Difficulties and developments in biological nutrient removal technology and modelling

Filamentous bulking and the long sludge age required for nitrification are two important factors that limit the wastewater treatment capacity of biolo gical nutrient removal (BNR) activated sludge systems. A growing body of ob servations from full-scale plants indicate support for the hypothesis that a significant stimulus for filamentous bulking in BNR systems is alternatin g anoxic-aerobic conditions with the presence of oxidized nitrogen at the t ransition from anoxic to aerobic. In the DEPHANOX system, nitrification tak es place externally allowing sludge age and filamentous bulking to be reduc ed and increases treatment capacity. Anoxic P uptake is exploited in this s ystem but it appears that this form of biological excess P removal (BEPR) i s significantly reduced compared with aerobic P uptake in conventional BNR systems, Developments in the understanding of the BEPR processes of (i) pho sphate accumulating organism (PAO) denitrification and anoxic P uptake, (ii ) fermentation of influent readily biodegradable (RB)COD and (iii) anaerobi c hydrolysis of slowly biodegradable (SB)COD are evaluated in relation to t he IAWQ Activated Sludge Model (ASM) No.2, Recent developments in BEPR rese arch do not yet allow a significant improvement to be made to ASM No.2 that will increase its predictive power and reliability and therefore it remain s essentially as a framework to guide further research. (C) 1999 LAWQ Publi shed by Elsevier Science Ltd. All rights reserved.