Low Intensity Chemical Dosing in the Everglades Nutrient Removal Project.

By: Dr. Phil Bachand

Since the 1960’s, the composition of the fauna and flora within many areas of the Florida Everglades has changed. Marshes in the northern Everglades historically consisting of sawgrass (Cladium jamaicense) and shallow open water sloughs have been replaced by cattail (Typha spp.) stands. In the sloughs, natural algal mat communities have disappeared and composition of periphyton communities has changed. These changes have been attributed in part to increased nutrient loads from upstream farmlands. Recent Everglades restoration efforts have focused on reducing phosphorus loads to the Everglades by using storm water treatment areas (STAs) to remove phosphorus from the agricultural runoff entering the Everglades. These STAs are large constructed wetlands specifically designed for phosphorus removal that will eventually cover more than 40,000 acres. Yet, preliminary data on both phosphorus concentrations in the agricultural runoff and the predicted phosphorus uptake rates of the STAs suggest that the predicted phosphorus concentrations from the STAs will still exceed desired threshold levels. In response to these predictions, researchers currently are investigating several biological and chemical treatment technologies that would be used in conjunction with the STAs.

The Duke Wetland Center is involved in one such technology, the application of low concentrations of chemical coagulants directly within the STAs to enhance phosphorus removal directly within these constructed wetlands. Ferric chloride and alum are the two coagulants being studied. They both are commonly used in wastewater treatment specifically for phosphorus removal. Alum is also used in lakes and reservoirs for the same purpose. Wetland Center scientists are designing a series of chamber experiments that will be conducted within the Everglades to test whether these coagulants can be used effectively in the Everglades environment to reduce phosphorus concentrations to desirable levels. If these coagulants work, these experiments will also be used to determine the desired doses and the hydraulic retention times for each, as well as determine the fate of added iron or aluminum in the Everglades soils.

Given the unique Everglades environment, the application of this technology has several hurdles it must overcome. First, the Everglades system has exceptionally high organic carbon levels. These high levels may interfere with the ability of the coagulants to effectively remove phosphorus or may themselves be an inexhaustible supply of phosphorus back into the system. The added coagulants may also adversely affect plant communities. Second, aluminum is potentially toxic. However, under the slightly alkaline and well-buffered (high carbonate concentrations) conditions found in the Everglades, this is not expected. Finally, iron may act as a micronutrient and stimulate growth of some plant species. During the course of these experiments, Center researchers hope to address these operational and ecological questions to determine if this technology can be used to mitigate the phosphorus enrichment problem within the Everglades without causing adverse ecological effects.