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

Abstract

A second order kinetic model was developed to predict the rate and extent of NH 4 + removal as struvite from anaerobic digester effluents. Alternative to this, NH 4 + can be recovered from struvite and the remaining Mg 2+ and PO 4 3 - can be recycled back to the wastewater to fix more NH 4 + . The NH 4 + solution was retained and the remaining Mg 2+ and PO 4 3 - were returned back to be mixed with wastewater. In a five-step process, NH 4 + recovery was initially 92% and progressively decreased to 77% in the fifth stage, due to loss of Mg 2+ and PO 4 3 - at each step in the supernatant. Finally, economic analysis of recycling nutrients was performed and compared to the one step process. The cost of NH 4 + recovery was calculated as $0.36/kg NH 4-N which is lower than $7.7/kg NH 4-N the cost of one step process without considering the market value of struvite obtained in one step process.