Reduction of total dissolved solids of pre-treated flowback water by two-stage nanofiltration: A gel layer based model to predict the system performance and scale up

Abstract

Huge quantities of water with additives are required in the hydrofracking process employed for shale gas extraction. The flowback water returned to the surface after the operation is rich in total dissolved solids (TDS) and the organics. An appropriate pre-treatment can remove the organics and a suitable membrane system can be used to reduce the TDS. In this work, a commercial nanofiltration flat sheet membrane was used in cross-flow mode to lower the TDS of the pre-treated flowback water. Two stages of nanofiltration were carried out successfully to bring down the TDS to 3.45 g/l for the first stage and 1.85 g/L for the second stage which is below the desired level prescribed by the Bureau of Indian Standards. The membranes showed an overall rejection of 73% for TDS. The maximum rejection of TDS was obtained at 758 kPa transmembrane pressure (TMP) and 120 L/h cross flow rate (CFR) for both the stages. At the optimum operating condition, the membrane showed a flux decline ratio (FDR) of 0.35 and 0.31 along with an initial flux recovery of 80% and 85% for both the stages, respectively indicating the membrane to have a good anti-fouling property. A transient two-component gel layer controlled model was implemented for the quantification of the membrane performance for both the filtration stages. The gel layer thickness after one hour operation was found to be 19.2 µm for stage 1 and 8.8 µm for stage 2 at the optimum operating condition. The operating parameters, cross flow rate and transmembrane pressure drop were optimized using the criteria of the minimum energy consumption. The optimal operating curves were thus generated for a filtration area of 1 m2 and a filtration volume of 10 m3 and for three different final permeate concentrations of 1.2 g/L, 1.5 g/L and 1.8 g/L. Thus, using the predicted curves, one can find out the optimum TMP, CFR and area required for a target permeate concentration after the second stage of filtration where the total specific energy of the system will be minimum.