The Egret Marsh Algal Turf Scrubber® (ATS) pilot was conducted for Indian River County, Florida, to evaluate whether source water from the Lateral C Canal would support healthy attached-algae growth and nutrient recovery. The study was performed in 2005 as an early technical evaluation supporting development of the later full-scale Egret Marsh ATS facility.
Unlike the larger facility-scale demonstrations described elsewhere on this site, this project was a short-duration bench-scale evaluation. The pilot used a 100-foot-long by 1-foot-wide mobile ATS floway and was operated under field conditions at the Egret Marsh facility. Because water-quality monitoring consisted of a limited number of grab samples and the project experienced multiple pump-related shutdowns, nutrient-removal conclusions were based primarily on algal productivity and harvested biomass nutrient recovery rather than influent-effluent water-quality comparisons.
Facility Summary
Facility: Egret Marsh Bench-Scale Algal Turf Scrubber® Pilot
Location: Indian River County, Florida
Technology: Algal Turf Scrubber® / attached-algae floway
Scale: 100-foot-long by 1-foot-wide pilot floway
Floway Slope: 0.5%
Average Inflow: 30,500 GPD
Influent TN: 1.28 mg/L during the March 2, 2005 to April 28, 2005 monitoring period
Influent TP: 110 µg/L during the March 2, 2005 to April 28, 2005 monitoring period
Status: Pilot testing completed
Operating Period: February 5, 2005 to April 28, 2005
Source Water: Lateral C Canal water associated with the Egret Marsh facility
Operating Context: Source-water suitability testing; algal productivity evaluation; preliminary nutrient-recovery assessment; design support for full-scale Egret Marsh ATS planning
Application: Evaluation of attached-algae growth potential, nutrient recovery through harvested biomass, potential growth limitations, and possible toxic or inhibitory source-water effects
Operating Context
Indian River County evaluated the bench-scale ATS pilot as part of its planning for nutrient removal from runoff waters associated with the Lateral C Canal, a tributary to the Main Canal. The project was intended to determine whether the canal water would support a productive attached-algae community and whether any nutrient limitations, trace-element limitations, toxic influences, or growth-inhibiting factors might affect future full-scale ATS design.
The mobile pilot unit was located in the southeastern portion of the Egret Marsh facility. Water was pumped from the Lateral C Canal to a 100-foot-long by 1-foot-wide floway equipped with a surge box, flow meter, HDPE liner, and geogrid growing matrix. The pilot was intended to emulate key features of a full-scale ATS floway at a reduced scale.
Testing began in March 2005 and continued through April 2005. Because electrical power was not available at the site, the system relied on a diesel diaphragm feed pump. During the short operating period, pump failures caused the floway to dry out on several occasions, resulting in loss of the algal crop. In each case, the algal community recovered quickly, providing useful information on source-water suitability and biological resilience, but limiting the value of direct influent-effluent water-quality comparisons.
Operational Significance
The principal value of the Egret Marsh bench-scale pilot was not as a full treatment-performance demonstration, but as an early source-water and biological-response test. The pilot showed that Lateral C Canal water supported vigorous development of an attached algal turf community. The report concluded that no deleterious or toxic materials were evident in the source water during the testing period that would prevent algal growth or development.
The project also provided important information for full-scale design. Algal production during the bench-scale test exceeded ATSDEM model projections. However, the report cautioned that the short-duration bench system had not reached ecological stability and had not yet developed a mature grazer community. As a result, measured biomass production was closer to gross production during early start-up, while the model projections represented a more conservative estimate of net production after grazing and respiration. The report therefore recommended that full-scale sizing continue to use established ATSDEM design constants rather than adjusting the model upward based on the short bench-scale results.
Performance Summary
The report cautioned against using the limited grab-sample influent and effluent data to calculate treatment performance. Because the floway was short, monitoring was limited, and the system experienced intermittent pump shutdowns and dry-out events, direct water-quality removal estimates would have been misleading.
Instead, the report estimated nutrient recovery using harvested algal biomass. Over the 40-day testing period, with downtime deducted, approximately 23.67 pounds of dry algal biomass were harvested from the 100-square-foot floway. Based on measured tissue nutrient content, this represented approximately 0.43 pounds of nitrogen and 0.11 pounds of phosphorus recovered in harvested biomass.
Expressed on an areal basis, the calculated biomass-based nutrient recovery rates were approximately:
Nitrogen recovery: 189 g/m²-year
Phosphorus recovery: 50 g/m²-year
These values were based on harvested biomass nutrient content, not direct influent-effluent water-quality reduction. Using average flow and water-quality conditions for the short test period, the report estimated that the recovered biomass was equivalent to an average reduction of approximately 0.04 mg/L total nitrogen and 11 µg/L total phosphorus.
The grab-sample water-quality data showed that total nitrogen averaged approximately 1.28 mg/L and total phosphorus averaged approximately 110 µg/L during the test period. These concentrations are useful for understanding source-water context, but the report emphasized that treatment performance should not be judged from grab-sample influent-effluent comparisons.
Biological Response and Source-Water Suitability
The algal community responded favorably to the field water conditions. Early growth was dominated by the filamentous diatom Melosira sp., which commonly appears during ATS start-up. Green algae, including Rhizoclonium sp. and Cladophora sp., became more noticeable during the latter portion of the bench-scale operation.
The report noted comparatively low nitrogen, magnesium, and potassium levels in the harvested biomass, although growth rates remained high during the short test. The report recommended that these parameters be closely monitored during future full-scale operation. The report also cautioned that low available nitrogen in the latter reaches of a full-scale floway could encourage nitrogen-fixing organisms and affect measured nitrogen removal rates.
Lessons Learned
The Egret Marsh bench-scale pilot provided a conservative but useful early evaluation of Lateral C Canal water for ATS treatment. The pilot demonstrated that the source water could support rapid algal turf development and that no obvious toxicity or growth inhibition was observed during the testing period.
At the same time, the project highlighted the limitations of short-duration bench-scale field testing. Intermittent pump shutdowns, repeated crop dry-out events, limited grab-sample monitoring, and the lack of a mature biological community limited the usefulness of direct water-quality removal comparisons. The most defensible performance information came from measured algal productivity and nutrient recovery in harvested biomass.
The study supported continued design and development of the full-scale Egret Marsh ATS facility, while recommending that full-scale sizing remain based on established ATSDEM projections rather than short-term bench-scale production rates.
Photographs
Reports and Publications
Technical report: Egret Marsh Bench-Scale Algal Turf Scrubber® Final Report — May 23, 2005
Related HydroMentia ATS facilities and demonstrations include Egret Marsh ATS, Osprey Marsh ATS, PC-South/Osprey Marsh ATS Pilot, and S-154 ATS.