Environmental and economic evaluation of an anaerobic digester treating waste activated sludge under different operational conditions. W. R. M. Leite*, E. W. Maffazzioli*, B. S. Magnus*, A. Dal Mago** and P. Belli Filho* * Department of Sanitary and Environmental Engineering, Federal University of Santa Catarina, Campus Trindade, Florianopolis, Santa Catarina, Brazil. (E-mail: engleite@gmail.com; brunasmagnus@gmail.com; ewmaff@yahoo.com.br; belli@ens.ufsc.br) ** Catarinense Company for Water and Sanitation, 83, Emilio Blum St., Downtown, Florianopolis, Santa Catarina, Brazil. (E-mail: adalmago@casan.com.br) Abstract This study compared the performance of a pilot anaerobic sludge digester under different operating strategies for the purpose of solids removal and stabilization. The influence of increasing organic loading rate - OLR (OP I) and then the effect of hydraulic retention time - HRT reduction (OP II and OP III) in anaerobic process were evaluated. Production of methane, organic matter degradation was used to measure and compare the system´s performance. The experimental time with HRT between 7 and 5 days showed the highest VS efficiency removals (higher than 70%), which positively influenced process stability. The applied OLR of 2.5 and 3.5 kgVS· m -3 ·d -1 resulted in higher yields of methane during OP I. Comparatively, HRT reduction resulted in a better system performance than the increasing OLR approach. The digester provided solids volume reduction over 50% and consequently cost savings which improve the overall sludge management. Keywords Sludge, stabilization, organic loading rate, solid retention time, economical analysis. INTRODUCTION Faced with the need for a sustainable final disposal for sludges produced in wastewater treatment plants (WWTP), an approach for strategies to their management is desired. Depending on the technology chosen, approximately 30 to 60% of operating costs of WWTP is related to the residual sludge treatment and disposal (Turonskiy and Mathai, 2006; Muhammad et al., 2011). Since the amount of sludge and its hygienic characteristics are strongly related to the overall costs of managing and sludge reuse, respectively, treatment processes to reduce the volume and pathogen control are recommended (Mathioudakis et al., 2009). Anaerobic digestion is a suitable method for treatment of sludge prior its disposal, being known as the most known and important technique for sludge stabilization (Metcalf and Eddy, 2003), resulting in destruction of volatile solids (VS) and biogas production, which results to lower costs for disposal. In this way this paper aims to present and compare the performance of a pilot anaerobic digester pilot under different operating conditions in order to achieve the stability and reduction of solids. An environmental-economic analysis was also developed to estimate the costs and savings of managing this waste using the digester. METODOLOGY The previously thickened secondary sludge (AD) was obtained from the largest WWT in Florianopolis, Santa Catarina, Brazil. The 100-L single stage high-rate anaerobic digester worked in mesophilic conditions (35 o C). The influent flow was determined according to the operational strategy adopted (Table 1).