Pilot-Scale Engineered Wetland Design for Produced Water Treatment

Abstract

Summary Engineered wetlands are a promising technology for treatment of oilfield and gasfield produced water. The engineering optimization of natural treatment processes, adapted from chemical engineering reactor design principles, shows that engineered wetlands are a viable alternative to mechanical produced water treatment systems. Engineered wetlands incorporate a subsurface flow gravel bed reactor, lined with an impermeable liner and equipped with an aeration system to enhance oxygen delivery. Design parameters include biodegradation rate coefficients for biochemical oxygen demand (BOD) and individual organic compounds, produced water flowrate, temperature, and influent and required effluent concentrations. An oil and gas company in the Caspian region is seeking to use engineered wetlands for treatment of oilfield and gasfield produced water. The oilfield produced water concentrations are high in total dissolved solids (TDS: 43048 mg/L), BOD (14693 mg/L), and oil and grease (1213 mg/L) and moderately high in total benzene, toluene, ethylbenzene, and xylenes (BTEX; 3.067 mg/L). The gasfield produced water concentrations are moderately high in TDS (16110 mg/L), BOD (9910 mg/L), and oil and grease (557 mg/L) and high in total BTEX (12.393 mg/L). Pilot-scale engineered wetland treatment systems have been designed on a flowrate basis of 100 m3/d using the first-order kinetic mass reaction model recently published by Kadlec and Wallace (2009). This model uses a modified tanks-in-series number that accounts for both hydraulic effects and weathering effects in the biodegradation rate. Hydraulic effects are caused by dispersion in the saturated gravel bed. Variation in the biodegradation rate is because of rapid biodegradation at the upgradient portion of the engineered wetland, in which short chain/low molecular weight organics are oxidized at rapid rates, and slower biodegradation at the downgradient portion of the engineered wetland, in which long chain/higher molecular weight organics are oxidized at lower rates. This advanced design process, combined with BOD surface area loading criteria, enables the optimization of engineered wetland design for produced water treatment.