Effect of Temporal Variation in Chemical Composition on Methane Yields of Rendering Plant Wastewater

Abstract

The effect of temporal variation in chemical composition on methane yields of rendering plant wastewater was studied in batch experiments at 37 °C. In total, 14 grab samples were collected from Monday through Friday (Day 1 to 5) from a rendering plant located in Queensland, Australia. Each day, three samples were collected: early morning (S1), midday (S2) and afternoon (S3). Chemical analyses showed that a significant different in total solids (TS), volatile solids (VS), and chemical oxygen demand (COD) was noticed among the samples. TS content ranged from 0.13% to 1.82% w/w, while VS content was between 0.11% and 1.44% w/w. Among the samples, S2 of Day 3 had the highest COD concentration (10.5 g/L) whilst S1 of Day 1 had the lowest COD (3.75 g/L) and total volatile fatty acid (VFA) concentration (149.1 mg/L). In all samples, acetic acid was the dominant VFA and accounted for more than 65–90% of total VFAs. Biochemical methane potential studies showed that the highest methane yield of 270.2 L CH4/kgCODadded was obtained from S3 of Day 3. Whilst the lowest methane yield was noticed for S1 of Day 1 (83.7 L CH4/kgCODadded). Results from kinetic modelling showed the modified Grompetz model was best fit than the first order model and a large variation was noticed between the experimental and the modelled data. Time delay ranged from 2.51 to 3.84 d whilst hydrolysis constant values were close to 0.21 d−1. Thus, the study showed that chemical composition of incoming feed to a biogas plant varies throughout the week and is dependent on the chemical composition of organic materials received and the amount of steam used for rendering process.