Sixth International Conference on Thermal Engineering: Theory and Applications May 29-June 1 2012, Istanbul, Turkey Yvan Dutil Tel.: +1-418-653-2910 E-mail: yvan@t3e.info 1 Analysis of thermal disposal techniques for wastewater sludge N. Lacroix, D. R. Rousse, Y. Dutil* Chaire de recherche industrielle en technologie de l’énergie et efficacité énergétique (t3e). École de Technologie Supérieure, Université du Québec, 1100, rue Notre-Dame, ouest, Montréal, Canada Abstract Sludge management has become a source of important discussions in Québec province. The environmental authority, MDDEP, is planning to ban organic matter elimination by 2020. In this context, agricultural spreading receives increasing attention. However, incineration is used to dispose of more than 50% of the province total sludge production. In order to prevent contaminated sludge spreading on fields, MDDEP has developed a system to categorize fertilizing residuals. In order to determine sludge field valorization limitations, a grade is given according to the levels of chemical contaminants, pathogens and odor potential. When limits are exceeded, other disposal approaches must be used. In order to improve thermal disposal techniques, anaerobic digestion and gasification will be compared to a combination of both technologies. The objective is to determine if biomethanation followed by gasification of digestate could be more efficient than any of the two techniques taken individually. The three approaches are evaluated on a financial and energetic point of view. The research is currently underway and results will be presented at the conference. Keywords: Wastewater, sludge, combustion, gasification, biomethanation, anaerobic digestion. 1. Introduction Quebec province generates about 245 000 dry tons of municipal sludge. Incineration is by far the most important treatment used. In 2007, 52% of total sludge dry weight was incinerated and 30% was landfilled [1]. Agricultural sludge spreading has a very low volume and this can be partly explained by the apprehensions of soil contamination. Figure 1 shows the repartition of sludge disposition. Fig. 1. Distribution of sludge disposal methods in Québec province (2007, MDDEP) Sludge use on agricultural land is regulated by the provincial environmental authority, MDDEP. Sludge is part of what is called fertilizing residuals (FR). Agricultural use is restricted according to the grade the sludge receives. This grade is calculated according specific criteria and is composed of three categories. The first one is chemical contaminants that are regulated (C1 or C2). Second is pathogen levels (P1 or P2) and last is odor potential (O1, O2 or O3). Higher the score is, the more restrictions will apply. As an example, a FR such as municipal sludge that received a score of C1-P1-O1 will face none of little limitations for spreading on fields while one that received C2-P2-O3 will be highly limited. If the limit of any of the three categories is exceeded, the residual is then considered "out of category". Agricultural and silvicultural uses are then forbidden. Therefore, the land application is not suitable for every situation. In addition, the regulation focuses only on elements involved in biology, heavy metals and dioxins/furans. Limitations for non-regulated substances are much less explicit and rely partly on declarations from industrials. This contributes to the majors concerns about agricultural use of sludge in Québec. In the end, some sources of FR must not return to the environment without treatment. Also, it is generally accepted that municipal sludge must not return into the food chain by direct growing of crops for animal or human consumption. That mainly explains why the thermal approach is still very important.