Effect of Membranes on Refractory Dissolved Organic Nitrogen MinGu Kim, George Nakhla* ABSTRACT: A 500-day comparative study with a novel membrane bioreactor (NMBR), anaerobic–anoxic–aerobic (A 2 /O) process, and University of Cape Town-adapted MBR (UMBR) investigated the effect of membrane on effluent dissolved organic nitrogen (efDON) using synthetic (SWW) and municipal wastewater (MWW). The runs, comparing an NMBR and A 2 /O process, indicated 0.3 mg/L lower efDON in the former than the latter. However, NMBR and UMBR achieved similar efDON quality, with an average of 0.8 mg/L, and the DON reduction by membrane averaged 0.4 mg/L, while the A 2 /O efDON was slightly higher than DON in the aeration tank, by 0.08 mg/L, on average. The efDON during the MWW run increased by as much as 0.8 mg/L compared with the SWW run. The efDON is a component of a protein found in soluble microbial products, and it followed a cyclical temporal pattern during the runs. Membrane fouling propensity increased the efDON. This study presents evidence that membranes are effective in reducing efDON. Water Environ. Res., 82, 281 (2010). KEYWORDS: dissolved organic nitrogen, membrane bioreactor, bio- logical nutrient removal. doi:10.2175/106143009X12487095236757 Introduction Concern with dissolved organic nitrogen in effluent (efDON) from biological nutrient removal (BNR) plants is increasing, as a result of stringent total nitrogen effluent quality requirements. According to an intensive review by Pehlivanoglu-Mantas and Sedlak (2006), efDON accounts for up to 80% of the total nitrogen in the nitrification–denitrification process effluent. Based on data collected from seven plants in the United States, Pagilla et al. (2006) also reported that the percentage of efDON ranged from 20 to 85% of the effluent 5 mg/L total nitrogen. Reduction of efDON is challenging, as a result of the complexity of its nature. Awobamise et al. (2007) tested the biodegradability of efDON during 30 days and observed that, out of an initial DON of 1 mg/L, biodegradable DON increased in the first 20 days, from less than 0.1 to 0.5 mg/L, indicating that 50% of DON is not removed by biological processes. This is consistent with the observation by Urgun-Demirtas et al. (2007), where 18 to 61% of efDON was bioavailable in the first 14 days. Pagilla et al. (2006) observed that the molecular weight of DON ranged from 1 to 1000 kD, with the low molecular weight compounds being more biodegradable. Thus, various physical or chemical treat- ments, attempted to break the chain of high-molecular-weight DON compounds, revealed the feasibility of anion exchange, coagulation, ozonation, and advanced oxidation, to some extent (Pehlivanoglu-Mantas and Sedlak, 2006), though their application typically is limited to industrial wastewaters. Parkin and McCarty (1981) suggested that solids retention time (SRT) can be a key parameter for controlling efDON, because more influent DON is biodegraded at longer SRTs. However, increased biomass endogenous respiration increases DON, suggesting the occurrence of an optimal SRT. O’Shaughnessy et al. (2006) also reported that longer SRT and higher temperature within ranges of 10 to 17 days and 5 to 17uC, respectively, led to efDON reduction. Membrane bioreactors (MBRs) have demonstrated superior performance over the conventional biological wastewater treat- ment process, in terms of better nitrification and capture of solids and colloidal organic matter. Several MBR performances, with respect to efDON, are presented in Table 1. Apparently, efDON is associated with SRT. As the SRT varied from 15 to 26 days (Lesjean et al., 2002) and 20 to 50 days (Mouthon-Bello and Zhou, 2006), the efDON decreased with increasing SRT, not only in terms of concentration, but also as the percent of total nitrogen, in both cases. However, as apparent from Table 1, the average efDON from three BNR systems was 1.8 mg/L, which seems relatively high compared with the ,1 mg/L efDON observed in 68% of the 188 tested conventional BNR plant samples in Maryland and Virginia (Pagilla, 2007). Despite the few reports tabulated above, the advantages of MBR systems on efDON are not delineated thoroughly, particularly as they relate to the relative contribution of membranes and long SRTs. In this study, a comparative assessment of MBR and a conventional BNR system at identical SRTs was undertaken, with special focus on investigating the effect of membrane on efDON to shed light on the membrane role in DON reduction. The objectives of this study were to assess the effect of several factors on effluent organic nitrogen, including membrane size exclusion, system total hydraulic retention time (HRT), anaerobic HRT, feed type, biomass concentration, and soluble microbial product, and to delineate any correlation between membrane fouling and efDON. Experimental Setup System Description. The tested laboratory-scale processes were a novel MBR (NMBR), anaerobic–anoxic–aerobic (A 2 /O) process, and University of Cape Town-adapted MBR (UMBR), as presented in Figure 1. The NMBR process, patented in North America (Nakhla and Patel, 2008), consists of four units— anaerobic reactor, clarifier, anoxic reactor, and aerobic reactor. A schematic diagram of the system is depicted in Figure 1a. In the Department of Civil and Environmental Engineering, University of Western Ontario, London, Ontario, Canada. * Department of Civil and Environmental Engineering, University of Western Ontario, 1151 Richmond Street, London, ON., Canada N6A 5B9; e-mail: gnakhla@eng.uwo.ca. March 2010 281