ARTICLE Monovalent Cations and Their Influence on Activated Sludge Floc Chemistry, Structure, and Physical Characteristics F. Kara, 1 * G.C. Gurakan, 2 F.D. Sanin 3 1 Selcuk University, 42031 Konya, Turkey 2 Department of Food Engineering, Middle East Technical University, Inonu Bulvarı, 06531 Ankara, Turkey 3 Department of Environmental Engineering, Middle East Technical University, Inonu Bulvarı, 06531 Ankara, Turkey; telephone: þ90-312-210-26-42; fax: þ90-312-210-26-46; e-mail: dsanin@metu.edu.tr Received 31 August 2007; revision received 11 November 2007; accepted 14 November 2007 Published online 13 December 2007 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/bit.21755 ABSTRACT: Multivalent cations have been known to be important components of activated sludge floc structure due to their bridging ability of the negatively charged sites on the biopolymer network. Recently in batch systems it was found that excess concentration of monovalent cations led to the deterioration in settleability, dewaterability of sludges and effluent quality of the system. In this study, effect of influent monovalent cations (potassium and sodium) on activated sludge floc structure was investigated in semi-continuous reactors. Results revealed that the increase in concentration of both ions correlated to the general increase in total EPS concentration. The zeta potential values were affected by the cation type and dose in such a way that sludge from sodium reactors had always higher zeta poten- tial values (higher negative charge) than the sludges from potassium reactors. Flocs from sodium reactors were more fragile and weak and the capillary suction time values of these sludges were higher compared to those from pota- ssium reactors. The findings of this research conclude that the floc structure is significantly weakened with the increase of monovalent cations. Even though EPS is produced, it is unable to bind the floc components together. With this, the physical properties of sludge deteriorate for both cations. Biotechnol. Bioeng. 2008;100: 231–239. ß 2007 Wiley Periodicals, Inc. KEYWORDS: activated sludge; capillary suction time; EPS; floc strength; hydrophobicity; potassium; sodium Introduction Activated sludge process is one of the most commonly used biological processes for municipal wastewater treatment. In this process, microorganisms in wastewater are grown under aerobic conditions to convert influent organic matter into carbon dioxide, ammonia-N, and new biomass. The quality of the effluent from activated sludge systems is highly dependent on the efficiency of solid–liquid separation process. Separation of solid biomass from the treated wastewater can only be achieved if effective bioflocculation occurs in the aeration tank. Activated sludge flocs are composed of microorganisms, debris, extracellular polymeric substances (EPS) and inorganic cations (Bruus et al., 1992; Eriksson and Alm, 1991; Higgins and Novak, 1997a,b). Microbial EPS are the key components for the aggregation of microorganisms in flocs. The EPS which is made up of protein, polysaccharide, lipids, and nucleic acids provide a biopolymer matrix for enmeshment of the microbes and cations (Brown and Lester, 1979; Eriksson and Alm, 1991; Higgins and Novak, 1997a; Sobeck and Higgins, 2002; Urbain et al., 1993). There are several bioflocculation mechanisms suggested, each emphasizing the importance of EPS. One of the widely accepted mechanisms is the polymer bridging of the microbial surfaces and EPS in the system (Jorand et al., 1995; Leppard, 1992; Ries and Meyers, 1968; Tenney and Stumm, 1965). The biopolymers have functional groups such as hydroxyl, carboxyl, and phosphate contributing to the overall negative charges on the floc surface. For this reason double layer compression by positively charged ions has been shown to be an important mechanism in floc formation by Zita and Hermansson (1994). Also, hydrogen bonding, hydrophobic interactions, polymer gel formation *Currently a Ph.D. student in the Department of Biotechnology, Middle East Technical University, Inonu Bulvari, 06531 Ankara, Turkey. e-mail: fkara@metu.edu.tr Correspondence to: F.D. Sanin Contract grant sponsor: Turkish Scientific and Technical Research Foundation (TUBITAK) Contract grant number: ICTAG-C085 103I032 ß 2007 Wiley Periodicals, Inc. Biotechnology and Bioengineering, Vol. 100, No. 2, June 1, 2008 231