The concentrations of CODt and CODf in the effluent remained below 100 mg L1 during the first month of operation at 2 d HRT, resulting in a COD removal efficiency of over 90% (Fig. 3a). In the second month effluent COD concentrations slightly increased even though HRT was raised to 4 days. After two months of operation, COD removal increased again when HRT was reduced to 2 days