**4. Conclusion**


### **4.1 Discussion**

When recovering phosphorus from incineration ash, the biggest problem is considered to be economic feasibility. It is still difficult to secure economic feasibility because the recovery cost is higher than that of phosphate rock. However, natural resources are finite and resource recovery technology needs to be developed to prepare for resource depletion. In particular, when recovering phosphorus from incineration ash, it is possible under acidic and basic conditions.

Although acidic conditions are favorable for phosphorus extraction efficiency from incineration ash, there is a disadvantage in that the purity of phosphorus crystallization is low due to the elution of many other heavy metals. In addition, the process is complicated because a separate pH control is required under acidic conditions. On the other hand, in basic conditions, phosphorus extraction efficiency is low, but phosphorus crystallization with high purity can be obtained because the elution concentration of heavy metals is low. In the future, considering these problems, it is judged that the phosphorus recovery method should be applied according to the site conditions of the sewage treatment plant and the e of phosphorus crystal (MAP or HAP).
