THE TREATMENT OF IRON OXALATE LEACH LIQUORS IN A UASB WITH SULFATE REDUCTION

The presence of small amounts of iron (> 0.013% Fe) in sand creates pr oblems in the manufacture of high quality glass. Removal by hot sulphu ric acid is possible, but creates environmental problems, and is costl y. Hence organic acids such as oxalic have been investigated since the y are effective in removing iron, and can be degraded anaerobically. T he aim of this work was to identify key intermediates in the anaerobic degradation of oxalate in an upflow anaerobic sludge blanket reactor (UASB) which was removing iron from solution in the sulphide form, and to determine the bacterial species involved. 2-bromoethanesulfonic ac id (BES) and molybdenum were selected as suitable inhibitors for metha nogenic and sulphate reducing bacteria (SRB) respectively. 40mM molybd enum was used to inhibit the SRB in a reactor with a 12hr HRT. Total S RB inhibition took place in 20 hrs, with a complete breakthrough of in fluent sulphate. The lack of an immediate oxalate breakthrough confirm ed Desulfovibrio( vulgaris subspecies oxamicus was not the predominant oxalate utilising species. Nevertheless, high concentrations of molyb denum were found to inhibit oxalate utilising bacteria in granular rea ctors but not in suspended population reactors; this observation was p uzzling, and at present cannot be explained. Based on the intermediate s identified, it was postulated that oxalate was degraded to formate b y an oxalate utilising bacteria such as Oxalobacter formigenes, and th e formate used by the SRBs to reduce sulphate. Acetate, as a minor int ermediate, existed primarily as a source of cell carbon for oxalate ut ilising bacteria. Methanogenic inhibition identified that 62% of the C H4 in the reactor operated at 37 degrees C originated from hydrogenotr ophic methanogenesis, whilst this figure was 80% at 20 degrees C. Poss ible irreversible effects were recorded with hydrogenotrophic methanog ens. (C) 1997 IAWQ. Published by Elsevier Science Ltd.