Biological mechanism of acetate uptake mediated by carbohydrate consumption in excess phosphorus removal systems

Abstract

The basic mechanism of substrate utilization has yet to be conclusively identified in the anaerobic-aerobic activated sludge process for enhanced phosphorus removal. Various researchers have reported PHB production and accumulation by the cell when fed with acetate under anaerobic conditions. Acetate is converted to acetyl-CoA, and the energy required for this conversion is supplied by the hydrolysis of intracellular polyphosphates. Synthesis of PHB from acetyl-CoA does not require ATP, but reducing power as NADH is essential. Some researchers have suggested that circulating some acetyl-CoA through the tri-carbonic acid (TCA) cycle would produce the requisite reducing power, but the TCA cycle is generally believed to accomplish complete oxidation of acetyl-CoA by transport of electrons to terminal electron acceptors like oxygen or nitrates. Therefore, investigation for alternate sources of NADH was undertaken. Four anaerobic batch experiments were conducted with anaerobic-aerobic acclimatized sludge fed with acetate. Samples were analyzed for COD and acetate uptake, P release, CO 2 production and for intracellular carbohydrate, protein and PHB content. PHB was synthesized by the sludge corresponding to acetate and COD uptake, along with orthophosphate release. Protein synthesis was not observed but intracellular carbohydrates were consumed. From the results, it is concluded that intracellular carbohydrates are consumed via the Embden-Meyerhoff-Paranas pathway and converted to acetyl-CoA, which provides NADH required for PHB synthesis. As observed carbohydrate consumption is greater than theoretical requirement for PHB synthesis, the consequent excess NADH available makes the possibility of some acetyl-CoA being circulated through the TCA cycle very remote.