Is A BASIX Rainwater Tank The Same As An OSD Tank?

No. A BASIX rainwater tank stores roof water so it can be used in the building, while an on-site detention tank, or OSD tank, holds stormwater back during a storm and releases it slowly so the street drainage is not overloaded.

Most residential developments in Sydney need both. Treating one as the other is a common reason a stormwater plan comes back from council with questions.

What BASIX asks for

BASIX sets water and energy targets for new residential buildings in NSW, apartments included, and a BASIX certificate is lodged with the development application.

A rainwater tank is one of the common ways to meet the water target. Roof water is collected, stored and reused for toilet flushing, the laundry and the garden. The commitments on the certificate, including the tank size, the roof area connected to it and what the water is used for, become part of the approval and are checked at completion.

What OSD does

OSD answers a different question. The council, or Sydney Water in some catchments, limits how fast a site can discharge stormwater during a storm, called the permissible site discharge.

The OSD tank stores the water the site produces above that rate and lets it out through an orifice plate sized to match it. After the storm passes, the tank drains empty so it is ready for the next one. The method and the numbers come from the council's stormwater policy, and how they fit into a full site design is covered in [stormwater management for Sydney sites](/blog/stormwater-management-sydney-sites-osd-infiltration-compliance).

Why one tank cannot do both jobs

A rainwater tank is meant to be full. That is how it saves water, and a well-used BASIX tank spends much of the year holding water for the toilets.

A full tank has no room left for a storm. If the BASIX volume were counted as detention, the site would have no detention at the exact moment the street drainage needs it. So the two volumes are designed and counted separately, even when the storms and the roof are the same.

When a combined tank is accepted

Some councils accept one tank divided into two zones. The BASIX volume sits at the bottom, below the orifice, and the detention volume sits above it. Only the volume above the orifice counts as detention.

Other councils want two separate tanks. The council's development controls decide which, and the answer has to be known before the tank is drawn, because it changes the size, the shape and the levels.

Where the tanks go

On an apartment site the tanks usually end up under a driveway, under landscaping or inside the basement, and each of those is a structural question as well as a civil one.

A tank under a driveway is a structure that carries vehicle loads. A tank inside the basement has to be coordinated with the slab and walls around it. Councils want the OSD outlet to drain to the street by gravity, so a detention tank set deep in a basement below the kerb level needs a different answer, and basement drainage in general is covered in [basement stormwater pump-out systems](/blog/basement-stormwater-pump-out-systems).

Title and maintenance

Councils commonly require the OSD system to be protected by a positive covenant and a restriction on the title, so that a later owner cannot remove it, fill it or build over it.

Both tanks need maintenance once the building is occupied. The OSD orifice and trash screen have to be kept clear, or the tank fills and overflows early. The rainwater tank needs its pump and first flush device looked after, or the toilets end up on mains water and the BASIX saving disappears. Developments that also connect to a recycled water network have a third system to coordinate, as discussed in [recycled water integration in Sydney's new developments](/blog/recycled-water-integration-sydney-new-developments).

What the drawings show

We size both tanks from the start of the civil design and show them as two separate functions on the drawings, with their volumes, orifice and overflow levels, inlets and access openings marked. The council and the certifier should be able to see at a glance which volume saves water and which volume protects the street.

That clarity is most of the battle. The wider drainage design they sit in is set out in [stormwater management for urban developments](/blog/stormwater-management-urban-developments).

How we approach it

The civil and structural design for the tanks comes out of one office, so the tank that works hydraulically is also the tank the slab can carry and the builder can form. One tank saves water and the other protects the street, and a site needs both jobs done even when they share the same concrete box.

ACSES provides civil engineering, including stormwater and OSD design, across Sydney. Talk to us about a project.

Building Relationships Beyond Structures