BOD analysis of industrial effluents 5 days to 5 min

phương pháp mới nhằm xác định BOD từ 5 ngày xuống 5 phút

BOD analysis of industrial effluents: 5 days to 5 min Shikha Rastogi a , Pratima Rathee a , T.K. Saxena b , N.K. Mehra c , Rita Kumar a, * a Centre for Biochemical Technology, Delhi University Campus, Mall Road, Delhi 110 007, India b National Physical Laboratory, Dr. K.S.Krishnan Marg, New Delhi 110 012, India c Department of Zoology, Delhi University, Delhi 110 007, India Abstract Wastewater generation and its subsequent treatment is a major problem for every industry and for the society as well. Prior to treatment, the wastewaters need to be monitored so as to permit their discharge into the local water resources. Amongst all the parameters for which the wastewaters are monitored, biochemical oxygen demand (BOD), is one of the most important and fre- quently used parameters for estimating the level of water pollution. The control of wastewater treatment plants is very difficult or even impossible using the classical determination method for BOD because of its high time consumption (3–5 days). The need for fast, portable and cost-effective methods for environmental monitoring has stimulated the production of a variety of field analytical tools such as biosensors. Biosensors are device that have several unique features such as compact size, simple to use, one step re- agentless analysis, low cost and quick-real time results. The conventional BOD measurement requires 3–5 days, which a microbial BOD biosensor senses within minutes. A number of microbial BOD sensors have been developed nationally and internationally. The drawback of such developed sensors is that they cannot be used for all types of industrial and domestic wastewaters. Our developed BOD biosensor is based on a pre-tested, synergistic formulated microbial consortium. It is capable to sense the BOD load of a wide variety of synthetic as well as industrial wastewaters having low–moderate–high biodegradability within minutes. The sensor BOD values show a good linear relationship with the BOD values obtained using the conventional method upto a GGA concentration of 90 mg/l (r ¼ 0:938). BOD values of real wastewater samples from different industries viz, distillery dairy and tannery were analysed using the developed sensor. The BOD sensor results were found to be comparable with those obtained using the conventional 3-day method. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 82.47.Rs Keywords: BOD biosensor; Immobilized microbial membrane; Biochemical oxygen demand; Microbial consortium; Industrial wastewaters 1. Introduction The biochemical oxygen demand (BOD) test is a crucial environmental index to determine the relative oxygen requirements of wastewaters, effluents and pol- luted waters. It measures the molecular oxygen utilized during a specified incubation period for the biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidize inorganic material such as sulphides and ferrous ions. It can also be a measure of oxygen used to oxidize reduced forms of nitrogen (nitrogenous demand), unless their oxidation is pre- vented by an inhibitor [1]. The conventional BOD test requires a five day incubation period at 20 ° C and de- mands skill in determination, thereby, making it un- suitable for process control. Thus, it is necessary to develop a measurement method that could circumvent the weaknesses of the conventional method. The fast, portable and cost effective methods for environmental monitoring has stimulated the development of a variety of field analytical tools such as biosensors. Biosensors are devices that transduce a selective biochemical re- sponse to a measurable signal. Several biosensor meth- ods for BOD measurement have been developed. The first report of BOD biosensor was published by Karube et al. in 1977. After that, several kinds of mi- crobial BOD sensors have been developed and various modifications have been carried out [4,6,8,9,12,15,16, 18–29]. Most of the above reported BOD sensors con- sisted of a synthetic membrane with single or a random * Corresponding author. Tel.: +91-11-7666156/157/7602; fax: +91- 11-7667471. E-mail address: rita@cbt.res.in (R. Kumar). 1567-1739/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S1567-1739(02)00199-2 Current Applied Physics 3 (2003) 191–194 www.elsevier.com/locate/cap combination of immobilized microorganisms serving as biocatalyst. A rapid and reliable BOD sensor should aim at being highly capable of analysing a sample of com- plex constituents with relatively low selectivity. Thus the sensor can respond to all kinds of biodegradable organic solutes in the samples. It is also important that the sensor should give results comparable to those obtained using the conventional BOD method. Design and de- velopment of a BOD sensor, based on a pre-tested for- mulated, synergistic microorganism in combination with an oxygen electrode, was therefore considered in the present course of study. The developed BOD biosensor is capable of assimi- lating most of the organic matter present in different types of wastewaters as well as industrial effluents. The aim of present study was to obtain good agreement between results of the sensor BOD measurement and those obtained from the conventional BOD analysis. 2. Materials and methods A formulated, synergistic and pre-tested microbial consortium used as a reference seeding material for BOD analysis [10,14] was incorporated as the biocom- ponent in the developed BOD sensor. The microorgan- isms comprising the microbial consortium were harvested in their log phase of growth from the respec- tive broth cultures. The cell pellet was suspended in 50 mM phosphate buffer solution (pH 6.8) to obtain the cell slurry. This cell slurry was immobilized on charged nylon membrane by filtering small aliquots under moderate vacuum. The immobilized microbial membrane prepared in the above said manner was left to dry for 18–20 h at room temperature. Finally, the dried membranes were transferred in buffer solution and kept at 4 °C, till further use. The immobilized microbial membrane was then coupled to the electrode by holding the membrane against the teflon gas permeable mem- brane by means of a nylon net to get a complete elec- trode assembly. The response of the electrode was measured in term of current (nA) obtained on a mul- timeter. In a few minutes after the electrode assembly was immersed into a buffer solution, (50 mM K 2 HPO 4 – KH 2 PO 4 buffer, pH 6.8) the current become constant because the diffusion rate of oxygen into the microbial film from the bulk of the solution reaches equilibrium with the consumption rate of oxygen by endogenous respiration of the immobilized microbes. This current level is named as Ôinitial basal currentÕ. When appro- priate aliquots from a standard BOD solution were added into the stabilized electrode assembly, the current level decreased as the biodegradable compound diffuse into the microbial film from the bulk of the solution. Then, in few minutes, the current of the dissolved oxy- gen (DO) probe reached another constant current level known as the Ôfinal basal currentÕ. The difference be- tween the initial and final basal current values was de- fined as change in current (DI). Because the magnitude of the DI is proportional to a concentration of imme- diately biodegradable organic compounds in a sample in a certain range, an unknown BOD concentration in a sample is predictable based on the magnitude of DI observed. The developed BOD sensor was calibrated using a standard glucose–glutonic acid (GGA) solution [1]. BOD of the industrial effluents were measured using the BOD sensor and BOD 5 test based on the dilution method described in JISK0102 [5] and standard methods [1]. The results obtained using the two measurement systems were compared. 3. Results and discussion The overall characteristics of a BOD biosensor are determined by the characteristics of the microbial membranes used in combination with the electrochemi- cal sensor [3]. First amongst them is the effect of cell population on BOD values. A number of BOD biosen- sors have been developed using single organism or a random combination of organisms, but mixed cultures are shown to be an efficient biodegrading agent for organic compounds in aqueous solution, with good ki- netics, sensitivity, stability and reproducibility [7]. Keep- ing this in view, a formulated microbial consortium, developed and extensively tested with a wide range of synthetic as well as industrial wastewaters was used as biocatalyst for the construction of the BOD biosensors in the present study [11,14]. The BOD values as observed by the developed BOD biosensor reflect the concentrations of the dissolved organic substances which are assimilated/metabolized by the immobilized microbes [17]. The assimilation of organic substances in turn depends on the metabolic and physiological state of the immobilized biocatalyst, i.e., the microorganisms in use, their type, their phase of growth and density on the support. The prepared im- mobilized microbial membrane was therefore charac- terized w.r.t. the above stated parameters. The findings are depicted in Table 1. The developed and characterized BOD sensor was used to analyze the BOD values of different concentra- tions of the standard GGA solution, the BOD 5 of which were simultaneously carried out. These values were utilized to plot a calibration curve as illustrated in Fig. 1. A linear relationship was observed between the cur- rent difference (between initial steady state current and final steady state current) and the 5-day BOD of the standard solution upto a concentration of 90 mg l À1 . The linear range of the sensor is defined as the substrate 192 S. Rastogi et al. / Current Applied Physics 3 (2003) 191–194 range that gives a signal directly proportional to the concentration [2]. The lower limit of detection was 1 mg l À1 BOD, by the developed sensor. The current was reproducible within Æ5% of the mean in a series of 10 samples having 44 mg l À1 BOD, using standard GGA solution. The developed BOD sensor was used to estimate the BOD of real wastewater samples. For the same samples, 5-day BOD was also determined by the conventional method for comparison with BOD values estimated by the sensor. The wastewater samples analyzed were those emanated from dairy, distillery and tannery industries. Each of these wastewaters were diluted appropriately with buffer depending on their BOD load. Table 2 shows the comparative BOD values for different industrial wastewater as examined by both the methods. While estimating the BOD values with the sensor, different dilutions of the same wastewater showed varied results. This is so because the change in composition of a wastewater sample due to dilution could apparently in- fluence the bacterial respiration rate [13]. Moreover, the high molecular weight substances that are impervious to the immobilized microbial membrane may remain un- noticed while estimating the BOD with the help of the sensor. In addition, the immobilized bacteria might assimi- lates various organic substances in distinct metabolic pathways or procedures, resulting in different levels of oxygen consumption. If wastewater samples with differ- ent composition are analyzed, the immobilized bacteria could show different respiration rates even although the samples have the same BOD 5 values. Moreover, there might not be a universal standard solution that would be suitable for the calibration of real wastewater samples of different compositions. Acknowledgements We acknowledge the Ministry of Environment and Forests, New Delhi for financial assistance. The author Shikha Rastogi greatfully acknowledge the CSIR, New Delhi for Senior Research fellowship. 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Rastogi et al. / Current Applied Physics 3 (2003) 191–194 . 32,000 0 .5 32,000 1.0 31,000 Tannery (outlet) 57 5 50 2 .5 60 5. 0 90 10.0 50 25. 0 32 Tannery (inlet) 3440 1000 0 .5 1400 0. 65 1380 1.0 150 0 S. Rastogi et al correlation of BOD sensor with conventional BOD 5 . Table 2 BOD 5 vis-  aa-vis BOD sensor Sample COD (mg l À1 ) BOD 5 (mg l À1 ) BOD sensor % Sample BOD value (mg