The possibility of controls on NTA and EDTA was mooted by the DoE in 1991 following a review of the environmental impacts of detergent ingredients. The idea then was that voluntary agreements which at least imposed a ceiling on their consumption at current levels might be needed (ENDS Report 199, pp 23-24).
Both chemicals are powerful chelating agents, and concern has focussed on the possibility that once discharged into the aquatic environment they may form highly stable soluble complexes with metal ions present in water and sediments, increasing metal concentrations in drinking water.
Two studies have been sponsored by the DoE since 1991. In the first, carried out by Warren Spring Laboratory (WSL), samples of river water from 25 sites and effluent and sludge from ten sewage treatment works were analysed for the two chemicals.1NTA levels in rivers were found to range from below the detection limit of 2µg/l to 43µg/l, while EDTA levels ranged between less than 2µg/l and 129µg/l. Concentrations of both compounds were an order of magnitude higher in "poor" class 3 rivers than in cleaner class 1 and 2 watercourses, indicating the influence of sewage effluent inputs.
Concentrations of both chemicals were much higher in sewage effluents, ranging from less than 2µg/l to 740µg/l for NTA and from 60-1,640µg/l for EDTA.
The analyses also showed that the bulk of the chemicals leave sewage works in effluent rather than sludge. The calculated mass of neither compound in sludge exceeded one tonne per year at any of the ten works. The calculated annual mass flows via effluent were much higher, ranging up to 119 tonnes for NTA and 275 tonnes for EDTA from one large works serving an urban/industrial catchment.
WSL also carried out a computer simulation to examine the interaction of the two chemicals with six heavy metals at five of the river sites. The exercise suggested that only about 5-25% of the soluble metal ions present would form complexes with NTA or EDTA because the latter are present at relatively low, or sub-stoichiometric, concentrations. WSL believes that NTA and EDTA loads at these sites could be increased without prejudicing compliance with EC limits on metals in drinking water.
WSL warned, however, that its conclusions were "highly speculative" because based on very limited data, and recommended further sampling and modelling. This does not appear to have been commissioned, although the DoE has now asked the Building Research Establishment to carry out an environmental hazard assessment of the two chemicals.
In the second study commissioned by the DoE, AEA Technology investigated the uses and environmental fate of NTA and EDTA.
Sales statistics are compiled annually by CEFIC, the European chemical industry federation. These show that NTA sales in the UK totalled 4,771 tonnes in 1991, with 86% being used in industrial cleaners and 12% in textile detergents.
NDTA sales in 1991 were 4,535 tonnes, with industrial and commercial detergents taking 34% and household detergents 9%. Several other industries are also consumers, including photochemicals (18%), textiles (6%), pulp and paper (6%), agriculture (6%), metal plating (3%), and cosmetics (2%).
Some industry sources are understood to have cast doubt on the NDTA figures. CEFIC's end-use data for the chemical are broken down only for Europe as a whole. AEA assumed that sales for each sector in the UK were identical to those for Europe, but this approach failed to take account of restrictions imposed on NDTA in some countries.
AEA calculated that 87% of NTA sales and 95% of NDTA sales are discharged to sewer. After taking into account their degradability in sewage treatment processes, the report estimates that 780 tonnes of NTA and 4,000 tonnes of EDTA are discharged annually to the UK aquatic environment.
These figures appear low in relation to the mass flows in sewage effluents calculated by WSL. At the ten sewage works studied, the calculated annual discharges of NTA and EDTA were 194 and 661 tonnes - 25% and 17%, respectively, of the totals estimated by AEA for the whole of the UK. WSL's calculations were based on spot effluent samples, so that variability in NTA and EDTA levels in effluents may account for the discrepancy - but there may also be other explanations.
AEA found little evidence of a recent reduction in consumption of either chemical, or of the substitution of EDTA by the more degradable NTA. The DoE is now understood to be contemplating inviting industry to participate in voluntary agreements to control their consumption.
WSL's study has also posed the question whether controls are needed for NTA and EDTA discharges from sewage works. The DoE asked water companies for a possible explanation of the high NTA levels in two sewage discharges - but they replied that since there was no specific NTA limit in their discharge consents the matter had not been investigated.