Fluoride removal from brackish groundwater by direct contact membrane distillation

The direct contact membrane distillation applied for fluoride removal from brackish groundwater was investigated. The self-prepared polyvinylidene fluoride membrane exhibited high rejection of inorganic salt solutes. The maximum permeate flux 35.6 kg/(m2 x hr) was obtained with the feed solution at 80 degrees C and the cold distillate water at 20 degrees C. The feed concentration had no significant impact on the permeate flux and the rejection in fluoride. The precipitation of CaCO3 would clog the hollow fiber inlets and foul the membrane surface with increasing concentration factor when natural groundwater was used directly as the feed, which resulted in a rapid decline in the module efficiency. This phenomenon was diminished by acidification of the feed. The experimental results showed that the permeate flux and the quality of obtained distillate kept stable before concentration factor reached 5.0 with the acidified groundwater as feed. The membrane module efficiency began to decline gradually when the feed continued to be concentrated, which can be mainly attributed to the formation of CaF2 deposits on the membrane surface. In addition, a 300 hr continuous fluoride removal experiment of acidified groundwater was carried out with concentration factor at 4.0, the permeate flux kept stable and the permeate fluoride was not detected.