Biochemical Oxygen Demand And Chemical Oxygen Demand
- required for oxidation of inorganic and organic matter.
- essential for the livelihood of micro organisms.
- can be measured by
- BOD – Biochemical oxygen demand
- COD – Chemical oxygen demand
Biochemical Oxygen Demand
- measures the quantity of oxygen consumed by microorganisms during the decomposition of organic matter.
- indirect measure of biodegradable organic compounds in water.
- determining degree of H2O pollution.
- Important measurement in operation of sewage treatment plant.
- Comparing BOD of incoming sewage & effluent- efficiency, effectiveness of treatment is judged.
- For example, in a typical residential city raw sewage has a BOD value of around 300 mg/L. If the effluent from the sewage treatment plant has a BOD. of about 30 mg/L, the plant has removed 90 percent of the BOD
- DO Is measured prior to incubation.
- allowed to stand for five days at a controlled temperature of 20 °C (68 °F).
- At the end of the five-day period, the remaining dissolved oxygen is measured.
- BODt = (DOi – DOf) × D.F.
- BODt = Biochemical oxygen demand at t days.
- DOi = initial dissolved oxygen before incubation.
- DOf = final dissolved oxygen.
- D.F.= dilution factor= volume of the bottle/volume of the sample.
- Dilution is required.
- Pretreatment if toxic wastes.
- Long period of time.
- Seeding for industrial water.
Chemical Oxygen Demand
- Measure of oxygen equivalent of the organic matter content of the sample that is susceptible to oxidation by a strong chemical oxidant (acid + heat).
- COD test results are used for monitoring and control of discharges, and for assessing treatment plant performance.
- Expressed in mg/l or ppm.
- Potassium dichromate, a strong oxidizing agent is used under acidic conditions to find the amount of organic compound in waste water sample.
- Acidity is usually achieved by the addition of sulphuric acid.
- The amount of Cr3+ is determined after oxidization is complete, and is used as an indirect measure of the organic contents of the water sample.
- To do so, the excess potassium dichromate is titrated with ferrous ammonium sulphate (FAS) until all of the excess oxidizing agent has been reduced to Cr3+.
Accuracy Of The Test
- It is important that no outside organic material be accidentally added to the sample to be measured.
- To control for this, blank sample is used created by adding all reagents (e.g. acid and oxidizing agent) to a volume of distilled water. COD is measured for both and the two are compared i.e. COD of blank – COD of sample.
- Chemical Oxidant is not specific to oxygen-consuming chemicals that are organic or inorganic, both of these sources of oxygen demand are measured in a COD assay.
- It does not distinguish between Biodegradable and Non-Biodegradable organic matter.
- The test does not measure the oxygen demand caused. by the oxidation of ammonia into nitrate
COD vs BOD
- Faster process control.
- COD and BOD do not necessarily measure the same types of oxygen consumption.
- COD is always greater than BOD measurements.
- COD is a more stable measurement method.
- How can a correlation be determined?
- Collect empirical data
- COD and BOD data for the same water sample collected over the same period of time.
- Graph data
- Graph COD and BOD data to determine whether or not a correlation exists.