Lithium iron phosphate batteries, as a clean energy source, generate industrial wastewater during the production process of lithium batteries. The wastewater from lithium battery factories is mainly divided into cathode wastewater and anode wastewater. The main pollutants in the wastewater are ternary materials, lithium cobaltate, lithium iron phosphate, carbon powder, NMP solvent, and deionized water solvent.

① Cathode wastewater and anode wastewater have different properties and need to be collected separately for pretreatment;

② Cathode wastewater contains valuable raw materials that can be recovered and purified after separate treatment;

③ Cathode wastewater contains heavy metals such as cobalt, nickel, and manganese. The engineering design needs to consider the removal of these heavy metal ions separately;

④ Cathode wastewater and anode wastewater have high concentrations of organic matter, generally biodegradable, and contain macromolecular organic matter that is not easily aerobically biodegraded. It is high-concentration, difficult-to-biodegrade organic wastewater.

1) Introduction to biochemical treatment process technology;

2) Key points of ammonia treatment technology for wastewater from lithium iron phosphate production lines;

3) Application of electrocoagulation technology in the treatment of wastewater from lithium iron phosphate production lines;

4) Research on "zero discharge" technology for wastewater from lithium iron phosphate production lines.

4.1. Water volume and quality

1) The total amount of production wastewater is 350T/d, including 100m3/d of cathode wastewater, 200m3/d of anode wastewater; and 50m3/d of domestic sewage;

2) Relevant influent water quality indicators are shown in Table 1.

3) The effluent water quality meets the discharge limits for indirect discharge of new enterprises in Table 2 of the "Emission Standards for Pollutants in the Battery Industry" (GB 30484—2013). The emission standards for each indicator are shown in Table 2.

4) Treatment process: It is proposed to collect cathode wastewater and anode wastewater separately, and after pretreatment by three-stage sedimentation in the workshop, discharge them into the wastewater treatment plant regulating pool. Cathode & anode wastewater is pumped into the coagulation sedimentation tank, and part of the pollutants are removed through different process parameter controls. The treated effluent flows into the pre-acidification regulating pool; after being collected in the pre-acidification regulating pool with domestic sewage discharged from the workshop, it is treated by "UASB anaerobic reactor + A/O pool + secondary sedimentation" process, and finally discharged into the municipal sewage pipe network through a standardized discharge outlet; the effluent water quality of this lithium battery industrial wastewater station meets the discharge limits for indirect discharge of new enterprises in Table 2 of the "Emission Standards for Pollutants in the Battery Industry" (GB 30484—2013).

3) Process principle analysis:

(1) Coagulation sedimentation: Because the cathode wastewater contains low concentrations of cobalt, nickel, and manganese ions, they are all heavy metal pollutants and can react with OH- to form water-insoluble precipitates. According to the provisions of "Design Code for Chemical Treatment of Heavy Metal Wastewater" CECS92:97, the optimal pH for the precipitation separation of the above heavy metal wastewater hydroxides is 9～12; based on the above chemical characteristics, the hydroxide chemical precipitation separation method is used for cathode wastewater while adjusting the pH value to 9～12; in the operation and management of this system, long-term experience has shown that when the cathode coagulation sedimentation is adjusted to pH=9.5, by adding basic aluminum chloride and cationic PAM for coagulation sedimentation, the COD removal rate is above 20%, and the removal rate of cobalt, nickel, and manganese ions is above 97%, saving the amount of chemical addition while meeting the emission standards; the anode coagulation sedimentation tank controls the pH at 7～8, and by adding PAM and PAC, the COD removal rate is as high as 60% or more; the heavy metal removal rate is above 96%;

(2)The anaerobic treatment process shows a serious imbalance in the nutrient source (C:N:P) ratio of the lithium battery factory's industrial wastewater. Additional nitrogen fertilizer and glucose are needed. After the system is put into operation, domestic sewage will be added simultaneously to increase the biodegradability of the wastewater; although the BOD/COD ratio of the mixed wastewater of domestic sewage and pre-treated anode and cathode wastewater is greater than 0.3 after mixing and adjustment, it can be used for biological treatment, but the organic matter concentration of the mixed wastewater is still very high, and the wastewater has poor biodegradability. Direct aerobic treatment results in low treatment efficiency due to excessive organic load. At the same time, aerobic biodegradation requires sufficient air supply to create a favorable environment for microbial growth and reproduction, resulting in high energy consumption and large sludge production, causing large secondary pollution. Therefore, this system adopts anaerobic biological treatment. The bacteria involved are hydrolytic bacteria, acid-producing bacteria, and methanogenic bacteria. Under anaerobic conditions, without the need for power, most of the organic matter is degraded to a level suitable for aerobic biodegradation, and the produced methane can be used for secondary combustion and recovery. The UASB consists of three parts: a sludge reaction zone, a gas-liquid-solid three-phase separator, and a gas chamber. The mixed wastewater enters from the bottom of the anaerobic sludge bed, mixes and contacts with the sludge in the sludge reactor, and is consumed and decomposed by the anaerobic bacteria in the sludge into organic matter (carbohydrates, biogas). Due to the upward movement of biogas produced during the anaerobic process, the sludge in the upper part of the sludge bed is kept in suspension, and the lower sludge layer is also stirred to some extent; the biogas continuously aggregates and rises, forming some gas attached to the sludge particles in the upper part of the sludge bed, and then the biogas concentration becomes higher and higher, carrying some mud and water into the gas-liquid-solid three-phase separator. In the three-phase separation, when the biogas encounters the baffle plate at the bottom of the separator, it is reflected to the surroundings of the baffle plate, and then passes through the water layer into the biogas collection chamber. The solid-liquid mixture is reflected into the sedimentation zone of the three-phase separator, the sludge in the wastewater flocculates, the particles gradually increase in size, and the sludge separated by the sedimentation zone settles to the anaerobic reaction zone under the action of gravity for circulation. This process can achieve a removal rate of organic matter concentration of more than 50%, and also has a certain treatment effect on total nitrogen; it further reduces the organic load entering the aerobic tank. (3)Aerobic treatment process: The wastewater after anaerobic treatment enters the aerobic tank for further degradation of pollutants. Because the effluent ammonia nitrogen must be less than 30 mg/l and the total nitrogen must be less than 40 mg/l, the aerobic treatment process must have a mature biological denitrification process while removing COD. Therefore, the A/O (nitrification-denitrification) process is selected for this aerobic treatment; the A/O denitrification process creates a biological environment with alternating anaerobic and aerobic conditions, so that aerobic heterotrophic bacteria, denitrifying bacteria, and nitrifying bacteria are all in an alternating anaerobic/aerobic environment, forming a mixed bacterial community that can more efficiently remove organic matter and denitrify; under anaerobic conditions, denitrifying bacteria use organic carbon in wastewater as an electron donor and nitrate as an electron acceptor for "anaerobic respiration", reducing nitrate nitrogen in the return flow to nitrogen gas and releasing it into the atmosphere, thereby degrading total nitrogen and ammonia nitrogen in the wastewater; under aerobic conditions, a large number of aerobic bacteria decompose organic matter, while nitrifying bacteria oxidize ammonia nitrogen in the wastewater into nitrate; then return to the anoxic tank to prepare for denitrification; this process can achieve a removal rate of organic matter concentration of more than 90%, a total nitrogen removal rate of more than 70%, and a total phosphorus removal rate of more than 50%.

(4)Sludge reduction and utilization: This project will produce a certain amount of physical and chemical sludge and biological excess sludge during the wastewater treatment process. Some raw materials in the anode and cathode sludge can be utilized. If not properly handled and disposed of, it will cause secondary pollution. In order to reduce sludge production, the dewatered sludge after coagulation and sedimentation of the cathode and anode should be discharged into the thickening tank separately and pumped into the dewatering machine for filtration treatment by an air pump; according to the summary of operating experience, after the implementation of this project, the cathode sludge production will be about 150 kg/d (sludge water content is about 70%), and the anode sludge production will be about 600 kg/d (sludge water content is about 65%). Because the physical and chemical sludge can be directly dewatered by a box-type filter press, the dewatered sludge is sent to a qualified unit for secondary separation and purification; while the excess sludge from the secondary sedimentation tank of the biological part and the UASB anaerobic reactor are regularly discharged into the biological sludge thickening tank for thickening. Because the biological sludge has high sludge activity and high viscosity, it is recommended to use a screw press filter for the biological excess sludge. The biological excess sludge production is about 235 kg/d (sludge water content is about 80%), and the dewatered sludge is directly transported for composting.

4.2 Main Structures and Equipment Parameters

1) Anode and cathode wastewater equalization tank. One cathode wastewater equalization tank, concrete structure, with a top cover, effective volume of 48m3, dimensions of 3.2m×3.4m×5.0m, effective water depth of 4.5m. Equipped with 2 lifting pumps; one electromagnetic flow meter; one anode wastewater equalization tank, concrete structure, with a top cover, effective volume of 90m3, dimensions of 5.9m×3.4m×5.0m, effective water depth of 4.5m. Equipped with 2 lifting pumps and one electromagnetic flow meter.

2) Anode and cathode coagulation sedimentation tank. One anode coagulation sedimentation tank, concrete structure, with a top cover, dimensions of 4.6m×2.5m×5.0m, equipped with 2 sets of liquid alkali, PAC, and PAM dissolving and preparation devices, one online pH meter, and 2 sludge pumps; one cathode coagulation sedimentation tank, concrete structure, with a top cover, dimensions of 3.2m×2.5m×5.0m. Equipped with 2 sets of liquid alkali, PAC, and PAM dissolving and preparation devices, one online pH meter, and 2 sludge pumps.

3) Pre-acidification equalization tank. One pre-acidification equalization tank, concrete structure, with a top cover, dimensions: 3.1m×9.4m×5.0m, effective water depth 4.5m. 2 lifting pumps, 1 liquid level controller; 1 electromagnetic flow meter.

4) UASB tank. One anaerobic reactor, concrete structure, with a top cover, plane dimensions of 3.8m×6.6m×8.0m, effective volume: 188m3. Equipped with one BS-1300 type water distribution system, one three-phase separator, one water weir, one biogas collection device, and 2 anaerobic circulation pumps;

5) A/O (nitrification-denitrification). One nitrification-denitrification tank, concrete structure, with a top cover, dimensions of 10.7m×6.6m×5.0m, effective water depth of 4.5m. Equipped with one submersible mixer, 112 sets of rotary mixed aerators, 2 nitrification liquid return pumps, and one online dissolved oxygen meter.

6) Secondary sedimentation tank. One secondary sedimentation tank, concrete structure, with a top cover, dimensions 2.0m × 6.6m × 5.0m. Equipped with one set of sedimentation tank components, four sludge pumps, and one set of water collection weirs.

7) Sludge thickening tank.

(1) One each of cathode and comprehensive sludge thickening tanks, concrete structure, with a top cover, dimensions 1.3m × 2.5m × 5.0m, effective water depth 4.5m.

(2) One anode sludge thickening tank, concrete structure, with a top cover, dimensions 2.5m × 2.5m × 5.0m, effective water depth 4.5m. Equipped with four pneumatic diaphragm pumps for comprehensive, cathode, and anode sludge feeding, three box-type filter presses, one screw press filter, and two sets of cationic PAM and anionic PAM dissolution and preparation devices;

8) One accident pool, concrete structure, with a top cover, dimensions: 7.1m × 9.4m × 5.0m, effective water depth 4.5m.

9) Standardized discharge outlet. One standardized discharge outlet, brick-concrete structure, dimensions L × B × H = 8.3 × 1.4 × 1.5m. Equipped with one set of online monitoring equipment. (To be continued)