Cartridge Media Filtration: StormFilter for Tight Urban Sites

The tight-site problem

Bioretention is the default tertiary treatment measure in most NSW council WSUD guidelines — but it needs surface area, level difference and sunlight. On urban infill sites, the numbers often simply don’t work: a boarding house covering 80% of its lot, a childcare centre with every square metre of outdoor space committed to play area, or a commercial site where the only “spare” land sits over a basement. When a rain garden physically cannot fit, cartridge media filtration is usually the next option on the list.

How cartridge filtration works

A cartridge filtration system is an underground vault — a pit, chamber or precast tank — containing one or more replaceable cartridges filled with filter media. Stormwater enters the vault, passes through the media under hydraulic head, and exits via an underdrain or outlet manifold. The media does the same job as the sandy loam in a rain garden, but in an engineered, compact and serviceable package:

• Fine sediment is physically strained and trapped in the media
• Nutrients and metals are adsorbed onto media surfaces, with media blends selected to target phosphorus, nitrogen or heavy metals
• Hydrocarbons are captured by sorptive media components
• Flows beyond the treatment design rate bypass internally or via an upstream diversion, so the system does not choke the drainage network in large storms

The StormFilter and similar systems

The best-known cartridge system on Australian projects is the StormFilter, supplied in NSW by Ocean Protect. Based on published product information, StormFilter cartridges are siphon-actuated: as the vault fills, a siphon primes and draws water evenly through the full media surface, then breaks when the storm passes, which helps shed accumulated sediment from the cartridge surface between events. Cartridges are rechargeable — at maintenance the spent media is replaced rather than the whole structure.

Other cartridge and media systems are available, including the SPEL Filter (an upflow cartridge system that received SQIDEP verification in late 2022) and the Jellyfish Filter (a membrane-based cartridge device, also distributed by Ocean Protect). As an independent design consultancy we have no supplier alignment — the right device is the one that fits the site hydraulics, achieves the modelled performance and satisfies the council.

Designing a cartridge system into a tight site

Cartridge vaults solve the footprint problem but introduce their own design constraints:

• Hydraulic head. Cartridges need a driving head (commonly in the order of 500–700 mm depending on cartridge height) between inlet and outlet. On flat sites this can be as limiting as a rain garden’s depth requirement.
• Number of cartridges. Each cartridge treats a finite flow rate (typically a few litres per second). The MUSIC model and the hydraulic design together determine how many are needed.
• Pre-treatment. A pit insert or GPT upstream keeps leaves and litter off the cartridges and stretches the media replacement interval.
• Access. The vault needs a maintenance opening a truck can reach. A vault under a landscaped rear corner with no vehicle access becomes a lifelong servicing problem.

What about council acceptance?

Most NSW councils accept cartridge filtration where bioretention is demonstrably impractical, but policies vary by council and this must be checked early. Common themes:

• Councils increasingly expect proprietary device performance to be supported by independent field verification under Stormwater Australia’s SQIDEP protocol, rather than laboratory or manufacturer data alone
• Some councils cap the proportion of the treatment target a proprietary device may deliver, or require a written justification for not using vegetated measures
• MUSIC modelling parameters for proprietary nodes may be prescribed by the council or required to match the verified performance claims
• A maintenance schedule, servicing agreement and often a positive covenant are standard conditions

Maintenance is the hidden line item

Cartridge media is consumable. Typical servicing involves inspection once or twice a year and media replacement at intervals that depend entirely on sediment load — anywhere from one to several years. When comparing a cartridge vault against a rain garden, compare whole-of-life cost, not just construction cost: the vault usually wins on land value and loses on recurrent servicing. We cover that comparison in detail in a separate post.

Contrive Consultants designs WSUD systems and prepares MUSIC and DRAINS models accepted by NSW councils, including cartridge filtration layouts for constrained urban sites. Browse our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us for advice on your site.