Effects of electron acceptors on sulphate reduction activity in activated sludge processes

Abstract

The concentration of sulphate present in wastewater can vary from 10 to 500 mg SO₄^2-/L. During anaerobic conditions, sulphate is reduced to sulphide by sulphate-reducing bacteria (SRB). Sulphide generation is undesired in wastewater treatment plants (WWTPs). Previous research indicated that SRB are inhibited by the presence of electron acceptors (such as O₂, NO₃ and NO₂). However, the contact times and concentrations used in those studies are by far higher than occur in WWTPs. Since sulphide can influence the biological nitrogen and phosphorus removal processes, this research aimed to understand how the different electron acceptors commonly present in biological nutrient removal (BNR) systems can affect the proliferation of SRB. For this purpose, a culture of SRB was enriched in a sequencing batch reactor (approx. 88% of the total bacteria population). Once enriched, the SRB were exposed for 2 h to typical concentrations of electron acceptors like those observed in BNR systems. Their activity was assessed using three different types of electron donors (acetate, propionate and lactate). Oxygen was the most inhibiting electron acceptor regardless the carbon source used. After exposure to oxygen and when feeding acetate, an inactivation time in the sulphate reduction activity was observed for 1.75 h. Once the sulphate reduction activity resumed, only 60% of the original activity was recovered. It is suggested that the proliferation of SRB is most likely to occur in BNR plants with an anaerobic fraction higher than 15% and operating at sludge retention times higher than 20 days (at a temperature of 20 °C). These results can be used to implement strategies to control the growth of sulphate reducers that might compete for organic carbon with phosphate-accumulating organisms.

Keywords: Biological nutrients removal; Electron acceptor inhibition; Sulphate reduction activity; Sulphate-reducing bacteria; Wastewater treatment.

Fig. 1

Sulphide (circles), sulphate (diamonds), acetate (triangles), propionate (plus signs), lactate (multiplication signs) and soluble COD (dashes) profiles of the SRB enrichment culture observed during a typical cycle in the parent reactor

Fig. 2

FISH microbial characterization of the biomass present in the parent reactor: a DAPI (all living organism), b EUB MIX (all bacteria), c SRB385 (most sulphate reducers) and d DBB660 (Desulfobulbus). e Overlap of a–d

Fig. 3

DGGE band pattern and phylogenetic tree of the biomass enriched in the parent reactor

Fig. 4

Profiles observed in the control tests showing the concentrations of sulphide (circles), sulphate (diamonds), acetate (triangles), propionate (plus signs), lactate (multiplication signs), soluble organic COD (dashes) profiles in the control test performed with a lactate, b propionate or c acetate as carbon source

Fig. 5

Likelihood of lactate sulphate reducers growing in a BNR plant at different anaerobic contact times (1 to 2 h), anaerobic fractions (5 to 20%) and 8 days (blue), 15 days (orange), 20 days (grey) or 25 days (yellow) SRT at 20 C. Considering that the sludge is flocculent, i.e., there is no limitation of substrate and considering a minimum lag phase of 0.4 h for the SRB as observed in this study (Colour figure online)