Sensor Instrumentation Limitations

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.