GPTs and Pit Inserts: Where the SPEL Stormsack Fits

Primary treatment: the first line of defence

Every stormwater treatment train starts with the big stuff. Gross pollutants – litter, bottles, leaves, twigs and coarse sediment – are the most visible form of stormwater pollution, and NSW councils commonly adopt a target of around 90% reduction in average annual gross pollutant load for new development (exact targets vary by council). Gross pollutant traps (GPTs) and pit inserts are the devices that deliver this, and they also protect the finer treatment measures downstream from clogging.

The two broad families of devices

Pit inserts (at-source capture)

A pit insert is a basket or bag fitted inside a stormwater pit, directly under the grate. Runoff falls through the insert before entering the pipe network, so pollutants are captured at the point where they enter the system. Because they sit in standard pits, inserts need no extra land, no separate structure and minimal additional excavation – which makes them popular on duplex, townhouse and small commercial sites.

In-line GPTs (end-of-pipe capture)

An in-line GPT is a dedicated underground structure on the pipe network, typically near the site outlet, using screens or vortex action to capture pollutants from the whole upstream catchment. Devices in this family – such as the SPEL Ecoceptor and Vortceptor – suit larger catchments such as subdivisions, large commercial sites and car parks where a single capture point is more economical than many inserts.

The SPEL Stormsack: a typical pit insert

One widely used pit insert on NSW projects is the SPEL Stormsack, a filter bag system that fits into new or existing stormwater pits. Key characteristics based on the manufacturer’s published information:

• A reinforced geotextile filter bag supported by a frame that sits under the pit grate
• Standard sizes to suit common pit dimensions (such as 450×450 mm, 600×600 mm and 900×900 mm), with custom sizes available
• Captures gross pollutants, litter, leaves and coarse sediment, with bag options that also target oil and grease
• Designed with overflow capacity so the pit still functions hydraulically if the bag is full
• Maintenance by lifting the grate and removing or emptying the bag – pollutants are trapped just below the surface, which keeps cleaning quick

Comparable products exist from other suppliers – for example Ocean Protect’s OceanGuard pit basket – and as an independent design consultancy we model and specify whichever device best fits the site, the catchment and the council’s requirements.

Where pit inserts fit in the treatment train

It is important to be realistic about what a pit insert does and does not do. In a typical treatment train:

• Pit inserts deliver primary treatment. They are very effective on gross pollutants and coarse sediment, and they make a meaningful contribution to total suspended solids.
• They do not, on their own, meet nutrient targets. Dissolved nitrogen and phosphorus pass through a filter bag. A rain garden, bioretention basin or media filtration cartridge is still needed downstream to achieve TP and TN reductions.
• They protect downstream assets. Keeping leaves and litter out of a bioretention system or cartridge vault extends its life and reduces maintenance cost – one reason many designs pair an insert with a tertiary device.

In MUSIC modelling, pit inserts are represented with treatment performance parameters appropriate to the device, and several councils specify how proprietary devices may be modelled. Always check the local policy: some NSW councils accept manufacturer performance claims supported by independent verification (such as Stormwater Australia’s SQIDEP protocol), while others apply their own default parameters. Council acceptance of proprietary devices varies, and confirming the policy early avoids redesign later.

Maintenance: the make-or-break factor

A full filter bag treats nothing. Councils know this, which is why consent conditions typically require a maintenance schedule, and often a positive covenant, for treatment devices on private land. For pit inserts, plan on inspection every three to six months and after major storms, with more frequent cleaning during construction and leaf-fall periods. Build the maintenance regime into the strata or owner documentation from day one.

Contrive Consultants designs WSUD systems and prepares MUSIC and DRAINS models accepted by NSW councils, including treatment trains that combine pit inserts, GPTs and bioretention. See our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us to discuss your project.