Phosphorus recovery from incinerated sewage sludge ash (ISSA) and transferring into phosphate fertilizer

Abstract

The interest in recovery of phosphorus (P) from secondary P-sink is stimulated by global P scarcity and the current one-way flux of P-resources. Among the dominant P-sinks, incinerated sewage sludge (ISSA) is a promising secondary source for P recovery. In Hong Kong, approximately 1200 tonnes of dewatered sewage sludge is produced and sent to the T·Park sludge treatment facility for incineration each day. The ISSA is eventually disposed by landfilling. However, it has been found that the P content in this ISSA is comparable to those in P ores (5-40 % P2O5) and has a high recovery potential. In previous research studies of P recovery from ISSA using acid leaching. Both P and metal(loid)s were non-selectively extracted together making the recovered P unusable. In this PhD study, Hong Kong ISSA was firstly characterized, followed by exploration of P leaching with three kinds of leaching agents: mineral acids, organic acids and chelating agents. Based on this, an extensive experimental campaign was carried out to develop a novel two-step leaching method. This method involved the use of a pre-extraction procedure using 0.02 mol/L of EDTA at a liquid to solid ratio (L/kg) of 20:1 for 120 min, followed by extraction with 0.2 mol/L sulphuric and at a liquid to solid ratio of 20:1 for 120 min. This resulted in the production of a high purity P-extract. The pH of P-extract was raised to 4 by adding Ca(OH)2 and the precipitate formed contained significant calcium phosphates. A second method developed in this study used a modified biochar to sorb P from the acid-extract. This biochar was produced through pyrolysis of sugarcane biomass and modified by MgCl2 at a temperature of 700°C for 1 hour under an N2 atmosphere. Sorption experiments found this biochar had a high P recovery efficiency, due to its positively charged surface with a consistent layered porous structure and a high surface area. P sorption of the optimal biochar was 131 mg P/g biochar. Post-sorption, this biochar was P-laden and was a high plant-available P fertilizer. Along with struvite produced from ISSA, which was produced by our research team, three kinds of recycled phosphate fertilizers (RPF) were used to test their agronomic effectiveness by soilless cultivation and soil cultivation, separately. The results showed that these three kinds of RPF had comparable agronomic effectiveness to that of commercial fertilizers and posed no harm to plants in terms of heavy metal contamination. The biochar-P increased the germination rate significantly and enhanced the root system development. The study results showed that the P in the ISSA can be successfully extracted for use as a P fertilizer.