How to size a commercial septic tank

Learn how to get the right size septic tank for a commercial building

This step by step guide to sizing a commercial septic tank is similar to the guide to residential septic tank sizing. However the flow from a commercial premises is very different, therefore you need to follow the below steps to ensure you get it right.

Step 1 - Calculate the flow per day.

To calculate the maximum flow per day you must first download British water flows and loads 4.

Once you have this go to page 3 where you will find a table detailing different activities and their relative flow per person per day.

Step 2 - Find the application that suits your business.

On the left hand side of the table you will find the list off applications, find the one (or ones) that suit your business. For example: Office with canteen, note down the litres per person for each of the activities that suit your business. For example you might have:

• 20 hotel (guests only no meals): 80 Litres per person
• And 2 day staff: 90 Litres per person

Step 3 - Calculate the total Litres per activity

To get to the total litres per day, you now must caculate the total litres per activity. Therefore using the hotel example we have the following:

• 20 hotel guests per day: 80L per guest x 20 = 1600L
• 2 day staff at 90 Litres each: 180 Litres
• Total Max flow per day: 1780 Litres

Step 4 - Calculate your septic tank size

Once you have performed the above step, you can calculate the tank size needed by adding 2000L to the maximum flow per day as follows:

• Total flow per day for 20 hotel guests with no meals and 2 day staff: 1780 Litres + 2000 = 3780 Litres

Step 5 - Round up to the nearest available tank size

For the example described, there is a requirement for a 3780L tank, this may not be available, but you should be able to find a 3800 or 4000L tank. You may think that oversizing a septic tank is good and try and opt for a larger model. However this can lead to septicity and therefore odour in your tank.

Download the Excel Calculator

Now you understand the principle behind septic tank sizing for commercial businesses, you can download my excel spreadsheet which will calculate the size required for any application. You can use this for sizing a tank to be used in a hotel, office, factory or any commercial application stated on the flows and loads document.

What if i want a quick guide for a septic tank for a small office?

Since this is a common application, below is a table to help you get the right size septic tank for your office:

Office Staff Without Canteen:

Office Staff With Canteen:

Are there different styles of septic tank?

Yes, there are 2 main styles. The "onion" shaped septic tanks, these are made from GRP (glass reinforces plastic) or the cylindrical style "Millenuim" septic tanks which are made from rotationally molded Polyethylene. Unfortunately, 2 chamber septic tanks are much less common these days, however, in my opinion, they offer superior settlement to the single chamber tanks on the market.

Both styles of tanks can be used for a commercial premises.

Onion style septic tank

Cylindrical style