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Septic Tank 03

The document describes various sewage treatment processes including septic tanks, Imhoff tanks, ponds, lagoons and ditches. It provides details on the septic tank process, including sedimentation, anaerobic sludge digestion, gas venting and sludge removal. Design criteria for septic tanks and absorption systems are outlined. An example problem calculates the dimensions of a septic tank for a hostel with 200 students based on given criteria like detention time and sludge volume.

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Reymond Igaya
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57 views16 pages

Septic Tank 03

The document describes various sewage treatment processes including septic tanks, Imhoff tanks, ponds, lagoons and ditches. It provides details on the septic tank process, including sedimentation, anaerobic sludge digestion, gas venting and sludge removal. Design criteria for septic tanks and absorption systems are outlined. An example problem calculates the dimensions of a septic tank for a hostel with 200 students based on given criteria like detention time and sludge volume.

Uploaded by

Reymond Igaya
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as PDF, TXT or read online on Scribd
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OTHER TREATMENT PROCESSES

Other processes include:


 Septic tank
 Imhoff tank
 Ponds
 Lagoons
 Ditches
TREAT SEWAGE SO THAT IT IS MADE HARMLESS.
SEPTIC TANK
Process:
 Horizontal continuous flow bed tank (with longer
detention time)
 Extended sedimentation tank
 12-36 hours detention
 Closed tank – Anaerobic decomposition of settled
sludge
 Three purposes
Sedimentation
Digestion tank
Storage of digested sludge
 Digestion - Evolution of gases – gas vent
 Cleaning/ Removing digested sludge – once in a year (6 months to 2
years)
 Thus Two processes-Sedimentation and Anaerobic Sludge digestion
Construction details:
 Long, continuous flow tank of RCC and sides water proof.
 Facility for gas vent through vent pipe.
 Bottom (slope) to facilitate pumping of sludge.
 Manhole with for removal of sludge
 Inlet and outlet,Tee’s will be provided
 Sometimes baffles to reduce disturbances.
Septage:
 Effluent from septic tank disposal
 Soak pit/ Cess pool
 A closed circular or rectangular pit through which effluent
will be soaked or absorbed into the surrounding soil
 Two- Filled or Empty type
 Under drain /absorption /dispersion trenches.
 3-4 distributions@2m spacing, L shall not be more than30cm
Design criteria(Septic Tank):
 Quantity of sludge -30 l/capita/ years - Sludge storage
 Depth - 1.2 – 1.8 m
 Length: Breadth - 2-4 times
 Min width - 120cm
 Outlet ~ in let level difference = 5-7.5 or up to 15 cm
 Detention time = 24 hours
 Free board: 30cm
Design criteria for absorption system:
 Calculation of Total subsurface soil area required for soak pits/
dispersion trenches
 Lq = 130/√t
t- standard percolation rate for the soil min/cm
Lq-Maximum rate of effluent application in Lpd/m2 of
leaching surface
 Total trench area required = Total flow per day/ Lq
COMPONENTS OF A SEPTIC SYSTEM

Distribution Box
Septic Tank

Leach Line
Problem:
 Design a septic tank for the hostels of the Engineering
college:
 Number of Hostels = 3
 Students / Hostel = 200
 Water supply = 120 lpcd
 Assume Detention period 24 hours and
 Sludge volume = 30l/cap/year
Solution:
Design for one hostel:
 Waste water = 200x120x80/100 = 19.2 m3
 Let us assume that sludge withdrawal once in a year
 Volume required for sludge storage for one year
= 200x30
= 6000l= 6m3
 Capacity of septic tank = 25.2 m3
 Assuming a depth of 1.8 m , Area = 25.2/1.8=14 m2
3B2 = 14m2,
B2 = 14/3,
B = 2.16 say 2.20m
 Length = 6.60m
 Dimension = 6.6x2.2x2.1m (1.8m+0.3m free board)
 26.16m3 > 25.2 m3
 Assuming 1.5m depth, Area = 16.8 m2
 3B2= 16.8, B= 2.36 say 2.4 m
 Provide 7.2x2.4x1.8m septic tank
Design of absorption system:
Lq = 130/√t
Say t=20min/cm
Lq = 130√20 = 29lpd/m2
Area required = 200x120x0.8/29 =
662m2
L = 30m
W=1m
TREAT SEWAGE SO THAT IT IS MADE HARMLESS.

DISPOSE OF SEWAGE (MAKE IT GO AWAY)

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