Phosphorus (P) recovery and reuse as fertilizer from incinerated sewage sludge ash (ISSA)

Phosphorus (P), an essential macromineral, is fundamental to all living things. Alarmingly, P is consumed in excess in most countries, and the known P source will be run out within several decades. Therefore, looking for alternative P sources from P-bearing wastes is significant and urgent, which can not only recycle P resources but also avoid the huge cost of disposing these wastes. P-rich incinerated sewage sludge ash (ISSA) is a by-product of the incineration process of dewatered sewage sludge generated from sewage treatment plants. The P in the ISSA, if economically and environmentally recycled, can achieve the sustainable management of sewage sludge and P resources. In this chapter, the comprehensive physical and chemical characteristics of Hong Kong ISSA are discussed. The leachability and the associated environmental risk of metal(loid)s from the ISSA are highlighted in an attempt to improve the public perception of sewage sludge incineration from an environmental perspective. Afterward, a series of laboratory-scale processes for extracting P from the ISSA to form fertilizer products (Ca-P and struvite) are introduced. Especially, an innovative two-step extraction method and a struvite crystallization technology are developed to obtain a P extract with high purity. In addition, the characteristics and reuse values of P-free ISSA are evaluated. Lastly, future research needs are assessed with emphasis placed on P reclamation from the ISSA and application in agriculture. This chapter reviews the state-of-the-art recycling options of ISSA and thereby proposes innovative technologies to use ISSA as a valuable resource by extracting P and producing fertilizers.