During the initial years of the development of Biological Nutrient Removal (BNR) technology, it was assumed that the bacterial species responsible of the removal of phosphorus (BioP organisms) could not use nitrates as a final electron acceptor and could thus not denitrify. The carbon taken up in the form of Volatile Fatty Acids (VFA) in the anaerobic zone was thus deemed to be unavailable for denitrification in the anoxic zone. This was reinforced through experiments in which BioP organisms cultured in the high-rate Phoredox system in which no nitrification took place, did not denitrify when nitrates were added. Many researchers (e.g. Dold and Barker) have since shown that in BNR systems such as the 3-Stage Bardempho system, where nitrates are recycled to the anoxic zone which follows the anaerobic zone, a high degree of phosphorus uptake through denitrification does occur. In addition, the partial diversion of primary effluent directly to the anoxic zone has significantly improved phosphorus uptake under anoxic conditions.

Full-scale operations at the Westbank, British Columbia, plant showed a substantial uptake of phosphorus in the anoxic zone in the absence of oxygen. The Westbank configuration includes side stream primary sludge fermentation, VFA rich fermenter supernatant addition directly to the anaerobic zone and diversion of a portion of primary effluent to the anoxic zone. This configuration stimulates P-uptake under anoxic conditions, demonstrates the efficient use of carbon and is instrumental in achieving an annual average effluent Total-P concentration of less than 0.17 mg/l.

The phenomenon of denitrification by BioP organisms was included in the Biowin Model developed by Dold (Biowin Manual). This paper describes experiments and full-scale plant observations to establish the role of BioP organisms in the removal of nitrates in the anoxic zone of a plant which also receives a portion of the primary effluent and verification of the Biowin model.

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