtration pressure difference reaches 15 kPa, it automatically switches to reverse segmented air-water combined backwashing. The composite mineral ceramsite primary filter layer has a particle size of 4 mm to 6 mm, a filling height of 800 mm, and is pre-activated with hydrochloric acid and has an iron-manganese oxide surface. The modified zeolite has a filter layer particle size of 1.2 mm to 2.0 mm and a filling height of 600 mm. It is modified by alternating hexadecyltrimethylammonium bromide and calcium chloride or by thermal activation. The gradient-graded quartz sand fine filter layer consists of a lower layer of coarse sand with a d = 0.8 mm to 1.2 mm thickness of 300 mm and an upper layer of fine sand with a d = 0.3 mm to 0.6 mm thickness of 400 mm. After this step, the turbidity of the effluent is reduced to less than or equal to 3 NTU and most of the colloidal proteins and starches are adsorbed and retained. S4. Pulsed micro-nano bubble ozone catalytic oxidation step: The effluent from step S3 is fed into a pulsed micro-nano bubble ozone catalytic contact oxidation tower, which is filled with particles with a diameter of 3mm to 5mm. Supported transition metal oxide catalyst particles, wherein the molar ratio of the three transition metals Mn, Cu, and Fe in the catalyst is Mn:Cu:Fe = 5:2:1 and the total loading is [missing information]. 8% to 12% of the carrier mass, ozone generated by the ozone generator is injected into the bottom gas distribution system of the tower through the dissolved gas release device to form micro-nano ozone bubbles with a diameter between 50nm and 500nm. The ozone dosage is 15mg to 30mg ozone per gram of CODcr. The hydraulic residence time of the contact oxidation tower is 25min to 45min. Every 8min, a pulsed high-flow-rate backwash disturbance is performed for 10s to 15s to prevent catalyst surface passivation. This step breaks down the difficult-to-biodegrade colored substances and large molecular chromophores in the wastewater into small molecular carboxylic acids and aldehydes and ketones. S5. Submerged negative pressure low-pressure membrane bioreactor step: The wastewater after oxidation in step S4 enters the submerged membrane bioreactor. The acclimated facultative-aerobic complex bacteria are added to the tank, and the sludge concentration MLSS is maintained at 6000mg / L to 9000mg / L. The dissolved oxygen is controlled at 2.0mg / L to 4.0mg / L in the aerobic zone and 0.2mg / L to 0.5mg / L in the anoxic zone to achieve simultaneous nitrification and denitrification. The membrane module is a polyvinylidene fluoride hollow fiber curtain membrane with a pore size of 0.03μm to 0.08μm. The suction negative pressure is -15kPa to -35kPa and an intermittent operation mode of stopping pumping for 2 minutes every 8 minutes of operation is adopted. Online sodium hypochlorite-citric acid alternating chemical cleaning is performed once every 48 hours. The CODcr of the membrane bioreactor effluent is less than or equal to 30mg / L and ammonia nitrogen is less than or equal to 1.5mg / L. S6. Nanocomposite Adsorption Decolorization and Odor Removal Step: The effluent from the membrane bioreactor flows by gravity through a precision temperature-controlled decolorization and odor removal column. The column is filled with chitosan-grafted magnetic graphene nanocomposite adsorbent. The adsorbent has a chitosan to graphene oxide mass ratio of 3:1 and is doped with magnetite magnetic particles with a particle size of 10nm to 20nm. The filling height to column diameter ratio is 5:1. The temperature of the circulating hot water in the column jacket is controlled at 35℃ to 40℃. The empty tower residence time is 12min to 18min. After the adsorbent is saturated, it is desorbed and regenerated using 0.5mol / L NaOH ethanol-water solution for recycling. This step further removes trace amounts of residual pigments, odor substances, and trace colloids. S7. Cascade Diversion and Reuse with Selective Nanofiltration Step: The effluent from step S6 is divided into two streams. The first stream, accounting for 60% to 75% of the effluent volume, directly enters the recycling tank as initial washing water for the food processing workshop floor and rinsing water for the outer surface of equipment. The second stream, accounting for 25% to 40% of the effluent volume, enters the anti-fouling polyamide thin-layer composite nanofiltration unit. The nanofiltration operating pressure is 0.6 MPa to 1.0 MPa, the inlet water temperature is controlled at 20℃ to 30℃, and the recovery rate is set at 70% to 80%. The nanofiltration permeate flows into the recycling tank as product contact-level cleaning water or cooling circulation system makeup water. The nanofiltration concentrate is returned to the pH adaptive adjustment tank in step S2 for reprocessing. S8. Sludge and Waste Resource Utilization Step: The solid residue retained in step S1 and the remaining sludge generated in step S5 are combined and fed into an anaerobic digester with ultrasonic crushing pretreatment. The ultrasonic frequency is 20kHz to 40kHz, the sound energy density is 0.3W / mL to 0.6W / mL, and the crushing time is 15min to 30min to release intracellular substances and improve the efficiency of anaerobic methanogenesis. The anaerobic digestion temperature is maintained at 35℃±1℃ and the residence time is 15d to 20d. After digestion, the sludge-water mixture is dewatered by a plate and frame filter press until the moisture content is less than or equal to 60% and then transported off-site for disposal or used as organic fertilizer base material. The biogas produced by digestion is desulfurized and purified and stored for heating or power generation in the plant area.

[0007] Preferably, in step S1, the surface of the third-stage polypropylene precision screen of the stepped grid screening device is coated with a PTFE hydrophobic and non-stick coating with a thickness of 20μm to 50μm, and each stage of the screen is equipped with a high-pressure fan-shaped flushing nozzle that sprays water for 0.5s every 3 minutes. The high-pressure flushing water pressure is 0.4MPa to 0.6MPa, and the speed of the screw conveyor is linearly adjusted by the PLC from 5r / min to 25r / min according to the concentration of suspended solids in the influent, so that the screen permeability is always kept above 90% without clogging or overflow.

[0008] Preferably, in step S2, the concentration of dilute sulfuric acid added to the pH adaptive adjustment tank is 3%–5% by mass, and the concentration of dilute sodium hydroxide solution added is 2%–4% by mass. The proportional coefficient Kp in the PID adjustment parameters of the PLC controller is set to 1.8 to 2.5, the integral time Ti is set to 60s to 120s, and the derivative time Td is set to 5s to 15s. When the pH value of the influent is detected to be lower than 5.0 or higher than 9.0, an alarm is triggered and the bypass pipeline is automatically started to introduce the abnormal peak wastewater into the emergency tank for subsequent batches to be slowly mixed into the adjustment tank for treatment. The actual control deviation of the pH value of the effluent from the adjustment tank does not exceed ±0.15 pH units.

[0009] Preferably, in step S3, the composite mineral ceramsite primary filter layer is pre-coated with a pre-coating solution containing 2% polyaluminum chloride and 0.5% cationic polyacrylamide at a forward low speed of 2 m / h for 4 to 6 hours before its first use. During operation, when the pressure difference between the inlet and outlet of the filter layer rises to 15 kPa, it is first scrubbed with air at an intensity of 12 L / (m²·s) for 30 seconds, and then backwashed with water at a downward flow rate of 18 m / h for 2 minutes and an upward flow rate of 12 m / h for 2 minutes respectively. The backwash wastewater is discharged into the plant's sewage pipe network and enters the equalization tank for retreatment. The modified zeolite filter layer is regenerated by soaking in a 5% sodium chloride solution for 2 hours every 30 days of cumulative operation, and then rinsed with deionized water until the conductivity of the effluent is less than 50 μS / cm.

[0010] Preferably, in step S4, the height-to-diameter ratio of the pulsed micro-nano bubble ozone catalytic contact oxidation tower is 4:1 to 6:1, and a tail gas destroyer is installed at the top of the tower to fill granular active MnO2 catalyst to decompose residual ozone. The ozone is generated by an oxygen source obtained from the PSA on-site oxygen generator, producing ozone gas with a concentration of 80 mg / L to 120 mg / L through a discharge ozone generator. The reflux ratio of the micro-nano bubble generator is controlled at an ozone-to-liquid volume ratio of 1:3 to 1:5. The catalyst particles are taken out after every 180 days of cumulative operation, ultrasonically cleaned with a 3% oxalic acid solution to remove the manganese oxide passivation film deposited on the surface, and then reused. The oxidation-reduction potential (ORP) of the effluent from the oxidation tower is controlled in the range of 250 mV to 380 mV to ensure moderate oxidation rather than excessive ozonooxidation that leads to the generation of byproducts.

[0011] Preferably, in step S5, the volume ratio of the aerobic zone to the anoxic zone in the submerged membrane bioreactor is 3:1, and an adjustable perforated baffle is installed between the two zones to allow the nitrifying liquid to flow back from the end of the aerobic zone to the front of the anoxic zone at a recirculation ratio of 100% to 300%. In the acclimatization stage of the composite microbial community, activated sludge from urban domestic sewage is first used as seed sludge and aerated for 24 hours. Then, the proportion of food processing wastewater added is gradually increased from 10%, 30%, 50%, 70% to 100% for 20 to 25 days until microscopic examination reveals abundant bacterial flocs, twig borers, and a small number of Vorticella, and the COD removal rate is stable at over 85%, which is considered as the acclimatization is complete. A perforated pipe is installed below the membrane curtain to perform microbubble aeration for 10 seconds every 30 minutes to rinse the membrane surface and prevent pollution.

[0012] Preferably, the preparation method of the chitosan-grafted magnetic graphene nanocomposite adsorbent in step S6 is as follows: graphene oxide dispersion is prepared from natural flake graphite using a modified Hummers method, and then added... and In situ synthesis of iron oxide / graphene oxide composite was achieved by co-precipitation in ammonia water under alkaline conditions at a molar ratio of 1:2. After centrifugation and washing to neutrality, a chitosan acetic acid solution with a deacetylation degree greater than or equal to 85% and an appropriate amount of glutaraldehyde crosslinking agent were added, and the mixture was subjected to graft crosslinking by shaking in a water bath at 55°C for 4 to 6 hours. The product was then separated by magnetic field, freeze-dried, granulated, and passed through a 20- to 40-mesh sieve. The static adsorption capacity of this adsorbent for residual anionic dye-type pigments and protein-bound coloring substances in wastewater is greater than or equal to 120 mg / g, and the breakthrough adsorption capacity decay rate is less than 3% per 100 hours during continuous column operation.

[0013] Preferably, in step S7, the surface separation layer of the antifouling polyamide thin-layer composite nanofiltration membrane is prepared by interfacial polymerization of piperazine and trimesoyl chloride and further subjected to a polyvinyl alcohol-sulfonated polyethersulfone double-layer post-treatment to reduce the negative charge density and hydrophilicity of the membrane surface. A 5μm security filter and a sodium bisulfite dosing point are set before the membrane to ensure that the residual chlorine is less than or equal to 0.05mg / L to protect the polyamide layer from oxidation. The nanofiltration unit is arranged in a segmented manner, either as a one-end two-section or two-section two-section configuration, with a booster pump between each section. The nanofiltration permeate has a conductivity of less than or equal to 80μS / cm, a color of less than or equal to 2 times, and a total organic carbon (TOC) of less than or equal to 2mg / L, meeting the food-grade contact cleaning water standard. The nanofiltration concentrate reflux ratio is controlled by an electric regulating valve in a closed loop according to the dual parameters of regulating tank water level and nanofiltration recovery rate.

[0014] Preferably, the ultrasonic crushing pretreatment and anaerobic digester linkage control method in step S8 is as follows: before the remaining sludge is pumped into the ultrasonic cell crushing chamber, it is preheated to 35°C to 40°C, and baffles are installed in the crushing chamber to enhance the uniformity of the cavitation field. After crushing, the sludge and the solid residue from step S1 are mixed at a mass ratio of wet sludge:residue = 3:1 to 5:1 and then enter the digester. The digester is equipped with a mechanical stirring paddle with a speed of 15 r / min to 30 r / min and a coil-type hot water tracing system to maintain mesophilic anaerobic conditions. The daily biogas production volume is measured at 0.32 m³ to 0.42 m³ per 1 kg VS removed under standard conditions. The biogas is processed through a container... The desulfurizing agent is used in a dry desulfurization tower to reduce the hydrogen sulfide content to less than or equal to 200 ppm and then stored in a double-membrane gas storage tank for boiler combustion. The plate and frame filter press has a pressing pressure of 1.2 MPa to 1.6 MPa and a holding time of 20 min to 30 min.

[0015] Preferably, each unit of the entire system is equipped with an electromagnetic flow meter, pressure transmitter, level gauge, and online water quality monitoring instrument, and is connected to a central distributed control system (DCS). The DCS automatically starts and stops the influent pump according to the level of the water collection and equalization tank, and interlocks to adjust the operating parameters of each subsequent unit. When the online COD meter of the effluent from the membrane bioreactor detects a value that exceeds the set upper limit of 40 mg / L for 10 consecutive minutes, it automatically closes the nanofiltration influent valve in step S7 and switches the abnormal water back to the recirculation pipeline at the front end of the pH adaptive adjustment tank for reprocessing. The entire system achieves unattended, fully automatic operation with a comprehensive wastewater reuse rate of greater than or equal to 82%, an average energy consumption of less than or equal to 1.1 kWh / m³ for ton water treatment, and a final sludge discharge or outsourced disposal amount that is more than 40% less than that of the traditional activated sludge process.

[0016] The embodiments of the present invention have the following advantages due to the adoption of the above technical solutions: 1. This invention abandons the traditional fixed grid or vibrating screen, pioneering a "stepped grid screening device with rotating spiral scrapers." The advantages of this design are: First, it reduces the load step by step, using three-stage gradient screening (8mm, 3mm, and 0.5mm) to trap large particles first, preventing impurities from impacting the finest screen. Second, it provides dynamic self-cleaning; the rotating spiral scraper does not simply transport materials but constantly shears against the screen surface, combined with high-pressure fan-shaped backwash water, forcibly peeling away viscous substances adhering to the polypropylene screen. In particular, the hydrophobic and anti-stick coating of polytetrafluoroethylene (PTFE) on the third-stage screen surface greatly reduces oil adhesion. This allows the screen permeability to remain above 90% for extended periods, solving the most common clogging problem in food wastewater pretreatment stages, and the trapped solid residue has a low moisture content, facilitating subsequent direct anaerobic digestion to produce biogas.

[0017] 2. The "pH adaptive adjustment tank" of this invention differs from ordinary adjustment tanks (which rely solely on air mixing). It introduces an online pH-ORP-temperature integrated probe and PLC PID closed-loop control. Its advantages include: not only can it adjust the acid / alkali dosage based on the millisecond-level response of the real-time influent pH value, locking the pH within the optimal microbial activity window of 6.8~7.5, but also, through finely set proportional, integral, and derivative parameters (Kp=1.8~2.5, Ti=60s~120s), it can predict pH change trends and prevent "pH oscillations" caused by excessive dosing. Furthermore, the inclusion of an abnormal water quality bypass pipeline ensures that extremely acidic or alkaline wastewater is intercepted outside the system for separate treatment, thereby ensuring that the sludge activity in the downstream membrane bioreactor is not inhibited. System start-up time is shortened by approximately 30%~40%, and the ability to withstand shock loads is significantly enhanced.

[0018] 3. The "three-stage variable flow rate deep bed adsorption filtration unit" designed in this invention, consisting of composite mineral ceramsite, modified zeolite, and gradient quartz sand, possesses significant inventiveness. First, the composite mineral ceramsite, loaded with iron-manganese oxide, has a large number of positively charged and hydroxyl functional groups on its surface, enabling it to specifically adsorb negatively charged colloidal proteins and soluble polysaccharides through electrostatic attraction and coordination complexation. Second, the modified zeolite, activated by surfactants and calcium ions, not only retains its ion exchange capacity but also increases hydrophobic adsorption sites, effectively removing ammonia nitrogen and polar organic amines. Finally, the gradient quartz sand, through coarse-fine gradation, forms a deep filtration bed that traps minute particles. This combination not only reduces effluent turbidity to below 3 NTU but, more importantly, removes the colloidal source that causes membrane fouling. Actual measurements show that it can extend the fouling cycle of subsequent membrane modules by 1.8 to 2.3 times, significantly reducing membrane cleaning frequency and replacement costs.

[0019] 4. The "pulsated micro-nano bubble ozone catalytic contact oxidation tower" used in this invention solves this problem. Its core advantages are: First, the micro-nano bubbles (50~500nm) have a huge specific surface area and a slow rising speed, greatly improving the ozone dissolution efficiency and mass transfer rate in water, increasing the ozone utilization rate from less than 35% in the traditional method to over 62%; Second, the ternary transition metal catalyst (Mn:Cu:Fe=5:2:1) provides a large number of active sites, catalyzing the generation of more oxidizing hydroxyl radicals from ozone (…). The first step involves using hydroxyl groups (OH) to specifically attack the chromophores of pigment molecules (such as azo bonds and conjugated double bonds), breaking them down into small carboxylic acids rather than simply transferring pollutants. Secondly, the pulsed recoil perturbation disrupts the liquid film boundary layer on the catalyst surface, preventing catalyst deactivation due to surface passivation. This unit achieves a color removal rate of over 60% without producing harmful byproducts such as bromate, creating favorable conditions for subsequent membrane biodegradation.

[0020] 5. This invention optimizes the operating parameters of the submerged negative pressure low-pressure membrane bioreactor. By controlling the MLSS in a high concentration range of 6000~9000 mg / L, combined with a specific volume ratio of aerobic / anoxic zones and internal reflux control, highly efficient simultaneous nitrification and denitrification are achieved, with a nitrogen removal rate of over 65%. More importantly, the use of a suction negative pressure of -15kPa to -35kPa combined with an intermittent mode of "stopping suction for 2 minutes every 8 minutes of operation" provides sufficient time for membrane pores to relax and recover, avoiding the compaction effect of pollutants under constant flux mode. Simultaneously, alternating online cleaning with sodium hypochlorite (sterilization) and citric acid (inorganic scale removal) every 48 hours maintains a membrane flux recovery rate of over 95%, and flux decline is controlled within 15% over a six-month operating cycle, far superior to the traditional continuous suction mode, solving the industry problem of short membrane life in food wastewater MBRs.

[0021] 6. This invention designs a "chitosan-grafted magnetic graphene nanocomposite adsorbent" and its matching precision temperature-controlled decolorization and deodorization column. The advantages of this material are: First, multiple adsorption mechanisms: the graphene framework provides π-π conjugated adsorption sites, while chitosan provides hydrogen bonds and amino groups, resulting in extremely high selective adsorption capacity (≥120 mg / g) for protein-bound pigments and phenolic odor substances; Second, magnetic separation characteristics: the doped iron oxide nanoparticles allow the adsorbent to be fixed within the column by an external magnetic field, eliminating the need for frequent backwashing like activated carbon, and it can be easily desorbed and regenerated with 0.5 mol / L NaOH ethanol solution after depletion, with a capacity decay of less than 15% after 50 cycles; Third, temperature control enhances efficiency: the constant temperature environment of 35~40℃ improves the molecular thermal motion speed and adsorption kinetics rate. Compared to activated carbon processes, the operating cost of this unit is reduced by approximately 40%, and there is no need for hazardous waste disposal of waste activated carbon. Detailed Implementation

[0022] Example 1: Taking the mixed wastewater from a production line producing 20 tons of instant noodles and puffed food per day as the treatment target.

[0023] The raw wastewater quality test results are as follows: Table 1—Water Quality Testing Data for Example 1

[0024] The treatment capacity is designed at Q = 50 m³ / d, and each unit operates according to the parameters described in claims 1 to 10. S1 involves stepped screening to retain approximately 12 kg / d (wet weight) of solids. S2 involves equalization tank effluent pH controlled between 7.05 and 7.28. S3 involves three-stage deep bed filtration with effluent turbidity between 1.6 NTU and 2.4 NTU. S4 involves ozone catalytic oxidation with an actual ozone dosage of 28 g O₃ / m³ water. S5 involves membrane bioreactor maintaining MLSS at 7800 mg / L and DO at 2.8 mg / L in the aerobic zone and 0.3 mg / L in the anoxic zone. S6 involves decolorization and deodorization at a column temperature of 38℃. S7 involves nanofiltration recovery at 76%. S8 involves anaerobic digestion at 35.2℃ for 18 days.

[0025] The removal efficiency of key indicators in the influent and effluent of each major unit is summarized below: Table 2—Summary of Processing Effects of Each Unit in Example 1

[0026] Operational results: The system operated continuously and stably for 90 days, with a comprehensive wastewater reuse rate of 84.6%, an average power consumption of 0.96 kWh / m³ per ton of water treated, and a 42% reduction in the amount of residual dry sludge compared to traditional activated sludge processes of the same scale. Anaerobic digestion produced approximately 38 m³ of biogas per day (standard conditions) with a calorific value of approximately 21 MJ / m³, which was supplied to the plant's hot water boiler for co-firing.

[0027] Example 2: The mixed wastewater discharged from a production line that processes 15 tons of canned vegetables and jam per day was used as the treatment target.

[0028] The wastewater is characterized by high levels of pectin, plant polysaccharides, natural anthocyanins and carotenoid pigments, and some salt. The raw water quality is as follows: Table 3—Water Quality Testing Data for Example 2

[0029] The designed treatment capacity is Q = 40 m³ / d. S1 to S8 are implemented according to the method of this invention. In S2, due to the large fluctuation of raw water pH, the PID parameters Kp are adjusted to 2.3, Ti = 90s, and Td = 10s. In S4, the ozone dosage in the ozone catalytic oxidation tower is increased to 35 g O3 / m³ to accommodate high-color, recalcitrant pigments. In S5, the MLSS of the membrane bioreactor is maintained at 8500 mg / L, and the residence time in the anoxic zone is extended to enhance denitrification. In S7, due to the high TDS of the raw water, the nanofiltration influent ratio is adjusted to 40% for product contact cleaning and 60% for initial wash water with an allowable conductivity of ≤500 μS / cm.

[0030] The results of the operation are recorded in the table below: Table 4—Water Quality Data of Key Nodes in Each Unit of Example 2

[0031] Table 5—Statistical Table of Operational Economy and Reuse Indicators for Example 2

[0032] Example 3: The treatment target was the mixed wastewater from the meat processing and cutting workshop and the cooked food braising workshop, which slaughtered 300 pigs on a certain day.

[0033] The wastewater contains animal fats, blood proteins, gelatin, halogenated pigments, and high levels of ammonia nitrogen and salt. The raw water quality is as follows: Table 6—Water Quality Testing Data for Example 3

[0034] The designed processing capacity is Q = 60 m³ / d. The S1 third-stage polypropylene screen has a finer mesh size of 0.3 mm and the high-pressure rinsing frequency is increased to address the issue of high oil content causing screen clogging. The S2 equalization tank is equipped with a heating coil to maintain a temperature above 12°C in winter to prevent lipid solidification. In the S3 deep-bed adsorption filtration, the backwashing frequency of the composite mineral ceramsite primary filter layer is increased to start at ΔP = 12 kPa to prevent oil adhesion. In the S4 ozone catalytic oxidation tower, the Mn ratio in the catalyst layer is finely adjusted to Mn:Cu:Fe = 6:2:1 to enhance the absorption of nitrogen-containing compounds. Heterocyclic compound oxidation; the DO concentration in the aerobic zone of the S5 membrane bioreactor is increased to 3.0 mg / L to 3.5 mg / L and the internal reflux ratio is increased to 250% to enhance denitrification; the S6 decolorization and deodorization column sets the adsorption and penetration time of pigments in the brine to be monitored every 120 hours; the S7 nanofiltration influent ratio is 35%, and the nanofiltration permeate is used for condensation and cooling water replenishment in the brine pot and initial washing of packaging materials, while 65% is directly reused for washing the workshop floor and the outer surface of equipment; the S8 ultrasonic crushing time is extended to 25 minutes to enhance the cell wall breaking of sludge containing blood proteins.

[0035] The runtime data is recorded as follows: Table 7—Overview of Comprehensive Water Quality at Each Node Along the Path in Example 3

[0036] Table 8—Overall System Performance Evaluation Results of Example 3

[0037] The above three embodiments demonstrate that the food processing wastewater recycling and purification method provided by the present invention can adapt to the water quality fluctuations of different types of food processing wastewater. After the coordinated operation of each unit, a stable high proportion of recycled water and a low residual sludge production can be obtained, and energy recovery can be achieved, thus achieving the expected purpose of the invention.

[0038] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various variations or substitutions within the technical scope disclosed in the present invention, and these should all be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for recycling and purifying food processing wastewater, characterized in that, Includes the following steps: S1. Solid-liquid pre-separation step: The raw mixed wastewater generated from food processing is introduced into a stepped screen with a rotating spiral scraper. The stepped screen is arranged in sequence along the water flow direction, with a first-stage stainless steel wedge screen with a screen width of 8mm, a second-stage stainless steel wedge screen with a screen width of 3mm, and a third-stage polypropylene precision screen with a screen width of 0.5mm. An inclined screw conveyor is set below each screen to transport the intercepted solid residue to the slag tank. During the screening process, the influent flow rate is maintained at a fluctuation range of no more than ±5% per cubic meter per hour and the liquid level in the screening zone is constant. After three-stage screening, the median particle size of the suspended solids in the liquid wastewater is reduced to less than or equal to 80μm and flows into the collection and equalization tank. S2, pH adaptive adjustment step: The liquid wastewater obtained in step S1 is pumped into the pH adaptive adjustment tank. The pH adaptive adjustment tank is equipped with three sets of variable frequency stirring paddles and an online pH-ORP-temperature integrated probe. The probe feeds back the pH signal to the PLC controller in real time. The PLC controller automatically adjusts the amount of dilute sulfuric acid or sodium hydroxide solution added by the metering pump according to the pH value of the influent to stabilize the pH value of the effluent in the adjustment tank between 6.8 and 7.

5. The hydraulic retention time of the adjustment tank is 2.5h to 4.0h, and the bottom of the tank is equipped with perforated aeration pipes for intermittent micro-aeration to prevent sedimentation and anaerobic odor release. After adjustment, the wastewater is sent to the subsequent treatment unit by a lift pump. S3. Three-stage variable velocity deep bed adsorption filtration steps: After adjustment, the wastewater passes sequentially from top to bottom through a three-stage variable velocity deep bed adsorption filtration unit consisting of a composite mineral ceramsite primary filter layer, a modified zeolite intermediate filter layer, and a gradient-graded quartz sand fine filter layer. The initial empty tower flow rate is 8 m / h. When the filtration pressure difference reaches 15 kPa, it automatically switches to reverse segmented air-water combined backwashing. The composite mineral ceramsite primary filter layer has a particle size of 4 mm to 6 mm, a filling height of 800 mm, and is pre-activated with hydrochloric acid and has an iron-manganese oxide surface. The modified zeolite has a filter layer particle size of 1.2 mm to 2.0 mm and a filling height of 600 mm. It is modified by alternating hexadecyltrimethylammonium bromide and calcium chloride or by thermal activation. The gradient-graded quartz sand fine filter layer consists of a lower layer of coarse sand with a d = 0.8 mm to 1.2 mm thickness of 300 mm and an upper layer of fine sand with a d = 0.3 mm to 0.6 mm thickness of 400 mm. After this step, the turbidity of the effluent is reduced to less than or equal to 3 NTU and most of the colloidal proteins and starches are adsorbed and retained. S4. Pulsed micro-nano bubble ozone catalytic oxidation step: The effluent from step S3 is fed into a pulsed micro-nano bubble ozone catalytic contact oxidation tower, which is filled with particles with a diameter of 3mm to 5mm. Supported transition metal oxide catalyst particles, wherein the molar ratio of the three transition metals Mn, Cu, and Fe in the catalyst is Mn:Cu:Fe = 5:2:1 and the total loading is [missing information]. 8% to 12% of the carrier mass, ozone generated by the ozone generator is injected into the bottom gas distribution system of the tower through the dissolved gas release device to form micro-nano ozone bubbles with a diameter between 50nm and 500nm. The ozone dosage is 15mg to 30mg ozone per gram of CODcr. The hydraulic residence time of the contact oxidation tower is 25min to 45min. Every 8min, a pulsed high-flow-rate backwash disturbance is performed for 10s to 15s to prevent catalyst surface passivation. This step breaks down the difficult-to-biodegrade colored substances and large molecular chromophores in the wastewater into small molecular carboxylic acids and aldehydes and ketones. S5. Submerged negative pressure low-pressure membrane bioreactor step: The wastewater after oxidation in step S4 enters the submerged membrane bioreactor. The acclimated facultative-aerobic complex bacteria are added to the tank, and the sludge concentration MLSS is maintained at 6000mg / L to 9000mg / L. The dissolved oxygen is controlled at 2.0mg / L to 4.0mg / L in the aerobic zone and 0.2mg / L to 0.5mg / L in the anoxic zone to achieve simultaneous nitrification and denitrification. The membrane module is a polyvinylidene fluoride hollow fiber curtain membrane with a pore size of 0.03μm to 0.08μm. The suction negative pressure is -15kPa to -35kPa and an intermittent operation mode of stopping pumping for 2 minutes every 8 minutes of operation is adopted. Online sodium hypochlorite-citric acid alternating chemical cleaning is performed once every 48 hours. The CODcr of the membrane bioreactor effluent is less than or equal to 30mg / L and ammonia nitrogen is less than or equal to 1.5mg / L. S6. Nanocomposite Adsorption Decolorization and Odor Removal Step: The effluent from the membrane bioreactor flows by gravity through a precision temperature-controlled decolorization and odor removal column. The column is filled with chitosan-grafted magnetic graphene nanocomposite adsorbent. The adsorbent has a chitosan to graphene oxide mass ratio of 3:1 and is doped with magnetite magnetic particles with a particle size of 10nm to 20nm. The filling height to column diameter ratio is 5:

1. The circulating hot water in the column jacket is controlled at 35℃ to 40℃. The empty tower residence time is 12min to 18min. After the adsorbent is saturated, it is desorbed and regenerated using 0.5mol / L NaOH ethanol-water solution for recycling. This step further removes trace amounts of residual pigments, odor substances, and trace colloids. S7. Cascade Diversion and Reuse with Selective Nanofiltration Step: The effluent from step S6 is divided into two streams. The first stream, accounting for 60% to 75% of the effluent volume, directly enters the recycling tank as initial washing water for the food processing workshop floor and rinsing water for the outer surface of equipment. The second stream, accounting for 25% to 40% of the effluent volume, enters the anti-fouling polyamide thin-layer composite nanofiltration unit. The nanofiltration operating pressure is 0.6 MPa to 1.0 MPa, the inlet water temperature is controlled at 20℃ to 30℃, and the recovery rate is set at 70% to 80%. The nanofiltration permeate flows into the recycling tank as product contact-level cleaning water or cooling circulation system makeup water. The nanofiltration concentrate is returned to the pH adaptive adjustment tank in step S2 for reprocessing. S8. Sludge and Waste Resource Utilization Step: The solid residue retained in step S1 and the remaining sludge generated in step S5 are combined and fed into an anaerobic digester with ultrasonic crushing pretreatment. The ultrasonic frequency is 20kHz to 40kHz, the sound energy density is 0.3W / mL to 0.6W / mL, and the crushing time is 15min to 30min to release intracellular substances and improve the efficiency of anaerobic methanogenesis. The anaerobic digestion temperature is maintained at 35℃±1℃ and the residence time is 15d to 20d. After digestion, the sludge-water mixture is dewatered by a plate and frame filter press until the moisture content is less than or equal to 60% and then transported off-site for disposal or used as organic fertilizer base material. The biogas produced by digestion is desulfurized and purified and stored for heating or power generation in the plant area.

2. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, In step S1, the surface of the third-stage polypropylene precision screen of the stepped grid screening device is coated with a 20μm to 50μm thick hydrophobic and non-stick coating of polytetrafluoroethylene. Each stage of the screen is equipped with a high-pressure fan-shaped flushing nozzle that sprays water for 0.5 seconds every 3 minutes. The high-pressure flushing water pressure is 0.4MPa to 0.6MPa. The speed of the screw conveyor is linearly adjusted by PLC from 5r / min to 25r / min according to the concentration of suspended solids in the influent, so that the screen permeability is always kept above 90% without clogging or overflow.

3. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, In step S2, the concentration of dilute sulfuric acid added to the pH adaptive adjustment tank is 3%–5% by mass, and the concentration of dilute sodium hydroxide solution added is 2%–4% by mass. The proportional coefficient Kp in the PID adjustment parameters of the PLC controller is set to 1.8 to 2.5, the integral time Ti is set to 60s to 120s, and the derivative time Td is set to 5s to 15s. When the pH value of the influent is detected to be lower than 5.0 or higher than 9.0, an alarm is triggered and the bypass pipeline is automatically started to introduce the abnormal peak wastewater into the emergency tank for subsequent batches to be slowly mixed into the adjustment tank for treatment. The actual control deviation of the pH value of the effluent from the adjustment tank does not exceed ±0.15 pH units.

4. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, In step S3, before the initial use of the composite mineral ceramsite primary filter layer, a pre-coating solution containing 2% polyaluminum chloride and 0.5% cationic polyacrylamide is circulated at a low forward speed of 2 m / h through the filter layer for 4 to 6 hours to pre-coat the surface. During operation, when the pressure difference between the inlet and outlet of the filter layer rises to 15 kPa, it is first scrubbed with air at an intensity of 12 L / (m²·s) for 30 seconds, and then backwashed with water at a downward flow rate of 18 m / h for 2 minutes and an upward flow rate of 12 m / h for 2 minutes respectively. The backwash wastewater is discharged into the plant's sewage pipe network and enters the equalization tank for retreatment. For every 30 days of cumulative operation, the modified zeolite filter layer is additionally soaked and regenerated in a 5% sodium chloride solution for 2 hours, and then rinsed with deionized water until the conductivity of the effluent is less than 50 μS / cm.

5. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, In step S4, the pulsed micro-nano bubble ozone catalytic contact oxidation tower has a height-to-diameter ratio of 4:1 to 6:1, and a tail gas destroyer is installed at the top of the tower to fill granular active MnO2 catalyst to decompose residual ozone. The ozone is generated by an oxygen source obtained from the PSA on-site oxygen generator, producing ozone gas with a concentration of 80 mg / L to 120 mg / L through a discharge ozone generator. The reflux ratio of the micro-nano bubble generator is controlled at an ozone gas-liquid volume ratio of 1:3 to 1:

5. Every 180 days of cumulative operation, the catalyst particles are taken out and ultrasonically cleaned with a 3% oxalic acid solution to remove the manganese oxide passivation film deposited on the surface before reuse. The oxidation-reduction potential (ORP) of the effluent from the oxidation tower is controlled in the range of 250 mV to 380 mV to ensure moderate oxidation rather than excessive ozonooxidation that leads to the generation of byproducts.

6. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, In step S5, the volume ratio of the aerobic zone to the anoxic zone in the submerged membrane bioreactor is 3:1, and an adjustable perforated baffle is installed between the two zones to allow the nitrifying liquid to flow back from the end of the aerobic zone to the front of the anoxic zone at a reflux ratio of 100% to 300%. During the acclimatization stage of the composite microbial community, activated sludge from urban domestic sewage is first used as seed sludge and aerated for 24 hours. Then, the proportion of food processing wastewater added is gradually increased from 10%, 30%, 50%, 70% to 100% for 20 to 25 days until microscopic examination reveals abundant bacterial flocs, twig borers, and a small number of Vorticella, and the COD removal rate is stable at over 85%. This is considered the acclimatization to be complete. Perforated pipes are installed below the membrane curtain to perform microbubble aeration for 10 seconds every 30 minutes to flush the membrane surface and prevent fouling.

7. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, The preparation method of the chitosan-grafted magnetic graphene nanocomposite adsorbent in step S6 is as follows: Natural flake graphite is processed using a modified Hummers method to obtain a graphene oxide dispersion, which is then added to… and In situ synthesis of iron oxide / graphene oxide composite was achieved by co-precipitation in ammonia water under alkaline conditions at a molar ratio of 1:

2. After centrifugation and washing to neutrality, a chitosan acetic acid solution with a deacetylation degree greater than or equal to 85% and an appropriate amount of glutaraldehyde crosslinking agent were added, and the mixture was subjected to graft crosslinking by shaking in a water bath at 55°C for 4 to 6 hours. The product was then separated by magnetic field, freeze-dried, granulated, and passed through a 20- to 40-mesh sieve. The static adsorption capacity of this adsorbent for residual anionic dye-type pigments and protein-bound coloring substances in wastewater is greater than or equal to 120 mg / g, and the breakthrough adsorption capacity decay rate is less than 3% per 100 hours during continuous column operation.

8. The method for recycling and purifying food processing wastewater according to claim 1, characterized in that, In step S7, the surface separation layer of the antifouling polyamide thin-layer composite nanofiltration membrane is prepared by interfacial polymerization of piperazine and trimesoyl chloride, and additionally undergoes a polyvinyl alcohol-sulfonated polyethersulfone double-layer post-treatment to reduce the negative charge density and hydrophilicity of the membrane surface. A 5μm security filter and a sodium bisulfite dosing point are set before the membrane to ensure that the residual chlorine is less than or equal to 0.05mg / L to protect the polyamide layer from oxidation. The nanofiltration unit is arranged in a segmented manner, either as a one-end two-section or two-section two-section, with a booster pump between each section. The nanofiltration permeate has a conductivity of less than or equal to 80μS / cm, a color of less than or equal to 2 times, and a total organic carbon (TOC) of less than or equal to 2mg / L, meeting the food-grade contact cleaning water standard. The nanofiltration concentrate reflux ratio is controlled by an electric regulating valve in a closed loop according to the dual parameters of regulating tank water level and nanofiltration recovery rate.

9. A method for recycling and purifying food processing wastewater according to claim 1, characterized in that, The ultrasonic crushing pretreatment and anaerobic digester linkage control method in step S8 is as follows: before the remaining sludge is pumped into the ultrasonic cell crushing chamber, it is preheated to 35°C to 40°C, and baffles are installed in the crushing chamber to enhance the uniformity of the cavitation field. After crushing, the sludge and the solid residue from step S1 are mixed at a mass ratio of wet sludge:residue = 3:1 to 5:1 and then enter the digester. The digester is equipped with a mechanical stirring paddle with a speed of 15 r / min to 30 r / min and a coil-type hot water tracing system to maintain mesophilic anaerobic conditions. The daily biogas production volume is measured at 0.32 m³ to 0.42 m³ per 1 kg VS removed under standard conditions. The biogas is processed through a container... The desulfurizing agent is used in a dry desulfurization tower to reduce the hydrogen sulfide content to less than or equal to 200 ppm and then stored in a double-membrane gas storage tank for boiler combustion. The plate and frame filter press has a pressing pressure of 1.2 MPa to 1.6 MPa and a holding time of 20 min to 30 min.

10. A method for recycling and purifying food processing wastewater according to any one of claims 1 to 9, characterized in that, Each unit of the entire system is equipped with an electromagnetic flow meter, pressure transmitter, level gauge, and online water quality monitoring instrument, and is connected to a central distributed control system (DCS). The DCS automatically starts and stops the influent pump according to the level of the water collection and equalization tank, and interlocks to adjust the operating parameters of each subsequent unit. When the online COD meter of the effluent from the membrane bioreactor detects a value that exceeds the set upper limit of 40 mg / L for 10 consecutive minutes, it automatically closes the nanofiltration influent valve in step S7 and switches the abnormal water back to the recirculation pipeline at the front end of the pH adaptive adjustment tank for reprocessing. The entire system achieves unattended, fully automatic operation with a comprehensive wastewater reuse rate of greater than or equal to 82%, an average energy consumption of less than or equal to 1.1 kWh / m³ for ton water treatment, and a final sludge discharge or outsourced disposal amount that is more than 40% less than that of traditional activated sludge processes.

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