-Line Daumer. Optimization of struvite precipitation in synthetic biologically treated swine wastewater -Determination of the optimal process parameters. Journal of Hazardous Materials, Elsevier, 2013Elsevier, , vol. 244-245, pp. 357-369. <10.1016Elsevier, /j.jhazmat.2012 h i g h l i g h t sStruvite precipitation is mainly influenced by the quantity of MgO added. Optimized parameters favoured struvite formation despite of high [Ca 2+ ] without adding other reagents. 90% of the total dissolved phosphorus is recovered as large crystals of struvite. Raman spectroscopy and solid dissolution show co-precipitation of ACP and presence of CaCO 3 in solid.Keywords: Struvite P-recovery MgO Calcium phosphate Swine wastewater Raman a b s t r a c t A sustainable way to recover phosphorus (P) in swine wastewater involves a preliminary step of P dissolution followed by the separation of particulate organic matter. The next two steps are firstly the precipitation of struvite crystals done by adding a crystallization reagent (magnesia) and secondly the filtration of the crystals. A design of experiments with five process parameters was set up to optimize the size of the struvite crystals in a synthetic swine wastewater. More than 90% of P was recovered as large crystals of struvite in optimal conditions which were: low Mg:Ca ratio (2.25:1), the leading parameter, high N:P ratio (3:1), moderate stirring rate (between 45 and 90 rpm) and low temperature (below 20• C).These results were obtained despite the presence of a large amount of calcium and using a cheap reactant (MgO). The composition of the precipitates was identified by Raman analysis and solid dissolution. Results showed that amorphous calcium phosphate (ACP) co-precipitated with struvite and that carbonates were incorporated with solid fractions.

An experimental design was set up to understand the influence of five process parameters on the kinetics of struvite precipitation in synthetic swine wastewaters. The responses studied were the kinetics of phosphorus (P) removal, the struvite precipitation rate and the dissolution rate of amorphous calcium phosphates (ACP). The kinetic study showed that the P-removal was complete in less than 1 h and was influenced positively by the added MgO. The precipitation of struvite with MgO was confirmed to follow a first-order kinetic. This study showed that ACP co-precipitated with struvite during the first 30 min. Afterwards, ACP dissolved to maintain the phosphates balance limiting the struvite growth. An initial Mg:Ca > 1.5 induced a complete dissolution of ACP in 1 h. Another experiment was conducted and it validated the results of the statistical model. This experiment also determined that 7-10 h was the best time to recover large crystals. After 10 h, the crystals were broken by stirring.

A sustainable way to recover phosphorus (P) in swine wastewater involves a preliminary step of P dissolution followed by the separation of particulate organic matter (OM). The next two steps are firstly the precipitation of struvite crystals done by adding a crystallization reagent (magnesia) and secondly the filtration of the crystals. To develop the process successfully at an industrial scale, the control of the mechanisms of precipitation is the key point in order to obtain high value-added products, that is, big struvite crystals easy to harvest and handle. Experiments with process parameters optimized previously in a synthetic swine wastewater were performed on real swine wastewater to assess the role of the OM on struvite crystallization. After 24 h, with a pH increase to 6.8 only, 90% of the initial P was precipitated and 60% was precipitated as struvite. 80% of the solid recovered was in the fraction> 100 µm. The other forms recovered were brushite, amorphous calcium phosphate, NaCl, KCl and OM. The influence of OM on struvite precipitation in acidified swine wastewater was negative on the reaction kinetics but positive on the size of the struvite crystals. The presence of colloidal particles increased the size of the struvite crystals but slowed down the kinetics due to the viscosity induced by the repulsive force of the colloids. The maximum size of single struvite crystals (200 µm) was observed with the presence of particulate OM.

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These experiments were performed to verify the potential ways of treating pig farm effluents using aeration, struvite precipitation and filtration. MgCl 2 brine was added as a source of magnesium for struvite precipitation. Following aeration, precipitated struvite was separated from the effluent by filtration through a compressed straw filter. The filter plus struvite can be composted to yield struvite-enriched compost. A series of 5 batch pilot-scale experiments (effluent volumes from 200-400 l) was carried out and laboratory tests on the same effluent were performed to validate the pilot-scale results. Following 24 hours of aeration the pH increased to from 8.5 to 9, providing good conditions for struvite precipitation. Filtration proved to be efficient for separating precipitated struvite and total suspended solids. The efficiency of orthophosphate phosphorus removal in the pilot system varied from 63 to 96%, ammonia nitrogen removal varied from 22 to 79%, but magnesium removal efficiency was low due to the high natural Mg concentration, suggesting that the addition of Mg might not be necessary. Chemical oxidation demand declined significantly as a result of very high bacterial activity and the treated effluent no longer had an unpleasant smell.