WPL’s easy to install diamond sewage treatment plant case study


Courtesy of WPL Ltd

If you are lucky enough to be involved with a building project in the beautiful countryside of Great Britain, one thing you will find most properties have in common here is that they are likely to be off-mains drainage.

When involved with an off-mains building project, it is important that you do know where your wastewater goes and what happens to it, usually by means of a domestic sewage treatment plant or septic tank, for example.

One such project in Awbridge, Hampshire involved the conversion of a barn and outbuildings to holiday and long-term rental accommodation.  The property also included an existing thatched cottage.  With the addition of these dwellings there would also be an increase in wastewater produced from the toilets, kitchen and bathrooms.

The homeowners chose a WPL DMS3 Diamond sewage treatment plant for up to 11 people, which would be able to cope with all the wastewater from the entire property, new and existing.  They chose a local drainage installer who was able to calculate the correct size plant, based on the number of bedrooms at each property, and also to contact the Environment Agency to make sure that the treatment plant was registered as exempt from the need for an environmental permit.  The WPL Diamond plant achieves a guaranteed process performance to a 95th percentile standard of 20mg/l Biological Oxygen Demand (BOD), 30mg/l Suspended Solids and 20mg/l Ammoniacal Nitrogen.

For the installer, Independent Drainage based in Dorset, there are many reasons why they recommend the Diamond over other systems.  These include it’s compact design and easy installation, and for the homeowner the low visual impact (it’s completely below ground) and the absence of odours.  There are also no moving parts inside the tank making installation and on-going maintenance simple.  The treatment process is biological by continuous aeration provided by a blower housed in a small discreet weatherproof box.  The blower can also be installed in an outbuilding/shed.

In Awbridge, the installation in total was less than three days, although the final making good had to wait a few days as the usual British summer weather meant it rained on day three! But the tank was in, but not connected, within a few hours of the installation starting.  There was existing pipework to connect to which was identified during a site survey by the installer before the installation.  This designated where the tank was to be sited.  Unusually the tank was to be installed underneath a concrete courtyard rather than the garden.  This is no problem for a Diamond tank as it’s installed completely below ground, but access is still required to the tank for inspection purposes via the Diamond’s lid, which is covered by a galvanised steel access cover.

The Diamond tank is so called due to the conical design at the bottom.  This is beneficial to the treatment process as well as the installation as the excavation hole is much smaller than other tanks.  It can be stepped in towards the bottom of the hole resulting in a speedy excavation and it also requires less backfill material (shingle and/or concrete depending on the local water table).  With the tank in place, it is then backfilled with shingle to steady it as it is being filled with water from a hosepipe. As the tank fills up it settles into place and the backfilling can continue up to the outlet pipe.

Next the blower is installed in the adjacent outbuilding to where the Diamond is situated, as there is an existing power supply here.  The air hose is then routed from here through the wall and into a conduit running into a dug out channel and then through the side of the Diamond to join onto the air supply downpipe inside the tank.  The power failure alarm is also mounted on the exterior of the outbuilding so it is visible in the event of a power cut.

Finally connection to the inlet and outlet pipes is then made, and the system can be commissioned and tested.  Backfilling is completed and in this instance the top level is concreted over to restore the courtyard to its original appearance, with the addition of a new access cover for the plant and also an inspection cover for the wastewater inlet pipe.

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