Using COD to Estimate BOD

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The COD test is the most commonly used test to determine what the BOD is likely to be.


[edit] Advantages

  • Both CODand BOD are oxygen demand tests so theoretically these should be similar
  • Corrrelation is adequate for setting dilutions
  • Shortened test procedures can get useful data in around 10 minutes

[edit] Disadvantages

  • The standard test takes 2 hours to get useful data.
  • The test produces hazardous wastes which must be disposed of.
  • This test is generally not required so it is an extra step.

[edit] Correlation Methods

These two correlation methods are applicable to all methods of correlating BOD. For the sake of simplicity, COD is used as the example method.

[edit] Simple Ratio Method

Average the previous 10 COD results for a particular sample point, for example; with the influent. Find all of the corresponding BOD values for each of the TSS results averaged, then average those. Divide the BOD values by the COD values and multiply this ratio by the COD value for the day's BOD that is being estimated.

\frac{BOD_{average}} {COD_{average}}  COD_{current} = BOD_{estimated}

This method works the same way for TSS, TOC or potentially any method that is a good estimator for BOD.

[edit] Linear Regression Method

Use a computer spreadsheet to do a linear regression on the historical COD and BOD values. A free spreedsheet program, Open Office, can help with this task. Commonly used in industry, Microsoft Excel, can also perform linear regression in single or multiple variables.

If linear regression is used to determine the likely BOD, it is also possible to correlate the BOD against multiple values, for example COD, Ammonia, and TSS.

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