An investigation into fouling, wetting and their mitigation using commercial Antiscalant in membrane distillation desalination

Abstract

An experimental investigation into membrane fouling and loss of hydrophobicity was carried out in direct contact membrane distillation (DCMD) desalination of high salinity seawater. The membrane used was a commercial hydrophobic polypropylene flat-sheet with nominal pore size of 0.2 microns and a porosity of 73-75%. Extended runs of up to 91 hours in batch mode were conducted at various temperatures and flowrates, namely 50-70 ⁰C and 1.5-2.5 L/min respectively. Some runs consisted of injecting antiscalant. The results obtained showed that the membrane becomes significantly fouled at temperatures above 50 ⁰C often leading to significant distillate flux decline and a modest reduction of salt rejection, indicating that partial membrane wetting has occurred. It was found that the flux declined by 74% and 92% for temperatures of 60 ⁰C and 70 ⁰C, respectively at feed flowrate of 1.5 L/min. The effect of flowrate on the overall flux decline was not significant. The loss of hydrophobicity was studied using distillate conductivity and used membrane surface contact angle. In this work, both partial and surface membrane wetting were observed. The modest membrane wetting occurred at 70 ⁰C and 1.5 L/min and led to the lowest salt rejection of around 86%. However, at flowrate of 2.5 L/min regardless of the feed temperature studied, the salt rejection was higher than 99%. The morphology of the membrane and the chemical composition of the fouling components were studied using SEM and EDS. These were mainly carbonates of Ca and Mg and to a lesser extent, sulfates of Ca. Sodium chloride deposits and traces of silicates and iron were also observed. The presence of calcium sulfate in the membrane surface was associated with the lowest salt rejection experiments, indicating that this foulant could be responsible for the membrane wetting mechanism. Experiments performed with commercial antiscalant injected in the feed with a dose of 4 ppm showed that fouling and wetting were almost completely prevented. This work has successfully shown that fouling in membrane distillation desalination can be mitigated using commercial antiscalant. However, the importance of developing high performance membranes was recommended to reduce the demand of costly antiscalant injection.