THE CARIBBEAN S MOST BEAUTIFUL WASTEWATER TREATMENT SYSTEM: CONSTRUCTED WETLAND AT ANTIGUA, W. I. Gregory L. Morris * Laura Carbó Gregory L. Morris Engineering P.O. Box 9024157, San Juan, PR 00902 Tel. (787) 723-8995 www.gmaeng.com SUMMARY Design considerations and construction techniques are presented for an 83,000 gallon per day (gpd) constructed wetland for secondary wastewater treatment, Antigua, West Indies. The design eliminates almost all mechanical equipment and energy requirements, and was additionally designed as a zero-odor and zero-discharge system. The site has been extensively landscaped for ornamental use and as an educational resource for school children, and is definitely a contender for the world s most beautiful wastewater treatment plant. Keywords: constructed wetland, secondary wastewater treatment, Antigua INTRODUCTION Antigua, West Indies, located in the Lesser Antilles, depends heavily or the tourist trade which enters the island through its International Airport. Stanford Development Company planned the construction of several projects related to the International Airport including a championship cricket field and viewing stands, restaurant, international bank, and several commercial, retail and office facilities. Together these will produce wastewater volumes that cannot be handled by septic treatment systems normally used on the island.
Our firm, Gregory L. Morris Engineering, was contracted to design a wastewater treatment facility which addresses the following factors. Large pulses of hydraulic and organic loads will be generated on the days of championship cricket matches. Low energy use is desired given the high cost of electricity on Antigua. It was also desired to minimize the use of mechanical equipment that could be subject to failure since replacement parts are not readily accessible. The site available for the treatment facility was located upwind of the project s main commercial area, including the international bank building. Therefore, it must be an odorfree design. The treatment system must be either attractive, or otherwise have effective visual shielding from the airport, cricket stands, and bank building. The system should be environmentally sound, with the ability to recycle all effluent to irrigation. A wastewater treatment system consisting of a primary treatment stage followed by a constructed wetland was determined to be capable of meeting these criteria. DESIGN APPROACH A conceptual process schematic of the process which was designed is presented as Figure 1. The system may discharge to existing natural drainage or may operate as a zerodischarge system using the treated effluent to irrigate the non-treatment landscaping areas on the treatment plant site (berms, paths, roadside areas, etc). Figure 1: Schematic diagram showing the overall process sequence in the secondary treatment system. U.V. Disinfection Open-water wetlands Gravel-bed wetlands Primary sedimentation Tangential screen with odor-control cover Flow splitterprimary tank odor control PROCESS FLOW SCHEMATIC Irrigation of treatment site landscaping
Site. The site available for the treatment system was the side of a steep limestone hill. Therefore the system was designed as a series of terraced units; all wastewater is pumped to the top of the hill where the primary treatment system is located, and thence flows by gravity through the entire treatment process to the point of discharge into an existing drainage ditch. The only energy use within the treatment system itself is for small blowers to maintain negative pressure inside the primary tank (if required), and electricity for the UV disinfection system. Primary Treatment. The first treatment element is a tangential screen for the removal of coarse debris and plastics. The volume of screenings is very small, and they fall into a container which is periodically hauled to the landfill. A grit chamber operating by plain sedimentation immediately follows the tangential screen. Two units are provided in parallel, allowing on to be cleaned while the other operates. There are two Imhoff-type primary treatment tanks which operate in parallel. To prevent the escape of odors the primary treatment tank was designed as a completely enclosed chamber with hatches for access. Additionally, an air exhaust system was designed to withdraw a small volume of air from within the primary tank, thereby creating a slight negative air pressure within the tank. The exhausted air is then passed as upflow through a grassed soil bed to scavenge odors. Flow Splitting. Effluent from the two primary tanks enters a flow splitter box which divides the flow equally into 2-, 3- or 4-parts, depending on how many gravel-bed wetlands are in use. Upon exiting from the gravel-bed wetland, a second flow splitter diverts water to either of the openwater wetlands, or will split effluent evenly between the two. Proper flow splitting is essential to balance loads among treatment units. The flow splitters for this project consist of identical v-notch weirs upstream of outlet valves leading to each process unit. By closing any single or combination of outlets, the flow is evenly split among the weirs with outlets that remain open. The flow splitter boxes are open (covered with a grate) to facilitate visual inspection. The flow splitter immediately following the primary tank is also covered with
plexiglas to prevent the escape of odors. Gravel-bed Wetland Beds. The gravel-bed wetlands were selected as the first phase of the secondary treatment process for several reasons. Partially-treated wastewater will lie beneath the bed, thereby avoiding the release of odors. Gravel-bed wetlands have higher treatment capacity in relation to the bed surface area. Without standing water, it will not be possible for mosquitoes to breed in the initial part of the treatment system. Because the wetlands are cut into permeable limestone rock, plastic liners were used and were protected from rock by spreading 2 of sand as a base before liner installation. A typical section through the gravel bed is illustrated in Figure 2 showing the water level typically maintained within the bed. Open-water Wetland Beds. Open-water wetlands were selected for the second treatment phase as a polishing process. Because water exiting from the gravel beds will already have received a high level of treatment, it will be possible to maintain small fish in the open-water wetlands, which will control possible mosquito breeding. No odor problems are anticipated at the 4 stage. The pond acts as an equalization basin because it has a larger storage volume per unit area than the gravel-bed wetlands. A sectional view along the centerline of an open-water wetland unit is illustrated in Figure 3. Figure 2: Sectional view of gravel-bed wetland. Figure 3: Longitudinal section through open-water wetland showing substrate for plant rooting. Flow is from left to right, and the elbow at the outlet can be swiveled to establish the water level within the wetland. Water level 3 below top of gravel bed 3/4 dia gravel 21 deep Landfill liner Limestone bedrock 1 sand over liner liner 1 sand under liner liner Outlet sump 5
Outlet Level Control. The water levels in both the gravel bed and the open water wetlands are controlled by PVC-elbows (6 dia) located in sumps at the outlet of each wetland cell. By rotating the elbow the overflow level in the outlet pipe can be adjusted to keep the level within the wetland at the desired level (see outlet shown in Figure 3). Although not shown, there is also an overflow pipe which is directly connected to the outlet sump to prevent flow from overtopping the terrace berms. CONSTRUCTION The treatment system was constructed by Stanford Development Company with the assistance of several local sub-contractors and suppliers. The construction sequence was, generally, as follows: (1) rough grading of site including terraces and site for primary treatment unit; (2) construction of primary treatment unit and final grading of terraces and pond footprints; (3) installation of piping and splitter boxes; (4) placement of sand below, impermeable liner, and watertight test of ponds; (5) place sand above liner, gravel and second watertight test; (6) fill pond and plant of treatment vegetation; (7) complete installation of disinfection and related equipment, and ornamental landscaping, and (8) begin operation. Two of the construction steps are shown in Figure 4. Figure 4: Left- placement of sand layer on top of liner prior to placement of gravel. Right Planting of gravel-bed wetland using potable water with a small addition of fertilizer to provide nutrients prior to operating the cell with wastewater. Minor construction problems were encountered with the elevation of a flow splitter box, easily fixed, and one liner developed leakage problems following construction. Not all of the landscaped terraces between cells were diverted to a storm drain, meaning that during a heavy rainfall some surface runoff will enter the treatment cells. 6 Following start-up odors were tracked and eliminated. The principal odor came from raw sewage crossing the tangential screen, which was remedied by constructing a frame and Plexiglas cover.
Once this problem was corrected a subsequent odor audit was performed which identified several locations for the installation of weather-stripping. The use of tight fittings and weatherstripping eliminated the need to operate the small odor-control blowers. An overview of the completed treatment system, in start-up operation, is shown in Figure 5. Landscaping was performed under the direction or staff of Stanford Development Company. Vegetation planted in the treatment cells were selected in consultation with the authors, and the remaining plants were selected based other landscaping criteria. Figure 5: Completed treatment system in start-up operation. One of the open-water wetlands is in the foreground and the gravel-bed wetlands occupy the terraces above. ACKNOWLEDGEMENTS The design and construction of the system presented in the paper was financing by Stanford Development Company, Antigua.