Predominance of syntrophic bacteria, Methanosaeta and Methanoculleus in a two-stage up-flow anaerobic sludge blanket reactor treating coffee processing wastewater at high organic loading rate.

•Effective anaerobic treatment of CPW using UASB reactors in series.•Effluent recirculation and proportion of nutrients provided operational stability.•Balanced amounts of acetate and hydrogen-utilizing methanogens at high OLR.•Proteobacteria and Candidatus Cloacimonetes increased in response to OLR variation.