A foreign-funded enterprise in Guangdong produces 160 million pieces of connectors per year, and supports 3 electroplating production lines . It discharges 253m3 of electroplating wastewater every day. It requires 95% of the water after treatment, 240m3, for electroplating production line to be reused. Only 5% is allowed. The amount of water is discharged to the external environment after physical and chemical treatment and biochemical deep treatment.
1. Status of pollution sources
1.1 Introduction and treatment of production process
Pre-water quality analysis The plant's three electroplating production lines produce a total of 6 silk products, and the electroplating categories are: Cu, Sn, Ni, Zn, cyanide-plated Au. The three electroplating production lines have a total of 5 silk products with KAu (CN) 2 electroplated gold (Au). Since Au is a precious metal, the cleaning water is discharged into the wastewater treatment system before passing through a two-stage separate ion exchange bed located beside the gold-plated cylinder. For recycling, the concentration of KAu(CN)2 in the gold cylinder is 22gL, that is, the concentration of CN-(4) is 4g/L, and the amount of enthalpy in the cleaning process is 0.2Lh. The retention efficiency of ion exchange unit per stage is 95%. At 24h, the CN-content of the final residue entering the wastewater treatment system is: 4gL×0.2L/h×5%×5%×24h×5 wire=0.24gd, and the daily discharge wastewater volume of the system is 13m3, then the CN in the wastewater is discharged. - The concentration is only 0.0185 mg / L, far below the emission standard of 0.5 mgL. Au(CN)2- is saturated with ion exchange and sent to a professional company for precious metal recovery and cyanide treatment. Because of its extremely low content in wastewater, it has reached the standard without treatment, so it is not necessary to consider the treatment of CN-.
Similarly, it is calculated that the content of Au in the effluent wastewater is also extremely low, and it is not necessary to include the pollution factor. By analyzing the production process and pollution sources of the project, the main pollution factors affecting the environment are pH, Cu, Ni, Zn, COD, SS and so on. The wastewater quality 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 implements the first-level standard of Guangdong Provincial Standard “Water Pollutant Emission Limit†(DB44262001), namely: pH: 6～9; Cu0.5mgL; Ni1.0mgL; Zn2.0mgL; COD100mgL; SS70mgL; BOD520mgL1. 3 Water quantity and treatment requirements The wastewater treatment station is designed according to the processing capacity of 253m3d, and 95% of them are 240m3 for production, and only 5% of the water is discharged to the external environment, about 13m3d.
2. Description of wastewater treatment process
2.1 The wastewater treatment process consists of two systems:
System 1 is a treatment and recycling system for washing wastewater, and is composed of an activated carbon adsorption device, a strong acid ion exchange column and a strong alkali ion exchange column. The activated carbon adsorption device mainly removes organic pollutants, chromaticity and suspended solids in water, and is often used as a pretreatment for pure water process; a pure water device consisting of a two-bed ion exchange column, namely a strong acid ion exchange column and a strong alkali ion exchange column. The cations and anions in the water are separately removed, and after deionization, the electroplating line is returned for cleaning.
From the water balance diagram, it can be considered that the 240m3 of washing water generated on the electroplating production line is recycled to the outside environment for production by the activated carbon device and the pure water device, and the 12.6m3d eluate produced by the activated carbon and the resin is regenerated. The 0.4m3d concentrate discharged from the acid-base concentration cylinder is replenished 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 above-mentioned deionized pure water system regeneration, and the other is a small amount of intermittent discharge liquid (0.4m3d) of the acid-base concentration cylinder of the electroplating production line. The amount is 13.0 m3d. Both types of wastewater are rich in heavy metals (no complex), which are formed by adjusting the pH value and adding coagulant and flocculant two-stage coagulation reaction through solid-liquid separation facilities such as coagulation sedimentation tanks and filter tanks. The metal hydroxide precipitate is separated from the water. According to the conventional practice, the treated water can be discharged, but the first biochemical unit is added to the project for advanced treatment of wastewater.
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 several kinds of methanesulfonic acid such as CH3SO3, Sn(CH3SO3)2, Pb(CH3SO3)2 and the like. The sulfonate, in addition to adding the smoothness of the plated parts, also adds various brighteners to the plating cylinder. These additives also contribute to CODBOD5. Conventional detection of BOD5 values ​​takes too long. Because of the good correlation between BOD5 and COD values ​​and TOC, BOD5 values ​​are often obtained indirectly by detecting TOC values. The general data shows that the TOC maximum in the cleaning water of the electroplating line can reach 3mgL, and the wastewater BOD5TOC=1.8. Therefore, the amount of TOC produced in the daily system can be calculated as: washing water: 3 mg/L x 240 m3 = 720 g; tap water: 3 mg L x 12.6 m3 = 37.8 g.
The daily TOC is 757.8g, and the TOC is discharged through the 13m3 wastewater discharge system. The discharge concentration is 757.8g13m3=58.3mgL. According to the correlation between BOD5 value and TOC, the BOD5 concentration of the discharged wastewater can be calculated. It is 104.5mgL, which is far beyond the control requirement of BOD5 environmental emission standard 20mgL, so it is necessary to set biochemical unit at the end of the process. After calculation, the unit needs to remove BOD51.36kg per day, using biological contact oxidation method, the volume of the packing is 1.2m3, and the volume load of BOD5 is 1.13kgm3.d.
3. Engineering investment and operation results
The total investment of the wastewater treatment project is about 7 million RMB; the results of wastewater quality testing of the wastewater treatment system after the project operation show that it is far below the emission standard value and design value.
4 Conclusion
(1) The treatment of electroplating wastewater cannot simply consider that the biochemical unit is not suitable for application. The pollution factor should be carefully screened before 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. The effluent effect.
(2) The project has achieved 95% wastewater reuse rate of the system, and the discharged wastewater has excellent water quality and meets the requirements of clean production.
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