STRUVITE CONTROL AND IMPLICATIONS FOR THERMAL DRYING

Struvite accumulation has become a nuisance for many plants, especially as phosphorus removal has become more prevalent. Struvite is a crystalline material consisting of equimolar concentrations of magnesium, ammonium, and phosphorus. Struvite can deposit on pipes, creating a hard scale that reduces pipe capacity and can also accumulate on dewatering equipment, affecting the dewatering operation by blinding belts on belt filter presses or altering the balance of centrifuges.The anaerobic digestion process enhances conditions for struvite formation. Anaerobic digestion solubilizes particulate phosphorus and nitrogen, increasing the dissolved concentrations of PO43- and NH4+. When the digested solids are pumped or dewatered, the turbulence from these operations strip carbon dioxide from the liquid, resulting in an increase in pH, enhancing conditions for struvite formation.Both prevention and controlled precipitation methods have been used for struvite control. Prevention strategies attempt to modify conditions in the solution, through control of pH or constituent concentrations, to minimize struvite formation and precipitation. Conversely, controlled precipitation attempts to maximize the formation of struvite, but to confine it to a desired location in the treatment system.The Elledge wastewater treatment plant in Winston-Salem, North Carolina currently uses iron salts to minimize struvite formation in the centrifuge and centrate piping. The Utility Commission is considering installation of a heat drying process to dry dewatered biosolids. There is anecdotal information that the added iron could affect the explosivity limit of solids during the drying process, which is a safety concern; consequently, continued use of iron salts for struvite prevention may be problematic. To address this concern, the staff has performed testing on the dewatered cake to determine the effects of iron addition on explosion potential during the drying process.This paper investigates the state of the practice of struvite control methods, including the use of iron salts, chelating agents, and controlled precipitation. It also compares explosivity evaluation results for biosolids treated with and without iron addition and discusses recommendations for struvite control for treatment plants considering thermal drying processes.