How do you calculate the size of a septic tank?

To calculate the size of a septic tank, start with a minimum base volume of 2,000 liters. Then, consider the source of waste and refer to the Flows and Loads guide from British Water. It provides the liters discharged per person per day for different types of properties. For example, a standard residential dwelling may have 150 liters per person per day. Multiply this by the population to get the daily volume. Add it to the base volume to determine the required storage volume.
For instance, a five-person household would be 5 x 150 = 750 liters per day. Adding this to the base volume of 2,000 liters gives a required storage volume of 2,750 liters. It's important to choose a tank with a volume equal to or larger than this.
Consider the specific requirements of your property. For example, in an office building with 10 full-time staff members, if each person generates 90 liters per day, the calculation would be 10 x 90 = 900 liters per day. Adding this to the base volume gives a required storage volume of 2,900 liters.
Keep in mind that the desludging interval will depend on the tank manufacturer and may vary. It's essential to gather this information from the customer before making a decision.