Utilization of activated sludge and shell wastes for the preparation of Ca-loaded biochar for phosphate removal and recovery

The activated sludge-based biochar modified by metals performs well for the phosphate adsorbent. The cost of the modification materials and the follow-up treatment of the spent adsorbents are significant limits for the large-scale application of the developed adsorbent. In this study, wasted egg shells and oyster shells, rich in calcium (Ca), were employed to load Ca onto the surface of the activated sludge-based biochar. The results revealed that different forms of Ca on egg shell-modified activated sludge biochar (ESB) and oyster shell modified activated sludge biochar (OSB) induced different adsorption performances. ESB had a greater phosphate adsorption capacity than OSB, where the maximum adsorption capacities were 154.18 mg/g and 129.03 mg/g, respectively. The saturated biochar could slowly release phosphate at different levels of pH; in fact, the phosphate release efficiency of ESB exceeded that of OSB. Economic analysis suggested that the cost of phosphate removal by ESB was $1.13 per kg, and by OSB was $1.22 per kg. The phosphate adsorption using the Ca-loaded biochar revealed good prospects for sustainable development with the advantages of the high efficiency of phosphate removal, low cost and resourceful employment of wastes.