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.

Effects of Morphology on the Bulk Density of Instant Whole Milk Powder.

The chemical and physical properties of instant whole milk powder (IWMP), such as morphology, protein content, and particle size, can affect its functionality and performance. Bulk density, which directly determines the packing cost and transportation cost of milk powder, is one of the most important functional properties of IWMP, and it is mainly affected by physical properties, e.g., morphology and particle size. This work quantified the relationship between morphology and bulk density of IWMP and developed a predictive model of bulk density for IWMP. To obtain milk powder samples with different particle size fractions, IWMP samples of four different brands were sieved into three different particle size range groups, before using the simplex-centroid design (SCD) method to remix the milk powder samples. The bulk densities of these remixed milk powder samples were then measured by tap testing, and the particles' shape factors were extracted by light microscopy and image processing. The number of variables was decreased by principal component analysis and partial least squares models and artificial neural network models were built to predict the bulk density of IWMP. It was found that different brands of IWMP have different morphology, and the bulk density trends versus the shape factor changes were similar for the different particle size range groups. Finally, prediction models for bulk density were developed by using the shape factors and particle size range fractions of the IWMP samples. The good results of these models proved that predicting the bulk density of IWMP by using shape factors and particle size range fractions is achievable and could be used as a model for online model-based process monitoring.

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.

Phosphorous recovery through struvite crystallization: Challenges for future design.

Phosphorous (P) is an essential element for living organisms and is predicted to be depleted within the next 100 years. Across the world, significant phosphorous losses due to its low utilization efficiency become one of the main reasons for water pollution. Struvite crystallization has been found to be a promising recovery technique to mitigate these problems, as the recovered precipitate can be used as a slow release fertilizer or raw material for chemical industry. Although this technique has been widely investigated over the past two decades, there are currently few real applications in industry. This paper addresses this issue by reviewing key aspects relevant to process design to pave the way for future application. It will help to narrow down struvite process design options and thus reduce the voluminous calculations for a detailed analysis. Struvite process development, research trend, product application and process economics are reviewed and a conceptual process design is provided. This analysis provides comprehensive information that is essential for future industrial struvite crystallization process design.

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.

Substance flow analysis of phosphorus within New Zealand and comparison with other countries.

Phosphorus (P) is an essential and non-renewable resource that is estimated to be depleted within the next 70-140 years. Increasing P scarcity and environmental pollution requires the development of a proper P management system. One fundamental step is to understand how P flows within society. Thus, a substance flow analysis (SFA) of P was conducted to distinguish and quantify different P flows within New Zealand, and principal component analysis (PCA) was used to show where the P flow in New Zealand was significantly different from the most efficient P utilization countries. Aside from having low P recovery, New Zealand imports a significant amount of P, while re-exporting a much smaller fraction of it compared with other countries, resulting in a high accumulation. The SFA analysis showed that although diffuse sources of P pollution pose a greater environmental risk to water bodies, promoting P recovery from point sources of pollution, such as from waste treatment facilities and landfill sites, would still improve the P management efficiency in New Zealand, which could decrease the annual P rock imports by 20%.