Keeping flamingos in the pink

With tens of thousands of visitors flocking to see its flamingos and their other water companions, the Wildfowl and Wetlands Trust must try to keep the ponds clean and odour-free. Is using recycled glass for filtration the answer?

The Wildfowl and Wetlands Trust (WWT) at Slimbridge has more than 150 acres of land that is home to 1,500 ducks and swans as well as about 1,000 flamingos. These waterfowl attract about 200,000 visitors a year to Slimbridge.
The Trust has recently participated in a WRAP trial where recycled glass filtration media (RGFM) was used alongside silica sand in a water treatment process.
Why? Well, according to Neil Woodward, WWT centre operations manager at Slimbridge: "Where the flamingos are, there's a lot of contamination."
As a result, the water in which they stand for much of the day gets contaminated with excrement and food.
Maintenance of the ponds, which are clay-lined, involves de-silting them every year, and digging them out every three to four years.
The Slimbridge centre draws most of its process water from the adjacent Gloucester and Sharpness canal. And, the used water is treated in the existing reed beds before it is discharged into the River Severn. The centre uses two million gallons a day, says Woodward. "We need to ensure the water we return to the River Severn is as clean if not cleaner than out initial intake."
Also with the huge number of visitors flocking in daily, it is vital that the water is as clean as possible and odour-free. Also, clean water helps keep the birds in a healthy condition, says Woodward.

Development plans
There are also plans for the centre to have a viewing areas where visitors can see what is happening underwater, a development that will entail the water maintaining a clear appearance. WWT wants to raise more than £26M over the next 15 years to help fund its development plans.
Woodward, a big fan of recycling, jumped at the chance for the centre to participate in one of three trials using recycled glass for filtration that WRAP funded and which Water Development Services (WDS) carried out. The other two trials involved waste-water filtration at a cardboard manufacturing factory and a food processing company.
The WRAP-sponsored trial at Slimbridge entailed a GTFP with one filter vessel charged with a traditional silica sand and a second one filled with a selection of recycled glass in various filtration grades.
Christopher Dean, project manager at WDS, says water was pumped out of the pond adjacent to the flamingo house into a balance tank in order to ensure a continuous flow of water. A flow meter on top of the tank controlled the water intake, which closed when the tank was full. Although the balance tank maintained an even flow into the filters, it did allow the bacteria levels to rise steeply. Despite that, the filtration process still reduced the BOD/ COD levels to an acceptable level.
Because the container, housing the filtration equipment and the tank were located outside the flamingo house, Woodward says this was good for the visitors to see that the problem of the dirty water in the pond was being addressed.
The control system for the trial process was managed remotely, says Dean, such that the WDS team were able to both monitor and change the operating parameters, the influent configuration, and the backwash regime as required.

Filtration process
Two filters were used in the trial - one filled with silica sand and other initially with AFM, one type of recycled glass filtration media (RGFM). The influent was pumped in - the flow being controlled using Bürkert process valves - to the two filters for the filtration process. The treated water was then collected in a tank, before discharging back into the pond. Samples from the treated water were taken two or three times a week by WWT staff for testing the performance of the filters.
Dean says that several different grades and supplies of RGFM were trialled.
Although AFM was the most expensive, they all performed with more or less the same efficiency for this particular application.
A significant advantage of using recycled glass, RGFM, is that it can be kept consistently clean, by backwashing whereas sand clogs up after a few months and never returns to a clean condition after backwashing.
Woodward was pleased with the results of the trial and would be taking them into consideration when the plans for the Centre are taken forward next year.
He would also be interested in using recycled glass in developing their reed bed filtration systems.

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