WETLANDS CREATION USING SECONDARY TREATED WASTEWATER
J. Mitch Allan
INTRODUCTION
The concept of using treated sewage effluent as a freshwater
source for the creation and restoration of wetland ecosystems qualifies as an
alternative wastewater management technology for meeting the objectives of the
1977 Clean Water Act Amendments promoting the use of land treatment processes
that reclaim and reuse municipal wastewater. Wetlands reclamation projects are
cost effective and in this particular case will allow the treatment facility to
bypass more expensive dilution requirements. Depending on site conditions,
energy requirements are minimal compared with landscape irrigation,
agricultural, or industrial reclamation projects often requiring extensive
pumping or additional treatment. Wetlands projects are also consistent with
EPA's multiple use policy supporting wastewater management practices which
combine open space, recreational and education considerations with such
management.
During the past century, over eighty percent of the wetlands within the San
Francisco Bay region have been drained or filled to make way for agricultural,
residential, industrial and other uses. In addition, massive diversions of
freshwater to the Central Valley and Southern California threaten the remaining
natural wetlands, particularly the upper reaches of the estuary, which are
subject to reduced outflow of fresh water and increased salinity levels. This
loss of wetland habitat has resulted in greatly reduced wildlife and migratory
waterfowl populations throughout the bay region. In 1974 the Mt. View Sanitary
District (MVSD), near Martinez, California initiated a full-scale pilot wetlands
enhancement program on their low-lying reclaimed tidelands. The objective of the
program was to demonstrate the feasibility of utilizing sanitary plant effluent
to create a wetlands environment for wildlife and migratory waterfowl habitat.
Management techniques were tested to improve water quality and wildlife habitat
and for recreation and education. The unique aspects of this project is the use
of reclaimed wastewater as the sole hydrologic source for the site.
PROJECT OVERVIEW
The restoration area is located adjacent to the Mt. View Sanitary
Facility and is 20.3 acres (8.2 ha). The wetlands plots cover 15.2 acres (6.1
ha) with a capacity of 3.8 acre feet providing a 4.8 day retention time at plant
flow of 1.6 million gallons a day. The site consists of five interconnected
wetland ponds with tributary edge habitat. All of the treatment plant discharge
passes through the ponds and marshes into Peyton Slough which then discharges
into tidal waters of Suisun Bay through a tide gate.
This area was originally
a tidal marsh/estuarine complex with saline water flowing in from the San
Francisco Bay. The soil was predominantly saline. Little of this original area
is still present. The restoration area is currently surrounded by a Chevron oil
processing facility, a county dump, an eight lane interstate highway and a small
residential community. The nearest wetland area is on the other side of the
interstate. The project area has no hydrologic connection to this natural wetland.
All of the wastewater flowing through the wetlands complex is powered by gravity; no
pumps of any kind are used. Since a prime design objective is to provide a
combination of open water and vegetated habitat, this method of channelization
directs the flow through the vegetation and provides for circulation. A series
of weirs between each of the wetlands allows for manipulation of water flow.
METHODS
The engineering involved in this project took the view that a
well-designed project is one that requires little maintenance and can avoid
nuisance problems. The purpose of the wetland construction was to provide
wetland habitat for waterfowl and help improve water quality and not necessarily
to mimic other natural biomes. Construction took place primarily with an
emphasis on the physical structure of the wetlands and the expected hydrologic
flow. The following are aspects of the restoration of primary importance to the
engineers.
Levees - The area was divided into six wetland plots separated by
levees. Each of the levees are at least ten feet wide, steep-sided and compacted
during construction. Many wetland organisms such as muskrats, gophers,
crayfish and other small mammals tunnel into levees. Proper design, to
discourage degradation and minimize maintenance was crucial.
Plot Design - A multiplot system of wetlands was created to allow for
variations in depth and residence time. Depth determines
whether emergent vegetation or open water will be present. Depth and vegetation
will also effect temperature and dissolved oxygen. Keeping dissolved oxygen
values high is important to reduce odor problems that can occur in anaerobic
conditions.
Disease Vectors- Avian botulism (Clostridium botulinum) is a deadly
waterfowl disease that can be prevented by removing organic debris that collects
behind the weirs, assuring circulation and avoiding anaerobic conditions.
Another vector problem concerns the presence of mosquitoes. Originally this
problem was addressed by attempting to avoid stagnant water through
manipulations in the hydrologic design. This approach was only moderately successful and
as a result mosquito fish (Ganbusi afinis) were added to the wetlands. The addition of mosquito fish
caused an immediate drop in the mosquito populations. It also caused a drop in
the presence of zooplankton. This decrease in zooplankton grazing on algae
caused a significant increase in algal abundance.
RESULTS
The structural aspects mentioned in the methods section were all that
was done to restore this area. Vegetational secession and establishment of
wildlife occurred primarily unassisted and unmanaged. The results discussed
below were quantified two years after construction of the wetland complex was
completed.
Both submerged, emergent and terrestrial macrophytes had
established themselves in the restoration area. Sixty-seven species were
identified on the site and none were planted by the MVSD. Twelve of these
identified species were emergents including Typha spp., Scirpus spp. and Carex
spp.. A relatively low abundance of native plants were present on the site. Only
twenty-nine of the sixty-seven species were native to California. Whether this
low native density was representative of the surrounding landscape or was a
function of the restoration method was not evaluated.
The presettlement
condition of this area as a brackish marsh is still evident in the saline
quality of the soil. Ten of the identified species were considered saline
tolerant. The remaining plants in the site are field annuals, perennials,
herbs and shrubs. The vegetation serves as food and habitat for a variety of
animals. These also improve water quality through increased nutrient cycling.
The surface area of the wetlands is approximately 63% open water combined with
37% covered with emergent vegetation. Little or no upland
habitat is present with most of the ponds being enclosed by steep levees.
Algal growth
within the wetlands has been prodigious and beneficial. The growth of algae
helps to remove ammonia from the water, oxygenate it and act as a food source
for the zooplankton. Algal growth is highest in the summer. Few problems with
nuisance algae, such as filamentous mats, blue-green growth
or other odor producers has occured. Such growths can be a problem in nutrient rich environments.
Algal density increased significantly with the introduction of the mosquito fish
which preyed on algae grazing zooplankton. In future years increased predation
on the mosquito fish by egrets may help restore a balance with less algal
abundance.
Twenty-one species of large vertebrates, not including birds, live
at the MVSD wetlands; ten species of mammals, four species of amphibians, four
species of reptiles, three species of fish. One study found heavy use of the
levees by mice (Mus musculus and Reithrodontomys megalotis) and muskrats
(Ondatra zibethica). All of the animals present came to the MVSD wetlands on
their own.
Eighty-five species of birds either live in or stop at the wetlands
to feed during migration, a relatively large variety for such a small area,
especially when compared with the surrounding, highly developed, area. Of the
eighty five species 15 are ducks, 32 species of water and shorebirds, 30
passerine species, and 6 species of raptors. There are a number of bird families
that have inhabited the wetlands for several generations.
More than 34 species
of aquatic invertebrates live in the wetlands including bugs, beetles, flies and
zooplankton. The volume of invertebrates and rate of reproduction are
impressive. During the summer of 1977, up to 3.8 lbs/hr of zooplankton (mainly
Daphnia) were trapped in an outlet weir. There has been some interest in selling
this food source to local fish stores for supplementary income.
Biochemical
oxygen demand (BOD), a measure of the amount of oxygen needed by microbes
engaged in the process of organic decomposition, was used as a quantitative
measure of water quality. Water entering and leaving the wetlands tended to be
equal in BOD. Although the BOD was equal at the input
and output of the system it is important to note that human waste is the primary
organic substrate at the input and algae serves this purpose at the outflow This
shows that nutrient cycling is occurring within the wetland.
The dissolved
oxygen (DO), a measure of oxygen dissolved in the water, varies diurnally and
seasonally. The highest DO occurs in the summer during periods of peak algal
production and the lowest DO occurs in the winters in early morning. An overall
increase of DO in the effluent is due primarily to algae. Suspended Solids
(SS), the turbidity of the water, was also measured to assess water quality. SS
has increased and this is due mainly to higher densities of algae and not
inorganic sources such as silt and clay that can have negative effects on the
ecosystem. Nitrogen concentrations measured as nitrate, ammonia and organic
nitrogen significantly decrease as water flows through the wetlands system.
Phosphorous (measured as PO4) decreases as well, probably accumulating in the
sediment and plants.
CONCLUSIONS AND CRITIQUE
The Mt. View Sanitary District was fairly successful
in meeting the goals of habitat creation for megafauna. A great number and
variety of species use the wetlands either permanently or temporarily. The
wetlands have also proven to be beneficial to the larger community that uses
this site for recreational wildlife observation. The wetlands are a popular site
for the local chapter of the Audubon Society. This environment also provides an
educational environment for local high schools and colleges.
The designers
were moderately successful in improving the water quality of the treatment plant
effluent. There has been an overall increase in the suspended solids of the
effluent. This natural turbidity has less of a detrimental
effect than inorganic suspended solids would. The net decrease in nitrogen is
very beneficial to the water quality and serves as evidence that this tertiary
treatment provides functional as well as aesthetic benefits.
The in house
analysis of the project (Demgen & Nute, 1979) states that the designers do not
feel that phosphorous is the limiting nutrient in this environment, yet there is
no documented case of blue-green algae blooms, a common occurrence in nitrogen
limited environments. Long term phosphorous build up in the sediment and
vegetation is an issue that is important and unfortunately not addressed. Will
there be periodic harvesting of the vegetation or removal of the sediment?
Discussion with current scientists working at the site show that they are
looking at disposing of this excess phosphorous tied up in vegetation through
incineration or composting. The difficulty of composting is the potentially high
concentrations of heavy metals that are found in the municipal waste stream.
This raises other issues of the safety of effluent water for use by wildlife.
Although no detrimental effects have been observed there is very little research
in the area of how pollutants could potentially effect the physiology of
organisms on the site.
Another issue not addressed in their analysis
is the chlorination of the effluent previous to its entry to the wetland. This
provides reduced coliform concentrations in the effluent but may prove
detrimental to the wetland environment. They recently remedied this problem by
installing a UV radiation treatment to the effluent treatment process, negating
the need for chlorination.
Although much information was given on the
structural aspects of the wetland, I would be very interested in finding out
more about the functional processes that are occurring. Nutrient cycling,
organismal interaction, annual vegetational surveys, nesting success of
waterfowl and rates of predation are all aspects that could provide a better
picture of this wetland restorations success or failure. These aspects
are outside the scope of the project as it was intended but may provide more
information on the success of this project in areas other than those defined by
its progenitors.
One of the greatest successes of this project comes from the
impact it can have on the desirability of wetland restorations. This restoration
is one of the unique situations where a wetland environment was established for
reasons other than mitigation or replacement. The project is also a recognition
that natural systems have the ability to function beneficially in a utilitarian
as well as aesthetic manner.
LITERATURE CITED
Demgen F. and W. Nute, Welands Creation Using Secondary Treated Wastewater,
American Water Works Assoc. Proceedings, 1979 V.1, p. 727-739
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