Chitta,

These two tests measure different criteria.  Respirometers measure the rate of oxygen uptake eg mg O2 per minute.  Useful to indicate the level of biological activity eg healthy bacteria or the presence of toxins. 

The BOD test was used by the UK Royal Commission (1908 – 1915) to indicate degrees of stream pollution by measuring how much dissolved oxygen (mg/L) would be consumed in 5 days (longest distance to the sea) at 68°F (summer stream temperature).  It is still used in sewage effluent discharge licence conditions.  

The BOD test is also one of the key parameters used to measure the strength of wastewaters for the design of treatment plants.  Conversion to kg BOD/h (product of Flow x BOD) can be used to calculate aeration requirements and also the size of biological reactors. 

Thanks Michael. My point is BOD gives idea about the biodegradable substance present in waste water. However respirometer can also quantify the biodegradable substance from oxygen uptake rate curve. In market instruments are also available which claims that it is better than BOD5 test . 

Chitta,

I misunderstood your point.  You are correct, BOD is in effect a single 5 day point on the oxygen uptake curve and respirometry can be used to determine “on-line” equivalent BOD values.  I am familiar with the Oxi-Top respirometer used for BOD analysis which measures the pressure drop caused by oxygen depletion with NaOH addition to remove the CO2 generated.  Check samples sent to independent NATA accredited laboratories demonstrated satisfactory correlation.  However, it is not an official “standard method” and results can be challenged in legal disputes.  

Michael is absolutely correct.  there is no reason why a respirometer  cannot be used as you suggest and with careful calibration it can be used to get a very good estimate of BOD but it is the acceptability of the data in both the public domain and the scientific community that is the issue. The BOD test is, by necessity, a very closely defined method and in many jurisdictions it is written into laws regarding effluent compliance and into performance standards for treatment plants. To be honest I think this question would be best addressed to the local  environmental regulator who may have the last say on the matter.

As stated by both Michael and Alan, a respirometer might be used to determine BOD5 values. Meanwhile, as to the question about the more reliable technique, that will depend on the purpose of your study. For comparison purpose the BOD5 is a standard test that allows comparison between different treatments. Meanwhile, if you want to have more information about your specific sample the respirometer technique should provide you with more data and let you estimate the biodegradability of your substance on a wider scale.

I agree with Michael Lever, as tests they have different purpose. One (BOD5) tells you (indirectly) about the biodegradable organic matter in your wastewater under the tested conditions, while the other (respirometry) the rate of oxygen uptake (you use this test to compare). Of course you can use the respirometer (the apparatus) to measure BOD5.