How to Calculate the Size and Capacity of Septic Tank | Septic Tank Size Calculation

In this Article today we will talk about the How to Calculate the Size and Capacity of Septic Tank | Septic Tank Size Calculation | What is Septic Tank | What is Septic Tank Process | Septic Tank Capacity Calculation Formula | Standard Size of Septic Tank | Size of Septic Tank for 5 Users | British Standard Calculation for Septic Tank | Septic Tank Capacity Per Person

What is Septic Tank and It’s Process:

Location: Mostly septic tanks are suitable for rural and urban areas where proper sewer system are not available. What is Septic Tank Process

At present, every domestic/commercial building having the septic tank, in order to retain the sewage waste for sometime before its gets down to sewer line to avoid any blockage.

Basically, the septic tank is constructed of concrete/brick wall with the regular shape and it should have a minimum of two chambers. What is Septic Tank Process

Process: The effluent may be either liquid or solid waste received from household to septic tank through the inlet pipe. The septic tank was designed to store the wastewater at the minimum period of 18 hours to 24 hours, it is called detention period.

During this time period, the sludge was decomposed by the biological action of anaerobic bacteria and the solid waste settled down as sludge. Then the surface water runs to the soak pit through an outlet pipe. Then the settled sludge shall remove after 1 or 2 years.

Others Post: How to Calculate Water Tank Capacity for Building

Do.s:

1. The tank must be built in the proper location and size.
2. Reduce your water usage to keep your septic tank from filling up too quickly and to save water.
3. Sludge removal should be done on a regular basis and at a specific time.
4. Inspect the septic tank on a routine basis.
5. Build the septic tank while taking care not to disturb the groundwater.
6. If a septic tank blockage occurs, act quickly to resolve the issue.
7. The manhole should allow easy access to the tank. Septic Tank Capacity Calculation Formula

Don’t Dos:

1. The flushed wastewater may back up into our home due to insufficient tank capacity, so tank capacity is extremely crucial or important.
2. To avoid unnecessary blockage, do not flush rubbish, sanitary items, paper, or other solid or hard material.
3. Other sources of water should not be allowed to enter the septic tank.
4. To protect the environment, wastewater should not be discharged into the natural ground. Septic Tank Capacity Calculation Formula

Shape of Septic Tank:

The simplest form of septic tank is a single box, rectangular or cylindrical shape. The shape of the septic tank affects the flow velocity of the wastewater and sludge accumulation.

The dimension of septic tank should not be too deep. This is because it may cause a short circuit of the inlet and outlet flows. Size of Septic Tank for 5 Users

The septic tank has a large surface area and sufficient depth should always be desired. The rectangular single box tank (three times its width and length) is most favorable.

In some cases, a cylindrical shape with the appropriate size may also be the best choice.

How to Calculate the Size and Capacity of Septic Tank in Gallons?

How do we calculate the gallon volume a septic tank can deliver based on the simple measurements we can make? British Standard Calculation for Septic Tank

Generally, the residential septic tank is typically 4.5 feet wide x 8.0 feet long x 6 feet high. Your septic tank may be different size. The best practice is to find and measure the septic tank and to make correct calculations.

Tanks are generally buried 4 inches to 4 feet deep, depending on local site conditions, shape, slope and other factors.

Here is the basic math for calculating the capacity (volume) of the septic tank in gallons. Measurements taken on the internal dimensions of the septic tank are in feet.

Notes To The Table Above:

These simple constants and the formula for sizing a rectangular or round tank can help convert the size of a septic tank into cubic feet of sewage:

• One gallon = 231 cubic inches
• One gallon = 0.1337 cubic feet.
• 1 cubic foot = 7.48 gallons
• Volume of a cylinder = Pi x radius² x depth

Where,

Radius = half the diameter

Pi = 3.1416

• Volume of a rectangle = Length x Width x Depth

Septic Tank Size Calculation based Per User Consumption:

As previously stated, the capacity of a septic tank should be properly designed because insufficient tank storage capacity may result in wastewater backup to the house or force us to clean the water frequently, as well as increase maintenance costs.

Size of Septic Tank for 5 Users:

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 Liters

Total – 475 Liters/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 x 3 = 1425 Liters

Let us take 2000 Liters capacity minimum for a home. Minimum depth of septic tank should not be than 1.8m.

Sludge settled down per person – 30 Liters/Year. So here we take sludge removal 2 years once.

So Total Sludge = 30 Liters x 5 Persons x 2 Years = 300 Liters

Total Tank Capacity = 2000 + 300 2300 Liters

We know that,

1 Cum = 1000 Liters = 2300/1000 = 2.3 Cum

Area Required @ 1.8 m 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 4B x B = 1.2 Sqm where B = 0.54 m

Note: Breadth of the tank should minimum 750 mm, so that L= 4 x 075 = 3 m

L = 3 m; B = 0.75 m; Depth = 1.8 + 0.3 = 2.1 m (free board should be provided at least 300mm)

Tank Capacity = 3 x 2.1 x 075 = 4.725 Cum = 4.725 x 1000 = 4725 Liters

British Standard Calculation for Septic Tank:

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)

= 2900 Liters

We have illustrated the British standard formula just for your knowledge. After Reading this Article you must read about: How To Calculate the Estimate of Masonry Water Tank

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Conclusion:

Full article on How to Calculate the Size and Capacity of Septic Tank | Septic Tank Size Calculation | What is Septic Tank | What is Septic Tank Process | Septic Tank Capacity Calculation Formula | Standard Size of Septic Tank | Size of Septic Tank for 5 Users | British Standard Calculation for Septic Tank | Septic Tank Capacity Per Person. Thank you for the full reading of this article in “The Civil Engineering” platform in English. If you find this post helpful, then help others by sharing it on social media. If any formula of BBS is missing from this article please tell me in comments.