An Observation on Sludge Granulation in an Enhanced Biological Phosphorus Removal Process Ying Hui Ong 1 , Adeline Seak May Chua*, Boon Pin Lee 1 , Gek Cheng Ngoh 1 , Mohd Ali Hashim 1 ABSTRACT: A sequencing batch reactor (SBR) seeded with floccu- lated sludge and fed with synthetic wastewater was operated for an enhanced biological phosphorus removal (EBPR) process. Eight weeks after reactor startup, sludge granules were observed. The granules had a diameter of 0.5 to 3.0 mm and were brownish in color and spherical or ellipsoidal in shape. No significant change was observed in sludge granule size when operational pH was changed from 7 to 8. The 208-day continuous operation of the SBR showed that sludge granules were stably maintained with a sludge volume index (SVI) between 30 to 55 mL/g while securing a removal efficiency of 83% for carbon and 97% for phosphorus. Fluorescent in situ hybridization (FISH) confirmed the enrichment of polyphosphate accumulating organisms (PAOs) in the SBR. The observations of sludge granulation in this study encourage further studies in the development of granules-based EBPR process. Water Environ. Res., 84, 3 (2012). KEYWORDS: Enhanced biological phosphorus removal, sequencing batch reactor, polyphosphate accumulating organisms, sludge granules, Candidatus Accumulibacter phosphatis. doi:10.2175/106143011X13184219229335 Introduction Sludge granules are formed when microbial cells aggregate into compact, dense, and fast-settling granules with a diameter ranging from 0.5 to 10.0 mm (Tay et al., 2006). In comparison to flocculated sludge, applications of sludge granules in wastewater treatment have gained increasing attention in recent years because of its good settling property, higher biomass retention, and tolerance to higher carbon loading and toxicity (de Bruin et al., 2004; de Kreuk et al., 2005; Dulekguren et al., 2003; Jiang et al., 2006; Lemaire et al., 2008). Sludge granulation could help overcome problems found in flocculated sludge wastewater treatment systems, such as sludge bulking, large footprint of treatment plant needed, and huge amount of waste sludge produced (Gerardi, 2002; Lin et al., 2003). A comprehensive body of literature on use of sludge granules in anaerobic wastewater treatment has been growing since 1980s (Fang et al., 1995; Lettinga et al., 1984; Wu et al., 2001). This technology has been widely adopted in both domestic and industrial anaerobic wastewater treatment opera- tions because of its high organic removal efficiency and low operational cost in terms of aeration energy consumption (Tay et al., 2006). The upflow anaerobic sludge blanket (UASB) reactor is one of the best known anaerobic wastewater treatment process using sludge granules as a core component. Since late 1990s, research on sludge granules has moved toward aerobic granules. This wastewater treatment system has been reported to be effective in simultaneous removal of chemical oxygen demand (COD) and nitrogen (Beun et al., 1999; Tay et al., 2002; Yang et al., 2003; Yilmaz et al., 2008). Recent studies have reported on the formation of sludge granules in enhanced biological phosphorus (EBPR) process (Ahn et al., 2009; Dulekgurgen et al., 2003; Lin et al., 2003; Wu et al., 2010). The EBPR process is a modified conventional activated sludge process with an anaerobic stage before an aerobic stage. This process is capable of removing both organic matter and phosphorus from municipal wastewater (Kawaharasaki et al., 2002; Seviour et al., 2003). The alternating anaerobic-aerobic condition in EBPR process enables the selection and pro- liferation of polyphosphate accumulating organisms (PAOs), microbial populations responsible for phosphorus removal. Under anaerobic conditions, PAOs incorporate significant amounts of carbon substrate and store them as polyhydroxyalk- anoate (PHA) while polyphosphate is released as orthophos- phate (PO 4 32 ). During the aerobic conditions, PAOs degrade intracellular organic matter, and phosphorus is taken up to replenish their internal polyphosphate storage. Net phosphorus removal is achieved through waste of sludge that contains high levels of polyphosphate. In the study carried out by Lin et al. (2003), phosphorus- accumulating microbial granules were developed at different phosphorus:chemical oxygen demand (P:COD) ratios in a se- quencing batch reactor (SBR) fed with synthetic wastewater and operated for EBPR. They further investigated the effect of P:COD on process performance, aerobic respirometric activity of the granules, size of granule, and density of granule. Dulekgur- gen et al. (2003) reported that with the formation of sludge granules in their EBPR process, the settling time and initial SBR volume were reduced by half while influent COD concentration doubled. Ahn et al. (2009) demonstrated that operational pH can affect sludge granule stability. In their microassessment of sludge granules in the EBPR process, Wu et al. (2010) found that the positively charged sludge particles formed during the release of phosphorus in anaerobic condition could stimulate sludge granulation. Despite several reports on sludge granulation in EBPR process, the main factors affecting formation of granules remains unclear. In an examination of the effect of operational pH on EBPR operated in a tropical climate, unexpected sludge granula- tion was observed. Sludge granules were stably maintained 1 Department of Chemical Engineering, Faculty of Engineering, Univer- sity of Malaya, Kuala Lumpur, Malaysia. * Department of Chemical Engineering, Faculty of Engineering, Univer- sity of Malaya, 50603 Kuala Lumpur, Malaysia; e-mail: adeline@um. edu.my. January 2012 3