1-1
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.
Brix, H. 1999. How 'green' are aquaculture, constructed wetlands and conventional
wastewater treatment systems? Water Science and Technology 40(3):45-50.
Foster, J. B. and F. Magdoff. 1998. Liebig, Marx, and the depletion of soil fertility:
relevance for today's agriculture. Monthly Review 50(3):32-45.
Gopal, B. 1999. Natural and constructed wetlands for wastewater treatment: Potentials
and problems. Water Sci. Technol. 40(3):27-35.
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.
Hamersley, M. R., B. L. Howes, D. S. White, S. Johnke, D. Young, S. B. Peterson and J.
M. Teal. 2001. Nitrogen balance and cycling in an ecologically engineered septage
treatment system. Ecol. Eng. 18(1):61-75.
Hammer, D. A. and R. L. Knight. 1994. Designing Constructed Wetlands For Nitrogen
Removal. Water Sci. Technol. 29(4):15-27.
Kadlec, R. H. and R. L. Knight. 1996. Treatment Wetlands. Lewis Publishers, Boca
Raton.
Mitsch, W. J. 1993. Ecological engineering: a cooperative role with the planetary lifesupport system. Environmental Science and Technology 27(3):438-445.
1-6
Mitsch, W. J. and S. E. Jørgensen. 1989. Ecological engineering: an introduction to
ecotechnology. John Wiley and Sons, Inc., New York.
Narkis, N., M. Rebhun and C. Sheindorf. 1978. Denitrification at various carbon to
nitrogen ratios. 13:19-25.
Odum, H. T., W. L. Silver, R. J. Beyers and N. Armstrong. 1963. Experiments with
engineering of marine ecosystems. Contributions to Marine Science 9:373-403.
Petersen, J. E. 1992. Alienation and ecotechnology: an ecologically designed landscape.
Annals of Earth 10(2):16-18.
Todd, J., E. Brown and E. Wells. 2003. Ecological Design Applied. 20:421-440.
Todd, J. and B. Josephson. 1994. Living machines: theoretical foundations and design
precepts. Annals of Earth 12(1):16-25.
Todd, J. and B. Josephson. 1996. The design of living technologies for waste treatment.
Ecol. Eng. 6(1-3):109-136.
Van der Ryn, S. 1995. The Toilet Papers: Recycling Waste and Conserving Water.
Ecological Design Press, Sausalito, CA.