Emerging investigator series: phosphorus recovery from municipal wastewater by adsorption on steelmaking slag preceding forward osmosis: an integrated process

Abstract

Phosphorus is a critical non-renewable mineral essential for sustainable crop production. However, it is a primary cause of eutrophication in natural water bodies. Hence, a pragmatic approach that allows wastewater treatment plants to efficiently recover phosphorus from wastewater is needed. In this study, we investigated phosphorus recovery from municipal wastewater using an industrial waste product (steelmaking slag) as an adsorbent to selectively separate phosphorus from wastewater before its concentration by the forward osmosis (FO) process. Phosphorus adsorbed on slag was extracted, thereby regenerating the adsorbent medium. The resulting phosphorus-rich solution was further enriched by FO prior to chemical precipitation for phosphorus recovery. Batch test results showed that the equilibrium of phosphorus sorption on the slag medium was achieved within 45 min with an adsorption capacity of 3.8 mg g⁻¹. In continuous-flow column tests, the slag exhibited a stable phosphorus removal efficiency of 82% from wastewater, following which approximately 88% of the phosphate could be extracted from the adsorbent medium. When the phosphorus-rich solution extracted from the adsorbent was passed through the FO membrane, FO could retain 98% of the phosphate. However, the water flux through the FO membrane declined gradually to approximately 20% of the initial value at a water recovery of 90%, which could be attributed to the increasingly concentrated feed solution. The enrichment of phosphate in the concentrated feed by the FO process provided a favorable condition for spontaneous precipitation of amorphous calcium phosphate. This study demonstrates that the proposed combined adsorption–FO process can effectively remove, and thus recover, phosphorus from municipal wastewater.