The effect of several pH control chemicals on the dry batch digestion of the organic fraction of municipal solid waste

Abstract

Methane production in dry batch digestion of the organic fraction of municipal solid waste (MSW) was studied as a more complete method for management of MSW than landfilling. For start-up of the dry digestion of the organic fraction of MSW the addition of a methanogenic inoculum is essential. The first start-up of a dry anaerobic digestion batch reactor appeared to be unbalanced when a seed/substrate solids ratio of 0.08 was applied. Higher seed/substrate solids ratios were not employed because the total solids concentration is too low to speak of dry digestion under these conditions. The imbalance resulted in pH values below 6, and organic acid concentrations of 60 g COD (chemical oxygen demand) per liter. At these conditions methane production was negligible. To accelerate the start-up of the dry digestion process, several pH control chemicals were investigated. NaHCO 3 gave the best results when the buffer/ substrate solids ratio amounted to 0.06. The potential methane yield of 80 1 STP of methane per kg of organic fraction was obtained within 6 months. Ca (OH) 2 had a minor effect on the pH. CaCO 3 did not control the pH and inhibited the biogas production. The amount of pH control chemicals which were found to provide optimum pH control resulted in toxic levels of the cations for the methane production. The period required for adaptation of the methane-producing bacteria to these inhibitory cation concentrations limits the benefits of buffer additions during dry digestion.