Lowering the fresh water footprint of cooling towers: A treatment-train for the reuse of discharged water consisting of constructed wetlands, nanofiltration, electrochemical oxidation and reverse osmosis

The reuse instead of discharge of cooling tower water can significantly lower the industrial fresh water footprint. The reuse of cooling tower water requires the removal of different chemical fractions from the cooling tower water, such as dissolved minerals, phosphate, organic carbon and industrial chemicals. However, there is no stand-alone water treatment technology that can remove all these fractions simultaneously. Therefore, the aim of this study was to assess the removal of these fractions with an innovative pilot-scale technology train that was fed with real cooling tower water consisting of green and grey technologies: constructed wetlands, nanofiltration, electrochemical oxidation and reverse osmosis. In addition, attention was paid to the fate of emerging industrial contaminant benzotriazole, the performance of the nanofiltration membranes and potential production of unwanted by-products by electrochemical oxidation. The experiments showed that 1) The treatment-train was capable of reaching the desired water quality; 2) Benzotriazole was completely removed by the constructed wetlands that acted as pre-treatment before nanofiltration as a result of biodegradation; 3) the production of inorganic carbon species in the constructed wetlands resulted in increased fouling of the subsequent nanofiltration membranes; 4) these membranes mainly retained divalent ions and did not retain monovalent ions, which resulted in a permeate stream whose EC was too high for reuse applications and therefore required further treatment by reverse osmosis; 5) electrochemical oxidation of the nanofiltration concentrate was more efficient than direct electrochemical oxidation of cooling tower water in terms of degradation of recalcitrant humic acids. In addition, less unwanted chlorinated by-products were produced as a result of different ions ratios due to retention of ions by the nanofiltration membranes. Before the full-scale application of the studied treatment train for cooling tower water treatment, it is recommended to identify strategies to improve the water recovery, to include innovative new nanofiltration membranes that can make the use of reverse osmosis redundant and to adopt new insights in preventing unwanted by-product formation during electrochemical oxidation.