Ozonation strategies to reduce sludge production of a seafood industry WWTP J.L. Campos * , L. Otero, A. Franco, A. Mosquera-Corral, E. Roca Department of Chemical Engineering, School of Engineering, University of Santiago de Compostela, Lope Gómez de Marzoa s/n, E-15782 Santiago de Compostela, Spain article info Article history: Received 22 May 2008 Received in revised form 24 July 2008 Accepted 29 July 2008 Available online 1 October 2008 Keywords: Activated sludge system Excess-sludge Ozone Wastewater abstract In this work, several alternatives related to the application of ozone in different streams of a seafood industry WWTP were evaluated to minimize the production of waste sludge. The WWTP was composed of two coagulation-flocculation units and a biological unit and generated around of 6550 kg/d of sludge. Ozone was applied to sludge coming from flotation units (110 g TSS/L) at doses up to 0.03 g O 3 /g TSS dur- ing batch tests, no solids solubilization being observed. Ozone doses ranging from 0.007 to 0.02 g O 3 / g TSS were also applied to the raw wastewater in a bubble column reaching a 6.8% of TSS removal for the highest ozone dose. Finally, the effect of the pre-ozonation (0.05 g O 3 /g TSS) of wastewater coming from the first flotation unit was tested in two activated sludge systems during 70 days. Ozonation caused a reduction of the observed yield coefficient of biomass from 0.14 to 0.07 g TSS/g COD Tremoved and a slight improvement of COD removal efficiencies. On the basis of the capacity for ozone production available in the industry, a maximum reduction of sludge generated by the WWTP of 7.5% could be expected. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Excess-sludge treatment and disposal supposes between 50 and 60% of operating costs of wastewater treatment plants (WWTPs). In Europe, the production of sludge reached 10 billions kg in 2005 and this production will increase due to the fact that waste- water treatment is expanding by the building of new WWTPs and the upgrading of existing plants because of more stringent effluent criteria (Paul and Debellefontaine, 2007). Main alternative methods for sludge disposal in EU are land application (45%), landfill (38%) and incineration (17%) (Ginestet, 2007). Land application of sludge is restricted to avoid health risks due to the presence of heavy metals or pathogens while building and operating costs of landfill are increasing due to the lower land availability and the stricter regulation. Incineration may also be the final option for sludge disposal but this process generates ash and discharges heavy metals to the atmosphere (Balogh and Nollet, 2008). Therefore, an affective strategy to reduce sludge disposal costs could be to minimize its production within the WWTP. Technologies for reducing sludge generated from both primary and biological treatments are gaining attention (Ødegaard, 2004; Benabdallah et al., 2007; Low and Chase, 1999; Liu and Tay 2001; Wei et al., 2003). Anaerobic digestion is the most used pro- cess to reduce sludge from the primary settling together with the biological excess-sludge. However, enzymatic hydrolysis of solids is the step which limits the solids degradation efficiency. For this reason, many researchers have focused their efforts to apply sev- eral pre-treatment methods to improve the rate of hydrolysis, thus enhancing solids removal (Goel et al., 2003a; Bougrier et al., 2006; Sievers et al., 2004). Bougrier et al. (2006) studied the effects of ultrasounds, ozonation and thermal pre-treatment and found that thermal treatment was the most efficient in terms of solids solubi- lization and improvement of biogas production. Minimization of the excess-sludge coming from the biological reactor may be achieved by either changes in operational condi- tions (Extended aeration, membrane bioreactor, improved aera- tion, chemical uncoupler...) or by treatment of the recycled activated sludge (Acid or base hydrolysis, thermal treatment, ozon- ation, mechanical disintegration...) (Mahmood and Elliott, 2006). Changes in operational conditions are mainly focused on promot- ing disappearance of suspended organic matter by mechanisms like maintenance, endogenous respiration, and decay of cells or grazing by higher organisms while the treatment of recycled acti- vated sludge is focused on promoting lysis of biomass by adverse environmental conditions (pH, oxidant conditions or temperature). Recently a technical and economical evaluation of various routes to allow wastewater treatment plants to produce less sludge showed that ozonation is the most feasible technology to achieve high reduction percentages of solids (Ginestet, 2007). Different strategies for ozone treatment can be applied (Paul and Debelle- fontaine, 2007): (1) Suspended solids may be directly pumped from the biologi- cal unit towards an ozonation contactor for partial oxidation and then pumped back to the biological tank for further bio- logical degradation. 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2008.07.056 * Corresponding author. Tel.: +34 981 563100x16777; fax: +34 981 528050. E-mail address: joseluis.campos@usc.es (J.L. Campos). Bioresource Technology 100 (2009) 1069–1073 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech