Constructed Wetlands Treating Storm Water Runoff

Storm water runoff is defined as the water sourced from rain or melting snow flowing over the surface of the land. The quantity of the runoff is largely dependent on the type of land on which the rain falls and flows over. In forests and grassy field, due to the pervious nature of the soil, most of the rain is either absorbed or evaporated, producing small amount of runoff. On the contrary, the developed areas with their impervious surfaces generate large amounts of storm water runoffs. As they flow over the surfaces, they become rich in pollutants. The storm water runoffs are harmful to the ecology influencing water quality, aquatic biota and temperature of natural water bodies like streams and lakes (Ten Towns Great Swamp Watershed Management Committee, n.d.).
In order to avoid the harmful effects of storm water runoff, storm water management techniques are employed. There are a number of storm water best management practices available to treat the storm water runoff, which are both structural systems which are engineered and constructed and non structural pollution prevention techniques. Constructed wetlands are one of the methods of treating the runoff flow (Sayre, et al., 2006).
Constructed wetlands are ecosystems that either are enclosed by water body or have water present near the soil surface, for a time period ranging from few months to entire year, resulting in soil which is saturated and sustain aquatic vegetation. (Mitsch & Gosselink, 2000; Noller, et al., 1994) They are deemed as the effective methods of pollutant amputation (Pennsylvania Storm Water Best Management Practices Manual, 2006).
The use of natural wetlands for treatment of waste water has been around for a century now but the use of constructed wetland for the treatment of storm water runoff is fairly recent, starting in 1950s (Kadlec & Knight, 1996). Wetlands operate by improving the water quality through the means of physical process like sedimentation, adsorption, filtration, etc, chemical processes like precipitation, chemical adsorption, etc. and biological processes like vegetation uptake, bacterial mediated reactions, etc., which may function individually or collectively (Mitchell, et al., 2002; Dunbabin & Bowmer, 1992).
The functions of wetlands include ground water recharge and discharge, floor water variation, sediment stabilization and retention, nutrient removal, aquatic and wildlife diversity and abundance. The vegetation in the wetlands has the potential to influence water conditions through a number of processes like shading, sedimentation deposition, transpiration, etc. As the constructed wetland evolves there are some unpredictable long term effects as well. Solid decomposition is one of the mechanisms. In the presence of sufficient carbon source, the decomposition of solids does not result in sedimentation of the constructed wetland. Another process is the attraction of colloidal particles like suspended solids to the aquatic root hairs as a result of opposite electrical charges. Once attached, these particles are slowly absorbed by the plants and microorganisms (Campbell & Ogden, 1999).
In storm water wetlands, there are variations in design leading to systems with different amounts of water storage. They include shallow wetland, the extended detention shallow wetland, pond/wetland system and pocket wetland. (Iowa Storm water Management Manual, 2008).
The system of treatment wetlands is such that the physical features assist the chemical and biological processes in eliminating or transforming pollutants. There are three types of configuration in treatment wetlands floating aquatic plant, free water surface and sub surface flow. The floating aquatic plant depends on the removal of pollutant by harvesting of plants (Mays, 2001). In free water surface constructed wetland, the water is allowed to flow over the surface covered with plants. In sub-surface flow, the water flows at beneath the gravel substrate surface which supports the plant growth (Higgins & Maclean, 2002).
According the Iowa Storm Water Management Manual (2008), all storm water wetland has the capability to remove 80% of the total suspended solids, 40% of total phosphorus, 30% of total nitrogen, 70% of fecal coliform and 50% of heavy metals, as long as the size, design, construction and maintenance are as per recommendation. Undersize or poorly designed wetland systems still remove total suspended solids to an extent but phosphorous and nitrogen removal is not viable. (Iowa Storm water Management Manual, 2008)
According to EPA’s National Urban Runoff Program, the first flush of urban storm water runoff has a higher contamination than sewage waste water. In 1977, the use of constructed wetland was established as an effective storm water runoff treatment facility in the EPA publication. The advantages of aquatic plantation in reducing the pollutant content of the urban storm water runoff discharge into a lake in Minnesota were included in the publication. It proofed to be a commencement point of a series of similar development all over United State, especially in Florida and mid Atlantic states (Campbell & Ogden, 1999).
At airports the storm water runoff contains glycol and other de-icing material along with the usual pollutants (Higgins & Maclean 1998). The chief glycols which are used as antifreeze agents are ethylene glycol, 1, 2 -propylene glycol and di-ethylene glycol. The amount of storm water generated by airports depends on weather and the type of aircraft. A small corporate aircraft is estimated to lead to few hundred litres of storm water. (Pellon 1995). Despite the efforts, most of the glycol and other de-icing materials make their way to the storm water runoff (O’Connor & Douglas 1993). There are not many off-site possible options for disposing of the glycol-contaminated water at an airport. Although a limited amount of it can be collected at de-icing pads that could be sent to waste water treatment facilities, the major portion of it enters the storm water runoff, and due to the large areas of airports, the volumes are huge. Owing to the large quantities of storm water, there has to be some method of handling the runoff on-site. (Higgins & Maclean, 2002). The release of untreated glycol containing runoff from the airports can cause hydraulic and chemical disturbance in the treatment processes (Higgins 2000a, b). The constructed wetlands are the only viable option for a large amount of glycol containing storm water runoff at the airports (Higgins & MacLean 1999). Among the different forms of wetland system, the sub surface flow constructed wetland systems are suitable for the storm water runoff from airports. The reason is that since there are no open water surfaces, the issue of attracting waterfowl is resolved (Higgins 2000a).
At the Edmonton International Airport in the province of Alberta in Western Canada, it was estimated that the 80 to 90% of the glycol used becomes the content of the surface storm water runoff. As the de-icing of the most aircrafts is done in the winter season, the glycol is accumulatively trapped in snow and releases instantly once the snow melts. Initially, the Edmonton International Airport use to release the contaminated storm water runoff into a nearby stream, after holding it at the impoundment areas for some time. This way the levels of glycol were low when released into the stream. At times when the volume of contaminated runoff was too large, they had to be discharged into the stream with high content level. Odor was an additional problem. (Higgins & Maclean, 2002).
In order to deal with this issue, the airport initiated a number of studies to assess glycol management options. The study revealed that the constructed wetland would be the most feasible option. In order to remove the glycol contamination, a massive Sub Surface Flow wetland system was installed. During the feasibility study, the properties of glycol and other de-icing substance was examined. Other aspects that were studies included the conditions which may lead to highest glycol contamination, other alternatives for treating the storm water runoff, the impact that the presence of ammonia and nitrate contamination can have on the size of the system and effects on wetland size and cost if a glycol recovery facilities is build to reduce the level of glycol in the runoff.
The feasibility study on constructed wetland included a treat ability test at pilot scale. The feed water was used to generate the airport’s runoff in worst case scenario. The test was conducted using two pilot SSF wetland cells, each of 1.2m2, which were connected in series and contained 0.6m thick gravel layer and vegetation of adaptive cattails. The residence time of the water in the cells ranged from two to four days. The result was that 99% of the glycol was removed. The feasibility study confirmed that the constructed wetland would be the best choices, as there was enough space at the airport site, it could be expected to function under worst case scenario, it would be able to successfully remove the ammonia and nitrates and it would be the most cost effective alternative (Higgins & Maclean, 2002).
The Sub Surface Flow wetland was design and build as a part of a larger storm water management system for an airport property of more than 500 hector. (Higgins & Maclean, 2002). The standard, first-order plug flow design methods (Kadlec and Knight, 1996; Reed et al., 1995) and the data from the pilot treat-ability test was used in the designing. It has twelve cell gravel bed systems. The surface area is a little over 2.7 hector. The wetland is situated in west of the airport facilities, kilometers away on a flight path. The contaminated storm water runoff is discharged into the surge pond through extensive network of sewers and ditches. Form the surge pond (the Gun Club Surge Pond) the waters are delivered over a long period of time to the Sub Surface Flow wetland through pipeline and feed pump, where pollutants are either removed or degraded. Relatively uncontaminated runoff is discharged to 440,000m 3 dry storm water Detention Pond, without going through the wetland. The water which is cleaned up in the wetland is discharged into the natural water bodies via a weir. The wetland does not function in the winter times and the contaminated runoff during that period is stored in the Gun Club Surge Pond for later treatment.
The dimensions of the twelve cells are 47.5m by 47.5m each and are arranged in six trains of two cells each. These cells are such that they are sloping away from a central berm in three rows. The square cells are basins and the soil is scooped out for their construction being formed into berms surrounding them. The slope of the berms is 2:1 on the interior and 3:1 on the exterior side. There is also a 3m grassed tops between the rows of cells. There is a “footprint” of 4.5 hectors for wetland and its ancillaries. An aluminum irrigation piping which is situated along the central berm is used to pump the contaminated water into the aluminum inlet distributor pipes. These pipes are present on the surface of the first or primary cells of each of the six trains at their upstream ends. The water is then directed to the surface of the cells by the inlet distributors. It is where the water infiltrates the gravel substrate in each cell and flows underground along the length of the cells, into permeable PVC outlet distribution pipes. These pipes are buried in the cell’s gravel at the deepest and furthest downstream ends.
In each of the six primary cells, the water is evenly distributed by adjusting thirty adjustable nozzles (3/4″ ball valves). In the cells, there are bio-films in the gravel particles which aid in controlling of pollution through microbially mediated processes. From the primary cells, the water exit into control structures which are round, flat-bottomed, concrete sewers with manholes. They are located in the centers of the downstream berms of the wetland. The risers on rotating elbows control the water level in the upstream primary cells. The flow in the secondary cells is the same as in the primary cells. The only difference is that the effluent from the secondary cells control structures is discharged into a peripheral ditch. The ditch encompasses the wetland on three sides. The water is directed towards receiving water from here via an outlet weir.
In the wetland, the maximum contaminated water flow rate received depends on the temperature of the water and concentration of glycol in it. At a yearly water temperature of 13 degree Celsius and the contamination level of 1400 mg EG/L, the maximum design flow rate is approximately 1300 m3/day. The SSF wetland can potentially treat polluted water up to 1500 m3 per day in warmer temperature or when the concentration of glycol is low. Lower volumes are treated in cooler temperature. The wetland cells have been each planted with approximately 750 cattail clumps which have been transplanted from local storm water ditches. (Higgins & Maclean, 2002).
In 2002, another Sub Surface Flow Wetland system was constructed at Heathrow Airport London, United Kingdom for treatment of glycol-contaminated storm water runoff. (Worrall et al, 2001). It was designed to manage runoffs from two catchment areas of 600 ha. In one, there is an aerated surge pond with a floating curtain, which separates clean and dirty runoffs. The dirty runoff is discharged into an aerated surge pond. An extra treatment is employed by floating the rafts of reeds in the exit channels which encircles the surge pond. In an aerated balancing pond, situated in front of a large, multi-cell SSF wetland, the water coming from the phytoremediation raft system joins the water from the second catchment area.
Airport authorities and airlines, in the previous years, have ignored the environmental impacts of contaminated storm water runoffs with de-icing chemicals. The massive SSF wetland treatment system described above have addressed the environmental problems, by treating the runoff effectively and economically. Also, it is evident by these examples that constructed wetlands can be designed such that it does not provide habitat for or attract waterfowl. (Higgins & Maclean, 2002).
Also, the above cited examples have shown that the constructed wetlands can be used for treating storm water which is contrary to the previously held assumption. The wetlands have also introduced some modern trends in the designing of constructed wetlands. These include multi-train configurations as oppose to the earlier constructed wetland designs which involve single flow trains and relatively low aspect ratio cells as oppose to earlier designs which were constructed with long and narrow cells. (Higgins & Maclean, 2002).
The above cited example case study show the effectiveness of constructed wetlands in treating storm water runoffs. The data from many similar projects testify to the fact that the constructed wetland improves the storm water runoff quality. The mechanisms at work in wetlands are quite effective in the removal of pollutants. In addition to that, the wetlands support wildlife habitat and landscaping (Surrency, n.d.). Hence, there are dual benefits attached with this type of green technology.