Biological Nutrient Removal Systems – Egret Marsh Regional Stormwater Park
Research indicates that eighty to ninety percent of the nitrogen and around eighty
percent of the phosphorus in the IRFWCD canal water is present in dissolved forms.1 2
Dissolved nitrogen is commonly removed through biological means while dissolved
phosphorus can be removed either chemically or biologically. Phosphorus removal by
chemical precipitation is very expensive and has been ruled out as a viable method to
reduce phosphorus in the IRFWCD’s canal waters.3 Of the several biological options to
remove dissolved nitrogen and phosphorus, an Algal Turf Scrubber (ATS) system was
determined to be the most cost-efficient.4
The County’s first ATS is under construction and is named Egret Marsh Regional
Stormwater Park (Egret Marsh). In December 2009, construction was about nintey
percent complete and final completion is anticipated in January 2010. Egret Marsh will
remove dissolved nutrients from ten million gallons of IRFWCD’s canal water each day
and it will produce a usable byproduct of harvested algae.
Polluted water from a large IRFWCD canal (the Lateral D Canal) will be pumped into
Egret Marsh for treatment. First, the water will enter an ATS, the heart of Egret Marsh’s
treatment system. From there, cleaned water will flow by gravity through a series of
three polishing ponds and then into a wood stork habitat area. Finally, it will be returned
to the canal system where it will eventually flow into the Lagoon.
ATS technology was developed by Dr. Walter Adey, Director of Marine Laboratories at
the Smithsonian Institute in Washington, D.C. Originally designed to clean water in
saltwater aquariums, the technology is now applied to remove pollution from stormwater
runoff. An ATS is simple in concept, but very complex in the algae’s intercellular
workings. The ATS consists of a large, gently sloped surface over which is placed a
plastic geomembrane that is overlaid with a rough geotextile grid. Nutrient-rich waters
are discharged onto the surface and an algal turf is cultured on the rough grid. The
algal turf covers the surface with dense mats and as water flows over the algae, it
removes nitrogen, phosphorus, color, and a variety of other pollutants from the water.
The water is pulsed over the mat in waves and it is collected in a trough at the bottom of
the sloped surface.
Fundamentally, the ATS is a large algae farm and from time-to-time the algae must be
harvested. This is accomplished using a small all-terrain vehicle or tractor pushing a
scraper or dragging a chain that dislodges the algal turf, which is then washed into a
collection trough where it is removed by an automatic rake at a centralized harvesting
station. The harvested algal mass is conveyed to an impervious pad where it is
available for further processing. The harvested algae may be composted and used as a
1
Egret Marsh Stormwater Park 10 MGD Algal Turf Scrubber (ATS) Final Basis of Design, Hydromentia, Inc., June
20, 2005, p. 13
2
Id.
3
Final Engineering Report – Egret marsh Regional Stormwater Park, Carter Associates, Environmental Consulting
& Technology, Inc., p. 17
4
Id.
soil supplement or mixed with other material and fed to cattle. Other uses for the
nutrient-rich algae such as methane gas generation will be researched at Egret Marsh.
This photograph shows the influent channel to an algal turf scrubber. At Egret Marsh, an
automated siphon will discharge Lateral D Canal water onto the algal turf scrubber surface
through pipes. Cycling between water and air over the algal cells is believed to trigger cellular
enzymatic reactions that breakdown and remove some color from the water. The black surface
of the influent channel is the geomembrane liner. In the background is the algal turf scrubber
where algal mats can be seen growing on the rough surface of a grid-like material covering the
geomembrane. Here, algae remove large amounts of nitrogen, phosphorus, color, and other
pollutants from the water. Many wading birds are drawn to the ATS surface where they eat
small organisms that colonize the algal mass.
The three large polishing ponds following the ATS are connected in series and they will
remove by settling, any algal material escaping the ATS, together with some of the
residual nutrients and remaining settable solids. Each pond will have littoral zones
planted along portions of its bank for wildlife habitat. The ponds will harbor large
populations of fish, insects, waterfowl, and other animals and they will create a park-like
atmosphere.
A habitat for the endangered wood stork will be constructed at Egret Marsh and it will
receive water from the last polishing pond. It will be operated and managed in a
manner conductive to the wood storks’ feeding requirements and it will offer the birds a
safe, clean, and well-stocked wetland system that will greatly assist their survival. The
wood stork habitat will be a valuable ecological enhancement and an important
educational component. It will
demonstrate that high quality wetlands
may be established and used as
effective wildlife habitat systems when
they are properly loaded from a
nutrient perspective. The habitat will
provide a needed home and refuge for
the birds, amphibians, insects, and
animals that Egret Marsh will attract.
It will become a valuable nature
showcase, demonstrating a successful
effort to recreate a rapidly vanishing
ecosystem.
A wood stork.
Egret Marsh is estimated to remove 21,780 pounds per year of total nitrogen and
11,555 pounds per year of total phosphorus.5
Stormwater Division will construct at least two more regional nutrient removal systems
similar to Egret Marsh. One will serve the North Relief Canal and the other will serve
the South Relief Canal.
5
2004 Section 319(h) Grant Application prepared by Indian River County and Hydromentia, Inc., p. 5
LATERAL D
CANAL PUMP
STATION
APRIL 2009