Going underground

Selecting and installing underground rainwater harvesting tanks can be a complicated process for self-builders, here David Stagg of Graf UK provides advice

People’s increasing interest in rainwater harvesting, which was particularly noticeable during Covid lockdowns, means there is now a perplexing plethora of storage tank options on the UK market. 

Many are manufactured in the UK, while some are imported from Germany and other European countries. They come in many different shapes and sizes, with some suitable for over-ground storage and some for the more discreet underground.

Perhaps the most important consideration when choosing which underground one is right for you, is the groundwater level on the site you are going to be installing the system on. 

If the water table is high, there are low-profile shallow tanks which can be completely submerged in groundwater and still be guaranteed to stay in the ground, even though the pressure of the groundwater surrounding them could cause them to pop up out of the ground. 

Tanks installed in groundwater should have vertical hole features so the groundwater can surround them and pass up through them without building up pressure from underneath and around the sides, which pushes against the tank and causes it to pop upwards. 

It is nevertheless important to design the drainage around the tank to try and remove the groundwater from immediately around it. A perforated pipe that circles the tank can collect groundwater and divert it to the nearest drainage point, or ideally somewhere several metres away.

Another important consideration is whether the rainwater harvesting tank you choose requires a concrete or a granular gravel backfill. Sometimes this depends on the type of tank, or the depth in which the tank is being installed; it can also depend on the loading which is going to be imposed on top of the tank. 

Tanks that require a concrete surround typically take longer to install and so the labour costs can be more expensive. Concrete is also more expensive than gravel. Tanks that require a gravel surround can generally be installed in as little as half a day. But because the gravel can simply be backfilled and compacted in layers, you don’t have to wait for it to set like concrete so it’s a much quicker process. 

Whichever installation method is used, it’s vital the manufacturer’s instructions are followed so their warranties will stand. Many tanks are provided with warranties of 10+ years but builders and contractors should follow the instructions to the letter to ensure responsibility lies with the manufacturers even once the tank is in the ground.

Another important consideration is whether the tank filter is an internal, integral part of the structure or it’s an external pre-tank filter which requires two access covers and two manhole lids. If the filter is inside the tank, one lock on the top gives easy access to the filter. It helps if the filter package is manufactured by the same company that manufactured the tank. 

When it comes to backfilling the tank with gravel or concrete, it’s important to fill them with water beforehand. Most manufacturers suggest this is done in stages, such as filling the tank with 300 mm of water and surrounding it with 300 mm of aggregate. And repeat until the tank is completely full of water and completely backfilled.

It should go without saying that one of the most important considerations when installing a rainwater harvesting tank is to keep it as clean as possible during installation, although this is often hard to do.

Cleanliness inside the tank is of paramount importance because its water is going to be used for your washing machine, flushing toilets and your outside tap. If dirt or soil, or bits of gravel or stone, find their way into the tank during installation because the access points have not been covered, the whole system could be affected. 

It could mean the tank has to be completely cleaned out as soon as it starts to be used. 

Ensure the tank is installed with the access covered or the lid temporarily already on, and that the people installing it are aware it’s not a septic tank! Yes, it has happened!

Finally, consider what type of lid you require. On most tanks, you’ll have two options – a standard lid with pedestrian loading for a garden or a driveway-loading lid. These are usually cast iron and will take a light traffic load, typically a car or van. 

If the tank is being installed in an access road or in a commercial property where lorries and HGVs may pass over it, then this must be considered to ensure the lid, and the tank underground is capable of withstanding the load above it.

David Stagg is technical product specialist at Graf UK