Novel Recovered Compound Phosphate Fertilizer Produced from Sewage Sludge and Its Incinerated Ash

Current disposal methods of sewage sludge (SS) and its incinerated ash (ISSA) do not allow for the recycling of the phosphorus (P) present. The aim of this study was to fabricate a P compound fertilizer by using an optimal SS-derived biochar (SSB) with enhanced inherent Ca-P content to load P from the P extract of ISSA. For this purpose, SS was impregnated with four agents (MgCl, CaCl, MgO, and CaO) at mass ratios of 20 or 10% and pyrolyzed at 700 or 300 °C. Characterization of the produced SSBs indicated that those with 20% MgCl, CaO, and MgO added and pyrolyzed at 700 °C exhibited high surface areas, abundant mesopores, and abundant plant-valuable P. When in contact with the P extract, these three SSBs showed the highest P adsorption efficiencies among the obtained SSBs but CaO/MgO-impregnated SSBs have more metal coprecipitation. The postsorption SSBs were P-enhanced and alkalinity decreased, which are favorable for fertilizer application. Furthermore, a pot experiment verified that these three postsorption SSBs had a comparable P fertilizer effect to that of the commercial P fertilizer. Therefore, the MgCl-impregnated SSB with P loading from the ISSA extract has potential as a P fertilizer alternative that can support global P sustainability by reclaiming P from SS and ISSA.