Water Solutions - Correlation between KMnO4, COD, BOD, DOC and TOC

Water Solutions - Correlation between KMnO4, COD, BOD, DOC and TOC

What is the relation between Permanganate Value, Chemical Oxygen Demand and Dissolved Organic Carbon?

When designing an ion exchange installation, the Permanganate value (PV) of a water is used to determine the organic loading on the anion resin. However, this value is often not given and only BOD, COD or DOC values may be available. The following information is intended to help in converting BOD, COD and DOC values into PV values.

PV (Permanganate value) and COD (Chemical Oxygen Demand)

PV and COD are a measure of the amount of reduced compounds in a sample, which have been oxidized by a strong oxidizing agent. Although inorganic substances such as Fe2+, S2- may also be subject to oxidation, for most natural and industrial waters, the matter to be oxidized is organic in nature. Therefore these values can be used to characterize the organic load of a water.

For converting the COD value into a PV value, the following should be taken into account:

• For COD determinations, the organic matter is almost completely oxidized (conversion >90%) due to the stringent oxidizing conditions (K2Cr2O7 in excess, 2 hours, 150 DegC, catalyst Ag). In contrast, the Permanganate test is a much milder (KMnO4, 10 min., 120 deg C) and only the readily-oxidizable compounds will be converted. Conversion is only around 30-50% for natural waters; with industrial waters conversions vary even more (10% - 80%).
• The PV can be expressed either as ppm KMnO4 or ppm O2 whilst COD is ppm O2.

Therefore the following ratios have to be taken into account:

Equation 1:  PV(ppm KMnO4) = PV(ppm O2) * 4

Equation 2: PV(ppm O2) = COD(ppm O2) * 0.4

To express PV as ppm KMnO4, the value obtained as ppm O2 has to be multiplied by 4 (Equation 1). Equation 2 assumes that in the permanganate test (expresses as ppm O2 ) only 40% of the oxidizable matter are converted whereas the COD conversion amounts to 100%. This means that the COD as ppm O2  has to be multiplied by 0.4 to yield the PV as ppm O2. Equations 1 and 2 can be combined in the following equation (Equation 3):

Equation 3: PV(ppm KMnO4) = COD(ppm O2) * 1.6

Biological Oxygen Demand (BOD)

The BOD characterizes the biological biodegradability and is closely related to the PV, as it describes this part of the COD that is more readily oxidized. Depending on the type of water, the BOD5 (biodegradability in 5 days, expressed as ppm O2) from 0.5 up to 3 times the PV (as ppm O2), and for most waters a value of 1.5 can be applied. This results in the following equations:

Equation 4: PV(ppm O2) = BOD5(ppm O2) * 1.5

Equation 5: PV(ppm KMnO4) = BOD5(ppm O2) * 6
 
Dissolved and Total Organic Carbon (DOC and TOC)

DOC and TOC are a measure of the amount of dissolved organic carbon and total organic carbon present respectively. Except for municipal wastewaters, sludge-type fluids and other types of contaminated waters, the TOC value which is measured after filtration through a 0.45  m pore size filters corresponds approximately to the DOC.

Equation 6: DOC(ppm C) = TOC(ppm C)

As the oxidative parameters PV, COD and BOD5 characterize the organic load of a water, they correlate fairly well with the key parameters for organic load, DOC and TOC. For industrial waters, the ratio of COD (as ppm O2) to DOC (as ppm C) varies according to the composition from 4.0 (e.g. for a very reduced compound such as methanol) to 1.3 (very oxidized compounds such as formic acid). For natural waters, the ratio of COD to DOC has been found to be ~ 3, so equation 7 can be applied:

Equation 7:  COD(ppm O2) = DOC or TOC(ppm C) * 3

Combining Equation 7 with the ratio of COD to PV given by Equation 3, the following equation may be used to convert the DOC (or TOC) of a water into the PV:

Equation 8: PV(ppm KMnO4) = DOC or TOC(ppm C) * 4.8

Example:
A river with a TOC content of 4 ppm C, corresponds to:

COD(ppm O2) = 4 ppm C * 3  = 12 ppm O2

PV(ppm KMnO4) = 4 ppm C * 4.8  = 19.2 ppm KMnO 4