Abstract
This paper presents the results of batch anaerobic co-digestion of matooke, cassava, and sweet potato peels and vines. These agricultural wastes and others form the biggest portion of household wastes in developing countries. However, they have remained an unexploited resource amidst the ever increasing needs of clean energy and waste disposal challenges. Efforts to use them individually as biogas substrates have been associated with process acidification failure resulting from their fast hydrolysis. The aim of this work was to exploit agricultural wastes is co-digestion among themselves and assess their effect on methane yield and its kinetics, pH and hydraulic retention time (HRT). Sixteen ratios of Matooke peels (MP), cassava peels (CP) and sweet potato peels (SP) were assessed in duplicate. Methane yield and its kinetics, pH and HRT demonstrated dependence on the proportion of substrates in the mixture. Depending on the ratio mixture, HRT increased to 15 days compared to less than 5 days for single substrates, hydrolysis rate constant (k) reduced to a range of 0.1 - 0.3 d-1 compared to single substrates whose k-values were above 0.5 d-1, pH was maintained in the range of 6.38 - 6.43 and CH4 yield increased by 15% - 200%. Ratios 2:1:0, 2:0:1, 0:1:2, 1:1:1 and 1:1:4 were consistent all through in terms of model fitting, having a positive synergetic effect on HRT, hydrolysis rate constant, lag phase and methane yield. However, more research is needed in maintaining the pH near the neutral for process stability assurance if household wastes are to be used as standalone substrates for biogas production without being co-substrates with livestock manure.
Highlights
Clean energy shortage and waste disposal challenges at household level in developing countries are a reality and solutions to them are not likely to be realised in the near future amidst the ever growing population
Depending on the ratio mixture, hydraulic retention time (HRT) increased to 15 days compared to less than 5 days for single substrates, hydrolysis rate constant (k) reduced to a range of 0.1 - 0.3 d−1 compared to single substrates whose k-values were above 0.5 d−1, pH was maintained in the range of 6.38 - 6.43 and CH4 yield increased by 15% - 200%
Co and Mn were not detected in Matooke peels (MP) while in cassava peels (CP) and sweet potato peels (SP) were above the recommended range
Summary
Clean energy shortage and waste disposal challenges at household level in developing countries are a reality and solutions to them are not likely to be realised in the near future amidst the ever growing population. Unlike livestock manure that already contains anaerobes, household wastes and in particular of agricultural origin like matooke peels, cassava peels and sweet potato peels among others are not straightforward substrates to deal with in biogas production. They are liable to acidifying the anaerobic digestion process due to their fast degradability at the hydrolysis stage [6] [7]. A substrate ratio(s) working in a stable state with controlled hydrolysis rate would be obtained to enable use of household wastes as standalone substrates for biogas production
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