septic tanks

What size do you need?

Depending on your habitation and how many people are living under your roof, defining your needs in terms of septic tank size could be quite challenging. Read the article below to choose what water treatment tank is more suitable for you.

septic tanks

Septic tank sizes

Overview

In the UK, a house not connected to the mains/sewers will require treatment using one of the following solutions:

Septic tank
Sewage treatment plant
Cesspool
Non-standard system such as reed bed

In any such case, you, as the homeowner, are responsible for meeting the general binding rules.

To make sure your septic tank works as it is designed to, it needs to be capable of handling the flow of effluent that your premise is producing.

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How to determine septic tank size?

Step 1: Determine “P” (Minimum Population) from the number of bedrooms you have.

The volume of effluent coming from your house is determined by the number of people using the system. This is calculated based on the number of bedrooms in your home.

*Reference: Flows & Loads from British Water

**Please note that the abbreviation for Minimum Population is “P”.

Small one and two-bedroom houses or flats

Flat with one bedroom = minimum 3P system
Flat with two bedrooms = minimum 4P system

Single house

Up to and including three bedrooms: P = 5, add 1 P for each bedroom thereafter.

House with three bedrooms = minimum 5P system
House with four bedrooms = minimum 6P system (5+1)
House with five bedrooms = minimum 7P system (5+2)
House with six bedrooms = minimum 8P system (5+3)

*Reference: Flows & loads from British Water for larger domestic housing, commercial and industrial premises.

Step 2: Calculate the number of litres of wastewater produced per day

British Water estimates that one person creates 150L per day. To calculate the number of litres per day for your property, just multiply the number of people x 150L.

Example: three bedrooms house = 5P (see step 1)

5 P x 150L = 750L per day

Step 3: Add 2,000L to your litres produced per day

Carrying on with the example above: 750L (step 2) + 2,000L = 2,750L

Therefore, the minimum size septic tank required for this example is 2,750L. For more information on our septic tanks or on our

wastewater treatment solutions, do not hesitate to download our ebooks.

Choosing the correct size septic tank

Given the choice, it’s recommended to round up the tank size required. If a septic tank is too small, your soakaway system will suffer as a result. If insufficient settlement time occurs, small suspended solids might block your soakaway. In which case, you will have to replace it or convert to a new sewage treatment unit.

Septic tank size calculator

Number of bedrooms	P	Minimum size of a septic tank	Tricel recommendation
1	3	2450	Tricel Vento 6
2	4	2600	Tricel Vento 6
3	5	2750	Tricel Vento 6
4	6	2900	Tricel Vento 6
5	7	3050	Tricel Vento 12
6	8	3200	Tricel Vento 12
7	9	3350	Tricel Vento 12
8	10	3500	Tricel Vento 12
9	11	3650	Tricel Vento 12
10	12	3800	Tricel Vento 12