Multi-stage digestion of municipal solid waste to fuel gas

Abstract

Multi-stage digestion experiments on shredded municipal solid waste were carried out with the object of developing improved practical processing systems especially for solid waste. On review it was found that multi-stage digestion was understood even by early workers primarily as a means of separating two broad groups of microorganisms, namely, the acid formers which solubilize cellulosic materials to organic acids and the methane fomers which convert the solubilized materials to fuel gas. Initially in this work, a series of ten 3.8 l jars were used to simulate a plug flow reactor of 38 l total working volume. Following pressing, the solubilized material from this acid-forming stage was fed to a 45 l continuous stirred tank reactor (CSTR) anaerobic digester. This 83 l system processed up to 0.5 kg/day waste. Recovery of 48 percent of the input total volatile solids (TVS) was obtained from the plug flow unit after solids separation by pressing; 33 percent of the input TVS was converted to fuel gas. Later in this work, experiments were carried out in ten 208 l steel reactors operated as a plug flow system with 2080 l total working volume and followed by a 2270 l CSTR type reactor. This 43501 system processed up to 23 kg/day solid waste. Acetic acid inhibition of the acid-forming microorganisms at levels under 10,000 mg/l appeared to limit solubilization in the plug flow unit. Continued experiments should focus on building a microorganism population tolerant to higher levels of acetic acid. Signficant solubilization of the solid waste within 3 days was judged by torque measurements on the stirrer. Anaerobic digestion of soluble material in the CSTR unit was rapid. Continued experiments with the soluble feed to the CSTR unit should take advantage of the potential for separation and recycling of methane-forming microorganisms.