Biological sulphate reduction with synthesis gas

The objectives of this thesis are (1) to study the feasibility of using synthesis gas as electron donor and carbon source for biological sulphate reduction and (2) to develop criteria for design and operation of gas- lift bioreactors for sulphate reduction using immobilized biomass.

At appeared that sulphate-reducing bacteria, grown on H ₂ /CO ₂ , formed stable biofilms on pumice particles. Biofilm formation was not observed when basalt particles were used. However, use of basalt particles led to the formation of aggregates of sulphate-reducing biomass. The sulphate-reducing bacteria grown on pumice particles easily adapted to free H ₂ S concentrations up to 450 mg/L. These high free H ₂ S concentrations caused reversible inhibition rather than acute toxicity. When free H ₂ S concentrations were kept below 450 mg/L. a maximum sulphate conversion rate of 30 g SO₄^2-/L.d could be achieved after only 10 days of operation. Gas to liquid mass transfer capacity of the reactor determined this maximum sulphate conversion rate.

Furthermore, biological sulphate reduction appeared to be applicable within a pH range of 5.5 to 8.0, with an optimum near pH 7.5. The pH affected aggregate configuration and diameter. At pH 7.0, the average Sauter mean diameter of the aggregates was 1.5 mm. Moreover, phase-contrast and SEM microscopy showed highly branched aggregate surfaces. A pH increase led to increased surface irregularity without affecting the particle diameter. A pH decrease caused a decreased surface irregularity and changed the aggregate Sauter mean diameter from 1.50 mm at pH 7.0 to 2.26 at pH 5.5. However, the pH did not have a significant effect on the biomass composition. Examination of the bacterial composition of the aggregates by phase-contrast microscopy, SEM microscopy as well as enrichments showed that at all pHs Desulfovibrio sp. and Acetobacterium sp. were the most abundant micro-organisms.

When sulphate reduction was carried out with synthesis gas as electron donor and carbon source, the reactor performance was strongly affected. Addition of 5% CO negatively affected the overall sulphate conversion rate, i.e. it dropped from 12 - 14 g SO₄^2-/L per day to 6 - 8 g SO₄^2-/L per day. However, a further increase of CO to 10 and 20% did not further deteriorate the process. With external biomass recycling the sulphate conversion rate could be improved to 10 g SO₄^2-/L per day. Therefore biomass retention clearly could be regarded as the rate limiting step. Furthermore, CO affected the aggregate shape and diameter. SEM photographs showed that rough aggregates, pre-grown on H ₂ /CO ₂ , changed into smooth aggregates upon addition of CO. Addition of CO also changed the aggregate Sauter mean diameter (d32) from 1.7 mm. at 5% CO to 2.1 mm at 20% CO. After addition of CO, a layered biomass structure developed. Acetobacterium sp. were mainly located at the outside of the aggregates, whereas Desulfovibrio sp. were located inside the aggregates.

Additionally, thermophilic (55 °C) sulphate and sulphite reduction was studied. The results of the experiments clearly demonstrated that sulphate conversion rates up to 7.5 g SO₄^2-/Lper 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 SO₄^2-/L per day. Under the applied conditions, a strong competition for hydrogen between hydrogenotrophic sulphate-reducers, designated as Desulfotomaculum sp., and hydrogenotrophic methanogens was observed. Growth of the mixed culture was totally inhibited at a H 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.

Based in the abovementioned results it is concluded that both aims of the thesis are attained. First, biological sulphate reduction appears to be feasible. Additionally, a number of criteria for design and operation of gas-lift bioreactors for sulphate reduction were developed and discussed. Finally, several recommendations for future research are given.