Comparison between ozone and ultrasound disintegration on sludge anaerobic digestion C.M. Braguglia * , A. Gianico, G. Mininni Water Research Institute, CNR, Area di Ricerca RM1-Montelibretti, Via Salaria km 29.300, 00015 Monterotondo (Roma), Italy article info Article history: Received 30 September 2009 Received in revised form 26 May 2010 Accepted 18 July 2010 Available online 16 August 2010 Keywords: Ultrasound Ozone Sewage sludge Anaerobic digestion Dewaterability abstract This paper deals with the comparison of ultrasound (mechanical) and ozone (chemical) pre-treatment on the performances of excess sludge semi-continuous digestion. Sludge solubilisation has been investi- gated by varying specific energy input. For each pre-treatment, long anaerobic digestion tests were carried out by two parallel digesters: one reactor, as control unit, was fed with untreated waste activated sludge, and the other one was fed with disintegrated sludge. To evaluate and compare the efficacy of both pre-treatments, the specific energy was maintained approximately the same. The digestion tests were carried out to investigate the feasibility of anaerobic digestion performance (total biogas production, volatile solids removal, sludge dewaterability) and to assess the heat balance. Results obtained from the digestion of sonicated sludge at 4% disintegration degree (w2500 kJ/kg TS) showed that the ultrasound pre-treatment may be effective both in increasing VS destruction (þ19%) and cumulative biogas production (þ26%). On the contrary, the digestion test with ozonized sludge (ozone dose of 0.05 g O 3 /g TS corresponding to w2000 kJ/kg TS) did not indicate a significant improvement on the digestion performances. By doubling the ozone dose an improvement in the organics removal and cumulative biogas production was observed. Relevant differences in terms of colloidal charge and filterability were discussed. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Sludge management in an economically and environmentally acceptable manner is one of the critical issues facing modern society, due to the very fast increase in sludge production, as a result of the implementation of the Directive 1991/271. The challenge in the coming years will be both to sustain agricultural use of good quality sludge, and to reduce as much as possible disposal of polluted sludge especially in landfill. Upgrading of activated sludge treatment might be required in order to increase stabilization minimizing digestion residence time and maximizing biogas production. Waste activated sludge is very difficult to digest due to the rate-limiting cell lysis (Lafitte-Trouque and Forster, 2002). In the last years a renewed interest for the anaerobic digestion of sludge raised from the possibility of a significant gain in solids degradation and energy recovery by applying an appropriate sludge pre-treatment that leads to the breakage of flocs, cell walls, and bacteria membranes enhancing the hydrolysis of sludge vola- tile solids (Carballa et al., 2007; Khanal et al., 2007; Müller et al., 2003). This paper deals with the comparison of ultrasound (mechan- ical) and ozonation (chemical) pre-treatments. Ultrasonic pre- treatment leads to cavitation bubbles formation in the liquid phase. These bubbles grow and then violently collapse causing high shearing forces in the surrounding liquid phase and formation of radicals. Ozone, on the contrary, is used for water and wastewater treatment due to its strong oxidative properties. During sludge ozonation, because of the complex composition of sludge, ozone decomposes itself into radicals and reacts with the whole matter: soluble and particulate fractions, organic or mineral fractions (Böhler and Siegrist, 2004). If activated sludge is exposed to low- dose ozone at less than 0.02 g O 3 /g TS, ozone firstly destroys the floc, leading to the disruption of the compact aggregates (Chu et al., 2009). Moreover, Zhang et al. (2009) found that, by applying ozone dose at 0.05 g O 3 /TS, the soluble COD increased linear with the ozonation time during the first 105 min and then stagnated, probably because of the balance of sludge lysis and mineralization, showing that too long time was unnecessary. Extra ozone dose or long treatment time resulted in mineralization of dissolved organics and should be avoided. As regards the effect of pretreatments on the filterability, high- energy ultrasonic treatment can disrupt flocs and increase the number of fine particles and bound water. Therefore, only low * Corresponding author. Tel.: þ39 06 90672798; fax: þ39 06 90672787. E-mail address: braguglia@irsa.cnr.it (C.M. Braguglia). Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman 0301-4797/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvman.2010.07.030 Journal of Environmental Management 95 (2012) S139eS143