The behaviour of nitrilotriacetic acid during the anaerobic digestion of sewage sludge

In laboratory simulations of the anaerobic sludge digestion process, the impact of the detergent builder nitrilotriacetic acid (NTA) on digester efficiency has been studied. It was concluded that NTA at concentrations up to 30 mg l⁻¹ had no adverse effect on anaerobic digestion. However, analysis of the digested sludge indicated that only 29–45% of the influent NTA was removed during treatment. To differentiate between biological and physical processes of removal, biological activity in the digesters was arrested by the addition of sodium azide. Effluent NTA concentration did not increase indicating that biological activity was not responsible for the removal observed.Batch experiments undertaken to evaluate NTA solubility in digested sludge (containing azide) at two solids concentrations indicated a decrease in soluble NTA with increasing solids concentration.It is concluded that the removal of NTA observed during anaerobic digestion was not biological and in part was the result of adsorption onto the solid phase.