Lecture 6b Sewage Treatment

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Lecture 6bSewage Treatment Constructed Wetlands
http//agen521.www.ecn.purdue.edu/AGEN521/epadir/w
etlands/graphics.html

• Using Wetlands for sewage treatment.
• By Jennie Swenson Terry Cooper

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On Site Sewage Treatment Systems Septic Tank
Systems
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1855-First U.S. sewage treatment system

• 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.

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/0508
17/050817-5.asp
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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
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Ocean Dumping
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.

Mirfield Sewage Sludge Incinerator -UK
Fertilizer
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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.
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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 basin- even a wet soil
can be used.
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Subsurface Flow System (SFS)

• Water flows below media- No water on soil
  surface but subsoil is saturated
• Sand, gravel, rock
• Grasses, trees
• Minimal land

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Subsurface Flow System
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Free Water Surface (FWS)

• Water flows over soil media
• Water lt18
• Sedges, reeds, rushes
• Land intensive

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Free Water Surface
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Aquatic Plant System (APS)

• Similar to FWS
• Water gt18
• Water hyacinth,
• duckweed,
• pennywort
• Fish

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Aquatic Plant System
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Constructed Wetland Scales
Subsurface Flow
Free Water Surface
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Major Mechanisms of Pathogen Removal

• Sedimentation
• Predation
• Adsorption
• Inactivation

• Bacteria -gt
• Viruses -gt

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Percent RemovalFecal Coliform

• Type Range Avg
• 29 Subsurface 99.9 - 78.2 97.6
• 8 Free Water 99.9 - 81.6 91.8
• 4 Aquatic Plant 98.5 - 43.2 79.5

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Reasons Cited for High Removal Rates

• Long retention time
• Low effluent loading rate
• Vegetation
• Increase microbial population
• Root excretions
• Aeration of media

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Reason Cited for Low Removal Rates

• Insufficient sunlight
• Lack of maturity
• Excessive wildlife
• High turbidity
• Resuspension of solids
• Water soluble humic substances

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Spring Hills Wastewater System Innovative
Technology

• 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.
• The unauthorized discharge needed to be
  corrected, but the cost of compliance was of
  great concern.
• Solution Spring Hills 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.
• 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.
• 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 Citys financial
  capability.
• With the application of constructed wetland
  technology, the costs became affordable.

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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
Outflow to lift station and drip irrigation
Wetland Cell Typical Cross Section
adaped from Widseth Smith Norlting and Associates
report dated 11/98
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Dyad Problem

• 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.
• Solution
• 1.33 x 62.4lbs/ft3 83lbs/ft3 30x40x8/12
  800ft3
• 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???

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Everstekoog constructed wetland
The End
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