The feasibility of a near-complete and biosafe conversion of human- and food waste into biogas was investigated in the context of ESA’s MELiSSA loop (Micro Ecological Life Support System Alternative). The treatment comprises of a series of processes, i.e. a mesophilic lab-scale CSTR (continuously stirred tank reactor), an upflow biofilm reactor, a fibre liquefaction reactor containing the rumen bacterium Fibrobacter succinogenes and a hydrothermolysis system in near-critical water. In the one-stage CSTR, a biogas yield of 75% with a specific biogas production of 0.37 L biogas g-1 added VS (volatile suspended solids) at a HRT (hydraulic retention time) of 15 to 25 days was obtained. When the SRT (solid retention time) was uncoupled from the HRT, and all solids were completely retained in the methane reactor, a more complete biogas conversion was observed at a SRT of above 20 days, corresponding to a 10% increase of degradation on a total COD basis. Solids present in the CSTR effluent were subsequently treated in two ways. Firstly, hydrothermal treatment at temperatures of 250 to 350°C resulted in extensive liquefaction and a complete sanitation of the waste. The effluent of the hydrothermal treatment was subjected to a second methanogenesis, originally using a low-load methanogenic biofilm reactor (MBR), and showed extra methane production. The hydrothermal treatment produced no compounds inhibitory to methanogenesis. Additional conversion of up to 60% of the remaining COD was thus achieved. Secondly, application of the cellulolytic Fibrobacter succinogenes converted the remaining cellulose contained in the CSTR effluent into mainly acetate and propionate. Subsequent anaerobic digestion of the Fibrobacter hydrolysate in the upflow biofilm reactor yielded up to 30% extra conversion of the remaining COD.
Thus, the biogas in the sequence CSTR – Fibrobacter – MBR resp. CSTR – Hydrothermal – MBR corresponds with conversions of more than 90% of the original organic matter. Combining a long solid retention system in the main CSTR and hydrothermal treatment seems most promising to obtain optimall conversion and sanitation of the waste.