4. Conclusions
When the slag is applied in the phosphorus removal process for wastewater, the main factor determining the slag replacing period was the capacity to maintain the high pH level of the solution. The phosphorus removal efficiency increased as the slag grain size decreased, however, the coarse-grained slag coated by sodium hydroxide showed significantly higher removal efficiency than the fine-grained slag without coating. It can be attributed to the higher pH level and faster coagulation caused by sodium hydroxide coating. The return process could solve the problem of high concentration of aluminum and sodium in the initial column effluent and, as a result, it extended the lifespan of the slag.