SPECIATION AND FATE OF ETHYLENEDIAMINETETRAACETATE (EDTA) IN MUNICIPAL WASTE-WATER TREATMENT

The speciation of EDTA in sewage effluents leaving wastewater treatmen t 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 under taken to quantify the speciation of EDTA in influents and effluents of sewage treatment plants. Chemical equilibrium calculations are of onl y limited use for this purpose because several weeks are needed to rea ch thermodynamic equilibrium in wastewater due to slow metal exchange processes. In the effluents from treatment plants that precipitate pho sphate, concentrations of dissolved Fe (0.05 mu m- and 0.45 mu m-filte rable) correlated with the concentrations of EDTA. An operational sche me, using sunlight or artificial light sources for specific photoconve rsion of FeEDTA species, was applied to distinguish between photo-degr adable (=FeEDTA) and photo-resistent EDTA species. Field studies condu cted at three municipal wastewater treatment facilities showed that ED TA speciation changes from the input to the output because FeEDTA is f ormed from other metal-EDTA complexes after addition of iron(II)-conta ining solutions into the aeration tanks. With respect to total amounts of EDTA, fractions of FeEDTA in the influents and effluents varied fr om 10 to 55% and from 20 to 90%. Mass balances comprising sampling per iods of several days showed that no significant elimination of EDTA oc cured by biological or chemical processes during sewage treatment, whe reas the chemically related phosphate substitute nitrilotriacetic acid (NTA) was efficiently degraded (>90%). As long as the speciation of E DTA in wastewaters is dominated by FeEDTA, and aerobic conditions are maintained, the remobilization of common heavy metals out of sewage sl udge is unlikely to occur.