Preliminary investigation into factors affecting controlled struvite crystallization at the bench scale

A preliminary investigation of factors affecting controlled struvite crystallization was conducted in bench-scale studies using a new crystallization reactor design, operating under a range of reactor and feed strength conditions. In 21 different experiments, it was found that the bench-scale reactor functioned well in terms of achieving good particle classification, although its performance was somewhat restricted in terms of P removal. Reactor operation was generally somewhat compromised because of scale effects. As a preliminary to further (ongoing) pilot-scale investigations, it was found that, for a given set of operating and feed strength conditions, there was a specific value of pH (pHlim) beyond which the reactor would not operate effectively. The pH value is an important operating parameter in struvite crystallization, and the mode of pH operation plays a major role in determining struvite crystal quality. When operated at pH values exceeding pHlim, poor quality (i.e., dendritic) crystals were produced and reactor plugging often occurred. A qualitative assessment of temperature effects on crystallization reactions suggested that low temperatures may indeed favour struvite crystal growth. Reactor seeding was not found to be a factor of any particular importance since, once struvite crystallization reactions were underway, the reactor appeared to be "self-seeding".