Kinetic study and characterization of sewage sludge for its incineration R. Rodriguez, D. Gauthier, S. Udaquiola, G. Mazza, O. Martinez, and G. Flamant Abstract: The sewage sludge from San Juan, Argentina, was characterized in an attempt to improve the thermal treatment efficiency and to reduce the environmental impact of incineration. The ash content is about 50% dry basis, and its weight loss at 105 8C was high. Taking into account the moisture content, it required drying for an autothermal combustion, thus imposing an additional combustible for incineration. The sludge contains high concentrations of several trace elements, and all but Hg are two to fivefold more concentrated in its ash. Thermogravimetric analyses were carried out on dry sam- ples of sludge in inert and oxidative atmosphere. Three peaks can be observed in all differential thermogravimetric curves during the organic matter decomposition. The pyrolysis curve is over the combustion curve and parallel to it up to high temperatures. A kinetic model of the sludge weight loss during its incineration, based on these studies and combining the multi-step pyrolysis and combustion processes, is proposed and discussed. Key words: sewage sludge, incineration, thermal analysis, pseudo-mechanistic models. Re ´sume ´: Les boues d’e ´puration de San Juan, en Argentine, ont e ´te ´ caracte ´rise ´es afin d’ame ´liorer l’efficacite ´ du traitement thermique et de re ´duire l’impact environnemental de l’incine ´ration. Les cendres contiennent environ 50 % d’humidite ´ sur sec et sa perte de poids a ` 105 8Ce ´tait e ´leve ´e. En tenant compte du taux d’humidite ´, ces boues doivent e ˆtre se ´che ´es pour une combustion adiabatique, exigeant ainsi l’usage de combustible additionnel pour l’incine ´ration. Les boues contiennent de fortes concentrations de plusieurs e ´le ´ments traces, et tous, sauf le Hg, sont de deux a ` cinq fois plus concentre ´s dans les cendres. Les analyses thermogravime ´triques, sous atmosphe `re inerte et oxydante, ont e ´te ´ effectue ´es sur des e ´chantillons secs de boues. Durant la de ´composition, toutes les courbes thermogravime ´triques pre ´sentaient trois pics de la matie `re orga- nique. La courbe de pyrolyse se situe au-dessus de la courbe de combustion et elle y est paralle `le jusqu’a ` des tempe ´ratures e ´leve ´es. Nous proposons et analysons un mode `le cine ´tique de la perte de poids des boues durant leur incine ´ration, base ´ sur ces e ´tudes et combinant la pyrolyse a `e ´tapes successives et les proce ´de ´s de combustion. Mots-cle ´s : boues d’e ´puration, incine ´ration, analyse thermale, mode `les pseudo me ´canistes. [Traduit par la Re ´daction] Introduction The treatment of municipal sewage sludge is becoming a concern because its production has noticeably increased and is expected to keep increasing. This increase is due to both world population growth and the development of wastewater treatment, which has become a necessity (Go ´mez-Rico et al. 2005). Most municipalities in Argentina are currently adopt- ing land filling and biocomposting as main alternatives for the final disposition of sewage sludge, but these conven- tional options are being progressively restricted because sewage sludge contains different toxic substances (both or- ganic and inorganic). The current trend indicates an increas- ing interest in thermal processes, particularly incineration, due to its many advantages: reduction of volume, destruc- tion of organic and pathogenic micropollutants, and energy production (Thipkhunthod et al. 2005). While producing thermal energy, incineration also generates gases and solid residues (powders, ash). Heavy metals (HM) are the main pollutants in the solid residues and their concentration and leachability determines the final disposal (Yao et al. 2004). The gas pollutants are nitrogen and sulphur oxides, highly toxic organic compounds such as dioxins and furans, several halogenic acids and also several vaporized heavy metals. The sludge composition varies greatly from one place to an- other. For instance, the heavy metals’ concentration depends strongly on the local industrial activity, which has a tremen- dous effect on the potential hazard of the incineration resi- dues. Therefore, the sewage sludge characterization is then of prime importance (Conesa et al. 1997; Font et al. 2001; Calvo et al. 2004). The energetic balance of the wet sludge combustion proc- ess is governed by the moisture content. The net energy re- leased is lower than during dry sludge combustion, since energy is required for the evaporation of the moisture. If the net energy is not enough for autothermal combustion, Received 16 November 2006. Revision accepted 4 December 2007. Published on the NRC Research Press Web site at jees.nrc.ca on 17 April 2008. R. Rodriguez 1 and S. Udaquiola. Lab. Tec. Amb, Instituto de Ingenieria Quı ´mica, Universidad Nacional de San Juan, Libertador 1109 (O) 5400, San Juan, Argentina. D. Gauthier and G. Flamant. PROMES-CNRS, 7 Rue du Four Solaire, 66120 Odeillo Font-Romeu, France. G. Mazza. Depto. De Quı ´mica, Fac. de Ingenierı ´a, Univ. del Comahue, Buenos Aires 1400, CP:8300, Neuque ´n, Argentina. O. Martinez. CINDECA-CONICET, PROIRQ, Fac. de Ingenierı ´a (UNLP), 1 y 47, La Plata, CP:1900, Argentina. Written discussion of this article is welcomed and will be received by the Editor until 30 September 2008. 1 Corresponding author (e-mail: rrodri@unsj.edu.ar). 247 J. Environ. Eng. Sci. 7: 247–257 (2008) doi:10.1139/S07-052 # 2008 NRC Canada