Dual-stage forward osmosis/pressure retarded osmosis process for hypersaline solutions and fracking wastewater treatment Ali Altaee a,1 , Nidal Hilal b, ⁎ a Faculty of Engineering and Physical Sciences, University of West of Scotland, Paisley PA1 2BE, UK b Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, UK HIGHLIGHTS • Dual-stage FO/PRO is proposed for hypersaline water treatment and power generation. • Two designs were suggested: PRO–FO and FO–PRO systems. • PRO–FO system generates higher power than the FO–PRO system. • Increasing draw solution flow rates increased the permeate flow rate and TDS. • Treated hypersaline water is suitable for RO treatment or discharge to sea. abstract article info Article history: Received 26 March 2014 Received in revised form 8 July 2014 Accepted 10 July 2014 Available online xxxx Keywords: Hypersaline solution Forward osmosis Pressure retarded osmosis Fracking wastewater Hypersaline solution with high TDS is not suitable for direct treatment by the conventional membrane and ther- mal processes. The current study proposes a dual-stage FO/PRO process for hypersaline solution treatment and power generation. The treatment process reduces the concentration of saline wastewater and hence renders it suitable for disposal directly to sea or treatment by the conventional membrane and thermal processes. The draw and feed solutions in the FO process were the hypersaline solutions and wastewater effluent, respectively. Five concentrations were evaluated for the process treatment with different concentrations ranging from 53 g/L to 157 g/L. The performance of FO membrane was estimated using pre-developed computer software. The results showed that a higher power can be generated from the PRO-FO system than from the FO-PRO system without compromising the concentration of hypersaline solution after dilution. The study also showed that although in- creasing the flow rate of draw solution resulted in an increase in the permeate flow rate, it caused a reduction in the dilution of draw solution. On the other hand, the study showed a negligible improvement in the performance of FO membrane upon increasing the feed solution flow rate. Finally, the simulation results showed that the con- centration of diluted draw solution was suitable for the conventional membrane and thermal treatments or dis- charge to seawater after the dual-stage FO membrane treatment. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Hypersaline solution is a wastewater generated from industrial activities such as oil and gas industries. It is characterized by the high- concentration of Total Dissolved Solids (TDS) which is normally more than seawater concentration of 35 g/L. Such wastewater is difficult to treat by the conventional water treatment technologies such as Reverse Osmosis (RO) and Bioreactors [1–5]. Shale gas industry is one of the ac- tivities which largely contribute to hypersaline wastewater generation. Typically, water and additives are injected at high pressure into the gas well so as to open fractures in the shale and develop a flow path for the gas to escape. Despite the rapid growth in shale gas industries over the last few years, the capacity for treating and handling of the fracking wastewater has remained underdeveloped [1,2]. The characteristics and composition of fracking wastewater vary from place-to-place and time-to-time through- out the production cycle. Drilling water, for instance, contains rock cuttings which are carried back to the land surface while flowback water contains high concentrations of additives. The volume of the flowback has been reported to vary from 1500 m 3 to 4500 m 3 per well per week, but decreases with time upon the completion of fracking operation [6]. In ad- dition, there is a large volume of production water which is collected dur- ing the production life of the gas well. Practically, production water is retained in the gas well and exposed to the shale formation for long peri- od of time [6]. Fracking wastewater, in general, contains large amounts of suspended solids, high salinity (TDS), fluid additives, and other naturally occurring metal ions [1,3,5]. TDS is of particular importance because of its Desalination 350 (2014) 79–85 * Corresponding author. E-mail address: ali.altaee@uws.ac.uk (A. Altaee). 1 Tel.: +44 7986517994. http://dx.doi.org/10.1016/j.desal.2014.07.013 0011-9164/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal