Concentrating primary reverse osmosis concentrate by direct contact membrane distillation

In the present work, a thermally driven membrane process-direct contact membrane distillation was investigated for concentrating the primary reverse osmosis concentrate to minimize the water loss. The primary reverse osmosis concentrate used in the work was obtained from the RO process of the direct drinking water preparation system designed for the 29th Olympic Games with 50% recovery. The feed water was Beijing tap water. Membrane fouling during the DCMD process was discussed, and acidification and accelerated precipitation softening was used as the pretreatment method to control the calcium scaling. Experimental results showed that acidification can alleviate and even eliminate CaCO₃ scaling, but CaSO₄ crystallization still occurred and led to a sharp decline of module efficiency. Accelerated precipitation softening enabled a high removal efficiency of Ca^2 +, so both the CaCO₃ and CaSO₄ scaling was efficiently controlled. After proper pretreatment to control calcium scaling, the primary reverse osmosis concentrate could be concentrated 40 times and then the whole recovery was enhanced to 98.8%.