Stormwater Detention Tanks (OSD Tanks)
Underground fibreglass (FRP) OSD tanks for on-site stormwater detention, from 5,000L to 150,000L, made in Australia by Mullaly.
What is an OSD tank?
OSD stands for on-site stormwater detention. When a site is built on, roofs and paving replace soil and grass, so more rain runs off and it runs off faster. An OSD tank holds that extra runoff during a storm and lets it out slowly through a controlled outlet, so the site does not overload the council drainage system downstream.
A detention tank is meant to empty after each storm, ready for the next one. That is the difference from a rainwater or retention tank, which keeps water for reuse (more on that below).
Mullaly makes underground stormwater detention tanks in fibreglass, from 5,000L up to 150,000L for some applications, single or double wall. Larger storage volumes are made up of several OSD tanks connected together.
How an on-site detention tank works
An on-site detention system has three parts:
- Collection. Roof and surface drainage brings runoff to the tank.
- Storage. The tank holds the runoff that can't be released straight away.
- Outlet control. An outlet, usually an orifice sized by the engineer, limits the flow leaving the site. An overflow handles storms bigger than the design storm.
Two figures drive the design. The permissible site discharge (PSD) is the most the site may release, set so the downstream drainage can cope. The site storage requirement (SSR) is the volume needed to hold back the runoff above that rate. Both come from the local council or drainage authority's rules and are worked out by the project's civil or hydraulic engineer, who usually certifies the design.
Our job is the tank: we build it to the storage volume, inlet and outlet levels and site loads in the engineer's design.
Who sets OSD requirements
There is no single national OSD rule. Each council or drainage authority sets its own requirements in its development control plan, stormwater policy or engineering guidelines, and they differ in when OSD is needed, how PSD and SSR are calculated, and how the system must be certified and maintained. Check the rules for the site, or ask the project engineer, before sizing a tank.
Council figures are often given in cubic metres. One cubic metre is 1,000L, so a 20m³ storage requirement is a 20,000L tank, and 120m³ is 120,000L.
Detention tanks and retention tanks
The two are often mixed up, and councils treat them differently. A detention tank slows the water down: it fills during a storm, releases the water slowly through a controlled outlet and drains empty before the next storm. The aim is a lower peak flow leaving the site. A retention tank keeps the water: it stays in the tank until it is used on site for irrigation, toilet flushing or wash-down. The aim is reuse and less runoff overall.
A site may need one, the other or both. Where the council allows it, one tank can hold a retention volume at the bottom for reuse, with a detention volume above it that drains away after each storm. Mullaly fibreglass tanks are used for both. See our stormwater retention tanks page for tanks that store runoff for reuse.
Mullaly stormwater detention tank sizes
| Item | Detail |
|---|---|
| Single tank capacity | 5,000L to 150,000L (the largest sizes for some applications) |
| Larger volumes | Several tanks connected with PVC or fibreglass pipework |
| Diameter | Up to 3.28m |
| Wall | Single wall or double wall |
| Installation | Underground, and select above-ground installations |
| Structure | Integral, high-profile fibreglass ribs, designed for the soil and surface loads on each site. Not limited by the water table |
| Material | Fibreglass (FRP) with resin that contains no fillers. No protective lining or coating needed |
| Accessories | Access risers, manways, inspection ports, FRP inspection ladders, baffles, and inlet, outlet, overflow and vent points |
Full details are in the water storage tank technical data sheet (TD_TDS_003). Tell us the storage volume and levels from your engineer's design and we will recommend tank sizes.
Why a fibreglass detention tank
Usable ground above
The tank goes underground, so the surface stays free for car parks, driveways and landscaping.
Not limited by the water table
Integral fibreglass ribs give the tank its strength, and the tank is designed for the soil and surface loads on its site.
Watertight
Designed to be 100% watertight, so groundwater stays out of the detention volume.
Corrosion proof
Fibreglass is rust and corrosion proof and resists the surrounding soil and the stored water. No lining or coating needed.
No ongoing tank maintenance
The tank structure needs no ongoing maintenance or inspections. The outlet control and any screens still need to be kept clear, as most councils require.
Light to transport
A large tank ships on one truck bed and can be installed in tight footprints and remote places.
Installing an underground stormwater detention tank
Each tank is supplied with Mullaly's installation manual. Fibreglass tanks are light for their size, so a large-capacity tank ships on one truck bed and is easy to install in remote places and in tight footprints between buildings or boundaries. Because the tank is not limited by the water table, it suits sites with high groundwater; tell us the groundwater level when you ask for a quote.
Where the outlet can't drain to the council system by gravity, the detention tank needs a pump-out. See our stormwater pump stations, made in one piece from the same fibreglass as our tanks.
Other uses for Mullaly tanks
Besides stormwater detention, Mullaly fibreglass tanks are used for rainwater harvesting and water reuse, overflow and emergency storage at pump stations, and fire protection water. For those uses, and for factory wash-down water, see our industrial water tanks and underground water tanks. Where runoff from a refuelling area or workshop may carry fuel or oil, treat it with an Aquator oil water separator before it reaches the tank or drain.
Water storage tank photos
OSD tank questions
What is an OSD tank?
An on-site stormwater detention tank. It stores runoff from a developed site during a storm and releases it slowly through a controlled outlet, so the site does not add to flooding downstream.
Is a detention tank the same as a rainwater tank?
No. A detention tank empties after each storm so its volume is ready for the next one. A rainwater or retention tank keeps water for reuse. Some councils allow a combined tank with separate detention and retention volumes.
What size OSD tank do I need?
The site storage requirement comes from your council's rules and is calculated by your civil or hydraulic engineer. Send us the volume and levels from the design and we will recommend a tank or set of tanks.
How big can a Mullaly detention tank be?
Single tanks run from 5,000L to 150,000L for some applications, in diameters up to 3.28m. Larger volumes are made up of several tanks connected with PVC or fibreglass pipework.
Does an OSD tank need a pump?
Only if the outlet can't drain to the council system by gravity. In that case a pump station empties the tank. See our stormwater pump stations.
Can the tank go in high groundwater?
Yes. Mullaly tanks are not limited by the water table. Tell us the groundwater level when you ask for a quote, and follow the installation manual supplied with the tank.
Does an OSD tank need maintenance?
The fibreglass tank structure needs no ongoing maintenance or inspections. The outlet control, overflow and any screens or pits still need to be checked and kept clear, as most councils require of on-site detention systems.
Downloads
- FRP water storage tank technical data sheet (PDF)
- Single-wall water storage tank installation manual (PDF)
More documents are in the resource centre.
Download the water storage tank brochure
Enter your email and the brochure opens straight away. We will also email you the link.
Request a quote for a detention tank
Send us the storage volume, inlet and outlet levels and site details from your engineer's design. We reply to every enquiry within 2 business days.
Or call 02 4964 8270 or email sales@mullaly.com.au