Anaerobic degradation kinetics of particulate organic matter in untreated and sonicated sewage sludge: Role of the inoculum M.C. Tomei * , C.M. Braguglia, G. Mininni Water Research Institute, CNR, via Reno 1, 00198 Roma, Italy Received 19 April 2007; received in revised form 7 December 2007; accepted 12 December 2007 Available online 29 January 2008 Abstract Degradation kinetics of particulate matter in anaerobic digestion of secondary sludge, untreated and sonicated, was investigated by carrying out batch tests at different feed/inoculum ratio (F/I) (in the range of 0.1–4.0). Particulate COD degradation data were analysed using the four equations most widely utilized to model the hydrolysis process and the related kinetic parameters were evaluated. The increase of F/I results in a correspondent increase of the process rate up to one order of magnitude in the investigated interval for both untreated and sonicated sludge. The maximum step increase is observed in the range of 0.1–2.0 while for F/I varying from 2.0 to 4.0 only a modest enhancement of the process kinetics is detected. The effect of sonication on kinetics is not appreciable at low F/I, due to the low fraction of fed sludge and to the consequent strong substrate limitation, whereas at high F/I a slight increase is evidenced. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Sewage sludge; Particulate matter degradation; Hydrolysis kinetics; Ultrasound treatment 1. Introduction Anaerobic treatments are extensively employed to stabi- lise sewage sludge in wastewater treatment plants of med- ium and large capacities (>50,000 p.e.). The main objective of sludge anaerobic digestion is the degradation and destruction of organic substances, with consequent reduction of the odorous emissions and pathogens. This conversion is catalyzed by a large number of bacteria, that operate in synergy, catalyzing different chemical reactions so the metabolic pathways involved in the anaerobic degra- dation are quite complex. Detailed and accurate anaerobic process models have been proposed in the last decades (Batstone et al., 2002; Siegrist et al., 2002) but still it remains a bottleneck for the evaluation of the kinetic parameters for complex substrate matrices as it is the sew- age sludge. Data available are essentially referred to the degradation of simpler substrates or substrates having a more specific interest for industrial application and to the treatment of concentrated wastewater. In the last years a renewed interest for the anaerobic digestion of sludge raised from the chance of a possible increase in energy recovery and solids degradation by applying an appropri- ate secondary sludge pre-treatment like sonication (Wang et al., 1999; Tiehm et al., 2001; Lafitte-Trouque ´ and For- ster, 2002; Braguglia et al., 2006). In fact, ultrasound treat- ment leads to the breakage of the cell walls and bacteria membranes, so improving the bacterial eso-enzymes release into solution and enhancing the biocatalysis of the hydro- lytic reactions. In order to evaluate the anaerobic digestion perfor- mance, both in terms of removal efficiency and energy recovery, an accurate model and reliable kinetic and stoi- chiometric parameters are required. In the literature mod- els there is a general agreement on considering the hydrolysis of particulate matter as kinetically limiting step of the whole process (Li and Noike, 1992; Shimizu et al., 1993). With the aim to give a contribution to anaerobic sludge digestion modelling, in the present study degradation 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2007.12.035 * Corresponding author. Tel.: +39 06 8841451; fax: +39 06 8417861. E-mail address: tomei@irsa.cnr.it (M.C. Tomei). Available online at www.sciencedirect.com Bioresource Technology 99 (2008) 6119–6126