Effects of operational factors on the performance of a bio-ZVI system for enhancing anaerobic treatment of sulphate-rich wastewater

Despite the fact that zero valent iron (ZVI) has been used to improve anaerobic digestion (AD) processes, the ability of ZVI to improve AD treatment is dependent on operational factors that result in improved methane (CH4), which has been contradictory in different studies. In this study, the effects of ZVI concentration, chemical oxygen demand (COD), and sulphate (SO4 2- ), and the initial pH in the bio-ZVI system on the AD treatment of SO4 2- -rich wastewater were investigated to obtain key parameter effects on the performance of ZVI. The concentration of ZVI, COD, and SO4 2- , and the initial pH were varied using batch assays. The result shows the increase of ZVI concentration stimulates an increase of CH4 production. However, CH4 production was further reduced when the ZVI dosage was increased to 24 g/L due to the excessive production of OHions via the oxidation reaction of ZVI. Consequently, the pH values could not be in the suitable pH range for methane-producing archaea. Effects of the initial pH on the ability of ZVI, the initial pH was varied at 5, 6, and 7. The result shows the initial pH of 5 cannot improve the CH4 production whereas the initial pH of 6 or 7, the CH4 production was higher than in the control. The COD of 2 g/L (SO4 2- 1 g/L), 4 g/L (SO4 2- 2 g/L), and 6 g/L (SO4 2-3 g/L) were investigated to study the effects of COD concentration on the ZVI performance. The result showed that the bio-ZVI reactor of 6 gCOD/L was improved and the CH4 production level with and without ZVI addition were ca. 0.196 ± 0.002 and 0.058 ± 0.007 L/ gCODadded, respectively. The result indicated that the increased COD and SO4 2- concentration affected the increase of sulfide levels. When the pH was increased by ZVI oxidation, the level of undissociated hydrogen sulfide decreased, resulting in the CH4 production being higher than in the control. These results indicated that pH, COD and SO4 2- concentration play a key parameter for the ZVI performance in AD treatment of SO4 2- -rich wastewater