Practitioner's Tool / Constructed Wetlands
Constructed wetlands are gaining in popularity as the wastewater system of choice in the Philippines for many point sources of sewage pollution, including those from public markets, hospitals and slaughterhouses. Constructed wetlands come in many shapes, sizes and flow patterns. The one pictured here is a subsurface flow constructed wetland, otherwise known as a planted gravel filter or reed bed. This type of constructed wetland is a passive wastewater treatment system, which in many cases requires no energy inputs. The wetland pictured here was engineered for the Dumaguete Public Market by the Bremen Overseas Research and Development Agency, a German nongovernmental organization operating in the Philippines as BNS Borda. BNS Borda has been responsible for much of the proliferation of this technology through their comprehensive services that include initial consultation, design, construction management and post-construction supervision. Visit BNS Borda online.
Contents
How it Works
A differentiation between two types of constructed wetlands has to be made: the subsurface and the surface flow constructed wetlands.
Subsurface flow constructed wetlands work by delivering pretreated wastewater (either from a septic tank or anaerobic filter) to the wetlands. For the Dumaguete system, wastewater flows from the market through a series of grit and grease traps to an anaerobic baffled reactor and then to the wetlands. As the pretreated effluent flows through the media in the wetlands, the bacteria living on the gravel and plant root surfaces break down the organic matter in the wastewater. The wetlands plants, specifically chosen for this purpose, play an important role by delivering small amounts of oxygen to the root nodules, which provides micro environments where aerobic bacteria can flourish. The plants also add aesthetic value, making this an ideal spot for visitors to stroll and relax in the outdoors.
The drawing on the left illustrates the flow pattern of the wastewater through the wetlands. Note that a coarser grade of gravel is used at the inlet and outlet of the wetlands. This is to provide even distribution throughout the system. Also note that the outlet pipe is at an elevation slightly below the surface of the wetlands cell. This sets the water level so it is always below the top of the gravel, hence the name, subsurface flow wetlands. Keeping the wastewater below the gravel surface eliminates flies and mosquitoes and keeps odors to a minimum.
The drawing also illustrates the main components of the wetlands system:
- Excavated basin: Typically the basin is excavated to a depth of 1 to 1.5 meters--ideal for maximum root penetration.
- Liner: Excavations should be lined with an impermeable material such as plastic or concrete to prevent the wastewater from entering into the soil where it might impact groundwater.
- Media: Locally available media is most effective for solving local wastewater problems, but most constructed wetlands use gravel or crushed rock as the medium of choice. Other examples of media include lava rock and even crushed glass.
- Plants: Plants that almost always grow naturally in wetlands are called obligate plants. Wetlands obligate plants can withstand continued saturation and are necessary for treatment wetlands. Always use locally available species that can be harvested (with permission) from nearby natural wetlands. Never import plant species that are not native to the location where the wetlands will be installed.
- Inlet and outlet piping: Piping is usually perforated 6 inch PVC that is used to distribute the wastewater evenly through the media. An outlet structure specifically designed for wetlands allows the operator to vary the water depth in the wetlands cell, typically accomplished with a pipe on a swivel fitting. Swivel the pipe down to lower the water level and raise the pipe to its maximum elevation (just below the gravel surface) to kill any non-wetlands plants (weeds).
Surface Flow Constructed Wetlands. Pictured to the right is the surface flow constructed wetlands for the city of Arcata, California, home of 18,000 people. The wastewater system is an ideal example of how wastewater treatment can enhance the natural environment and give back to the community. The treatment wetlands of the Arcata Wastewater Treatment Plant are responsible for filtering out suspended solids, nutrients, organic matter and metals, which is accomplished using wetlands that cover approximately 3.5 hectares at an average depth of .75 meters. These wetlands are completely covered in aquatic plants that slow the flow of the water as well as assist in the absorption of the nutrients, metals, and organic compounds in the water. Many thousands of people visit the wetlands and nature center every year, adding to the tourism opportunities of the area. For more information on the Arcata Marsh Wetlands, visit the Arcata Marsh site on appropedia.org
Treatment Efficiency
Constructed wetlands are considered secondary treatment systems, meaning they must be preceded by appropriate pretreatment and primary treatment devices, such as septic tanks, grease traps and other units that remove the heavier solids, fats, oils and grease from the wastewater stream. As secondary treatment systems, constructed wetlands will produce an effluent that is suitable for discharge to the environment under most circumstances, although additional treatment including filtration and disinfection might be required to meet site-specific discharge requirements.
Additional Functions and Values of Constructed Wetlands
Wastewater treatment wetlands may provide additional values to communities to help cover the costs of the project or otherwise enhance the environment:
- Treated water from the wetlands may be filtered and disinfected for reuse for landscape irrigation or toilet flushing.
- Wetland cells may be used for hydroponic gardening and production of economically viable plants, such as flowers for the cut flower industry.
- Constructed wetlands can be designed to improve habitat for birds and butterflies.
- When designed for aesthetics as well as wastewater treatment, wetland cells may be incorporated into the environment to enhance landscaping features.
Constructed wetlands projects may provide significant eco-tourism opportunities by accomplishing two main objectives:
- Addressing wastewater treatment issues: High value tourists will not come and spend their vacation money in places where the environment is fouled by sewage. Addressing wastewater treatment issues is one important step in improving the environment to make it possible to attract tourists.
- Attracting nature enthusiasts: Adding a habitat component to a constructed wetlands project may help attract ecotourists such as birdwatchers and other nature enthusiasts. Birding enthusiasts will go to great lengths to see their prized species. Wetlands systems can be fitted with nature trails, self-guided tours and informational kiosks. Local tour guides can be trained in species identification for additional livelihood and income opportunities.
A Cost-Effective Approach to Wastewater Treatment
Many local governments, housing developers and others are turning to constructed wetlands systems as there may be a significant cost advantage over more traditional wastewater treatment methods. Where land is abundant, relatively level and free of excessively high groundwater or flooding, wetlands systems may be the best option. While the land requirements are rather large, the total capital costs for installation are significantly lower than mechanized wastewater treatment systems for similar flow volumes. Operation and maintenance costs are also much lower than for mechanized systems.
Dumaguete City Market Constructed Wetlands Project
The Dumaguete City Market Constructed Wetlands Project in the Philippines serves as an example of a successful wetlands project. The market's wastewater system is supported by funds collected from the public restroom, which covers all operation and maintenance expenses. Pictured on the right are the Dumaguete constructed wetlands basins under construction. Note the large rock at the inlet and outlet locations. The interior of the cells will be filled with pea gravel that is 2 to 5 centimeters in diameter. Also note that this project uses concrete as the impermeable liner material. Concrete has advantages as it may be easier to obtain than plastic and can be installed by local crews. Plastic liner is somewhat difficult to install and requires trained personnel with specific tools and equipment.
Project details:
- Flow volume: 80 cubic meters per day
- Land area: 55 square meters
- Number of stalls: 1,255
- Treatment Modules:
- - Settling tank
- - Anaerobic baffled reactor
- - Anaerobic filter
- - Planted gravel filter
- - Polishing pond
- Total Cost:
- - Treatment plant: 3 million Philippine pesos
- - Cost per stall: 2,390 pesos
Baywan Fisherman's Village Constructed Wetlands Project
The Bayawan Fisherman's Village in the Philippines is a good example of a low-cost and sustainable treatment system configuration using constructed wetlands. The Bayawan project utilizes a two-cell constructed wetlands configuration in which sewage first flows through clustered septic tanks and then to a settled sewer system prior to entering the treatment system.
Project details:
- Number of people served: 3,690
- Number of households: 738
- Area required: 2,650 square meters
- Wetland area per home: (sq. mtrs) 3.5
- Construction cost: 5,750,000 Philippine pesos
- Cost per home: 7,791 pesos
The Bayawan project is a joint effort between Bayawan City and the German Technical Cooperation Agency, which provided technical assistance for the project. For more information, have a look at the SuSanA library.
Links
There are many great reference materials available on the topic of constructed wetlands. In addition to the few mentioned within this post, the sites listed below are helpful resources:
