International Journal of Environmental Science and Development, Vol. 1, No. 4, October 2010 ISSN: 2010-0264 327 Abstract — A hybrid model of activated sludge reactor has been developed in laboratory scale to explore the feasibility of carbon oxidation along with nitrogen removal through nitrification and denitification in the same reactor. The reactor was operated initially under suspended growth only and then hybrid system containing 20 g/L of tyre tube beads. The reactor was operated continuously with the high strength composite synthetic wastewater containing COD and NH 4 + -N in the range of (1000 – 3500) mg/L and (500 – 1750) mg/L respectively that follows COD : NH 4 + -N equals to 2 : 1. About 80% removal of COD was achieved at a HRT of 8 hrs. even under purely suspended growth condition. At least 12 hrs. HRT was needed to achieve more than 90% NH 4 + -N removal for an influent NH 4 + -N of 1000 mg/L or more. Nitrite and nitrate formation was promoted with the addition of attached biomass by means of bio-carriers. The maximum extent of nitrite and nitrate formation was about 43.9% and 35.3% of influent NH 4 + -N respectively. Index Terms— ammonia nitrogen removal, COD removal, hybrid bioreactor, shaft-type. I. INTRODUCTION The aerobic hybrid bioreactor is recently being used to augment the additional treatment capacity of conventional activated sludge process. This system essentially comprises biomass both in suspended growth and attached phase, either under fixed or moving condition. Obviously, the introduction of attached biomass by means of carrier materials or fixed media enhances the total microorganism concentration in the reactor resulting in better removal of carbonaceous organic mater (expressed in terms of COD). Apart from heterotrophic carbon oxidation, a considerable amount of nitrification can also take place depending on the availability of dissolved oxygen (DO) concentration by the biomass attached with the media. With the progress of nitrification, DO level deteriorates reaching to a limiting value for aerobic oxidation, which may inculcate denitrification by the facultative denitrifiers present in the suspended growth stage. Furthermore, DO concentration gradually decreases towards the inner surface of the biofilm, creating an anoxic zone. Therefore, denitrification can take place at the inside of the Manuscript received on June 24, 2010. Debabrata Mazumder is with the Civil Engineering Department, Bengal Engineering and Science University, Shibpur, Howrah – 711 103, West Bengal, India; Phone: 0091-033-2668-4561 (Extn. 654); Fax: 0091-033-2668-4564; e-mail: debabrata@ civil.becs.ac.in). . biofilm in presence of organic carbon derived from endogenous decay of the heterotrophs in addition to some amount of residual organic carbon. In the year 1996, one of the two streams of the Annapolis, Maryland, USA step aeration activated sludge treatment plant of capacity 37,000 m 3 /d was modified by single-sludge anoxic-aerobic operation, and then fixed-film media were integrated into the aerobic zone to enhance nitrification [14]. The integrated Ringlace media increased the nitrification rate per unit volume to 225% of that observed in the control section, attaining a value of 1.75 kg NH 3 -N /d per linear meter at 15 o C. The media also increased denitrification in the aerobic media section permitting a potential of 25% or more reduction in the volume of the anoxic zone. Nitrification-denitrification was studied by Jih et. al. (2001) in a single-sludge reactor system treating high-strength nitrogen wastewater [6]. The NH 4 + -N removal efficiencies (98 - 82%) were observed to be higher than total nitrogen removal efficiencies (71 - 43%). The hybrid activated sludge-biofilm system like Integrated Fixed Film Activated Sludge (IFAS) has recently become popular for enhanced nitrification and denitrification in aerobic zones because it is an alternative to increasing the capacity of treatment plant units for accomplishing year round nitrification and nitrogen removal [16]. Simultaneous nitrification and denitrification (SND) via the nitrite pathway has also become a promising method that can significantly reduce the energy and COD demand for nitrogen and phosphorus removal [18]. The South-Budapest Municipal Wastewater Treatment Plant (SBWWTP) based on the high-load activated sludge process (ASP) was upgraded into nutrient removal in 1998-1999 in Hungary [11]. The study conducted over 25 months revealed that besides the efficient pre-denitrification obtained in the AS basin, significant ammonium oxidation was observed in the aerated zone. In the same year, a series of bench-scale nitrification/denitrification tests were carried out with both suspended growth and hybrid bioreactors [15]. The hybrid reactor was filled with plastic (polyethylene) media to evaluate the effects of biofilm. The results from the reactor operated under a MCRT of 10 days demonstrated that the hybrid system showed greater stability in nitrification and higher denitrification efficiency than the suspended growth system. A laboratory-scale fluidized-bed reactor with an external aeration loop was used for nitrification of high-strength ammonium wastewater (up to 500 mg NH 4 -N/L) [2]. The Simultaneous COD and Ammonium Nitrogen Removal from a High-strength Wastewater in a Shaft-type Aerobic Hybrid Bioreactor Debabrata Mazumder