COMPLICACIONES DE LA IMPACTACIÓN DE LAS TERCERAS

Free-water surface systems (FWS) are designed to simulate natural wetlands, being of shallow water depth and having water flowing over the surface of the soil. They are also designed to maximize wetland habitat values and reuse opportunities whilst providing water quality improvements (US EPA, Sept 1993). The potential treatment processes operating in FWS wetlands, such as sedimentation, biological assimilation and microbial and chemical decomposition, were

described more fully in Chapter Three. To enable some of these processes to operate, decaying plant litter provides carbon for denitrification and atmospheric reaeration at the water surface supplies additional oxygen (Reed et al, 1995). Many authors have described the advantages of free-water surface wetlands over subsurface wetlands (Richards et al, 1992; US EPA, Sept, 1993; Reed et al, 1995). The main advantages are: their greater pollutant removal potential; their capacity for wildlife enhancement, for conservation, and for public amenity; their relative ease of construction; and their comparatively low cost compared to SF wetlands. The pollutant removal capacity of constructed FWS systems is one of the main reasons cited by engineers for developing such systems over SF systems. Reed et al (1995), for example, found that at inlet concentrations greater than 6 mg/1 nitrogen removal was more efficient in FWS wetlands than in SF wetlands and that most of the nitrate produced would be denitrified. They also reported that a constructed FWS wetland in Areata, California, which had a residence time of three days, removed 95% of the influent E.Coli.

In addition to their pollutant removal potential, free-water surface wetlands have a greater potential for beneficial habitat values than SF wetlands because the water surface is exposed and accessible to birds and animals (Reed et al, 1995). Evidence for this assertion is also provided in a US EPA Publication which describes seventeen treatment wetlands in greater detail (US EPA, Sept 1993). For most of these wetlands the aim was two-fold: to provide environmental enhancement; and to improve the quality of the water.

The effectiveness of the role of these constructed wetlands as wildlife areas can be seen in schemes such as that at the Mount View Project which treats 1.3 million gallons of secondary wastewater per day from 16,000 residents in the area. There is now a total of 85 acres of wetland containing abundant wildlife with 15 species of birds and 34 species of fish, mammals, reptiles and amphibians resident.

other examples of the benefits for wildlife can be seen in the Orlando Easterly Wetlands Project in Florida, the West Jackson County Scheme, in Mississippi and the Des Plains River Wetlands Project in Illinois. The Orlando Easterly Wetlands Project uses 1200 acres of what was previously drained land. It takes stormwater and discharge effluent for disposal and attracts more species than the surrounding natural wetlands. This is considered to be due to the higher productivity rates within the system.

The West Jackson County scheme consists of three constructed wetland treatment systems (combined area 56 acres) built in 1990-1991 to treat 2.6 million gallons of water per day. Wildlife variety has been greatly enhanced with forty-five wetland plant species and with food and cover being provided for various animals and for sixty-two bird species. In the Des Plains River Wetlands Project, which consists of four constructed wetlands, vegetation and bird populations have also increased greatly. There has been a 500 % increase in bird species and a 4,500 % increase, overall, in the number of birds.

Richards et al (1992) state that another advantage of free- water surface wetlands is that they are simpler to design and construct and they require less complex inlet distribution structures than SF wetlands. The water depth can also be selected, and potentially varied, so that the optimum residence time and mix of wetland species can be achieved. This should enable the treatment of the poor quality water entering the constructed wetland to be optimised.

However, despite all these advantages, free-water surface wetlands are not without their problems. In some areas mosquitos cause problems, particularly to local residents. In addition, Richards et al (1992) state that unlike subsurface flow wetlands, where the substrate is protected by layers of decomposing vegetation, free-water surface wetlands are subject to freezing and reduced efficiency in the winter months.

4.4 POLLUTANT REDUCTION BY DIFFERENT TYPES OF WETLANDS

4.4fi) Comparisons between the pollutant removal

efficiency of natural versus constructed wetlands The pollutant removal efficiency of 11 natural and 15 constructed wetlands when fed with stormwater was examined by Strecker et al (1992) (Diagram 4.1):

Diagram 4.1 Comparison of site average pollutant removals

for natural and constructed wetland systems Pollutant TSS NH3 TP TPb 100- 80- N=11 4 0 N=14 N=4 N=6 N=15 N=11 N=5 O) B -20 2 -40 <D Q_ -60 N=7 -80

Natural Constr Natural Constr Natural Constr Natural Constr

Percentiles 9 0

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Strecker et al (1992) state that although there was

insufficient data to allow multiple regression analysis to be undertaken a number of trends could be noted:

"...constructed wetlands were generally found to have a

higher average removal performance than natural

wetlands, with less variability.." (page 19)

As Diagram 4.1 shows, the performance of the constructed wetlands, particularly for phosphorus and ammonia removal, was much better than that of the natural systems. This is to be expected as the constructed wetlands were designed specifically for stormwater treatment (Strecker et al 1992).

Another source of data that allows the performance of natural and constructed wetlands to be compared is the US EPA "Wetlands Treatment Data Base" (Brown, 1994). Of the 203 systems that they examined, 24% were natural wetlands and 76% were constructed wetlands. However, unlike the data considered by Strecker et al (1992), the data base mainly contained wetlands treating wastewater (Diagram 4.2):

niagr-am 4.2 The treatment efficiency of natural versus constructed wetlands using the US EPA database

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When data from 35 constructed and 29 natural wetlands were examined, the same trend that Diagram 4.1 revealed was also evident: that the constructed wetlands achieved greater pollutant reductions. The greatest differences were in the treatment of total suspended solids where the natural wetland reduced the concentration by 49.3% whereas the constructed wetland achieved a 72.9% reduction. The greater reductions in the TSS level and in phosphorus recorded in the constructed wetlands are probably due to the effect of specific design features, such as sediment traps and the distribution of water over the inlet end of the wetland; features that are not usually found in natural wetlands.

The evidence from Diagrams 4.1 and 4.2, of the lower reported treatment efficiency of natural wetlands, together with the reservations expressed in Section 4.2 about the use of such wetlands for stormwater treatment, makes the use of natural wetlands for this purpose inadvisable.

4.4fiil Comnarisons between the nolTutant removal