Co-Processing of Organic Fraction of Municipal Solid Waste and Primary Sludge – Stabilization and Disinfection

Abstract

Batch mesophilic digesters were fed a mixture of the organic fraction of municipal solid waste (OF-MSW) and primary sludge (PS), and operated under non-mixing condition at a 30-day influent mass retention time. The reactors were started in a slurry mode and brought to the target level of 26-30% total solids in about 3 months from start. Two feed characteristics, the level of PS solids in the feed, and the particle size of the OF-MSW, were the variables used to determine their effect on methane production during digester operation. Higher biodegradable volatile solid reductions and methane yields matched increased fractions of primary sludge in the feed (5%, 10%, and 15% as dry solids). Incomplete digestion, limited to fermentation, took place in reactors that did not receive primary sludge. The particle size did not have any effect on specific weight of the feed but the amount of primary sludge did. Addition of vitamins and micronutrients to the inhibited reactors did not stimulate methane production, which narrowed the range of possible causes to the lack of available macronutrients, namely nitrogen. On the other hand, possible un-ionized ammonia (NH3) inhibition of reactors fed with the highest primary sludge-amended feed hindered organic conversion rates. Elimination of pathogenic microorganisms from the high-solids residue via long-term storage, followed by low-dose lime disinfection, was also investigated. Storage alone reduced fecal coliform and Salmonella sp. bacteria to below detection, but bacterial spores of anaerobic Clostridium perfringens survived. Subsequent liming of the residue caused irreversible inactivation of the spores because of high pH and, possibly, free ammonia (NH3) inherent to sludge.