Abstract:

The effects of shock doses of Cr(VI) on activated sludge process operating at a fixed dilution rate (0.12 h-1) without recycle has been investigated. Different levels of Cr(VI) (1.0, 2.0, 3.0, 5.0, 10.0, 25.0 and 50.0 mg/l) were suddenly dosed to activated sludge units, operating at steady state with a synthetic wastewater devoid of chromium and continued until a new steady state has been reached. The results have shown that Cr(VI) even at very low dosages of 1.0 and 2.0 mg/l was toxic to the activated sludge process when shock injection was applied. The process performances observed at these levels of Cr(VI) with acclimatised cultures in previous studies have never been reached. It has been observed that shock Cr(VI) doses at low levels have immediately influenced the reactor biomass concentration, but not the COD removal efficiency. However, higher levels of Cr(VI) (25.0 and 50.0 mg/l) caused immediate adverse effects, and dramatic reductions in COD removal efficiency have been observed. The time elapsed to reach a new steady state after the shock, has been observed to increase with increasing Cr(VI) dosages. However, the shock injection of 50.0 mg/l Cr(VI) caused complete destruction of microbial population and lead to oscillations in in effluent COD concentration without a new steady state taking place even after 1000 h (=120*hydraulic retention time).

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