Vinicius Moreira and Brian D. LeBlanc

Developing new or enhancing the effectiveness of existing wastewater treatment systems is essential in situations where expansion of a livestock production system is limited by land availability, thereby limiting space for land application during waste recycling. LSU AgCenter researchers are using floating systems to grow various plant species and create favorable conditions for plants and microbial communities to thrive and create a concentrated wetland treatment in wastewater lagoons. Hydroponic phytoremediation of wastewater is built around the idea that plant roots and rhizomes will remove nutrients from the water they float upon and enhance naturally occurring microbial communities.

In a recent study AgCenter researchers evaluated floating systems at a human sewage treatment facility in southeast Louisiana. Facility managers were concerned primarily with elevated levels of biological oxygen demand, total suspended solids and fecal coliforms. Evaluation of the three-stage wastewater treatment system indicated sludge accumulation in the first stage of treatment may have been a limiting factor in the system’s overall ability to treat wastewater effectively. Remediation of the sludge depth issue in the system’s first pond would have required extensive dredging and would have placed a high financial burden on the operator.

Seeking a cost-effective alternative, the operator contracted with Martin Ecosystems, of Baton Rouge, to install a novel solution using a floating island plant treatment system. Martin Ecosystems installed BioHaven Floating Treatment Wetlands to cover approximately 0.7 percent of the 5.1-acre pond that had an average depth 3 feet. The objective of the AgCenter study was to monitor the impact of floating wetlands on effluent water quality in the wastewater treatment system.

In April 2011, 38 floating wetland modules measuring 5 feet by 8 feet and four layers thick were installed in the first stage wastewater-treatment lagoon, for a total of 1,520 square feet. The floating wetlands were installed below the inflow pipe located in Stage 1 of the wastewater treatment system. Modules were connected to each other to form four tiers. The first tier, nearest the influent pipe, consisted of 16 modules; the second tier, 12 modules; the third tier, six modules; and the fourth tier, four modules. The tiers were placed in successive arches moving away from the influent source to ensure that raw wastewater entering the system would pass through the island matrixes and hanging root systems to maximize the water-treatment interface and ultimately increase lagoon retention time prior to exit at the out-flow pipe.

During the evaluation period, floating wetland modules contained either three species of wetlands plants or only vetiver grass. Water quality characteristics monitored included biological oxygen demand, phosphate, total suspended solids, fecal coliforms, ammonia, nitrate, nitrite and dissolved oxygen each month for 27 successive months. Monthly samples included raw wastewater at the inflow pipe and first-stage effluent just before entering the secondary treatment pond. Results from influent and effluent data were compared as well as pre- and post-floating wetland treatment design.

Water quality evaluation indicated significant declines in the 27-month average concentrations of fecal coliforms (58.9 percent), total suspended solids (65.9 percent), biological oxygen demand (71.6 percent) and ammonia (19.2 percent) in effluent wastewater after interaction with the planted floating wetlands. In contrast, the nitrate and dissolved oxygen contents of the effluent increased nearly five-fold, and nitrite increased nearly three-fold compared to raw wastewater. Effluent phosphate concentration also increased by 12.6 percent, but it was not significantly different from the influent wastewater.

A facility review of noncompliance events obtained from the Louisiana Department of Environmental Quality (DEQ) was conducted using discharge monitoring reports and noncompliance reports for January 2007 through April 2015. From January 2007 to March 2011, a period that preceded the installation of floating wetlands, the treatment facility filed 28 non-compliance reports in the DEQ system. Most of these were the result of excessive discharge of total suspended solids, biological oxygen demand and fecal coliforms. The floating wetlands were initially planted and installed in April 2011, and plants were actively growing and maintained until April 2015. During a 48-month period, regardless of the plants employed, the operator had only five noncompliance events compared with 28 for the 51-month period immediately preceding floating wetland installation.

The limitations of this evaluation of a wastewater treatment facility make it difficult to draw definitive conclusions about the magnitude of effluent improvement, but it does appear that floating wetlands may have improved treatment in both first and second stages and can have a beneficial effect on final-stage effluent water quality. This evaluation documented significant concentration reductions in four parameters, three of which (fecal coliforms, total suspended solids and biological oxygen demand) had been responsible for the facility’s noncompliance issues prior to the floating wetland installation.

A previous AgCenter pilot study of a replicated and controlled dairy wastewater three-stage treatment system produced similar trends. The 17-month pilot study evaluated floating wetlands by sampling only every three months, using influent and effluent sampling as well as control comparisons. The data indicated that floating wetlands appeared to improve treatment effluent for total suspended solids by 10 percent, total solids by 7 percent, chloride by 11 percent, chemical oxygen demand by 10 percent, and total nitrogen, ammonia and ammonium nitrogen by 7.5 percent.

Other factors may also have contributed in various stages of treatment. But without a control treatment, floating wetland contributions to water treatment could not be quantified, although data suggest that floating wetlands played an important role in improving treatment at the sewage treatment facility. This observation needs more scientific support. Overall, the two studies suggest that floating wetlands have significant potential as a component of wastewater treatment. This may be especially valuable where space and lagoon construction or enlargement are needed. Further studies under controlled conditions are necessary to accurately quantify the efficacy and costs of floating wetlands.

Vinicius Moreira is an associate professor in the School of Animal Sciences, and Brian D. LeBlanc has the Roy and Karen Pickren Professor in the School of Plant, Environmental and Soil Sciences and is a specialist with Louisiana Sea Grant at LSU.

(This article appears in the fall 2017 issue of Louisiana Agriculture.)

Read an earlier article about this project in the spring 2010 issue of Louisiana Agriculture Improving Dairy Wastewater Treatment for Louisiana’s Environment.

Ryegrass being harvested from floating islands in the dairy wastewater lagoon at the LSU AgCenter Southeast Research Station to determine yield. Photo by Vinicius Moreira

Floating treatment wetlands were lifted off the wastewater lagoons for forage harvesting to determine yield and nutrient content. Photo by Vinicius Moreira

Vetiver (Chrysopogon zizanioides) seedlings were transplanted onto floating treatment wetlands to evaluate plant survival and wastewater treatment efficiency. Photo by Vinicius Moreira

Ryegrass on floating islands in a dairy wastewater lagoon as part of an experiment at the LSU AgCenter Southeast Research Station. Photo by Vinicius Moreira

Ryegrass seedlings soon after planting in a dairy wastewater lagoon as part of an experiment at the LSU AgCenter Southeast Research Station. Photo by Vinicius Moreira