Abstract
Recovering phosphorus from wastewater is crucial for maintaining a circular nutrient economy. Struvite (MgNH₄PO₄·6H₂O) precipitation provides a promising pathway for phosphorus recovery while producing a slow-release fertilizer. If the recovered struvite can be directly applied in agriculture depends largely on its particle size. However, the mechanisms driving the formation of large directly usable spherical struvite particles, otherwise known as ooids or pisoids, remain poorly understood. This study investigates the key parameters controlling struvite crystallization and ooid development in a stirred tank reactor (STR) using real anaerobic digester centrate at pilot scale. Experiments were performed at different initial phosphate concentrations (73, 149, and 305 mg/L PO₄-P) and pH values (7.5, 8.0, and 8.5). The resulting median crystal sizes ranged from 107 µm to 744 µm (265.9 ± 201.4 µm, 1σ), with the largest particles forming at 73 mg/L PO₄-P and pH 8.5. Lower phosphate concentrations and higher pH values both favored the formation of larger particles; however, phosphate concentration was the dominant factor controlling ooid development, while pH mainly influenced overall precipitation efficiency. Thin-section microscopy under polarized light revealed that ooid formation proceeds through a dual process of agglomeration and crystal growth on pre-existing nuclei, creating a branching internal framework. This characteristic growth-morphology, described here for the first time, is referred to as the monstera stage . The resulting spherical structures — newly termed as crystal ooids — represent the structural core for further ooid development. These findings demonstrate that struvite ooid formation in STRs can be controlled by the factors pH-value and phosphate concentration, enabling the production of coarse-grained, agriculturally usable struvite-based fertilizers and contributing to sustainable phosphorus recovery from wastewater systems.