By Mark A. Reinsel
High nutrient loading and eutrophication of surface waters continue to be topics of great concern in the wastewater treatment community. Floating treatment wetlands (FTWs) have been shown effective in substantially reducing nutrient levels in several studies involving smaller-scale lagoon treatment plants.
The key feature of these floating islands, developed by Floating Island International (FII), is their high surface-area-to-footprint ratio, which enables them to perform a wetland's function in a fraction of the space. Applications include polishing of municipal wastewater, direct treatment of raw wastewater, ponds and lakes impacted by septic systems and/or waterfowl, and waterways degraded by agricultural runoff.
Constructed of post-consumer polymer fibers and typically vegetated with native plants, these floating wetlands mimic the ability of natural wetlands to purify water by incorporating a "concentrated wetland effect." FTWs have been studied for removal of total nitrogen, ammonia, phosphorus, BOD and TSS, all parameters of concern for a healthy aquatic ecosystem.
In recent projects, total nitrogen removal was studied in four small wastewater systems equipped with FTWs, which were enhancements of wastewater lagoons at different scales. Three studies included controls, which were parallel lagoons treating the same influent wastewater but without the floating wetlands. The earliest study conducted was for a Montana Board of Research and Commercialization Technology (MBRCT) grant.
Table 1 shows concentrations, percent removals and removal rates in pounds of total nitrogen removed per year per cubic foot of FTW material. Key parameters are: a) the removal rate of the FTW with the lagoon (e.g., 0.9 lb/yr/ft3 for the MBRCT Test Pond system) and b) the removal rate of the FTW by itself, which is the difference between the FTW and the control (e.g., 0.9 – 0.4 = 0.5 lb/yr/ft3 for the MBRCT Test Pond system).
Total nitrogen removal ranged from 40% to 87% in the four systems. The MBRCT, Rehberg Ranch and Wiconisco studies included "control" lagoons treating the same influent wastewater. FTW total nitrogen removal was better than the control by 52% and 21% in the MBRCT and Rehberg Ranch studies.
The Rehberg Ranch and Wiconisco FTWs are small municipal systems treating average flows of 12 and 16 gallons per minute (gpm), respectively. The Rehberg Ranch system, installed in late 2009, is the latest-generation FTW with a pump for circulation and aeration. The Wiconisco system was one of the first full-scale FTWs installed in 2005.
The McLean's Pit system in New Zealand has FTWs but no parallel lagoon to serve as a control. Monitoring data at McLean's Pit, which treats landfill leachate, was limited to the percent removal and removal rate.
The Wiconisco and Rehberg Ranch FTWs treat municipal wastewater that contains high ammonia concentrations but low initial nitrate concentrations. In all four systems listed, ammonia was reduced to nearly zero while the nitrate concentrations (including the nitrate converted from ammonia by biological nitrification) were still relatively low after treatment.
For total nitrogen removal, both aerobic and anoxic conditions (either in different locations or in treatment stages) are required. Nitrification is the primary mechanism for ammonia removal in these systems. Denitrification (biological reduction of nitrate to nitrogen gas under anoxic conditions) is the primary mechanism for nitrate removal.
Although the Rehberg and Wiconisco FTWs were aerated for ammonia removal, both systems achieved anoxic conditions (and subsequent nitrate removal) in the presence of dissolved carbon in the wastewater, which acts as a food source for denitrifying bacteria. Both the Wiconisco (Pennsylvania) and Rehberg Ranch (Montana) FTWs are located in cold-weather climates, which has traditionally limited biological nitrate removal.
Ammonia removal ranged from 60% to 87% in the four systems. The Rehberg Ranch FTW removed 26% more ammonia than the control lagoon (84% vs. 58%), while the Wiconisco FTW was 9% better than the control. The highest ammonia removal rate, 2.5 lb/ft3/yr, was observed with the Wiconisco FTW.
Total phosphorus removal ranged from 42% to 91%. FTWs in the MBRCT and Wiconisco studies improved phosphorus removal by 9% and 5% compared to the control lagoons. Efforts are currently underway at Floating Island International to further enhance FTW phosphorus removal through biological treatment and chemical adsorption.
Fate of Nutrients
FII researchers have estimated that approximately 80% of FTW efficacy is due to bacteria attached to plant roots and the FTW polymer matrix itself, with the other 20% attributed to nutrient uptake by plants. Plants create the platform for biological activity in a biofilm, while also contributing nutrient uptake and aesthetic benefits.
A 30-year comprehensive study of a natural wetland near Houghton Lake, MI, provides a natural example of how wetland treatment systems function over time. Removal of nitrogen (an average of 95%) and phosphorus (94%) in this system has been well documented ("The Houghton Lake Wetland Treatment Project," Ecological Engineering, R.H. Kadlec, 2009).
Nitrogen is efficiently removed at Houghton Lake, as ammonia is quickly converted to nitrate, which was subsequently reduced to background levels or below. The fate of nitrogen is difficult to quantify, because removal mechanisms include both nitrogen gas venting and nitrogen fixation.
Following initial adsorption, accretion of decomposition residuals is the principal mechanism for phosphorus removal, as determined by Kadlec, and stores about 80% of the removed phosphorus. This accretion accumulated to depths of 10-30 cm over the 30-year period. As this occurred, the wetland displayed no tendency to lose its ability to sequester phosphorus. It is believed that FTWs can remove phosphorus in a similar manner.
Removal of Other Parameters
Biochemical Oxygen Demand (BOD) removal ranged from 30% to 92%. BOD removal was high with the Wiconisco and Rehberg Ranch FTWs but removal in the control lagoons was nearly as high. Data indicate that FTWs offer BOD removal equivalent to traditional lagoons while providing improved nitrogen and phosphorus removal.
Total suspended solids (TSS) removal in three studies equipped with FTWs ranged from 54% to 93%. For Rehberg Ranch (the only study with a control), the FTW improved TSS removal by 10%.
Conclusions
The need to reduce nutrient levels in wastewater is increasingly critical as rivers, lakes and coastal waters become more nutrient-loaded worldwide, which may create a demand for cutting edge, "green" technologies such as floating islands. Although traditional wastewater lagoons effectively remove BOD and TSS (as shown by the "control" lagoons in these studies), their ability to remove nitrogen and phosphorus from municipal wastewater is limited. FTW technology enhances these lagoons with the "concentrated wetland effect," facilitating compliance with increasingly stringent wastewater nutrient, BOD and TSS criteria.
It is not necessary to remove wetland vegetation for the FTW system to effectively remove nutrients. FII personnel recommend allowing plants to flourish, as harvesting them would likely inhibit the accretion process, which is the primary mechanism for phosphorus removal.
For more information on the floating island systems, visit www.floatingislandinternational.com.
About the Author: Mark A. Reinsel, Ph.D., P.E., is President of Apex Engineering. Dr. Reinsel has 27 years of experience in consulting, industry and academia. His consulting work focuses on treating mining and other industrial wastewaters through biological, chemical and physical processes. He may be contacted at [email protected].
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