Interplay of digester supernatant composition and operating pH on impacting the struvite particulate properties

Although the reactive crystallization of struvite to recover phosphorus from digester supernatant in wastewater treatment plants has been extensively investigated in the past, the process frequently confronts challenges in terms of consistent product quality and optimal operating costs. Therefore, further investigation is necessary to better understand the complex interaction of digester supernatant composition and operating parameters. The present work has focused on investigating the interplay of the digester supernatant composition and the operating parameters on the particulate properties of the struvite product obtained from a complex wastewater system. Under different set of conditions, the obtained particulate properties and the relevant phosphorus removal/recovery efficiency were examined. It has been found that smaller struvite crystals were preferable in forming round granules that can facilitate the transportation as well as spreading to the field. High concentration of polymers present in the supernatant with low supersaturation level (pH = 7.5, N/P= 6.3) supported the formation of smaller struvite crystals with narrow size distribution, by inhibiting the crystal growth. But, this effect became less significant, as the supersaturation level was increased or the polymer concentration was decreased, which led to the formation of irregular X-shaped crystals (with dendritic surface). These X-shaped crystals cannot pack tightly despite their larger particle sizes. The P-removal/recovery rates were between 87.5-94% for all conditions investigated in this study. Considering the cost-efficiency and the desired crystal size for the formation of struvite granules, low pH 7.5 and low N/P molar ratio 6.3 were suggested for the proposed struvite crystallization process.