New insights into interactions of organic substances in poultry slurry with struvite formation: An overestimated concern?

The high content of organic substances in strength agro-industrial wastewater has been documented to be among the major barriers hampering nutrient recovery efficiency of struvite precipitation. However, our results in this study show that the previously reported negative impacts of organic substances in high-strength agricultural wastewater on struvite precipitation might be overestimated. This study is the first to test the influence of three forms of organic substances from real high-strength wastewater that contains a complex of particulate, colloidal and soluble organic substances, on nutrient recovery efficiency and product quality through struvite precipitation at varying pH conditions. Our results demonstrated that the inhibition of organic substances on struvite formation only happens at the pH levels of <9.0 with recovery reduction of PO₄ (5-15%) and NH₄ (6-13%). The inhibitory effect of the organic substances at the optimal pH range (9.5-10) reported from the literature review is only ≤5%. Moreover, the transformation in the contents of humic- and protein-like substances with an increment in pH was characterized and may contribute to mitigate the inhibition of nutrient recovery. Even though the particulate and colloidal organic substances slowed the precipitation reaction, they substantially increased the particle size (i.e., 70% and 40%, respectively) of the formed struvite. The presence of organic substances in all tested forms does not significantly influence the purity and crystalline structure of struvite which can still be used as a slow-releasing fertilizer. Regarding the relocation process of organic substances during struvite precipitation under varying pH conditions, understanding the interaction between organics and heavy metals which in turn affect the dynamics of heavy metals in solution and precipitates remains limited; thus, additional research is needed.