Use of lime, limestone and kiln dust to stabilize reverse osmosis treated water

The City of Pembroke Pines is embarking on an alternative water supply (AWS) project that includes the potential of using treated wastewater for aquifer recharge. The concept includes the use of reverse osmosis membranes, ultraviolet disinfection and advanced oxidation processes downstream of activated sludge and microfiltration. One of the problems is that the permeate leaves the process grossly under-saturated, because with respect to minerals, virtually everything in the water is removed by the reverse osmosis membranes. The practical natural minimum hardness level for water is 40 mg L−1 as CaCO3, while the permeate water was <7 mg L−1. As a result, a post-treatment system needed to be designed to restore minerals to the water to achieve stability so the water does not dissolve metals, other piping and treatment tank materials. Traditionally reverse osmosis plants for potable water systems use caustic soda, polyphosphates, orthophosphates and other chemicals to address the stability issue. These are costly and for an aquifer recharge project, the costs seemed high. For this project, the research focused on alternative solutions to restore hardness, alkalinity and pH using lime, limestone and kiln dust. All three resolved the pH and stability issues for the pilot process.