A review of methods for the thermal utilization of sewage sludge: The Polish perspective Sebastian Werle * , Ryszard K. Wilk Institute of Thermal Technology, Silesian University of Technology at Gliwice, 44-100 Gliwice, Konarskiego 22, Poland article info Article history: Received 10 November 2009 Accepted 15 January 2010 Available online 11 February 2010 Keywords: Sewage sludge Pyrolysis Gasification Combustion Co-combustion abstract On the basis of demographic projections, it is estimated that the quantity of sewage sludge which will be produced in Poland between 2010 and 2018 will increase from 612.8 thousand tons (dry basis, d.b.) to 706.6 thousand tons (d.b.). Currently, the predominant method for the disposal of this sludge is its storage and agricultural application. However, the legislation taking effect in the next few years will effectively block these avenues of sewage-sludge disposal. Therefore, effective methods of thermal sewage-sludge utilization must be developed. Here we review the state of knowledge and technology in thermal methods for the utilization of municipal sewage sludge to obtain useful forms of energy such as pyrolysis, gasification, combustion, and co-combustion. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Municipal sewage sludge is generated in wastewater treatment plants in the process of sewage-treatment [1,2]. The waste stream of sewage sludge is rapidly growing, generating wastes which require management in compliance with the law [3]. According to Ministry of the Environment of Poland, sewage sludge belongs to group No.19, including the wastes from the treatment installation and the equipment for waste management, wastewater from wastewater treatment plants and from the treatment of drinking water and water for industrial purposes [4]. Poland is inhabited by 38.2 million people with an average population density of 122 persons per square kilometer, and has a territory of 322,577 km 2 , of which 311,904 km 2 is occupied by land. In the year 2008, in 3090 (both municipal and industrial) Polish sewage-treatment plants more than 1100 thousand tons (d.b.) of municipal and industrial sewage sludge was produced. In 2008, wastewater treatment plants serviced only 63% of the population (87% in urban areas and 26% in rural areas, where about 39% of the population lives); by compar- ison, in the countries of Western Europe, more than 78% of the population is serviced by waste treatment plants. In Poland, only 456 cities and 559 rural communities possessed a modern waste- water treatment plant with enhanced nitrogen and phosphorus removal. In these plants, 918 hm 3 of waste was treated, which accounts for 73% of the waste discharged through urban and rural sewerage systems. This ratio fluctuates around 70% in most coun- tries. The average unit index of sewage sludge generated in Polish wastewater treatment plants amounts to 0.25 kg d.b./m 3 of treated wastewater. Table 1 presents the quantities of municipal sewage sludge produced in Poland in the years 1999–2007 along with the anticipated amount in the future. According to the Polish Environmental Policy [5] and the objectives of the National Waste Management Plan 2010 (NWMP) [1] as well as the National Urban Wastewater Treatment Program (NUWTP) [2], the quantity of sewage treated in Poland is system- atically increasing. A measurable effect of this is, first of all, the increasing proportion of the population being served by waste- water treatment, but also the growth of the amount of produced sewage sludge (and sewage). On the basis of demographic projec- tions, it is estimated that the quantity of sludge which will be produced in Poland between theyears 2010 and 2018 will increase from 612.8 thousand tons (d.b.) to 706.6 thousand tons (d.b.) [1]. Currently, the predominant method for the disposal of sewage sludge is its storage and agricultural application. Sewage sludge is deposited in landfill sites dedicated exclusively to this sludge, in lagoons or jointly with municipal waste. Fig. 1 presents the current structure of the sewage-sludge management system [1]. In terms of the commitments derived from the introduction of European Union (EU) Directives, this structure of sewage-sludge utilization in Poland is highly unfavorable. The main problems are the high percentage of stored sewage sludge and a lack of installa- tions for its thermal utilization. Thermal processes can be used for the conversion of large quantities of sewage sludge (e.g., in large urban areas) into useful energy. Processes for thermal utilization of * Corresponding author. Tel.: þ48 32 237 29 83; fax: þ48 32 237 28 72. E-mail address: sebastian.werle@polsl.pl (S. Werle). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene 0960-1481/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.01.019 Renewable Energy 35 (2010) 1914–1919