jesc.ac.cn Journal of Environmental Sciences 21(2009) 1753–1760 Leaching of phosphorus from incinerated sewage sludge ash by means of acid extraction followed by adsorption on orange waste gel Biplob Kumar Biswas, Katsutoshi Inoue * , Hiroyuki Harada, Keisuke Ohto, Hidetaka Kawakita Department of Applied Chemistry, Saga University, Honjo 1, Saga 840-8502, Japan. E-mail: bipshi2001@yahoo.co.uk Received 27 January 2009; revised 22 April 2009; accepted 27 April 2009 Abstract Ashes from sewage sludge incineration have a high phosphorus content, approximately 8% (W/W), which indicates a potential resource of the limiting nutrient. Incineration of sewage sludge with subsequent recovery of phosphorus is a relatively new sludge treatment technique. In this article, the leaching of phosphorus by using sulfuric acid as well as hydrochloric acid by means of several batch experiments was presented. At the same time a selective recovery of phosphorus by adsorption was also discussed. The effects of acid concentration, temperature and time on extraction were studied. The phosphorus leaching increased with the increase in acid concentration and temperature. Kinetic studies showed that the complete leaching of phosphorus took place in less than 4 h. Selective adsorption of phosphorus by using orange waste gel provided a hint for recovery of this natural resource, which eventually could meet the ever-increasing requirement for phosphorus. The overall results indicated that the incinerated sewage sludge ash can be treated with acid to efficiently recover phosphorus and thus can be considered a potentially renewable source of phosphorus. Key words: sewage sludge ash; phosphorus; metals; acid extraction; recovery DOI: 10.1016/S1001-0742(08)62484-5 Introduction Phosphorus is a limited non-renewable resource, which is indispensable as an essential nutrient for the growth of organisms in most ecosystems, and can not be replaced by other elements. It is a very important element for many industries as well. The demand for phosphorus fertilizer alone increased with the increase in the world’s population from 9×10 6 to 40×10 6 metric tons between 1960 and 2000 and was expected to increase further to 20×10 6 metric tons by 2030 (Tilman et al., 2001). The commercial products of phosphorus including fertilizer are principally produced from phosphate rock. However, there have been some alarming reports that deposits of high-grade phosphate ores are likely to be depleted in the next few decades (Franz, 2008; Runge-Metzger, 1995; Yoshida and Galinada, 2002). Therefore, it is very important to develop a sustainable method to recycle and conserve the phosphorus used in society to compensate for such global exhaustion of phosphate ores and to cope with a quite limited supply of phosphate in future, which can eventually lead to conserve phosphorus resource. Phosphorus can be recycled from any phosphorus-rich residues such as phosphorus-rich ash, sewage sludge and agricultural residue (Pettersson et al., 2008a). However, in decision-making processes about ma- terials management, recycling costs need to be compared with disposal costs (Inyang et al., 2003). * Corresponding author. E-mail: inoue@elechem.chem.saga-u.ac.jp Sewage sludge is one of the end-products of the phys- ical, chemical and biological operations employed in modern wastewater treatment systems. The upgrading of treatment plants results in increased volumes of biological or chemical sludge, which may be more difficult to handle than a simple primary sludge. In addition, sludge volumes increase annually and more chemical sludge is produced with high metal and phosphate contents, there is the possibility to loss such elements associated with disposal (Tateda et al., 1997). One option for sludge handling may be the digestion of the sewage sludge utilizing its energy content to produce biogas. However, the volume of the residual material remains almost the same as that of the original sludge. Moreover, this system requires additional processes to separate phosphorus which remains in the residue after digestion. Because of these considerations, incineration has grown in popularity as a final disposal process for sludge. Solid sludge incineration of municipal solid waste, which results in approximately 70% and 90% reduction in mass and volume respectively (Kosson et al., 1996), is a component of integrated waste management in many countries. However, after incineration, the inorganic constituents (phosphorus and metals) are left in the ash (Scott, 1980). One of the recovery methods is to use the sewage sludge as a fertilizer directly on soil/land. A decade ago there was a great interest in spreading sludge on agricultural land due to the potential for recycling valuable components