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Biological sulfate (SO42−) reduction was examined in anaerobic sequential batch reactors (ASBRs) operated under different hydraulic retention times (HRTs) ranging from 12 to 36 h and COD (Chemical Oxygen Demand)/SO42−) ratios of 2.4, 1.6 and 0.8. Competition between SO42− reducing bacteria (SRBs), methane producing archaea (MPAs) and homoacetogens (HACs) was examined in controls and cultures treated with linoleic acid (LA). The ASBR performance was influenced by the COD/SO42− ratio in control cultures with a SO42− reduction of 87% at a COD/SO42− ratio of 0.8. At a 12 h HRT, in both control and LA treated cultures, greater than 75% SO42− removal was observed under all the conditions examined. In control reactors operating at a 36 h HRT, high levels of MPAs belonging to Methanobacteriales and Methanosarcinales were detected; however, in comparison, under low COD/SO42− ratio and with decreasing HRT conditions, a relative increase in SRBs belonging to Desulfovibrio and Desulfatibacillum was observed. Adding 0.5 g·L−1 LA suppressed Methanobacteriales, while increasing the LA concentration to 1 g·L−1 completely suppressed MPAs with a relative increase in SRBs. HACs belonging to Bacteroidetes were observed in the control and in cultures operated at 12 h HRT with a COD/SO42− ratio of 1.6 and fed 0.5 g·L−1 LA; however, with all other LA levels (0.5 and 1.0 g·L−1) and HRTs (12, 24 and 36 h), HACs were not detected.

Taguchi statistical design, an orthogonal array (OA) method, was used to study the impact of the COD/SO42− ratio, hydraulic retention time (HRT) and linoleic acid (LA) concentration on sulfate (SO42−) reduction in an anaerobic sequencing batch reactor using glucose as the electron donor. Based on the OA, optimum condition for maximum SO42− reduction was evaluated. Increasing the COD/SO42− ratio and HRT caused decreasing SO42− reduction while increased SO42− reduction was observed with increasing LA concentration (1 g L−1). In control (not fed LA) cultures, higher SO42− reduction (87% ± 3%) was observed at a low COD/SO42− ratio of 0.8. This indicates that increasing SO42− reduction was observed at increasing SO42− loading rates. In general, results from this study reveal that limiting the substrate concentration with high SO42− levels (low COD/SO42− ratio) favors high SO42− removal. Surface plots were used to evaluate the significant interactions between the experimental factors. Accuracy of the model was verified using an analysis of residuals. Optimum conditions for maximum SO42− reduction (97.61%) were observed at a COD/SO42− ratio of 0.8 (level 1), 12 h HRT (level 1) together with 1000 mg L−1 LA addition (level 3). In general, the Taguchi OA provided a useful approach for predicting the percent SO42− reduction in inhibited mixed anaerobic cultures within the factor levels investigated.

Due to the toxicity effects and endocrine disrupting properties of phenolic compounds, their removal from water and wastewater has gained widespread global attention. In this study, the photocatalytic degradation of phenolic compounds in the presence of titanium dioxide (TiO2) nano-particles and UV light was investigated. A full factorial design consisting of three factors at three levels was used to examine the effect of particle size, temperature and reactant type on the apparent degradation rate constant. The individual effect of TiO2 particle size (5, 10 and 32 nm), temperature (23, 30 and 37 °C) and reactant type (phenol, o-cresol and m-cresol) on the apparent degradation rate constant was determined. A regression model was developed to relate the apparent degradation constant to the various factors. The largest photocatalytic activity was observed at an optimum TiO2 particle size of 10 nm for all reactants. The apparent degradation rate constant trend was as follows: o-cresol > m-cresol > phenol. The ANOVA data indicated no significant interaction between the experimental factors. The lowest activation energy was observed for o-cresol degradation using 5-nm TiO2 particles. A maximum degradation rate constant of 0.0138 min−1 was recorded for o-cresol at 37 °C and a TiO2 particle size of 13 nm at a D-optimality value of approximately 0.98. The response model adequately related the apparent degradation rate constant to the factors within the range of factors under consideration.