Energetic feasibility of a two-stage anaerobic digestion system compared to a single-stage system treating whey and glycerin

Splitting anaerobic digestion processes into two reactors may be biologically advantageous, but their separation in two stages significantly increases energetic costs. Therefore, the aim of this work was to study the energetic feasibility of a two-stage system producing hydrogen and methane by anaerobically co-digesting whey and glycerin in a sequencing batch reactor. The highest methane productivity (185.9 molCH(4).m(-3).d(-1)) was achieved when the reactor was operated in fed-batch mode at 11.8 kgCOD.rn(-3).d(-1). The energy produced by treating all the whey from a dairy industry was estimated. We proposed a system of three reactors operating in parallel (194.4 m(3) each) that produces hydrogen continuously, followed by three other reactors of 5.2 m(3) each, also in parallel, to produce methane from the effluent of the acidogenic reactors. The total energetic yield of the two-stage system would be 7.0 MJ.kgCOD(removed)(-1) and the monthly savings could reach up to US$ 8170, which leads to a technically feasible system. However, care must be taken when using a two-stage system for treatment of easily biodegradable substrates (whey and glycerin), because it will only have a better overall energetic yield if the single-stage system has a poor performance, i.e. a yield of 8.8 molCH(4).kgCOD(removed)(-1) or less.