SEMESTER: 4TH
SUBJECT: SANITARY ENGINEERING
UNIT/CO-4
TITLE OF THE UNIT: SEWAGE TREATMENT AND DISPOSAL
TOPIC: INTRODUCTION, OBJECTS and CLASSIFICATION
What is sewage treatment?
• The artificial process of changing or removing the objectionable constituents present in sewage to make it less dangerous and less offensive is called sewage treatment
Aerobic decomposition
Treatment units
• Contact beds
• Oxidation ponds
• Trickling filters
Anaerobic decomposition
• It is caused by anaerobic bacteria in absence of oxygen. This type of bacteria can survive without oxygen. This is also called as putrefaction
Treatment units
• Septic tanks
• Imhoff tanks
• Sludge digestion tanks
Objectives of sewage treatment
• To reduce volume of sewage
• To destroy the pathogens
• To recover its industrial value
• To enable its safe and satisfactory disposal
Classification of sewage treatment
• Preliminary treatment
• Primary treatment
• Secondary treatment
• Final or tertiary treatment
Preliminary treatment
• Removal of floating and heavy settle able inorganic solids
• Removes oils and grease
• Reduces BOD by 15 to 30%
• Treatment units like screens, grit chambers, skimming tanks and sedimentation tanks are used
Primary treatment
• In this large suspended organic solids are removed which is done in sedimentation tanks
Secondary treatment
• The effluent coming out from primary treatment is further treated. This is also called biological treatment which is carried out in aerobic or anaerobic conditions
Final or tertiary treatment
This consists of removing the organic load left after the secondary treatment to kill pathogenic bacteria by chlorination
Effluent and sludge
Effluent: It is a clear sparkling liquid. The disposal of effluent does not create any special problem.
Sludge: The suspended solids which accumulate at the bottom of clarifiers or settling tanks etc., are called sludge. Sludge is a combination of sewage with different proportions of water
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