Inhibitory effect of high calcium concentration on municipal solid waste leachate treatment by the activated sludge process

This research focused on the inhibitory effects of Ca on the aerobic biological treatment of landfill leachate containing extremely high Ca concentrations. When the Ca concentration in leachate to be treated was more than 4500 mg l⁻¹, the total organic carbon removal rate was significantly reduced and the processing time to achieve the same removal efficiency was 1.4 times that in the control treatment without added Ca. In contrast, the total nitrogen and ammonia nitrogen (NH₄⁺–N) removal efficiencies were positively related to the Ca concentration, increasing from 65.2% to 81.2% and from 69.2% to 83.7%, respectively, when the dosage of added Ca increased from zero to 8000 mg l⁻¹. During aerobic treatment, the reductions of solution Ca concentration were in the range of 1003–2274 mg l⁻¹ and were matched with increases in the Ca content in the residual sludge. The inhibition threshold of Ca in the leachate treated by the activated sludge process appeared to be 4500 mg l⁻¹, which could be realized by controlling the influent Ca concentration and using an appropriate sludge return ratio in the activated sludge process.