Influence of reagent choice on strategies of nutrient recovery by struvite crystallisation

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Two different approaches of magnesium ammonium phosphate (MAP) crystallisation were compared. Anaerobically treated wastewater containing high phosphate levels (100 to 120 mg PO4 3- - P/l) was shown to be treated most efficiently using air stripping combined with MgCl2 and NaOH reagent addition. Residual PO4 3- -P was 8 to 15 mg PO4 3- -P/l yielding 1 to 2,5 mm spherical pure MAP crystals. At lower phosphate levels (< 30 mg PO4 3- -P /l) a novel approach using ureolytic induced MAP formation with MgO addition (Precipur) resulted in somewhat lower 5 to 7 mg PO4 3- -P /l effluent levels. The separated matter contained besides MAP residual MgO and Mg(OH)2. Results show that depending on initial PO4 3- -P levels and fluctuating influent levels of both ammonium and magnesium, reagent choice is a determinative factor to achieve a cost effective full scale MAP treatment.

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