Treatment of Sewage

TREATMENT PROCESSES ARE CLASSIFIED AS:

1. Preliminary treatment
2. Primary treatment
3. Secondary or biological treatment
4. Complete final treatment

Preliminary Treatment: Separating floating materials, heavy inorganic solids.

Processes used:

1. Screening: Floating matter removal
2. Grit chambers or detritus ranks: To remove grit or sand
3. Skimming tanks :To remove oils or greases

Primary Treatment: Removal of large suspended organic solids

Processes used:

Sedimentation tank: To remove suspended solids .

Secondary Treatment: The effluent from sedimentation tank should be stabilized aerobically or anaerobically to get clearer effluents .

Aerobic biological units

Fillers : Trickling filters

Aeration tanks: Activated sludge process

Oxidation ponds

Anaerobic biological units

Septic tank

lmhoff tank

Sludge digestion tank

Final or Tertiary treatment : To kill pathogenic bacteria chlorination of sewage

Screen: Screen is a &vice with uniform openings of size ranging from 3 mm to 50 mm used to retain. Separate large floating substances present in waste water.

Design of screens: Screen’s are designed based on approach velocity “Va”

Va=0.6 to 1.2 m/sec

c/s area of screen device b x d = /Va

depth “d” 0.8 m to 1 m

fix “b” c/s area/depth

Let n = number of openings

n+1 = number of bars

b = n x s + (n+1)

s=size of openings [25 mm]

=dia of bar[ 10 mm]

Head loss through screen,

Preliminary treatment
Grit chambers: To separate out the grit,gravel sand, egg sells etc of size 2 mm or larger. It is a settling tank with lesser detention time of I min and flow vel: 0.2 to 0.3 m/sec.The flow vel. Should neither be far low as go cause settling of lighter organic matter nor should it be so high as not to cause the settlement of the entire silt and grit present

Design of Grit chambers:

Grit chambers are designed based on

• D. T
• VH 
• SLR (or) SOR, ‘Vo’

Volume of Grit chamber = Q x DT

Surface area of Grit chamber = ⍬/Vo

C/s area of Grit chamber = ⍬/VH

Detritus tanks:

• To separate out not only larger grit but also very fine sand particle.
• Detention time  should be kept between 3 to 4 min

Skimming tank:It is a chamber so arranged that the floating matter like oil, fat, grease etc., rise and remain on the surface of the waste water (Sewage) until removed, while the liquid flows out continuously under partitions or baffles.

• Efficiency of skimming tank can be increased by passing chlorine  gas
• Detention time should be 3 ta 5 min

Primary treatment:

1. Sedimentation tank: principles same as explained in “water supply engineering"
2. Detention time : 2 hours

• velocity of flow 0.3 m/min
• Plain sedimentation tank removes 60 to 65% suspended solids