BACKGROUNDChemostats are characterized by an increase in viscosity of the reaction broth and are prone to non-ideality. The present study is mainly focused on the identification of non-idealities, namely dead zone and bypassing of a real chemostat or continuous stirred tank bioreactor. RESULTSExperimental studies were conducted in non-reactive mode to study the hydrodynamic characteristics of a real chemostat involving Lactobacillus sp. isolated from Indian curd. The influence of change of relative viscosity on non-ideality parameters was studied. A simulated solution of PVA having the same viscosity range as the growth medium of Lactobacillus sp. were used to understand the hydrodynamic behavior of the reactor by conducting residence time distribution (RTD) experiments in non-reacting mode. Both steady and unsteady state analyses have been carried out with variation of the two main process parameters, namely dilution rate and substrate concentration. CONCLUSIONA clear portrait of the comparison between non-ideal and ideal systems has been cited here, and it was observed that the real or non-ideal system lags behind the ideal one in many aspects. The present study adds to the knowledge of biochemically reacting systems. (c) 2014 Society of Chemical Industry