In the static granular bed reactor (SGBR), wastewater enters at the top of the reactor through the inlet flow distribution system and passes downward by gravity through the dense bed of active anaerobic granules. The downflow mode of operation allows the influent wastewater to be mixed with the bulk liquid by the countercurrent flow of biogas and liquid. Thus, high concentrations of organics in the influent wastewater are immediately dispersed and diluted. Biogas induced mixing sufficiently reduces dead volumes and short-circuiting and eliminates the need for a mechanical agitation mixing systems or recirculation pumping.