We get asked this question quite often: what is the difference between a retention tank and a detention tank?

Both tanks are used to collect rainwater or stormwater. However, a retention tank is designed to keep the water for use at a later date, while a detention tank eventually drains the water shortly after it is collected.

Rainwater vs. Stormwater

For the purpose of clarity, it is worth distinguishing between rainwater and stormwater.

Rainwater quite simply refers to the rain that falls on the roof, which is often harvested into a storage tank before it gets in contact with the ground. On the other hand, stormwater refers to rainwater falling on roofs down to the gutters and downpipes or runoff from ground surface areas such as driveways, gardens, lawns, footpaths and roads. Naturally, rainwater is of much higher quality since it contains fewer contaminants (e.g. soil, debris, chemical fertilisers, oil residues, etc.).

Harvesting rainwater can help reduce your water bills, as you are essentially utilising a free, renewable resource. A water retention tank would be a good investment for this particular purpose.

Water Retention Tanks

A typical retention tank is designed to harvest both rainwater and/or stormwater, and retain the water rather than simply draining it off. The harvested rainwater can then be used for many purposes such as:

• irrigating your garden
• flushing toilets
• topping up your swimming pool without using mains water
• washing your cars and driveway
• washing clothes
• drinking (if the water system is properly maintained and water is treated)

Stormwater is generally unsafe for drinking, but it can often be utilised for low-risk purposes such as toilet flushing and garden watering. If treated, stormwater can even be used for filling swimming pools.

Investing in retention tanks makes sense, even more so if you’re in a rural area where there is no access to reticulated water supply or where there is a greater need for water for agricultural use.

Stormwater Detention Tanks

Detention tanks also collect rainwater and/or stormwater, but they are typically utilised as part of an effective stormwater system for new homes. Local councils require new homes to have an effective drainage system, which would help in managing the flow of runoff entering bodies of water.

As the name suggests, a water detention tank is designed to “detain” water, that is, to say, it holds the water for a limited period only. The tank is intended to drain the water and remain empty except during a rainfall event and for a short period thereafter.

Stormwater detention tanks have a large inlet pipe and a smaller outlet, which prevent sudden surges of runoff by averaging the out peak flow. This takes some pressure off our ageing stormwater network and consequently prevents flooding.

Why should you use a detention tank?

Its biggest help is assisting your local area’s stormwater drainage facilities to drain manageable loads of water runoff during heavy downpours. Increasing the capacity of existing drainage facilities to keep up with increased stormwater flows has its constraints (i.e. cost and space), which is why many councils have passed on to new home owners some of the responsibility for their stormwater runoff.

To put simply, whether you need a stormwater detention tank and to what extent you need it depends on your local council. How big the detention tank basically depends upon a number of things, such as the size of your property, the building structures within your property, and the surface area of your roof. Your local builder should be able to tell you the right size you need, and your local council should have some table or equation for this as well.

A detention tank can be a standalone facility, or it can be made to work double as a retention/detention system. It can also be used in addition to a rainwater tank which, in heavy downpours, will fill up quickly. The addition of a retention tank, in this case, keeps space available for more water to be detained in the event of prolonged or consecutive rainfall. This solution is preferable as it maximises your rainwater/stormwater collection. This type of system may need to be designed for your property and submitted for council approval.

Because stormwater often carries small debris, blockage often occurs which makes the detention tank ineffective. We at Rotational Plastics have designed our detention tanks with a double-filter system, which features one filter at the inlet point to arrest large debris and another filter by the tank outlet to catch smaller particles.

Materials used for water tanks

Rainwater/stormwater tanks come in different sizes, shapes and materials. Below are some of the most commonly used materials for retention and detention tanks.

• Galvanised steel. Steel water tanks have a zinc coating that protects it from corrosion. They are built with capacities from about 30,000 litres to a million litres, making them a suitable solution for large storage requirements. Over time, the zinc coating may leak into the water which results in rusting. This problem is often overcome by adding liners made of food-grade polyethylene, although it is expensive to fabricate and is vulnerable to wear over time.
• Fiberglass. The quality of fibreglass tanks depend on the maker. In general, they are rigid but light and thin, making them prone to cracking. This type of material needs a food-grade coating to make it safe for storing potable water.
• Polyethylene. The most popular water tank material by far, polyethylene is exceptionally strong and durable but relatively lightweight. These properties make polyethylene water tanks easy to install and cost effective. Polyethylene is a high-quality plastic that is recyclable and reusable, making it an environment-friendly material. It is a food-grade plastic so it’s non-corrosive and does not need a liner, which tends to disintegrate over time. The flexibility of plastic allows the tank to be moulded into various shapes and dimensions. Poly tanks can be made in storage capacities of up to about 50,000 litres.

Need more information about retention and detention tanks? Get in touch with us on 0800 768 284!