Simultaneous organic carbon and nitrogen removal in an anoxic–oxic activated sludge system under various operating conditions Kashif Rasool a , Dae Hee Ahn b,⇑ , Dae Sung Lee a a Department of Environmental Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu 702-701, Republic of Korea b Department of Environmental Engineering and Biotechnology, Myongji University, 116 Myongji-ro, Cheoin-gu, Yongin, Gyeonggi-do 449-728, Republic of Korea highlights  A modified pre-denitrification anoxic-oxic process was proposed.  Efficient pre-denitrification using returned activated sludge was achieved.  Sludge concentration, dissolved oxygen, pH and alkalinity profiles were monitored.  Proposed process was suitable for achieving COD and nitrogen removal. article info Article history: Received 25 September 2013 Received in revised form 17 March 2014 Accepted 21 March 2014 Available online 1 April 2014 Keywords: Activated sludge Anoxic–oxic Nitrification Recycling sludge Wastewater treatment abstract This study investigated a bench-scale anoxic–oxic activated sludge system for integrated removal of COD and nitrogen. The experimental unit includes four chambers and continuous feeding in first chamber without recycle of nitrified liquid from aerobic to anoxic chamber unlike the conventional anoxic–oxic process. Recycled excessive sludge was used for the purpose of recycling nitrified mixed liquor. Synthetic wastewater with average loading rates of 0.53 kg COD/m 3 /d and 0.067 kg NH 4 + -N/m 3 /d was fed to the reactor system at hydraulic residence times (HRT) of 24 and 18 h. The results of 100 days operation showed high removal efficiencies of organic matter of about 97% as total COD and more than 99% removal of ammonia–nitrogen. In anoxic–oxic operation phase, total inorganic nitrogen (TIN) removal was about 66% by pre-denitrification. Moreover, the solid liquid separation through final clarifier was excellent without any suspended solid in the effluent. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The contribution of nutrient-rich wastewater to the eutrophica- tion of inland and coastal waters has resulted in increased demands for COD, and nutrient removal from municipal and industrial wastewater (Chen et al., 2006; Klees and Silverstein, 1992). Nitro- gen is becoming increasingly important in wastewater manage- ment because nitrogen can have many adverse effects on the environment (Iqbal et al., 2010; Xiankai et al., 2008). Moreover, nitrates in wastewater streams have raised concerns due to severe impacts on human and animal health. Different agricultural land uses in the suburban areas cause a distinct difference in NO 3  con- tamination and the anoxic subsurface system associated with the geological settings and pavement coverage function to buffer NO 3  contamination due to active denitrification and less nitrification. Activated sludge process, being cost effective, is normally adopted for treatment of organics and nutrients rich industrial and domestic wastewater (Kulkarni, 2013; Naseer et al., 2013). Bio- logical nitrogen removal process uses nitrifiers and denitrifiers to achieve nitrogen removal from the wastewater. Nitrification demands a very efficient oxygen supply coupled with adjustment for changes in the alkalinity of the wastewater due to the forma- tion of hydrogen ions, whereas, denitrifying bacteria essentially need a carbon source as an electron donor. The primary nutrients which should be removed to prevent deterioration of water bodies are carbon, nitrogen and phosphorous. Carbon is not considered difficult to remove biologically. On the contrary, one of the most significant problems with treatment of many wastewaters is a lack of organic carbon, as the removal of both nitrogen and phospho- rous involve heterotrophic conversions, requiring an electron donor. Thus, treatment plants treating wastewaters containing low COD:N ratios experience difficulties in removing residual nitrogen and phosphorous due to a shortage of organic substrate (Naseer et al., 2013). This has led to an increased interest in the http://dx.doi.org/10.1016/j.biortech.2014.03.108 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Contact address: Department of Environmental Engi- neering and Biotechnology, Myongji University, Republic of Korea. Tel.: +82 31 330 6692; fax: +82 31 336 6336. E-mail addresses: dhahn@mju.ac.kr (D.H. Ahn), daesung@knu.ac.kr (D.S. Lee). Bioresource Technology 162 (2014) 373–378 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech