Cypress Wastewater Restoration Project
Visit to Mandeville Wastewater Assimilation Project
June 10, 2010
Mandeville is a fifty-minute car trip from New Orleans over the ‘longest bridge in the world.’ Traveling over the bridge one cannot help but imagine the extensive engineering required to complete a twenty-three mile long bridge. The bridge, almost destroyed by an escaped barge many years ago, now holds a long line of travelers.
On coming into town it is clear that this area was developed more recently than New Orleans. Extensively landscaped yards host a range of brick and wooden homes built within the last thirty years or so. Each house is surrounded by substantial green space and the tree -lined streets are shady and cool. This area did not sustain flood damage and the shaded winding streets appear to be new.
We find our way to the Mandeville Treatment plant where both the mayor and the head engineer greet us.
The city of Mandeville has chosen to design a biological wastewater treatment system in which nitrogen enriched and treated sewer effluent is discharged into the surrounding wetlands providing nutrients for the surrounding Cypress-tupelo swamp while purifying the wastewater.
Examples of signs include those which explain how wetland plants cycle nutrients, plant identification, and descriptions of wildlife in the area.
Our first stop is the overflow pond, complete with turtles and a small alligator.
The overflow pond (pictured above) provides a holding area for excess water created during runoff from rainstorms or storm surge. This biological treatment facility accepts sewage from eight existing treatment plants, which in the past, had discharged directly into local water bodies. This plant has increased storm water drainage capacity for the city and decreased both the point source and non-point source pollution.
An extensive pipe system (on the left of the boardwalk) carries treated fresh water and nutrients out to the marshlands. Fresh water is important to offset the salinity of the ocean water introduced to this marshland from post-hurricane tidal surge. Hurricane Katrina introduced an eight-foot tidal surge, followed three weeks later by a seven- foot tidal surge during Hurricane Rita. These storms and the resulting salt water severely degraded the wetlands causing plant loss and soil erosion.
One of the goals of this treatment facility is to preserve natural and scenic marshland and provide an intact wetland ecosystem to absorb storm surge and protect the surrounding community. It is estimated that 25-50% of Cypress-tupelo swamps have been lost due to logging and salt-water intrusion, causing the population of Louisiana to be more vulnerable flooding from hurricanes. Intact swamps act as a barrier to slow the force of post-hurricane storm surges and protect the floodwalls and levies.
The swamps play a critical role in the history of New Orleans. When we interview older local residents, they tell stories of spending the entire day in the swamp catching catfish, crawfish, garfish, and logger head turtles in the bayou. Restoration of these wetlands through wastewater assimilation will not only help protect the residents from storm surge, it also will restore the possibilities for residents to fish and gather seafood which once played a critical role in regional diets and is the foundation of the world renown New Orleans cuisine.
The Process of Wastewater Treatment.
Wastewater enters the plant from the city of Mandeville and surrounding areas where it is filtered to remove non-biodegradable materials. It is then aerated and piped through a series of three ponds like the one pictured below. Large waste particles settle out into the ponds. . The current ponds have been in existence for over twenty years and are just now showing signs of algae growth.
The next step in the process is called ‘nitrification’ where ammonia is removed from the water by running the water through this sprinkler system (pictured below) to oxidize the water. It is then drained and through the large rocks below.
From here the ‘nitrified’ water travels through a series of pipelines and is discharged into the marsh.
On our tour we witnessed both fish and alligators enjoying assimilation water. Wetlands serve as nurseries for small fish and crustations.
The subtropical climate in Louisiana provides optimal conditions for use of nutrient assimilation as a method for wastewater treatment. In a recent study on Cypress tree growth at the Cypriere Perdue Swamp (St. Martin Parish) from 1920 to 1990, there was a significant difference in tree growth (mean basil increment) of baldcypress trees in the nutrient rich waters of the wastewater treatment site compared to a Cyprus forest with no treatment assimilation project (Hesse and Doyle, 1998).
Here we see young Cyprus trees established at the Mandeville treatment site.
This project with increase the cities wastewater treatment capacity from two million gallons daily to four million gallons daily while preserving the Chinchuluba Basin floodplain. The benefits include increased growth of marsh grasses, plants and trees, which in turn provide habitat to waterfowl.
The city planners and engineer (Dave) at Mandeville have successfully demonstrated how we can develop sustainable treatment systems which restore and preserve native ecosystems while providing a critical service for local communities. Thanks Dave!
Dave DeGeneres, Director of Public Works, Mandeville, Louisiana
Report by Greg Gordon, Director of Environmental Services (2001)
Mandeville Aquatic Ecosystem Restoration Project
CONSERVATION HABITATS & SPECIES ASSESSMENTS LA CWCS–DEC 2005