Compact accelerated precipitation softening (CAPS) with submerged filtration: Role of the CaCO₃ "cake" and the slurry

A new concept for compact accelerated precipitation softening (CAPS), comprising in-tank mixing and filtration, is presented. Softening with CAPS is both fast and efficient, because CaCO₃ precipitation is achieved in a well-stirred calcite slurry and in a compact cake formed on the filter elements. Other advantages of the CAPS process are its compactness, technical simplicity, and low cost. In this study, laboratory CAPS units were run continuously (up to 600 h) for tapwater softening. The effects of the initial concentration of the CaCO₃ slurry, the pumping speed, and the pH and the individual effects of the slurry and the cake and of the filter cake load were analyzed in terms of the reduction of the saturation index (SI) and the precipitation capacity. It was found that calcium reduction in the cake was 10-100 times faster than that in the slurry. In addition, turbidity was reduced significantly because of the microfiltration capability of the cake. CAPS can be used as a stand-alone water-treatment process or in conjunction with pressure-or electricity-driven membrane processes (UF, NF, RO, ED) as an effective pretreatment routine for increasing recovery and decreasing fouling rates.