Swimming pool backwash discharge

Practically all modern swimming pools in the UK operate with a system of pumps and filters (sand). With the pool water circulating though the sand, deposits (such as hair, oil or dead skin etc) can build up. The water flows in through the top of the filter housing and makes its way down through the sand bed where the sharp edges of the sand catch the many of the particulates prior to recirculation. Many filters will remove particles as small as 20 to 25 micrometres, reducing the extent of visible suspended solids.


How does pollutant load effect swimming pool backwash discharge?

The rate at which the build-up in the sand filters occurs depends on the “pollutant load”. This will vary considerably depending on the installation, number of users and cleanliness.

For example, an indoor, small domestic pool, where the users always shower before use, will have minimal load.

An outdoor pool, with nearby trees and sudden high public use during times of sunny weather can be exposed to considerable load.

As the sediment builds up, it acts as a filter aid by reducing the porosity of the sand bed thus trapping smaller particle sizes. However it will eventually blind the filter and lower the permeability unacceptably and require backwashing.

So what is backwashing?

This is where the flow of water is reversed, back through the filter, to purge the system of the pollutant sediment. It is common for some sand to be lost in the process.

What problems does backwashing present?

For backwashing to be effective it needs to be at a far greater flow rate than takes place during normal operation. As would be expected the first flush of backwash can be regarded in the same category as sewerage effluent.

Sudden high flows of sewerage effluent can present a number of problems (see the next question).

Backwashing effectively removes water from the system, requiring additional clean water input to the system, so it typically has a direct operational cost and sustainability issue to address.

swimming pool backwash discharge and pumps at Dormy House

Where can backwash be discharged?

This is a complex issue and depends on consideration of many site and proposal factors. In the UK, it is the role of civil engineering consultants to provide advice on backwash discharge and drainage. The discharge of backwash is typically excluded by a pool consultant/contractors brief with the notes “for discharge refer to civil engineering consultant” along with the rest of the drainage and external work associated with the project.

What are the restrictions on swimming pool backwash discharge generally?

Discharge is often to a public foul sewer. Permission for such discharge is required under the Water Industry Act Section 106. The Sewerage Authority may have capacity limitations on its network and require, at considerable cost, the sewers to be reconstructed. Often restrictions are placed on the maximum flows and times of discharge. This can include low flows over night when public sewer flows are at the lowest.

For many sites a public sewer is unavailable and an alternative discharge system has to be installed. Swimming pool backwash can potentially cause a pollution risk to groundwater or watercourses. A system should always be designed and specified by a civil engineering consultant with the necessary experience of hydrogeology.

Some systems can be designed such that the backwash water can be treated and diluted with rainwater such that the water can be reused for landscape watering. By employing professional civil engineering consultants with experience of water sustainability these options can be explored further. Considerable cost savings can be achieved in the long term by experienced design.

Where swimming pools are open to the paying guests or the public they are usually treated as a “commercial operation” and as such the backwash needs a trade waste licence. The application can vary across the UK, but generally a holiday cottage with a small pool is not included, where as a private school, which allows public charged use does require a licence. A professional civil engineering consultant should be able to assist and will work with the Sewerage Authority to obtain an agreement on your behalf either way.

My pool contractor says swimming pool backwash discharge to the sewer without any agreement is fine as he does it all the time….

Is he providing professional advice, or is this based on getting away with it previously? What if it caused flooding for others or the Sewerage Authority found out? Would he accept all fault and pay all damages or expect you to?

swimming pool backwash discharge and sand filters at Dormy House

Swimming pool backwash discharge and basement developments

It is becoming increasingly common in London for swimming pools to be constructed in basements, with low level plant rooms. The backwash should never be discharge straight into the typical basement pumping station. It is critical that a civil engineering consultant is employed to advise on the interface between the pool systems and the drainage for the development.