Circular economy approach: Nutrient recovery and economical struvite production from wastewater sources by using modified biochars.

The enrichment of phosphorus (P) and nitrogen (N) in aquatic systems can cause eutrophication. Moreover, P rocks may become exhausted in the next 100 years. A slow-release fertilizer called struvite (MgNH 4 PO 4.6H 2 O) can reduce surface runoff. However, the high cost of raw material or chemicals is a bottleneck in their economical production. Therefore, incinerated sewage sludge ash, food wastewater, and bittern were combined as the sources of P, N, and Mg, respectively. Sawdust biochar was used to enhance the adsorptive recovery of nutrients. First, recovery kinetics was studied by comparing bittern-impregnated biochar (BtB) with the Mg-impregnated biochar (MgB). Subsequently, the synergistic physical and chemical interactions were observed for P and N recovery. Almost complete PO 4 3–P recoveries were achieved within 10 min for both biochars. However, NH 4 +-N recovery was stable after 2 h, with 26% recovery by MgB and 20% recovery by BtB. Biochars activated with steam (steam-activated biochar) and KOH (KOH-activated biochar) gave superior activities to those of unactivated biochars and activated carbon (AC) nutrient recovery and struvite purity. Moreover, the activated biochars showed a lower risk of surface runoff, similar to that of AC. Therefore, activated biochars can be used as an alternative to AC for economical struvite production from a combination of wastewater sources. [Display omitted] • Phosphorus and nitrogen enrichment spur eutrophication; P rocks may vanish within a century. • Struvite (MgNH 4 PO 4.6H 2 O) reduces runoff but faces cost hurdles. • Incinerated sewage sludge ash, food wastewater, and bittern cut expenses for P, N, and Mg; sawdust biochar aids recovery. • BtB and MgB show rapid PO 4 3–P recovery in 10 min, stable NH 4 +-N recovery in 2 h. • Steam- and KOH-activated biochars surpass unactivated ones and AC, offering economical struvite from wastewater. [ABSTRACT FROM AUTHOR]