Defining the best pre-treatment method for sewage sludge and corn straw

Biogas is an environment-friendly fuel representing an expanding field in the renewable energy sector. While some co-substrates are readily used by the microbiota, complex materials such as cellulose-rich fibres are not fully degradable and require a treatment prior to their introduction into the biogas plant. Here we optimize five different pre-treatment methods (I-V) for two representative co-substrates, corn straw and sewage sludge. We test their resulting methane potential according to a standardized procedure (VDI 4630) and compute an energy balance considering also the energy input for pre-treatments in order to identify the best method for each co-substrate. The pre-treatments include three traditional approaches (I) enzymes (II) steam explosion and (III) high temperature/sodium hydroxide treatment and two innovative methods, (IV) the hydro-thermal carbonisation (HTC) and (V) the implementation of anaerobic rumen fungi (Neocallimastigomycota) that increase biomethanation by degrading recalcitrant (hemi)celluloses through potent exo-enzymes. First results compare the three traditional pre-treatments and demonstrate a significant change in the overall methane potential. More precisely, pre-treatments led to +42% (II), +25.6% (III) and +9% (I) methane yield when applied to corn straw and +30 % (III), + 4.7% (I) and -5.5% (II) when applied to sewage sludge. Preliminary findings for the innovative techniques indicate that i) during HTC, a very auspicious liquid fraction for amendment to biogas reactors is generated and that ii) anaerobic fungi are capable of good growth on the corn straw substrate.