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Final Programmatic Report
for the National Fish and Wildlife Foundation project (2003-0093-020)
Sewage Treatment Using Constructed Wetlands: Phase 2
SUMMARY
This constructed wetland retrofit project carried out by Centro Ecológico Akumal (CEA) allowed for timely research and development to take place, which will serve to further the design and implementation of these systems. Constructed wetlands have a sensible and emerging role in the Riviera Maya, and in other similar geographical regions, especially with those nations which share the Mesoamerican Barrier Reef System. Still little is known about the functioning of these systems in the climate and conditions specific to this region. This project allowed for the evaluation of mature wetlands and innovations for their improvement. Regional wetlands were shown to have limited capacity for removing contaminants from waste water. This project demonstrated that further studies must be done to determine the key parameters, such as media type, aeration, and plant species and coverage, to improve wetland efficacy. Finally, as a result of this project, an ongoing dialogue was created among owners, designers, and groundskeepers, as we seek new ways of designing and managing our systems.
RESULTS
Initially, we proposed to: 1) replace the limestone gravel with a material that does not dissolve, 2) introduce a new aeration technology to increase efficiency, 3) replace and research optimal plants for this climate, and 4) train a maintenance team to upkeep septic and wetland systems.
The Limestone Gravel
Though limestone is generally not recommended to use in constructed wetlands, it is the only gravel material available in the Yucatan Peninsula, and in addition represents the major cost of construction. Though the mineral is usually believed to negatively affect plant growth, we found that more importantly it seems to inhibit biofilm development. Microbes are known to grow in films on surfaces, and their structure is key on the roots and gravel for the system to be effective. Upon excavation, the wetland gravel looked and smelled clean. It is believed that the surface conditions of the limestone, such as pH or alkalinity, inhibit microbial growth and assimilation of nutrients, thereby adding to the poor performance seen in regional wetlands (Fig. 3). Though chemical tests of the water indicate reasonable ranges for microbial and plant activity, further research is needed on the bio-geochemical interaction taking place at the surface to better understand the effectiveness of the gravel material.
The visual lowering of the gravel height by 10-15 cm in many wetlands led to the assumption that chemical weathering was taking place, dissolving the gravel and diminishing the volume. Though pH values below the carbonate equilibrium value (pH=6.4) were never noted in the wastewater influent nor effluent, they are buffered by the high alkalinity. But when it rains, the CO2-saturated rainwater causes carbonic acid to form and dissolve the limestone. This is likely upon the unsaturated, top layers of gravel that come into direct contact with rain. The high pH levels (8.2-9.0) observed after heavy rains are evidence that carbonate disassociation has occurred to some extent. To fully define the carbonate chemistry, we will be measuring alkalinity and calcium concentrations, in relation to rain and the brackish wastewater.
Surface depressions are also caused by plant organic acids in litter dissolving the gravel. The overall effect of dissolution seems to be limited at the surface, and may lead to ponding and odors until a new cap of gravel is added. The inner gravel of the wetland that is saturated with wastewater seems stable with time, and retains its size and porosity.
An additional drawback to the limestone is its high initial affinity for phosphate, which dramatically decreases with time. In the mature wetlands evaluated (3-7 years), the average phosphate removal was only 1% (Fig.3). Sorption onto gravel is the major removal mechanism of phosphorous, though it only represents a short-term sequestration. Washing and re-using gravel is an option, though sorption bottle experiments indicated only a partial recharging of sites (CEA 2004).
Current research suggests that the most sustainable phosphorous removal may be through various stages or mixtures of media that are selected for longer-term retention (Brix et al. 2001).
Alternative Media
To test alternative media that could be used in place of the gravel, we performed bottle experiments on shredded PET-plastic, broken glass, and a volcanic rock using partially treated blackwater and greywater. Limestone and only wastewater were used as controls. The PET plastic demonstrated the most rapid organic matter removal (COD) and the best biofilm development, and so we focused on this material in our further research. Though pellets are the ideal shape, they could not be produced locally, so we returned to shredding recycled bottles by hand and testing porosity. Colleagues at the Monteverde Institute in Costa Rica have recently completed a similar project where PET wetlands performed better than crushed rock due to better root penetration (Dallas 2004). Their communication has been helpful in the design of a pilot-scale PET bed for greywater on CEA property. This system will continue to be tested through its development and its viability compared to a conventional design.
Aeration
In general, the removal of nutrients in constructed wetlands is limited by a lack of oxygen (EPA 2000). An innovative design by North American Wetland Engineers consists of a small electric or solar-powered compressor in a sealed box, which pumps air down to a perforated hose snaked on the bottom of the wetland.
Fig.1 Aeration Pump
Fig.2 CEA Retrofit Wetland Oct 2004.
We installed an A/C pump in the retrofit CEA wetland (Fig. 1, 2) but due to technical problems with the regulator and the tenuous electricity system in the region, it has not functioned well since its installation. AJ Rossman of Draker Solar (the group that designed and installed the pump) will return in December 2004 with students from University of Vermont to teach a hands-on course and change the pump to DC-solar power.
Two other aeration systems operating in homes in the region were evaluated chemically. They run for 12 hours per day, and reduce organic matter (as COD) to less than 20 mg/L and ammonium to 0 mg/L. This is compared to systems without aeration, which discharge on average
91.6 mg/L of COD and 28.6 mg/L of nitrogen (Fig.3). Aeration does not seem to significantly affect the removal of phosphorous.
Though there are concerns with using mechanical systems with natural treatments, especially in regions without the expertise to maintain them. Hybrid designs such as these may serve to be the most viable. Aerated wetlands require less than half the space and materials of a regular bed, and the cost of the pump is more than compensated in these savings. Unlike unaerated cells, these systems have demonstrated levels of carbon and nitrogen removal suitable for discharge in sensitive environments.
Figure 3. Influent and effluent concentration averages of 7
constructed wetlands in Akumal, ± std. error of the mean.
Plants
To survey plant species, cover, and root penetration we evaluated the two wetlands on CEA property, as well as 50 others in the region. We also drew upon the initial wetland plant survey by State Botanist Edgar Cabrera and Dr. Mark Nelson of Planetary Coral Reef Foundation (Nelson 1998). As shown in Fig. 4, the majority of the species are non-native plants with roots that reach less than half of the depth of the wetland cell. Stratification was particularly evident on the walls of excavated wetlands, where the root zone, and area of greatest microbial activity, is seen at the top 25 cm and a dark, anaerobic layer underneath.
Avg. root
Rank
Common name
Scientific Name
Depth (cm)
Native
1
Taro, elephant ear
Alocasia macrorhiza, Xanthosoma roseum
16
N
2
Papiro, umbrella plant
Cyperus alternifolius
30
N
3
Helecho, fern
Acrostichum danaefolium
28
Y
4
Platano, banana
Musa paradisiacal
26
N
5
Ixora, jungle flame
Ixora coccinea
20
N
6
Platonillo, canna lilly
Canna sp.
12
N
7
Tulipan, hibiscus
Malvaviscus arboreus
15
Y
Fig.4. Characteristics of highest ranked plant species.
The US Environmental Protection Agency has also identified this issue of root depth in wetlands that not only limits treatment but causes a preferential flow path in the underlying layer of gravel (2000). These effects are also caused by the irregular and incomplete planting exhibited by the average 66% above ground plant cover in the regional constructed wetlands.
Though there is little evidence in the literature to suggest that plant diversity significantly affects performance, most designers agree that biodiversity is an indicator of system health, creating a myriad of niches both within the wetland for internal processes and externally for habitat and beauty. Experience with the two CEA wetlands suggests that biodiversity is difficult to sustain, whether naturally evolved or managed. In Wetland I, 70% of the species was reduced from 54 species in 1997 to 16 species in 2004. In Wetland II, 83% of the species was reduced from 63 to 11 species. The majority of lost species were understory plants, most notably Typha dominguensis, cattail, initially the highest ranking species, but driven out by shade or competition.
In the retrofit wetland, we selected plants with the most extensive root zone (wetland fern, papiro, zacate, and reed) and those that have a secondary value as a product (bamboo, papaya, ginebre, and bird-of-paradise, Fig.5). The plants have become well established by their first year, with certain species already showing signs of dominance over the traditional wetland plants like cattail and reeds. The natural function of these plants is important to sustain within the constructed wetland, and may require continued management until they are more firmly established.
Maintenance
With the assistance of our property administrator, we reviewed the best maintenance practices of wetlands for our groundstaff. CEA also published a Manual for the Operations and Maintenance of Constructed Wetlands in both English and Spanish (2003). Though many of the regional wetlands do not function well due to a lack of care, it was more effective to personally visit and work with owners and caretakers than to disseminate the manual. Many people were motivated to renovate their own systems, from which we continue to expand our experience and understanding of this method. CEA found that continuous interaction with wetlands owners helps ensure more effective management.
Fig. 5 Papaya
bird-of-paradise
Ginegre
EVALUATION
We evaluate our success in the project through internal working groups and the external feedback from colleagues following this report. Due to arising circumstances, many of the proposed ideas changed form, but we feel positive about the progress that was made in relation to the goals outlined above. We have a solid base with which to continue our research and development of this technique, which will be communicated through our work in the community, upcoming workshops, and our website.
PARTNERSHIPS
This work manifested through the kind support of volunteers, many of whom came to learn more about ecological systems, and many experts themselves. We thank Dr. Mark Krekeler of George Mason University and Dr. Rebecca Ferrell of Metropolitan State College of Denver for their help with the chemical analyses, and AJ Rossman and Curt Sparks of North American Wetland Engineers for their engineering services. We also recognize the pioneering work of Planetary Coral Reef Foundation to bring constructed wetlands to this region, and our new friends at the Monteverde Institute for their research in alternative designs. The continuing support of the National Fish and Wildlife Foundation is also greatly appreciated.
FUTURE
We will continue to monitor our systems and work towards their optimization, and provide the means for others to do the same. We hope to work further with local engineers, architects, and government agencies to address the design and regulation of ecological waste treatments at the municipal and state level. By creating a model on our property and in our town, we serve to demonstrate and live the fruits of sustainable waste practices – clean water, healthy coral and economy.
PUBLICATIONS
Brix, H., Arias, C.A., and Bubba, M. 2001. Media Selection for Sustainable Phosphorous Removal in Subsurface Flow Constructed Wetlands. Water Science and Technology, 44(11-12), pp 47-54.
Centro Ecológico Akumal, 2003. A Manual for the Operation and Maintenance of Constructed Wetlands. www.ceakumal.org
Centro Ecológico Akumal, 2004. The Use and Efficiency of Constructed Wetlands in the Riviera Maya. www.ceakumal.org
Dallas, S.C. and Ho, G. 2004. Subsurface Flow Reedbeds Using Alternative Media for the Treatment of Domestic Greywater in Costa Rica. IWA, 6th Specialist Conference on Small Water and Wastewater Systems.
Environmental Protection Agency, USA. 2000. Constructed Wetlands Treatment of Municipal Wastewaters. EPA/625/R-99/010.
Nelson, Mark. 1998. Limestone Wetland Mesocosm for Recycling Saline Wastewater in Coastal Yucatan, Mexico. Dissertation, University of Florida.
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