Influence of inoculum to substrate ratio on the anaerobic digestion of a cassava starch polymer

Abstract

The objective of this work was to assess the anaerobic digestion of a cassava starch polymer (CSP) under different inoculum to substrate ratios, evaluating the material decomposition level and the energy production from the biogas generated in this process. The experiment was designed with 6 treatments in triplicate, which consisted of volatile solids (VS) of the inoculum to VS of the CSP ratios (VSI/VSCSP) of 0.04, 0.08, 0.20, 0.60, 1.00, and a control treatment without the addition of CSP. Five destructive reactors were built for each treatment to monitor the degradation kinetics in the digestion process. Higher total solids (TS) and volatile solids (VS) removal values were found in lower ratio treatments. The maximum VS removal values found were greater than 85% in the 0.08 treatment and greater than 90% in the 0.04 treatment. The digestion process yielded a product with a high biogas content, rich in methane and hydrogen. The treatment with ratio of 0.08 resulted in a production of 1384 mL biogas per gram of VS removed. The results of the biogas analysis showed up to 44% hydrogen content in the acidogenic phase and 87% methane after stabilization of the methanogenic phase. The data from the 0.04, 0.08, and 0.20 treatments were fitted to the bisigmoidal model, using an equation adapted from the Gompertz function. Specific production rate was estimated for each phase of the digestion process. The rapid dissolution in water and easy degradation of the sugars of CSP into volatile acids indicates the possibility of studying anaerobic digestion in reactors with acidic and methanogenic phases physically separated, thus, optimizing the parameters to obtain more hydrogen and methane.