What is more important in selecting the right system to do the job, catchment area or treatable flow?

In recent years the industry has seen some proprietary GPT providers sizing their units using potential catchment pollutant loadings rather than treatable flow rate (TFR). This practice not only confuses those wishing to compare different proprietary systems but, significantly, it enables those GPT providers to provide under-sized alternatives that compromise the optimal pollutant removal efficiency.

The recognised industry standard for achieving optimal pollutant removal and retention rates falls within a designed treatable flow rate of between a 1:3 months to a 1:1 year ARI discharge. This is the flow rate at which the GPT will operate without by-pass to ensure maximum pollutant removal rates. Systems sized using catchment area invariably result in smaller units, which have a lower-designed TFR and go into by-pass prematurely.

Consequently, the product does not treat the catchment's optimal pollutant loading and may even cause flooding as under-sized units usually have insufficient by-pass capacity at the designed peak discharge flows. Also, they usually have inadequate storage capacity for the expected pollutant loadings.

Ecosol always ensures that its units are appropriately sized for optimal capture efficiencies with minimal hydraulic impact at peak design discharge flows. We have ten different sized units each with a TFR capacity dependent on the size and slope of the outlet pipe.

The flow generated by any catchment is matched to one of these ten standard units to ensure it can be treated, with flows greater than the TFR by-passing. It should be noted that the location and level of imperviousness in a catchment can greatly affect the quantity of runoff and volume of pollutants.

Are trash racks an effective solution? It seems cheaper to simply put some nets across the water channels?

No. Trash racks suffer from very low capture rates (between 5-15%) and often fail to collect smaller gross pollutants such as cigarette butts etc. More importantly, they do not have an overflow mechanism and are prone to block, potentially causing serious flooding during major storm events. They also require frequent cleaning, which is usually difficult and costly, and are also visually unattractive. Ecosol's Net Tech product is the exception to most netting units available in that is it is very affordable, has an effective overflow mechanism and captures more than 91% of gross pollutants greater than 19mm.

Even in the 1990s, many engineered at-source litter baskets and in-line/end-of-line gross pollutant traps (GPTs), while a significant improvement on earlier devices, still suffer from the above and other limitations, including:

Not surprisingly, these first-generation trash racks, and also the early-design litter baskets, have been superseded by more cost effective, efficient, and unobtrusive designs.

When comparing different units, can we use the capital cost per cubic metre of pollutants captured as a guide?

No. This is a mistake many clients and consulting engineers make when trying to compare the performance and cost-effectiveness of different proprietary products. There are several flaws with this approach.

Whilst the capital cost can be readily found the ongoing cleaning and maintenance costs are often not added to determine a more accurate whole of life cost.

Also, the effectiveness of the cleaning regime is a critical factor in the equation. If a unit is cleaned only once in the year, when in fact it requires cleaning say three times, it will obviously have been in by-pass for much of the year and so the amount of pollution collected will be less than if it had been more regularly cleaned. This would make a nonsense of any attempt to compare different units using the level of captured pollutants as a key factor. Other factors such as differences in catchment characteristics, hydrological conditions, and different methods of cleaning will also affect any comparisons.

How do I decide between installing filters before stormwater enters the drainage network or down the line?

Capturing pollutants at source is an extremely efficient, and usually inexpensive, way of managing stormwater pollution. At-source treatment devices prevent gross pollutants from entering the drainage network and, therefore, reduce the likelihood of drain blockages. However, it is important to carefully consider the required cleaning frequency of the devices as this can add significantly to the whole-of-life cost.

Depending on the expected pollutant loadings, and the number of required gully pits, specific to your project it may be more cost-effective to install a smaller in-line/end-of-line gross pollutant trap, such as the RSF 4200 (click here for more details).  

What is the estimated life of the unit and especially the internal filtration system and what materials are used?

All materials used in the manufacture of Ecosol products are selected to specifically match their operating environment. The units are manufactured using the following materials:

Material	Useful Life (years)*
Reinforced concrete Pit, cover slab and risers	50
Ductile Iron Access lids	20
Stainless Steel - Grade 304 and 316 Internal filtration structure	15
Stainless Steel (RSF 100) Support frame	15
Alucobond (RSF 100 and RSF 1000) By-pass flaps	15
Polyethylene (Pe) Mesh Filtration mesh	5

*The quoted useful life is representative for the materials under normal working conditions assuming the recommended cleaning and maintenance regime has been followed. However, it is not in any way offered as a guarantee.

We would like to treat the flow close to the source of rubbish such as in shopping centre car parks wherever possible. Does this require having lots of litter baskets in the side entry pits and regular cleaning, making it a costly option?

At-source litter baskets are a very popular, inexpensive treatment measure and are usually installed at hot spots (commercial areas with high gross-litter loadings) such as shopping centres. However, their effectiveness does depend on a regular, thorough cleaning regime as they will collectively have a smaller holding capacity than an in-line/ end-of-line gross pollutant trap. Often, the deciding factor on choosing between an at-source or in-line/ end-of-line solution is the ease with which the units can be cleaned.

Do we really need a pollution trap when the majority of what is caught is organic leaves and such?

Yes. In nature, much of the organic material such as leaves tend to break down and return to the soil. However, communities usually have lots of impervious surfaces such as roadways, paths and car parks. These surfaces dump vast quantities of leaf and other organic litter into watercourses, when historically this would not have occurred.

Have you undertaken any independent field testing?

Yes. All Ecosol's products have been extensively tested in the field. Many of these tests have been carried out by independent bodies and others in conjunction with local councils and authorities (click here to see some of the reports).

While field testing is a valuable way for councils to gain a better understanding of what is happening in their catchment, it is important to understand that comparisons between different proprietary products solely on the basis of field tests usually undertaken under differing test regimes and in different catchments will almost certainly provide inconsistent results. Ecosol believes that it's almost impossible to ensure any two units are treating similar size flows that contain the same concentrations of pollutants, even when the sites are located near to each other. Each site has different hydrologic and hydraulic conditions and often there are varying levels of other activity within the catchment at the time of testing.

What is a "Hydraulically Driven Barrier"?

This is a unique phenomenon with the Ecosol RSF 4000 product. When the unit is operating within its under the low-flow conditions, i.e. at its TFR, the circulating effect of flows within the unit drives a barrier upstream along the by-pass channels. As this water meets the flows entering the inlet to the capture silo, a unique hydraulically driven barrier is created ensuring all flows up to the TFR are directed into the capture silo.

As flows of a greater magnitude enter the unit, the hydraulic barrier gradually breaks down and in major pipe discharges, allows the excess flows to by-pass without remobilising the captured pollutants. However, it is important to note that the unit continues to collect and filter flows at least equivalent to the TFR.

How easy is it to install an RSF 4000?

Very easy. This is one of the great benefits of the RSF 4000. Unlike most other proprietary systems that come in as many as six parts, all requiring on-site assembly, our units (up to our RSF 4750, and also the RSF 4900 (site-specifics permitting)) arrive on-site with the internals already fitted ready to be dropped into place. This reduces significantly the on-site installation time and cost. For example, an RSF 4300 can usually be in place within 30 minutes of arrival on site. Another two hours connecting pipes and backfilling and the unit is fully commissioned.

Does the unit capture and retain nutrients and suspended solids and what are the efficiency removal rates?

Nutrients and suspended solids are captured by the RSF 4000 units, albeit at relatively low rates. This is mainly due to their attachment to the finer silts and sediments that are captured at much-higher rates. However, it must be understood that the RSF 4000, like all other GPT units, is a primary filtration device and that secondary treatment processes, such as sand filters, may also need to be considered if high efficiency removal of these constituents is required. Such secondary measures by themselves are usually not appropriate as they can block very quickly if the larger pollutants have not already been filtered from the flow by some form of primary filter.

What's the difference between a wet and dry system?

Most GPTs, such as the Ecosol RSF 4000, work as wet sumps that enhance pollutant retention and have settling qualities in certain flow conditions providing secondary benefits such as the capture of finer silts and sediment. Baffle boards also provide some retention of hydrocarbons (free oils and grease) in oil spills.

Dry systems are not as common today. However, in the right environment they do have some advantages. Systems such as the Ecosol RSF 100 and RSF 1000 are dry systems and their obvious advantage is the self draining of the captured pollutants making the unit easier to clean and reducing decomposition of pollutants.

Ecosol's solutions-driven approach and range of products enable us to offer the best product for a specific project.

Will mosquitos breed in the units?

Mosquitoes can and do breed anywhere near stagnant water. However, in many cases there is a constant flow through an Ecosol unit thereby removing the opportunity for mosquitoes to breed.

As a rule of thumb mosquitos breed in fairly clear oxygen rich water. Turbidity, due to organic pollution, results in a diminished light penetration, and at a certain depth anaerobic processes may take over. Consequently there is usually a lowering of oxygen levels making the water unsuitable for breeding for a large majority of mosquito breeds although there are a number of exceptions. Also free-floating hydrocarbons on the surface also inhibit mosquito breeding.

After extended dry periods will the captured pollutants decompose and omit odours and any leachate?

This process can occur in any wet sump system. However, if the unit is cleaned on a regular basis, the opportunity for it to occur is reduced significantly.

This problem is less likely to occur in the RSF 4000 unit due to the cleaning process whereby the unit is fully dewatered and all pollutants removed. In other wet-sump systems there is usually no dewatering facility so the water remains in the unit for extended periods of time, together with many of the fine settled toxicants. Also, each RSF 4000 unit is supplied with bolt-down, gas tight lids to prevent unauthorised access and the release of any odours.

How often do the units need to be cleaned?

This varies between the different types of units. The cleaning frequency depends heavily on weather patterns, pollutant load and activity within the catchment. Under normal operating conditions, we recommend the units are cleaned every 3-4 months.

Can units be cleaned by simply removing the basket rather than using the suction cleaning?

Cleaning your GPT is essential to ensure the it continues to capture and retain pollutants to its designed specification with out premature bypass.

The methods for cleaning GPTs has been widely debated for many years. Ecosol's units have been specifically designed for cleaning by vacuum method, which we believe offer far greater benefits than, for example, having a removable internal filtration basket. After many years of industry experience we have confirmed that this is the safest, most efficient, and cost-effective method, because:

	Captured pollutants or contaminates are retained within the unit
	Pollutants do not need to be manually handled and can be safely stored and transported;
	Free oils and grease (hydrocarbons) are able to be efficiently removed and stored;
	Harmful pollutants such as fine sediments, heavy metals, and nutrients are not remobilised;
	No additional machinery other than the eductor truck is required;
	The internal filtration system is unlikely to suffer any damage during the cleaning process; and
	All pollutants are removed from the capture silo, unlike alternative methods.

What makes the Ecosol unit unique to other units being cleaning by vacuum method is its dewatering baffle board system that is installed at the inlet of the unit enabling excess water to be dewatered back upstream of the unit when the unit is cleaned. This avoids the unnecessary cost of removing and disposing of stormwater retained within the unit and ensures pollutants only are efficiently removed. At the conclusion of the clean the dewatering board is removed and the excess water is then re-filtered through the unit.

Isn't the basket or bucket grab method the most cost-effective cleaning method for a gross pollutant trap?

No. Our experience shows that this method fails in several key areas:

	To avoid spillage pollutants must be drained prior to transportation and so purpose-built draining beds are needed - this presents serious safety issues as the public may be unnecessarily exposed to harm. Some draining methods involve suspending the removable basket over the GPT but this process releases the captured nutrients, heavy metals, and hydrocarbons back into the drainage network.
	Additional equipment (e.g. pumps, crane, etc.) is generally required, thereby increasing the cleaning costs.
	Over time the removable basket or bucket inevitably becomes damaged and needs replacing, adding to the ongoing costs.
	The removable basket is often very difficult to return to the GPT after cleaning.
	Oils and grease are usually not removed with this method.
	Units cleaned by this method ultimately need to be cleaned by vacuum method at least once a year to remove the fine sediments and hydrocarbons.

Can your end of line units work in a submerged outlet situation? If not, what is the maximum allowable influence by tide?

Situations where there is a standing water level above the outlet pipe invert level creates a problem for any GPT device. There can be no guarantee that the capture efficiency and the ability to retain pollutants will not be compromised. The extent to which this is a problem depends on the extent of inundation.

If there is only a small water depth at the outlet of say 100 to 150mm, this can be overcome by adding vertical guide plates at the entry to the capture silo. This will ensure the low flows are separated from the standing water level that would be present in the by-pass channels. In more severe circumstances we can provide a specially-designed unit that incorporates vertical hinged by-pass flaps to ensure the captured pollutants will remain in the unit even during a rising water level from downstream.

I am a MUSIC user. Does Ecosol provide input values for its units?

Yes, we have prepared MUSIC input values for the RSF 100, RSF 1000 and RSF 4000 - these can be found at the bottom of each product page, or if you prefer, contact us.