Recovery of Ammonia and Production of High-Grade Phosphates from Side-Stream Digester Effluents Using Gas-Permeable Membranes

Abstract

Phosphorus recovery was combined with ammonia recovery using gas-permeable membranes. In a first step, the ammonia and alkalinity were removed from municipal side-stream wastewater using low-rate aeration and a gas-permeable membrane manifold. In a second step, the phosphorus was removed using magnesium chloride (MgCl2) and reduced amounts of alkali. The side-stream wastewater contained 730 mg N/L, 140 mg P/L and 2900 mg/L alkalinity. The process recovered approximately 79–93% of the ammonia and 80–100% of the phosphorus. Surprisingly, the phosphates produced were very-high grade (42–44% P₂O₅) with a composition similar to the bio-mineral newberyite. However, lower grade phosphate products (27–29% P₂O₅) were produced whenever the N recovery step was bypassed or carbonate alkalinity was added. Therefore, removal of ammonia and alkalinity are important considerations for production of very-high grade phosphate product.