Presentation on theme: "The Use of Wetlands as Water Treatment Systems David Chervek BAE 558, Spring 2005."— Presentation transcript:
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. 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.
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
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. Research and the use of constructed wetlands have increased rapidly over the last 15-20 years. *Photo courtesy of USGS
How Does It Work? 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.
To be able to use Darcy’s Law, a few assumptions need to be made. – –Uniform Flow – The flow in a wetland won’t be uniform due to precipitation gains and evaporation losses. Also, unequal porosity may cause preferential flow. To allow the use of Darcy’s 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.
The typical subsurface system consists of, n Liner n Inlet structure n Bed (including media and plants) n Outlet structure Liner The liner goes under the entire system and can be a manufactured liner or clay. n This prevents the wastewater from infiltrating into the ground before it is treated. n A berm around the system prevents runoff from entering the system. *From Ogden, M., Constructed Wetlands For Wastewater Treatment
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.
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 Darcy’s 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.
Bed Slope n n 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 n n 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.
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.
Outlet n n The outlet structures used are similar to the inlet structures. n n 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.
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, n n The breakdown and transformation by the microbial population clinging to the surface of the media and plant roots n n 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.
The subsurface wetlands have proved to be effective at greatly reducing concentrations of, n n 5-day biochemical oxygen demand (BOD 5 ) n n Total suspended solids (TSS) n n Nitrogen n n Phosphorus n n 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. BOD 5 is removed by the microbial growth on the media and the plant roots. BOD 5 is the basis for determining the area of wetland required using a first order plug flow (first in, first out) model. Where, C e = Effluent BOD 5 (mg/L) C o = Influent BOD 5 (mg/L) K T = K 20 (1.06) (T-20) = Temperature dependent rate constant (d -1 ) K 20 = Rate constant at 20 B C = 1.04 d -1 t = Hydraulic residence time (d) T = Temperature of liquid in the system ( B C)
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 (m 2 or ft 2 ) d = Average depth of liquid in bed (m or ft) Q = Average flow rate (m 3 /d or ft 3 /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 BOD 5 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 BOD 5 is actually created by the plant litter and other organic materials. As a result, the above equations cannot be used for final design BOD 5 < 5 mg/L.
TSS The results for TSS removal have been similar to BOD 5 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 BOD 5 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.
Phosphorus n nSignificant 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 n nOne log to two log reductions in fecal coliforms have been achieved. n nThis is usually not enough to satisfy local regulations, however, so some sort of after treatment is needed. n nThe reduction is enough to significantly reduce the scope of the after treatment process.
How do we determine the size? Let’s look at an example. Say we want to design a system for a family of four. The BOD 5 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 n nFlow rate for a family of four is 360 gal/day or 48.1 ft 3 /day. n nA 2 feet deep bed with an effective liquid depth of 1.8 feet. n nThe media is small gravel with a hydraulic conductivity of 5000 ft 3 /ft 2 /day and a porosity of 0.34. The temperature of the water going through the system is about 20 B C (68 B F). n nOur equation is, 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.
Are we ready to build? 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.
Wetland Treatment Applications What types of wastewater can be treated with constructed wetlands? n nDomestic wastewater n nStorm water runoff from parking lots or farmland n nWastewater from livestock operations n nWastewater from mining and oil operations n nLandfill 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
Constructed wetlands offer several advantages over tradition water treatment systems. n nWetlands are less expensive to build and operate than mechanical systems. n nThere is no energy required to operate a wetland. n nWetlands 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. n nWetlands can also provide wildlife habitat and be more aesthetically pleasing than other water treatment options. n nSubsurface wetlands produce no biosolids or sludge that requires disposal.
The advantages of a subsurface wetland over the free water surface wetland include, n nNo exposed water surface to attract mosquitoes or for people to come in contact with. n nFewer odors. n nDue 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. n nBetter performance in colder climates due to the insulating effect of the upper media layer.
However, free water surface systems may be preferred in some instances, n n In areas where land is cheap and media costs high, a free water surface system can be cheaper. n n Free water surface systems are normally cheaper for larger systems (>60,000 gal/day). n n 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.
What are the disadvantages? n n For wetland systems in general, the amount of land required. Some locations may not have the appropriate space. n n The effectiveness will vary with temperature. n n 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.
What questions remain? n nWhether to use the same plant throughout or a combination of plants? n nAnd in what quantity? n nAre there other plants that may be more effective? n nHow to size the systems for different climates? n nHow long will a system last? n nHow 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. n nCan we develop more sophisticated models for design?
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., Trimble, S.W., 2004, Environmental Hydrology, Second Edition, Lewis Publishers, Boca Raton, FL