SEPTIC TANK
SIZE CALCULATION BASED PER USER CONSUMPTION
As above said,
the capacity of septic tank should be designed correctly because insufficient
tank size may result in wastewater backup to the house or it coerces us to
clean the water often and also it will increase the maintenance cost.
Let us estimate
the Tank size for a Home – 5 Members.
Daily Water
Usage for a Home – 5 Person
1. Cooking – 5 Liters
2. Bathing & Toilet – 85 Liters/Person, So for 5
person – 425 liters/Day
3. Washing cloths & Utensils – 30 Liters
4. Cleaning House – 10 Liters
5. Other – 5 Litres
Total – 475
Litres/Day Approximately
We take
detention period as 3 days. So the septic tank should have the capacity to
retain the household wastewater at least 3 days.
Total wastewater
in 3 days – 475×3 = 1425 Liters
Let us take 2000
Liters capacity minimum for a home. Minimum depth of Septic tank should
not be less than 1.8m.
Sludge settled
down per person – 30 liters/year. So here we take sludge removal 2 years once.
So total sludge
= 30 litres x 5 persons x 2 years = 300 Litres
Total tank
capacity = 2000+300 = 2300 Liters
We knew that 1
Cum = 1000 Liters = 2300/1000 = 2.3 Cum
Area required @
1.8m depth = 2.3/1.8 = 1.2 Sqm
Length &
breadth ratio of Septic Tank is 4:1 or 2:1
L:B ratio taken
as 4:1
So 4BxB = 1.2
Sqm where B=0.54m
Note – Breadth
of the tank should minimum 750mm, So that L = 4×0.75 = 3m
L – 3m; B –
0.75m; Depth = 1.8+0.3 = 2.1m (free board should be provided at least 300mm)
Tank capacity =
3 x 2.1 x 0.75 = 4.725 Cum = 4.725x 1000 = 4725 Liters
British Standard
Calculation:
As per British
standard, they follow the below formula to calculate the wastewater flow for a
septic tank.
C=A+P(rq+ns)
Where
R – Detention
period of Sewage in Days
Q – Sewage Flow
in liter per day
N – Number of
years
S – Sludge
accumulation in liters per person/year
Simplification
of (rq+ns) = 180 Liters
Now we rewrite
the formula as C=A+180P
C = 2000+(180×5)
C= 2900
Liters
