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other systems based on UV Visisible Spectrometry use the full spectrum involving measurement of organics over the entire spectrum starting from 200 nm to 750 nm with adaptive ranging capabilities. The system takes care of single bond organics also. Any change in matrix needs recalibration of the system to assess the factor for determination of COD & BOD. Preferred tools such as multi-wavelength dual beam scanning in UV-Visible range with library of industrial matrices in analyser with multi-point local adaptive calibrations help to have matrix change adaption seems better for analyser. The measured values are determined from the spectral data . The calculation is based on methods and characteristics that were achieved from a multitude of measurement and long time analysis and the observation from UV spectra. Thus, correlations vary with the change in waste matrix and these characteristics need to be frequently established in the beginning for better data quality visà- vis the actual values monitoring using laboratory technique. Any change in the waste matrix requires revalidation of the characteristics.

Some manufacturers have developed COD Analyser working on the same principle as the Laboratory technique for monitoring of COD. Many other technologies using direct/indirect methods have been introduced for monitoring of BOD & COD.

The standards for industrial discharges as notified in the EP(Act) 1986 and the water Act 1974 specify BOD and COD as the controlling parameter besides other specific parameters. Limited instrumentation for direct monitoring of these parameters i.e. BOD & COD on real time basis is available. The methods available require intensive infrastructure, besides using chemicals in the process which are released/discharged and can be a likely source of pollution. Besides the measurement is in batches, as it may take upto 02 hours to analyze one sample depending upon the concentration. These are indirect methods available for estimating BOD & COD. The 02 methods most commonly used for estimating BOD & COD in water and waste water samples are:

1) Deriving from TOC values and

2) Using UV Absorption spectrophotometry

In India TOC is not specified as a control parameter to industries or CETPs/STPs, therefore the values of BOD & COD has to be interpolated from TOC values.

In the first method TOC is measured and based on the laboratory validation as regards to the observed ratio of TOC:BOD & TOC:COD a correlation factor is established. The method of TOC measurement is approved. In the field TOC is monitored online using any of the approved principles i.e. persulfate or heated persulfate oxidation method, high temperature combustion method and assessment made through NDIR technique. Based on repeatable empirical relationship established between TOC, BOD or COD for a specific waste water source accompanying BOD or COD can be estimated from the recorded TOC values. This relationship between TOC:BOD & TOC:COD must be established for each set of matrix condition. Any change in waste water matrix impacts correlation between these pollutants and hence necessitates regular validation of the relationship between these pollutants.

Considering the need of skilled manpower, the requirement of gases and other peripheral requirements beside high O&M cost for operating TOC Analysers, need of an alternative method was felt.

The other method developed and deployed as a surrogate method is based on UV-Visible Spectrophotometry. This UV-Vis spectral absorbance technology has been found to be less labour intensive in comparison. The trade offs in the ability to compensate for the various interferences should be taken into consideration. Some Instrument Suppliers have incorporated features in their system to compensate for the interferences to improve their data quality. Some of the systems based on UV-Vis spectrophotometry use a single wavelength (i.e 254nm) or few wavelength bands to estimateCOD/BOD values . These system have measurement limitations The method is suitable for fairly stable water matrix. Turbidity interferes in the measurements of COD and BOD. The measure is an indirect method as absorbance at specified wavelength is
measured and correlated with COD & BOD.

In spite of the inherent advantages of on-line sensors/monitors, their wide application is still limited
due to the following reasons :

1. On-line monitoring suffers from more problems than laboratory-based methods because to date, the majority of on-line monitoring technologies developed are direct adaptations of traditional, laboratory-based analytical methods which were not originally designed for field applications.

Instead they are required to operate in extreme and variable measurement environments. Consequently, these methods require frequent calibration and maintenance.

2. In addition the analysers are often influenced from cross responses due to matrix variations between the standards and samples analysed, as the measurement conditions are not controlled.

3. Changes in sample matrix affect on-line analysers making it difficult to obtain continuous, reliable measurement in the field.

4. There are also significant economic and logistic costs associated with maintaining remote equipment, as it is difficult for operators to detect problems such as sensor fouling.

5. The problems associated with conventional on-line analysers are due to the fact that univariate linear calibraton models derived from Gauss’s theory of least squares are employed to determine unknown concentrations. Therefore, the samples and standards must be measured with equal care, under the same measurement conditions, to obtain reproducible and accurate results. Since such consistent measurement conditions are rarely present in the field this affects the operating conditions required for reliable performance and causes a high degree of unreliability in the results from online instrumentation.

6. Due to this reason the users/regulatory authorities need to frequently validate their online results with laboratory based methods.

7. The cost associated with maintenance of these conventional instruments has also greatly limited
their wide spread application.

NOTE : The industry must take full preview of available technologies while product selection and above referred limitations have to be curtailed.

The organic carbon in water and wastewater is composed of a variety of organic compounds in various oxidation states. Some of these carbon compounds can be oxidized further by biological or chemical processes, and the biochemical oxygen demand (BOD) and chemical oxygen demand (COD) may be used to characterise these fractions.

The presence of organic carbon that does not respond to either the BOD or COD test makes them unsuitable for the measurement of total organic carbon. Total organic carbon (TOC) is a more convenient and direct expression of total organic content than either BOD or COD, but does not provide the same kind of information. If a repeatable empirical relationship is established between TOC and BOD or COD, then TOC can be used to estimate the accompanying BOD or COD. This relationship must be established independently for each set of matrix conditions, such as various points in a treatment process. Unlike BOD or COD, TOC is independent of the oxidation state of the organic matter and does not measure other organically bound elements, such as nitrogen and hydrogen, and inorganics that can contribute to the oxygen demand measured by BOD and COD. TOC measurement does not replace BOD and COD testing.

1. TOC Analyser Configuration
Conductivity and Non dispersive infrared (NDIR) are the two common detection methods used in modern TOC analysers.

2. Conductivity based TOC analysers
Direct conductivity provides an all encompassing approach of measuring CO2. This detection method uses no carrier gas, is good at the parts per billion (ppb) ranges, but has a very limited analytical range.

Membrane conductivity relies upon the filtering of the CO2 prior to measuring it with a conductivity cell. Both methods analyse sample conductivity before and after oxidization, attributing this differential measurement to the TOC of the sample.
Conductivity analysis assumes that only CO2 is present within the solution. As long as this holds true, then the TOC calculation by this differential measurement is valid.

3. Non Dispersive Infrared (NDIR) TOC Analysers
The principal advantage of using NDIR is that it directly and specifically measures the CO2 generated by oxidation of the organic carbon in the oxidation reactor, rather than relying on a measurement of a secondary, corrected effect, such as used in conductivity measurements.

Merits
i. The low temperature techniques have the advantage of allowing a large volume of sample to be analysed thereby improving the low limit of detection.
ii. Also the blank value is very low as long as the reagents are pure, which makes the analysis more accurate
Limitations
i. Usually the particulates are more difficult to oxidise by nature or organics escape exposure to the reagents by being within the interstitial spaces of the particles.
ii. High molecular weight compounds such as proteins may be slow to oxidise with the low temperature techniques.

4. Combustion Technique
The combustion technique uses heat at 680oC or higher temperature in a stream of air, oxygen or nitrogen and usually in presence of a catalyst. Dissolved organics and particulate organics are expected to oxidise fully to carbon dioxide under these conditions. The catalysts vary from cupric oxide, cobalt oxide or platinum on an alumina support.

Analysis range
The range of TOC measurement varies with oxidation method and detection technique. A combustion/TOD method may measure upto 100% carbon in a sample, whereas the NDIR and conductivity detectors vary in range from as low as 0.5ppb to 25,000 ppm.