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.
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
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:
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.
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:-
A river with a TOC content of 4 ppm C, corresponds to:-
COD(ppm O2) = 4 ppm C * 3 = 12 ppm O2PV(ppm KMnO4) = 4 ppm C * 4.8 = 19.2 ppm KMnO 4
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ReplyDeleteIn water quality analysis, KMnO4 consumption, COD (Chemical Oxygen Demand), BOD (Biochemical Oxygen Demand), and TOC (Total Organic Carbon) are all related to organic matter and oxygen demand, with KMnO4 consumption often used as a quick indicator for oxidizable matter, while COD and BOD measure the oxygen required for chemical and biological oxidation respectively, and TOC measures the total organic carbon content.
ReplyDeleteHere's a more detailed breakdown:
1. KMnO4 Consumption (Permanganate Index):
What it measures:
KMnO4 consumption, or the permanganate index, is a method for assessing water quality, particularly for freshwater and treated drinking water, by determining the amount of oxygen demand by potassium permanganate.
How it works:
A water sample is heated with a known amount of potassium permanganate and sulfuric acid, and the amount of permanganate consumed is measured.
What it indicates:
It provides a measure of the oxidizable organic matter in the water, acting as a quick indicator for the presence of organic pollutants.
Relationship to other parameters:
KMnO4 consumption can be used to estimate COD, BOD, or TOC values.
2. Chemical Oxygen Demand (COD):
What it measures:
COD measures the amount of oxygen required to chemically oxidize the organic matter and other oxidizable substances in water.
How it works:
A water sample is treated with a strong oxidizing agent (like potassium dichromate) under specific conditions, and the amount of oxygen consumed is measured.
What it indicates:
High COD values suggest a high concentration of oxidizable organic matter and potential pollution.
Relationship to other parameters:
COD is a measure of the total oxygen demand, including both biodegradable and non-biodegradable organic matter, while BOD focuses on the oxygen demand from biological degradation.
3. Biochemical Oxygen Demand (BOD):
What it measures:
BOD measures the amount of oxygen required by microorganisms (bacteria) to decompose the organic matter in water through biological processes.
How it works:
A water sample is incubated under controlled conditions, and the change in dissolved oxygen over time is measured.
What it indicates:
High BOD values suggest a high concentration of biodegradable organic matter, which can lead to oxygen depletion in the water body.
Relationship to other parameters:
BOD is a measure of the oxygen demand from biodegradable organic matter, while COD includes both biodegradable and non-biodegradable organic matter.
4. Total Organic Carbon (TOC):
What it measures: TOC measures the total amount of carbon present in organic compounds in water.
How it works: The sample is oxidized, and the resulting carbon dioxide is measured.
What it indicates: TOC provides a measure of the overall organic carbon content in the water.
Relationship to other parameters: TOC can be used to estimate COD and BOD, and the COD/TOC ratio can provide information about the biodegradability of the organic matter.
Correlations and Relationships:
COD and BOD:
The relationship between COD and BOD depends on the type and concentration of organic matter in the water. In general, COD is higher than BOD because it measures the total oxygen demand, including both biodegradable and non-biodegradable organic matter.
COD and TOC:
There is a correlation between COD and TOC, but it's not always a simple linear relationship. The COD/TOC ratio can vary depending on the type of organic matter and the oxidation conditions.
KMnO4 consumption and TOC:
KMnO4 consumption can be used as a quick and inexpensive indicator of organic matter, and it can be correlated with TOC levels.
KMnO4 consumption and COD:
KMnO4 consumption can be used to estimate COD values, especially in drinking water applications.