Enhancing performance in anaerobic high-solids stratified bed digesters by straw bed implementation

Anaerobic high-solids single-stage stratified bed digesters have been found to be simple and flexible design candidates for small-scale reactors located in medium- to low-technology environments. In the present study, wheat straw was used as the starter material for the stratified bed. Upon green mass feeding, the anaerobically stabilised straw bed functioned both as a biofilm support and as a particulate filter. It enabled a direct onset of 7 kg VS m−3 batch loads, added twice a week, and permitted a low but consistent bed permeability during feeding at an average superficial flow velocity of 1 m d−1 to be achieved. Fed-batch tests with sugar beet tops in pilot- and laboratory-scale setups at an average loading rate of 2 kg VS m−3 d−1 resulted in average biogas production rates of 1.2–1.4 m3 m−3 d−1 and methane yields of 0.31–0.36 m3 kg−1 VSadded. At the end of the laboratory-scale feeding trial, the 200 day old straw bed had compacted to 50% of its initial volume, without any negative effects on performance being detectable.