Biochemical Oxygen Demand And Chemical Oxygen Demand

Biochemical Oxygen Demand And Chemical Oxygen Demand

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 And Chemical Oxygen Demand

Biochemical Oxygen Demand

Introduction

• measures the quantity of oxygen consumed by microorganisms during the decomposition of organic matter.
• indirect measure of biodegradable organic compounds in water.

Significance

• 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

Dilution Method

• 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.

Where,

• 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.

Kinetics

Limitations

• Dilution is required.
• Pretreatment if toxic wastes.
• Long period of time.
• Seeding for industrial water.

Chemical Oxygen Demand

Introduction

• 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.

COD TEST

• 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.

Limitations

• 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.