Behaviour of NTA and EDTA in biological wastewater treatment

Abstract

The behaviour and diurnal load variations of nitrilotriacetate (NTA) and ethylenediaminetetraacetate (EDTA) were investigated in winter and in summer in the sewage treatment plant Zu¨rich-Glatt. The results are discussed using the activated sludge model of the IAWPRC task group extended by the degradation kinetics of NTA and by its adsorption behaviour onto activated sludge. The plant had a sludge age of 3.6 d in winter and 4.8 d in summer and was in winter only partly nitrifying. In the investigated lane, the average daily loads of NTA and EDTA in the primary effluent were during the two investigated periods 13 ± 7kg NTA d −1 (0.5 ± 0.3g NTA person −1 d −1) and 1.3 ± 0.6kg EDTA d −1 (0.05 ± 0.02g EDTA person −1 d −1), respectively. The influent concentration varied from 300 to 1500 μg NTA 1 −1 and from 30 to 150 μg EDTA 1 −1. In both seasons NTA was biologically degraded up to 97%, while no elimination was observed for EDTA. During the measurements in the winter period, an additional 17 kg of NTA (120% of a daily load) were added to the effluent of the primary clarifier. During 4–5 h the biological NTA degradation was saturated and four times more than the daily average of NTA was degraded. Due to the increased NTA concentration (up to 2000 μg 1 −1 in the effluent of the secondary clarifier) zinc and lead were remobilized from the activated sludge, and their effluent concentration in the secondary clarifier increased by 200 and 50%, respectively. The concentration of copper did not increase during the NTA shock loading.