Download Pathogen Removal in Constructed Wetlands

Lecture 6b
Sewage Treatment & Constructed Wetlands
http://agen521.www.ecn.purdue.edu/AGEN521/epadir/wetlands/graphics.html
Using Wetlands for sewage treatment.
By Jennie Swenson & Terry Cooper
On Site Sewage Treatment Systems – Septic Tank Systems
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The wastes generated by some
60% of the U.S. population are
collected in sewer systems and
carried along by some 14 billion
gallons of water a day.
Some 10% is allowed to pass
untreated into rivers, streams,
and the ocean.
The rest receives some form of
treatment to improve the quality
of the water (which makes up
99.9% of sewage) before it is
released for reuse.
1855-First U.S. sewage
treatment system
Untreated sewage discharge is a persistent
problem that seems to be getting worse in an
era of regulatory neglect.
http://enr.construction.com/features/_gallery/050817/050817-5.asp
Sewage Treatment Technology
• Saved more lives than
any other technological
development
• A sewage treatment plant
is nothing more than a
LARGE MICROBIAL
CULTURE FLASK
• The result of this process
converts most of the
nutrients to chemicals
like carbon dioxide,
nitrate, sulfate,
phosphate; i.e., minerals
• Raw sewage is rich in
organic nutrients such as
human excrement, and
food and industrial
wastes.
• Since microbes grow
and utilize nutrients
most efficiently under
AEROBIC CONDITIONS,
sewage treatment plants
are designed to provide
excess OXYGEN for the
microbes.
Hong Kong Sewage Treatment Plant
Land Spreading
• Finally, there is
always some
material that can
not be easily
degraded by
microbes which
SETTLES OUT at
various stages in
the treatment
process.
• This material is
called SLUDGE
and it must also
be disposed of as
part of the
sewage treatment
process.
Ocean Dumping
Mirfield Sewage
Sludge Incinerator -UK
Fertilizer
Alternative Sewage Technology
• Constructed
Wetlands
– Engineered
system
– Utilize
natural
processes
– Treat
wastewater
Constructed wetlands are small artificial wastewater treatment systems
consisting of one or more shallow treatment cells, with herbaceous
vegetation that flourish in saturated or flooded cells. They are usually
more suitable to warmer climates. In these systems wastewater is
treated by the processes of sedimentation, filtration, digestion,
oxidation, reduction, adsorption and precipitation.
3-System Designs
1) Subsurface
Flow System
2)Free Water
Surface
3)Aquatic
Plant System
The Water holding
structure is constructed
in basin or channel.
Some form of
subsurface barrier limits
seepage in first basineven a wet soil can be
used.
Subsurface Flow System (SFS)
•Water flows
below media- No
water on soil surface
but subsoil is
saturated
•Sand, gravel, rock
•Grasses, trees
•Minimal land
Subsurface Flow System
Free Water Surface (FWS)
•Water flows over
soil media
•Water <18”
•Sedges, reeds,
rushes
•Land intensive
Free Water Surface
Aquatic Plant System (APS)
•Similar to FWS
•Water >18”
•Water hyacinth,
duckweed,
pennywort
•Fish
Aquatic Plant System
Constructed Wetland
Scales
Subsurface Flow
Free Water Surface
Major Mechanisms
of Pathogen Removal
• Sedimentation
Bacteria ->
• Predation
Viruses ->
• Adsorption
• Inactivation
Percent Removal
Fecal Coliform
#
29
8
4
Type
Range
Subsurface
99.9 - 78.2
Free Water
99.9 - 81.6
Aquatic Plant 98.5 - 43.2
Avg
97.6
91.8
79.5
Reasons Cited for
High Removal Rates
• Long retention time
• Low effluent loading rate
• Vegetation
– Increase microbial population
– Root excretions
– Aeration of media
Reason Cited for
Low Removal Rates
• Insufficient sunlight
• Lack of maturity
• Excessive wildlife
• High turbidity
– Resuspension of solids
– Water soluble humic substances
Spring Hill’s Wastewater System – Innovative Technology
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Description: The City of Spring Hill, population 77, had nonconforming septic tanks
connected by a community sewer that ultimately discharged to the Sauk River
without further treatment.
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The unauthorized discharge needed to be corrected, but the cost of compliance
was of great concern.
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Solution: Spring Hill’s new wastewater treatment system consists of a subsurface
flow constructed wetland followed by disposal by drip irrigation. The treatment
system is capable of treating 9,200 gallons per day of domestic wastewater.
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The construction cost of the treatment and disposal system was approximately
$285,000. The sewage collection system, designed by the city engineer, added
another $310,000 to the total capital cost of the system.
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The original Preliminary Engineering Report recommended regionalization at a
capital cost of $805,000 plus approximately $200,000 of improvements at the
regional pond system. The cost of the original plan, at over $25,000 per
connection, was beyond the City’s financial capability.
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With the application of constructed wetland technology, the costs became
affordable.
Cross section of Spring Hill wetland treatment cell plan
List of plants include:
broadleaf cattail (Typha latifolia), hardstem bulrush (Scirpus acutus),
river bulrush (Scirpus fluviatilis), duck potato (Sagittaria latifolia), wild
iris (Iris versicolor), big bluestem (Andropogan gerardi), switchgrass
(Panicum virgatum),
gravel
mulch
Inflow from
septic tanks
rock
Wetland Cell Typical Cross Section
Outflow to
lift station
and drip irrigation
adaped from Widseth Smith Norlting and Associates report dated 11/98
Dyad Problem:
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Calculate the amount of soil erosion
from this 3 acre field in tons per acre.
BD = Mass/Vol
BD soil = 1.33 g/cc
Area of soil measured = 30 ft x 40 feet. The soil was 8 inches
thick over this area.
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Solution
1.33 x 62.4lbs/ft3 = 83lbs/ft3 & 30x40x8/12 = 800ft3
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83s/ft3 = wt/800ft3 = wt = 83x800 = 66400 lbs/3acres
= 22133 lbs./acre or ÷2000lbs/ton = 11 tons/acre or 2 x sustainable rate- of 5 tons / acre
note: some soil did leave the field and was not in our calculation, I wonder how much???