Environmental Research 97 (2005) 245–257 Persistent organic pollutants (POPs) in the conventional activated sludge treatment process: fate and mass balance $ Athanasios Katsoyiannis, Constantini Samara à Department of Chemistry, Aristotle University of Thessaloniki, Environmental Pollution Control Laboratory, Thessaloniki GR-54124, Greece Received 6 January 2004; received in revised form 2 September 2004; accepted 15 September 2004 Available online 28 October 2004 Abstract The fate and the mass balance of persistent organic pollutants (POPs) during the conventional activated sludge treatment process were investigated in the wastewater treatment plant of the city of Thessaloniki, northern Greece. The POPs of interest were 7 polychlorinated biphenyls and 19 organochlorine pesticides. Target compounds were determined at six different points across the treatment system: the influent, the effluent of the primary sedimentation tank, the effluent of the secondary sedimentation tank, the primary sludge, the activated sludge from the recirculation stream, and the digested/dewatered sludge. The distribution of POPs between the dissolved and the adsorbed phases of wastewater and sludge was investigated. A good linear relationship between the distribution coefficients, K d , and the octanol–water partition coefficients, K ow , of the solutes was observed only in raw wastewater, suggesting that other factors affect the phase distribution of organic compounds in treated wastewater. For all POPs, a significant increase in partitioning with a decreasing solids concentration was observed, revealing an effect from non-settling microparticles remaining in the ‘‘dissolved’’ phase during the separation procedure. A good linear relationship was also revealed between log K d and the dissolved organic carbon (DOC) content of wastewater, suggesting that DOC favors the advective transport of POPs in the dissolved phase. Almost all POPs showed good mass balance agreements at both the primary and the secondary treatment. The losses observed for some species could be attributed to biodegradation/biotransformation rather than volatilization. The relative distribution between the treated effluent and the waste sludge streams varied largely among different compounds, with p-p 0 -DDE being highly accumulated in the waste sludge (98%) but almost 60% of a-HCH remaining in the treated effluent. r 2004 Elsevier Inc. All rights reserved. Keywords: Chlorinated pesticides; PCBs; Phase distribution; Sewage sludge; Wastewater treatment plant 1. Introduction Persistent organic pollutants (POPs) constitute a class of man-made chemicals with a pronounced persistence against chemical/biological degradation, environmental mobility, a tendency for bioaccumulation in human and animal tissue, and significant impacts on human health and the environment, even at extremely low concentra- tions. The chemical properties of POPs, such as low water and high fat solubility, stability in the face of degradation processes, and a low vapor pressure, are the hallmarks of their efficacy as pesticides and for their persistence at the environment (Kim and Smith, 2001). Despite their ban in the USA and Europe in the mid- 1970s, some of them were still in use into the 21st century in developing countries (Pandit et al., 2001), until the Stockholm Convention for POPs banned the production and use of those chemicals worldwide in 2001. ARTICLE IN PRESS www.elsevier.com/locate/envres 0013-9351/$ - see front matter r 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.envres.2004.09.001 $ This work was funded partially by the Water and Wastewater authorities of the city of Thessaloniki, Greece (E.Y.A.Th. S.A) and by the Greek General Secretariat for Research and Technology (G.S.R.T.), under the framework of Project PENED-2001, Common Minister Decision 9385/12-09-2002 (Code: 01 ED 489). à Corresponding author. Laboratory of Physical Chemistry, Depart- ment of Chemistry, Aristotle University of Thessaloniki, Thessaloniki GR-54124, Greece. Fax: +30 2310 997747. E-mail address: csamara@chem.auth.gr (C. Samara).