Pet food industrial wastewater contains high concentration of oil and grease (O&G) which is difficult to treat using conventional biological systems. In this study, anaerobic treatability of pet food wastewater was evaluated in a hybrid packed bed reactor-upflow anaerobic sludge blanket (PBR-UASB) treatment system. The PBR was packed with sol-gel/alginate beads containing immobilized enzyme as media which hydrolyzes the O&G in the wastewater as a pretreatment step while the UASB reactor was seeded with anaerobic sludge. The PBR was operated at 4 L/d for 64 days (phase I) and 8 L/d for another 57 days (phase II). The UASB reactor was operated at an HRT of 2.5 d and 1.25 d with an OLR of 1.27 and 2.5 kgCOD/m3.d during phase I and II respectively. Overall COD and O&G removal efficiencies of about 90% were achieved without any sludge floatation and with methane to COD yield of 80%. Despite the loss of immobilized lipase, the performance of PBR was satisfactory at oil loading of 0.9 kgO&G/m3.d which hydrolyzed 41% of O&G. Moreover, there was no sludge flotation observed during the period of operation due to pre-hydrolysis of O&G in the PBR. Further studies on system performance at higher organic loading rate and lower HRT are in progress.
In recent years, anaerobic treatment has attracted more attention than other treatment systems because it produces a valuable by-product, biogas, and produces less biomass (Lettinga, 1996). High-rate anaerobic reactors, such as, upflow anaerobic sludge blanket (UASB) (Tagawa et. al, 2002; Cammarota, et. al, 2001; Del Nery, et. al, 2001), hybrid UASB (Kim, et. al, 2004), and expanded granular sludge bed (EGSB) (Petruy and Lettinga, 1997; Rinzema, et. al, 1993) are the most widely used reactors for treating oily wastewaters. However, anaerobic treatment of oily wastewater from food industries was hampered by inhibition of LCFA (Rinzema et al, 1994) and sludge flotation/washout (Hwu, et al, 1998). Some researchers believed that the failure is essentially due to inhibition of methanogens and acetogens by LCFA (Hanaki, et. al, 1981; Rinzema, et. al, 1994) where LCFA disappear from solution and accumulate in solid biomass. Consequently, LCFA adsorb onto the membrane/cell wall of bacteria which damages the microbial cell transport function or protective function. On the other hand, other researchers believed that the biomass physically adsorbs fat/lipid causing biomass flotation and washout which also reduces LCFA bioavailability (Salminen and Rintala, 2002) and biogas release from the biomass.
In order to lessen the aforementioned problems, enzymatic hydrolysis could be used (Cammarota, et. al., 2001; Masse, et.al, 2001) prior to anaerobic treatment where lipases catalyze the hydrolysis of oil and grease (O&G) into free LCFA and soluble glycerol. The breakdown of O&G prior to anaerobic process might accelerate and stabilize the anaerobic digestion of oily wastewater (Masse, et.al, 2001). Besides, previous studies (Jeganathan et.al, 2006) proved that the lipase pretreatment enhances the anaerobic treatability for oily wastewater. To date, no study reported on oily wastewater treatment using immobilized lipase despite numerous studies being focused on free lipase hydrolysis of wastewater with synthetically added fat. Nevertheless, free lipases are generally soluble and unstable and hence can be used only once in solutions. On the other hand, commercially available lipases are very expensive. These problems can be easily overcome by immobilizing the lipases onto a strong, non-reactive matrix such as sol-gel/caalginate matrix. Immobilized enzymes have the advantages of multiple usage, controlled reactions, and thermostability (Matsumoto and Ohashi, 2003). In addition, for continuous operation in packed bed reactors, immobilized enzymes yield higher dosage per unit volume of reactor and hence provide higher volumetric productivity compared to free enzymes (Hartmeier, 1988). However, continuous supply of immobilized lipase is necessary due to activity loss and media breakage during continuous operation.
In light of this, the present work was intended to evaluate the feasibility of a hybrid packed bed reactor-upflow anaerobic sludge blanket (PBR-UASB) treatment system for high strength oily wastewater. The two-stage system was operated using real pet food wastewater at different HRTs and loading rates. The PBR was operated using sol-gel/alginate beads containing immobilized enzyme as media which hydrolyzes the O&G in the wastewater and the UASB reactor was operated with anaerobic sludge at anaerobic and mesophilic (35oC) conditions. Evaluation objectives include system performance at various loading rates and the role of PBR and UASB.