Removal of ammonia as struvite from anaerobic digester effluents and recycling of magnesium and phosphate

A second order kinetic model was developed to predict the rate and extent of NH4+ removal as struvite from anaerobic digester effluents. Alternative to this, NH4+ can be recovered from struvite and the remaining Mg2+ and PO43- can be recycled back to the wastewater to fix more NH4+. The NH4+ solution was retained and the remaining Mg2+ and PO43- were returned back to be mixed with wastewater. In a five-step process, NH4+ recovery was initially 92% and progressively decreased to 77% in the fifth stage, due to loss of Mg2+ and PO43- at each step in the supernatant. Finally, economic analysis of recycling nutrients was performed and compared to the one step process. The cost of NH4+ recovery was calculated as $0.36/kg NH4-N which is lower than $7.7/kg NH4-N the cost of one step process without considering the market value of struvite obtained in one step process.