The influence of nitrilotriacetic acid on heavy metal transfer in the activated sludge process—II. At varying and shock loadings

Abstract

In laboratory simulations of the activated sludge process, the impact of shock loads of the detergent builders sodium tripolyphosphate (STPP) and nitrilotriacetic acid (NTA) on the transfer of heavy metals to the effluent, has been studied. Experiments have also been undertaken in which the influent concentration of each builder was doubled, in an endeavour to simulate the effect of a “wash day”. It is apparent from the data presented that NTA, when present under either of these conditions, resulted not only in deterioration of metal removal by the activated sludge process but caused a mobilisation of heavy metals from the sludge. Copper and lead were the metals most extensively affected. The mobilisation of heavy metals from the mixed liquor to the effluent in activated sludge units acclimated to NTA, upon a doubling of the influent NTA concentration, resulted from a failure by the activated sludge unit to completely degrade the additional NTA. The concentration of NTA in the effluent rose to 4.7 mg l −1 following the increase in influent concentration. Ten days were required before effluent NTA concentrations returned to values equivalent to those prior to the increase in influent concentration. When condensed phosphates were added as shock load or upon the doubling of their influent concentration only the removal of copper was adversely affected.