SURVEY OF CONCENTRATIONS OF NTA AND EDTA IN UK RIVERS AND AT SEWAGE TREATMENT WORKS
Report No DWI0418
1992

EXECUTIVE SUMMARY

The Department of the Environment commissioned Warren Spring Laboratory to make a snapshot determination of the nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid (EDTA) contents of a sample of river waters, sewage effluents and sludges from the UK. Sampling was carried out on 29th April and 6th May 1992 at 25 river sites and 10 sewage treatment works.

Detection limits of 2 µg/l were achieved for both NTA and EDTA.

NTA concentrations in river water ranged from less than 2 µg/l to 43 µg/l with a log normal distribution. In Class 3 rivers (NWC or Scottish Classifications) the mean concentration was 16 µg/l and the median concentration was 10 µg/l. There were no identifiable differences between the means for the Class 1 or Class 2 rivers for which both the overall mean and the medians were less than 2 µg/l.

EDTA concentrations in river water ranged from less than 2 µg/l to 129 µg/l with a log normal distribution. In Class 3 rivers (NWC or Scottish Classifications) the mean concentration was 57 µg/l and the median concentration was 47 µg/l. In Class 2 rivers the mean concentration was 28 µg/l and the median concentration was 21 µg/l. For Class 1 rivers the mean concentration was between 6 and 7 µg/l with a median of less than 2 µg/l.

In sewage effluents the concentrations of both NTA and EDTA were much greater than those in rivers, with ranges of less than 2 to 740 µg/l for NTA and 60 to 1640 µg/l for EDTA. In contrast to the concentrations in river waters and sewage effluents, the NTA contents of sewage sludges exceeded that of EDTA. A greater proportion of the NTA is retained in sludge than is EDTA. NTA concentrations ranged from 1.4 to 23 mg/kg dry solids, compared to less than 0.2 to 3.8 mg/kg dry solids for EDTA. The mass flows of these substances in sludges from sewage works were found to be less than 1 t per year, even from the largest works, which is much less than that within effluents.

Simulations of the interactions of NTA and EDTA with toxic metals in 5 rivers were performed, to investigate competitive binding to specific metal ions. Taking typical values of soluble metal ion concentrations the metals were found to be in stoichiometric excess, so that only between about 5 and 25 percent of the total soluble metal ions were complexed. Complexation is unlikely to be uniform across metal species. NTA preferentially complexes with Cu but also forms complexes with Pb, while in the presence of an excess of Ni only this metal is complexed by EDTA. In other circumstances EDTA will form complexes with Pb, Cu, Zn and Cd. The implications of these observations in terms of water quality standards for drinking water are discussed in the report.

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