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INTRODUCTION
Biological nitrogen removal (BNR) processes are generally accepted in existing wastewater treatment plants (WWTPs) for nitrogen removal due to their economic advantages compared with chemical treatment methods (Fan et al. 2009). Among these BNR processes, the anoxic-oxic (A/O) process utilizes activated sludge (AS) technology that has been widely applied in most WWTPs on account of rich practical experiences obtained in its long-term application (Chan et al. 2009).
During the A/O process, the nitrogen removal is dependent on aerobic nitrification (ammonium is converted into nitrate) and anoxic denitrification (the nitrate is reduced to N2). Higher nitrification capability can produce more nitrates as electron acceptor for denitrification, thereby improving nitrogen removal efficiency. Thus, nitrification is a prerequisite for nitrogen removal. However, it is difficult to maintain the steady and high-efficiency nitrification in an AS system, because the slow-growing nitrifiers may be washed out of the system if the sludge retention time (SRT) is shorter than that needed for their proliferation (Nogueira et al. 2002). In addition, the nitrate recirculation flow from the aerobic to the anoxic zones is one of the important factors affecting nitrogen removal efficiency in the A/O process (Baeza et al. 2004). Higher nitrate recirculation flow would bring more nitrates back to the anoxic zone for denitrification. Nevertheless, this can increase the electricity consumption for recirculation and the operational cost. Hence, an effective method is required to modify the A/O process with AS technology for improving nitrification and reducing the nitrate recirculation flow, in order to ensure effluent nitrogen concentration satisfying the increasingly stringent discharge standard, and responding to requirement of energy conservation.
The integrated fixed-film activated sludge (IFAS) process is a hybridized involving microorganisms both in suspended sludge and on the biofilm carriers, and is particularly attractive for nitrogen removal because the slow-growing nitrifiers could be retained on the biofilm, which favors achieving longer SRT to strengthen nitrification (Regmi et al. 2011; Bassin et al. 2012). In addition, Fu et al. (2010) has verified that nitrifiers and denitrifiers can co-exist in one microbial community in a single aerobic reactor with free-floating carriers, facilitating the occurrence of simultaneous nitrification and denitrification (SND). As a novel BNR technology, SND can remove a part of nitrogen in the oxic reactor, which may reduce nitrate recirculation...