Present state and strategies are summarized on the recovery of anions in the wastewater, based on crystallization engineering. Anion which gives undesirable influence to the environment has been removed by coagulation and precipitation methods to produce a large amount of sludge, requiring wide space for its disposal. Recently steep increase in the price of the resources and deep concern about drying-up of resources have accelerated recovery of ions in the wastewater. Consequently many researches have been done to selectively separate a desirable ion. Crystallization is one of the suitable processes for this purpose, because it can remove and recover ions as the form of purified and stable crystals. In application of crystallization to the wastewater treatment, solid-liquid equilibrium, metastable zone width (MZW), degree of supersaturation, crystallization kinetics (crystal growth, nucleation and so on) and selection of seeds and desirable seed amounts should be given. Examples and strategies on phosphate and fluoride recovery are shown.
