Methane Production from Solid Potatoes by a Procedure Simulating a Bench-Scale Sequencing Batch Reactor Anaerobic Process

It is well known that anaerobic digestion (AD) is a multi-step process in which complex substrate molecules are firstly hydrolysed to simpler soluble monomers, in the hydrolytic-acidogenic step, then transformed into volatile fatty acids (VFAs), and finally converted into methane and carbon dioxide, in the acetogenic-methanogenic step.1–3 Normally, bacteria involved in these steps are quite different and therefore present maximum growth rate under different conditions. For example, hydrolysis is reported to be the rate-limiting step in the whole AD process with complex substrates, requiring often slightly acid pH.4 On the other hand, methanogenesis has its optimum for slightly alkaline pH. Two-stage AD appears to be very suitable for substrates with high insoluble COD (Chemical Oxygen Demand) contents like fruit and vegetable wastes, owing to the possibility of carrying out the stages in different environments. This operative mode increases stability, efficiency and conversion rate.5–9 The hydrolysis can be improved by inoculating the reactor with microorganisms producing hydrolytic enzymes specific for the substrate.7 However, in spite of these advantages, two-stage AD processes are not commonly applied in industrial application,10 probably because there is the need for more exhaustive studies to motivate higher plant investment. Recent interesting reviews on conventional, two-phase (high-rate) and temperature-phased anaerobic digestion processes are reported by Demirel et al.11 and Lv et al.12. The aim of this work is the proposal of a procedure to simulate an anaerobic sequencing benchscale reactor (ASBR). Solid potatoes, i.e. complex substrates, are employed as energy crops. A first stage process conducted by Saccharomyces cerevisiae (SC) yeasts was compared with a process led by the heterotrophic biomass grown on the same substrate, then both hydrolysed systems were sent to the acetogenic-methanogenic reactors, inoculated with an industrial anaerobic sludge and selected lyophilized anaerobic bacteria, to point out potentiality in biogas conversion and methane yield.13,14