Basic design and operation principles of the Wastewater Gardens® system
The system works by a gravity flow of wastewater from toilets, showers, and kitchens into a properly designed and sealed septic tanks and then into the specially engineered subsurface flow wetland cell which keeps the wastewater below the surface of the gravel, preventing odor and eliminating the risk of human contact with the sewage. The system normally consists of three wastewater treatment phases. First one is taking place in septic tank equipped with special filter, where anaerobic bacteria commence a biological breakdown of the waste, and solids settle out to the bottom of the tank.
The 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 provides the second stage of the treatment. The gravel allows for adequate residence time for the wastewater and provides an enormous surface area where a wide variety of chemical, biological and physical mechanisms cleanse the wastewater by removing organic compounds, suspended solids and excess nutrients. 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. In addition, the plants directly take up and transform wastewater into a lush garden. The presence of aerobic and anaerobic zones supports a wide variety of helpful organisms: bacteria, protozoa, algae and fungi (10-100 billion per 1 g of soil). These organisms uptake the nutrients from the wastewater and use them for their own life processes. There are also other mechanism for example: adsorption, decomposition, sedimentation. Discharge of the treated wastewater to a final leachfield, where fruit trees or other landscape plants can utilize remaining nutrients and water, or water pond provides the third treatment stage.
There is no odor from the system 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). The absence of exposed water also means that there is no mosquito breeding and it prevents accidental contact with 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.
Locally available plants adapted to wetland conditions are selected to make a diverse and beautiful ecosystem. Even small systems may support 30-50 species of plants. The biodiversity indices of some Wastewater Garden systems studied in Mexico were comparable to tropical forest systems and greater than many natural wetlands. Everything that is above the dry gravel is hygienic and safe to use. For example, the fruits grown can be safely eaten by humans, fodder crops can be grown for animal food trees grown for timber, fuel wood or wicker production, or flowers cut for commercial sale.
© 2002 Carpathian Heritage Society
& Natural Systems, Dr Andrzej Czech