Thermochemical treatment of sewage sludge ashes for phosphorus recovery C. Adam a, * , B. Peplinski b , M. Michaelis c , G. Kley a , F.-G. Simon a a Federal Institute for Materials Research and Testing (BAM), Division IV.3 Waste Treatment and Remedial Engineering, Unter den Eichen 87, 12205 Berlin, Deutschland, Germany b Federal Institute for Materials Research and Testing (BAM), Division I.3 Structural Analysis, Unter den Eichen 87, 12205 Berlin, Deutschland, Germany c Federal Institute for Materials Research and Testing (BAM), Division I.4 Process Analytical Technology, Unter den Eichen 87, 12205 Berlin, Deutschland, Germany article info Article history: Accepted 30 September 2008 Available online 25 November 2008 abstract Phosphorus (P) is an essential element for all living organisms and cannot be replaced. Municipal sewage sludge is a carrier of phosphorus, but also contains organic pollutants and heavy metals. A two-step ther- mal treatment is suggested, including mono-incineration of sewage sludge and subsequent thermochem- ical treatment of the ashes. Organic pollutants are completely destroyed by mono-incineration. The resulting sewage sludge ashes contain P, but also heavy metals. P in the ashes exhibits low bioavailability, a disadvantage in farming. Therefore, in a second thermochemical step, P is transferred into mineral phases available for plants, and heavy metals are removed as well. The thermochemical treatment was investigated in a laboratory-scale rotary furnace by treating seven different sewage sludge ashes under systematic variation of operational parameters. Heavy metal removal and the increase of the P-bioavail- ability were the focus of the investigation. The present experimental study shows that these objectives have been achieved with the proposed process. The P-bioavailability was significantly increased due to the formation of new mineral phases such as chlorapatite, farringtonite and stanfieldite during thermo- chemical treatment. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Phosphorus (P) is an essential element for all living organisms, including plants and animals, and cannot be replaced. Conse- quently, the agricultural sector requires large amounts of phospho- rus for food production (consumption of fertilizer-P in the EU-15 >1.2 million tons of P per year). However, P is a non-renewable re- source, becoming scarce in the near future, and should be saved by application of recycling processes to P-bearing waste streams. Sewage sludge is a carrier of P but is often contaminated with or- ganic pollutants and heavy metals (Marani et al., 2003; Harrison et al., 2006). Thus, conventional agricultural application of sewage sludge is a controversial issue. In order to protect the farmland from accumulation of pollutants, sewage sludge is often disposed or energetically used in power plants or cement industries, but P is in many cases irreversibly lost for recovery technologies. The European Project SUSAN (Sustainable and Safe Re-use of Municipal Sewage Sludge for Nutrient Recovery) is aimed to devel- op a sustainable and safe strategy for nutrient recovery from sew- age sludges using thermal treatment. A schematic of the strategy is presented in Fig. 1. Mono-incineration of the sludges completely destroys the organic pollutants in a first step. The incineration res- idues are ashes with high phosphorus content, but still contain heavy metal compounds above the legal limits for agricultural use. The fate of elements during incineration has been intensively studied (Chandler et al., 1997). Matrix or lithophilic elements are silicon, calcium, aluminium and iron, which form stable oxides or anions (i.e., silicates or alumosilicates). The fraction of volatile ele- ments consists of elements forming acid gases like halogens or sul- phur, volatile trace metals (e.g., mercury), and trace metal compounds (e.g., heavy metal chlorides). The behaviour of the var- ious metals is different. Fig. 2 displays the gaseous fraction of six heavy metal chlorides and oxides (Cd, Zn, Pb, Cu, Ni and Cr as pure substances in N 2 atmosphere) as a function of temperature calcu- lated with the thermodynamic modelling program HSC-Chemis- try Ò (Roine, 2002). Except for copper, the heavy metal chlorides are almost complete in the vapor phase below 1000 °C. The respec- tive heavy metal oxides have vapour pressures which are lower by several orders of magnitude and are not visible in Fig. 2, except for CdO and PbO. Therefore, the heavy metal oxides are mainly affili- ated with the group of lithophilic elements. However, conversion of the oxides to chlorides enables evaporation of these elements even at moderate temperatures. A review of the factors influencing the vaporization behaviour (i.e., temperature, chemical composi- tion, moisture and atmosphere) can be found elsewhere (Zhang and Kasai, 2004). Evaporation rates for heavy metals in fly ash from municipal solid waste incineration have been measured by Jakob et al. (1996). The evaporation of heavy metals from sewage sludge ash in the presence of a chlorine donor (e.g., magnesium chloride) at temperatures between 800 and 1000 °C, i.e., far below the melt- ing point of the ashes, was investigated by Adam (2004) and Kley 0956-053X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2008.09.011 * Corresponding author. Tel.: +49 (0) 30 6392 5843; fax: +49 (0) 30 6392 5917. E-mail address: christian.adam@bam.de (C. Adam). Waste Management 29 (2009) 1122–1128 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman