Toxicity of trivalent chromium in the anaerobic digestion process

The toxicity of trivalent chromium to an anaerobic digestion system was studied using completely mixed anaerobic digesters. One of the digesters was operated as a control, one was dosed in a stepwise manner with increasing concentrations of chromium, allowing acclimatisation of the bacterial population and the other two digesters were used for shock injections of chromium but with no acclimatisation. The results have shown that a total concentration of 1140 mgl−1 trivalent chromium was toxic to anaerobic digestion process when injected in stepwise manner. On the other hand, a toxic effect of the metal was observed at 500 mgl−1 concentration when shock injection was applied, resulting in the process failure. However, a shock injection of 400 mgl−1 of trivalent chromium only caused a short term inhibition of the process with recovery taking place within a week.