Water Research 37 (2003) 4561–4572 Mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. Effect of pre-treatment at elevated temperature Hariklia N. Gavala*, Umur Yenal, Ioannis V. Skiadas, Peter Westermann, Birgitte K. Ahring The Environmental Microbiology and Biotechnology Group (EMB), BIOCENTRUM-DTU, Bldg 227, Technical University of Denmark, Lyngby 2800, Denmark Received 26 June 2002; received in revised form 5 June 2003; accepted 19 June 2003 Abstract Anaerobic digestion is an appropriate technique for the treatment of sludge before final disposal and it is employed worldwide as the oldest and most important process for sludge stabilization. In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion. Furthermore, thermal pre-treatment is suitable for the improvement of stabilization, enhancement of dewatering of the sludge, reduction of the numbers of pathogens and could be realized at relatively low cost especially at low temperatures. The present study investigates (a) the differences between mesophilic and thermophilic anaerobic digestion of sludge and (b) the effect of the pre- treatment at 70  C on mesophilic and thermophilic anaerobic digestion of primary and secondary sludge. The pre- treatment step showed very positive effect on the methane potential and production rate upon subsequent thermophilic digestion of primary sludge. The methane production rate was mostly influenced by the pre-treatment of secondary sludge followed by mesophilic and thermophilic digestion whereas the methane potential only was positively influenced when mesophilic digestion followed. Our results suggest that the selection of the pre-treatment duration as well as the temperature of the subsequent anaerobic step for sludge stabilization should depend on the ratio of primary to secondary sludge. r 2003 Elsevier Ltd. All rights reserved. Keywords: Activity; Kinetics; Mesophilic digestion; Pre-treatment; Primary sludge; Secondary sludge; Thermophilic digestion 1. Introduction The residue generated during the primary, the secondary and the tertiary wastewater treatment is often called ‘‘sludge’’. Anaerobic digestion is an appropriate technique for the treatment of sludge before final disposal and it is employed worldwide as the oldest and most important process for sludge stabilization [1]. In general, mesophilic anaerobic digestion of sewage sludge is more widely used compared to thermophilic digestion, mainly because of the lower energy require- ments and higher stability of the process. However, the thermophilic anaerobic digestion process is usually characterized by accelerated biochemical reactions, higher growth rate of microorganisms and accelerated interspecies hydrogen transfer resulting in an increased methanogenic potential at lower hydraulic retention times [2]. Also, the enhanced hygienization effect of the thermophilic process [3,4] complies with the EU policy for elimination of pathogens, originating mainly from humans and animals; it has been reported that ARTICLE IN PRESS *Corresponding author. Fax: +45-45883276. E-mail address: hng@biocentrum.dtu.dk (H.N. Gavala). 0043-1354/$-see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0043-1354(03)00401-9