Sulfate in effluent is a challenging issue for wastewater reuse around the world. In this study, sulfur (S) removal and transformation in five batch constructed wetlands (CWs) treating secondary effluent were investigated. The results showed that the presence of the plant cattail (Typha latifolia) had little effect on sulfate removal, while the carbon-rich litter it generated greatly improved sulfate removal, but with limited sulfide accumulation in the pore-water. After sulfate removal, most of the S was deposited with the valence states S (-II) and S (0) on the iron-rich gravel surface, and acid volatile sulfide was the main S sink in the litter-added CWs. High-throughput pyrosequencing revealed that sulfate-reducing bacteria (i.e. Desulfobacter) and sulfide-oxidizing bacteria (i.e. Thiobacillus) were dominant in the litter-added CWs, which led to a sustainable S cycle between sulfate and sulfide. Overall, this study suggests that recycling plant litter and iron-rich filling material in CWs gives an opportunity to utilize the S in the wastewater as both an electron acceptor for sulfate reduction and as an electron donor for nitrate reduction coupled with sulfide oxidation. This leads to the simultaneous removal of sulfate, nitrate, and organics without discharging toxic sulfide into the receiving water body.
The microseepage of natural gas from subsurface hydrocarbon reservoirs is a widespread process in petroleum basins. On a global scale, microseepage represents an important natural source of atmospheric methane (CH4). To date, microseepage CH4 flux data have been obtained from ~20 petroleum systems in North America, Europe, and Asia. While the seasonal variations of gas flux due to soil methanotrophic activity are known, the role of geological factors in controlling gas fluxes has been poorly investigated. Here we present new microseepage data from the Dawanqi oil‐gas field located within the Tarim Basin (China), a petroleum system characterized by intense faulting and shallow (<700 m) reservoirs. We measured CH4 fluxes from the ground at 51 sites along three transects by using a closed‐chamber connected to a portable gas sensor using off‐axis integrated cavity output spectroscopy. Our results indicate that the highest CH4 fluxes occur over faults and/or shallow reservoirs, especially those that were not developed and that have higher fluid pressures. Microseeping CH4 is thermogenic, like that occurring within the Dawanqi reservoirs, as demonstrated by 13C enrichment (δ13C from −46.3‰ to −30.7‰) in the chamber. Mean and range microseepage values (17 mg m−2d−1; from −1.4 to 330 mg m−2d−1) are similar to those reported for other petroleum fields with active tectonics. Our results confirm that dry soil over petroleum fields can be a net source of atmospheric CH4 and its flux is primarily controlled by faulting, and reservoir depth and pressure. These factors shall be considered in global bottom‐up seepage emission estimates.
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