Study On The Law Of Heavy Metals Migration In Activated Sludge System

Heavy metals in municipal water get into sludge in many ways during treatment in wastewater treatment plant (WWTP) using activated sludge process. The highly concentrated heavy metals in municipal sludge limit its land utilization greatly.The concentration of heavy metals in dewatered sludge of WWTP of Beijing are monitored regularly .The results show that the concentration of B, Cd, Pb and Hg exceed the consent level occasionally, especially B, Cd and Hg in Qinghe WWTP. It is also found that the influencing heavy metals mainly exist in suspended solids. The concentration of B, Cu, Pb and Zn in sludge rise the same ratio as that in influence water, meanwhile other elements do not have such phenomenon. The bio-enrich rate of Cu and Zn in activated sludge is higher than that of other elements.The basic information that heavy metals migration in each construction of activated sludge process is investigated. The results revealed that after primary sedimentation tank, the influencing B and Zn are reduced by 20 percent, meanwhile a 50% of Cu. In bio-reaction tank of A~2O process, the bio-sorption rate of heavy metal by activated sludge is quite rapid: about 80% influencing metals are removed in 30 minutes. Sludge digestion process can reduce volume and weight of waste sludge without enriching heavy metals in it.The model that activated sludge absorb heavy metals is analyzed. Result indicates that surface adsorption and cell absorption each contribute 50% to the whole absorption. Diluted acid as low as 0.1mol/l can be used as a chemical agent to remove heavy metals from sludge . A model based on multi-metal competitive sorption that well fit the realistic data is established in this study. The research also found that the practicing pH, DO and ORP have little influence to the metal bio-sorption.Pre-biosorption process is set up to resolve the problem of over concentrated heavy metals in municipal sludge. Evaluation from economic view shows that, compared to the conventional process, this method will gain back the investment within one year.