Optimizing electrocoagulation pre-treatment efficiency during simultaneous treatment of different produced water streams in a FO-MD hybrid system

Abstract

In this study, the electrocoagulation (EC) process was explored and optimized for the pre-treatment of key contaminants present in hyper-saline produced water (PW) streams before their treatment through forward osmosis-membrane distillation (FO-MD) hybrid system. Due to their hyper-saline nature and the associated high temperatures, PW streams can be treated cost-effectively and sustainably using low-pressure dual membrane filtration systems like FO-MD hybrids. Desalter effluent (DE) stream was used as the FO feed solution, and WOSEP outlet (WO) stream served simultaneously as the FO draw solution and MD feed solution. The key foulants were identified in the WO and DE streams. Silicon, calcium, and sulfur were found as the key elements contributing towards FO and MD membrane fouling in the form of CaSO4, SiO2, and CaSiO3. Applying EC for 10 min with 100 mA current (7 mA/cm2 current density) at a neutral pH of 7 demonstrated the highest removal efficiencies for silicon (97 %), calcium (3.6 %), and sulfur (12.2 %) with only 0.023 kWh/m3 electric energy requirement. FO-MD hybrid efficiency was re-assessed with pre-treated WO and DE streams. The FO flux showed more stability and increased by 32.5 % and MD by 28.9 %, with a 10 % reduction in specific reverse solute flux. EC is thus a robust and effective process for hyper-saline hazardous PW pre-treatment and can substantially improve the FO-MD hybrid system efficiency.