FEASIBILITY OF EXPANDED GRANULAR SLUDGE BED REACTORS FOR THE ANAEROBIC TREATMENT OF LOW-STRENGTH SOLUBLE WASTEWATERS

The application of the expanded granular sludge bed (EGSB) reactor for the anaerobic treatment of low-strength soluble wastewaters using ethanol as a model substrate was investigated in laboratory-scale reactors at 30 degrees C. Chemical oxygen demand (COD) removal efficiency was above 80% at organic loading rates up to 12 g COD/L . d with influent concentrations as low as 100 to 200 mg COD/L. These results demonstrate the suitability of the EGSB reactor for the anaerobic treatment of low-strength wastewaters. The high treatment performance can be attributed to the intense mixing regime obtained by high hydraulic and organic loads. Good mixing of the bulk liquid phase for the substrate-biomass contact and adequate expansion of the sludge bed for the degassing were obtained when the liquid upflow velocity (V-up) was greater than 2.5 m/h. Under such conditions, an extremely low apparent K-s value for acetoclastic methanogenesis of 9.8 mg COD/L was observed. The presence of dissolved oxygen in the wastewater had no detrimental effect on the treatment performance. Sludge piston flotation from pockets of biogas accumulating under the sludge bed occurred at V-up lower than 2.5 m/h due to poor bed expansion. This problem is expected only in small diameter laboratory-scale reactors. A more important restriction of the EGSB reactor was the sludge washout occurring at V-up higher than 5.5 m/h and which was intensified at organic loads higher than 7 g COD/L . d due to buoyancy forces from the gas production. To achieve an equilibrium between the mixing intensity and the sludge hold-up, the operation should be limited to an organic loading rate of 7 g COD/L . d and to a liquid upflow velocity between 2.5 and 5.5 m/h. (C) 1994 John Wiley and Sons, Inc.