Integration of accelerated precipitation softening with membrane distillation for high-recovery desalination of primary reverse osmosis concentrate

Accelerated precipitation softening (APS) was integrated with direct contact membrane distillation (DCMD) to establish a desalination process for high-recovery desalting of primary reverse osmosis (PRO) concentrate. The APS process, between the PRO concentrate and the DCMD process, involved pH adjustment with sodium hydroxide along with calcite seeding, followed by microfiltration to avoid seeds clogging of the DCMD module. The PRO concentrate was obtained from the RO process of the direct drinking water preparation system designed for the 29th Olympic Games with 50% recovery. pH adjustment to 10.10, 5 g/L calcite dosage and 200 r/min agitation rate was determined for APS treatment via initial small-scale calcium removal tests. Elemental analysis revealed that APS treatment enabled 92% removal of calcium, 58.4% removal of total hardness, 4.4% removal of magnesium, 1.1% removal of sulfate and 1.6% removal of silica. Compared with the sharp decline found in the DCMD process of the PRO concentrate, the permeate flux declined only 20% within 300 h running after APS treatment. Then the PRO concentrate was concentrated 40 times and the whole recovery was enhanced to 98.8%. The permeate conductivity varied between 2.0 and 4.0 μS/cm during the DCMD process, and slightly increased to 6.0 μS/cm at the end due to partial wetting phenomenon.