PHYSICOCHEMICAL AND BIOLOGICAL PERFORMANCE OF EXPANDED GRANULAR SLUDGE BED REACTORS TREATING LONG-CHAIN FATTY-ACIDS

The influence of reactor hydrodynamics, temperature and co-substrate o n the performance of anaerobic digestion of long-chain fatty acids (LC FA) and sludge characteristics in expanded granular sludge bed (EGSB) reactors was investigated. Based on similar liquid superficial upflow velocities (upsilon(up)) of 3.4-4 mk in the absence of co-substrate, C OD removal efficiencies of 66 and 73% were attained in thermophilic (5 5 degrees C) runs at hydraulic retention times (theta) of 3 and 6 h re spectively. The corresponding mesophilic (30 degrees C) removal effici encies were 44 and 69%. The highest methane conversion achieved was on ly 15% in a thermophilic reactor operated at a hydraulic residence tim e (theta) of 6 h. A floating layer consisting of sludge granules and f at-like, non-degraded LCFA frequently clogged the gas collector. At a a of 0.6 h, both reactors failed due to severe washout of sludge granu les. When operated at a constant theta of 24 h, in the presence of glu cose and acetate, and at three upsilon(up), 4, 7 and 1 m/h, COD remova l efficiencies of 82-89% were obtained and no significant washout or f lotation of granules or fatty matter was observed. The higher upsilon( up) resulted in the lower methane conversion. Methane conversion ratio s 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 conv ersion ratio, 70%, was achieved at a upsilon(up) of 1 m/h at both temp eratures. Many white granules were present in the sludge bed due to cu mulative LCFA adsorption onto their surface. (C) 1998 Elsevier Science Ltd.