Many new housing developments are now choosing to integrate an underground rainwater harvesting system into the structure of the property from the outset.
This is a powerful environmental statement and brings attractive benefits to prospective homeowners. In this article we examine what an underground rainwater harvesting system tank actually does, and the benefits of having one installed.
The Features Of An Underground Rainwater Harvesting Tank
The most obvious feature of an underground rainwater harvesting tank is that it is designed to be installed below ground level, rather than above ground either externally, or within the property. This makes the system more advantageous for some purposes than others, as we shall see below. While every underground rain harvesting system will be unique to the individual property, all have certain features in common. These are:
1) Rainwater Collection Pipes
Rainwater run-off is captured from the roof of a building from its’ drainage system. This can either by done directly from the rainwater downpipes (RWPs) or, more commonly from the below ground stormwater drainage pipes. Different types of filter collectors are available for both options. The filters should be as fine as possible and ideally less than 1mm (1000 microns); this will ensure that very little organic matter or debris reaches the tank. Leaf guards can also be used where many trees are present as this prevents an accumulation of decomposing leaves within the gutters. Where the tank is being fed by water from two different sources, e.g. from a house and an external garage, the pipes should all ideally combine in to one common pipe; this enables all of the water to be filter by a single filter collector.
2) Storage Tank
The underground tank itself is almost always of one-piece design and is normally constructed from either high density polyethylene or GRP (Fibreglass). It is usually buried up to 1.2 metres under the ground and encased in either compacted stone or concrete. The tank is accessed from the ground via an inspection cover for maintenance purposes, though in most cases, once an underground tank is installed it needs very little human interference. The tank incorporates the following important components:
- Calmed Inlet pipe: This pipe reaches to the bottom of the tank where it terminates in a special flow calming diffuser. This calms the flow of water as it enters the tank and also helps to oxygenate that water as is does so.
- Overflow pipe: An overflow unit incorporates a water trap and a non-return valve that prevents the tank from flooding in the event of backflow from the drain. The overflow pipe should connect to a storm drain or soakaway.
- Float switch: Allows users to monitor the water level in the tank and prevent it running dry. There is usually an emergency water mains inlet that is triggered when the tank water drops below a certain level.
- Pump: A pump is submerged in the tank that feeds water into the outlet pipe on demand for use in domestic systems.
- Outlet pipe: A pressurised pipe from the pump that conveys water in a regulated flow to the property. This links the underground tank via a network of pipes to water-using systems on the property, such as external taps, washing machines, toilets etc.
3) Power Supply & Controls
The power supply is located above ground typically within a utility room or similar, with wires and sensors connecting it to the pump and float switch. There is also normally a user control interface that allows operators to shut down the system for maintenance and to identify / diagnose any faults.
Why Choose An Underground Tank?
Why choose an underground rain harvesting tank, rather than an above ground alternative? Once installed, it takes up no space at ground level so imposes little restriction on the design of the garden, as lawn, paving etc can be laid over the top of it.
Getting water in to it is generally easier as the pipework can be routed below ground as required.
Water stored below ground is kept cold and dark so there is no potential for growth of algae.
An underground tank also avoids the necessity of having a large and unattractive external tank in the property’s garden. Owners of existing properties are often deterred from installing an underground system because of the cost and disruption of installing the tank itself. However, this is not an issue when an underground system is incorporated into a property at the construction stage. It is then a far more cost-effective option.
Underground systems can be used to feed water to almost any domestic property, and are most commonly used for supply the house with water for toilets, laundry and outside use, though are also often used solely for garden purposes. They can connected up to sprinklers and external taps, eliminating the need for the user to access mains water to maintain their garden. This brings cumulative cost savings for the property owner, especially in houses with large gardens, as well as reducing pressure on the local water mains.
