Upload
akhilt29
View
2.086
Download
4
Embed Size (px)
Citation preview
Natural Alternatives to Conventional Wastewater
Treatment
By: Heather Stovall June 2007
Abstract
Since water is such an important resource and there is a strong likelihood that the world will face a fresh water shortage in the near future, we must find ways to more efficiently manage our limited sources of water. One of these ways is to reuse and recycle water by purifying wastewater. To treat wastewater, many wastewater treatment facilities currently use large amounts of energy and chemicals which are harmful to the environment. This project will research various ways of treating wastewater in order to find out if there are more natural and sustainable alternatives to conventional wastewater treatment. Several examples of alternative wastewater treatment are Living Machine technology, Ecoparqué in Mexico and engineered or constructed wetlands.
Natural wetlands already function to filter water and trap sediment before it enters
a body of water. This document concentrates on engineered wetlands as the main alternative to conventional wastewater treatment because they use little energy, provide wildlife habitat and can effectively treat wastewater. A theoretical design of a wetland serves as an example of how to design a wastewater treatment wetland. The sustainability of the wetland is then compared to conventional treatment using the Ecological Footprint model. In rural areas or for small communities, wetlands are cost effective and sustainable. In urban areas where land is scarce, conventional treatment might still be the only option.
Acknowledgements
I would like to thank all of my committee members for their help with my senior
project. They are: Harold Leverenz from the Department of Civil and Environmental
Engineering; Loren Oki, a UC Davis faculty member of the Landscape Architecture
Department specializing in water quality; Steve Greco, a UC Davis faculty member of the
Landscape Architecture Department specializing in ecology, and Randy Dahlgren, a UC
Davis faculty member of the Land, Air and Water Resources Department specializing in
soil science. They either directed me to or lent me the sources I used for the major part of
this project. They took time out of their busy schedules to meet with and give me advice.
With their expert guidance, I was able to complete my senior project. I would also like to
thank my family for their support and editing help.
ii
Table of Contents
Abstract
Acknowledgements ii
Table of Contents iii
List of Figures iv
Chapter 1: Introduction: The Future of Water Use 1
Chapter 2: Conventional Wastewater Treatment 4
Chapter 3: Natural Wastewater Treatment 7
Aquatic Floating Plant Systems
Ecoparqué
Treatment Wetlands
Chapter 4: Design of Treatment Wetland in Alamo, California 21
Chapter 5: Research Conclusions 32
Bibliography 36
iii
List of Figures and Tables
List of Figures
3.1 Living Machine 8
3.2 Restorer 9
3.3 Ecoparqué 9
3.4 Surface flow and subsurface flow wetland diagram 16
3.5 Bird watching in the Arcata Marsh 17
3.6 Map of the Arcata Marsh 18
4.1 Grading Plan 24
4.2 Master Plan 25
4.3 Section 26
List of Tables 2.1 Principal constituents of concern in wastewater treatment 5
2.2 NPDES permit requirements 6
3.1 Minimum HRTs 15
3.2 Plants in the Arcata Marsh 19
4.1 Ecological Footprint model land categories 27
4.2 Wastewater treatment energy use 29
5.2 Maximum effluent concentrations 33
iv
CHAPTER 1
The Future of Water Use
1
Water scarcity world wide is and will continue to be caused by the growing
human population and also an increase in the amount of water used per capita. In the 20th
century, the world population tripled and the uses of water resources increased by a factor
of 6. The most important uses of water are for drinking and for food production.
Currently over a billion people have insufficient access to safe drinking water and half
the world has inadequate sanitation (HRH 2002). Groundwater levels are falling and all
types of water bodies including rivers, lakes and oceans are becoming increasingly
polluted.
Global warming, sometimes called global climate change, is another factor that
will most likely affect water scarcity in the future. It is caused by greenhouse gas build-
up, mostly carbon dioxide, in the atmosphere which traps heat. Global warming is
predicted to cause weather patterns to change such as creating drought in areas that
normally get adequate amounts of rain (Natural Resources Defense Council 2006).
Many issues that cause water scarcity could be avoided with better water
management. One of the reasons that water is not used efficiently in the United States is
that the federal government subsidizes the cost of water, so water is inexpensive. If
people had to pay the actual cost of water, they would use it more efficiently (Landry
1999). For example, the water for farms in the Central Valley is heavily subsidized; if it
were not, farmers might keep water on site instead of letting it flow off site through tail
ditches to a nearby body of water. Also, farmers might be inclined to purchase purified
wastewater, which would be a good way to reuse water. The industrialized, developed
countries are the most inefficient with their water use and could reduce water problems
for themselves in the future by conserving water now.
2
In the United States, when water begins to become scarce, one of the first sources
of water people will turn to on a large scale will be graywater. Graywater is defined as
water from a household excluding water from kitchens and toilets. In California it is legal
to use graywater from bathroom sinks, showers and washing machines to irrigate
landscape (California Gray Water Law, 1997). All that is needed to use graywater is a
simple filter and additional piping to divert the water from the sewers. Graywater makes
up 50% of the wastewater generated by households (Phelps 2007) and so will be an
important source of water for reusable purposes. The other 50% of household wastewater
comes from toilets and kitchen sinks and will also be important to reuse. Purifying
wastewater so that it can be reused or even deposited back in the environment is a
complicated process. The rest of this document focuses on the sustainable treatment and
reuse of wastewater.
There are several ways to make wastewater treatment a more sustainable process.
For a process to be sustainable, it should maintain and promote biodiversity,
renewability, and resource productivity over time (Office of Biorenewables Programs
2007). “If we are to live sustainably, we must ensure that we use the essential products
and processes of nature no more quickly than they can be renewed and that we discharge
wastes no more quickly than they can be absorbed.” (Wackernagel 1996) Another aspect
of sustainability is maintaining the health of humans, the environment, the economy and
the community. The wastewater treatment options that are compared in this document are
conventional wastewater treatment, Living Machines, and treatment wetlands. On site
wastewater systems such as leach fields were not looked at because they are typically for
individual residences in rural areas.
3
Conventional Wastewater Treatment
CHAPTER 2
4
There are many types of wastewater including domestic, commercial, industrial
and agricultural. (Crites, 1998) Each type of wastewater has a different chemical makeup,
so for simplicity’s sake this document will focus on domestic wastewater treatment.
Although wastewater technology has improved recently, cities usually wait until their
current wastewater treatment plants, which can be 70 years old, begin to fail before
updating their technology.
The constituents in untreated wastewater can be divided into three types: physical,
chemical and biological. Physical constituents are the particles or solids in the effluent.
Effluent is defined as liquid waste that is untreated, partially treated or completely
treated. Chemical constituents include nutrients and heavy metals. Biological constituents
include coliform organisms and other microorganisms such as bacteria, protozoa,
helminthes and viruses (Crites 1998). These constituents need to be removed for various
reasons (Table 1).
Table 2.1 Principal constituents of concern in wastewater treatment (Crites 1998)
5
Conventional wastewater treatment goes through 3 stages. In the first stage, which
is primary treatment, solids settle out of the wastewater. The solids are called sludge and
are usually taken to the landfill. The next stage is called secondary treatment, where
dissolved or suspended materials are converted to a microbial biomass so that they can be
separated from the water. The third step called tertiary treatment is where particles and
nutrients such as phosphorus and nitrogen are removed (Water Environment Federation
2007). Finally, the water is disinfected, usually with chlorine and sometimes ozone or
ultraviolet (UV) radiation. In order to discharge treated wastewater to surface water a
National Pollutant Discharge Elimination System (NPDES) permit is required (Reed
1990).
Table 2.2 NPDES permit requirements for Central Contra Costa Wastewater Treatment Facility (Discharge Prohibitions 2007)
Constituent Units Monthly Max
Weekly Max
Daily Max Instantaneous Min
Instantaneous Max
BOD mg/L 25 40 50 - - TSS mg/L 30 45 60 - - pH s.u. - - - 6 9 Oil and grease mg/L 10 - 20 - - Copper μg/L 14 - 20 - - Lead μg/L 3.5 - 8.2 - - Mercury μg/L 0.018 - 0.046 - - Cyanide μg/L 2.8 - 6.4 - - Acrylonitrile μg/L 6.3 - 13 - - Dioxin μg/L 0.014x10-6 - 0.028x10-6 - - Enterococci Bacteria
35 colonies /100 ml
- - - -
6
CHAPTER 3
Natural Wastewater Treatment
7
Aquatic Floating Plant Systems:
Living Machines were created by John
Todd, an ecological designer and founder of the
nonprofit organization Ocean Arks International.
Living Machines are a series of tanks with plants
and other organisms contained in them. Wastewater
is then pumped through these tanks to naturally t
the water. They mimic wetland ecology to treat
wastewater, but require less space and do it more
efficiently than a wetland because the conditions can be controlled so they are more ideal.
For example, the organisms have more oxygen than in a wetland because air is bubbled
through the tanks. Some Living Machines also produce beneficial by-products such as
methane gas, edible and ornamental plants and fish (Todd 1994).
reat
Figure 3.1 Living machine (www.livingroutes.org)
The United States Congress had the EPA (Environmental Protection Agency) do a
study of four different Living Machines and found that they are not any less expensive
than conventional wastewater treatment. Because the tanks are aerated about the same
amount as in conventional wastewater treatment, the same amount of energy is used. In
colder areas, Living Machines must be located inside greenhouses which also use energy
to heat and cool them. If a renewable source of energy is used, then Living Machines
would be more sustainable and cost effective. However the EPA found that the plants do
not significantly contribute to the treatment but just make the wastewater treatment more
aesthetic (US EPA 1997). Living Machines are still being built. One advantage is that
8
they provide educational value. The Living Classroom, an environmental education
center that is being built in San Francisco, will be completed in fall of 2007 and will have
a Living Machine among its other sustainable features (Brun 2007).
Ocean Arks International, the same organization
that created Living Machines, developed Restorers,
which are floating rafts of plants and other organisms
that clean the water. Restorers can be used to clean
polluted lakes or to treat wastewater by building
artificial ponds (Ocean Arks International). This company
also designs treatment wetlands, although they are not the
pioneers for treatment wetlands.
Figure 3.2 Restorer (Ocean Arks International)
Ecoparqué
Ecoparqué, a combination of a park and a wastewater treatment plant (Figure 3.3),
is located in Tijuana, Mexico, and was created in response to poor sanitary conditions and
a need to treat wastewater
from the city. The w
used to go straight into
Tijuana River, which run
along the US Mexico border.
Oscar Romo, the Coastal
Training Program
Coordinator of the Tijuana
astewater
the
s
Figure 3.3 Ecoparque (www.nmsu.edu/~frontera/may01/feat3.html)
9
River National Estuarine Research Reserve, came up with an environmentally friendly
solution in the creation of Ecoparqué (Bedar 2000).
Ecoparqué treats the wastewater generated from a neighborhood of 1,200 home
and uses no chemicals. The wastewater flows by gravity to Ecoparqué. A microcriba
filters out larger organic matter, which is then composted with tiger worms and used in
Ecoparqué. Two biofilters, which are large tanks filled with bacteria colonies, treat the
water. A clarifier settles the solids out of the water. The operators test the water and if it
does not meet their standards, they re-circulate it through the biofilters. The water, which
still has nutrients in it, is then used to irrigate the plants that make up Ecoparqué (Bedar
2000).
There was once a problem in Ecoparqué because there was a high level of
industrial solvents and heavy metals in the water. This was caused by small industrial
facilities within some homes. The problem was solved by explaining the dilemma to the
residents and once they understood that it was a problem, the water became cleaner
(Bedar 2000).
An important feature of Ecoparqué is that smaller decentralized wastewater
treatment systems can be used to successfully serve small communities within a city and
that the water can be reused to irrigate parks. People there are willing to use a park even
though it is irrigated with treated wastewater. Also, in some circumstances, the nutrients
can be left in the water if it is going to be used to irrigate plants which could make use the
nutrients.
10
Treatment Wetlands:
A wetland is defined as “an area that is regularly saturated by surface water or
groundwater and is characterized by a prevalence of vegetation that is adapted to life in
saturated soil conditions" (US EPA, 1994). Wetlands can be divided into two categories;
marshes and swamps (Reed 1990). Marshes are characterized by the presence of
emergent, non-woody species of plants, and swamps are characterized by the presence of
woody plants. Wetlands are most often found near bodies of water and are the transitional
area between aquatic and terrestrial ecosystems. They filter the water by trapping
sediment and organic matter and are sometimes referred to as the “kidneys of the
landscape.” Wetlands can be sinks (more of a substance goes into them than comes out)
of some materials and sources (more of a substance goes out of them than comes in) for
other materials. Wetlands lessen both flooding and droughts (Mitsch 2000). They
ameliorate flooding because they give the water a place to flood into, and they lessen
droughts because they store water organisms to survive during dry periods. They also
provide valuable habitat for fish and other wildlife, such as various bird species.
In the past, the United States federal government policies encouraged and
subsidized the conversion of wetlands to other uses such as agriculture and by the mid
1980s half of the wetlands in the United States equaling 117 million acres had been
destroyed. Wetlands today are protected; they are the only habitat type that is completely
regulated on both private and public land in the United States. They are also protected in
other parts of the world (Committee on Characterization of Wetlands 1995).
11
The importance of wetlands became widely recognized in the 1970’s. Due to
public concern about water pollution, the Clean Water Act was passed in 1972 and
amended in 1977. It set regulations controlling the quality of water or any substances
entering into surface water. The passing of this law is one of the main reasons that
constructed wetlands were considered as a way to purify wastewater (Reed 1990).
The first use of wetlands to treat wastewater was simply discharging the effluent
into existing natural wetlands. This practice raised concerns about possible negative
effects wastewater could have on the wetland ecosystem, so constructed wetlands became
a new way to treat wastewater (Mitsch 2000).
Many wetland plants can be successfully propagated and the hydrological
conditions can be simulated, so it is fairly simple to design and build wetlands (Reed
1990). The exact species of plants are not important, because most of the wastewater
treatment is done by other organisms that live within the wetland, such as bacteria.
Treatment wetlands are more effective at higher temperatures, but still work at low
temperatures. They can function when winter temperatures are well below freezing
(Gearheart 1993). Sometimes in very cold climates, the wastewater is stored during the
winter (US EPA 1993). Wastewater is fairly warm (55 to 86 degrees Fahrenheit in warm
climates) and so treatment wetlands have a warmer temperature than a natural wetland
would. A warm temperature could have a negative effect on some species of fish that
might be living in the wetland. However, the warmer temperature could be beneficial to
wastewater treatment because the optimal temperature for the bacteria which perform the
treatment is 77 to 95 degrees Fahrenheit (Crites 1998).
12
Treatment wetlands differ from natural wetlands in that they are more
continuously flooded, whereas in natural wetlands the water level fluctuates seasonally
(Crites, 1990). Engineered wetlands can be used to treat anything from raw sewage to
secondarily treated water, in which case they are called polishing or tertiary wetlands
(Reed 1990).
Engineered wetlands are best planted with plugs, which are small young plants,
but can also be planted with rhizomes or clumps of emergent plants with the tops cut off.
Clumps of emergent plants have the best survival rate, but take more time. Rhizomes are
pieces of stem that grow horizontally below the soil and easily develop new roots and
shoots. Rhizomes have a lower success rate, but take less time to plant and cost less
money. Plugs are the best because they have a fairly good success rate although not as
good as clumps of emergent plants and are fairly fast and inexpensive to install. The
wetland should ideally be planted in fall so that they are kept moist during the winter, but
can be planted in any season as long as they are kept moist (Gearheart 1993). If seeds are
used, they should be planted in spring to take full advantage of the growing season
(France 2003). Plant propagation materials should be collected from a local source,
because they are already adapted to the local environment (Gearheart 1993).
In wastewater treatment, odors are caused by hydrogen sulfide which is created
by anaerobic conditions. Using certain plants can affect the odor of a wetland. For
example, cattails should not be used widely because they create anaerobic conditions.
Odor can be reduced by “pretreatment to reduce the total organic loading on the aquatic
system, effective effluent distribution, step feeding of influent waste stream and
supplemental aeration.” (Gearheart 1993)
13
The ideal soil type on which to construct a wetland is clay. The bottom of the
wetland must be sealed so contaminants do not leak into the ground water. A liner, like
the type used to create ponds, can be used. Also, sometimes a layer of clay or bentonite
can be laid down, compacted and topped with gravel (France 2003).
Wetlands are very effective at removing organic matter, both particulate and
dissolved. The roots of the plants provide air and habitat for organisms such as bacteria,
fungi, and aquatic invertebrates that break down the organic matter. Organic matter is
measured in biochemical oxygen demand (BOD) and total suspended solids (TSS) (Reed
1990). Biological oxygen demand measures the amount of oxygen that organisms in the
water are using (Gearheart 1993), and a high BOD would mean that the water requires
further treatment. Wetlands can handle an organic loading of up to 200 lbs/acre/day
without the BOD rate becoming too high and an even greater amount of organic matter
before the TSS becomes too high (Reed 1990).
There are several reasons that organic matter needs to be removed. The first
reason is that a high amount of suspended solids decreases the clarity of the water making
it difficult for aquatic organisms to catch prey. Another reason is that it can clog the gills
of fish and kill them. Also suspended particles block light needed by photosynthetic
organisms (Menesini 2006).
Nitrogen is removed by microbes which change ammonia to nitrate and nitrate
into nitrogen gas. Nitrogen is also fixed from the air by plants, and so removal of nitrogen
is most effective at higher levels of nitrogen input. Phosphorus has no atmospheric source
or sink, but is bound to the substrate of the wetland. Both of these nutrients are also taken
up by the plants, but unless the plants are harvested, they return to the water or soil on an
14
annual basis (Reed 1990). It is important that nitrogen and phosphorus are removed
because when the water is discharged into a natural water body with high amounts of
nutrients, eutrophication occurs. Eutrophication occurs when there is an excessive growth
of organisms, such as algae, causing a depletion of oxygen, which causes the death of
other organisms such as fish that need a certain amount of oxygen in the water to live. If
high amounts of nitrogen get into the drinking water, it is dangerous for humans because
nitrogen can bind to oxyhemoglobin in the blood preventing it from being able to carry
oxygen. This is called Blue Baby Syndrome because babies are affected more severely
than adults. Blue Baby Syndrome can cause the baby to have a blue tint because tissues
are deprived of oxygen (Park 2005).
Metals are primarily removed by sedimentation after chemical reactions cause
them to precipitate out. Some removal is also achieved by plant uptake. Metals should be
removed because they can be toxic to organisms (Menesini 2006).
Fecal coliform organisms such as E coli need to be removed from the wastewater.
Their presence indicates that other pathogens are likely to be present which could cause
diseases such as dysentery, typhoid fever and hepatitis A (US EPA 2006). The removal
mechanism for fecal coliform organisms is not exactly known but is thought to be one or
more of the following: sedimentation, adsorption, temperature ingestion or denaturing.
Wetlands remove 99% of fecal coliform organisms after about 6 days and 99.9% after 10
days (Gearheart 1993); therefore wetlands can effectively remove pathogens that are
hazardous to human health.
15
Each substance has a minimum hydraulic retention time or the time a particle of
water takes to travel through the wetland for the substance to be removed to acceptable
levels (Table3.1).
Table 3.1 Minimum HRTs (Crites 1990)
Substance removed Minimum Hydraulic Retention Time (HRT) to remove BOD 5-7 days TSS 5 days Nitrogen 3-5 days Phosphorus 21 days
There are two types of man-made treatment wetlands: subsurface flow and
surface flow. Both types of wetlands have two main similarities: having low energy
requirements because they use solar energy, and providing habitat for wildlife. As
mentioned previously, over half the wetlands in the United States have been destroyed.
Since wetlands are the
primary resting grounds
for migrating birds,
they have fewer and
fewer places to stop,
rest and eat. Wetlands
also provide habitat for
fish, mammals, reptiles
and amphibians.
Figure 3.4 Surface flow and subsurface flow wetland diagram (Mitsch 2000)
Subsurface flow
wetlands are made by
16
placing a layer of aggregate or gravel at a depth of 1.5 to 3.3 feet (Leverenz 2002). The
wastewater flows through the aggregate and the plants are rooted to it (Figure 3.1).
Surface flow wetlands consist of wastewater flowing across the soil and much of the
microbial activity happens on and around the stems of the plants (Gearheart 1993). A
surface flow wetland has the potential for problem with mosquitoes, but the subsurface
flow wetland’s water is underneath the aggregate, so it does not have a problem with
mosquitoes. However, the advantage of surface flow wetlands is that they are cheaper to
build than subsurface flow wetlands (Leverenz 2002).
Built Examples:
The Arcata Marsh is the
most well known wastewater
treatment wetland in
California. It provides tertiary
treatment for the entire city of
Arcata. Not only does it treat
wastewater, it also provides
wildlife habitat and its paths
are used extensively for public
recreation such as walking and bird watching (Figure 3.2). Arcata first built a small
experimental marsh in 1979 to test if a marsh could effectively treat their wastewater.
This experiment was successful and became the nursery for planting the Arcata Marsh
which was completed in 1986 (Couch 2007).
Figure 3.5 Bird watching in the Arcata Marsh (http://www.birdandhike.com/jlboone/Me/DCP_4793a.jpg)
17
Much of the wastewater treatment in Arcata is the same as conventional
wastewater treatment with the wetland providing tertiary treatment. First a bar screen
filters out items such as plastic or rags which cannot be composted. Then the solids settle
out in a clarifier. The solids go into an anaerobic digester which produces methane that is
used to heat the digester. During the summer, the excess methane is burned off in a flare.
If the marsh had more funds, they would get the equipment to use the excess methane to
generate electricity. The sludge is then placed in drying beds and composted with
hydrocotyle, a plant that is harvested from the wetlands. The compost is used as a soil
amendment in city parks (Couch 2007).
The wastewater goes into oxidation ponds where more solids settle out of the
water and organic matter within the water is broken down. Then the water is piped into
the marsh. It is treated with chlorine both before entering the marsh and after leaving it. It
is probably not necessary to disinfect the water twice, but must be done because it is a
city imposed standard. The main purpose of the marsh is to remove the algae from the
water. The marsh tries to maintain close to 100 % vegetation cover, so that little to no
sunlight reaches the water and no algae can grow. After the water flows through the
marsh, it is discharged into Humboldt Bay (Couch 2007). See Figure 3.3 for a map of the
Arcata Marsh and where the water flows.
18
Figure 3.6 Map of the Arcata Marsh (http://www.gamlyn.com/creative/c_3_arcatamarshmap.php)
The main differences between conventional wastewater treatment and the Arcata
Marsh are that the solids are used for compost instead of taken to the dump. Also, in the
final treatment step, instead of bubbling oxygen through the water in concrete tanks as in
conventional treatment, a wetland is used. The roots of the plants provide the oxygen and
the sun is the source of energy. The Arcata Marsh has two ponds where final effluent is
mixed with sea water and fish such as steelhead and cut throat trout are raised and
released into the wild (Couch 2007).
Table 3.2 Plants in the Arcata Marsh (Friends of the Arcata Marsh)
Common Name Scientific Name
Brass buttons Cotula coronopifolia Bulrush Scirpus acutus Broadleaf Cattail Typha latifolia Common mare’s-tails Hippuris vulgaris Common rush Juncus effusus Cordgrass Spartina densiflora
19
Duckweed Lemna miniscula Jaumea Jaumea carnosa Marsh pennywort Hydrocotyle ranunculoides Marsh rosemary Limonium californicum Pickleweed Salicornia virginica Saltbush Atriplex patula Sea arrow-grass Triglochin maritimum Umbrella sedge Cyperus eragrostis Water parsley Oenanthe sarmentosa Water plantain Alisma plantago-aquatica
There are three plants that dominate the marsh; two of which are useful (Figure
3.2). The marsh pennywort is an important plant because it provides food for the ducks
and the bulrush oxygenates the water. The cattail which has taken over much of the
marsh is unwanted in large quantities because it creates anaerobic conditions. Some
common birds that are found in the marsh are mallards, marsh wrens, northern harriers,
ospreys, peregrine falcons, song sparrows, snowy egrets, white-tailed kites and wood
ducks (Friends of the Arcata Marsh).
In Petaluma, California, a wastewater treatment wetland is currently being built
and should be finished in 2008. It used the Arcata Marsh as a model, but there are a few
differences. The Petaluma Wetlands disinfect their water with UV radiation instead of
chlorine (Petaluma Wetland Alliance (PWA) 2005). In addition, instead of discharging
the treated water into a bay, Petaluma plans on using it to irrigate city parks and golf
courses (Martin 2006).
20
CHAPTER 4
Design of Treatment Wetland
in Alamo, California
21
Alamo, California was chosen as the location for the theoretical design of a
wastewater treatment wetland. Alamo is a town located about one hour east of San
Francisco and has a population of 16,000 (City-Data.com 2007). The average temperature
ranges from 38 to 84 degrees Fahrenheit and the average precipitation during the year is
23.6 inches (Hoare 2007). The majority of the rainfall occurs during the winter, which
causes an additional amount of water to flow into the wetland during this time. Reuse of
the water for irrigation is difficult because there is more water available at a time when
plants do not need to be irrigated.
The site chosen for the Alamo Wetland is a 4.7 acre parcel of land in a residential
neighborhood. It was an empty lot which was recently turned into a neighborhood park.
The wetland is theoretically designed as if the city decided to build a treatment wetland in
the empty lot instead. The slope of the site is about 3.5% with the north end being higher.
According to a soil survey, the soil type on the site is Clear Lake Clay, a poorly drained
soil and Lodo Clay Loam, a well drained soil with sandstone and shale underneath (US
Dept. of Agriculture 1977).
The wetland will treat the wastewater from about 2,020 houses which is over 12%
of the town’s population. The wetland design is a subsurface flow wetland mainly
because subsurface wetlands have less odor than surface flow wetlands which would be
an advantage because it is in close proximity to residential neighborhoods. Also, the
proposed design will not have a problem with mosquitoes as there are no pools of water
exposed to the surface. (Leverenz 2002). The size of the aggregate is between 0.12 and
1.25 inches with the aggregate around the inlets is a little bigger with a maximum size of
22
2 inches. The larger size will prevent clogging (Crites 1998). The aggregate will consist
mostly of recycled concrete which has been broken up. One advantage of using recycled
concrete is that it would avoid having to take up landfill space for old concrete. Another
advantage is the cost factor because natural rock has become more expensive and scarce.
Quite often recycled concrete is available closer to the construction site, thus reducing
transportation energy and cost.
The site was re-graded (Figure 4.1) so that the wetland cells have a slope of less
than 0.75%. The cells could have been tiered, but are not so that the trail that runs
through the wetland is ADA accessible. ADA stands for the American Disabilities Act
and ensures that public places are accessible to people with disabilities. As can be seen on
the master plan, (Figure 4.2) and the section (Figure 4.3) the main trail is 6 feet wide and
winds from the parking lot through the cells and then loops back. There are also three
foot wide dirt paths around each cell for maintenance access. The site can also be
accessed from the north side via a ramp and the site can be walked through on the way
somewhere else. The slopes on the north and south sides of the site are planted with
vegetation both to prevent erosion and to make the site aesthetically pleasing.
23
FIGURE 4.1
GRADING PLAN
FRO
M SEW
ERIN
FLUEN
T PIPE
EFFLUEN
T PIPETO
GO
LF CO
URSE
Fig
ure
4.2
MA
STER
PLA
N
Wetland Cell Path PathWetland CellPath
Figure 4.3 Section
To determine the size of the wetland several equations were used.
To determine the surface are for a subsurface wetland the following equation was used:
A = (Q)(t)(3.07)/[(n)(d)]
A = Surface area in acres Q = Average daily flow through wetland in Mgal/day t = Detention time in days d = depth of water in wetland in ft n = plant based void ratio, 0.65 to 0.75 typically Since the site chosen for the wetland is a specific area, Q, the average daily flow was
solved for:
Q = (A)(n)(d)/[(t)3.07]
A = 2.66 acres (combined area of cells) n = .7 d = 3 ft t = 5 days Q = .364 Mgal/day (or 364,272 gal/day) (Crites 1998) In subsurface wetlands storms will probably not affect the performance of the wetland,
unless it is an extremely wet climate. In surface flow wetlands it would (EPA 1999).
To determine the how many residences the wetland could serve this equation was used:
26
Household flow = 40 gal/home/day + (35 gal/person/day)(number of persons/home) = 40 + (35)(4) = 180 gal/home/day Q/180 = 2023 residences (Crites 1998)
The experimental wetlands in Arcata took two years to become completely
vegetated (Gearheart 1993). Another source states that it takes 2-3 years for plants to
become established and 4-6 years for an adequate amount of litter and sediment to
develop (Mitsch 2000). The Alamo Wetland would be planted in late fall right before the
rainy season. This way the plants will have a chance to establish themselves before
wastewater is applied. In the spring, half of the houses will have their wastewater directed
through the wetland. The next spring the wetland vegetation will be more mature and the
other half of the houses will have their water piped to the wetland. If wastewater flows
are increased gradually, then plants will be better able to adapt to continuous flooding
(Crites 1990). The primary plant species used in the wetland is bulrush Scirpus acutus.
Also present are pickleweed, common rush and sedge, all of which are native to
California.
The proposed Alamo wetland is split into cells. If some cells need to be
maintained or drained, then the rest of the wetland can continue functioning. During the
winter when there is an increase in the amount of water, all of the cells will be used.
During the summer about half the cells will be used. Which ones are used will alternate,
creating dry periods. Season fluctuation in a wetland creates greater biodiversity. It also
will make the wetland more effective at treating the wastewater, because allowing it to
dry out will oxygenate the sediment that normally has no oxygen. The wetland is an
27
average of 3 feet deep, with interspersed deep and shallow areas to slow the water down
(Mitsch 2000).
The bank of the wetland has a slope of 5:1. Plants that are adapted to grow in
shallow water will thrive farther up the slope and different species of plants will grow in
the deeper water, creating greater biodiversity than one depth of water would. The slope
of the substrate should be less than 1% so that the water flows slowly through the
wetland. The ratio of the length to the width, also known as the aspect ratio should be a
minimum of 2:1 with longer being better (Mitsch 2000). The cells of the proposed Alamo
wetland have an aspect ratio of 6:1. Longer wetland cells are more expensive to build
because more berms have to be built, so the cells usually don’t get longer than 10:1.
There are multiple influent points where the wastewater enters the wetland so that the
wastewater is distributed throughout the wetland and multiple collection points. If there
were only one outlet for the water, then the velocity of the water would be too high
because water which was spread over a large surface area would be channeled to one
point.
No disinfection of the water is planned since the wetland should be able to
remove at least 99% of the fecal coliform organisms. The water that has passed through
the wetland will be monitored for fecal coliform organisms as well as BOD, TTS, heavy
metals and toxins. If a disinfection system is needed or if the residents demand it, then a
UV disinfection system can be installed.
The treated water from the wetland would then be used to irrigate the 88 acre
Round Hill Golf Course locate 0.7 miles away. Subsurface irrigation will be used so that
there will be very little human exposure to the water. In the winter when little to no
28
irrigation is needed, the water will percolate through the soil until it reaches the
groundwater. The golf course grass is Perennial Rye grass (Golf Link 2007) which is
moderately tolerant to salts and so is suitable for irrigation with recycled water (Asano
2007).
To determine the sustainability of this wetland, the Ecological Footprint model
was used. The Ecological Footprint is a concept developed by William Rees which
measures the land area needed to sustain a person or population of people based on their
resource consumption and waste produced. It looks at sustainability on a broader level
rather than just looking at one aspect such as loss of rainforest habitat or the extinction of
the tiger salamander (Wackernagel 1996).
The Ecological Footprint model divides land use into 4 categories (Table 4.1).
Table 4.1 Ecological Footprint model land categories (Wackernagel 1996)
Energy land is calculated to compare the sustainability of wetlands to conventional
treatment.
29
Table 4.2 Wastewater treatment energy use (Leverenz 2004)
Treatment process
Parameters Units
Conventional activated sludge
Subsurface flow constructed
wetland Total energy consumption
kWh/1000 gal 4 < 0.4
Fraction of energy used for aeration
% 56 0
Fraction of energy used for pumping
% 20 100
Fraction of energy used for other processes
% 24 0
The local power company gets 13% of its electricity from renewable sources
(Pacific Gas and Electric 2007). Renewable energy has an average ecological footprint of
0.1 hectares for 100 gigajoules(GJ) per year and fossil fuel has a footprint of 1.0 hectares
for 100 gigajoules per year (Wackernagel 1996). The wetland uses 0.4 kWh/1000 or a
total of 145.6kWh (.524 GJ) (Table 3.3). Conventional wastewater treatment uses
4kWh/1000gal which would mean 364kWh (1.31 GJ) to treat the same amount of water
as the wetland treats.
Wetland footprint calculation: 0.524 GJ(13%)(0.1/100) + 0.524GJ(87%)(1/100) = 0.004627 hectares or 0.0114 acres Conventional footprint calculation: 1.31 GJ(13%)(0.1/100) + 1.31 GJ(87%)(1/100) = 0.01157 hectares or 0.0286 acres
Conventional wastewater treatment’s ecological footprint is 40% larger than the
ecological footprint of the wetland. The area calculation is an underestimate because the
30
Ecological Footprint model cannot take every variable into consideration. For example,
the only pollutant in the model is carbon dioxide (Wackernagel 1996).
31
32
CCHHAAPPTTEERR 55
Research Conclusions
Conventional wastewater treatment takes up the least land area, and so it would
better for urban areas and areas where land is more expensive. Using standard treatment,
the cost for a small community to achieve the same level of treatment as large
communities is much higher (Gearheart 1993). The biggest obstacle to overcome for this
non-conventional type of wastewater treatment is the residents’ aversion to having waste
near their home, or out in the open and not contained. They would rather not think about
where their waste goes and what happens to it. Natural systems are thought to be inferior
to conventional systems, but the naturally treated water quality is equal to or better than
that of conventional systems (Table 5.1).
Table 5.1 Maximum effluent concentrations ( Leverenz 2004)
Treatment process
Parameters Units
Typical domestic
wastewater
Conventional activated sludge
Subsurface flow constructed
wetland BOD mg/L 190 10 10 COD mg/L 430 70 70 TSS mg/L 210 6 10 Ammonia-N mg/L 25 25 10 Total N mg/L 40 35 10 Total P mg/L 10 10 5 Turbidity NTU N/A 5 5 Fecal coliform CFU/100 mL 105 23 23
Different components of the various processes of wastewater treatment have
advantages and disadvantages. Some features can be more sustainable, such as finding
alternatives to using dangerous chemicals to disinfect the water. Disinfecting with
chlorine or ozone can have a negative affect on human health, because they form
carcinogenic compounds. UV light does not use any chemicals, but for it to be used to
33
kill pathogens in water, the water must be relatively clear and might have to be filtered.
Disinfecting wastewater with UV light uses more energy than conventional disinfection
techniques, but if a renewable form of energy is used, then it would be more sustainable.
One advantage that chlorine and ozone have over UV disinfection is that they stay in the
water and prevent pathogens for a period of time after disinfection occurs whereas UV
disinfection does not.
Using purified wastewater for irrigation provides an alternative to using potable
or drinkable water for irrigation. When treated wastewater is used for irrigation, the water
standards are more relaxed because it does not have to comply with aquatic toxicity
requirements. However, reclaimed water has a higher salinity and will eventually
increase the salinity of the ground water or not be appropriate for irrigation in some
situations (City of Davis 2003).
Each of the two types of engineered wetlands has advantages and disadvantages.
Subsurface flow wetlands have less odor, and prevent mosquito problems. Surface flow
wetlands create a better habitat for wildlife and are cheaper to build. Using wetlands to
treat wastewater takes up more land than conventional treatment, but uses less energy.
Conventional treatment is the best option in some urban situations where land is scarce.
Wetland treatment should definitely be considered for wastewater treatment in more rural
areas and for small satellite communities which are far away from a wastewater treatment
plant where it would be expensive to build a long pipeline. However, wetlands can also
treat the wastewater from an entire city as at the Arcata Marsh. The advantages of
wetlands also include attractive landscape, wildlife habitat creation, minimal sludge
34
generation, low operation and maintenance cost and educational uses (Ocean Arcs
International).
35
36
BBIIBBLLIIOOGGRRAAPPHHYY
Asano, T. (2007). Water Reuse. United States of America. Metcalf and Eddy, Inc. Brun, B. (2007). Heron’s Head Park gets Living Classroom. Sustainable Industries Journal. Committee on Characterization of Wetlands. (1995). Wetlands Characteristics and Boundaries. Washington, D. C. National Academic Press. Crites, R. W., Tchobanoglous, G. (1998). Small and Decentralized Wastewater
Management Systems. United States of America. McGraw-Hill Companies, Inc. France, R. L. (2003). Wetland Design. New York, NY. W. W. Norton & Company, Inc. Gearheart, R. A. (1993). Use of Constructed Wetlands to Treat Domestic Wastewater, City of Arcata, California p. 142-162. In US EPA. Municipal Wastewater Treatment Technology Recent Developments. Noys Data Corporation. Leverenz, H. (2002). Review of Technologies for the Onsite Treatment of Wastewater in California. Davis, CA. Center for Environmental and Water Resources Engineering Department of Civil and Environmental Engineering. University of California, Davis. Martin, G. (2006). Green Technology to make Sewage a Less Dirty Word. San Francisco Chronicle. Menesini, M. M. (2006). More than 35,000 Tons of Pollutants Kept Out of the Environment Last Year. Pipeline Vol. 10 No. 2. Contra Costa County Sanitary District. Mitsch, W. J. and Gosselink, J. G. (2000). Wetlands. Columbus, Ohio. John Wiley and Sons. Petaluma Wetlands Alliance (PWA). (2005). “Moving the Wastewater Treatment Plant to Construction.” Petaluma, CA. PWA Reed, S. C. (1990). Natural Systems for Wastewater Treatment. Alexandria, VA. Water Environment Federation. Todd, N. J. and Todd, J. (1994). From Eco-Cities to Living Machines: Principles of Ecological Design. Berkeley, CA. North Atlantic Books. United States Department of Agriculture. (1977). Soil Survey of Contra Costa Count, California. United States. National Cooperative Soil Survey.
37
United States Environmental Protection Agency (US EPA). (1999). Constructed Wetland Treatment of Municipal Wastewaters. United States. EPA. United States Environmental Protection Agency (US EPA). (1994). Great Lakes Report to Congress. United States. EPA. http://www.epa.gov/glnpo/rptcong/1994/glossary.htm Wackernagel, M. and Rees, W. (1996). Our Ecological Footprint. Canada. New Society Publishers. Other Sources: Bedar, M. A. and Sharp, T. W. (Producers), and Bedar M. A. (Director). (2000). Ecoparqué. [DVD] Environ Mental Productions. California Gray Water Law. (1997). http://www.owue.water.ca.gov/docs/Revised_Graywater_Standards.pdf City of Davis. (2003). Status Report on Municipal Wastewater Treatment Facilities. Davis, CA. City of Davis Department of Public Works. http://www.cityofdavis.org/pw/water/pdfs/wastewater-status.pdf City-Data.com. (2007). Alamo, California. http://www.city-data.com/city/Alamo- California.html Couch, D. (2007). Interview with senior wastewater technician. Arcata, CA Discharge Prohibitions. (2007). Central Contra Costa Sanitary District Collection System and WWTP Friends of the Arcata Marsh. Arcata Marsh and Wildlife Sanctuary. http://www.humboldt.edu/~ere_dept/marsh/ Golf Link. (2007). Round Hill Golf Course. Alamo, CA. Hillclimb Media.
http://www.golflink.com/golf-courses/golf-course.asp?course=1008 HRH, Prince of Orange. (2002). No Water No Future. World Summit on Sustainable Development. Johannesburg. http://www.nowaternofuture.org/ Landry, C. (1999). Water, water everywhere, waiting for a market. Orange County Register. http://www.perc.org/perc.php?id=162 Leverenz, H. (2004). Comparison of Natural, Conventional, and Technological Wastewater Management Alternatives for Water Reuse.
38
Natural Resources Defense Council. (2006). Consequences of Global Warming. Natural Resource Defense Council. http://www.nrdc.org/globalWarming/fcons.asp Ocean Arks International. http://www.oceanarks.org/ Office of Biorenewables Programs. (2007). Glossary of Biorenewables Terms. Iowa State University of Science and Technology. http://www.biorenew.iastate.edu/resources/glossary-of-biorenewables-terms.html Park, Julian, Finn, John and Cook, Richard. (2005). “Environmental Challenges in Farm Management.” United Kingdom. The University of Reading. http://www.ecifm.rdg.ac.uk/#Intro Pacific Gas and Electric Company adds More Geothermal Energy to Renewable Energy Mix. (2007). California. PG&E. http://www.pge.com/news/news_releases/q2_2007/070510a.html United States Environmental Protection Agency (US EPA). (2006). Monitoring and Assessing Water Quality. United States. EPA. http://www.epa.gov/volunteer/stream/vms50.html Water Environment Federation. (2007). Alexandria, VA www.wef.org/publicinfo/newsroom/wastewater_glossary.jhtml Hoare, R. (2007). World Climate. Butte and Tuttle Ltd. http://www.worldclimate.com/cgi-bin/data.pl?ref=N37W122+2200+040064C
39