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A pilot-scale constructed wetland treatment system (CWTS) was designed and constructed for simulated fresh to hypersaline natural gas storage produced waters. The system incorporated three types of wetland reactors: freshwater reducing, freshwater oxidizing, and saltwater. Each type of reactor was designed specifically to promote biogeochemical reactions that transform or transfer targeted constituents. As the system acclimated, hydrosoil conditions developed, which sustained the biogeochemical reactions necessary for the function and performance of the system. The characteristics of hydrosoil in the freshwater reducing cells included redox conditions within the ideal range for sulfate reduction (150 to 250 mV), near-neutral pH, and relatively high concentrations of acid volatile sulfide (AVS). These conditions were favorable for removing metals from the simulated gas storage produced waters. In addition, fine-grained sediments in the freshwater reducing cells provided surfaces for binding metals, and organic matter supported sulfate-reducing bacteria by providing an energy source. As designed, the organic content and amount of AVS were elevated, and redox potential was less in the freshwater reducing cells than in the saltwater cells and freshwater oxidizing cells. The results of our study confirm that the hydrosoil of CWTSs can be designed to achieve specific conditions and functions, and that hydrosoil characteristics can be manipulated to attain the desired treatment performance. |
