The Use of Wetlands as Water Treatment Systems

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The Use of Wetlands as Water Treatment Systems

• David ChervekBAE 558, Spring 2005

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Introduction

• Global population growth is creating a two-part
  problem with water supplies.
• An increase in the amount of potable water needed
  for consumption.
• An increase in the amount of wastewater created.
• A practical and cost-effective solution is needed
  that can treat the wastewater and protect the
  aquifers that the population relies on for their
  drinking water.
• Scientists and engineers have studied the water
  treatment effect of natural wetlands for many
  years, resulting in the development of
  constructed wetlands for treating wastewater.

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There are two types of constructed wetlands

• Free water surface wetlands, like most natural
  wetlands where the water surface is exposed to
  the atmosphere.

Photo courtesy of Earthpace Resources
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Subsurface wetlands, where the water surface is
below ground level.

Photo courtesy of USGS
The use of subsurface constructed wetlands for
water treatment began in Western Europe in the
1960s and in the U.S. in the 1980s. Research
and the use of constructed wetlands have
increased rapidly over the last 15-20 years.
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How Does It Work?
The basis for the hydraulic design of the system
is Darcys Law, Where, Q Flow rate in
volume per unit time. K Hydraulic conductivity
of the media. A Cross-sectional area of the
bed perpendicular to the flow. dh/dl The
hydraulic gradient.
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• To be able to use Darcys Law, a few assumptions
  need to be made.
• Uniform Flow The flow in a wetland wont be
  uniform due to precipitation gains and
  evaporation losses. Also, unequal porosity may
  cause preferential flow. To allow the use of
  Darcys Law, these issues can be mitigated by
  using the average Q and careful construction of
  the bed to minimize preferential flow in the bed.
• Laminar Flow A very coarse media with a high
  hydraulic gradient will result in turbulent flow.
  By keeping the media size below 4 cm or designing
  for minimal hydraulic gradient, laminar flow can
  be assumed.

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From Ogden, M., Constructed Wetlands For
Wastewater Treatment

• The typical subsurface system consists of,
• Liner
• Inlet structure
• Bed (including media and plants)
• Outlet structure
• Liner
• The liner goes under the entire system and can be
  a manufactured liner or clay.
• This prevents the wastewater from infiltrating
  into the ground before it is treated.
• A berm around the system prevents runoff from
  entering the system.

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• Inlet
• The inlet can be a manifold pipe arrangement, an
  open trench perpendicular to the flow, or weir
  box. The manifold arrangement can be a pipe with
  several valve outlets or a simple perforated
  pipe.
• Coarse gravel allows rapid infiltration of the
  water.
• The inlet purpose is to spread the wastewater
  evenly across the treatment bed for effective
  treatment.

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Bed

• Media
• Many different media sizes have been tried for
  the bed, but gravel less than 4 cm diameter seems
  to work best.
• Larger diameters increase the flow rate, but
  result in turbulent flow, precluding the use of
  Darcys Law for design.
• Smaller media gives a reduced hydraulic
  conductivity, but has the advantage of more
  surface area for microbial activity and
  adsorption.
• Soil is sometimes used to remove certain
  materials due to the ability of reactive clays to
  adsorb heavy metals, phosphates, etc. The
  tradeoff is a greatly reduced flow rate.
• The depth of the media is usually between 1-3
  feet and most commonly 2 feet.

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Bed

• Slope
• Systems have been designed with bed slopes of as
  much 8 percent to achieve the hydraulic gradient.
  Newer systems have used a flat bottom or slight
  slope and have employed an adjustable outlet to
  achieve the hydraulic gradient.
• Aspect Ratio
• The aspect ratio (length/width) is also
  important. Ratios of around 41 are preferable.
  Longer beds have an inadequate hydraulic gradient
  and tend to result in water above the bed surface.

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Bed

• Plants
• Three types of plants are normally used
• Cattails, which are a favorite food of muskrats
  and nutria.
• Bulrush is also high on the mammals food list,
  but they should not be attracted to the wetland
  if the water surface is kept below the media.
• Reeds are used most often in Europe because they
  are not a food source for animals. However, they
  are not allowed in some areas due to their
  tendency to spread and push out native
  vegetation.
• The type used will also depend on the local
  climate and the substances to be removed.
• In some instances decorative plants are used,
  but results show them to be less effective and
  require more maintenance.
• Control of the water level can be used to
  increase root penetration and control weeds.

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• Outlet
• The outlet structures used are similar to the
  inlet structures.
• One preferred addition is making the outlet
  adjustable to allow the control of water level.
  The level could be lowered when a large amount of
  rainfall is expected or raised for maximum
  cross-sectional use of the media.

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Treatment

• Wetlands treat water in the following ways,
• Filtration and sedimentation Larger particles
  are trapped in the media or settle to the bottom
  of the bed as water flows through. Because these
  systems are normally used with a pretreatment
  system, such as a septic tank or detention pond,
  this is a small part of the treatment.
• The main treatment processes are,
• The breakdown and transformation by the microbial
  population clinging to the surface of the media
  and plant roots
• The adsorption of materials and ion exchange at
  the media and plant surfaces.
• The plants in the bed also provide oxygen and
  nutrients to promote microbial growth. The rest
  of the bed is assumed to be anaerobic.

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• The subsurface wetlands have proved to be
  effective at greatly reducing concentrations of,
• 5-day biochemical oxygen demand (BOD5)
• Total suspended solids (TSS)
• Nitrogen
• Phosphorus
• Fecal Coliforms
• Wetlands have also shown the ability for
  reductions in metals and organic pollutants.

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Biochemical oxygen demand is a measure of the
quantity of organic compounds in the wastewater
that tie up oxygen. BOD5 is removed by the
microbial growth on the media and the plant
roots. BOD5 is the basis for determining the area
of wetland required using a first order plug flow
(first in, first out) model. Where, Ce
Effluent BOD5 (mg/L) Co Influent BOD5
(mg/L) KT K20(1.06)(T-20) Temperature
dependent rate constant (d-1) K20 Rate
constant at 20B C 1.04 d-1 t Hydraulic
residence time (d) T Temperature of liquid in
the system (BC)
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The hydraulic residence time, t, can be
determined from the following equation,
Where, n The porosity of the media as a
fraction A The area of the bed (m2 or ft2) d
Average depth of liquid in bed (m or ft) Q
Average flow rate (m3/d or ft3/d)
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Combining these equations and rearranging,
results in an equation for the required
area, Note that the area required is
inversely proportional to the temperature, thus
the system should be designed for the coldest
temperatures to be encountered. The majority of
BOD5 is removed in the first couple of days in
the system and longer hydraulic retention times
(HRT) do not result in significant additional
removal. Reductions of up to 90 have been
achieved. Can the system ever achieve 100
removal? No, because some BOD5 is actually
created by the plant litter and other organic
materials. As a result, the above equations
cannot be used for final design BOD5 lt 5 mg/L.
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• TSS
• The results for TSS removal have been similar to
  BOD5 in that the majority is removed in the first
  few feet of the bed (or first couple of days) and
  a system properly sized for BOD5 removal would be
  properly sized for TSS removal.
• Nitrogen
• The removal of nitrogen in the form of ammonia
  and organic nitrogen requires a supply of oxygen
  for nitrification. This oxygen usually comes from
  the plant roots. Plant roots that do not
  penetrate close to the full depth of the bed
  leave a large anaerobic area and hence, a low
  reduction in ammonia. Oxygen can be added
  mechanically, but that increases costs. However,
  it may be feasible if significant ammonia
  reduction is a priority.
• There is actually the possibility of an increase
  in ammonia due to anaerobic decomposition of the
  organic nitrogen.
• Retention time is also a factor in ammonia
  removal in that a longer HRT can significantly
  increase the ammonia removal.
• Reductions of 90 plus have been achieved with
  full penetration of the plant roots and a HRT of
  7 days.

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• Phosphorus
• Significant phosphorus removal requires some
  tradeoffs due to the large contact areas needed
  for phosphorus retention. For significant
  phosphorus removal, sand or fine river gravel
  with iron or aluminum oxides is needed. These
  finer materials with their lower hydraulic
  conductivity require larger areas and may not be
  feasible if that is not a major goal.
• Fecal Coliforms
• One log to two log reductions in fecal coliforms
  have been achieved.
• This is usually not enough to satisfy local
  regulations, however, so some sort of after
  treatment is needed.
• The reduction is enough to significantly reduce
  the scope of the after treatment process.

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How do we determine the size?

• Lets look at an example.
• Say we want to design a system for a family of
  four. The BOD5 coming out of the septic tank is
  100 mg/L and we want to reduce it to 10 mg/L.
  What size system do we need?
• Criteria
• Flow rate for a family of four is 360 gal/day or
  48.1 ft3/day.
• A 2 feet deep bed with an effective liquid depth
  of 1.8 feet.
• The media is small gravel with a hydraulic
  conductivity of 5000 ft3/ft2/day and a porosity
  of 0.34.
• The temperature of the water going through the
  system is about 20B C (68B F).
• Our equation is,
• As stated above, we would like the aspect ratio
  to be around 41.
• This would result in a bed about 6.6 feet wide
  and 26.4 feet long.

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• Are we ready to build?
• Not just yet. We still need to apply Darcys Law
  to make sure the system can handle the flow we
  need. We will assume the bed is not sloped, so
  our hydraulic gradient is 0.005. If the bed were
  sloped 1 to 2 degrees, the gradient would be 0.01
  to 0.02. Applying Darcys Law,
• Plenty of capacity, but it is actually too high.
  The water may not be deep enough to reach the
  plant roots or may flow through too fast to be
  properly treated. You may try a finer media. If
  the capacity had been less than the required
  flow, surface flow would be possible and again
  proper treatment would not be achieved. This is
  an iterative process where you need to adjust
  length, width, slope, media, etc. until you
  achieve the proper flow. You want the capacity to
  be a little above the actual flow rate to account
  for peaks from precipitation.

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Wetland Treatment Applications

• What types of wastewater can be treated with
  constructed wetlands?
• Domestic wastewater
• Storm water runoff from parking lots or farmland
• Wastewater from livestock operations
• Wastewater from mining and oil operations
• Landfill leachate
• For the most common current use, treating
  domestic wastewater, the wetland is usually used
  in conjunction with a pretreatment process such
  as a standard septic tank. The septic tank
  removes the larger suspended solids to make the
  wetland more efficient and reduce the chance of
  the media getting clogged. The wetland outflow
  can then be sent to a standard leaching field for
  final treatment

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• Constructed wetlands offer several advantages
  over tradition water treatment systems.
• Wetlands are less expensive to build and operate
  than mechanical systems.
• There is no energy required to operate a wetland.
• Wetlands are passive systems requiring little
  maintenance. Normally, the only maintenance
  required is monitoring of the water level and
  rinsing the media every few years to remove
  solids and restore adsorption capacity.
• Wetlands can also provide wildlife habitat and be
  more aesthetically pleasing than other water
  treatment options.
• Subsurface wetlands produce no biosolids or
  sludge that requires disposal.

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• The advantages of a subsurface wetland over the
  free water surface wetland include,
• No exposed water surface to attract mosquitoes or
  for people to come in contact with.
• Fewer odors.
• Due to the greater surface area in contact with
  the water and greater root penetration of the
  plants, subsurface systems can be significantly
  smaller. Although the media cost can be
  expensive, it is usually offset by the smaller
  land area required, resulting in a lower cost for
  the subsurface system.
• Better performance in colder climates due to the
  insulating effect of the upper media layer.

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• However, free water surface systems may be
  preferred in some instances,
• In areas where land is cheap and media costs
  high, a free water surface system can be cheaper.
• Free water surface systems are normally cheaper
  for larger systems (gt60,000 gal/day).
• The subsurface systems are more suited to
  relatively constant flow, so free water surface
  systems may be preferred for storm runoff systems
  where peak flows are much larger than the average
  flow.
• There is no single design that gives maximum
  reduction on all contaminants. The target
  reductions will determine what plants are used,
  what media is used, the HRT, etc.

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• What are the disadvantages?
• For wetland systems in general, the amount of
  land required. Some locations may not have the
  appropriate space.
• The effectiveness will vary with temperature.
• For subsurface wetlands, there is limited
  wildlife habitat created as compared to the free
  water surface system. Due to the water surface
  being below ground, there is little wildlife
  habitat created and its main use is as a water
  treatment system.

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What questions remain?

• Whether to use the same plant throughout or a
  combination of plants?
• And in what quantity?
• Are there other plants that may be more
  effective?
• How to size the systems for different climates?
• How long will a system last?
• How do we remove more ammonia at a lower HRT?
  Promising research is being done on a
  recirculating system above the bed to increase
  ammonia removal.
• Can we develop more sophisticated models for
  design?

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References
Duggan, J., Bates, M.P., Phillips, C.A. 2000, The
efficacy of subsurface flow reed bed treatment in
the removal of Campylobacter spp., faecal
coliforms and Escherichia coli from poultry
litter, International Journal of Environmental
Health Research 11, pp. 168-180 (2001) Dusel,
Jr., C.E., Pawlewski, C.W., 2000, Constructed
Wetlands Offer Flexibility, Land and Water,
Inc. Joy, D., Weil, C., Crolla, A., Bonte-Gelok,
S., 2000, New technologies for on-site domestic
and agricultural wastewater treatment, Can. J.
Civ. Eng. 28(Suppl. 1) pp. 115-123
(2001) Kaseva, M.E., 2003, Performance of a
sub-surface flow constructed wetland in polishing
pre-treated wastewater a tropical case study,
Water Research 38, pp. 681-687 (2004) Mink, L.,
2002, Use of surface and subsurface wetlands for
treatment of municipal waste water, Research
Extension Regional Water Quality Conference
2002 Murray-Gulde, C., Heatley, J.E., Karanfil,
T., Rodgers, Jr., J.H., Myers, J.E., 2002,
Performance of a hybrid reverse
osmosis-constructed wetland treatment system for
brackish oil field produced water, Water Research
37 pp. 705-713 (2003) Nelson, M., Alling, A.,
Dempster, W.F., van Thillo, M., Allen, J., 2003,
Advantages of using subsurface flow constructed
wetlands for wastewater treatment in space
applications Ground-based Mars base prototype,
Adv. Space Res. Vol. 31, No. 7, pp. 1799-1804
(2003) Ogden, M, 2000, Constructed Wetlands For
Wastewater Treatment Reed, S. C., U.S. EPA 1993,
Subsurface Flow Constructed Wetlands For
Wastewater Treatment, A Technology
Assessment Sim, C.H. 2003, The use of constructed
wetlands for wastewater treatment, Wetlands
International Malaysia Office, First
Edition U.S. EPA, 2000, Wastewater Technology
Fact Sheet, Wetlands Subsurface Flow Ward, A.D.,
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Second Edition, Lewis Publishers, Boca Raton, FL