The effect of ‘High-pH pretreatment’ on RO concentrate minimization in a groundwater desalination facility using batch air gap membrane distillation

Abstract

This study compared the efficacy of two different scale control strategies: (i) ‘High-pH pretreatment’; and (ii) antiscalant (AS) addition for reverse osmosis (RO) concentrate minimization, in a lab-scale air gap membrane distillation (AGMD) unit. In contrast with previous studies that used a direct contact membrane distillation set-up, we systematically investigated the performance of batch AGMD configuration with and without the preliminary reduction of scale-forming constituents, using high salinity, high alkalinity, high SiO2 and high magnesium hardness RO concentrate. Results indicated that ‘High-pH pretreatment’ by means of the cold lime-soda ash process was more productive than only AS addition to the RO concentrate, resulting in significant salt precipitation when the concentration factor (CF) in the AGMD system increased above 1.3. The High-pH precipitation process provided significant concentration reduction of SiO2 (96%), magnesium (96%) and calcium (86%). Following ‘High-pH pretreatment’, pH re-adjustment and final AS addition, the use of AGMD allowed us to minimize the existing RO concentrate with an initial total dissolved solids (TDS) level of 10.8 g/L by a CF of 3.2. In addition, this chemical demineralization process enabled operation of the AGMD unit at a higher temperature, thus increasing permeate flux.