Research On Safety Of Wastewater Reuse In Municipal Water Treatment Plants

As economy develops, our population grows and life level improves continually, water demand is increasing, better water quality is being required, so contradiction between the water supply and water demand is becoming more serious. In recent years, in order to save water, Reusing wastewater in water treatment plants is currently the effective ways to solve water shortage problem. Wastewater contains large amounts of pollutants from source water and coagulant introduced during water purification, whose concentrations are several times or even hundreds of times higher than the original ones due to the concentration, and risk of finished water is significantly increased. So it is meaningful to evaluate, analyse and control the safety of wastewater reuse.Taking the two major urban surface source water plants in Northwest as study object, where wastewater was treated and reused, turbidity, CODMn, residual aluminum, ammonia, acrylamide monomer and other related water quality indicators were monitored in the finished water, the impacts of reusing wastewater on the water treatment processes and the quality of finished water were evaluated and analysed. The results show that levels of turbidity, ammonia, aluminum and CODMn were less influenced by the reuse of treated wastewater; The results also indicate the content of acrylamide monomer in the reused wastewater from both water plants A and B, which sometimes made the content of acrylamide monomer in finished water higher than the regulated limit.The control of acrylamide monomer in reused wastewater becomes one of the research focuses of wastewater reuse. Adding polyacrylamide (PAM) to sludge before dewatering can enhance the performance of sludge dewatering, however, acrylamide monomer can be introduced in the process of imposing PAM and remained in the reused water, which has strong toxicity to human and increases the toxic risk of finished water. In this research, a series of dewatering experiments on the optimization of PAM dose were performed in the laboratories to find out the possible approach to control the risk of acrylamide monomer in the reused wastewater. Under various conditions of sludge concentration, PAM of both anion type and cation type were added to the sludge at different dosages, the specific resistance to filtration (SRF) of the sludge was measured before and after imposing PAM, and the optimum PAM dosages under different experiment conditions were determined by considering the effects of PAM on SRF and the solid content after the sludge is dewatered together. The results showed that PAM of both cation type and anion type can be used to improve the performance of sludge dewatering, although the dewatering performance was improved at similar levels, the residual acrylamide monomer in finished water was lower using PAM of anion type. To meet the purposes of small SRF of sludge, low turbidity and residual acrylamide monomer in the supernatant, recommend to use anion type PAM, the optimum PAM dosages were 0.16g/L~0.25g/L for A water plant and 0.09g/L~0.16g/L for B water plant, which are less than or equal to the current values used in both water plants, the risk of acrylamide monomer in finished water will be decreased if the above optimum PAM dosages are used in the wastewater treatment process of water plants.