Anaerobic ceramic membrane reactor-double membrane process for reuse of wastewater from automobile parts production
The production process of auto parts is complicated, and the waste water produced contains resins, surfactants, paints, pigments, heavy metal ions and other pollutants. At present, the traditional process of coagulation-pretreatment (Fenton or hydrolysis) -biological treatment, followed by advanced treatment (such as adsorption, etc.) has many shortcomings, such as too many treatment units, poor load resistance, difficulty in achieving stable standards, and insufficient reuse of effluent. A Jiangsu auto parts production enterprise is mainly engaged in the production and processing of precision cavity molds, the production of special sealing materials, etc., producing coating, magnetic research, rinsing and cleaning wastewater. In this paper, based on the long-term experimental data, the process of air-floating-anaerobic ceramic membrane reactor was used. Some of the treated water meets the water quality requirements of ultra-pure water through sand filter-carbon filter-double membrane method (ultrafiltration + two-stage RO) -EDI process. Anaerobic membrane bioreactor (AnMBR) combines the characteristics of membrane bioreactor and anaerobic treatment technology. The efficient separation and intercepting effect of membrane component prevents the biomass from being lost from the reactor, and realizes the effective separation of SRT and HRT. Anmbr has the characteristics of high sludge concentration, strong impact load resistance, and eliminating the three-phase separator. In this project, the film material is silicon carbide material, and the traditional ceramic film with Al2O3 as the main material has greater film flux and more stable chemical properties. This paper introduces and analyzes the design characteristics of submerged anaerobic ceramic membrane bioreactor, and the selection of main equipment such as ultrafiltration and RO and the actual operation effect of six months, which can provide design basis for sewage treatment and reuse in similar enterprises.
1. Project overview
According to the actual production situation of the plant, the sewage discharged by each workshop and a small part of domestic wastewater are collected into the regulating tank in the sewage treatment area of the plant, and the designed maximum flow rate is 55m3/d. Each water quality index of raw water is shown in Table 1. The treated water quality meets the secondary standard in Table 1 of the pollutant discharge standard of urban sewage treatment plant ≥GB18918-2002. The maximum amount of water to be reused is 45m3/d. The purpose of reuse is to produce ultra-pure water to meet some production section water requirements (resistivity requirements equal to 18.2MΩ•cm).
2. Processing technology
The design process of wastewater treatment and reuse is shown in Figure 1. In this project, most SS, petroleum, anionic surfactants (LAS), heavy metals and some COD in raw water were removed by air flotation. Diatomaceous earth is used as coagulant in the air flotation unit of this project. Compared with traditional polyaluminum chloride and polyferric sulfate, diatomite is rich in natural resources and can deal with colloidal and granular pollutants, especially for the removal of chromaticity, dissolved phosphorus and heavy metals. Anaerobic membrane bioreactor can completely solve the problems of sludge loss and poor sludge settling performance in anaerobic treatment, and then improve the effect of anaerobic treatment. At the same time, the anaerobic membrane bioreactor equipment is compact, covers a small area, and saves the cost of civil construction and equipment. Nano-scale ceramic membrane can stop all microorganisms, colloid and part of macromolecular organic matter, ensure the SS and turbidity of the effluent is zero and remove most of the COD. In the final stage, part of the water in the intermediate tank 1 is used for backwashing of the ceramic membrane in the anaerobic membrane bioreactor, and the rest flows into the intermediate tank 2. Add sodium hypochlorite to the middle tank 2 to remove ammonia nitrogen and further reduce COD and chrominance to ensure that the effluent reaches the standard. Part of the produced water is pumped to the reuse water intake tank. In the reuse stage, the treated water is removed by sand filtration and carbon filtration in the intermediate sump 2, and the suspended particles and microorganisms generated in the intake tank and pipeline transportation are reused to avoid the organic pollution of membrane elements caused by their deposition in subsequent equipment. At the same time, the security filter is set before the ultrafiltration and the first stage RO membrane to intercept the colloids and microorganisms that appear in the ultrafiltration box due to secondary pollution. After two stage reverse osmosis and EDI desalination, the ultrafiltration water enters the ultra-pure water production tank for enterprise production.
The concentrated water produced by ultrafiltration, primary RO and EDI in this project COD< 150mg/L, directly discharged into the municipal pipe network into the park sewage treatment plant. The concentrated water produced by secondary RO is recycled into primary RO. The sludge produced by the system is treated by the stacked screw filter press and then commissioned to transport.
3. Main structures, equipment and design parameters
3.1 Incoming water comprehensive regulation pool
The water conditioning pond body is a steel concrete structure, the total size is 6.0m×2.0m×4.5m, the effective water depth is 4.4m, and the effective volume is 48m3. There is a submersible mixer in the pool, model QJB0.85/8-260/3-740/c/s, power 0.85kW. The regulating pool lift pump is a submersible pump, one used and one ready, model QDX3-18-0.55S, power 0.55kW. Before air intake, the pH is adjusted between 7 and 8 by adding hydrochloric acid or sodium hydroxide. The two dosing tanks are 1000L PE tanks, each tank is equipped with a dosing pump, model GX1800, power 1.5kW; Equipped with a mixer, speed 1440r/min, power 0.75kW.
3.2 Air float and air surface pool
Air float model No. CDAF-3, processing capacity 2 ~ 3m3/h, reflux pump motor 0.75kW, aeration motor 0.75kW, foam scraper 0.37kW. By automatic dosing system, diatomaceous earth is added in the first coagulation tank, and PAM is added in the second coagulation tank to help sedimentation. Two dosing tanks are 500L PE tanks, each tank is equipped with a dosing pump, model GX900, power 1.5kW, while equipped with a mixer, speed 1440r/min, power 0.75kW. The gas surface pool body is steel concrete structure, the size is 3.0m×1.0m×4.5m, the effective water depth is 4.0mm, the high height is 0.5m, and the effective volume is 12m3.
3.3 Anaerobic ceramic membrane reactor
The submerged anaerobic membrane bioreactor was used. It is mainly composed of anaerobic tank and membrane system. Anaerobic tank body is carbon steel anticorrosive structure, size Φ4×8m. A water inlet pipe, a water outlet, a biogas outlet, a biogas circulation outlet and a drain outlet are provided. The outside of the anaerobic tank is made of rock wool insulation material, the thickness is 50mm, and the outside is laid with colored steel plate. The film module adopts the latest research and development of silicon carbide material flat ceramic film (Boxin Jingtao Environmental Protection Technology Co., LTD.). The pore size of the ceramic membrane is 100nm, and the design membrane flux is 100L/ (m2•h). A total of 40 diaphragms are placed in the anaerobic tank, and the effective area of each diaphragm is 0.5m2, a total of 20m2. The diaphragm and membrane components are immersed directly in the anaerobic tank. A 30-mesh stainless steel mesh with a diameter of 30 mesh is arranged around the diaphragm to block the pollution of sludge on the surface of the membrane. At the same time, the biogas generated by the air circulation pump is washed on the surface of the film to control the sludge deposition on the surface of the film. Air circulation pump model YQB3-5, two use one standby, power 0.9kW. Membrane suction pump adopts self-priming pump, one use one standby, model 25ZX3.2-20, flow 3.2m3/h, suction 6.5m, 20m head, power 0.75kW. Membrane backwash pump adopts vertical pipeline centrifugal pump, one use one standby, model YG32-125, flow 6.5m3/h, head 20m, power is 1.5kW. The operation mode of the plate ceramic membrane is water for 9min, stop for 1min, and backwash for 1min. When the suction transmembrane pressure difference reaches 0.04MPa during operation, the membrane will be activated for maintenance cleaning, and the cleaning agent is sodium hypochlorite used to remove organic and biological fouling. Dosing adopts automatic on-line dosing method, dosing pump adopts diaphragm metering pump, model GX900, power 1.5kW. In order to maintain the alkalinity required for anaerobic reaction, it is necessary to set up an anaerobic alkalinity feeding tank next to the gas emergence pool and actually add sodium bicarbonate. Anaerobic alkalinity dosing tank is 1000L PE tank, dosing tank is equipped with a dosing pump, model GX1800, power 1.5kW; Equipped with a mixer, speed 1440r/min, power 0.75kW.
3.4 Intermediate pools 1 and 2
The pool body is a steel concrete structure, the size is 3.0m×1.0m×4.0m, the height is 4.5m, and the effective volume is 12m3. Both tanks are connected with film suction pumps. There is a sodium hypochlorite dosing tank in the middle pool 2. The dosing pump is diaphragm metering pump, model JWM32/1.0, power 60W. The intermediate pool 2 is provided with an external discharge pump, which is discharged to the Pap metering tank. Submersible pump, one use one standby, model CP50.75-50, power 0.75kW. Part of the water is discharged directly to the standard, and part goes into the reuse water tank. When the amount of water used for reuse is not enough, tap water will be added in the reuse tank to ensure the stable operation of the membrane.
3.5 Sand filter and carbon filter
Sand filter filter material is quartz sand. The inlet water is calculated at 2m3 per hour, the filtration rate is set to 7m/h, the diameter of the tank is 0.6mm, and the height of the tank is 2m. Carbon filter material is activated carbon. The diameter of the tank is 0.6mm and the height of the tank is 2m. The pump of sand filter and carbon filter tank adopts one use and one standby, model 25ZX3.2-20, flow 3.2m3/h, suction 6.5m, head 20m, power 0.75kW. Sand filter and carbon filter and ultrafiltration share a backwash pump.
3.6 Ultrafiltration
The nominal pore size of the ultrafiltration membrane is only 30nm, which is a pre-treatment for desalting purified water, and has a good retention ability for suspended matter, colloidal substances and microorganisms. There are two sets of ultrafiltration systems in this project. A single set of ultrafiltration equipment includes: 50 µl security filter, 2 sets, one for each inlet and backwash; Internal pressure hollow fiber membrane is adopted, the membrane element model is AQUCELL4050, the effective membrane area of a single branch is 3.6m2, the designed water yield is 60L/ (h•m2), there are 6 branches in total. Bracket, valve piping system, water production flowmeter, concentrated water flowmeter and backwash flowmeter each set. Instrument operating box 1 set. Ultrafiltration backwash pump using one standby, model 25ZX3.2-20, flow 3.2m3/h, suction 6.5m, 20m head, power 0.75kW.
3.7 Two-stage reverse osmosis system
A total of one stage reverse osmosis, two sets of two stage reverse osmosis, running at the same time. A single set of reverse osmosis equipment includes: 5 µsafety filter (pre-stage reverse osmosis); 2 sets of light vertical multistage centrifugal pumps, model CDLS2-15LSWSC, flow rate 2m3/h, head 112m, power 1.5kW; RO membrane element model is CPA2-4040, primary RO3, secondary RO2; One set of scale inhibitor and alkali adding system. Bracket, valve piping system, primary concentrated water flowmeter and secondary concentrated water flowmeter each set. Instrument operating box 1 set.
3.8 EDI
The EDI model is CP-1000S and two are running simultaneously. The size of a single unit is 616mm×266mm×262mm, and the product water flow is 0.8 ~ 1.2m3/h. Bracket, valve piping system, water production flowmeter, polar water flowmeter and concentrated water flowmeter each set. Instrument operating box 1 set.
3.9 Sludge thickening tank and stacked screw sludge dehydrator
The sludge thickening tank is a 5-square PE tank equipped with a cationic PAM dosing system. The dosing tank is equipped with a dosing pump, model GX900, power 1.5kW, and mixer, speed 1440r/min, power 0.75kW. Stacked screw sludge dewatering machine, model DL201, power 0.4kW, size 2500mm×800mm×1300mm. The filtrate produced is pumped to the regulating tank by the submersible pump equipped with the dehydrator.
4. Results and discussion
After 6 months of preliminary commissioning and operation of the project, the effluent water quality is basically stable, and all pollutants meet the discharge standards and reuse standards. The effluent water quality testing of each unit is shown in Table 2. The pH of raw water is adjusted between 7 and 8 with acid or base before air intake. The air flotation unit uses diatomite as coagulant with dosage of 2g/L, and PAM as coagulant aid with dosage of 5mg/L. The decolorization rate is about 40%, the removal rate of COD is about 20%, the removal rate of heavy metals is about 80%, and the removal rate of petroleum and anionic surfactants (LAS) can reach 94% and 90% respectively. The anaerobic reactor inoculated sludge was obtained from the anaerobic granular sludge of other plants (VSS about 70g/L). The amount of inoculation is about 1/4 of that of an anaerobic reactor. The flat film adopts the mode of water production without vacuum system, which reduces the operation energy consumption. In normal operation, due to the interception effect of ceramic membrane, the effluent COD is 90 ~ 110mg/L and the turbidity is 0.1NTU. In this project, biogas generated is used to carry out cyclic aeration of the diaphragm through the aeration pump, and the pollution of the membrane is greatly delayed by the shearing effect of the gas. At the same time, the backwash pump is run. The operating pressure of the silicon carbide ceramic film is lower than that of the traditional aluminum oxide ceramic film. After 2 months of actual operation, the transmembrane pressure difference reached 0.04MPa. At this time, sodium hypochlorite online cleaning is started. During online cleaning, the backwash pump is run at the same time, and the sodium hypochlorite concentration is 1000mg/L. After each cleaning, the membrane flux returned to normal. The actual operation results showed that the anaerobic sludge was not inhibited significantly after sodium hypochlorite cleaning, and the methanogenic activity recovered quickly. The whole operation and dosing are automatically controlled by PLC and touch screen. After 5 months of operation, the entire diaphragm was removed for restorative cleaning with 5000mg/L sodium hypochlorite and 2000mg/L citric acid impregnation. During winter operation, steam heating is used in the gas surface pool to maintain a temperature of 30 ~ 35 ° C to ensure the effect of subsequent anaerobic treatment. The actual operation results of half a year show that the anaerobic ceramic membrane reactor has strong load impact resistance, stable effluent water quality, can remove most of the COD in the raw water, and the sludge in the tank can adsorb the remaining heavy metals. Meanwhile, the cleaning frequency of the new nano-plate ceramic membrane is low, which also ensures the continuity of operation. Ammonia nitrogen increased slightly after anaerobic, indicating that there was a small amount of organic ammonia in the raw water and ammoniation occurred during the anaerobic process.
In the reuse stage, the turbidity of the ultrafiltration effluent is less than 0.2NTU, which meets the requirements of further reverse osmosis influent. The main function of RO is to remove most of the inorganic salts in the raw water. The conductivity of primary RO water is less than 100µS/cm, and the conductivity of secondary RO water is less than 22µS/cm. The conductivity of raw water is between 1.1 and 1.9mS/cm, so the desalting rate of secondary RO is above 98.8%. The electrical resistivity of EDI effluent is 18.2MΩ•cm, which meets the requirements of enterprise reuse to produce ultra-pure water.
5. Cost analysis
The total investment of the project is more than 900,000 yuan, of which nearly 150,000 yuan is invested in civil construction and more than 400,000 yuan is invested in equipment (including ceramic film diaphragm). The investment of pipe, pipe fittings, automatic control, cable bridge is more than 100,000, and the other is 200,000. Running cycle 24h/d. The entire sewage treatment cost is about 4.1 yuan /m (2 excluding sludge disposal cost), including electricity 1.3 yuan /m2, pharmaceutical cost 1.9 yuan /m2, labor cost 0.6 yuan /m2, maintenance cost 0.2 yuan /m2, and testing cost 0.1 yuan /m2. The operating cost of the reuse system is 3.02 yuan, including 1.2 yuan /m2 for electricity, 0.81 yuan /m2 for medicine, and 0.9 yuan /m2 for others (including filter material, filter element, membrane replacement, etc.). Waste water reuse saves the water cost of nearly 100,000 yuan (calculated at 5 yuan /m2).
6. Conclusion
In the actual project, a new technology - air flotation + anaerobic ceramic membrane bioreactor is used to treat automotive parts wastewater. The operation effect is stable for half a year, and the effluent water quality is stable, meeting the requirements of ≤ urban sewage treatment plant pollutant discharge standard ≥GB18918-2002. The new silicon carbide nano-plate ceramic membrane has the advantages of high membrane flux, strong corrosion resistance and low cleaning frequency. Part of the treated water is then used to produce ultra-pure water required for production through pre-filtration - ultrafiltration - secondary reverse osmosis -EDI, thereby improving the utilization rate of water resources and reducing environmental pollution, resulting in extremely significant economic and environmental benefits. This project case can provide reference and reference for the treatment and reuse of automotive parts wastewater.
