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I. Introduction to greenhouse-wetland treatment systems
Over the last several hundred years, humans have begun living in higher and higher densities, leading to high volumes of sewage output in small geographic areas.
This high density of sewage has led to the need to treat the wastewater we produce in order to protect human and ecosystem health (Foster and Magdoff 1998). An assortment of technologies including septic systems in rural areas and sewage treatment plants in urban ones has been developed to deal with this problem. The purpose of these systems is to remove pathogens, solid waste and organic carbon from the water. Some also remove nutrients such as nitrogen and phosphorus which cause eutrophication in aquatic systems.
There are, however, some problems with the current systems for sewage treatment. Septic tanks in particular do not effectively remove nutrients. Many larger treatment plants do not actively remove nutrients, and those that do generally rely on chemical treatment. Phosphorus removal can be achieved through chemical precipitation
(Kadlec and Knight 1996). Although nitrogen removal primarily relies on microbiological processes, methanol is often added to stimulate the removal of nitrate
(Narkis et al. 1978). Treatment plants also typically use chemicals such as chlorine or ozone to remove pathogens (Kadlec et al. 1996). Another difficulty of conventional wastewater treatment is the large energy input required.
A more fundamental problem with conventional wastewater treatment is its failure to take advantage of the potential resources embodied in wastewater. Our society is simultaneously investing fossil fuel energy and other resources to move nitrogen from wastewater to the atmosphere and to fix nitrogen from the atmosphere for agricultural uses (Foster et al. 1998). The nutrients in wastewater are an important resource that is 1-2 currently going unused. By changing the way we process wastewater, however, it is possible to take advantage of these resources.
Several alternatives to conventional systems exist. One which has been widely studied is the use of natural or constructed wetlands to treat wastes. In theory, treatment wetlands rely on natural wetland processes, thus requiring very low chemical and energetic input. If properly designed, wetland treatment can be very effective at nutrient removal (Hammer and Knight 1994). While the use of wetlands is a promising idea, there are several potential obstacles. To be effective these wetlands require a large land area. In addition, wastewater added to wetlands must be pretreated to remove solids, reducing the energetic savings (Gopal 1999). Another problem is that in temperate climates these marshes exhibit reduced functionality for much of the year (Hammer et al. 1994).
A third way of treating wastewater is a hybrid between sewage plants and wetlands. I will refer to these as greenhouse-wetland systems. This characterization includes a variety of systems including those referred to as Advanced Ecologically
Engineered Systems and marketed as “Living Machines”, “Solar Aquatic Systems” and a few other names. Greenhouse-wetland systems generally consist of a series of treatment tanks, as do conventional systems. Subsequent tanks in the process are designed and optimized to remove pathogens, organic matter, nitrogen and phosphorus using natural wetland processes. Planted tanks are located in a greenhouse ensuring year-round treatment even in temperate climates.
Greenhouse-wetland systems are a relatively new technology which is part of the emerging field of ecological engineering. Ecological engineering is the process of designing whole, complex ecosystems modeled on natural ecosystems to fulfill human 1-3 needs (Odum et al. 1963, Mitsch and Jørgensen 1989, Mitsch 1993). Self-organization is a principle of ecological engineering which is important to greenhouse-wetland systems
(Todd and Josephson 1994). Rather than including a predetermined set of components as in traditional engineering, self-organization entails allowing ecosystems to develop in ways that best serve their intended function. In greenhouse-wetland systems, this means, for example, that different tanks will develop different plant communities due to differences in light availability and nutrient loading. Another important principle of ecological engineering is to fulfill multiple functions simultaneously. Some of the functions of greenhouse-wetland systems are described below.
Ecologically engineered greenhouse-wetland systems are a fundamentally different form of wastewater technology because, in theory, the function of such systems extends beyond the removal of contaminants from water. Ideally, these systems treat waste as a resource which can produce useful products. For example, the tank ecosystems can be used for raising fish, crayfish, flowers, plants for landscaping, and even food
(Todd et al. 2003). Greenhouse-wetland systems also have important educational value.
The planted tanks and wetland beds are aesthetically pleasing, causing people to be interested in, rather than disgusted by, how their waste is treated. The systems are usually decentralized, located near the site of waste production. This too helps people make a connection between themselves and their waste, increasing awareness of the issue of wastewater treatment, and begins to break down alienation between people and their waste products (Petersen 1992).
Greenhouse-wetland systems are not perfect solutions to wastewater treatment.
They are an 'end of the pipe' solution, failing to address the problem of putting our waste 1-4 into clean water in the first place (Petersen 1992). The use of composting toilets could potentially eliminate the creation of sewage and therefore the need for sewage treatment
(Van der Ryn 1995). However, since greenhouse-wetland systems do not require a change in sewage infrastructure, they are a more feasible alternative in the short term. A second difficulty with greenhouse-wetland systems is that, depending on how they are constructed and operated, though they are intended to require low energy inputs, they may actually use more energy than conventional treatment systems (Brix 1999).
Greenhouse-wetland systems currently fulfill many treatment purposes in many places. Several are at educational institutions such as Stensund Folk College in Sweden
(Guterstam 1996) and a public school in Toronto (Todd and Josephson 1996). From 1996 to 1999, a greenhouse-wetland system treated all municipal wastewater in South
Burlington, Vermont. That system now treats brewery wastes (Todd et al. 2003). Several greenhouse-wetland systems are designed for waste products other than domestic wastewater. One system installed in Harwich, MA treated septage pumped from septic tanks (Hamersley et al. 2001). Another system in Nevada treats the waste from a chocolate factory (anonymous, 2003).
Developing a better understanding of how greenhouse-wetland systems function is critical to improving design and operation of these systems. With this goal, I examine in this paper several aspects of the performance of a particular greenhouse-wetland treatment system. The Oberlin College Living Machine, located in the Adam Joseph
Lewis Center for Environmental Studies, has been treating the wastewater produced in that building since February 2000. Chapter 2 analyzes inorganic nitrogen dynamics in the
Living Machine over a three-year time span. It addresses the influence of changes over 1-5 time in both input to the system and the system itself. Chapter 3 focuses on the use of an environmentally benign alternative to the traditional technique for determining organic nitrogen and the information that method provides about nitrogen processing in the
Living Machine. Chapter 4 focuses on a new technique for measuring the in situ metabolic activity of the Living Machine. This method is compared with the standard method of determining biological oxygen demand.
References
2003. Ethel M. Chocolates. Living Machines, Inc.
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Guterstam, B. 1996. Demonstrating ecological engineering for wastewater treatment in a Nordic climate using aquaculture principles in a greenhouse mesocosm. Ecol. Eng. 6(1-3):73-97.
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Van der Ryn, S. 1995. The Toilet Papers: Recycling Waste and Conserving Water. Ecological Design Press, Sausalito, CA.