SEPTIC TANK SIZE CALCULATION BASED PER USER CONSUMPTION

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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

C – Capacity in liters

P – Number of People

A – 2000 Liters as constant

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

We have illustrated the British standard formula just for your knowledge.

Hope you found this article useful. Happy Learning 🙂