Aquatic Pollution and Coastal Management

2.3.Measurement of Organic Pollution

Unit 2 : Sewage and domestic wastes, treatment and reuse

2.3.Measurement of Organic Pollution/Assessment of Sewage Impact
Organic pollution can be measured by a number of chemical and biological parameters, which are directly related to the level of organic matter present in the water.

Measurement of organic pollution is made by four ways ,these are

Biochemical Oxygen Demand (BOD)
BOD is defined as the rate of removal of oxygen by microorganisms in the aerobic degradation of organic matter in waters. It is measured as the difference in the oxygen between the initial sample and after its incubation for five days at 200C or for three days at 270C.

Chemical Oxygen Demand (COD)
It is a measure of the oxygen requirement of organic matter in the sample that is susceptible to oxidation by strong chemical oxidants like potassium permanganate, ferric sulphate, potassium iodate and potassium dichromate.

Total Oxygen Demand (TOD)
TOD refers to the total quantity of oxygen required to burn the whole sample. TOD analyzers are used to measure residual oxygen continuously in the stream of gases coming out as gas effluent after burning the sample.

Biological Indices of Organic Pollution
Biological indicators (bioindicators) are considered to be organisms which by their own presence or absence indicate the existence in abundance of a particular critical factor.

The structure of the biotic community is the reflection of conditions prevailing in waters and can be exploited for the biological assessment of the water quality with regard to organic pollution.

a) Shannon and Wiener’s Diversity Index

The index is based on the number and relative dominances of various species present in water.

Diversity index (H)= Epi x log2pi
pi = ni/N
ni= Number of individuals of a species
N= Total number of individuals of all the species
log2= log 10 x 3.322

The decline in the value of index is taken as an evidence of pollution
- Value above 3 indicate clean water
- Value less than 3 indicate pollution

b) Odum’s Index

Odom’s index = (Total number of species in a sample/ total number of individuals of all species) x 100

The values decrease with rise in the level of pollution.

c) Saprobic Index
Classical “Saprobic Systems” have been evolved originally and this forms a basis for the “Indicator Organisms Concept”.

The proliferation of polychaete, Capitella capitata, absence of interstitial copepods and the ratio between the micro and macro fauna are some of the basic characteristics for assessing sewage impact.