The TDS of seawater is roughly 40,000 mg/L, whereas the COD is around 30,000 mg/L.
According to definition of COD, it must be equal or greater than TDS of any given sample, since COD amounts to all compounds in a sample of water or wastewater.
What is the cause for lower COD than its TDS in highly saline solutions?
COD is an index of water pollution by organics and is a bellwether parameter for sea water quality discharge. Various methods differing in oxidizing agent, acidity,catalyst,and temperature have been reported to measure COD. The TDS is the sum of all dissolved solids which included COD also. Thus TDS should be higher than COD.
Sir, COD should include acid digestion of all the organics and inorganic compounds (Dissolved and suspended) in water or wastewater. Hence, COD should be greater that TDS. kindly correct me if I am wrong.
Shahul, your point has some meaning for further study.It is good, if focus more by doing experiment too.
COD is oxygen demand - it seeks to measure the amount of oxygen needed to completely oxidize all organic matter present. Mercury is added to bind up chloride, so that it is not oxidized, although a modification of the standard method is needed for the high concentrations in seawater. Sodium, calcium, magnesium, and sulfate have no oxygen demand, at least under the test conditions. Most of the TDS in seawater comes from these inorganic salts (percent levels) without oxygen demand, while COD is usually much lower as it measures only the organics. Thus even the added concentration of COD from the organic suspended solids would still leave total COD much lower than TDS in these samples. I hope this is helpful!
Chemical oxygen demand (COD) is defined as the amount of a specified oxidant that reacts with the sample under controlled conditions. The quantity of oxidant consumed is expressed in terms of its oxygen equivalence.
Oxidation of most organic compounds is 95 to 100% of the theoretical value. Pyridine and related compounds resist oxidation and volatile organic compounds will react in proportion to their contact with the oxidant. Straight-chain aliphatic compounds are oxidized more effectively in the presence of a silver sulfate catalyst.
The most common interferent is the chloride ion. Chloride reacts with silver ion to precipitate silver chloride, and thus inhibits the catalytic activity of silver. Bromide, iodide, and any other reagent that inactivates the silver ion can interfere similarly.
from APHA 1999 Standard Methods for the Examination of Water and Wastewater
TDS means Total Dissolved Solid ( NaCl ,KCl MgSO4 etc that is all inorganic thing consider as total dissolve solid . No relation between TDS and COD. Relation has Total solid ( TS ) and COD. Total solid means organic and inorganic substance . COD might be zero having high TDS
- Badges
- Science topic
- Similar topics
- Engineering
- Petroleum Engineering
- Oil