Combining Forward and Reverse Osmosis for shale gas wastewater treatment to minimize cost and freshwater consumption

Abstract

Shale gas production requires significant water demand for well exploitation and a great volume of wastewater is generated since nearly 70 % of the drilling water returns to the surface [1], as flowback water (FBW) and produced water (PW), with different salinities. Their treatment has a double benefit: treated water can replace freshwater (FW) and besides waste volumes are reduced. Conventional desalination technologies can be appropriate for FBW but not for the hypersaline PW. Forward Osmosis (FO) is a promising alternative to deal with PW which can be used as a standalone desalination process or as an advanced pretreatment for other technologies [2]. In this work, we propose a superstructure that combines FO with Reverse Osmosis (RO). Its objective is twofold: to minimize FW consumption in well exploitation as well as the volume of final waste. The superstructure comprises a RO unit; two FO units; and mixers and splitters allowing connections between the units. In the figure, the FO1 and FO2 units act as pretreatments for the RO and as waste concentrators aiming for zero liquid discharge (ZLD). In the F01 unit, FBW is diluted and the sludge from the previous pretreatment (where other contaminants apart from Total Dissolved Solids (TDS) are removed) becomes concentrated. In the FO2 unit, the PW is diluted and the brine from the RO unit becomes concentrated. We formulated a bi-objective NonLinear Programming (NLP) problem that aims simultaneously to minimize the specific total cost ($/m3 drilling water) and the FW consumption (m3). The proposed approach is applied to a case study that uses 8500 m3/day of drilling water. The solution shows the trade-off between the cost and FW consumption and highlights the potential of FO to offer a solution for the treatment of the hypersaline PW and simultaneously reduce the shale gas waste volume.