A Rocla CDS® separator on Chifley St in Smithfield – one of a series of seven approved for installation by Fairfield City Council along Prospect Creek – will see up to 50 tonnes* of pollutants pulled each year from this location alone.
Murray Powell from Optimal Stormwater, the company tasked with the design and then the installation of the separators (Gross Pollutant Traps), says the council has more Rocla CDS separators than any other type of GPT installed on their stormwater network. “They have a historical preference for the highest performing devices and the Rocla CDS sits at top of that category,” he says.
MUSIC modeling shows 5.28t gross pollutants and 32.1t total suspended solids but the design consultants, Optimal Stormwater, expect more – between 40 and 50t per annum. “Based on experience we’re expecting about 10% litter, 50% organics, 39% sediment and 1% oil,” says Murray Powell.
With the 38ha catchment area dominated by industrial activities – including automotive repairs and wreckers – this particular site was a high priority because, on inspection, the creek was found to be loaded with litter, had very poor water quality and oil was discovered inside the stormwater line.
“Looking at the streets above, there was evidence of litter and grass clippings coming down the line,” says Murray. “Grass is one of the most problematic of the pollutants because it’s so fine – so when it sits in water it breaks down, decays very fast and consumes all the oxygen in the water which then drives the pH down, causing acidity and anaerobic breakdown of other pollutants – releasing adsorbed pollutants back into the water. Grass clippings can have quite devastating effects inside GPTs and in slow flowing waterways or creeks…but better to catch them and remove them, than not catch them at all.”
It was also discovered that the upstream stormwater system – from years of development in the area – contained deposited sediment, which reduced the pipeline flow capacity. Installing the Rocla CDS meant the pipelines could be flushed downstream with the separator capturing the sediment, thereby restoring the original pipeline capacity.
Murray says that what makes the Rocla CDS the best choice is its non-blocking screening technology, which enables the treatment of a greater volume of water because the pollutants don’t block the screen. “The amount of pollutants removed is proportional to the volume of water treated, so if it’s bypassing because the screen is blocked, it doesn’t matter what flow rate you can claim,” he says. “It’s about the reliability of the non-blocking screen and the off-line storage of pollution, which means you don’t get a decreasing flow rate as you would with many other types of GPTs. Storage in the offline sump also minimises the potential for re-suspension and flushing of pollutants.”
Another benefit of the Rocla CDS separator is that it is fast and safe to maintain and clean. Add to that the fact that Rocla can customise the CDS product range in a number of ways so it can take as many pipe sizes and configurations (twin 1200 pipes with a 375mm side pipe in this case), or RCBC, drops and bends as required and you’ve got a very flexible, easily maintained solution.
With budgets for maintaining stormwater assets constantly under pressure, maintenance costs and lifecycle costs are an important concern for councils. Funding for cleaning stormwater assets is tight, and with more and more GPTs being added to asset registers, budgets are being stretched further.
“The Rocla CDS allows councils to spread out the maintenance frequencies by providing a large storage sump, and a product design that facilitates efficient and quick cleaning,” says Henry Ha, Rocla’s Water Quality Specification Engineer. “The self cleaning vortex action against the screen means that the screen rarely needs to be cleaned other than a quick hose down once the sump is emptied and the Rocla CDS dewatered during maintenance periods. The sump is easily accessed by an educator hose. The sump can even be enlarged if need be, if the asset owner wants to extend cleaning intervals. The sump in each Rocla CDS separator model can be almost doubled in capacity with Rocla’s know-how and collaboration early in the project.”
Murray says the ample pollutant storage volume of 11m3 for the Chifley Street Rocla CDS 2018L unit will allow quarterly emptying, which will fit in nicely with the council’s ongoing cleaning programme.
“If you look at lifecycle costs, with the Rocla CDS separator it might be 20% more expensive up front, but it stores twice the pollutants, therefore requires half the cleaning,” he says. “In terms of overall costs, around one third is design and construction with the other two thirds incurred over the life of the product. So the Rocla CDS may have a higher initial capital cost, but dramatically lower ongoing costs than most other devices.”
“The whole project took about five weeks with the majority of the device precast, which made it quicker, faster and safer,” says Murray. “The fact that the Rocla CDS is offline means you install the whole unit and, at the last minute, break into the pipe to add the diversion chamber. So rainstorms don’t matter for the installation of Rocla CDS separators, but they do for other GPTs where you have to break into the pipe at the start of construction.”
Principal: Fairfield Council, NSW
Designer/Contractor: Optimal Stormwater
Product: P2018L Rocla CDS® separator
For a video of the Rocla CDS separator cleaning process or for more information call your local Rocla representative or phone 131 004.