Speciation and fate of ethylenediaminetetraacetate (EDTA) in municipal wastewater treatment

Abstract

The speciation of EDTA in sewage effluents leaving wastewater treatment plants determines its ultimate fate in natural surface waters, since only the Fe(III)-EDTA complex (FeEDTA) is quickly degraded by direct photolysis, whereas other EDTA species are very slowly transformed, if at all, by biological or chemical processes. Field studies were undertaken to quantify the speciation of EDTA in influents and effluents of sewage treatment plants. Chemical equilibrium calculations are of only limited use for this purpose because several weeks are needed to reach thermodynamic equilibrium in wastewater due to slow metal exchange processes. In the effluents from treatment plants that precipitate phosphate, concentrations of dissolved Fe (0.05 μm- and 0.45 μm-filterable) correlated with the concentrations of EDTA. An operational scheme, using sunlight or artificial light sources for specific photoconversion of FeEDTA species, was applied to distinguish between photo-degradable (=FeEDTA) and photo-resistent EDTA species. Field studies conducted at three municipal wastewater treatment facilities showed that EDTA speciation changes from the input to the output because FeEDTA is formed from other metal-EDTA complexes after addition of iron(II)-containing solutions into the aeration tanks. With respect to total amounts of EDTA, fractions of FeEDTA in the influents and effluents varied from 10 to 55% and from 20 to 90%. Mass balances comprising sampling periods of several days showed that no significant elimination of EDTA occured by biological or chemical processes during sewage treatment, whereas the chemically related phosphate substitute nitrilotriacetic acid (NTA) was efficiently degraded (>90%). As long as the speciation of EDTA in wastewaters is dominated by FeEDTA, and aerobic conditions are maintained, the remobilization of common heavy metals out of sewage sludge is unlikely to occur.