Membrane distillation is a potential technology to recover more water from the RO brine concentrate for the purpose of high water recovery and zero liquid discharge. However, since calcium phosphate scaling has been widely reported in RO applications, it could also hinder high feed water recovery in the reclamation of wastewater by membrane distillation. In this study, the calcium phosphate saturated feed was used to mimic the RO concentrate, and the fouling mechanism of calcium phosphate on the membranes in direct contact membrane distillation (DCMD) was investigated. Membrane fouling was quantified by permeate flux decline and membrane wetting monitored by variation of the permeate conductivity. Influence of concentration, stream velocity, temperature and pH on the fouling behavior of the calcium phosphate suspension were presented. It was found that both the elevating temperature and turbulence could enhance the calcium phosphate fouling under the testing conditions, due to the crystallization incurred fouling in combination with the boundary effects. Low pH would suppress membrane as expected, and adjusting the suspension’s pH to as low as 4 could effectively inhibit the membrane fouling in the studied water recovery range. It was found that the calcium phosphate fouling will result flux decline and membrane wetting due to the lowering the membrane hydrophobicity (contact angle). Furthermore, the fouling incurred by the crystallization/precipitation of calcium phosphate has more influence on permeate flux than that incurred by the settlement of the suspended calcium phosphate.

膜蒸馏是从反渗透盐水浓缩物中回收更多水的潜在技术，目的是实现高水回收率和零液体排放。但是，由于磷酸钙结垢已在反渗透应用中得到了广泛报道，因此它也可能会阻碍通过膜蒸馏回收废水中的高给水回收率。在这项研究中，使用磷酸钙饱和进料模拟RO浓缩液，并研究了直接接触膜蒸馏（DCMD）膜上磷酸钙的结垢机理。通过渗透通量下降定量膜结垢，并通过渗透电导率变化监测膜润湿。提出了浓度，物流速度，温度和pH对磷酸钙悬浮液结垢行为的影响。发现在测试条件下，升高的温度和湍流都可以增强磷酸钙结垢，这是由于结晶引起结垢并结合边界效应。低pH值会抑制膜，如预期的那样，将悬浮液的pH值调节至低至4可以有效抑制在研究的水回收率范围内的膜污染。已经发现，由于降低了膜的疏水性（接触角），磷酸钙结垢将导致通量下降和膜润湿。此外，与悬浮的磷酸钙沉淀相比，磷酸钙结晶/沉淀所引起的结垢对渗透通量的影响更大。