Linda Benedict, Achberger, Eric C., Sheffield, Ron, Moreira, Vinicius R., LeBlanc, Brian D.
Vinicius R. Moreira, Brian D. LeBlanc, Eric C. Achberger, Ron E. Sheffield, Laura Zeringue and Claudia Leonardi
Regulations intended to assure public water safety in the United States have been in place since 1948 under the Federal Water Pollution Control Act. Until relatively recently, the Clean Water Act of 1972 was applied by U.S. Environmental Protection Agency and state agencies to limit easily identifiable discharges, also called point sources of pollution. Examples of point sources can be pipes and ditches that discharge pollutants from industries and municipal wastewater treatment plants into public waters.
Despite these laws, degradation of public waters continues. Notable examples include the hypoxia zones in the Chesapeake Bay and the Gulf of Mexico. To address increasing pollution, legislation is being gradually adjusted to address nonpoint sources, such as agricultural, urban and suburban stormwater rich in nutrients and pathogens. Recent revisions to the Clean Water Act, such as the confined animal feeding operations rules, are intended to limit runoff from animal operations from reaching public waters. The EPA’s most recent update of the confined-animal rule states that animal operations that discharge pollutants into public waters have to apply for a National Pollutant Discharge Elimination System permit.
Lake Pontchartrain, a 630-square-mile water body north of New Orleans, has in the past been plagued with poor water quality. Among other sources, dairy operations were identified by the Lake Pontchartrain Basin Foundation as significant nonpoint sources contributing to the lake’s impairment. This is an important problem because nearly 80 percent of the state’s dairy cows are located in southeast Louisiana parishes, which are north of the lake.
Unlike large-scale dairy operations predominant in other states, the Louisiana dairy industry consists of medium-size and small operations, averaging fewer than 150 milking cows each. Cows spend most of the year on pastures, and the only waste is washwater from the milking shed and adjacent areas. The collected wastewater contains residues of manure, milk and cleaning products, but it is highly diluted (Table 1), making it uneconomical to be recycled directly through land application. On the other hand, liquid manure lends itself well to hydraulic handling and lagoon treatment. Other reasons to choose enhanced treatment before land application include the need for less land to apply manure at recommended agronomic rates and a lower risk of environmental contamination. The former is particularly important in southeast Louisiana where significant urbanization has shifted land use away from agriculture and increased land prices.
With the local farmers in mind, the Dairy Wastewater Treatment Evaluation System was built by the LSU AgCenter with support from U.S. Department of Agriculture and Lake Pontchartrain Basin Foundation grants. Located at the Southeast Research Station in Franklinton, the system handles wastewater from the milking parlor and holding pens where some 200 Holstein dairy cows are held for approximately eight hours a day throughout the year.
The system incorporates the USDA's Natural Resources Conservation Service-recommended single-cell facultative lagoons – a type of stabilization pond used for biological treatment, the most common wastewater treatment technology in the region – augmented by aerobic lagoons and constructed wetlands. The system is replicated so researchers can compare treatments simultaneously.
Since the system was built in 2004, four studies have been completed, two are in progress, and others are scheduled or in the planning phase. Completed studies evaluated the improvement in water quality with each treatment stage, the use of pickerel weed (an emerging wetland plant), the profile of wastewater delivery in the anaerobic lagoons, and the use of different plant species on floating islands at the different stages in the system. More than 20 water quality characteristics have been analyzed in samples collected bi-weekly from every stage – raw wastewater, facultative lagoons, aerobic lagoons and constructed wetlands.
The first study carried out between June 2005 and May 2006 evaluated the incremental contributions of each additional stage in terms of wastewater treatment. Concurrently, the use of pickerel weed for nutrient removal was evaluated in the constructed wetlands. Although plant stands were affected by Hurricane Katrina and decimation was completed by feral hogs immediately after, the vegetative growth had little effect on nutrient removal from the wetlands’ effluent compared with floating plants spontaneously growing in control wetlands. That observation brought interest in testing the growth and nutrient removal ability of different forage species grown on floating islands.
In a follow-up study, the system’s original design – a three-stage treatment system – was compared with an alternative two-stage system in which facultative lagoon effluent was delivered directly into the constructed wetlands to evaluate whether wetlands could effectively replace aerobic lagoons. Constructed wetlands are 40 percent of the size of the aerobic lagoons, thus requiring less land permanently set aside for wastewater treatment. Effluents from the treatment showed efficacies of 39.2 percent of effluents from aerobic lagoons, with widely varying results ranging from 1 percent for nitrogen to 49 percent for chemical oxygen demand. That study demonstrated that anaerobic (facultative) lagoon-effluent-fed wetlands can outperform aerobic lagoons per unit of area for some characteristics.
Land requirements would be large for wetlands to maintain comparable removal efficacies of nitrogen in a two-stage treatment system. For situations in which nitrogen removal is required, a two-stage (anaerobic lagoon-aerobic lagoon) system or a three-stage (anaerobic lagoon-aerobic lagoon-constructed wetland) system is preferred.
Those studies have generated data that have allowed researchers and extension specialists to share reliable information with local farmers. For instance, nutrient content in raw wastewater was highly variable. That variability tended to be followed closely in facultative lagoon effluents, but less variation was observed for nutrients in the effluent from aerobic lagoons and constructed wetlands. Nitrogen concentration in raw wastewater, for example, ranged from 14 ppm to 306 ppm. In constructed wetlands effluent, however, nitrogen ranged from 5.6 ppm to 56 ppm. This highlights the importance of adequate collection and timely analysis of wastewater samples intended for land application on private dairy operations.
In systems such as the Dairy Wastewater Treatment Evaluation System, pollutants can be removed from wastewater by primary, secondary and tertiary methods that include physical, chemical and biological treatments such as sedimentation (solids removal), biodegradation to less-harmful compounds (organic matter breakdown and denitrification) and starvation (E. coli removal). Potential pollution capacity of the organic constituents in raw wastewater, measured as chemical oxygen demand, was approximately 90 percent removed over the entire system. Nutrient removal rates ranged between 50 percent for phosphorus to 80 percent for nitrogen, while E. coli counts decreased by more than 99.9 percent. Pollutant removal rates varied widely with water quality characteristics and treatment stage.
In spite of the low concentrations of pollutants, even small dairy grazing operations may eventually be required by law to reduce loads of nutrients and other potential contaminants before recycling wastewater onto fields. The studies carried out with the system are providing economically viable alternative treatment technologies to southeast Louisiana dairy producers.
E. coli in dairy wastewater treatment systems
Anaerobic/facultative lagoons, the standard dairy wastewater treatment system in Louisiana, are noticeably inefficient in eliminating pathogens. E. coli are commonly used as an indicator of presence of human and animal waste in water. The number of viable pathogens per 100 milliliters of water is used to judge the extent of that contamination. Although it was improved significantly over the facultative lagoon alone, E. coli removal rates were surprisingly low in effluent from the experimental system, considering wastewater was retained for more than 200 days. Researchers are using molecular biological techniques to investigate why that is happening. Not all E. coli are the same. While most strains are benign – some are part of our normal intestinal flora – others may cause disease. DNA fingerprinting techniques are helping determine if an E. coli strain comes from dairy cows or from wildlife. In fact, most E. coli found in the system effluent in the 2005-2006 study was of wildlife origin. Even though the treatment system achieved high removal rates of bovine strains of E. coli, the effluent may never pass water quality standards because of wildlife contamination. DNA fingerprinting also was able to document that some strains of common E. coli can survive in the system much longer than others. Because these survivors would likely enter the environment, they should be targeted to enhance wastewater treatment system efficiency.
‘Floating’ wetlands show promise
Earlier studies demonstrated that emerging wetland plants, such as pickerelweed, showed little to no advantage on wastewater treatment when compared with naturally occurring floating wetland plants such as duckweed. Removing nutrients directly from the wastewater should be more efficient than cycling the wastewater through sediment and soil. In partnership with Floating Islands Environmental Solutions, a company in Baton Rouge, AgCenter researchers are currently evaluating the potential of several plant species grown on artificial floating islands placed in each stage of the system. A grant from the Lake Pontchartrain Basin Foundation supports a project to compare different island coverage rates on dairy lagoons. It is expected that pollutant removal rates will be enhanced by the floating devices and require less maintenance than emerging plants would.
Vinicius R. Moreira, Associate Professor, Southeast Research Station; Brian D. LeBlanc, Associate Professor and Roy and Karen Pickren Professor of Water Resources, Callegari Environmental Center, LSU AgCenter, Baton Rouge, La.; Eric C. Achberger, Associate Professor, and Ron E. Sheffield, Associate Professor, Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.; John Westra, Associate Professor, Department of Agricultural Economics & Agribusiness, LSU AgCenter, Baton Rouge, La.; Laura Zeringue, Research Associate, Southeast Research Station, Franklinton, La.; and Claudia Leonardi, Biostatistician, Pennington Biomedical Research Center, Baton Rouge, La.
(This article was published in the spring 2010 issue of Louisiana Agriculture.)