VPE 511: Environment and Environmental Hygiene (2+1)

ALGAE

• The algae constitute the most ubiquitous form of plant life existing in freshwater; the majority are microscopic in size but some are visible to the naked eye, eg., the bright green thread-like algae often seen in ponds and streams.
• From the point of view of water supply, three groups of algae are of great importance, namely the Isokcnatae or Green Algae, the Myxophyceae or Blue-Green Algae, and the Diatoms.
• These and other groups of algae, which may only flourish moderately in the running water of rivers and streams, generally increase markedly in the comparatively still waters of lakes and storage reservoirs. This increase in useful in some ways.
• The aeration of the water by the algae helps to purify it and, as will be mentioned later, they play a vital role in the purification of water supplies by filtration processes but, if their growth becomes excessive, they may interfere with filtration.
• Such excessive increases in the algal population of open storage reservoirs can be controlled by the application to the water of a chemical algicide, such as copper sulphate which is used at the rate of 1kg to 4.5kg. CuSo₄ per 45,46,000 litres of water. Care must be taken to ensure the even distribution of the CuSO₄ throughout the reservoir.
• The usual method of application is to tow a bag of the along carefully planned lines in the reservoir .
• It is better policy to use Cuso₄ as a preventive against excessive algal multiplication in a reservoir than as a curative, in which case the dead algae undergoing decomposition may render the water unfit to use for time.
• Objectionable taste due to algal growths may be removed by dosing the water the with powdered activated carbon at the rate of 1 to 5 parts per million. The covering over of small reservoirs will effectively reduce algal growth therein.
• A great bulk of algae passing on from storage reservoirs to the filter beds during periods of algal abundance introduces mechanical difficulties in the filtration of the water, and unless the filtration plant is frequently cleaned the filter beds will become choked.
• The use of algicides in the filter beds is not practicable, as the purification of vary large quantities of dead algae may be even more objectionable than the presence of the living plants in the water.
• If it is necessary to remove offensive tastes after filtration about 0.5 part per million of potassium permanganate may be used.