and low cost of device. a number of pure cultures, e.g. trichosporon cutaneum, bacillus cereus, klebsiella oxytoca, pseudomonas sp. etc. individually, have been used by many workers for the construction of bod biosensor. on the other hand, many workers have immobilized activated sludge, or a mixture of two or three bacterial species and on various membranes for the construction of bod biosensor. the most commonly used membranes were polyvinyl alcohol, porous hydrophilic membranes etc. a defined microbial consortium can be formed by conducting a systematic study, i.e. pre-testing of selected micro-organisms for use as a seeding material in bod analysis of a wide variety of industrial effluents. such a formulated consortium can be immobilized on suitable membrane, i.e. charged nylon membrane. charged nylon membrane is suitable for microbial immobilization, due to the specific binding between negatively charged bacterial cell and positively charged nylon membrane. so the advantages of the nylon membrane over the other membranes are : the dual binding, i.e. adsorption as well as entrapment, thus resulting in a more stable immobilized membrane. such specific microbial consortium based bod analytical devices, may find great application in monitoring of the degree of pollutant strength, in a wide variety of industrial waste water within a very short time. biosensors can be used to indirectly measure bod via a fast (usually <30 min) to be determined bod substitute and a corresponding calibration curve method (pioneered by karube et al., 1977). consequently, biosensors are now commercially available, but they do have several limitations such as their high maintenance costs, limited run lengths due to the need for reactivation, and the inability to respond to changing quality characteristics as would normally occur in wastewater treatment streams; e.g. diffusion processes of the biodegradable organic matter into the membrane and different responses by different microbial species which lead to problems with the reproducibility of result (praet et al., 1995). another important limitation is the uncertainty associated with the calibration function for translating the bod substitute into the real bod (rustum et al., 2008). fluorescent a surrogate to bod5 has been developed using a resazurin derivative which reveals the extent of oxygen uptake by micro-organisms for organic matter mineralization. a cross-validation performed on 109 samples in europe and the united-states showed a strict statistical equivalence between results from both methods. an electrode has been developed based on the luminescence emission of a photo-active chemical compound and the quenching of that emission by oxygen. this quenching photophysics mechanism is described by the stern–volmer equation for dissolved oxygen in a solution: i 0 / i = 1 + k s v [ o 2 ] /i~=~1~+~k_~} i : luminescence in the presence of oxygen i 0 : luminescence in the absence of oxygen k s v : stern-volmer constant for oxygen quenching [ o 2 ] : dissolved oxygen concentration the determination of oxygen concentration by luminescence quenching has a linear response over a broad range of oxygen concentrations and has excellent accuracy and reproducibility. polargraphic method the development of an analytical instrument that utilizes the reduction-oxidation (redox) chemistry of oxygen in the presence of dissimilar metal electrodes was introduced during the 1950s. this redox electrode utilized an oxygen-permeable membrane to allow the diffusion of the gas into an electrochemical cell and its concentration determined by polarographic or galvanic electrodes. this analytical method is sensitive and accurate down to levels of ± 0.1 mg/l dissolved oxygen. calibration of the redox electrode of this membrane electrode still requires the use of the henry's law table or the winkler test for dissolved oxygen. software sensor there have been proposals for automation to make rapid prediction of bod so it could be used for on-line process monitoring and control. for example, the use of a computerised machine learning method to make rapid inferences about bod using easy to measure water quality parameters. ones such as flow rate, chemical oxygen demand, ammonia, nitrogen, ph and suspended solids can be obtained directly and reliably using on-line hardware sensors. in a test of this idea, measurements of these values along with bod which had been made over three years was used to train and test a model for prediction. the technique could allow for some missing data. it indicated that this approach was possible but needed sufficient historic data to be available. real-time bod monitoring until recently, real-time monitoring of bod was unattainable owing to its complex nature. recent research by a leading uk university has discovered the link between multiple water quality parameters including electrical conductivity, turbidity, tlf and cdom. these parameters are all capable of being monitored in real-time through a combination of traditional methods (electrical conductivity via electrodes) and newer methods such as fluorescence. the monitoring of tryptophan-like fluorescence (tlf) has been successfully utilised as a proxy for biological activity and enumeration, particularly with a focus on escherichia coli (e. coli). tlf based monitoring is applicable across a wide range of environments, including but by no means limited to sewage treatment works and freshwaters. therefore, there has been a significant movement towards combined sensor systems that can monitor parameters and use them, in real-time, to provide a reading of bod that is of laboratory quality. dissolved oxygen probes: membrane and luminescence the development of an analytical instrument that utilizes the reduction-oxidation (redox) chemistry of oxygen in the presence of dissimilar metal electrodes was introduced during the 1950s. this redox electrode (also known as dissolved oxygen sensor) utilized an oxygen-permeable membrane to allow the diffusion of the gas into an electrochemical cell and its concentration determined by polarographic or galvanic electrodes. this analytical method is sensitive and accurate to down to levels of ± 0.1 mg/l dissolved oxygen. calibration of the redox electrode of this membrane electrode still requires the use of the henry's law table or the winkler test for dissolved oxygen. test limitations the test method involves variables limiting reproducibility. tests normally show observations varying plus or minus ten to twenty percent around the mean.: 82 toxicity some wastes contain chemicals capable of suppressing microbiological growth or activity. potential sources include industrial wastes, antibiotics in pharmaceutical or medical wastes, sanitizers in food processing or commercial cleaning facilities, chlorination disinfection used following conventional sewage treatment, and odor-control formulations used in sanitary waste holding tanks in passenger vehicles or portable toilets. suppression of the microbial community oxidizing the waste will lower the test result.: 85 appropriate microbial population the test relies upon a microbial ecosystem with enzymes capable of oxidizing the available organic material. some waste waters, such as those from biological secondary sewage treatment, will already contain a large population of microorganisms acclimated to the water being tested. an appreciable portion of the waste may be utilized during the holding period prior to commencement of the test procedure. on the other hand, organic wastes from industrial sources may require specialized enzymes. microbial populations from standard seed sources may take some time to produce those enzymes. a specialized seed culture may be appropriate to reflect conditions of an evolved ecosystem in the receiving waters.: 85–87