Laboratory investigation of ethanol production from municipal primary wastewater solids

Abstract

Ethanol can be produced from a number of renewable resources such as starches, sugars, or lignocellulosic materials. Lignocellulosic waste materials such as municipal wastes or wastepaper are defined as such because they contain cellulose in combination with lignin. The cellulose component in these materials can be converted to ethanol in a two-step process where the cellulose is first converted to glucose sugars by hydrolysis (saccharification); the resulting sugars can in turn be converted to ethanol by fermentation. A preliminary investigation was performed to assess the conversion of the cellulosic component of municipal primary wastewater solids to ethanol. Primary wastewater solids collected from two full-scale facilities were tested. A laboratory experimental set-up was designed in order to perform the necessary conversions. The primary wastewater solids were characterized to contain 10% cellulose and 26% lignin. Conversion of the cellulose to glucose was achieved by Trichoderma reesei cellulases in enzyme hydrolysis. These experiments demonstrated that higher substrate concentrations, temperatures and enzyme concentrations resulted in higher rates of reaction. Conversion of the glucose to ethanol during fermentation was accomplished by the action of yeasts from Saccharomyces cerevisiae. The two conversion processes were then combined in the same vessel to obtain simultaneous conversion in a process known as simultaneous saccharification and fermentation (SSF). SSF experiments employed the cellulase system from Trichoderma reesei QM9414 and Saccharomyces cerevisiae in the same vessel and achieved ethanol concentrations between 1.5 and 2.3 g/l from nutrient media containing 100 g/l primary wastewater solids. This corresponded to overall conversion efficiencies of cellulose to ethanol in the range of 17–60% based on theoretical values.