Evaluating the potential for improving anaerobic digestion of cellulosic waste via routine bioaugmentation and alkaline pretreatment

In this study, two methods for improving anaerobic digestion processes were investigated. The first method was routine bioaugmentation and the second was alkaline pretreatment. Both of these methods were applied to a two-phase anaerobic digestion process for treating the residuals from sweet corn processing, which have a significant lignocellulosic fraction as well as some starch from the base of the kernels. The two-phase anaerobic digestion process was proposed as one component of a larger integrated anaerobic/aerobic waste treatment process in which four co-products would be generated namely, methane-rich biogas, fertilizer, single cell protein, and algal biomass. The first objective of this study was to determine whether bioaugmentation with a cellulolytic bioculture would result in increased methane production compared to a non-bioaugmented control condition. Batch tests were conducted to compare the biogas potential of sweet corn processing residues with and without bioaugmentation using a proprietary cellulolytic bioculture. The results indicated that bioaugmentation was beneficial to digestion performance, increasing the average methane production by 34% compared to non-bioaugmented controls (265 versus 199 ml/g VSadded). The average rate of methane production was also increased in the bioaugmented condition compared to non-bioaugmented controls. However, the observed total methane production was relatively low in comparison to the maximum theoretical production (415 ml CH4/g VSadded), suggesting there to be room for further improving digestion efficiency. The second objective of this study was to verify whether routine bioaugmentation with cellulolytic microorganisms benefited substrate hydrolysis and subsequent methane production compared to one-time bioaugmentation. It was hypothesized that through routine bioaugmentation with cellulolytic microorganisms, a microbial population better suited for degradation of lignocellulosic material could be achieved and maintained, thereby increasing the rate of hydrolysis and ultimately increasing the rate of methane production. Pursuant to this objective, a two-phase sequencing/semi batch experiment was conducted in which routine bioaugmentation with two sources of cellulolytic microorganisms was compared to one-time bioaugmented and non-bioaugmented conditions. Neutral detergent fiber (NDF) analysis and net soluble chemical oxygen demand (sCOD) generation suggested that routine bioaugmentation improved substrate hydrolysis by 22-25% in comparison to one-time bioaugmentation after 14 days of operation. Methane yields from routine bioaugmented conditions using a proprietary cellulolytic bioculture also showed 15% higher methane production was achieved in comparison to one-time bioaugmentation after 36 days of digestion. In this experiment, bioaugmentation with a proprietary cellulolytic bioculture was compared to bioaugmentation with dairy cattle rumen fluid. The rumen bioaugmentation culture produced higher methane yields than the proprietary bioculture (16-34%). However, both were below theoretical yields, suggesting that further optimization of the bioculture could improve process efficiency. After evaluating the relative benefits of routine and one-time bioaugmentation, it was apparent that although bioaugmentation improved digester performance, there was still a significant fraction of un-hydrolyzed material. Thus, a third objective was added to determine the benefit of alkaline pretreatment on substrate solubilization and the digestibility of the resulting hydrolysate. Two long-term pretreatment batch tests (29 and 68 days) were conducted to determine the extent and rate of substrate hydrolysis under elevated pH conditions. It was found that through alkaline pretreatment up to pH 12, volatile solids solubilization was increased 2-4 fold compared to non-pretreated controls. Rates of solubilization were dependent on the pH consistency, which fluctuated during the batch tests due to the production of amino acids and fatty acids and intermittent addition of base to re-establish the target pH of 12. A subsequent anaerobic digestion batch test using the resulting hydrolysate indicated that 50% more methane production could be achieved in the case of alkaline pretreatment compared to a non-pretreated control. In summary, results from this study indicated that both routine bioaugmentation with a cellulolytic bioculture and alkaline pretreatment were significantly beneficial (34-50% improvement) for the anaerobic digestion of sweet corn processing residues, by contributing to higher rates of substrate hydrolysis and subsequent methane production