Environmental Engineering (Part-4) (General Engineering) (SSC JE)

Invert of the sewer means inside bottom of the sewer.
Generally we can say that the invert is the lowest point upon which water can flow on the inside.

Generally chlorides are soluble in water. But silver chloride (AgCl₂) and lead chloride (PbCl₂) is insoluble in water.

Taste and odour–Caused by chemicals and dissolved gases like H₂S, methane etc.
• Taste and odour are objectinable because of carcinogenic nature.
• Taste and odour are measured by instrument known as osmoscope.
• Intensity of taste and odour is measure in threshold odour number (TON).
• Acceptable limit = 1 TON
• Cause of rejection = 3 TON

Grit chambers–After screening the sewage enters into the grit chamber, where sand, small stones and grits are removed and disposed of as a solid waste.
• Grit chambers of a sewage plant are usually changed after two weeks.

The most common cause of acidity in water is carbon dioxide. Acidity can be caused by weak organic acids such as acetic and tannic acids and strong mineral acids including sulphuric and hydrochloric acids, however, the most common source of acidity in unpolluted water is carbon dioxide in the form of carbonic acid.

Drinking water gets contaminated with lead when lead pipes are used for transportation of water. The prescribed upper limit concentration of lead in drinking water is about 0.01 ppm.
According to IS – 10500 : 2012

Sedimentation basins are generally made of reinforced concrete and may be rectangular or circular in plan, long narrow rectangular tanks with horizontal flow are generally preferred to the circular tanks.

• The capacity and others dimensions of the tank should be property designed, so as to effect a fairly high percentage of removal of the suspended materials. A plain sedimentation tank under normal condition may removes as much as 70% of the suspended impurities present in water.

Filtration– After screening and sedimentation the fine suspended particles and bacteria present in it. To reduce or to remove the remaining impurities still further to produced potable and polatable water.

• The water is filtered through bed of fine granular material, such as sand etc.
• The process of passing the water through beds of such granular materials is known as filtration.

• Filtration may help in removing odour, colour, turbidity and some pathogenic bacteria from the water.

Dissolved oxygen–
• Maximum quantity of oxygen present in water at a particular temperature is termed as saturated dissolved oxygen (DO).
• Deficiency of DO indicates the presence of biological activity.
• Minimum 4 ppm dissolved oxygen is required at all depth and at all temperature for the survival of aquatic life. It is calculated by Wrinkler's method.