8/11/2019 Water Pollution Indicators

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Water Pollution Indicators

The indicators on water pol ution are in three categories: physical (temperature, turbidity , total suspended/

dissolved solids), chemical (pH, dissolved oxy gen, nitrate, COD, BOD, pesticides, metals) and biological (EPT index,

Trent biotic index) indicators. Most of them have been explained already in the book and only a few indicators are

further explained here.

Oxygen saturation: the maximum amount of oxy gen dissolved in water at a given temperature is called thesaturation concentration. It is determined by Henry s law and varies from 14.6mg O2/L at 0

oC to 7.6 mg O2/L at

30oC. As a result, fish with high oxy gen demand prefer cold waters. The oxygen demanding wastes have greater

impacts on water quality in the summer as well as during dry seasons since lower river flow has less capacity to

dilute the waste. Under rare occasions, actual oxygen concentration may exceed the saturation value when algae

and macrophytes are photosynthesising and producing dissolved oxy gen. Such oversaturation is not stable and is

eliminated when sufficient turbulence occurs.

Dissolved oxygen (DO) can be measured by oxygen sensors (such as zirconia sensor, Clark-type electrode and

Optodes).

Biochemical oxygen demand (BOD) is the amount of oxy gen required by microorganisms to break down in organic

matters. It is used as an indicator of organic pollution of water. BOD is expressed in mg O2/L and can be measured

in different way s. The most common method is carried out by mixing water sample with a small amount of

microorganism seed (e.g., analogous to adding yeast to flour dough). The sample is tested to measure its dissolved

oxygen (DO) before it is sealed in the dark to prevent further oxygen to be dissolved or generated by

photosynthesis. After 5 day s incubation at 200C, the dissolved oxy gen is measured again and the difference

between those two DO measurements is the BOD (initial DO - final DO). Dilution of the sample with oxy gen

saturated water may be used if organic matters in the sample are highly concentrated and the dilution ration

should be included in the BOD calculation. Incubation durations other than 5 days have been tested before such as

1, 2, 5, 10 and 20 day s and BOD is commonly adopted which represents about 68% of the total BOD (in contrast,

90% of the BOD in 10 day s and 99% of the total BOD in 20 day s). BOD biosensor and software sensor are

alternative measurement methods which are much faster (e.g., 30 minutes), but they are less reliable than the

BOD method. The BOD test results are normally with 10 or twenty percent uncertainties. If the water contains

toxic chemicals capable of suppressing microbiological growth (e.g., industrial wastes, antibiotics in medical waste,

commercial cleaning agent, chlorination disinfectant residuals), the BOD test results would be much lower. The

amount and types of enzymes would also influence the BOD test results. In addition, some biodegradable organic

matters can only degrade anaerobically such as cellulose and BOD test (depending on aerobic microorganisms) willnot include those.

Typical BOD5 values: pristine water: up to 1mg/L; moderately polluted water: 2 to 8 mg/L; treated sewage (by

three stages): up to 20 mg/L; untreated sewage: 600 mg/L in Europe and 200 mg/L in USA (due to the dilution from

more water consumption).

Chemical oxygen demand (COD) measures organic matters which are both biodegradable and non-biodegradable,

hence it covers a more broad spectrum of organic matters. As a result, the COD in a water sample is always higher

the BOD. The ratio of COD/BOD provides a good indication to the proportion of organic matters that are

biodegradable (COD/BOD usually varies from 1.25 to 2.50). In rivers, BOD gives a more useful estimate of the

possible oxygen demand than COD and is more widely used as a measure of organic strength in the polluted water.

Biological indicators are useful to get a general reading of water quality without the high cost of lab scale analysis.

Various indices are used to represent the states of macro-invertebrates. They are in two types: 1) pollution indices

(existence or absence of indicator species); 2) diversity indices (to indicate the overall effect of all pollutants).

However, biological indices are not always reliable. Factors other than pollution can affect the biological indices.

Absence of some species may reflect its normal life cycle and quantitative sampling of biodiversity is difficult.