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Phosphorus removal by crystallisation has the advantages of fast reactions, high efficiency, and recyclable resources, and it has attracted much attention at globally in recent years. With extensive research, its theory and process technology have been continuously improved. Magnesium ammonium phosphate (MAP) crystallisation and calcium hydroxyphosphate (hydroxyapatite, HAP) crystallisation are two common methods for wastewater dephosphorisation, but there are few reviews of these two methods. In this paper, the research results concerning MAP and HAP crystallisation methods are comprehensively reviewed. The reaction principle, influencing factors, and engineering applications of the two products are summarised, and the two crystallisation methods are compared. Finally, the development of future phosphorus crystallisation technology is discussed. These prospects provide a basis for the promotion and application of phosphorus removal by crystallisation.

Marine dumping of dredged sediment results in the release of considerable amounts of nitrogen and phosphorus into the seawater. In this study, we evaluated the release pattern of nitrogen and phosphorus during sediment dumping through experimental simulation under different mixing ratios and temperatures. The concentration of NO3-N, PO4-P, and total phosphorus (TP) in seawater quickly increased in less than 1 h after mixing of sediment and seawater and remained stable thereafter. The concentration of NH4-N increased and peaked at 0.25 h, but quickly decreased thereafter. The concentration of NO3-N, NH4-N, PO4-P, and TP in seawater showed a positive correlation with the mixing ratio. The contribution of NO2-N from sediment to water was negligible and there was no obvious difference in the release amount among different mixing ratios. The release amount of each measured nutrient at 25°C was marginally higher than that at 15°C, suggesting that marine dredging and dumping should be performed in winter rather than in summer. The release amount of NO3-N, NO2-N, NH4-N, PO4-P, and TP in a 5000-m3 vessel reached 26.5, 17.6, 0.28, 1.68, and 1.68 kg, respectively. This study provides a detailed evaluation of nitrogen and phosphorus release pattern in marine sediment dumping process, which will be helpful for the conservation and selection of marine dumping areas.