Constructed Wetlands: A low Cost Waste Water Treatment System

IntroductionGlobal 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.

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.

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.

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

◦ Each chamber consists of four cells:◦ Within each cell are water hyacinth plants

The constructed wetland removes solids, dissolved solids, nutrients, and pathogens.

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

Types of constructed wetlands

Surface flow◦ FWS (Free Water Surface)

Subsurface flow◦ VSB (Vegetated Submerged

Beds) Vertical flow

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.

Surface Flow Wetlands

Surface Flow Wetlands

Horizontal Subsurface Flow Wetlands

• In this, Water flows below media.

• 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 1960’s and in the U.S. in the 1980’s.

*Photo courtesy of USGS

Vertical Flow Wetland

dldhKAQ

The basis for the hydraulic design of the system is Darcy’s 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.

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.

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.

Wetlands treat water in the following ways

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.

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)

tK

o

e TeCC

The hydraulic residence time, t, can be determined from the following equation,

Where,

n = The porosity of the media as a fractionA = 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)

QnAdt

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..

dnK

CCQAT

eo /ln

TSSThe 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.

NitrogenThe 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.

PhosphorusFor 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 ColiformsThis 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.

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).

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.

Advantages of Subsurface Wetland(SSW) over Free Water Surface wetland(FWS)

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)

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 40–50% and 40–90%, 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).

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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)].

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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.

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 ).