It gives introduction about Sewage

1. Sewage
Unit II

2. • Sewage:- Waste water from domestic area
• Consists--
• Black Water: Waste water originating from toilet, dishwashers, food preparation sink etc.
Highly contaminated with chemicals, pathogenic in nature.
• Grey Water: Waste water originating from non-toilet units ( bathroom and sinks) less polluted,
can be reused.
• Yellow Water: Urine collected using a specific channel, not mixed with black or gray water.
Black Water
Grey water
Yellow water

3. Sources of waste water
Domestic Area Non-Sewage area
(Home, public rest room, hotels, schools etc.) (Flood water, runoff, swimming
pools, car garages etc.)

4. Characteristics of Sewage
• Municipal sewage consists of 99.9% water with small quantities of impurities.
• Physical Characteristics: Characteristics which can be detected using physical senses.
I. Temperature: Temperature of sewage is higher than natural water due to industrial waste.
II. Colour: Yellowish grey, highly polluted sewage is black in colour.
III. Odour: Fresh sewage smell- oily or soapy, toxic sewage- objectionable (H2S)
IV. Solids: Floatable solids.
V. Turbidity: Suspended matter which restricts the visual depth.

5. Chemical Characteristics
• PH value: The PH of sewage is very high and decreases when it becomes septic.
• Dissolve Oxygen (DO): Dissolve oxygen level is high for less septic sewage and decreases
with the concentration of pollutants.
• Oxygen Demand : Amount of oxygen used by bacteria in the sewage.
• BOD: Biological Oxygen Demand- Amount of oxygen required for the stabilization of
organic material in the sewage under aerobic condition.
• COD: Chemical Oxygen Demand- Amount of oxygen required for the stabilization of organic
material chemically. (Chemical Oxidants are used)

6. Biological Characteristics
Bacteria: Sewage contains huge quantity of Bactria, some are harmless while
some are harmful ( Pathogenic/ disease-causing)
Expected range of bacteria in raw sewage – 5 lakh to 50 lakhs per ml.
Types of bacteria:
• Aerobic: which survives in the presence of oxygen.
• Anaerobic Bacteria: Which survives in the absence of oxygen.

7. Sewage Treatment
• Preliminary treatment
• Secondary treatment
• Tertiary treatment

8. Primary Treatment
• Consists of holding sewage in a basin- helps the solids to settle down.
• Units in Primary treatment are-
• Screen Chamber, Grit Chamber, Oil and Grease Chamber etc.

9. Secondary Treatment
• Removes dissolved and suspended biological matter.
• Aeration brings water and air in close contact in order to remove dissolved gases (such as
carbon dioxide) and oxidizes dissolved metals such as iron, hydrogen sulfide, and volatile
organic chemicals
• Clarification - to remove
suspended solids and
solid particles.

10. Tertiary Treatment
• Given after Primary and Secondary treatments before discharge of sewage into
the ecosystem.

11. On-site treatment of sewage
• For residential area away from municipal treatment plant must have own treatment unit.
• Treated sewage/ waste water- disposed onto own land without causing pollution.
• e. g. Septic tank