Chronic Toxicity Of Municipal Wastewater Containing Chromium For Sbr And Its Modelling Research

With the heavy metal pollution is on the rise, the long-term toxicity of heavy metal on the electroplating wastewater and other industrial wastewater or mixed in urban sewage on the biochemical system is also a concern. However, so far, most of the research is based on the experiment of acute toxicity of heavy metal on short-term. Long-term chronic toxicity of low concentration under emissions standards on the performance of biochemical system and the effect of inhibition kinetics of each functional bacteria are seldom reported. This makes biochemical sewage treatment system lack of scientific basis to be the stable running up to standard.According to the’electroplating pollutant discharge standards (GB21900-2008)’, the concentration of total chromium emissions requirements in table three was0.5mg L-1. There were three concentrations to set up,0.25mg L-1,0.5mg L-1,1mg L1. This research studied the long-term chronic toxicity of municipal wastewater containing chromium for step-feed SBR combined with intermittent aeration and traditional SBR. Experiments were run for46days, which run126days, run Ⅱ20days. The characteristics of the sludge, carbon nitrogen and phosphorus removal efficiencies and the system robustness are considerably different. The main conclusions are as follows:(1) The relative abundance of nitrifying bacteria, denitrifying bacteria and phosphorus accumulating bacteria were decreasing as the chromium concentration was increasing. The relative abundance of filamentous bacteria was increasing during the competition. The relative abundance of nitrifying bacteria and denitrifying bacteria in R4were33%and44%in R1, respectively. The relavive abundance of phosphorus accumulating bacteria and community diversity of the step-feed SBR combined with intermittent aeration were higher than that of the traditional SBR.(2) Low concentration of chromium could not raise SVI. The sludge of SBR was bulking immediately when the chromium was1mg L-1. Low concentration of chromium could stimulate microbial secreted more extracellular polymeric substances (EPS) against the toxic effect, but when the chromium concentration increased to1mg L"1, biological activity of microbial was inhibited, EPS began to decrease, and biochemical system was deteriorated.(3) Low concentration of chromium did not impact SBR biochemical system indicators removal rate, but slightly reduced the stability of the system. In the concentration of total chromium intake of0.5mg L-1, the step-feed SBR combined with intermittent aeration appeared significant inhibition of nitrogen and phosphorus removal on the35th day, while inhibition of traditional SBR delayed to40th days. In the concentration of total chromium intake of1mg L-1, the step-feed SBR combined with intermittent aeration presented obvious inhibition in one day. The impact of long-term low concentration of chromium for pollutants removal rates decreased as follows:NH4+-N> PO43--P> COD. The step-feed SBR combined with intermittent aeration was better than traditional SBR on the removal rate of total nitrogen, but the former was more susceptible to the suppressive effect of chromium.(4) From the inhibition coefficients of three kinds of functional bacteria, the difference of coefficients between growth and decay was nearly, but Cr (Ⅵ) inhibition coefficients were much smaller than that of Cr (Ⅲ) inhibition coefficients, which indicated that Cr (Ⅵ) inhibition is much greater than Cr (Ⅲ). Under the condition of Cr (Ⅲ), anti-impact abilities of three kinds of bacteria decreased as follows:ordinary aerobic heterotrophic bacteria> denitrifying bacteria> poly-phosphate accumulating organisms; under the condition of Cr (Ⅵ), anti-impact abilities of these bacteria were similar.(5) Based on the fully coupled activated sludge model (FCASM4) and combined with the results of inhibiton kinetics for heavy metal chromium, the corresponding reactions of ordinary aerobic heterotrophic bacteria, denitrifying bacteria and poly-phosphate accumulating organisms were coupled with Cr (Ⅵ) and Cr (Ⅲ) inhibition kinetics equations. Then the activated sludge model (FCASM4-Cr) for simulating the chromium containing wastewater treatment system was established, calibrated and validated. The results show that the chromium containing wastewater treatment system was well simulated by FCASM4-Cr which was better than FCASM4.