Anaerobic digestion process feedstocks

Another vital parameter in effective digestion is the availability of trace elements, or micronutrients, notably iron, cobalt, nickel and zinc, in the feedstock. These elements, when available in relatively small amonnts, can stimnlate methanogenic activities. The exact amount needed can vary for different types of wastewater, and prior trials are needed before they are added. More information on the importance of micronutrients on the anaerobic digestion process can be found in the literature (Banks, Zhang, Jiang, Haven, 2012 Demirel Scherer, 2011 Facchin et al., 2013). 

Pretreatments of the polymeric feedstocks to enhance the hydrolysis of the major components such as cellulose include size reduction, thermal-chemical treatments, and specific enzyme pretreatments (52,57). Although many of the pretreatment processes increase the rate and extent of potymer hydrolysis, the cost for treatment is prohibitive based upon the revenues generated for energy production alone in anaerobic digestion (32). 

Most often, the rates for feedstock destruction in anaerobic digestion systems are based upon biogas production or reduction of total solids (TS) or volatile solids (VS) added to the system. Available data for analyses conducted on the specific polymers in the anaerobic digester feed are summarized in Table II. The information indicates a rapid rate of hydrolysis for hemicellulose and lipids. The rates and extent of cellulose degradation vary dramatically and are different with respect to the MSW feedstock based on the source and processing of the paper and cardboard products (42). Rates for protein hydrolysis are particularly difficult to accurately determine due the biotransformation of feed protein into microbial biomass, which is representative of protein in the effluent of the anaerobic digestion system. 

According to the interviews performed as part of the assessment of the biomass industry, the gaseous-fuels-from-biomass industry is small. Corporate activities are focused on the anaerobic digestion of manures to produce gaseous fuel. Minimal activities are directed towards the development of other feedstocks and processes. 

Processes employed in biorefinery concepts to convert biomass feedstock into marketable products include biochemical (e.g., anaerobic digestion, microbial fermentation, enzymatic conversion), chemical (e.g., hydrolysis, transesterification, hydrogenation, oxidation), and thermochemical (e.g., pyrolysis, gasification) processes. 

Both digester systems exhibit extremely low levels of detectable cellulase activities (exoglucanase, endoglucanase, and -glucosidase) when compared to industrial saccharifying processes (See Table III) in which the hydrolysis of cellulose in the feedstock is optimized with respect to enzyme loading. Therefore, the data indicate the level of improvement that may be made to attain maximum rates for cellulose hydrolysis in the anaerobic reactor system. 

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