Enhanced uptake of phosphorus by activated sludge-effect of substrate addition

Abstract

Studies were undertaken to examine the effect of substrate addition upon the release and subsequent uptake of phosphorous by a biological wastewater treatment laboratory scale system. A train of six reactors were fed at a rate of 16 ml min −1 raw wastewater using a sludge recycle ratio of 0.75 (12 ml min −1). The first two reactors were simply stirred (anoxic) without the addition of air and the remaining four were aerated with excess air. Various substrates were added to the first reactor (anoxic) at different concentrations. It was determined that all the short chain substrates tested enhanced the release of phosphorus in the anoxic stages and subsequently led to increased biological uptake (removal) of phosphorus. The substrates tested included sodium acetate, acetic acid, butyric acid, ethanol and methanol. It was determined that 30 mg l −1 sodium acetate was the optimum dose for biological release and uptake of phosphorus and the addition of any greater concentration had marginal effect on the ultimate removal of phosphorus. The effect of these substrates showed some specificity regarding phosphorus release, with butyric acid causing the greatest release and sodium acetate the least. However as far as phosphorus uptake (removal) was concerned, this phenomenon of substrate-specificity was less significant. For all substrates, effluent phosphorus concentration was within ± 1 mg l −1 with an approximate mean concentration of 1 mg l −1 residual. Phosphorus released (approx. 14 mg l −1) at higher temperature (29°C) was 75% greater than at the lower temperature (24°C). Similarly the final residual phosphorus at 29°C was 33% lower than at 24°C.