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 COD and BOD data to determine whether or not a correlation exists.