Anoxic biological phosphorus removal in a full-scale UCT process

Abstract

Enhanced biological phosphorus removal is based on the selective enrichment of bacteria accumulating inorganic polyphosphate, obtained at a cyclic regime of alternating anaerobic and aerobic conditions. In the University of Cape Town (UCT) process for combined nitrogen and phosphorus removal, polyphosphate-accumulating bacteria will also be exposed to nitrate in the anoxic zone, i.e. an electron acceptor that may be utilized as well as the oxygen of the aerobic zone. During a 1-year study of the full-scale UCT process run at Öresundsverket, Helsingborg, special attempts were made to quantify the relative contribution of an anoxic phosphate uptake at full-scale conditions: the dominant chemical oxygen demand (COD) uptake in the anaerobic zone could be identified as poly-β-hydroxy-alkanoates (PHA). PHA accumulation was at its largest during a test period with acetate added as an extra carbon source. At least one-third of the COD consumed in the anoxic zone could be identified as PHA. The anoxic sludge contained increased amounts of polyphosphate and reduced amounts of free orthophosphate compared to the anaerobic zone, approaching the levels of aerobic sludge. The metal bound orthophosphate remained largely unaffected, at a level of 25–30% of the total phosphorus content. After correction for the sludge recycling of the system, the formation of inorganic polyphosphate in the anoxic zone itself was estimated to be 30% of the total. When the metabolic activity was tested under controlled conditions in batch, the anaerobic sludge of the plant showed a high denitrifying activity accompanied by a phosphorus uptake and a simultaneous consumption of intracellular PHA corresponding to 2 g-COD/g-N, i.e. half the theoretical value needed for denitrification when biomass growth is included. It is concluded that intracellular PHA played a major role as a carbon source for denitrification in this full-scale UCT process, with a corresponding phosphate uptake also in the anoxic zone. The biological nitrogen and phosphorus removal must, therefore, be regarded as interconnected.