Sustainable Struvite Control Using Carbon Dioxide

Struvite (MgNH4PO4·6H2O) scaling in wastewater treatment plants causes significant processing issues. Struvite formation can be controlled by acid addition since struvite solubility increases as pH decreases. Carbon dioxide from anaerobic digester gas can be used to lower the pH of wastewater to prevent struvite formation. In this work, bench-scale carbon dioxide injection illustrated the feasibility of using carbon dioxide to lower the pH of digester centrate for prevention of struvite formation. A pilot study was conducted to evaluate alternative designs for full-scale process application. Mixing a centrate stream with centrate that was presaturated with carbon dioxide reduces the pH to levels necessary to prevent struvite formation and was more effective than application of gaseous carbon dioxide. Experimental results validated an equilibrium model, which was adapted to produce a design protocol for implementation of this technology. Design parameters include measurable water quality parameters, including alkalinity and concentrations of struvite components. This work demonstrates that carbon dioxide is a sustainable alternative to traditional methods of struvite control and provides a simple methodology for full-scale implementation.