Structural, physicochemical and microbial properties of flocs and biofilms in integrated fixed-film activated sludge (IFFAS) systems Basuvaraj Mahendran a , Lori Lishman b , Steven N. Liss a, * a School of Environmental Sciences, University of Guelph, Guelph, ON, Canada N1G 2W1 b Wastewater Science and Technology Directorate, Environment Canada, Burlington, ON, Canada L7R 4A6 article info Article history: Received 27 February 2012 Received in revised form 25 May 2012 Accepted 26 May 2012 Available online 13 July 2012 Keywords: Integrated fixed film activated sludge Floc Biofilm Physicochemical properties Extracellular polymeric substances Microbial community abstract Integrated fixed-film activated sludge systems (IFFAS) may achieve year-round nitrification or gain additional treatment capacity due to the presence of both flocs and biofilms, and the potential for multiple redox states and long solids retention time. Flocs and biofilms are distinctive microbial structures and characterization of the physicochemical and structural properties of these may provide insight into their respective roles in wastewater treatment and contaminant removal in IFFAS. Flocs and biofilms were examined from five different pilot media systems being evaluated for potential full scale implementation at a large municipal wastewater treatment plant. Flocs and biofilms within the same system possessed different surface characteristics; flocs were found to have a higher negative surface charge (0.35 to 0.65 meq./g VSS) and are more hydrophobic (60%e75%) than biofilms (0.05 to 0.07 meq/g VSS; 19e34%). The EPS content of flocs was significantly higher (range of 2.1e4.5 folds) than that of biofilms. In floc-derived extracellular polymeric substances (EPS), protein (PN) was clearly dominant; whereas in biofilm-derived EPS, PN and polysaccharide (PS) were present in approximately equal proportions. Biofilm EPS had a higher proportion of DNA when compared to flocs. Biofilm growth was preferential on the protected internal surfaces of the media. Colonization of the external surfaces of the media was evident by the presence of small microcolonies. The structural heterogeneity of the biofilms examined was supported by observed differences in biomass content, thickness and roughness of biofilm surface. The biofilm on the interior surface of media was found to be patchy with clusters of cells connected by an irregular arrangement of interconnecting EPS projections. Biofilm thickness ranged between 139 mm and 253 mm. The pattern of oxygen penetration is expected to be complex. Nitrifiers and denitrifiers were predomi- nantly associated with the biofilms, and the latter were found to be dispersed throughout the film and arranged in micro-clusters, suggesting partial oxygen penetration. ª 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Present address: School of Environmental Studies and Department of Chemical Engineering, Richardson Hall, Room 251, Queen’s University, Kingston, ON, Canada K7L 3N6. Tel.: þ1 613 533 6933; fax: þ1 613 533 6934. E-mail address: steven.liss@queensu.ca (S.N. Liss). Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres water research 46 (2012) 5085 e5101 0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2012.05.058