Physicochemical and biological performance of expanded granular sludge bed reactors treating long-chain fatty acids

Abstract

The influence of reactor hydrodynamics, temperature and co-substrate on the performance of anaerobic digestion of long-chain fatty acids (LCFA) and sludge characteristics in expanded granular sludge bed (EGSB) reactors was investigated. Based on similar liquid superficial upflow velocities ( υ up) of 3·4–4 m/h in the absence of co-substrate, COD removal efficiencies of 66 and 73% were attained in thermophilic (55°C) runs at hydraulic retention times (θ) of 3 and 6 h respectively. The corresponding mesophilic (30°C) removal efficiencies were 44 and 69%. The highest methane conversion achieved was only 15% in a thermophilic reactor operated at a hydraulic residence time (θ) of 6 h. A floating layer consisting of sludge granules and fat-like, non-degraded LCFA frequently clogged the gas collector. At a θ of 0·6 h, both reactors failed due to severe washout of sludge granules. When operated at a constant θ of 24 h, in the presence of glucose and acetate, and at three θ up, 4, 7 and 1 m/h, COD removal efficiencies of 82–89% were obtained and no significant washout or flotation of granules or fatty matter was observed. The higher θ up resulted in the lower methane conversion. Methane conversion ratios of 49% (4 m/h) and 39% (7 m/h) in thermophilic, and 59% (4 m/h) and 53% (7 m/h) in mesophilic runs were obtained. The highest methane conversion ratio, 70%, was achieved at a θ up of 1 m/h at both temperatures. Many white granules were present in the sludge bed due to cumulative LCFA adsorption onto their surface.