How does a Wastewater Garden work?
A Wastewater Garden system normally consists of three treatment phases:
Septic tank
Wastewater Garden, the water-tight (lined with concrete, impermeable clay or geomembrane to hold wastewater in) comprising just one compartment (cell) in small systems and more compartments in larger applications, and
Discharge of the treated wastewater to a final leachfield or use as subsurface or drip irrigation.
Primary treatment, to separate solids, occurs in a conventional, watertight septic tank or settling lagoon. But then instead of passing directly into a leachfield, with its attendant problems of little further treatment, smell, clogging and large size, the nutrient-rich wastewater effluent is fed into a lined, subsurface flow wetland. In this type of wetland the sewage water is kept 2-4 inches (5-10 cm.) below the surface of a bed 20-40 inches deep (0.5 – 1 m deep) of gravel. The treatment compartments are planted with a wide variety of wetland plants, specially selected for the locality, into the gravel bed filled with sewage water. After treatment in the Wastewater Garden, the discharged water goes to a comparatively small subsurface discharge leachfield or the treated water can be recycled for further irrigation of lawns, shrubs, flowers or trees. Wastewater is generally held for a minimum residency time in the septic tank for 2-3 days, and in the Wastewater Garden for 5-7 days, during which time the wastewater is treated and purified by the plants and natural microbes
Since the system looks so simple, how can it give such good results?
Although the system does not normally use machinery (except pumps if necessary to get wastewater to the Wastewater Garden, or to bring several sources to a centralized treatment location) nor chemicals, the variety of natural mechanisms that do the water recycling and purification make Wastewater Gardens very effective. These mechanisms are biological, chemical and physical – and the great advantage is that all these mechanisms are totally natural, and do not require human intervention or assistance to keep working.
The water treatment mechanisms include physical filtration and sedimentation, biological uptake and transformation of nutrients by anaerobic (bacteria that flourish in the absence of oxygen) and aerobic (oxygen-needing bacteria) and plant roots and metabolism, as well as chemical processes (precipitation, adsorption and Decomposition) that purify and treat the wastewater.
What do the plants do?
The plants are the aerators of the system, since wetland plants pump air into their root systems. This helps maintain a population of air-breathing microbes (aerobic bacteria) which are part of the treatment process, and the roots of the plants support a wide variety of helpful organisms. In addition, the plants directly take up and transform wastewater into a lush garden.
How many and what kind of plants do I need in a Wastewater Garden?
Since the Wastewater Garden is dry only at the top 5-10 cm (2-4 inches) of its gravel bed, the rest of the gravel (normally 75 cm or 30 inches) holds the wastewater. All the plants in the Wastewater Garden are specially chosen because they must be able to live in these water-saturated conditions. This means they are “wetland plants”, but Planetary Coral Reef Foundation has conducted years of research to extend the range of plants which can thrive in Wastewater Gardens. These include many plants not normally found in natural wetlands, nor usually considered as wetland plants. Many of these plants are very beautiful and useful, and this allows us to design systems with many plants (with a high biodiversity) suitable for whatever climate and local conditions the Wastewater Garden will be used in.
Normally, 1-3 plants (depending on the size of the plants) are planted per square metre (10 square feet) of the Wastewater Garden, including some with deep roots capable of reaching down to the bottom of the gravel bed. It is natural for some plants to thrive, and others to die, especially as the Wastewater Garden develops with time and taller plants shade out lower ones. Plants may be added and vegetation pruned to keep the garden looking good. A mixture of shallow, medium and deep-rooted plants (trees, tall shrubs, palms) are planted to ensure that there is effective root penetration of the gravel bed of the Wastewater Garden.
How long can the systems last?
If properly maintained, the Wastewater Garden should last for decades. Eventually, the gravel in the system may become partially filled with organic matter, and will need to be cleaned, or taken out and replaced with fresh, clean gravel. The plants can then be transplanted back in, and the system can continue effective treatment for decades more.
Can I harvest from the plants growing in the Wastewater Gardens?
Anything that is harvested above the level of the dry gravel at the surface of the Wastewater Garden is safe to use since it has no physical contact with the wastewater.
Thus fruit, fibre (wood or handicraft, weaving materials), cut flowers or plants for medicinal uses may be harvested from the system. The only type of plant part which should not be used as root crops, since the roots have contact with the wastewater beneath the surface.
What maintenance is required?
The maintenance needs are pretty simple but important.
The septic tank at the beginning of the system needs normal maintenance: filters need checking every 3-6 months and washing if necessary. The septic tank should be pumped out when solids fill more than half its depth.
Water levels in the wetland cells need to be checked periodically, especially during periods of low occupation when evapotranspiration may exceed input into the treatment wetland. Until plants become well established it is important that water levels not be allowed to drop below their root zone.
The wetland plants need normal garden care – pruning for appearance and encouragement of new flowers. Leaf litter and prunings can be allowed to remain in the system providing mulch above the gravel, and encouraging an aerobic microbial community in the layer above the saturated gravel.
Should the wetland be planted before connection to the septic tank, it may be necessary to fertilize to help the plants get established; and maintenance of adequate water levels with tap water is required.
Ensuring that drainage is adequate around the Wastewater Garden so that runoff rainwater and soil do not wash into the system is extremely important. Wastewater Gardens are built with a berm higher than surrounding ground level, but one must check occasionally to make sure soil has not built up around the Wastewater Garden which will allow rain runoff to enter.
How do you live with a Wastewater Garden?
Do I need to change my water use habits? Are there chemicals I should avoid using once I have a Wastewater Garden?
Water usage: Wastewater Gardens are designed for robust operation and can handle periods of higher or lower wastewater input. However, the more water conservation is practiced in the house, the more effectively the wastewater will be treated because it will remain for a longer time in both septic tank (for separation of solids) and in the Wastewater Garden. The longer the Wastewater Garden retains the wastewater, more water will be used by the plants (for its cooling processes, evapotranspiration), and even better water purification will occur. Reducing the amount of water used in the house (through use low water-use toilets, showers and laundry machines) also saves money on water bills and will also improve the performance of your Wastewater Garden.
Chemicals: normally, household chemicals (such as cleaning products) are heavily diluted by the time they reach the septic tank and will not cause serious problems for both septic tank and Wastewater Garden. But the more aware householders are of avoiding using and especially flushing strong chemical-based products down drains that lead to the septic tank and Wastewater, the better such systems will operate since they are living systems. Sending fewer chemicals to the septic tank will ensure that the bacteria in the septic tank remain healthy. This will mean that the septic tank builds up solids more slowly, and will extend the time between when it needs to be pumped out. Tobacco, such as cigarette butts, should also not be flushed down the toilet as it is harmful to the natural bacteria that live in your septic tank.
What are the economic benefits of using Wastewater Gardens?
After installation, the low operating and maintenance costs of Wastewater Gardens are its great economic advantage over high-tech, mechanically based sewage treatment plants (STP). Maintenance costs are usually only 5-10% of a conventional STP, and is easily done by anyone, not a skilled technician who is required to adjust and repair conventional STP components. The long-life of the Wastewater Garden also has economic benefits, as a lifetime of 30-40 years is 2-3 times that which might be expected of an STP especially in tropical conditions.
The Wastewater Garden is also of economic benefit as it can supply some of the landscaping needs without need of additional potable water or fertilizer; and will also reduce overall water consumption since part of the green landscape is being watered by otherwise wasted water.
Where did the technology come from?
Dr. Mark Nelson working in collaboration with the Planetary Coral Reef Foundation (PCRF) (U.S.) and the eminent systems ecologist, Prof. H.T. Odum of the Center for Wetlands at the University of Florida, developed this innovative approach to wastewater treatment using man-made wetlands, employing high biodiversity and subsurface flow. The basic approach has been extensively tested and successfully applied in the United States and Europe over the past several decades. The Wastewater Garden is a totally ecological design and raises constructed (artificial) wetlands to a complete system.
Initial work by Dr. Nelson and the founders of PCRF with constructed wetlands for sewage treatment was in the self-contained experimental facility Biosphere 2 in Arizona. There, working with scientists from NASA, constructed wetlands were developed for the Biosphere 2 Test Module in 1987, and for the closure experiments in Biosphere 2 from 1991-1994.
What can you do with the water that is treated in a Wastewater Garden?
The Wastewater Garden uses some of the incoming water for plant evapotranspiration. This may amount to 10-50% of the wastewater, depending on the design of the system and local climatic conditions and represents direct use and recycle of the wastewater. The remaining treated water that leaves the Wastewater Garden though very low in suspended solids, coliform bacteria and organic compounds, still contains some nitrogen and phosphorus, which is useful to plants. Thus, the best options for final use are to employ subsurface irrigation of additional landscape shrubbery, gardens or trees; or to distribute the water through drip irrigation systems below covering mulch. The only use that is not advisable is for irrigation of leafy vegetables, for although the discharge water from the Wastewater Garden is usually highly reduced in bacteria, it is not completely disinfected since we normally don’t use a final disinfectant such as chlorine (which is harmful to the environment itself) or ultra-violet lights. The discharge water could also be used for flushing toilets but the cost of pumping it back into houses usually makes this option uneconomic.
What about graywater and graywater recycling?
Graywater refers to wastewater other than that from the toilet: laundry machines, showers, sinks and kitchen water. These can be separated from blackwater (the discharge from toilets) and recycled in separate systems if the local regulations permit it, and the separation is easy enough to be economically done. The advantages of separating graywater is that more “greening” can be accomplished with the wastewater and overall project costs will be lower the Wastewater Garden is smaller. In these cases, the Wastewater Gardens treats just the blackwater, or the blackwater and the kitchen’s wastewater, which can be a problem for graywater, reuse systems since it contains food particles, grease and oil.
Graywater recycle systems normally use a small tank to separate solids (a sedimentation tank) followed by subsurface irrigation trenches, which water landscape gardens and shrubs/trees. The Wastewater Gardens treating just the toilets (or toilets + kitchen) are smaller, and less expensive to build, and overall project costs are lower, and more areas around the buildings can be greened using this normally “wasted water”.
However, where the Wastewater Garden is retrofitted to existing buildings, it is sometimes too difficult and expensive to separate graywater, and the Wastewater Garden is designed to handle all the wastewater: graywater and blackwater.
How much space is required for a Wastewater Garden?
This depends on many factors including the climate (the warmer the climate, the smaller an area is needed since the plants and microbes are more effective year-round), the number of people and how much wastewater they produce, whether the system treats all the wastewater or the graywater is recycled separately, and what standard of treatment is required or desired.
However, as a rule of thumb, in warm, subtropical and tropical areas, treating just the blackwater may require 1-2 m2 (10-20 square feet) of Wastewater Garden per full-time resident; and for treatment of black and graywater, 2-3.5 m2 (20-35 square feet) may be required. In cold climates, these numbers may be twice as great, depending on the level of treatment required during the cold periods of the year when plants may be dormant and bacterial activity is lowered because of low temperatures.
Why is sewage harmful?
Pollution of water resources by improperly or inadequately treated domestic wastewater (sewage) contaminates drinking water supplies and so is a leading cause of human disease worldwide. Health problems related to sewage are widespread, ranging from children swimming in open sewage treatment ponds, failure of leachfields due to wet season inundation, and sewage effluent pollution of groundwater, rivers and lakes with adverse impact on drinking water quality and recreational use of these resources.
In addition to issues of human health, the release of nutrients from this wastewater causes eutrophication (excessive nutrients) in the environment, leading to a wide range of environmental problems. These include coral reef decline, oxygen depletion/fish kill and ecological degradation of rivers and lakes, and giving competitive advantage to weed species over native plants in ecosystems impacted by release of human wastewater.
How do Wastewater Gardens help improve human health and protection of the environment?
Small, rural and isolated communities have great expense and difficulty in maintaining the highly technical systems that they are given. It is frequently reported that maintenance soon becomes almost non-existent and inadequate sewage treatment results. “Wastewater” is in fact a valuable source of nutrients and water, upon which ecologically flourishing wetlands can exist. Wetland scientists have demonstrated that not only natural but also properly designed and constructed man-made wetland ecosystems are extremely efficient at utilizing and cleaning such nutrient-rich waters.
This approach turns out not only to be easy to maintain and efficient in turning what was previously “waste” into green plants and reusable water. Wetlands are also lower cost, in that there is far less reliance on complex technology, which is capital and maintenance-intensive, and uses much electricity/fuel. The use of ecologically constructed wetlands for human sewage treatment relies on the ability of green plants and non-pathogenic microbes rather than expensive machinery. In addition, designed wetlands create a “buffer” ecosystem between the human economy and the environment to mitigate negative impacts, illegal as well as unpleasant and unhealthy which otherwise would endanger our water supplies and thus human health, as well as negatively impacting the environment.
Why doesn’t the system smell? Why is there no malodor?
There is no odor from a Wastewater Garden because the sewage is never exposed to the air. It’s as simple as that. At every stage of the process, the sewage is kept under a layer of gravel or under a tight lid (in the septic tank or in the control box of the Wastewater Garden). The absence of exposed water also means that there is no mosquito breeding and it prevents accidental contact with wastewater. This makes Wastewater Gardens especially attractive from a public health standpoint.
Are the systems approved?
Subsurface flow wetlands have been reviewed and approved by the United States Environmental Protection Agency and by many European regulatory bodies. Tens of thousands of constructed wetlands have now been built all over the world. There are also approaching a hundred Wastewater Gardens in the United States, Mexico, Belize, the Bahamas, Poland, France, the Philippines, Indonesia and Australia. However, regulatory guidelines differ markedly from country to country, and even within countries, local governing bodies have greater or lesser familiarity with constructed wetlands such as Wastewater Gardens. In most areas, getting approval will not be difficult, but in some regions, education of local health department officials is required before permission to proceed is granted.
Can systems be built for large numbers of people?
There are examples of constructed wetlands that treat millions of litres (or gallons) of wastewater daily. Some systems have been built for towns with populations from 10,000-20,000 people and which cover many hectares of land.
What about salty (saline) water, is that a problem?
In some areas, especially ones near the coast, water supplies may be somewhat saline (salty). But there are many plants, which can handle salty water without difficulty, so Wastewater Gardens can be designed in these circumstances. The choice of plants will be adapted to the salinity of the water, which the Wastewater Garden receives. We have built many such systems in the Yucatan of Mexico, and a surprising range of plants can tolerate salinity up to 1/3 the salt content of seawater. If the water is even saltier, we choose more halophytic (salt-tolerant) plants, including mangrove species, which can handle water as salty as seawater.