Experimental Investigation of the External Nitrification Biological Nutrient Removal Activated Sludge (ENBNRAS) System Zhi-rong Hu, S. Sötemann, R. Moodley, M. C. Wentzel, G. A. Ekama Water Research Group, Department of Civil Engineering, University of Cape Town, Rondebosch Cape, South Africa; telephone: 27 21 650 2588; fax: 27 21 689 7471; e-mail: ekama@eng.uct.ac.za Received 3 September 2002; accepted 9 January 2003 DOI: 10.1002/bit.10664 Abstract: A systematic lab-scale experimental investiga- tion is reported for the external nitrification (EN) biologi- cal nutrient removal (BNR) activated sludge (ENBNRAS) system, which is a combined fixed and suspended me- dium system. The ENBNRAS system was proposed to intensify the treatment capacity of BNR-activated sludge (BNRAS) systems by addressing two difficulties often en- countered in practice: (a) the long sludge age for nitrifi- cation requirement; and (b) sludge bulking. In the ENB- NRAS system, nitrification is transferred from the aero- bic reactor in the suspended medium activated sludge system to a fixed medium nitrification system. Thus, the sludge age of the suspended medium activated sludge system can be reduced from 20 to 25 days to 8 to 10 days, resulting in a decrease in reactor volume per ML waste- water treated of about 30%. Furthermore, the aerobic mass fraction can also be reduced from 50% to 60% to <30% and concommitantly the anoxic mass fraction can be increased from 25% to 35% to >55% (if the anaerobic mass fraction is 15%), and thus complete denitrification in the anoxic reactors becomes possible. Research indi- cates that both the short sludge age and complete deni- trification could ameliorate anoxic aerobic (AA) or low food/microorganism (F/M) ratio filamentous bulking, and hence reduce the surface area of secondary settling tanks or increase the treatment capacity of existing systems. The lab-scale experimental investigations indicate that the ENBNRAS system can obtain: (i) very good chemical oxygen demand (COD) removal, even with an aerobic mass fraction as low as 20%; (ii) high nitrogen removal, even for a wastewater with a high total kjeldahl nitrogen (TKN)/COD ratio, up to 0.14; (iii) adequate settling sludge (diluted sludge volume index [DSVI] <100 mL/g); and (iv) a significant reduction in oxygen demand. © 2003 Wiley Periodicals, Inc. Biotechnol Bioeng 83: 260–273, 2003. Keywords: activated sludge; biological nutrient removal (BNR); fixed media; external nitrification (EN) INTRODUCTION To control eutrophication, the biological nutrient (N + P) removal activated sludge (BNRAS) system has become an established technology in wastewater treatment practice, and this development has been facilitated by an improved understanding of nitrification–denitrification (ND) and bio- logical excess phosphorus removal (BEPR) processes. However, implementation of the BNRAS system has brought with it a new set of difficulties (Ekama and Went- zel, 1999a), including: (1) the long sludge age required for nitrification; (2) filamentous organism bulking; and (3) the treatment/disposal of liquors/supernatants generated from sludge treatment. This research focuses on the first two, namely the long sludge age for nitrification requirement and filamentous organism bulking. Long Sludge Age Requirement for Nitrification In BNRAS systems, the growth rate of obligate aerobic nitrifiers is low compared with that of heterotrophic organ- isms, that is, both ordinary heterotrophic organisms (OHOs) and polyphosphate-accumulating organisms (PAOs), and thus the requirement to nitrify governs selection of the two linked design parameters: (1) system sludge age; and (2) aerated mass fraction. The need for N and P removal sets a requirement for the unaerated sludge mass fraction (anaero- bic for P removal and anoxic for N removal), which usually must be high (>40%), causing the aerated mass fraction to be reduced (<60%). To compensate for this, and ensure complete nitrification throughout the year, particularly dur- ing winter, long sludge ages need to be selected. For ex- ample, with a maximum specific growth rate of nitrifiers ( nm20 ) of around 0.45/day at 20°C, to guarantee nitrifica- tion throughout the year (at the minimum temperature of 14°C), the sludge age of the single sludge BNRAS system must be around 20 to 25 days if 50% to 60% of the sludge mass in the system is unaerated. Such long sludge ages result in large biological reactors per megalitre (ML) waste- water (WW) treated. If nitrification can be made independent of the activated Correspondence to: G. A. Ekama Contract grant sponsors: Water Research Commission (WRC) of South Africa; Water and Sanitation Services (WSSA), a South African subsidiary of Ondeo (France); Technology and Human Resources in Industry Pro- gramme (THRIP) of the South African Department of Trade and Industries (DTI); National Research Foundation (NRF) of South Africa; University of Cape Town (UCT) © 2003 Wiley Periodicals, Inc.