The general model for single-sludge wastewater treatment systems of the IAWPRC task group (Henze et al., 1987) was extended to describe the biological degradation of NTA and adsorption of NTA onto activated sludge based on literature studies and experiments undertaken at the Glatt wastewater treatment plant (waste water discharge: Q = 55 - 60 000 m3/d, 110 000 inhabitants) for the city of Zurich. During two days, the behaviour and diurnal load variation of nitrilotriacetate (NTA), zinc, lead and copper were analyzed on one lane (volume = 1 812 m3, Q = 14 700 m3/d) of the four parallel lanes used in secondary treatment.

The plant had a sludge age of about 3.6 days and was partly nitrifying (wastewater temperature = 10-11 °C). The average daily load of NTA for the investigated lane was 14 kg NTA/d, corresponding to 0.5 g NTA/person.d. The influent concentration varied between 300 and 1 500 µg NTA/l. NTA was biologically degraded up to 97 %.

Between 12 am and 2 pm of the second day 17 kg NTA (120 % of the daily load of one lane) had been added to the primary effluent. During 4 to 5 hours the biological NTA degradation was saturated and four times more than the daily average of NTA was degraded. Zinc and lead did increase in the secondary effluent during the NTA shock loading.

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