ORIGINAL PAPER Hydrodynamic behaviour and its effects on the treatment performance of panelled anaerobic baffle-cum filter reactor R. Renuka 1 • S. Mariraj Mohan 2 • S. Amal Raj 1 Received: 2 November 2014 / Revised: 31 March 2015 / Accepted: 25 April 2015 / Published online: 27 May 2015 Ó Islamic Azad University (IAU) 2015 Abstract The aim of this paper was to present the in- fluence of hydraulic behaviour in the treatment of sewage (domestic wastewater) using panelled anaerobic baffle-cum filter reactor (PABFR). The PABFR has five compartments of equal size in which the first three compartments operate as anaerobic baffled reactor (ABR) followed by anaerobic filters (AF). The combined reactor has a great potential for its application due to its panelled structure and arrangement of baffles inside each compartment. Hydrodynamic be- haviour of the reactor was determined by means of a pulse tracer test and by calculating the residence time distribution curves at different flow rates. At high flow rates [with hydraulic retention time (HRT) \ 4 h], the mixing pattern in ABR showed a completely mixed type with a maximum dead space of 14 % and as the flow decreased (HRT of 8 and 12 h), the ABR’s mixing behaviour was intermediate between plug flow and completely mixed. In the case of AF, as the flow increased, the dispersion was intermediate between completely mixed and plug flow, and as the flow rate decreased, the reactor became completely plug flow with the dead space ranging between 2.2 and 7.4 %. On the whole, PABFR treatment performance in terms of chemical oxygen demand removal was 90, 89 and 64 % for 12-, 8- and 4-h HRT, respectively, which clearly shows the cor- relation between mixing and reactor process performance. Thus, the outcome of this research in general highlights the importance of hydrodynamic behaviour for a better op- eration of the reactor. Keywords Residence time distribution Á Hydrodynamic behaviour Á Panelled anaerobic baffled reactor cum filter reactor Introduction The application of anaerobic process on wastewater treatment has been studied intensively over the last few decades. The advantages of this type of digestion are that it does not require aeration, the construction costs are low, and large amounts of biogas is produced (Qi et al. 2013). Unstirred plug-flow (PF) anaerobic reactors often exhibit problems due to the accumulation of organic acids which decreases the pH level, ultimately resulting in poor reactor performance (Barber and Stuckey 1999). Studies on con- tinuous stirred tank reactor (CSTR)-type anaerobic diges- tion have shown that the treatment performance is low due to high mixing frequency (Lindmark et al. 2014). Thus, optimum mixing is required to get a positive effect on anaerobic treatment performance. The hydrodynamic behaviour in any biological reactor is of fundamental importance for the efficiency of wastewater treatment processes. The hydrodynamics and degree of mixing that occur within a biological reactor strongly in- fluence the extent of contact between the substrate and bacteria, thus controlling mass transfer (Mansouri et al. 2012). Adequate mixing promotes transfer of a substrate to the micro-organism and also provides heat to maintain a uniform temperature, thus assuring the effective use of the & R. Renuka renuramakrishnan@gmail.com 1 Department of Civil Engineering, Centre for Environmental Studies, Anna University, Chennai 600025, Tamil Nadu, India 2 Department of Civil Engineering, Alagappa Chettiar College of Engineering and Technology, Karaikudi 630 004, Tamil Nadu, India 123 Int. J. Environ. Sci. Technol. (2016) 13:307–318 DOI 10.1007/s13762-015-0824-z