Answer to Question #233542 in Chemical Engineering for pavani

Question #233542

Explain about the BOD, COD & TOC?


1
Expert's answer
2021-09-22T00:22:12-0400

BOD, Biochemical Oxygen Demand

BOD is an indicator of the fraction of organic matter that can be degraded by microorganisms, typically expressed as the amount (mg/L) of oxygen consumed over five days at 20°C. BOD includes the readily biodegradable organic carbon (carbonaceous or cBOD) and occasionally ammonia (nitrogenous or nBOD). cBOD is the primary driver of BOD and includes soluble, particulate, and colloidal organic carbon compounds.

Weaknesses

  1. 1. Time: Five days
  2. 2. Precision: Wide precision ranges on the order of +/-10-20%, showing limited reproducibility
  3. 3. Interferences: Wastes containing suppressors to microbiological activity can inhibit and limit BOD results
  4. 4. Specificity: The application of BOD is specific to a single sample and is not linear

COD, Chemical Oxygen Demand

  1. COD is the amount of oxygen required for the chemical oxidation of compounds in water. This demand is determined using a strong oxidant, with most standard methods using dichromate and to a lesser extent permanganate.

Weaknesses

  1. 1. Time: Several hours
  2. 2. Oxidation limitation: Some organic molecules are relatively resistant to dichromate oxidation and may give a falsely low COD
  3. 3. Inorganic carbon: The oxidation step does not distinguish between organic and inorganic carbons
  4. 4. Additional inorganics: Chlorides, nitrite, ferrous iron, and sulfide may interfere
  5. 5. Hazardous chemicals: Dichromate is toxic due to its oxidizing power and its use is being restricted in Europe

TOC, Total Organic Carbon

  1. 1. TOC is a direct measure of all organic carbon in a sample. It is determined by oxidation of the organic carbon compounds. TOC analysers can be configured to measure different fractions of organic carbon including: TOC, Purgeable Organic Carbon (POC), Non-purgeable Organic Carbon (NPOC) and Dissolved Organic Carbon (DOC).

Depending on the organic carbon technique employed, Inorganic Carbon (IC) is removed or measured. Measuring for both inorganic carbon and organic carbon will provide Total Carbon (TC).

Strengths

  1. 1. Direct: TOC analysers directly measure the amount of organic carbon without interferences
  2. 2. Versatile: TOC analysers come in a variety of configurations
  3. 3. Time: Less than 10 minutes
  4. 4. Accuracy & Precision: TOC analysis often results in accuracy and precision within +/-5% over a large range of concentrations from 1 ppb to 50,000 ppm

Weaknesses

  1. TOC does not identify the overall oxidation state. Most regulations are still written for BOD and COD reporting.

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