Using Anaerobic Toxicity Assays to Aid in Co-substrate Selection for Co-digestion

Abstract

While there has been increasing interest in manure anaerobic digesters during the past decade, economic constraints remain a limitation to widespread use of the technology. Literature has suggested that co-digestion with other available organic co-substrates could increase the economic viability of manure anaerobic digesters by increasing methane production potential. However, co-substrates must be carefully selected to avoid adding complexities to the manure digestion process. Biochemical methane potential (BMP) assays have been reported to provide a good initial evaluation of potential co-substrates, but they are performed under conditions optimized for the microbes, and the co-substrate is assayed under diluted conditions. Anaerobic toxicity assays (ATAs) provide additional information that could be utilized with BMP results to assist with co-substrate selection. An ATA evaluates a substrate's ability to inhibit methane production and thus determine its potential toxicity. This article provides specific details about the ATA process used by the Agricultural Waste Management Laboratory at Iowa State University on potential digester materials, including the assay method and utilization of the results. The two potential digester materials presented in this article provide examples of a toxic and a non-toxic material. Specifically, normalized BMP results indicated that the industry process by-product produced 60 mL CH4/g VS, and that it was not toxic at material inclusion rates of up to 49%. Conversely, the enzyme process by-product produced normalized BMP results yielding 284 mL CH4/g VS, but based on ATA results was highly toxic at all inclusion rates. These results clearly indicate the need to thoroughly evaluate co-digestion co-substrates.