Xylem Water Solutions Canada - Helping nature take the strain. Wetland stormwater control and retention.

Helping nature take the strain. Wetland stormwater control and retention.

Necessity, they say, is the Mother of Invention. Which is partly behind an ingenious solution for tackling the large, irregular stormwater run off near urban areas. Large volumes of combined sewer overflow (CSO), polluted stormwater or urban run off occur in a relatively short time. It is important to contain these for treatment. Conventional concrete retention basins for managing these flows are considered expensive. Moreover, it is almost impossible to find suitable land for basins above ground, and underground basins are too costly.

This solution is flexible, effective and environmentally sound - a solution in fact that helps Nature do what comes naturally. The technique was developed in Sweden by Karl Dunkers. It has been applied, and praised, in tens of cases in North America and the Nordic countries. Basically it utilizes water-bodies close to urban areas to retain and equalize large flows of stormwater before it is pumped with constant flow for treatment. This takes place in existing treatment plants, in special Dunkers plate separator plants for stormwater or in natural or constructed wetlands. When wetlands are used, the technique meets new US Environmental Protection Agency guidelines calling for pre-sedimentation and equalization before discharge into open wetlands. Karl Dunkers' technique consists of a floating basin divided into compartments. The system features pontoons from which heavy plastic baffles are suspended. No basin floor is required - instead the natural lake bottom is used. No heavy engineering is required, minimizing visual impact. And what's more, no disturbing changes are made to the environment. Where treatment in wetlands is applied, the biological life is stimulated thanks to an equalized, pre-treated and circulated flow through the wetland.

Q question of balance
A six cell equalization basin with openings in the baffle walls. Stormwater enters the first cell (top left) where a continuously operating pump transports run off to the treatment plant or to the wetland for cleaning. An outlet in the last cell gives out directly to the lake.

The technique of creating floating basins using suspended baffles has helped prove that cost saving and environmental conservation can go hand in hand. Lake Ronninge in the municipality of Taby near Stockholm is a case in point. The quality of the water in the lake had been progressively worsening during the sixties and seventies with all the signs of excessive eutrophication and phosphorus levels rising to around 80 mg/m3. Something needed to be done quickly.

The proposal was put forward for an integrated treatment system combining a system for equalizing run off flow and a chemical treatment plant to reduce phosphate discharge to the lake .

Once again, instead of constructing costly, inflexible and unattractive concrete basins, special floating containment basins were recommended using the unique pontoon-suspended baffle system. During dry periods, the basin is filled with lake water. This water is pumped steadily from the first compartment to a chemical treatment plant to reduce the phosphorus concentration in the lake itself. When the run off is excessive, however, and the pump to the treatment plant cannot cope with the additional volume, a natural replacement occurs as the stormwater pushes the lake water out from the basin. During wet weather, urban run off enters the basin where it will immediately be pumped from the first compartment to the treatment plant. Once the run off has normalized again after the rain, the pumping from the first compartment is still going on, and the lake water progressively enters and fills the basin from the last compartment towards the pump compartment. Consequently, the natural lake water itself serves as the flow balance medium. Strategically placed ITT Flygt pumps and mixers serve to enhance the natural fluid dynamics of the system and to eliminate 'dead spots'.

Last exit to Brooklyn?
The Dunkers technique can even be applied to make full use of the density difference between salt and fresh water so that the natural salt water itself serves as the flow balance medium.

The City of New York has also successfully adopted this patented technique. During those torrential wet seasons that can plague the famous city, thousands of cubic meters of stormwater can easily overload the city's treatment plants. During such periods, the risk of large quantities of CSO reaching the waterways untreated is high. The Dunkers flow balance system in Fresh Creek, Brooklyn, is located at one of the largest CSO outfalls. Here at least a part of the CSO will be collected and stored until it can be pumped to the nearby 26th Ward sewage treatment plant for final treatment.

Under normal conditions, the Fresh Creek equalization basin is filled naturally with salt water. When stormwater enters the basin, the natural density difference between the fresh water and the salt water of the creek pushes out the salt water. The basin thereby balances the flow and retains the untreated CSO. This CSO can then be pumped by two ITT Flygt CS 3152 pumps to the existing sewage treatment plant to remove pollutants. As the stormwater flow recedes, the salt water gradually flows back into the basins. In this way, the salt water actually forms the 'bed' of the basin so, once again, major engineering works are eliminated. The flow is controlled through openings in the baffles optimizing the fluid dynamics and allowing natural sedimentation to take place. Bottom sediment is removed regularly.

No limits?
The system applied in Huddinge near Stockholm features no fewer than fifteen separate cells and a total capacity of 5.000 m3. This plant can handle run off up to 1500 m3/h, controlling and containing the flow until the normal treatment plants can handle the additional volumes.

The technique has been applied successfully for both large and smaller flows, and with conventional chemical plant treatment as well as with the more innovative technique of natural treatment through the wetlands. The size of the basin employed depends on a number of variables including the predicted run off volume, the pumping capacity and the area available. The flow balance principle remains unaltered. The plastic baffles are weighted to the bottom of the lake or creek. A perfect seal is not needed; large openings permit flow from one compartment to the next. The use of ITT Flygt pumps and mixers, specifically dimensioned and strategically placed, further enhances the natural dynamics of the installations.

One such system, installed in Huddinge near Stockholm features no fewer than fifteen separate cells and a total capacity of 5,000 m3. This plant can handle run off up to 1500 m3/h, controlling and containing the flow until the normal treatment plants can handle the additional volumes.

Controlling the torrents in Toronto
This technique is waiting to be applied, for example, in two projects in the Toronto region. A hitherto abandoned and polluted bay is to be effectively divided into two specific areas for handling stormwater. The unique baffle system is suspended from pontoons. The natural bed of the bay forms the floor of the basin. Openings in the baffles and specifically dimensioned and strategically placed ITT Flygt mixers and pumps allow controlled flow from one area to another.

Controlling the stormwater flow in this way balances the feed into the wetland area and creates an ongoing circulation of stormwater and lake water through the wetland. Furthermore, this allows conditions for stability in the sedimentation process and the storage of large volumes of stormwater. The circulation maintains a high oxygen level in the wetland which stimulates the natural growth of wetland vegetation. This natural cleaning process is effective and environmentally friendly. And because of this, the Toronto example not only provides a successful stormwater equalization and treatment plant, it also results in a previously uninviting bay now forming an attractive recreational area of park land.