Granulation of biomass in thermophilic upflow anaerobic sludge blanket reactors treating acidified wastewaters

The development of granular sludge in thermophilic (55°C) upflow anaerobic sludge blanket reactors was investigated. Acetate and a mixture of acetate and butyrate were used as substrates, serving as models for acidified waste‐waters. Granular sludge with either Methanothrix or Methanosarcina as the predominant acetate utilizing methanogen was cultivated by allowing the loading rate to increase whenever the acetate concentration in the effluent dropped below 200 and 700 mg COD/L, respectively. The highest methane generation rates, up to 162 kg CH4‐COD/m3 day, or 2.53 mole CH4/L day, were achieved at hydraulic retention times down to 21 min, with granules consisting of Methanothrix. The formation of Methanothrix granules did not depend on the type of seed material, nor on the addition of inert support particles. The growth of granules proceeded rapidly with adapted seed material, even when the reactors were inoculated with low concentrations. With mesophilic seed materials growth of granules took much longer. Thermophilic Methanothrix granules strongly resemble mesophilic granules of the “filamentous” type. Some factors governing the thermophilic granulation process are discussed.