How to size septic-tanks? Types and Importance of it in Wastewater Treatment

Private sanitation is widespread in the tier 2 and tier 3 cities of India, with more than 30% of new homes being used for on-site sanitation. Since the sewerage system is so bad, each home has its own septic tank for sewage decomposition.

Need to install a septic tank and wondering what size the septic tank should be?

The most important factor in installing a septic tank is the type and size of septic tank required. The correct dimensions of the septic tank are essential for the efficient operation of the private sewage treatment system. In this article, we will look at several factors that determine the size of septic tanks.

Septic tank basics

Your wastewater treatment plant is a self-contained compartment set up to hold and decompose the sewage in the compound of your home. The septic tank consists of two main parts.

Ground receiving field or drainage pipe and tank. When sewage enters the septic tank, solid waste collects at the bottom of the septic tank and forms a so-called sludge layer. Soap residues and fats form a cumulative layer on the surface. The middle layer contains sewage. When the septic tank is full, the sewage leaves the tank and enters the drainage area, where it is drained to the ground.

What are the main types of septic tanks?

Before considering the size of a septic tank, it is important to understand the following common types of septic tanks:

1. Glass fibre septic tank

2. Plastic septic tank

3. Concrete septic tank

The most common type is a concrete septic tank, but it is heavy and requires heavy and expensive equipment’s to install. Fiberglas and plastic septic tanks are lightweight and suitable for hard-to-reach places and remote areas.

Why septic tank size is important?

If the septic tank in your home is too small to meet your needs, it will not be able to handle the amount of sewage that flows out of your home. This can cause all sorts of problems like stinks, floods, blockages.

However, the most common result if the septic tank is too small is that the pressure generated causes the water to be released before it is efficiently washed. This means that the solid waste in the septic tank will not be fully decomposed and will accumulate more quickly, leading to the possibility of overflow and clogging.

If the septic tank is too large, it will not work if the proper amount of sewage does not flow. If your septic tank is too large for your home, it does not need enough collected liquid to produce bacteria that helps break down the solid waste in the septic tank.

What determines the size of the septic tank?

Several factors that determine the size of a septic tank are:

-Take into account water usage

The most reliable and effective way to determine the right septic tank size for your property is to calculate the amount of water you consume. The required size of the septic tank depends on the amount of water it can hold, which is diverted to the soil receiving field. Based on the total water usage of households, the recommended septic tank sizes are:

·       Less than 5000litres per day: 7200 litre septic tank

·       Less than 3500 litres a day: 6000 litres septic tank 

·       Less than 3000 litres a day: 4600litres septic tank 

·       Less than 2000litres a day: 3500 litres septic tank

-Take into account the size of your property

Another way to find the best septic tank for your home is to consider square feet. The bigger the house, the bigger the septic tank needed. For example, a house less than 1,500 square feet typically requires a tank of 3000 to 3800 litres. On the other hand, a large house of about 2,500 square feet requires a large tank that exceeds the 4000 litres range.

-Number of bedrooms in your property

Another factor that determines the size of a septic tank is the number of bedrooms in your home.

The more bedrooms you have in your property, the larger the septic tank you need. The recommended septic tank sizes depending on the number of bedrooms are:

·       12 bedroom house requires 3000 litres septic tank of size

·       A three-bedroom house needs a septic tank of 3800litres of size

·       A four-bedroom house requires a 4600 litres septic tank of size

·       A 56-bedroom house requires a 5700 litres septic tank of size

-Number of People

The larger the number of people in the house, the larger the septic tank needs to be. For example, if you have two people at home, you'll need a small septic tank.

When considering the size of the septic tank you need, be aware that the size of the septic tank determines the overall efficiency of the sewer system. If the septic tank in your home exceeds its capacity, sewage can flow back into your home, causing clogging, flooding, and harm to you and your family. Therefore, to avoid this inconvenience, it is important to evaluate the size of the septic tank to choose the right option for your home.

For household use of septic tanks and faecal sludge management in rural and urban India, size of the septic tank shall be 1500mm × 750mm × 1000 mm (5ft × 2.5ft × 4ft) in respect to their length, breadth and depth.

A minimum width of 750mm (2.5feet) and depth of 1 metre below the outlet capacity of 1000 litres is required. For rectangular septic tank, the length shall be 2 to 4 times the width and for circular septic tank, minimum diameter shall not be less than 1.35m and depth of 1 metre.

Empirical Calculations for sizing a septic tank

C = A + P (rq +ns)

Where,

C is capacity of septic tank, A = constant, P = number of users, r = minimum retention time,

q = average sewage flow in litres per person per day, n = number of years sludge is stored and

S = rate of sludge accumulation in litre per person per year.

In British code, A = 2000litres, rq + ns = 180litres, so rounding formula, we will get

C = 2000 litres + 180P.

Suppose we have to find capacity of septic tank size for 4 users, put this value

C = 2000 litres + 180 × 4 = 2900 litres,

For 4 users, minimum size of septic tank could be 2.2m × 1.0m × 1.30m (7.2ft × 3.3ft × 4.25ft). 1 to 1.5ft free space above water level inside tank, so overall height should be 6ft including depth having the tank capacity of 3000 litres of liquid wastages.

For installation of STP’s, ETP’s or other waste water plants, contact Netsol Water. Who is the one of the leading commercial RO plant manufactrers in India.