Strategies to optimize phosphate removal from industrial anaerobic effluents by magnesium ammonium phosphate (MAP) production

BACKGROUND: Owing to more stringent phosphate discharge requirements and the increasing prices of fertilizers, beneficial recovery and reuse of phosphate from industrial anaerobic effluents is becoming an important issue. Phosphate recovery by struvite or magnesium ammonium phosphate (MAP) permits its recycling in the fertilizer industry because struvite is a valuable slow release fertilizer. Two different approaches to MAP crystallization depending on initial levels of phosphate in the wastewaters were tested and compared. RESULTS: For low-phosphate-containing anaerobic effluents (< 30 mg PO43--P L-1), a novel approach using ureolytic induced MAP formation with MgO addition appeared to be suitable. The residual phosphate concentrations in the effluent ranged from 5 to 7 mg PO43--P L-1 and the separated matter contained residual amounts of Mg(OH)(2). High-phosphate-containing anaerobic effluents (1100 to 120 mg PO43--P L-1) were treated efficiently using air stripping combined with MgCl2 and NaOH reagents, yielding residual phosphate levels of 8 to 15 mg PO43--P L-1 and spherical pure MAP crystals of 0.5 to 2 mm. CONCLUSION: Results show that depending on the initial phosphate concentrations in the wastewaters and the ammonium and magnesium levels, the strategy selected for struvite crystallization is a determinative factor in achieving a cost effective technology. (C) 2008 Society of Chemical Industry