Available online at www.sciencedirect.com Journal of Hazardous Materials 154 (2008) 550–557 Wastewater treatment in a hybrid activated sludge baffled reactor Mostafa Tizghadam, Christophe Dagot ∗ , Michel Baudu Laboratoire des Sciences de l’Eau et de l’Environnement, Universit´ e de Limoges, ENSIL, Parc ESTER, 16 Rue Atlantis, F-87068 Limoges Cedex, France Received 17 August 2006; received in revised form 17 October 2007; accepted 19 October 2007 Available online 30 October 2007 Abstract A novel hybrid activated sludge baffled reactor (HASBR), which contained both suspended and attached-growth biomass perfect mixing cells in series, was developed by installing standing and hanging baffles and introducing plastic brushes into a conventional activated sludge (CAS) reactor. It was used for the treatment of domestic wastewater. The effects on the operational performance of developing the suspended and attached-growth biomass and reactor configuration were investigated. The change of the flow regime from complete-mix to plug-flow, and the addition of plastic brushes as a support for biofilm, resulted in considerable improvements in the COD, nitrogen removal efficiency of domestic wastewater and sludge settling properties. In steady state, approximately 98 ± 2% of the total COD and 98 ± 2% of the ammonia of the influent were removed in the HASBR, when the influent wastewater concentration was 593 ± 11 mg COD/L and 43 ± 5 mg N/L, respectively, at a HRT of 10 h. These results were 93 ± 3 and 6 ± 3% for the CAS reactor, respectively. Approximately 90 ± 7% of the total COD was removed in the HASBR, when the influent wastewater concentration was 654 ± 16 mg COD/L at a 3 h HRT, and in the organic loading rate (OLR) of 5.36 kg COD m -3 day -1 . The result for the CAS reactor was 60 ± 3%. Existing CAS plants can be upgraded by changing the reactor configuration and introducing biofilm support media into the aeration tank. © 2007 Elsevier B.V. All rights reserved. Keywords: Wastewater treatment; Activated sludge; Hybrid activated sludge baffled reactor; Biofilm 1. Introduction More than 80% of biological wastewater plants are based on the principle of activated sludge process, in which suspended bacteria oxidise the carbonaceous and nitrogen compounds to produce an effluent that is in accordance with legal standards, and that corresponds to a minimal environmental impact. The increase of organic and hydraulic loads, related to the improve- ment of wastewater collection, and the implementation of new European directives and national regulations, often leads to dis- charges which do not comply with the standards. A new deal for wastewater engineers is now to stretch the performance of existing infrastructure, which represents one of the most sig- nificant challenges to the practice of wastewater engineering [1]. Currently, innovative processes are on the market, like the sequencing batch reactor (SBR) [2], membrane processes [3] and attached or hybrid growth processes [4]. ∗ Corresponding author. Tel.: +33 55 542 3670; fax: +33 55 542 3662. E-mail address: dagot@ensil.unilim.fr (C. Dagot). Upgrading overloaded conventional activated sludge (CAS) treatment plants is a promising solution, particularly when they have space limitations or need modifications that will require large investment. Upgrading activated sludge treatment plants for the enhancement of the COD removal, nitrification, den- itrification and sludge settling properties, can be achieved by optimising the existing system by changing the bioreactor con- figuration, or/and changing to a higher biological capacity process, with a combination of suspended and attached-growth processes. In the first case, it has been definitely shown that the hydro- dynamics of the process have an effect on rates of pollution removal, on the global kinetics of organic matter degradation and on the sludge settling properties. The hydrodynamics of the process are primarily dependent on the geometric characteris- tics of the reactor system, such as the configuration and the size of the reactor, and on the characteristics of aeration dynamics [5–7]. Filamentous bulking can be controlled by changing the hydraulic regime of the reactor from complete-mix to plug-flow. Pilot scale experiments indicate that a plug-flow configuration produces sludge with significantly better settling characteris- 0304-3894/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2007.10.092