Methane production kinetics of pretreated slaughterhouse wastewater

Highlights: • Methane production increased significantly with the combination of both pretreatments regards slaughterhouse wastewater. • Thermal pretreatment produced two phases: a liquid (clarified) and a semisolid, which concentrated most pollutants. • The optimal volumetric ratio was 4:1, equivalent to a substrate/inoculum ratio of 4.26. • Methane production kinetics in the semisolid was complex and followed stepped curves. • The Luedeking-Piret and the Gompertz models adequately described methane production from semisolid. -- Abstract: The aim of this work was to analyze the methane production kinetics of pretreated slaughterhouse wastewater through biochemical methane potential (BMP) tests, where the volumetric ratio and, consequently, the substrate to inoculum ratio (SIR) were varied. The thermal pretreatment was autoclaving (60 min at 120^∘C/1.2 atm), and two phases appeared: liquid (clarified) and semisolid. Half of this semisolid also underwent a mechanical pretreatment (1 min with a cell disruptor). Afterward, BMP tests were carried out in serological bottles under five different volumetric ratios (1:1, 2:1, 3:1, 4:1 and 5:1), with RSWW, semisolid and semisolid after mechanical pretreatment as substrates. The volume of clarified liquid obtained after autoclaving was 84.5% of the RSWW, with removal percentages of COD, solids, proteins, and ammonium ranging from 50% to 89%, while the volume of the semisolid was only 4.35% of the RSWW, with the major portion of the pollutants concentrated in it. In the BMP tests, the methane generation was approximately fifteen times higher and lasted two times longer with semisolid as the substrate than that when using RSWW. The optimal volumetric ratio was 4:1 (SIR 5.67). The methane kinetics also changed: in RSWW, the kinetics followed the typical sigmoidal curve, whereas in semisolid, the kinetics were complex, showing a stepped curve. The cumulative methane production fit well to the Gompertz, Richards, logistic, Luedeking-Piret and modified first-order rate models, although none of them accurately described the second inflection point found at the end of the inhibited steady-state phase.