Membrane preconcentration as an efficient tool to reduce the energy consumption of perfluorohexanoic acid electrochemical treatment
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One of the key points for the large-scale implementation of electrochemical water treatment technologies lies in the need of reducing the energy consumption. The present work analyzes the removal of persistent perfluorohexanoic acid (PFHxA, 204 mg L−1) from industrial process waters using a strategy that combines membrane pre-concentration followed by electrooxidation of the concentrate. A mathematical model describing the nanofiltration (NF) system was developed and complemented with new and background experimental data of PFHxA and ion species rejections and total permeate flux through the NF270 and NF90 membranes. Similarly, the kinetics of PFHxA electrolysis on boron doped diamond anodes was determined at laboratory scale. Later, the model was used to simulate the NF-ELOX integrated process, where a commercial spiral wound unit (membrane area 7.6 m2) was implemented and the electrooxidation unit was scaled-up to pilot plant (anode area 1.05 m2). The obtained energy savings depended on a combination of the target PFHxA removal ratio at the end of the treatment train, the separation performance of the commercial membrane and the reduction of the electrolyte ohmic resistance in the electrooxidation stage, that was attained as a result of the increase of salts content in the concentrate. Only the tight NF90 membrane allowed to achieve high (99%) PFHxA removal ratios in the integrated NF-ELOX process, and the specific energy consumption was estimated at 11.6 kWh m−3, 59.2% less than when electrolysis alone was applied. Still, the electrolysis is the most energy demanding step, with 85.9% contribution to the total energy consumption. The strategy of combining membrane pre-concentration with electrochemical degradation could be extended to the treatment of other highly persistent organic compounds.