Anaerobic degradation of fat particles in slaughterhouse wastewater with and without hydrolysis pretreatment.

Lipids represent an important fraction of the particulate organic charge in slaughterhouse wastewater. Anaerobic treatment of slaughterhouse wastewater has been reported to be slowed down or impaired because of high concentrations of suspended solids, particularly fats. However, the fate of lipids during anaerobic digestion has been poorly defined, especially for wastewaters from the meat processing industry. The objectives of this thesis were thus (1) to evaluate the effect of hydrolysis pretreatment on the anaerobic digestion of fat particles in slaughterhouse wastewater; (2) to characterise and quantify neutral fat hydrolysis and long-chain fatty acid (LCFA) oxidation during anaerobic degradation of slaughterhouse wastewater with and without hydrolysis pretreatment; and (3) to determine the effect of particle size on fat hydrolysis. The efficiency of four pretreatments to hydrolyse and reduce the size of pork and beef fat particles during mixing at room temperature was tested: NaOH and three commercial lipases of plant, bacterial and animal origins. The most promising pretreatment was the pancreatic lipase PL-250 that could significantly reduce the initial average particle size (Din) of pork fat by a maximum of 40% after 4 h of mixing at room temperature. Approximately 35% of the neutral fat was hydrolysed after 5.5-h of pretreatment with 250 mg/l of PL-250 in a substrate containing approximately 2000 mg/l of pork fat particles. Most of the free LCFAs released during the hydrolytic pretreatment remained adsorbed on the fat particle surface. The effect of pretreatment with PL-250 on subsequent anaerobic digestion of the substrate was evaluated by feeding control and enzyme pretreated slaughterhouse wastewater containing pork fat particles to anaerobic sequencing batch reactors (ASBRs) operated at 25°C. The main conclusions from the experiment were: (1) Pretreatment with PL-250 only had a small effect on pork fat particle digestion at 25°C, marked by a decrease of about 5% in digestion time to achieve 80% reduction in initial neutral fat and free LCFA concentrations. (2) Anaerobic degradation of pork fat particles is mainly controlled by free LCFA oxidation and, in ASBRs operated at 25°C, near maximum oxidation rate is reached at low free LCFA concentration. Consequently, increasing the initial free LCFA concentration by prehydrolysing the substrate will have limited effect on fat degradation rate. (3) At Din ranging from 60 to 450 mum, pork fat hydrolysis rate in anaerobic reactors is not a function of particle size. The fat particles became more filamentous and plate-like as their size was increased. Bacteria could probably colonise the inside as well as the outside of the particles. Consequently, specific surface area (m2/m3) available for hydrolysis was not significantly increased by decreasing the pork fat particle size. (4) Neutral fat hydrolysis and free LCFA oxidation rates can be adequately modelled using first-order and Monod-type kinetics, respectively. The first-order hydrolysis rate constant averaged 0.63 +/- 0.07 d-1, while the maximum oxidation rate (kmax) and half-saturation concentration (Ks) were estimated at 164 +/- 37 mg free LCFA /l/d and 35 +/- 31 mg free LCFA/1, respectively. (5) Fat hydrolysis rate will be underestimated if based on the increase in soluble compounds with respect to particulate organics. An analytical method that removes bound LCFAs from solids surface must be used to measure lipid hydrolysis