Optimal Mix Ratio, Total Solids and Organic Loading Rates for Anaerobic Digestion

Abstract

Biogas technology is a sustainable and valid alternative to fossil fuels. It reduces greenhouse gas (GHG) emissions and can enhance energy security. pH, temperature, retention time, particle size, volatile solid content, inoculums, mix ratios (MR), total solid (TS) and organic loading rate (OLR) are among the factors that influence biogas production. Some studies have been done to determine optimal values of these factors for various feedstocks. The aim of this work was to determine the dependency of mix ratios (MR), total solids (TS) and organic loading rate (OLR) and their corresponding optimal values during anaerobic codigestion of cow dung (CD), pig (PM) and sheep manure (SM). Tests were done using a 0.15m 3 fixed dome laboratory reactor under a constant temperature of 35±1℃, 20 days operation period and gas yield was measured using water displacement method. A mix ratio of 1:1:2 (CD: PG: SM) lead with yield rate of 1.707m 3 /m 3 day followed by 3:1:3 with 1.300, then 1:1:3 with 0.782 and lastly 1:0:0 with 0.314m 3 /m 3 day. These values were significantly different at α=0.05 (LSD=0.392). There was an increase in biogas yield as TS increases from 6 to 8% followed by a decrease from 8% to 12%. Mean gas yields for 6, 8, 10 and 12% TS were 0.782, 0.644, 0.511 and 0.301m 3 /m 3 day respectively. TS 6 and 8%, 6 and 10%, 10 and 12 % were not significantly different from at α=0.05 but for 8 and 10% and 8 and 12% were significantly different (LSD=0.212). The biogas production rate for 1, 2, 3 and 4kgVS/m 3 day OLR were 0.599, 0.671, 0.769 and 0.386m 3 /m 3 day respectively. Mean yields for 1 and 2, 1 and 3, 1 and 4 and 2 and 3kgVS/m 3 day were not significantly different at α=5% but for 3 and 4, 2 and 4kg/m 3 day were significantly different (LSD=0.2417). It was then concluded that 1:1:3, 3:1:3 and 1:1:2 resulted in an increase in biogas production rate of 0.468, 0.986, and 1.393 respectively compared to pure cow dung. There was a 21.43% increase in biogas yield as TS increases from 6 to 8% followed by a 61.48% decrease from 8% to 12%. Increasing loading rate from 1-3kg VS/m 3 resulted in 29% increase in biogas production rate and further increase from 3 to 4 kg VS/m 3 day decreases biogas production rate by 50%. Mix ratio of 1:1:2, TS 6 and 8% and OLR 2 and 3kgVS/m 3 day were then recommended.