USE OF FERROUS SULFATE TO REDUCE THE REDOX POTENTIAL AND ALLOW THE START-UP OF UASB REACTORS TREATING SLOWLY BIODEGRADABLE COMPOUNDS - APPLICATION TO A WASTE-WATER CONTAINING 4-METHYLBENZOIC ACID

During the anaerobic treatment of slowly biodegradable wastewaters con taining dissolved oxygen, the natural communities of aerobic and anaer obic facultative bacteria present in reactors' sludges may not succeed in lowering the redox potential of their environment to a level compa tible with methanogenesis. In order to allow reactor start-up, it is p roposed to add a single initial dose of FeSO4 directly to the reactor liquor, together with an inexpensive easily biodegradable compound (i. e. carbohydrates, agroindustrial wastes...). Under the action of sulp hate reducing bacteria, FeSO4 is reduced to FeS which in turn lowers r edox potential. This method was tested on an Upflow Anaerobic Sludge B ed (UASB) reactor fed at an hydraulic retention time of 5.3 +/- 0.3 da ys with an aerobic synthetic wastewater (dissolved O-2, 5 mg.l(-1); re dox potential referred to the standard hydrogen electrode (E(h)), +119 to +250 mV; pH 7 to 7.4; 6 degrees C) containing p-toluic acid (4-met hylbenzoic acid) as sole carbon and energy source. During the first 37 days of reactor operation, sludge E(h) was high (+118 to +161 mV; pH 7.2 to 7.4; 30 to 35 degrees C) and no COD (Chemical Oxygen Demand) re moval occured. Soon after the addition of FeSO4 (0.15 g FeSO4. 7H(2)O per g of Volatile Suspended Solids [VSS]) and glucose (0.38 g per g VS S), sludge E(h) decreased to -95 mV (pH 7.3; 33 degrees C) while COD r emoval increased to 51.3%. An economical way to implement this method at full scale is suggested.