ENVIRONMENTAL ENGINEERING SCIENCE Volume 26, Number 3, 2009 © Mary Ann Liebert, Inc. DOI: 10.1089/ees.2007.0272 Model Based Optimization of Nitrogen Removal in a Full Scale Activated Sludge Plant H. Güçlü Insel, 1, * Erdem Görgün, 1 Nazik Artan, 1 and Derin Orhon 2 1 Istanbul Technical University, Environmental Engineering Department, Faculty of Civil Engineering, TR-34469, Maslak, Istanbul, Turkey. 2 Turkish Academy of Sciences, Piyade sokak No. 27, 06550, Çankaya, Ankara, Turkey. Received: November 9, 2007 Accepted in revised form: April 25, 2008 Abstract This paper evaluates model-based upgrading of an activated sludge plant treating wastewaters of leather tan- ning industrial district with an average capacity of 13,000 m 3 /d. Significant impact of temperature changes on biochemical processes, such as nitrification and denitrification were quantified and used in setting the aeration control for process optimization in terms of total nitrogen removal. Model evaluation based upon kinetic and stoichiometric parameters specifically determined for the tannery wastewater, indicated that an intermittent aeration type of an operation strategy was suitable for achieving the required effluent total nitrogen level of 100 mgN/L under all operating conditions at the process temperature range of 21°C–35°C. The simulation study also showed that the nitrification performance of the plant was improved by increasing the aeration intensity. The application of intermittent aeration provided a competitive advantage in meeting the total nitrogen stan- dards, improving the total nitrogen removal to around 60%. Key words: leather tanning; process optimization; nitrogen removal; intermittent aeration; process control; mod- eling 471 Introduction L eather tanning is one of the most important industrial category in terms of wastewater management. The tan- ning process requires a high amount of process water to- gether with a variety of different chemicals. Consequently, it generates a strong wastewater, reported to contain total chemical oxygen demand (COD) and total nitrogen levels in the range of of 3235–7420 mgCOD/L and 112–640 mgN/L, respectively (Orhon et al., 1999a). Hence, high treatment ef- ficiencies are required for both organic carbon and nitrogen loads before discharging to receiving waters. Activated sludge process is regarded as a suitable technology for treat- ing the organic pollutants from leather tanning wastewaters. However, the existing wastewater treatment plants designed for only organic carbon removal require appropriate process modifications and implementation of process control in or- der to comply with nitrogen discharge standards. Activated sludge models have been widely used in the in- terpretation of biologic performance and the implementation of plant-wide process optimization (Henze et al., 1987, 2000). These models have been utilized in activated sludge treat- ment plant design and control for nutrient removal from do- mestic sewage. However, full-scale industrial applications (textile, leather tanning, etc.) of activated sludge models are still lacking and more experience needs to be gained for full- scale system optimization. Model-based influent wastewater and biomass characterization are crucial in full-scale model- ing tasks considering the environmental factors (Murat et al., 2003; Görgün et al., 2007). For full-scale models, dissolved oxygen and therefore aeration, play a vital role on system performance, especially on the extent of nitrification and si- multaneous nitrification/denitrification processes taking place in biologic reactors. Recently, modeling studies have focused on evaluating the impact of aeration on the perfor- mance of different systems (Insel et al., 2006a, 2006b; Insel, 2007). This work was carried out for the treatment plant of an organized leather tanning industrial district, which was orig- inally designed and operated for organic carbon removal. The plant operation faces problems in meeting the recent reg- ulations now imposing a new discharge limitation of 100 mg/L for total nitrogen.The plant previously served as the study site of a number of similar investigations (Kabdasli et al., 1993; Ates et al., 1997; Orhon et al., 1998). Recently, Görgün et al., (2007) developed and calibrated a model to as- sess all specific biodegradation characteristics at three dif- ferent temperatures. The model was used to evaluate and verify the performance of the activated sludge system for the implemented operation conditions. This study was planned as a follow-up of this previous effort, with the main *Corresponding author: Istanbul Technical University, Faculty of Civil Engineering, Environmental Engineering Department, TR- 34469, Maslak, Istanbul, Turkey. Phone: +902122857302; Fax: +902122856587; E-mail: inselhay@itu.edu.tr.