2. Introduction
Global population growth is creating a two-part problem with water
supplies.
โฆ An increase in the amount of potable water needed for
consumption.
โฆ 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.
Some Facts & Figures:
๏ People generate 50-100 gallons of wastewater every day.
๏ Comes from sinks, showers, toilets, dishwashers, laundry, factory
waste, food service waste, and shopping centers
๏ Mostly water with organic solids and other things that are flushed.
3. Constructed wetlands offer several
advantages over tradition water treatment
systems:
๏ฎWetlands are less expensive to build and operate than mechanical
systems.
๏ฎThere is no energy required to operate a wetland.
๏ฎWetlands 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.
๏ฎWetlands can also provide wildlife habitat and be more aesthetically
pleasing than other water treatment options.
๏ฎSubsurface wetlands produce no biosolids or sludge that requires
disposal.
4. What are Constructed wetlands
๏ 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.
The constructed wetlands generally
consist of six chambers
โฆ Each chamber consists of four cells:
โฆ Within each cell are water hyacinth
plants
The constructed wetland removes
solids, dissolved solids, nutrients,
and pathogens.
5. 4. Perforated drains, drain rock,
texture transition and sand filter
1. Excavation and Forming
2. Waterproofing: Base, geo-textile
membrane
3. Distribution piping
CONSTRUCTION & INSTALLATION
7. 1. Free water surface(FWS)
wetlands, like most natural wetlands are
those where the water surface is exposed
to the atmosphere. Water flows over soil
media.
A channel (flow bed) is dug and lined with
an impermeable barrier such as clay or geo-
textile. The flow bed is then covered with
rocks, gravel and soil. Vegetation is also
planted. It is better to have plants that are
native to the area. After that the wastewater
is let into the flow bed by an inlet pipe. The
usual depth of the wastewater is 10 to 45cm
above ground level. As the water slowly
flows through the wetland, simultaneous
processes clean the wastewater and the
cleaned water is released through the outlet
pipe.
11. โข In this, Water flows below
media.
โข No water on soil surface but
subsoil is saturated
2. Subsurface wetlands,
where the water surface is below
ground level.
The use of subsurface constructed
wetlands for water treatment
began in Western Europe in the
1960โs and in the U.S. in the
1980โs.
*Photo courtesy of USGS
13. dl
dh
KAQ ๏ญ๏ฝ
The basis for the hydraulic design of the system is Darcyโs 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.
14. Typical SubsurfaceWetland System consists of :
๏ Liner
๏ Inlet structure
๏ Bed (including media and plants)
๏ Outlet structure
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 4:1 are
preferable. Longer beds have an inadequate hydraulic gradient and tend to
result in water above the bed surface.
15. Filtration and sedimentation โ 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.
Wetlands treat water in the following ways
16. The Subsurface Wetlands have proved to be effective at
greatly reducing concentrations of following parameters :
๏ 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.
17. 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)
๏จ ๏ฉtK
o
e T
e
C
C ๏ญ
๏ฝ
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)
Q
nAd
t ๏ฝ
18. 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.
.
๏จ ๏ฉ๏ ๏
dnK
CCQ
A
T
eo /ln
๏ฝ
19. TSS
๏ฎThe 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
๏ฎThe 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. Nitrate removal in a wetland takes place by plant uptake, de-nitrification
and microbial processes. A number of factors affect the rate of nitrate removal,
including hydraulic loading rate/hydraulic retention time, concentration of nitrate
in the inflow water, temperature of the water, soil conditions, vegetation
processes, and flow characteristics in the wetland.
Phosphorus
๏ฎFor 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
๏ฎThis is usually not enough to satisfy local regulations, however, so some sort of
after treatment is needed.
๏ฎThe reduction is enough to significantly reduce the scope of the after treatment
process.
20. Plants in FWS Wetlands (Macrophytes)
๏ The type of plant does not matter because primary role is providing structure
for enhancing flocculation, sedimentation, and filtration of suspended solids
๏ Even though plant type does not matter much, there are some common
varieties-
Sedges, Water Hyacinth, Common Cattail, Duckweed, Spatterdock, Waterweed
๏ In the past monocultures or a combination of two species were used
๏ Currently more diverse representative of natural ecosystem plantings occur
๏ The presence of macrophytes is one of the most conspicuous features of
wetlands and their presence distinguishes constructed wetlands from
unplanted soil filters or lagoons. The macrophytes growing in constructed
wetlands have several properties in relation to the treatment process that
make them an essential component of the design (Brix, 1997).
21. ๏ฎNo exposed water surface to attract mosquitoes or for people to come in
contact with.
๏ฎFewer odors.
๏ฎDue 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.
๏ฎBetter performance in colder climates due to the insulating effect of the
upper media layer.
Advantages of Subsurface Wetland(SSW)
over Free Water Surface wetland(FWS)
22. Literature Review
๏ It has been accepted as a low cost eco-technology alternative to
conventional treatment methods (especially beneficial to small
communities that cannot afford expensive treatment systems) and been
increasingly accepted by the general public for reuse purposes ( Green
and Upton 1995; White 1995 ; Billore et al. 1999 ; Fenxia and Ying
2009 ; Friedler 2008 ) .
๏ Constructed wetlands (CW), are now widely used as an accepted
method of treating wastewater (Gopal, 1999; Kivaisi, 2001; Vymazak,
2007; Rousseau et al, 2008) and are cheaper than traditional
wastewater treatment plants.
๏ This growing popularity has been largely due to the fact that pond and
wetland based systems offer the advantages of providing a relatively
passive, low-maintenance and operationally simple treatment solution
while potentially enhancing habitat, recreational, and aesthetic values
within the urban landscape (Knight et al., 2001; Lee and Li, 2009;
Rousseau et al., 2008).
๏ Because of high removal efficiency, low cost, water and nutrient
reuse, and other ancillary benefits, CTWs have become a popular
option for wastewater treatment (Ghermandi et al. 2007; Rousseau et
al. 2008; Llorens et al. 2009; Kadlec 2009). The design for CTWs is
based on free-water surface flow (FWS), horizontal subsurface flow
(HF), or vertical subsurface flow (VF) (Kadlec 2009)
23. ๏ Constructed treatment wetlands (CTWs) are used to remove a wide
range of pollutants such as organic compounds, suspended solids,
pathogens, metals, and excess nutrients (e.g., N and P) from various
wastewaters including stormwater runoff and municipal wastewater
(Ghermandi et al. 2007; Vymazal 2007; Snow et al. 2008; Cooper
2009; Kadlec 2009).
๏ In FWS-CTWs, removal efficiencies above 70% can be achieved for
total suspended solids (TSS), chemical oxygen demand (COD),
biochemical oxygen demand (BOD), and pathogens, primarily bacteria
and viruses (Kadlec and Wallace 2009). Removal efficiencies for N
and P are typically 40โ50% and 40โ90%, respectively (Andersson et
al. 2005; Vymazal 2007).
๏ Wetlands are extremely valuable to society. Wetlands can decrease
flooding , remove pollutants from water , recharge groundwater,
protect shorelines, provide habitat for wildlife , and serve important
recreational and cultural functions. Taken as a whole, it is estimated
that the aggregate value of services generated by wetlands throughout
the world is $4.9 trillion per year (Costanza et al. 1997).
Continue..
24. ๏ FWS constructed wetlands closely resemble natural wetlands it should be
no surprise they invariably attract a wide variety of wildlife, namely,
protozoa, insects, mollusks, fish, amphibians, reptiles, birds, and mammals
(Kadlec R.H. and Knight R., 1996; Knight R.L. et al., 1993).
๏ While a debate on the sustainability of FWS wetlands for wastewater
treatment is still ongoing, particularly for P removal, long-term records
provide evidence of the longevity of treatment wetlands (Kadlec and
Wallace 2009). Two FWS wetlands, the Brillion Marsh in Wisconsin and
Great Meadows Marsh in Massachusetts operated for over 70 years and
retained their treatment efficiency (Kadlec and Wallace 2009).
๏ Aquatic vegetation accelerates the pesticide removal compared to open
water systems [109, 110]. This reportedly occurs because of the increased
capacity for plant/biofilm sorption and subsequent immobilization,
breakdown, or uptake of pesticides [Kantawanichkul S, Wara-Aswapati S
(2005)].
Continue..
25. Continue..
๏ The contamination of water bodies with agricultural pesticides can
pose a significant threat to aquatic ecosystems and drinking water
resources [Schulz R, Peall SKC].
๏ Wetlands iri India support around 2400 species and subspecies of
birds. But losses in habitat have threatened the diversity of these
ecosystems (Mitchell and Gopal, 1990). Introduced exotic species like
water hyacinth (Eichhornia crassipes) and sal.vinia (Salvinia molesta)
have threatened the wetlands and clogged the waterways, competing
with the native vegetation.
๏ Twenty-two states have lost at least 50 percent of their original
wetlands. Since the 1970s, the most extensive losses have been in
Louisiana, Mississippi, Arkansas, Florida, South Carolina, and North
Carolina. Source: Wetlands, 2nd edition, Van Nostrand and Reinholdt,
1993.
26. Continue..
๏ Treated water from CWs can also be used for restricted or unrestricted
agricultural irrigation of crops depending on its effluent standard, and
it can also serve as a tool for recharging groundwater (Emmett et al.
1996 ; Rousseau et al. 2008 ).
๏ Large scale CWs can serve as a new habitat for the local species of
flora and fauna (Knight et al. 2001 ) .
๏ Large scale CWs designed from both ecological and engineering
perspective can add further value for recreational and various other
human use such as nature watching, walking, jogging, fishing,
picnicking, relaxing and art (photography, painting) (Gearheart and
Higley 1993 ; Knight 1997 ; Knight et al. 2001 ) .
๏ The major disadvantages of this systems is that it requires more space
than conventional systems & site selection is almost always an issue
due to unavailability of adequate land area and accessibility of the
site(Sundaravadivel and Vigneswaran 2010 ).
