The Use of Wetlands as Water Treatment Systems. David Chervek BAE 558, Spring 2005. 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.
David ChervekBAE 558, Spring 2005
Global population growth is creating a two-part problem with water supplies.
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.
Free water surface wetlands, like most natural wetlands where the water surface is exposed to the atmosphere.
*Photo courtesy of Earthpace Resources
*Photo courtesy of USGS
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.
Research and the use of constructedwetlands have increased rapidly over the last 15-20 years.
The basis for the hydraulic design of the system is Darcy’s Law,
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.
The liner goes under the entire system and can be a manufactured liner or clay.
*From Ogden, M., Constructed Wetlands For Wastewater Treatment
Three types of plants are normally used
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.
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.
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 plants in the bed also provide oxygen and nutrients to promote microbial growth. The rest of the bed is assumed to be anaerobic.
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.
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 at20BC = 1.04 d-1
t = Hydraulic residence time (d)
T = Temperature of liquid in the system(BC)
The hydraulic residence time, t, can be determined from the following equation,
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)
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 < 5 mg/L.
Let’s 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?
As stated above, we would like the aspect ratio to be around 4:1.
This would result in a bed about 6.6 feet wide and 26.4 feet long.
Not just yet. We still need to apply Darcy’s 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 Darcy’s 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.
What types of wastewater can be treated with constructed wetlands?
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
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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