Effects and modelling of ultrasonic waste-activated sludge disintegration Serkan S ¸ ahinkaya 1 & Mehmet Faik Sevimli 2 1 Department of Environmental Engineering, Nevs ¸ehir University, Nevs ¸ehir, Turkey and 2 Department of Environmental Engineering, Selçuk University, Konya, Turkey Keywords modelling; pretreatment; sludge disintegration; sonication; ultrasonic; waste-activated sludge. Correspondence Serkan S ¸ ahinkaya, Department of Environmental Engineering, Engineering and Architectural Faculty, Nevs ¸ehir University, Campus, 50300 Nevs ¸ehir, Turkey. Email: serkansahinkaya@gmail.com doi:10.1111/j.1747-6593.2012.00358.x Abstract Sonication is a well-known sludge pretreatment technique with the advantages of simple operation and high efficiency. However, it is an energy-intensive process. Hence, it is very important to predetermine its sludge disintegration efficiency at varying pretreatment conditions in order to minimize the ultrasonic energy con- sumption. In this study, it was found that the ultrasonic sludge disintegration occurred in two stages: rapid and subsequent slow disintegration stages. For this reason, it was aimed to develop a simple and accurate mathematical model to describe the two-stage sludge disintegration as a function of pretreatment condi- tions. Sludge concentration and ultrasonic density along with sonication period were involved in this model as independent variables. It was determined that the mathematical model can predict accurately the degree of sludge disintegration. Thus, the proposed model was seen to be very useful for evaluating the disintegra- tion efficiency and/or for process design using the operating parameters under different conditions. Introduction Large amounts of waste-activated sludge (WAS) are inevitably produced as the main by-product of activated sludge process during biological treatment of wastewater. Management, reduction and minimization of the waste sludge to be handled have a significant effect on the economical and operational conditions of wastewater treatment plants. Han- dling cost of the WAS may be as high as 30–40% of the capital cost and 50% of the operating cost of the treatment plants (Vlyssides & Karlis 2004). For the stabilization of highly putrescible biological sludge, aerobic and anaerobic sludge digestion processes are commonly utilized. While aerobic digesters are employed in small wastewater treatment plants, anaerobic digesters are used in big plants with the benefits of mass reduction, methane production and improved dewatering properties of the digested sludge. However, extremely slow hydrolysis of the WAS limits the digestion performance of both aerobic and anaerobic digest- ers. In order to improve the rate limiting hydrolysis stage and the stabilization degree, ultrasonic pretreatment (named briefly as ‘sonication’) (Apul & Sanin 2010; Pilli et al. 2011; Sahinkaya et al. 2011), ozone oxidation (Erden & Filibeli 2010), mechanical disintegration (Wett et al. 2010), alkaline pretreatment (Sahinkaya et al. 2011), thermal pretreatment (Sahinkaya et al. 2011), Fenton oxidation (Erden & Filibeli 2010), microwave irradiation (Saha et al. 2011) and biological hydrolysis with enzymes (Ayol et al. 2008) were investigated in half-scale and lab-scale reactors. Among these pretreat- ment methods, sonication is the most effective sludge disin- tegration method. Sonication is a well-known sludge disintegration method. When ultrasonic pressure waves pass through a liquid medium; the cavitation bubbles are formed, grow and col- lapse violently depending on power density, frequency and duration of sonication. This phenomenon, named as ‘acous- tic cavitation’, causes some physico-chemical results; forma- tion of hydromechanical shear forces, production of oxidizing agents such as OH • ,O • ,N • and H • , pyrolysis of volatile organic compounds in the bubbles and rise in the temperature of bulk solution (Wang et al. 2005). Among these mechanisms, hydromechanical shear forces are predominantly responsible for the sludge disintegration (Huan et al. 2009) and are most effective at low ultrasonic frequencies below 100 kHz (Pilli Water and Environment Journal. Print ISSN 1747-6585 1 Water and Environment Journal •• (2012) ••–•• © 2012 CIWEM.