Start-up strategies in manure-fed biogas reactors: Process parameters and methanogenic communities

• Cattle manure holds abundant and diverse methanogenic communities.
• CSTRs can be started up without seed, but manure and a slow temperature rise.
• This warrants a smooth start-up in terms of performance and community dynamics.
• Methanogenic communities in such reactors withstand upsets due to overload conditions.

Four start-up strategies were assayed in 75 dm3 continuous stirred-tank reactors to select the optimal conditions for the start-up of the demonstration BIO4GAS® plant. Two reactors were completely filled with manure and their temperature increased from 20 to 37 °C at either a slow or fast rate. The other two reactors were started at 37 °C with a seed sludge from a stably operating plant and their load (cattle manure) increased at a low or high rate. Reactor performance was monitored for 35 days. The composition and abundance of the methanogenic communities was determined using a phylogenetic microarray and quantitative PCR. All reactors performed successfully in terms of biogas production and experienced a steady start-up, with pH values above 7.3 and VFA/alkalinity ratios below 0.3, denoting stability. Similar methanogenic loads (averaging 5 × 107 g-1 of 16S rRNA gene copies) were detected in the cattle manure and seed sludge, however the methanogenic diversity was higher in the manure with dominance of Methanosarcina and subdominance of Methanocorpusculum and Methanobrevibacter. Starting-up the reactors with an initial full load of manure and progressively increasing the temperature entailed less changes in the sludge chemical environment and in the dynamics of the dominant methanogens. Using a rate of temperature increase of 0.61 K d−1 proved better than1.21 K d−1 with a methane yield of 103.8 dm3 kg−1 VS and a COD mass removal efficiency of 28.3%. The diverse methanogenic community in the manure easily adapted to reactor upsets due to forced overload conditions.