The wastewater quality of electroplating industry is complex. The wastewater contains heavy metal ions such as chromium, zinc, copper, nickel, cadmium and other toxic impurities such as acid and alkali. The industrial wastewater has the following characteristics:
(1) The components are complex, and the pollutants can be divided into inorganic pollutants and organic pollutants.
(2) The water quality has a large change range, various types of production wastewater pollutants, and the CODcr change coefficient is large.
(3) Wastewater is toxic and contains a large amount of heavy metal ions. If it is directly discharged without treatment, it will cause great pollution to the surrounding water bodies
Oxidation reduction method refers to the use of strong oxidants or strong reducing agents to oxidize or reduce toxic substances in wastewater into non-toxic or low toxic substances. In electroplating wastewater, hexavalent chromium mainly exists in the form of CrO42 - and Cr2O72 - under acidic conditions. Reduction reaction occurs under the action of ferrous ion, and the reduction reaction is fast. The reduced chromium exists in the form of Cr (OH) 3 precipitation under alkaline conditions Electroplating wastewater zero discharge manufacturer The sludge obtained is the mixed precipitation of trivalent chromium and iron hydroxide. When ferrous sulfate is used to reduce hexavalent chromium, considering that the reduction reaction is not complete, the amount of ferrous sulfate in actual operation is 2.5-3 times of the theoretical calculation amount, so the amount of sludge is large.
After two-stage sedimentation treatment, the PH value and heavy metal ion indicators in the wastewater have passed the standard, but the CODcr in the wastewater exceeds the standard due to the presence of additives and other organic substances in the wastewater (part of the CODcr in the wastewater is generated by ferrous and part of the CODcr is generated by organic substances). According to the measurement, the CODcr value after two-stage sedimentation is about 200 mg/l, while the national standard is 100 mg/l, Therefore, after two-stage precipitation pretreatment, the wastewater is treated by aerobic biochemical method to reach the national standard.
Electroplating additives mainly include leveling agent, stress relieving agent, surfactant, brightener, auxiliary brightener, etc., mainly including aldehydes, saccharin, decomposition products, etc., most of which are biochemical substances. Aerobic biological treatment process includes activated sludge process and biofilm process. Activated sludge process includes SBR and its improved type, AB process, etc; The biomembrane method includes contact oxidation method, biological filter, etc. Among them, SBR and its improved type and biological contact oxidation process are currently popular processes for biochemical treatment of sewage in the world.
Biological contact oxidation is a biofilm process between activated sludge process and biofilter process. The contact oxidation tank is equipped with fillers. Some microorganisms grow firmly on the surface of the filler in the form of biofilm, and some grow in flocculent suspension with water. At present, the biological contact oxidation method is to set the filler in the pool, immerse the oxygenated sewage into all the filler, and flow through the filler at a certain speed. The filler is covered with biofilm, and there is also some activated sludge in the sewage. The sewage is in contact with the biofilm and activated sludge, and the sewage is purified under the action of microorganisms. It can be said that biological contact oxidation method is a biological treatment method which is between activated sludge method and biofilm method, and has the dual efficacy of activated sludge and biofilm.
