Heat Treatment and Anaerobic Digestion of Refuse

Alkaline heat treatment was evaluated as a pretreatment process for increasing the anaerobic digestibility of municipal refuse. The goal was an increased methane yield from refuse lignocellulose. Batch digestibility assays were employed to ascertain the separate effects of solids concentration (0.79 - 10.0 percent), sodium hydroxide concentration (75 - 300 meq/l), and temperature (25 - 250°C) on the methane yield from refuse. Consumption of added alkali was observed and found to be relatively constant per unit mass of refuse solids. Various semi-continuous reactor configurations were evaluated. These studies resulted in investigation of a two-stage digestion system, with the second digester fed alkaline-heat-treated (180 meq/l NaOH, 200°C, 2.1 percent solids, 1 hour) effluent from the first. Such a system operated on newsprint was capable of increasing the methane yield by 80 percent, compared with the yield from a single-stage digester fed raw newsprint. A major limitation appears to be digester inhibition noticeable whenever the concentration of heat-treated lignin products exceeds 1 g/l.