Phosphorus recovery through struvite crystallisation: Recent developments in the understanding of operational factors.

Phosphorus is an essential element for life and is predicted to deplete within the next 100 years. Struvite crystallization is a potential phosphorus recovery technique to mitigate this problem by producing a slow release fertilizer. However, complex wastewater composition and a large number of process variables result in process uncertainties, making the process difficult to predict and control. This paper reviews the research progress on struvite crystallization fundamentals to address this challenge. The influence of manipulated variables (e.g. seed material, magnesium dosage and pH) and sources of variation on phosphorus removal efficiency (e.g. organics and heavy metal concentration) and product purity were investigated. Recently developed models to describe, control and optimize those variables were also discussed. This review helps to identify potential challenges in different wastewater streams and provide valuable information for future phosphorus recovery unit design. It therefore paves the way for commercialization of struvite crystallization in the future.

A new thermodynamic approach for struvite product quality prediction.

Struvite precipitation has drawn much attention in the last decade as a green chemical process for phosphorus removal and recovery. Product purity affects the usefulness, and thus price, of the product when recovered struvite is sold as fertilizer. However, there is currently little research on struvite quality, as well as on models for accurately predicting. This paper presents an alternative approach to the traditional thermodynamic model where the solid with the largest positive saturation index precipitates first, depleting the concentrations of constituent ions before the next solid can precipitate. In the new thermodynamic approach, all solids with a positive saturation index precipitate simultaneously, and deplete the common pool of available ions in tandem. It was validated against experimental data, compared with the traditional thermodynamic models and a previously developed empirical model. The proposed new approach was more accurate than other models, except when both the ammonium nitrogen and magnesium concentrations were very low, a condition not likely to be encountered in industry. Therefore, this model is more suited for predicting the performance of struvite precipitation under varying wastewater conditions.

Phosphate recovery from hydrothermally treated sewage sludge using struvite precipitation.

New technologies are needed to recover phosphate from organic wastes, such as sewage sludge. Sewage sludge can be hydrothermally treated to make it safe but this process is expensive. Recovering a valuable by-product, such as phosphate, could improve the economics of hydrothermal treatment. Therefore, the technical and preliminary differential cost analysis of combining hydrothermal treatment with phosphate recovery (by precipitation of magnesium ammonium phosphate (struvite)) was investigated. The effects of pH, magnesium ion dose, and either wet oxidation or thermal hydrolysis hydrothermal treatment were examined. Phosphate recovery was more sensitive to pH than magnesium ion concentration, with diminishing rates of recovery at high levels of both. Also, more struvite was recovered following wet oxidation treatment than thermal hydrolysis. Preliminary differential cost analysis showed that wet oxidation combined with precipitation at an optimal pH and magnesium ion dose could generate revenue.