Ultrasonic lysis of Biological Sludge: A Three-stage Process

ACTIVATED sludge processes are the dominating wastewater treatment method. Large quantities of hazardous excess sludge are generated in the processes, and elimination of the waste sludge has been a major problem for the last decade worldwide. The disposal of waste sludge is subject to various environmental, social and economic problems; and the cost consists about 50% of the operational cost of WWTP [Friedler, 2006]. Thus, various techniques allowing sludge reduction have been developed recently. Among which, ultrasonic treatment is one of the most promising technologies [degaard, 2004]. High-energy ultrasound has been shown to disrupt the flocs and lyze the bacterial cells, leading to a reduction in particle size in the solution and improving its biodegradability [Gonze, 2003; Bougrier, 2005; Dewil 2006; Nickel, 2007]. Sludge is treated by ultrasounds to provide organic materials for further biological treatment prior to anaerobic digestion [Tiehm, 2001; Mao, 2006; Onyeche, 2002; Neis, 2001] or recycling into aeration tank [Zhang 2007b; Yoon, 2003]. Numerous studies showed that such a pre-treatment could reduce the sludge digestion time from 20 days to 8 days and increase the biogas production [Tiehm, 2001; Mao, 2006]; or to reduce the mass of excess sludge by 50%-100% [Zhang 2007b; Yoon, 2003]. The effects of sonication on physico-chemical characteristics of sludge including the sludge size, viscosity, capillary suction times, organic contents, soluble organic contents, and supernatant protein and DNAs were reported [Bien 1997; Chiu 1997; Chu 2001; Bougrier 2006; Zhang 2007a; Na 2007]. Impacts of important parameters such as the ultrasonic frequency, power intensity, and during were investigated [Chiu 1997; Tiehm 2001; Gonze 2003]. The process was optimized in terms of time, power density and sludge concentration [Show 2007]. Kinetic models were developed for the ultrasonic sludge disintegration [Wang 2005; Li 2007]. A recent review summarizes the mechanisms, applications and effects of sludge sonication [Khanal 2007]. A threshold was found to exist but the accurate value was inconsistent in different studies [Tiehm 2001; Gonze 2003; Bourgrier 2005]. Mechanical shear forces resulting from acoustic cavitation were believed to be the main mechanism for sludge disintegration while reactive oxidants such as hydroxyl free radical had little influences. Hence low sound frequency (20–40 kHz) and high energy intensity were preferred for the operation. ABSTRACT: Ultrasounds have been known very effective for biological sludge disintegration, which can be used as a pretreatment method prior anaerobic digestion or sludge minimization. This paper is a preliminary investigation of the involution of sludge matrix during sonication. The sludge was treated with 0.8 W/ml ultrasounds at 20 kHz and the changes of the sludge size, supernatant chemical oxygen demand (SCOD), supernatant protein, nitrogen, phosphate, protein, and polysaccharides during sonication were recorded. The results showed that ultrasonic sludge disintegration could be treated as a three-stages process. The first stage lasted around 5 minutes; during which the macro-flocs were destroyed, and the entrapped matters and free-water were released. The second stage lasted 10–15 minutes; during which the micro-flocs were destroyed, the extracellular polymer substances (EPS) were dissolved, the bacteria were disinfected, and the capillary water was released. The primary particulates dominated the last stage; and their small size prevented complete disintegration. As a result, the increase of supernatant organics showed a saturation effect.