Advanced treatment of electroplating wastewater with biochemical unit

A foreign-funded enterprise in Guangdong produces 160 million pieces of connectors per year, with a total of 3 electroplating production lines, and discharges 253m3 of electroplating wastewater per day. It is required that 95% of the treated wastewater, that is, 240m3, be reused as cleaning water for electroplating production lines, and only 5% of the water is allowed to be discharged to the external environment after being subjected to conventional physical and chemical treatments and advanced biochemical treatments.

1. Pollution source status

1.1 Introduction to the production process and treatment

Previous water quality analysis The plant’s 3 electroplating production lines produced a total of 6 finished wires. The electroplating categories are: Cu, Sn, Ni, Zn, Au cyanide plating, etc. The 3 electroplating production lines have a total of 5 wires for finished products with KAu (CN) 2 electroplating gold (Au). Because Au is a precious metal, the cleaning water passes through a secondary separate ion exchange bed set beside the gold plating tank before being discharged into the wastewater treatment system. For recycling, the concentration of KAu(CN)2 in the gold cylinder is 22gL, that is, the CN-concentration is 4g/L. The entrainment amount in the cleaning process is set to 0.2Lh, and the retention efficiency of each stage of ion exchange unit is 95%. After 24h, the final residual CN- content that enters the wastewater treatment system is: 4gL×0.2L/h×5%×5%×24h×5 silk=0.24gd, the daily discharge of wastewater from the system is 13m3, then CN in the discharged wastewater -The concentration is only 0.0185mg/L, which is far lower than the 0.5mgL requirement of the emission standard. Au(CN)2- is saturated with ion exchange and sent to a professional company for precious metal recovery and cyanide-breaking treatment. Because of its extremely low content in wastewater, it has reached the standard without treatment, so there is no need to consider the treatment of CN-.

In the same way, it is calculated that the Au content in the discharged wastewater is also extremely low, and there is no need to include a pollution factor. By analyzing the production process and pollution sources of the project, it is determined that the main pollution factors affecting the environment are pH, Cu, Ni, Zn, COD, SS, etc. The quality of wastewater before treatment is roughly as follows: pH: 4-6; Cu50mgL; Ni50mgL; Sn50mgL; Zn50mgL; Au: very small amount; CN-: very small amount.

1.2 Emission Standards

The wastewater discharge standard is in accordance with the first-level standard of Guangdong Provincial Local Standard “Water Pollutant Discharge Limits” (DB44262001), namely: pH: 6-9; Cu0.5mgL; Ni1.0mgL; Zn2.0mgL; COD100mgL; SS70mgL ; BOD520mgL1.3 water volume and treatment requirements The wastewater treatment station is designed according to the treatment capacity of 253m3d. It is required that 95% of it, or 240m3, is reused for production, and only 5% of the water is discharged to the outside environment, which is about 13m3d.

2. Description of wastewater treatment process

2.1 The wastewater treatment process includes two systems:

System 1 is a treatment and reuse system for cleaning wastewater, which is composed of an activated carbon adsorption device, a strong acid ion exchange tower and a strong alkali ion exchange tower. Activated carbon adsorption device mainly removes organic pollutants, chroma and intercepted suspended solids in water, and is often used as the pretreatment of pure water process; a pure water device composed of two-bed ion exchange tower, namely a strong acid ion exchange tower and a strong alkali ion exchange tower , Respectively remove the cations and anions in the water, and return to the electroplating production line for cleaning after deionization.

From the water balance diagram, it can be considered that the 240m3 of cleaning water produced by the electroplating production line per day is used for production without being discharged to the environment after the activated carbon device and the pure water device. The 12.6m3d of cleaning water produced by the regeneration of activated carbon and resin The concentrated liquid 0.4m3d discharged from the deliquoring and acid-base concentration tank is supplemented with the same amount of tap water and discharged after being treated by the system 2.

System 2 mainly accepts two types of wastewater, one is the eluent (12.6m3d) produced by the regeneration of the above-mentioned deionized pure water system, and the other is a small amount of intermittent discharge (0.4m3d) from the acid-base concentration tank of the electroplating production line. ), the total amount is 13.0m3d. Both types of wastewater are rich in heavy metals (no complexes). After adjusting the pH value and adding coagulants and flocculants, the two-stage coagulation reaction will be formed through solid-liquid separation facilities such as coagulation sedimentation tanks and filter tanks. The metal hydroxide precipitates are separated from the water. According to conventional practices, the water can be discharged after treatment, but the project added a first-level biochemical unit for advanced treatment of wastewater before discharge.

2.2 The necessity of adding biochemical units. Generally, electroplating wastewater contains a certain amount of organic acids and salts. For example, the electroplating solution contains CH3SO3, Sn(CH3SO3)2, Pb(CH3SO3)2 and other methyl groups. Sulfonic acid and sulfonate, in addition to increase the smoothness of the plated parts, various brighteners are added to the plating tank, and these additives also contribute to CODBOD5. The routine detection of BOD5 value takes too long. Due to the good correlation between BOD5, COD value and TOC, BOD5 value is often obtained indirectly by detecting TOC value. General data show that the maximum TOC in the cleaning water of its electroplating production line can reach 3mgL, and its wastewater BOD5TOC=1.8. Therefore, the amount of TOC produced in the daily system can be calculated as: cleaning water: 3mg/L×240m3=720g; tap water: 3mgL×12.6m3=37.8g.

The daily TOC is 757.8g, and this amount of TOC is finally discharged out of the system through 13m3 wastewater, and its discharge concentration is: 757.8g13m3=58.3mgL. According to the correlation between BOD5 value and TOC, the discharged wastewater can be calculated The BOD5 concentration is about 104.5mgL, which has far exceeded the control requirement of the BOD5 environmental emission standard of 20mgL, so it is necessary to install a biochemical unit at the end of the process. It is calculated that the unit needs to remove 51.36kg of BOD per day, using the biological contact oxidation method, the filler volume is 1.2m3, and the BOD5 volume load is 1.13kgm3.d.

3. Project investment and operation results

The total investment of this wastewater treatment project is about 7 million RMB; the test results of the wastewater discharged from the wastewater treatment system after the operation of the project show that it is far below the discharge standard value and design value.

4. Conclusion

(1) The treatment of electroplating wastewater cannot simply be considered that the biochemical unit is not suitable for application. The pollution factors should be carefully screened before the design. Under the premise that there may be sufficient organic pollutants in the system, it is necessary to add a biochemical unit for advanced treatment. Can get the ideal effluent effect.

(2) This project has achieved a 95% waste water reuse rate of the system, and the discharged waste water quality is excellent, which meets the requirements of clean production.