Assignment-Relationship between BOD5 and COD

Relationship between BOD₅ and COD

Introduction

Fish breathe the dissolved oxygen. But the procedure of degradation by microorganism uses dissolved oxygen at the same time. Hence, the main focus of wastewater treatment is reducing the oxygen consumption of microorganism use (Brown and Caldwell, 2001).

Hence, the indicators were developed to quantify the oxygen consumption to fit the wastewater treatment requirement.

Definition of BOD₅

BOD is short for Biochemical Oxygen Demand. Biochemical oxygen demand (BOD) is a measure to indicate the amount of the oxygen dissolved in waters used by microorganisms to degrade substance (Penn et al, 2007).

BOD₅ is named as the five-day BOD. The BOD₅ term comes from the BOD₅ test which is a standardized test to show the organic degradation ability in waste water. It has set the experiment period as five days. Within this period, experimenters measure the water sample under a particular standard condition. The BOD of this sample is obtained by using a bioassay test method. In the test, although five-day period is not long enough for the microorganism entirely degrading particular kinds of compounds. It is still long enough and possible to indicate the subsistence and reproducing ability of the microorganism.

Definition of COD

COD is short for Chemical Oxygen Demand. The Chemical Oxygen Demand (COD) term is the oxygen requirement for potassium dichromate to oxidate organism in water under an acid solution.

The extent of sample oxidation in COD test can be affected by experiment period, reagent type and COD concentration.

Relationship

In Boyles’ research, he listed the comparison of COD and BOD as below,

Comparison of COD BOD
Parameter	COD	BOD
Oxidant Used	K2Cr2O7, Mn2 (SO4)3	Oxidation by microorganisms
Most Suitable Use	Rapid and frequent monitoring of treatment plant efficiency and water quality	Modeling treatment plant process and the effects of organic compounds on the dissolved oxygen content of receiving waters
Test Completion Time	1-1/2 to 3 hours	5 days (for standard BOD test)
Accuracy and Precision	5-10% relative standard deviation; may be higher when samples contain suspended solids; sample homogenization can be important	15% relative standard deviation; not considered highly accurate
Advantages	•Correlates with BOD on waste with constant composition. •Toxic materials do not affect oxidant. •Changes in the COD value between influent and effluent may parallel BOD content and supplement BOD results •Short analysis time	•Most closely models the natural environment when used with the proper “seed”
Disadvantages	•Interference from chloride ions •Some organic compounds are not oxidized completely	•Toxic materials kill microorganisms •Microorganisms do not oxidize all materials present in waste •Inaccuracies when used with improper “seed” •Lengthy test period

Boyles, 1997. ‘Table 1 Comparison of COD BOD and TOC’

As it shows, the BOD test process is more complex than the COD test. BOD₅ takes five days to give out the oxygen demand. At the same time, the COD test only requires several hours. The accuracy of both BOD and COD are higher than 85% which results could be trusted.

Davies, 2005. The Biological Basis of Wastewater Treatment

From the graph above, it shows that the biologically degradable part of COD is almost the BOD. That is one of the most relevant relationships between COD and BOD.

The BOD/COD ratio can also be seen as an indicator. The ration indicator can be used to describe the pollutants leaching from landfill (Lee and Nikraz, 2014).

Conclusion

BOD term is relevant to COD term. They are both common and basic terms to indicate the dissolved oxygen. Both COD and BOD tests are highly accurate.

It is possible to use COD test results to estimate the BOD in the same situation (Boyles, 1997).

Reference

Brown and Caldwell. (2001). ‘A Guidebook fro Local Governments for Developing Regional Watershed Protection Plans’. Brown and Caldwell. Available at: https://epd.georgia.gov/sites/epd.georgia.gov/files/related_files/site_page/devwtrplan.pdf [Accessed date: April 2^nd, 2016]

Penn, M. Pauer, J and Mihelcic, J. (2007). ‘Biochemical Oxygen Demand’. Hawley's Condensed Chemical Dictionary. Available at: http://www.eolss.net/sample-chapters/c06/e6-13-04-03.pdf [Accessed date: April 2^nd, 2016]

Boyles, W. (1997). ‘The Science of Chemical Oxygen Demand’. Technical Information Series, Booklet No.9. Available at: http://pbadupws.nrc.gov/docs/ML0510/ML051080100.pdf [Accessed date: April 2^nd, 2016]

Davies, P. (2005). ‘The Biological Basis of Wastewater Treatment’. Strathkelvin Instruments Ltd. Available at: http://www.s-can.nl/media/1000154/thebiologicalbasisofwastewatertreatment.pdf [Accessed date: April 2^nd, 2016]

Lee, A and Nikraz, H. (2014). ‘BOD: COD Ratio as an Indicator for Pollutants Leaching from Landfill’. Journal of Clean Energy Technologies. 2(3). Available at: http://www.jocet.org/papers/137-E30005.pdf [Accessed date: April 2^nd, 2016]