Experimental evaluation on concentrating cooling tower blowdown water by direct contact membrane distillation

Concentration of simulated cooling tower blowdown water (CTBD) by the desalination process of direct contact membrane distillation (DCMD) has been evaluated. A bench-scale DCMD setup was used to test the desalination performance. Both silica-free and silica-containing simulated CTBD water were prepared and concentrated. The concentration process using DCMD produced a permeate flux of about 30 L/m2 · h and a salt rejection of more than 99.95% under feed side temperature of about 60 °C. Membrane scaling was examined for the simulated feeds. It was exhibited that insoluble calcium carbonate scale formed on membrane under concentration factor of about 3.7–4.0 for silica-free simulated CTBD water. Silica, calcium carbonate and sulfate scaling precipitated together under concentration factor of about 3.2–5.0 for silica-containing simulated CTBD water. The scales resulted in the drop of both permeate flux and salt rejection, while the performance was recovered after membrane cleaning. The addition of antiscalant enhanced the concentration factor to about 8.0 and the corresponding water recovery to about 87%. The experimental results indicated that MD is a potential feasible technology for the water recovery of CTBD water.

► MD process is feasible for desalting simulated cooling tower blowdown water.
► Conductivity limit during the concentration process was about 17 000 μS/cm.
► Above this concentration, large amount of CaCO3 scale formed on the membrane.
► Antiscalant was effective at preventing silica, carbonate and sulfate scale.
► A cleaning procedure was developed to recover membrane performance after scaling.