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Why sizing your Septic tank/ Sewage treatment plant matters

In order to be compliant with the 2020 General Binding Rules it is important to ensure that your wastewater system is appropriately sized and designed to suit your application. For domestic purposes the calculations are relatively straightforward but nevertheless must be calculated accurately.

Whenever looking at the sizing for any of your projects the bible you must live by is the British Water Flows and Loads 4 which not only provides the calculations but also outlines the daily flow from each wastewater application.

The standard calculation for domestic properties is the number of bedrooms + 2, for example if you have a 4-bedroom property this would have a population equivalent (PE) of 6. As a standard manufacturers will have a PE on most of their domestic units making it easier to select a unit of suitable size.

Where more properties are involved the same will apply, i.e. 1st Property PE (bedrooms +2) + 2nd Property PE (bedrooms +2) = Combined PE requirements, so 2 no. 3 bedroom properties would have a Population equivalent of 10.

Regardless of how many bedrooms are actually occupied the calculations need to be taken off the peak expected rate in case these rooms are in use in future.

Sizing adjustments for package sewage treatment plants

There are other factors outlined in the British Water Flows and Loads 4 that need to be taken into consideration for larger projects, where multiple properties are involved, there is a criteria where if you are between a certain population range that adjustments can be done on the PE equivalent to account for the balance of the flow from domestic properties.

The first population range where you can adjust is when the total is between 13-25, this will allow you to multiple the total population x 0.9 to give the Population equivalent.

e.g. 1 no. 4 bedroom and 3 no. 3 bedroom properties would give a combined population of 21, this can then be times by 0.9 giving the new population equivalent of 18.9.

Likewise, when the population is between 26 and 50 then a reduction of 0.8 can be offered.

Why the sizing is important

Ensuring you have a correctly sized system in place is vital as these units have been specifically designed and undergone stringent testing in order to achieve their BS EN certification to handle a certain maximum daily flow which is outlined within the product specification.

For example, a Klargester BA BioDisc is a 6-population system therefore it is suitable to serve up to a 1 no. 4 bedroom property. If any more properties were connected this would not allow the system to operate efficiently and the treatment process would struggle to aerate enough to break down the effluent to the required standard to comply with the general binding rules.

To ensure your system is appropriately designed and sized why not give us a call on 01388 537030.

More RA Dalton Waste Water News

Durham Office

01388 537030

Bishop Auckland
County Durham
DL13 1DB

Crook Office

01388 537030

Unit 10c,
Beechburn Industrial Estate,
County Durham, DL15 8RA

Glasgow Office

01698 827628

Office 60
East Kilbride Business Centre
14 Stroud Rd
East Kilbride
G75 0YA

Chesterfield Office

01246 865412

Unit 2, Midway Business Centre
Bridge Street Industrial Estate
Clay Cross, Chesterfield
S45 9NU

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