Innovative Water Management Technology to Reduce Environment Impacts of Produced Water

Abstract

Clemson University with Chevron as an industry partner developed and applied treatment technology using constructed wetland systems to decrease targeted constituents in simulated and actual produced waters to achieve reuse criteria and discharge limits. Pilot-scale and demonstration constructed wetland treatment system (CWTS) experiments led to design strategies for treating a variety of constituents of concern (COCs) in produced waters including divalent metals, metalloids, oil and grease, and ammonia. Targeted biogeochemical pathways for treatment of COCs in pilot-scale CWTS experiments included divalent metal sulfide precipitation through dissimilatory sulfate reduction, metal precipitation through oxidation, reduction of selenite to insoluble elemental selenium, aerobic biodegradation of oil, nitrification of ammonia to nitrate, denitrification of nitrate to nitrogen gas, separation of oil using an oilwater separator, and sorption of ammonia to zeolite. Treatment performance results indicated that CWTSs can be designed and built to promote specific environmental and geochemical conditions in order for targeted biogeochemical pathways to operate. The demonstration system successfully achieved consistent removal extents even while inflow concentrations of COCs in the produced water differed by orders of magnitude. Design strategies used in the pilot-scale and demonstration CWTSs to promote specific conditions that can be applied to designing full-scale CWTSs include plant and soil selection, water-depth selection, addition of amendments, and hydraulic retention time (HRT). These strategies allow conditions within a CWTS to be modified to achieve ranges necessary for the preferred biogeochemical treatment pathways. In the case of renovating a produced water containing COCs that require different biogeochemical pathways for treatment, a CWTS can be designed with sequential cells that promote different conditions. For example, the pilot-scale CWTS for post-reverse osmosis produced water was designed to promote oxidizing conditions within the first wetland cell for nitrification of ammonia, and the subsequent three cells were designed to promote reducing conditions for denitrification of nitrate. By incorporating multiple wetland cells in a CWTS, the conditions within each cell can be modified for removal of specific COCs. In addition, a CWTS designed with multiple cells allows for convenient sample collection points so that biogeochemical conditions of individual cells can be monitored and performance evaluated. Removal rate coefficients determined from the pilot-scale CWTS experiments and confirmed by the demonstration system can be used to calculate HRTs required to treat COCs in full-scale CWTSs. The calculated HRTs can then be used to determine the surface area or "footprint" of a full-size CWTS for a given inflow rate of produced water.