Recovery of nitrogen and phosphorus by struvite crystallization from swine wastewater

Effects of the aeration rate and magnesium:phosphate (Mg:P) molar ratio on struvite crystallization in swine wastewater were investigated using a newly designed unit process. In addition, a feasible purification method to recover pure struvite from retrieved sediment and heavy metal content present in the recovered struvite was also examined. The process had reaction and settling zones and was operated in a continuous flow mode; the reaction time (RT) of the reaction zone was 4 h. Nine runs were conducted at different aeration rates and Mg:P ratios. Struvite formation was directly proportional to the aeration rate and reached a plateau at around 0.73 L/L∙min of aeration. Struvite formation increased significantly (P < 0.01) with increased Mg:P molar ratios and 0.8–1was found to be optimum for enhancing struvite crystallization. The recovered sediment contained 19% total phosphorus (TP) and 15% total nitrogen (TN); 65% and 67% of TP and TN in the sediment were recovered in the pure struvite by redissolving and recrystallizing using acid and alkali solutions, respectively. The heavy metal content in the struvite was also reduced after purification. Struvite formation was later verified by analyzing the morphology and composition employing scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively.

Research highlights
► Struvite formation increased significantly with Mg:P (0.8–1) molar ratio and aeration rate of 0.73 L/L∙min.
► Coprecipitation of metal ions with struvite occurred, the struvite retrieved contained low levels of heavy metals, below the legal limit.
► Furthermore, approximately 65% of TP and 67% of TN present in the sediment slurry could be recovered in pure struvite.