Thermophilic sulphate and sulphite reduction in lab-scale gas-lift reactors using H sub(2) and CO sub(2) as energy and carbon source

Feasibility of thermophilic (55 degree C) sulphate and sulphite reduction with H sub(2) and CO sub(2) gas-mixtures was studied in gas-lift reactors, which contained pumice particles as carrier material. Particular attention was paid to biomass retention and the competition between hydrogenotrophic sulphate-reducers and other hydrogenotrophic thermophiles. A model medium with defined mineral nutrients was used. The results of the experiments clearly demonstrate that sulphate conversion rates up to 7.5 g SO sub(4) super(2-)/L per day can be achieved. With sulphite, a reduction rate of 3.7 g S/L per day was obtained, which equals a sulphate conversion rate of 11.1 g SO sub(4) super(2-)/L per day. Under the applied conditions, a strong competition for hydrogen between hydrogenotrophic sulphate-reducers, tentatively designated as Desulfotomaculum sp., and hydrogenotrophic methanogens was observed. The outcome of the competition could not be predicted. Growth of the mixed culture was totally inhibited at an H sub(2)S concentration of 250 mg/L. Poor attachment of sulphate-reducing bacteria was observed in all experiments. The biomass concentration did not exceed 1.2 g/L, despite the presence of 50 g/L of pumice. The reason for this phenomenon remains to be understood.