DESISN OF A SEPTIC TANK AND
ASSOCIATED EFFLUENT
TREATMENT SYSTEM
NAME
: H.V.THENUWARA
INDEX NO
: 150636C
DATE OF PER
: 07/06/2019
DATE OF SUB
: 21/06/2019
DESIGN DATA SELECTION
SLS 745: Part 1:2004, “The code of practice for the designs and construction of septic tanks and
associated effluent disposal systems” would be used.
 Index No
=
150636
 User Category
=
Low income - Residents
 Maximum no of users =
51
 Percolation rate
=
100 mm/h
 Ground water table
=
5.5m below the existing ground level
 De-sludging interval =
4 years
 Site plan
No ground water wells or any other sources in close proximity.
=
Drinking water is supplied by the National Water Supply and
Drainage Board
 Soakage pit
=
no soakage pits in the land
Reference
Calculations
SLS 745:
part1:2004
AVERAGE DAILY FLOW ESTIMATION
Clause 4
Table 1
Output
Low income Residents
No of users=51
Per capita blackwater flow
= 40 l/p/day
Per capita greywater flow
= 120 l/p/day
Per capita all wastewater flow = 160 l/p/day
Appendix B
Average daily flow (Blackwater) = 40*51 l/day
= 2040 l/day
= 2.04 m3/day
Blackwater =
2.04 m3/day
Greywater =
6.12 m3/day
Average daily flow (Greywater) = 120*51 l/day
= 6120 l/day
= 6.12 m3/day
SLS 745:
part1:2004
Clause 5.2
Appendix C
C.1
C.2
All waste =
8.16 m3/day
Average daily flow (All waste) = 160*51 l/day
= 8160 l/day
= 8.16 m3/day
DESIGN OF SEPTIC TANK
Determination of working capacity of Septic Tank
The working capacity of septic tank shall be the sum of the
volumes required for settling and sludge digestion, sludge
storage and scum storage and shall be estimated as follows.
Volume required for settling
Vs = ts . Q
Time required for settling (ts)=1.5-0.3logQ
=1.5-0.3log(2.04)
=1.407 days>0.2
Vs= 2.87 m3
Hence
OK
C.3
Volume required for settling:
Vs=ts*Q
=1.407*2.04
=2.87 m3
Volume required for sludge digestion
Vd=0.741 m3
Vd=qs*td*P
Vol. of fresh sludge (qs)
For black waste only)
= 0.00055m3/day/person(
The required to sludge digestion(td)=33days(For 20’C)
Population equivalent (P)
=Q/0.05
=2.04/0.05
=40.8
C.4
Vd=0.00055*33*40.8
=0.741m3
Volume required for sludge storage
Vst=r*p*n
Vol. of digested sludge
Black waste)
=0.022 m3/year/person (For
Desludging interval
=4 years
Vst=3.59 m3
C.5
Vst =0.022*40.8*4
=3.59 m3
C.6
Design working capacity of Septic tank
V=Vs+Vd+1.5Vst
= (2.87+0.741+1.5*3.59) m3
=8.996m3
V=9 m3
Design capacity= 9 m3.
SLS 745.Part
1 2004
5.2.4(b)
Clause 5.2.3
(b)
1m3 < Design capacity < 12m3
So, we can use a single rectangular tank consisting of two
compartments with 2:1 volumetric ratio in order to increase
the settling characteristic.
SEPTIC TANK GEOMETRY
Clause 5.2.4
(d) (e)
B
2B
Clause 5.2.3
(a)
Clause 5.2.7
(a)
(b)
B
Minimum internal width (B) =750mm
Minimum internal depth below liquid level = 1m
Assume a depth (D) = 1 m
L*B*D = 9
3B*B*1 = 9
B = 1.732 m > 0.75 m
Width =1.8 m
Length =5.4m
Hence OK
Take internal width (B) = 1.8 m
Length (l) = 3*B = 5.4 m
Total tank volume = 1*1.8*5.4 = 9.72 m3
Height= 1.2 m
Clause 5.2.5
(b)
(c)
(d)
Free board
Assume a free board of 200 mm >= 200mm
Air space provided > 10% of the total tank volume
Air content = 0.2*1.8*5.4 = 1.944 m3 = 1.944*100/9 = 21.6%
Hence OK
So height of septic tank = 1+0.2 = 1.2 m
Clause 5.2.6
Inlet and outlet arrangement
Clause 5.2.8
Clause 5.2.9
(a) (b) (c) (d)
Inlet fitting extension into liquid = 20% x D = 0.2*1
=200 mm
Outlet fitting extension into liquid = 300 mm
Inlet and outlet pipe diameter
= 100mm
Invert of the outlet pipe below the invert of the inlet pipe
= 50mm (> 30 mm)
Access opening
Clause 5.3
Clause 5.3.1
Clause 5.3.1
(a)
(iii) Table 2
circular opening diameter for each compartment = 0.5m
Chamber partitions
Height of opening from bottom level of tank = 50 % D
= 0.5*1 = 0.5 m
Diameter of the opening = 100 mm
Vent pipe
Height of vent pipe = 1.5 m
Diameter of vent pipe = 25 mm
Suitable mosquito proof mesh has to be provided.
Diameter of opening to provide an air space in air chamber
between the two compartments = 25 mm
Inspection port
Table 3
Diameter of the inspection port for each compartment
= 180 mm
DESIGN OF SOAKAGE PIT
 Percolation rate
=
100 mm/h
 Ground water table
=
5.5m below the existing
ground level
 Soakage pit
=
no soakage pits in the
land
Average daily flow = 8.16 m3/day
(f)
Soakage pit geometry
Assume the shape of soakage pit to be circular.
Dimensions of Soakage pit
The effective area of a soakage pit shall be the area of the side
walls lying between a level 150 mm the invert of the inlet
pipe and the bottom of the pit.
Specific effective area = 8.4 m2/m3*day
(For a percolation rate of 100mm/h)
Required effective area = 8.4*8.16 = 68.5m2
Single soakage pit is adequate (Avg. daily flow < 30 m3/day)
Min. minor dimension = 900mm
Max. major dimension = 3m
Minimum depth to GWT = 3.0m (Percolation rate =
100mm/hr )
Assume minor dimension = 3m (diameter of the tank)
SLS 745:
Part1: 2004
Clause 5.3.1
68.5 = (2× π ×1.5 × H) + (π ×1.52)
H = 6.51 m > 5.5m
Assume 3 soakage pits with 15m distance apart
D=3m
H = 2.2 m
(2× π ×1.5 × H) + (π ×1.52) = 68.5/3
H = 1.67 m
Total height = 1.67 + 0.15 + 0.3
= 2.12 m
Actual depth to groundwater table from bottom of soakage pit
= (5.5m – 2.12 m) = 3.38 > 3 m
Hence ok
Diameter = 3.0 m
Height
= 2.2 m
Cover = 50mm cover