Water Treatment and Scaling in Distribution Pipes—The Impact of Silica and Phosphate on Calcium Carbonate Precipitation

Scaling due to the precipitation of CaCO3 in water distribution systems results in flow restrictions and head loss, damage to plumbing fixtures, and clogging membranes/filters in treatment systems. A potential method for controlling CaCO3 precipitation is using scale inhibitors (e.g., PO43−, Zn2+, etc.). A recent study found that in NaCl electrolyte solution, both phosphate and zinc inhibited the precipitation of CaCO3; however, neither phosphate nor zinc could reduce the precipitation of CaCO3(s) in natural groundwater. Two possible reasons identified for this were: (a) the presence of silica in groundwater which could enhance the precipitation of calcium, and (b) the possible precipitation of Ca-PO4 solids in groundwater systems. This study assesses silica's effect on the precipitation of CaCO3, both in the presence and absence of phosphate. This research involved laboratory batch experiments in 0.1 M NaCl solution containing calcium chloride and sodium bicarbonate. Later, similar batch experiments were carried out in synthetic groundwater. In the batch experiments, the mixing time was 30 min, and the concentration of Ca2+ was 75 mg/L. The effect of silica and phosphate on the precipitation of CaCO3 was evaluated by varying their concentrations. The precipitation of CaCO3 was slightly enhanced in the presence of silica (without phosphate) in NaCl electrolyte solution. On the contrary, the precipitation of CaCO3 decreased in NaCl electrolyte solution in the presence of phosphate (without silica). In the presence of both phosphate and silica, silica did not significantly affect phosphate's inhibiting ability on precipitation of CaCO3. Experiments carried out in synthetic groundwater also did not find any effect of silica on the precipitation of CaCO3. Further investigations under a broader range of conditions are needed to understand better the chemical processes leading to the different effects of phosphate on the precipitation of calcium carbonate in an electrolyte solution and natural groundwater.