ANOXIC BIOLOGICAL PHOSPHORUS REMOVAL IN A FULL-SCALE UCT PROCESS

Enhanced biological phosphorus removal is based on the selective enric hment 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 phosph orus 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 st udy of the full-scale UCT process run at Oresundsverket, Helsingborg, special attempts were made to quantify the relative contribution of an anoxic phosphate uptake at full-scale conditions: the dominant chemic al oxygen demand (COD) uptake in the anaerobic zone could be identifie d as poly-beta-hydroxy-alkanoates (PHA). PHA accumulation was at its l argest during a test period with acetate added as an extra carbon sour ce. At least one-third of the COD consumed in the anoxic zone could be identified as PHA. The anoxic sludge contained increased amounts of p olyphosphate and reduced amounts of free orthophosphate compared to th e 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 recy cling of the system, the formation of inorganic polyphosphate in the a noxic zone itself was estimated to be 30% of the total. When the metab olic activity was tested under controlled conditions in batch, the ana erobic sludge of the plant showed a high denitrifying activity accompa nied by a phosphorus uptake and a simultaneous consumption of intracel lular PHA corresponding to 2 g-COD/g-N, i.e. half the theoretical valu e needed for denitrification when biomass growth is included. It is co ncluded that intracellular PHA played a major role as a carbon source for denitrification in this full-scale UCT process, with a correspondi ng phosphate uptake also in the anoxic zone. The biological nitrogen a nd phosphorus removal must, therefore, be regarded as interconnected. (C) 1997 Elsevier Science Ltd.