Biological removal of EDTA in conventional activated-sludge plants operated under alkaline conditions

Semicontinuous activated-sludge (SCAS) units inoculated with activated sludge from a plant treating predominantly domestic wastewater were used to study EDTA removal from wastewater at different pH values. Almost complete removal of EDTA was found in units maintained at pH > 8•0, but high EDTA removal was obtained only in SCAS units run with sludge retention times ≥ 12 days. A maximum EDTA removal rate of 0•2 kg EDTA/m3/day was achieved in the SCAS units. A full-scale activated-sludge system operating under alkaline conditions provided high removal percentages, confirming the removal of EDTA in activated-sludge systems as established in the SCAS tests. The results in both laboratory and full-scale activated-sludge units showed that a reduction of more than 90% of the incoming EDTA was attainable. Ratios of biological oxygen demand to theoretical oxygen demand of > 0•6 in closed-bottle tests, inoculated with either sludge from a full-scale treatment plant or an SCAS unit, proved that EDTA was removed by biodegradation. These results clearly demonstrated that EDTA-containing wastewater is amenable to activated-sludge treatment in conventional plants operated under alkaline conditions.