# What size Sewage Tank do I need?

Whichever system you choose, to ensure it works as it was designed to do, it is important to size it correctly.

How to Size a Sewage Treatment Plant

### How to Size a Sewage Treatment Plant

Sewage treatment plants are sized by population so the first thing to find out is how many people the property can accommodate.

**Calculate the population of your property**

First you need to calculate the number of people your property can accommodate, not the number of people actually living there. This is defined in the British Water document “Flows and Lows 4” and states that a house with up to 3 bedrooms should be a minimum 5 population (P) system. Thereafter you add 1P per bedroom:

- house with 4 bedrooms = minimum 6 P system (5+1)
- house with 5 bedrooms = minimum 7 P system (5+2)
- house with 6 bedrooms = minimum 8 P system (5+3)

Multiple properties are sized individually and added together.

For example, for two houses (3 and 4 bedrooms, respectively) the plant is sized for a minimum of 11 P (5 + 6).

For populations between 13-25 the P is multiplied by 0.9.

For populations between 25-50 the P is multiplied by 0.8.

For example, for two houses (5 and 6 bedrooms) 7P + 8P = 15P x 0.9 = 13.5 and round up to 14 population plant

### Points to Remember

- The calculation gives you the minimum size, so always round up to the nearest size tank and then check the manufacturer specifications
- Subject to regulation, sewage treatment plants may discharge to a watercourse a stream; they may also discharge to a drainage field
- If the treatment plant is to serve more than one property, calculate the population for each property separately and then add them together
- Think about the loads – a holiday cottage may have people staying on a sofa bed – spa baths may increase the usage – larger, luxurious houses tend to have increased water consumption – all things that may affect the loads on the septic tank

Take a look at our range of Sewage Treatment Plants.

### How to Size a Septic Tank

**1 – Calculate the population of your property**

First you need to calculate the number of people your property can accommodate, not the number of people actually living there. This is defined in the British Water document “Flows and Lows 4” and states that a house with up to 3 bedrooms should be a minimum 5 population (P) system. Thereafter you add 1P per bedroom:

- house with 4 bedrooms = minimum 6 P system (5+1)
- house with 5 bedrooms = minimum 7 P system (5+2)
- house with 6 bedrooms = minimum 8 P system (5+3)

Multiple properties are sized individually and added together.

For populations between 13-25 the P is multiplied by 0.9.

For populations between 25-50 the P is multiplied by 0.8.

**2 – Calculate the number of litres of wastewater produced each day**

For a standard residential domestic dwelling British Water estimates that one person creates 150L of wastewater per day.

Example: 3 bedroom property = 5P x 150L = 750L per day

**3 – Add 2,000L to the litres per day**

To comply with Building Regulations and the Environment Agency binding rules, you must add 2,000L.

So to continue our example: 3 bedroom property = 5P x 150L + 2000L = 2,750L.

This means the minimum size septic tank for a 3 bedroom property is 2,750L. Always round up this figure to nearest available septic tank size.

### Points to Remember

- The calculation gives you the minimum size, so always round up to the nearest size tank and then check the manufacturer specifications
- Septic tanks are not allowed to discharge to a stream or ditch; they must discharge to a drainage field
- If the septic tank is to serve more than one property, calculate the population for each property separately and then add them together
- Think about the loads – a holiday cottage may have people staying on a sofa bed – spa baths may increase the usage – larger, luxurious houses tend to have increased water consumption – all things that may affect the loads on the septic tank

Take a look at our range of Septic Tanks.

### How to Size a Cesspool or Cesspit

Cesspools are sized by population so the first thing to find out is how many people the property can accommodate. For a domestic household the minimum volume is 18,000l for a 1 bedroom property (2 people).

*Please note that cesspools are not permitted in Scotland unless for a temporary solution whilst a permanent solution is installed.*

**Calculate the population of your property**

First you need to calculate the number of people your property can accommodate, not the number of people actually living there. This is defined in the British Water document “Flows and Lows 4” and states that a house with up to 3 bedrooms should be a minimum 5 population (P) system. Thereafter you add 1P per bedroom:

- house with 4 bedrooms = minimum 6 P system (5+1)
- house with 5 bedrooms = minimum 7 P system (5+2)

**Calculate the size of Cesspool**

Building Regulations state that you must make sure the tank has a minimum capacity of 18,000 litres per 2 users plus another 6,800 litres per each extra user. This will provide approximately 45 days storage.

For example, a 4 bedroom property = 6 P. 2P = 18,000 plus a further 4P x 6,800 = 45,200 litres

A cesspit should be emptied regularly by a registered waste carrier.

We recommend the use of a High Level Alarm which alerts when the tank needs to be emptied.

Take a look at our range of Cesspools.