Scaling control of forward osmosis-membrane distillation (FO-MD) integrated process for pre-treated landfill leachate treatment

Abstract

Membrane fouling is a major challenge for membrane-based wastewater treatment. In this study, we investigated the integrated forward osmosis-membrane distillation (FO-MD) process for landfill leachate treatment. However, severe scaling induced by the formation of calcium sulfate happened at the FO membrane selective layer. Herein, an organic phosphonic acid scale inhibitor, hexamethylene diamine tetra (methylene phosphonic acid) (HDTMPA), was added in draw solution (DS) to control the scaling behavior. HDTMPA chelated with Ca2+ to prevented the nucleation and growth of gypsum scale crystals and alleviated membrane scaling. For FO membrane cleaning, the water flux recovery rate was greater than 90% after three scaling-cleaning cycles owing to the formation of deformed crystals and loose scaling layer via addition of HDTMPA. The mechanism of scaling with the existence of HDTMPA were further explored. In addition, we conducted long-term FO-MD experiment using NaCl DS with HDTMPA. The efficiency of the integrated system was much superior to that of the FO alone system with an extreme high contaminant rejection of 99%. HDTMPA not only alleviated FO scaling, but also prevented wetting of the MD membrane, and then promoted the efficiency and operation stability of the integrated FO-MD system.