Journal of Environmental Sciences 19(2007) 553–558 Sludge granulation and efficiency of phase separator in UASB reactor treating combined industrial effluent Abdullah Yasar ∗ , Nasir Ahmad, Muhammad Nawaz Chaudhry, Aamir Amanat Ali Khan Directorate of Research and Development, University of the Punjab, 41-Kamran Block, Allama Iqbal Town, Lahore-Pakistan. E-mail: yasar.abdullah@gmail.com Received 15 May 2006; revised 16 August 2006; accepted 22 September 2006 Abstract Sludge granulation and the effect of gas-liquid-solid separator (GLSS) design on the efficiency of upflow anaerobic sludge blanket (UASB) and upflow anaerobic sludge filter (UASF) reactors, operating at HRTs ranging from 3 to 12 h were investigated. VSS/TS ratio gradually increased in both the reactors with increasing sludge age (from 0.5 to more than 0.7 for UASB reactor and 0.012 to 0.043 for UASF reactor). X-Ray diffraction analysis of the UASF sludge showed the presence of expanding clays revealing its additional absorption capability. Fuoraphyllite and albite precipitation related to excellular polymers of the microbial shell structure, showed the extended growth of microorganisms during sludge granulation. A gradual decrease (82%–69%) in COD removal with decreasing HRT was apparent in UASF reactor. In case of UASB reactor, this decrease was marginal because addition of GLSS device significantly improved (14%–20%) the overall efficiency of the UASB reactor. GLSS enhanced the efficiency of the UASB reactor by increasing the settleability of suspended particles and accelerating the coagulation of colloidal particles due to the velocity gradient. Key words: sludge granulation; gas-liquid-solid (GLS) phase separator; UASB; microbial growth Introduction The success of the upflow anaerobic sludge blanket (UASB) reactor lies in the development of dense sludge bed at the bottom of the reactor, where biological digestion takes place. The sludge bed is basically formed due to ag- gregation of suspended solids and bacterial population into flocs and granules (Hulshoff, 1989). These dense aggre- gates have good settling properties and are not susceptible to washout from the system under practical conditions. The granulation of sludge enables the treatment system to show good treatment performance at high organic loading rates (El-Mamouni et al., 1997; Mahadevaswamy et al., 2004). It also leads to the reduction in the reactor size, which renders the treatment system cost effective. Never- theless, parameters like temperature and upflow velocity substantially affect the sludge granulation (Barbosa and Sant Anna, 1989). Many researchers (Barbosa and Sant Ann, 1989; Singh and Viraraghavan, 1998) investigated the formation of sludge granulation at ambient temperatures (19–28°C) and an upflow velocity (V up ) of 0.478 m/h. They observed spherical granules after one month of operation. Where as the size of the granules was increased up to 8 mm in diameter after a period of 9 months. For a well performing biological wastewater treatment system (UASB reactor) it is extremely essential to ensure good contact between the incoming substrate and the *Corresponding author. E-mail: yasar.abdullah@gmail.com sludge mass in the system and to maintain a large sludge mass in the system. In order to qualify these conditions, the treatment system (UASB reactor) is equipped with a gas- liquid-solid (GLS) phase separator, a column and effluent draw-off facilities (Kansal et al., 2003). The GLS device also helps to improve the overall treatment efficiency of the reactor by dividing it into a settling zone (upper part) and a digestion zone (lower part). The wastewater is introduced uniformly through the bottom of the reactor, it passes through the sludge bed (digestion zone) and then enters into the settling zone. The enlarged part of the reactor causes substantial decrease in the upflow velocity, which in return facilitates the flocculation of suspended sludge and enhances its settling. With the time, mass of accumulated sludge on the slopes of phase separator exceeds the frictional force and slides back into the di- gestion zone and supplement the digestion of the organic matter of incoming wastewater. Various types (designs) of phase separators have been investigated for treatment efficiency (Cavalcanti, 2003; Sayed and Fergalal, 1995). The introduction of proper phase-separator design into the conventional UASB can significantly improve its treat- ment efficiency under comparable conditions (El-mitwalli, 2000; Cavalcanti, 2003). According to El-mitwalli (2000), an addition of vertically oriented reticulated polyurethane foam sheets in the upper part of the UASB reactor offers relatively higher efficiency because the presence of foam sheets prevents sludge bed flotation. Cavalcanti (2003) demonstrates that a UASB reactor having parallel plates