Long-Term Performance of High-Rate Anaerobic Reactors for the Treatment of Oily Wastewater JEGANAESAN JEGANATHAN, GEORGE NAKHLA,* AND AMARJEET BASSI Department of Chemical and Biochemical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B9 Complex oily wastewater from a food industry was treated in three different UASB reactors at different operating conditions. Although all three systems achieved fat, oil, and grease (FOG) and COD removal efficiencies above 80% at an organic loading of 3 kg COD/m 3 ‚d, system performance deteriorated sharply at higher loading rates, and the presence of high FOG caused a severe sludge flotation resulting in failure. Initially, FOG accumulated onto the biomass which led to sludge flotation and washout of biomass. The loss of sludge in the bed increased the FOG loading to the biomass and failure ensued. Contrary to previous findings, accumulation of FOG rather than influent FOG concentrations or volumetric FOG loading rate was the most important factor governing the high- rate anaerobic reactor performance. The critical ac- cumulated FOG loading was identified as 1.04 ( 0.13 g FOG/g VSS for all three reactors. Furthermore, FOG accumulation onto the biomass was identified mainly as palmitic acid (>60%) whereas the feed LCFA contained only 30% of palmitic acid and 50% of oleic acid. Introduction In anaerobic processes, fat, oil, and grease (FOG) is first hydrolyzed to free long-chain fatty acids (LCFA) and glycerol. LCFA is converted into acetate and hydrogen via -oxidation (1) prior to degradation to methane and carbon dioxide. Glycerol is degraded to 1,3 propandiol (2) and subsequently to acetate and hydrogen. Anaerobic digestion of FOG yields higher biogas production since the fraction of degraded substrate for lipids (0.948) is higher than that of carbohydrates (0.504) and proteins (0.710). In addition, anaerobic treatment of FOG produces less biomass because the fraction of substrate used for cell synthesis for lipids is 0.052 compared to 0.5 and 0.29 for carbohydrates and proteins, respectively (3). High-rate anaerobic reactors, such as upflow anaerobic sludge blanket (UASB) reactors (4-6), hybrid UASB reactors (7), and expanded granular sludge bed (EGSB) reactors (8, 9), are widely used for treating oily wastewaters. Treatment of complex (inhibitory/insoluble) wastewaters (e.g., contain- ing LCFA) in high-rate reactors causes operational problems and in some cases even failure (10, 11). Moreover, most of these literature studies were conducted with synthetic wastewater or low-strength wastewater with synthetically added fats to avoid the complexity and heterogeneity of FOG. The presence of FOG in wastewater causes two main problems for anaerobic treatment processes: (i) inhibition of methanogenesis due to LFCA (1, 12) and (ii) sludge flotation/washout (9, 11) and hence is limiting for gas production and removal of COD (13). There are contradictory reports on the mode of failure and no definite mechanism has been delineated so far. Some researchers believed that failure is mainly due to the inhibition of methanogens and acetogens (1, 12, 14) by LCFAs. LCFAs disappear from solution and accumulate in solid biomass (1) within 24 h and subsequently, adsorb onto the membrane/cell wall of bacteria which damages the microbial cell transport function or protective function. Koster and Cramer (15) reported that acetoclastic methanogenesis was inhibited by oleic (614 mg/ L), myristic (593 mg/L), lauric (320 mg/L), capric (447 mg/L), and caprylic (972 mg/L) acids. Angelidaki and Ahring (14) showed that oleic acid (100-200 mg/L) and stearic acid (>300 mg/L) inhibited the degradation of acetate, propionate, and butyrate at thermophilic temperature. Nonetheless, Hanaki et al. (1) found in batch assays that glucose fermentation was not affected by the presence of LCFA, if readily biodegradable COD was available. Most of these batch inhibitory studies were based on synthetic LCFAs which allowed the assessment of inhibitory effect of individual LCFAs but did not explore synergistic effects. On the other hand, other researchers concluded that the biomass physically adsorbs fat/lipid causing biomass flotation and washout which also reduces LCFA bioavailability (16) and biogas release. Hwu et al. (11) treated synthetic LCFA mixture in a laboratory-scale UASB reactor and concluded that sludge flotation occurred at relatively low influent concentrations (263 mg LCFA/L or 0.2 g COD/g VSS‚d) below the inhibition levels for methanogens, indicating that clearly in continuous systems, sludge flotation precedes inhibition. However, the effect of FOG/LCFA inhibition or sludge flotation on anaerobic sludge has essentially been studied in batch experiments in serum vials (1, 12, 14-15), or in continuous anaerobic reactors using synthetic LCFAs (7, 10- 11). So far, there is no report available on the anaerobic treatability of real, high-strength FOG wastewater in a continuous system on a sustained long-term basis which clearly establishes the loading criteria and/or describes the exact mechanism of failure of a high-rate anaerobic reactor treating oily wastewater. Hence, the main objective of this study was to investigate the treatability of complex oily wastewater from a rendering industry in three UASB systems at different operating conditions and delineate the failure mechanism. Evaluation objectives include system perfor- mance at various oil loading rates and organic loading rates. COD and FOG mass balances were performed and the accumulation of FOG on the biomass was calculated for each system. Experimental Section Reactor Setup. System 1 (Figure 1a) essentially consisted of a laboratory-scale UASB reactor which was made of Plexiglas with a working volume of 10 L and had two sections (lower part 50 cm high, 10 cm i.d.; upper part 20 cm high, 20 cm i.d.). System 2 (Figure 1b) also consisted of a laboratory- scale UASB reactor which was made of PVC with a working volume of 15 L (60 cm high, 20 cm i.d.). System 3 (Figure 1c) comprised a packed bed reactor (PBR) and a UASB in series with working volumes of 2 L (PBR 100 cm high, 5 cm i.d.) and 10 L (UASB same as system 1). The internal diameters of the UASBs were carefully selected (g10 cm) to prevent severe biomass flotation due * Corresponding author phone: 1 519 661 2111, ext. 85470; fax: 1 519 850 2129; e-mail: gnakhla@eng.uwo.ca. Environ. Sci. Technol. 2006, 40, 6466-6472 6466 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 20, 2006 10.1021/es061071m CCC: $33.50  2006 American Chemical Society Published on Web 09/07/2006