"An investigation was conducted of systems for mixing municipal solid waste (MSW) with municipal sewage sludge (MSS) in an anaerobic digester to produce usable fuel (methane gas). Adequate mixing is of paramount importance to the success of this biogasification process. Gas draft tubes and mechanical agitators were evaluated for use in a 387,500-L (100,000 gal), 10.7-m-diameter digester. Feed ratios of MSW to MSS were either 3:1 or 9:1. Loading rates of volatile solids varied from 1.25 to 3.125 g/L per day, and total solids in the feed were 4, 7, or 10 percent. Hydraulic retention time was 22.5 days, except for one 11 -day study. Problems that occurred during the study were dense scum formation (hard cellulose mats up to 1.5 m thick), heavy wear on the mixing systems as a result of the cellulose fibers and grit from the MSW/MSS mixture, and insufficient amounts of volatile solids in the mixed zone. The digester operated without problems as long as the total solids level was 5 percent or below; higher concentrations resulted in insufficient mixing and operational problems. Increased mixing power would improve the distribution of volatile solids, probably decrease scum formation, and result in increased gas production. But maintenance problems resulting from the nature of the MSW/MSS mixture and the increased energy costs of a higher-powered mixer would detract from system performance. MSW/MSS mixtures with high cellulose contents are therefore judged not to be amenable to anaerobic digestion using the same methods employed for municipal wastewater."