Optimization of substrate mixing ratios for the anaerobic digestion of chicken manure and slaughterhouse waste: a case study of Irvine’s Zimbabwe (Pvt) Ltd, Zimbabwe

Abstract

This study investigated the biogas potential of co-digesting chicken manure (CM) and slaughterhouse waste (SW) at Irvine’s Zimbabwe (Pvt) Ltd farm. Four different mixes of CM to SW as a percentage of total solids (TS) as follow: Mix 1 (100%: 0%); Mix 2 (67%: 33%); Mix 3 (33%: 67%) and Mix 4 (0%: 100%) were considered for co-digestion using cow dung as inoculum. The mixes were characterised in terms of carbon to nitrogen (C/N) ratio, chemical oxygen demand (COD), pH, TS, and volatile solids (VS). After 15 days of digestion, Mix 1 produced the highest biogas of 51 mL/gTS (24.4 m3/m3 feedstock) with a methane content of 30.2 mL/gTS (59.1%). The COD, pH, and TS values for the mixes after digestion ranged from 11,250 mg/L (Mix 1) to 25,000 mg/L (Mix 4), 5.54 (Mix 1) to 6.48 (Mix 4) and 11.5% (Mix 4) to 14.7% (Mix 2), respectively. Unlike findings from other studies that suggest that co-digestion increases gas production, mono-digestion of CM had the highest biogas potential and produced the most stable sludge. Therefore, mono-digestion of CM can be used for biogas production at Irvine’s Zimbabwe (Pvt) Ltd farm.