The  What? When? and Why? of Retention Tanks and Detention Tanks

In today’s building and development market, supplying a domestic above ground or below ground water tank is not enough for many councils and developments. As the urban sprawl continues across much of Melbourne and Sydney, and in fact, most of Australia, we are seeing more and more tanks requiring detention fittings as specified by the architect, council, development or storm water engineers. 

Where a traditional Aussie home once reigned supreme on a 800sqm-1000sqm block, they are slowly being demolished and being developed to medium density developments where three or four townhouses are being put on the same block. This effectively increases the non-permeable surfaces (any surface that does not allow water to penetrate, i.e. driveways and concrete) on the ground 3x or 4x, as well as increasing the roof space by almost four times as much than previously, where the home once stood. 

This increase in roof space and non permeable surfaces means that when it rains, a lot more of that rain ends up in in our gutters and drains, which all end up in the storm water drainage systems. When these area were first developed, they had never forecasted such large volumes of water and are in many cases inadequate for the new infrastructure going up today. As such, to stop a lot of this water entering the storm water systems in large volumes over short periods, rain water tanks in Melbourne and Sydney especially have started to see a secondary function where they are no longer retaining water for domestic use later, but rather detaining water in the tanks and slowly dispersing it through controlled systems where flow rates are known and the engineers can make sure our storm water systems are not over whelmed. 

What Are The Fittings We Need?

One of the most commonly asked questions when looking at water tanks on house plans is, what are the fittings we need? There are two reasons to have a fitting on a tank, to detain the water or to retain it. What exactly does this mean? 

RETENTION: the action of continuing to hold a substance

In retention water tanks, we have a fitting at low level on a water tank. The entire capacity of the tank is retained at the property and able to be used by the home-owner. The fitting is usually connected to a valve to be able to open and close the water flow and then connected to a water pump to be used for garden use, flushing toilets or to run a domestic household.

Now this is where its gets a bit more complicated…

DETENTION: the state of being detained : a period of temporary custody

A detention water tank is where we are trying to slow the flow of water down and detain it for a period of time., as it flows directly into the storm water overflow pipe. That is we do not want to retain the water for later use. By slowing down the water entering the storm water system from our roof, the detained water is controlled and prevents flooding in our streets and pits. In some cases, the entire water tank is used as a detention tank. In this scenario, the home-owner cannot use any of that water for their own domestic use, and as it rains, the water slowly enters the storm water system. In the other scenario, a water tank will have a fitting further up the rain water tank. Water above our detention fitting will slowly be distributed into the storm water pipe, and the water content below our detention fitting is retained on site for domestic use.

Confusing? See our example below.

In the picture below is a 3000 litre slimline rain water tank.  We are retaining 2000LT for the use of the home owner. We have 1000LT above a detention fitting that can fill quickly in the event of a large deluge and then will drain back to the 2000 litres over time. The quick fill and slow drain is done by 2 different sized pipes. The inlet pipe filling the tank in this instance is 100mm and the detention fitting at the 2000LT mark that is plumbed back to meet the overflow is 25mm slowing the storm water flow down. As you can see in the picture below, the detained water is connected to the water tank overflow system.

If you have any questions or have plans that show a detention system, ASC Water Tanks are specialists in Detention Water Tanks and Retention Water Tanks. We can check out your plans and make sure you getting the right product every time. In the example above, we used an above ground water tank for detention, however we also do underground detention water tanks and storm water calming systems.

ASC Water Tanks have been supplying Australian homes with rainwater tanks since 2007. We sell top-grade Australian Made water tanks to support our economy and to deliver a superior product to our customers and have industry experts able to assist in your next project.

To discuss how to set up your rainwater harvesting system at home or for your upcoming project, contact us here or give us a call on 1300 229 606.

Article Co-Authored By: Luke Stratford (BDM & Trade Sales Specialist) & Anant Yuvarajah (Director)