Constructed wetlands

last updated: 05/98

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1. Introduction

The use of wetlands to treat effluent is not a new idea. Thousands of years ago, natural wetlands were used by the Chinese and by the Egyptians to clarify liquid effluent. However, the first “constructed” wetland was not used until 1904 (in Australia). Even after that the use of such wetlands was slow to catch on. The first botanical treatment of waste was not reported in Europe until the 1950s; America’s research into the field did not begin until the 1970s. Nevertheless, it is now recognized that constructed wetlands are an economic way of treating liquid effluent (and even raw sewage - see the section on “New Generation” Reed Bed Filters in France).

Constructed Wetlands reduce concentrations of suspended solids, biochemical oxygen demand (BOD5), nitrogen, phosphorus, and coliform bacteria (often by up to 98%). Their simplicity and scalability make them effective for treatment of waste from small communities. If constructed on suitable topography, they require little energy input, which makes them suitable for both under-developed and rural sites. However despite the suitability of climate in developing countries, the spread of wetlands in such areas has been described as "depressingly slow" (P.Denny et al., 'Constructed wetlands in developing countries', Water Sci and Tech. 35 (5) pp167-174 1997).

2. Number of Wetland Systems Currently in Use

It has been claimed that there are "thousands of wetland-based wastewater treatment systems around the world" (K.R.Eddy and E.M.Angelo 'Biogeochemical indicators to evaluate pollutant removal efficiency in constructed wetland'’ Water Sci and Tech. 35(5) pp 1-10, 1997). However, although it is clear use of constructed wetlands is increasing, the precise number of such systems in operation is relatively difficult to obtain. Those figures which are available are summarized below:

USA and Canada
Constructed wetlands are still not in widespread use as treatment systems for wastewater. A 1996 survey of the USA and Canada showed 176 wetland treatment sites in use. A majority (116) of these were in sub-tropical or warm-temperate zones. However, the state with the greatest number of installations was the cold-temperate South Dakota (40 sites). The majority of wetlands in cold-temperate zones were of the FWF type (Free Water Flow - See Section3).

Northern Europe
In Northern Europe, Denmark is the leader in implementing constructed wetlands. A pioneer of SSF-type (Subsurface Flow - See Section 3) installations, the country has at least 130 wetlands, most of which treat municipal wastewater. By comparison, Sweden and Norway have shown much less interest in such systems and neither government has given final approval use of constructed wetlands for statutory water treatment. In 1996 Sweden had 6 FWF and 8 SSF wetlands for treatment of municipal or domestic wastewater. However, in most cases they were installed only to aid in removal of nitrogen, or to 'polish' water which had been treated by other means. Norway had almost twenty wetlands, the majority of them SSF-type installations.

Eastern Europe
The spread of constructed wetlands is greatest in the Czech Republic. Between 1989 and 1996, 26 systems were built. As a result of their success, 54 more such systems are currently being constructed. All systems are of the horizontal SSF-type and treat municipal waste (after initial mechanical pre-treatment). Hungary and Estonia are also known to be introducing constructed wetlands, but no numbers are currently available.

3.0 Types of Constructed Wetlands

Constructed wetlands usually comprise reeds (Phragmites australis) and/or bulrushes (Schoenoplectus) planted in gravel or sand. Constructed wetlands may use horizontal or vertical flow (see Figure 1). Although the trend is towards vertical-flow wetlands, most early installations were horizontal flow.

Figure 1 - Types of Constructed Wetlands


types of wetlands


Horizontal-flow wetlands may be of two types: free-water surface-flow (FWF) or sub-surface water-flow (SSF). In the former the effluent flows freely above the sand/gravel bed in which the reeds etc. are planted, and there may be patches of open water; in the latter effluent passes through the sand/gravel bed. In FWF-type wetlands effluent is treated by plant stems, leaves and rhizomes. Such FWF wetlands are densely planted and typically have water-depths of less than 0.4m. However, dense planting can limit oxygen diffusion into the water, and FWF wetlands are typically less effective at reducing BOD5 and phosphorus than SSF wetlands (in which effluent is treated by the roots). Nevertless, in either case, the area of plants required for waste treatment is typically 1-2 m2/person equivalent (equal to one person living continuously in catchment area for wetland treatment).

4. How Constructed Wetlands treat Waste

The treatment of waste by constructed wetlands is achieved by a large number of chemical and biological processes, many of the latter microbially-mediated. Table 1 (below) shows the main processes (and the sites at which they occur) affecting Carbon (usually measured as BOD5), Nitrogen (as NH4+ or NO3-), and Phosphorus.

Table 1 - Processes occurring in treatment of waste

Contaminant Site Process
BOD5 Stems and Leaves
Roots
Bed media (gravel/sand)
Bed media (gravel/sand)
Microbial respiration
Microbial respiration
Microbial respiration
Settling
Nitrogen Leaves
Algae in water column
Roots
Soil
Bed media(gravel/sand)
Volatilization (as N2 and N2O)
NO3 and NH4+ -> Soluble Organic Nitrogen
Ammonium -> Nitrate
Nitrate -> N2, N20, or NH4+
Settling
Phosphorus Stems and Leaves
Roots
Roots
Bed media (gravel/sand)
Bed media (gravel/sand)
Microbial Repiration
Microbial Repiration
Uptake
Sedimentation/Burial
Adsorption

5.0 Cold Climate Limitations

Although many of the world’s constructed wetlands are in the temperate and cold-temperate zones, these climatic conditions are not ideal for wastewater treatment. All chemical reactions slow as temperature drops and this is true for the processes occurring in constructed wetlands. This section examines a number of potential problems encountered by constructed wetlands in cooler climates.

5.1 Ice Formation

Ice has unpredictable effects on constructed wetlands, particularly those of the FWF-type. In snowy climates, if sufficient snow accumulates around plants etc., then the freezing of the water underneath is inhibited significantly. However, if ice forms around plant stems (and is held in place by them) then ice grows downwards into the water, causing water levels to drop rapidly. Constriction of flow can then lead to flooding, further freezing, and hydraulic failure. This can be avoided by operating FWF-type wetlands with a greater water depth in winter. Preliminary results from numerical models also suggest that covering FWF-wetlands with extruded polystyrene (XPS, 10 cm) is sufficient to prevent ice formation, even even if temperatures drop to -10°C for periods as long as three weeks.

The susceptibility of SSF-type wetlands to icing problems is less than that for FWF; the unsaturated surface layer functions well as an insulator. It is thought that vertical-flow systems may be even more resistant to icing problems than those using horizontal-flow.

5.2 Thaw Effects

A second problem with cold climate wetlands is the spring thaw. The severity of the problem will depend largely on the size of the catchment area for the wetland. If the catchment area is large, a thaw will markedly decrease the residence time of effluent within the system. This in turn will affect the level of BOD5 reduction and nutrient removal.

5.3 Biogeochemical Reactions and Nutrient Uptake

As mentioned at the start of this section, temperature affects the rate at which biogeochemical processes occur. In cold climates the rate at which biomass takes up nutrients will be significantly lower than in warm, subtropical or tropical climates. Indeed, the treatment area required to transfer 90% of nutrients to biomass increases from around 7 ha at 20°C to 35 ha at 0°C. However, this is not particularly important if nutrient recycling is not required. Figure 2 shows uptake of nitrogen and phosphorus for wetlands in Florida (sub-tropical), New Zealand (warm temperate), Sweden
(cold-temperate) and Canada (cold-temperate).

Figure 2 - Nutrient Uptake by Wetlands from different climatic regions

graph

6. Case Studies in Constructed Wetlands

6.1 “New Generation” Reed Bed Filters in France

Water Sci and Tech. 35 (5) pp 312-15 1997

Since the 1980s, French researchers have investigating the use of Reed Bed Filters (RBFs) to treat sewage from small communities. Currently, there are 15 RBF systems in France, each treating waste from populations from 100-250 people (p.e) each. The systems are capable of receiving raw sewage (usually screened to < 2cm). They typically comprise several Type A (primary treatment, 1.15 m2/p.e) and 2 Type B (secondary treatment, 1.05m2/p.e) filters. Variant systems sometimes use 1 Type C (horizontal) filter instead of a second Type B.



Figure 3. New Generation Reed Beds




wetlands in france


Typically there are a number of Type A filters. Each is loaded with waste for 3 to 4 days, before being rested for 6 to 8 days. These rest periods are critical to the functioning of the system; they allow mineralisation of Total Suspended Solids and maintain aerobic conditions in the gravel and rhizomes.

After 15 months of operation a detailed examination of the Montromant plant was undertaken over 48 hours. During the period of testing, temperatures varied from -8.5°C to +6.5°C . The results of the experiment showed the system to be highly effective at improving water quality (see Table 2)

Table2 - Performance of New Generation Reed Bed Systems (conc. in mg/l)


  Total COD d COD BOD5 TSS TP P-PO4 TKN
Raw Sewage 495 190 215 225 8.5 6.4 42.8
Filter A outflow 92 70 0 18 5.8 5.3 19.6
Final Outflow 58 40 16 12 5.6 5.1 10.1
Removal (%) 87.5 80 92.5 94.5 40 28 76




The application of raw waste to the A-type filters was shown to result in sludge accumulation of 1.5 cm/year, however this accumulation has not been shown to inhibit breakdown of waste, even at sludge-heights of 15 cm. Nevertheless, accumulation of such sludge should be allowed for in design of tank height (if it is not to be removed manually at a later stage).

6.2 Swiss studies

Water Sci and Tech. 35 (5) pp 307-14 1997

As of 1997, Switzerland had approximately 80 constructed wetlands. One of the longest-running facilities is the small plant (5.1 population equivalents) at the Centre for Applied Ecology at Schattweld. The system, which receives greywater and liquid human waste (urine), comprises a sand-filter and a horizontal-flow reed bed (see Figure 4). Between 1990 and 1995, its performance of the system was closely monitored and its efficiency examined.

Figure 4 - Swiss Wetland System

swiss wetlands


Average values for removal of BOD etc. appear in Table3 (below). After 10 years of operation, performance had not shown any notable degradation. Indeed, for a number of parameters (particularly NH4-N and Total-N) removal actually increased with time - and was at its highest in the final year of measurements.

Table 3 - Performance of Swiss System after 10 years use (Conc in mg/l)


 
Total COD
BOD5
TP
NO3-N
NH4-N
Min-N
Gray water 311 129.5 8.5 3 89.8 92.6
Sand filter out 31 0 3.1 50.7 1.9 62.2
Final Outfall 26.7 5.4 0.8 12.7 6.3 18.5
Removal (%) 91.4 95.8 90.6 -323.0 93.0 80.0

6.3 Waste Treatment and Habitat Creation - Slimbridge, UK

Applied Energy, Vol 57 No.2-3 pp.129-174, 1997

At Slimbridge in the UK, constructed wetlands have been exploited a tourist attraction. Wetlands are a valuable habitat for wildlife, and can support many rare species. Slimbridge is a center for people in the UK to see wetland birds, amphibians (such as frogs, newts and toads), and insects such as dragonflies. The wetlands are also used to treat all the liquid waste produced on-site (up to 4 000 m3/day). Waste passes through a settlement pond and then into a series of three parallel wetland beds, one planted with phragmites one with mosaics, and the third with irises. All act to remove BOD etc, but by using different species, three quite different habitats are created for wildlife. After recombination the waste then flows through two polishing beds before leaving the system.

6.4 Constructed Wetlands in Food Production (Theoretical Model)

As mentioned at the beginning of this report, constructed wetlands are not widely used in developing, tropical countries. However, this is the very environment in which such wetlands perform best. Indeed, constructed wetlands can form an integrated part of the food production system in such climates (see Figure 5).

Figure 5 - Wetlands Integrated within a Village Food Production Cycle

treatment of village waste with wetlands

The advantage of a hot climate is a continuous growing season, which means that the wetland biomass can also be harvested. For example, the annual production of papyrus in tropical conditions can be in excess of 100 tonnes/ha/year. The foliage can be sustainably cropped, while the papyrus stems can be used for matting and thatching roofs. Water that has passed through the wetland can be used to irrigate crops and/or introduced to a fishpond. In this final stage, remaining nitrates and phosphates stimulate the growth of phytoplankton - the favorite food of the Tilapia (Oriochromis niloticus L.), a food fish becoming increasingly popular in Europe.


Such systems may actually yield a profit for local communities, and would be a powerful tool in breaking the poverty cycle. Whilst few systems are in use, experiments have begun to be undertaken in Uganda - joint research between RIZA (a government agency from the Netherlands) and Uganda’s own National Water and Sewerage Corporation.

7. Conclusions

 

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