Abstract—Hybrid bioreactor having both suspended-growth and attached-growth bacteria is found a novel and excellent bioreactor system for treating the municipal wastewater containing inhibitory substrates too. In this reactor a fraction of substrate is used by suspended biomass and the remaining by attached biomass resulting in the competition between the two growths for the substrate. The combination of suspended and attached growth provides the system with enhanced biomass concentration and sludge age more than those in ASP. Similar to attached growth system, the hybrid bioreactor ensures considerable efficiency for treating toxic and refractory substances in wastewater. For the process design of hybrid bioreactor a suitable mathematical model is required. Although various mathematical models were developed on hybrid bioreactor in due course of time in earlier research works, none of them was found having a specific simplified solution of the corresponding models and without having any drawback. To overcome this drawback authors already developed a simplified mathematical model for process design of a hybrid bioreactor. The present paper briefly highlights on the various aspects of process design of an aerobic hybrid bioreactor for the treatment of municipal wastewater. Keywords—Hybrid bioreactor, mathematical model, process design, application, municipal wastewater. I. INTRODUCTION YBRID bioreactor is the biological system where both suspended and attached growth biomass occur simultaneously. Suspended sludge-biofilm process is an aerobic hybrid process which combine suspended sludge with attached growth process by the incorporation of biofilm media into aeration tank of activated sludge process [7], [1], [9], [15]. The aeration tank was made hybrid bioreactor also by providing plastic nets vertically inside the tank [4], [6]. In this reactor competition for substrates exist between two different growths of bacteria. However, a suitable mathematical model is needed in the process design that considers two growths for a single substrate in a completely mixed system. Slowly biodegradable substances in the wastewater are generally the aliphatic and aromatic organic compounds, inorganic salts, organic solvents etc which pose unique problems in wastewater treatment, due to their resistance to biodegradation and potential toxicity to the environment and human health. Those compounds are difficult to treat in conventional biological treatment process and are termed as S. Sarkar is with Heritage Institute of Technology, Anandapur, Chowbaga, Road, P.O. East Kolkata Township, Kolkata-700107(corresponding author to provide phone: 8697777755; e-mail: sushovan.sarkar@heritageit.edu). D. Mazumder is with Indian Institute of Engineering Science and Technology, Shibpur, P.O. – Botanic Garden, Howrah – 711 103 (e-mail: debabrata@civil.iiests.ac.in). refractory or recalcitrant. It is the hybrid bioreactor which takes up challenges for stabilizing those slowly biodegradable substances. Rational analysis for the appropriate process design for predictions pertinent to the performance of the said reactor is extremely needed. II. ROLE OF HYBRID BIOREACTOR IN WASTEWATER TREATMENT The combination of suspended and attached biomass improved effluent quality, solids settling and promoted nitrification. The hybrid bioreactor system can eliminate recirculation of biomass by omitting the secondary clarifier and thereby economizing the entire process. Further the process reduces the volume of the tank thereby making optimization in reactor design and also imparts stability to the treatment system. In the past there was a problem in increase in biomass in a conventional activated sludge reactor due to difficulty in separation of large quantity of biomass in the secondary clarifier. These problems are eliminated in the hybrid bioreactor because of the attachment of a portion of biomass to the support medium. It has been established that two to five fold increase in biomass concentration is possible in hybrid bioreactor compared to that in a conventional activated sludge process [4]. Hybrid bioreactor has been satisfactorily used for the treatment of wastewater containing slowly biodegradable /recalcitrant substances. Solid retention time is higher for the adhered biomass than for the biomass flocs in suspension thereby making compatible for growth of nitrifying microorganisms preferentially onto the support [1]. III. PROBLEM SCENARIO So far, a few numbers of model expressions for the hybrid bioreactor was developed and almost none of them considered the concurrent growth of both suspended and attached biomass except the model proposed by [10]. However, the numerical solution obtained by Regular Falsi method in that case was an approximate one. In other cases, model expression for hybrid bioreactor was developed using either a set of dimensionless algebraic equation [8] or some graphical tools [2], which lead to an approximate solution. Earlier the numerical analysis method like regular falsi technique, pseudo analytical tool [10], [11], normalized loading curve were used to calculate the effluent substrate concentration in a hybrid bioreactor and no satisfactory result was obtained. Out of all, pseudo analytical appeared to be effective, because it simplified the analysis of biofilm segment in as hybrid bioreactor. However, it also could not provide an accurate solution and ultimately it was difficult to predict the performance of the hybrid bioreactor. Process Design and Application of Aerobic Hybrid Bioreactor in the Treatment of Municipal Wastewater Sushovan Sarkar, Debabrata Mazumder H World Academy of Science, Engineering and Technology International Journal of Chemical and Molecular Engineering Vol:9, No:3, 2015 500 International Scholarly and Scientific Research & Innovation 9(3) 2015 scholar.waset.org/1307-6892/10001004 International Science Index, Chemical and Molecular Engineering Vol:9, No:3, 2015 waset.org/Publication/10001004