Constructed Wetlands as stand-alone secondary treatment systems. ... at town and village scale or to septic tank systems at household and small community ...
EVALUATION OF THE WASTE TREATMENT PERFORMANCE OF CONSTRUCTED WETLANDS WITH SPECIAL REFERENCE TO WILLIAMSTOWN CO. GALWAY WETLAND SYSTEM. A.M. Cawley1 and M. Healy2 1
Department of Engineering Hydrology, National University of Ireland, Galway 2 Department of Civil Engineering, National University of Ireland, Galway
ABSTRACT In Ireland, Constructed Wetland systems are increasingly being used to perform tertiary treatment on municipal waste effluent from small towns and villages located in areas whose receiving waters are deemed sensitive. The bedrock formation in the west of Ireland is primarily karst limestone and the overburden / soil cover there is very shallow making such waters highly sensitive to pollution sources, as little or no natural attenuation / treatment will occur. In a lot of cases, suitable surface streams for dilution are not present, particularly during the summer period and treated effluent must be discharged directly to groundwater. Therefore, further reductions in nutrient and bacterial levels are desirable. Constructed Wetland technology has been seen to offer a relatively low cost alternative to the more conventional tertiary treatment technologies, particularly when dealing with low population numbers. This paper examines the waste treatment performance, in terms of nutrient (phosphorus and nitrogen) reduction, of a recently constructed surface-flow wetland system at Williamstown, Co. Galway. Evaluation of the performance is based on over two years of water quality and hydrological monitoring data. The nitrogen and phosphorus mass balances for the wetland system are presented.
INTRODUCTION Over the past two decades much research attention has been focused on quantifying the effectiveness and improving the design of Constructed Wetlands in treating wastewater at either secondary or tertiary (polishing) stages. It is now recognized that constructed wetlands provide an effective and economic way of treating liquid effluents. The application of constructed wetland technology is being used worldwide to cleanse effluents from a variety of sources, for example, municipal waste effluent from cities and towns, sewage from single households and small rural communities, agricultural runoff, dairy washings, mine drainage, urban and motorway storm runoff, and landfill leachate. In Ireland application of wetland technology is in its infancy relative to North America and other European countries. The last five years have seen a significant introduction of constructed wetland technology as a sewage treatment system to single households and small rural communities and as a tertiary system to larger villages and towns. There is also a growing use of constructed wetlands in farmyard waste management and in the treatment of urban stormwater runoff, in particular treatment of motorway runoff. As a treatment option it has widespread appeal due to its relative low running cost and its visual and environmental acceptance by the general public. In Ireland as in quite a number of other counties, water authorities are reluctant to permit the use of Constructed Wetlands as stand-alone secondary treatment systems. In a lot of cases they are being recommended only as an add-on polishing system to either conventional secondary treatment systems at town and village scale or to septic tank systems at household and small community scale, particularly in areas of sensitive receiving waters. The primary reasons for this reluctance is the scarcity of reliable long-term performance data relevant to the particular country / climate and their general poor winter treatment performances.
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GENERAL CHARACTERISTICS OF CONSTRUCTED WETLANDS Wetlands have the natural capability of filtering out nutrients such as nitrogen and phosphorous from the water. Constructed wetlands mimic the functions of natural wetlands. A constructed wetland is essentially an engineered water impoundment with a bed composed of the substrate, water tolerant plants, the water column and a microbial population. The substrate can be sand, gravel or soil in which the wetland plants grow. Types of Constructed Wetland In general there are two types of Constructed Wetland system. The first, a Free Water Surface (FWS) wetland, has its water surface exposed to the atmosphere and has a shallow water depth not exceeding 0.4m (normal operating levels 0.3m). It has a soil layer as a rooting media to emergent plants and flow is horizontal over the bed. An impermeable liner normally lines it, with flow entering and leaving via inlet and outlet controls. The second, a Subsurface Flow (SF) wetland, is an excavated basin lined and filled with a permeable media (gravel or soil) that supports in its upper layer the root system of wetland vegetation. The water level is maintained just below the top of the porous media with lateral flow through the permeable media obeying Darcy’s flow law. In addition to the above types, a hybrid type wetland incorporates aquatic pond systems in series with either FWS or SF systems. Wetland Vegetation Plants are an integral part of the effluent treatment processes in constructed wetlands. Their submerged surfaces provide structure for the accumulation of microorganisms in biological wastewater treatment. They provide short-term storage of nutrients during the growing season by assimilating them as biomass and longer-term storage as litter store decomposing in the substrate. The emergent and floating vegetation also prevents the occurrence of eutrophication through shading in FWS systems. The plant rhizomes provide oxygen in the root zone for aerobic biological processes to take place. Wetland vegetation should have certain desirable characteristics: They should be active vegetative colonizers with spreading rhizome systems. Although this is of limited importance in Surface-Flow Constructed Wetlands (where there is limited soil / water contact), spreading rhizomes are of particular importance in Subsurface Systems. The wetland vegetation should also have considerable biomass or stem densities to achieve maximum assimilation of nutrients.
WILLIAMSTOWN FREE-WATER SURFACE WETLAND INTRODUCTION The village of Williamstown is located between the town of Dunmore and the village of Ballymoe in North County Galway. The bedrock formation of the region is primarily karst limestone overlain by a thin layer of glacial clay and sand. Catchment drainage is via a series of small streams that disappear under ground via a swallow hole located approximately 500m east of the site. This underlying aquifer system is classified as a Regionally Important Karst Aquifer of extreme vulnerability (Daly, 1985). As a consequence of the sensitivity of the receiving water body and the lack of suitable streamflow for initial dilution, the waste effluent requires the provision of tertiary treatment in addition to the conventional (20/30) secondary treatment. Constructed Wetland technology was seen to offer a relatively low cost, effective alternative to the more conventional tertiary wastewater treatment technologies particularly in this case where the P.E. is not very high. In the summer of 1998, construction of secondary treatment package plant (aeration chamber and clarifier) and a FWS Constructed Wetland was completed, consisting of two reed bed cells and one retention pond cell connected in series. The three cells of the wetland are constructed as shallow
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lagoons enclosed in boulder clay embankments and lined with a high-density polyethylene (HDPE) liner. Details of the wetland system are presented in Table 1 and a schematic in Figure 1. The wetland emergent plant species used were two varieties Phragmites Australis (Common reed) and Typha Latifolia (cattail/Bulrush). Floating vegetation planted in the pond system were a variety of water lily and Duckweed these did not survive the first years planting and the pond for the study period was unoccupied. The design mean inflow rate to the treatment works is 79 m3/day which represents a population equivalent of 330. The observed mean inflow rate (see Table 3) for 1998 – 1999 study period was 55m3/day. Analysis of the rainfall – inflow hydrographs showed that approximately 28% of this discharge was storm runoff from a number of roofs and yards that had not been separated. Therefore the actual P.E. discharging to the treatment system was 147 (based on flow volumes). The secondary treatment plant was designed to produce an effluent quality better than 20mg/l BOD, 30mg/l Suspended Solids and 10mg/l NH4. The measured effluent quality leaving the package treatment plant for the study period had a mean BOD of 18mg/l, TSS of 52mg/l, and NH4 of 2.2 mg/l.
SLUDGE DRYING Y AR ND CO SE
REED BED
T EN TM EA TR
V-Notch chamber
PL T AN
WETLAND 1 Area = 277m2 PUMPING STATION
V-Notch chamber
Overflow
POND Area = 300m2
V-Notch
OVERFLOW TANKS
20 m
lon g
10 0m m
uP V
C la
WETLAND 2 Area = 448m2 nd dra in
30
m
lon g1 00
mm
uP
VC
lan d
dra
in
Figure 1: Layout of Treatment Work and Wetland Table 1 Details of Wetland Configuration Wetland Cell
First Wetland Pond Second Wetland Combined
Cell Dimensions Length Width depth (m) (m) (m) 28 38.5 47
10 8 9 - 12
.3 .8 .3
43
Area
Volume
(m2)
(m3)
277 300 448 1025
83 240 134 457
Hydraulic Residence Time (mean flow 55 m3/day) (days) 1.51 4.36 2.44 8.3
National Hydrology Seminar 2000
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Table 2 Performance of Secondary Treatment Plant Parameter
Influent (mg/l) Mean = 311 St. dev = 162 Mean = 331 St. dev = 373
BOD TSS
Effluent (mg/l) Mean = 18 St. Dev = 11 Mean = 52 St. Dev = 57
Reduction % Mean = 94 St. Dev = 5 Mean = 83 St. Dev = 23
MATERIALS AND METHODS Hydrological monitoring The flow of water within the wetland system is controlled by V-notch weirs, which are located at the outlet of the first, second and third cells. An electromagnetic meter located at the inlet of the first cell and pressure transducers located in the V-notch chambers at the outlet of each cell allow continuous measurement of the flow rate through the system. Two ‘Tipping bucket’ rain gauges (located on site) allow the daily rainfall amount to be calculated. Met Eireann supplied potential evapotranspiration values as monthly totals. A rating relationship was derived for each of the V-notch weirs from on site volumetric flow measurements. The accuracy of the electromagnetic meter was also checked and found to be well within the 5% standard. Table 3 Seasonal and Annual Wetland Water Balance Summer Inflow 1998 & 1999 Winter Inflow 1999 Annual 1998 to 1999
Effluent Inflow Rate 41.5 m3/day 40.5 mm/day per m2 66.7 m3/day 65.1mm/day per m2 55.55 m3/day 54.2mm/day per m2
Rainfall 3.1 m3/day 3.02mm/day per m2 4.7 m3/day 4.54 3.9 m3/day 3.8mm/day per m2
P. Evaporation 2.1 m3/day 2.03 mm/day per m2 0.3 m3/day 0.3 mm/day per m2 1.2 m3/day 1.2 mm/day per m2
Total constructed wetland area of 1025m2.
Water Quality Monitoring An Auto-sampler was set at the wetland inlet to produce a composite daily sample based on discrete hourly samples. This allowed a more accurate measurement of the nutrient mass balance for the system as the influent to the wetland has considerably higher variance in concentration than the effluent. ‘Gulp’ samples were taken weekly from four points throughout the wetland (at the inlet to the system, and at the outlet of the first, second and third cells. Testing for Total Nitrogen, Total Phosphorus and Chlorophyll A was carried out by the EPA (Castlebar). Testing for Faecal Coliform, COD, TSS, Ammonia, Nitrate and inorganic phosphorous was carried out by the Department of Engineering Hydrology, NUIG and BOD5, COD, and TSS measurements was performed by Galway Co. Council. It is important to note that all testing was carried out in accordance with Irish EPA standards.
TREATMENT PERFORMANCE OF WILLIAMSTOWN CONSTRUCTED WETLAND The main emphasis of this study is to quantify the performance of the FWS wetland in nutrient reduction. Removal efficiencies for BOD, TSS and faecal coliforms are also quantified for completeness. PATHOGEN REMOVAL An overall reduction in faecal coliforms of 99.77% based on weekly measurements carried out in September to March 2000 indicates that the system has excellent pathogen removal capability. Faecal coliform reduction rates in the reed beds are higher than the rates recorded in the retention pond. This
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indicates that the pathogen attenuation mechanisms unique to reed beds (i.e., attenuation by the dense stands of reeds, predation by protozoa and the exposure to antibiotic excretions from the roots of the macrophytes within the reed beds) are of some significance. A retention time of 8 days will ensure high die-off rates in any case. SUSPENDED SOLIDS REMOVAL An overall reduction in suspended solids of between 84% and 90% was noted in the wetland. Most settlement occurred in the first cell, where reductions of up to 94% were recorded. However, due to the intense eutrophication in the retention pond (where the water remained hytertrophic 55% of the time), an increase in suspended solids was observed. Aside from the effect of algal blooms, the reduction in TSS is a function of the hydraulic retention time. BOD REMOVAL An overall reduction in BOD of 49% (st. dev. 28%) was achieved by the wetland system during the study period. No discernable trend was evident in terms of a reduction in treatment efficiency with decreasing ambient temperature (i.e. summer growing season v’s winter non-growing season). If anything the summer period proved to have the lower efficiencies, brought about by the formation of algal blooms in the detention pond. Higher efficiencies were also evident with higher BOD loadings. The primary treatment process is physical settlement as opposed to biological oxidation. NITROGEN REMOVAL Both FWS and SF constructed wetlands are very effective in nitrogen removal, with up to 70 to 80% removal being reported (Knight et al., 1993). Removal efficiencies are very dependent on residence time and temperature. Research carried out by Bachand et al. (2000) show that water temperature and organic carbon availability affect denitrification rates. Longer retention times also result in enhanced settlement of particulate organic nitrogen within the wetland. Nitrogen removal in constructed wetlands is accomplished primarily by physical settlement, denitrification and plant/microbial uptake. Plant uptake will not represent permanent removal unless plants are routinely harvested. The nitrogen mass balance indicates that, on average, 58% reduction in Total Nitrogen occurs from the inlet to the outlet of the system. The retention pond has the poorest reduction in Total Nitrogen (18%), with the first and third cells also recording reductions of 30% and 26%, respectively. Table 4 presents results of the nitrogen mass balance for the 1998 – 1999 study period.
1st Reed Bed Area: 277m2 Retn. Time: 1.5 d Retn. Pond Area: 300 m2 Retn. Time: 4.4 d 2nd Reed Bed Area: 448m2 Retn. Time: 2.4 d
Tot. N NO3-N TKN NH3 Tot. N NO3-N TKN NH3 Tot. N NO3-N TKN NH3
Table 4 Nitrogen Mass Balance Influent Effluent mg/l g/d mg/l g/d 31.9 1770 21.9 1231 19.1 1060 13.3 747 12.8 710 8.6 484 2.2 122 2.0 112 21.9 1231 17.6 1003 13.3 747 12.2 695 8.6 484 5.4 308 2.0 112 1.1 63 17.6 1003 12.7 739 12.2 695 9.2 535 5.4 308 3.5 204 1.1 63 0.8 47
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Denitrification Settlement Plant uptake
Reduction g/d 291 226 22
Denitrification Settlement Plant uptake
52 176 Nil
Denitrification Settlement Plant uptake
126 104 34
National Hydrology Seminar 2000
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Nitrification Nitrification is an aerobic process that involves the oxidation of Ammonia-N to Nitrate-N. Nitrate N composes 60% of total nitrogen entering the wetland, whereas in the effluent leaving the wetland it composes 69% of total nitrogen. These results on initial examination suggest nitrification as the dominant process, however closer examination of the nitrogen balance shows that the apparent increase in the ratio of Nitrate to total Nitrogen results from significant settlement of particulate (organic) nitrogen on to the substrate and its subsequent immobilisation in the predominantly oxygen deprived substrate. In the Pond due to wind and convective mixing, both ammonia and nitrification are marginally more pronounced producing negligible denitrification rates for the pond. Plant uptake Phragmites Australis accounted for approximately 94% and 63% of the total vegetative area in the first and third cells, respectively and Typha Latifolia accounted for the remainder. Annual cumulative nitrogen retention was estimated to be 88mg/m2/day for Phragmites Australis and 40mg/m2/day for Typha Latifolia. Per shoot the nitrogen content of Typha Latifolia was found to be four to five times that of the Phragmites. The total nutrient uptake by the wetland species plays a virtually insignificant role when compared to the total loading on the system, with an areal nitrogen loading rate of approximately 2.44g m-2 d-1 and cumulative plant uptake of 0.074g m-2 d-1, i.e., 33 times smaller than the loading. Denitrification Denitrification is an anaerobic process which occurs in the upper layers of the substrate and is highly temperature dependent (Bachand et al., 2000). This was also verified in the Williamstown wetland with direct correlations between observed denitrification rates and water temperature. A plot of denitrification rate and temperature for the second wetland illustrates the above, see Figure 2. The computed annual denitrification rate for the wetland system based on the mass balance presented in Table 4 is 0.469kg/d or 171kg/year. This represents an annual reduction rate of 26.5% of the influent nitrogen. An average denitrification rate of 457mg m-2d-1 (N) was computed for the overall wetland system. Operating a Free-Water Surface Constructed Wetland under Mediterranean conditions consisting of three macrocosms in replicate under a mean hydraulic loading rate of 68 m3/day, Bachand et al. (2000) calculated nitrate-N removal rates of 261-835 mg-N m-2d-1, which linearly reduced to zero during the winter period (with drop in temperature). Comparatively, for this study, an average nitrate depletion rate of 457mg m-2d-1 (N) was computed. In the Pond the denitrification rate was found to be negligible on account of wind and convective mixing.
Settlement Net settlement of particulate organic nitrogen (calculated from the difference in influent and effluent particulate organic Nitrogen values) accounts for the removal (as long term storage in the substrate) of 24% of the total incoming nitrogen. Settlement rates calculated from the nitrogen mass balance for the wetland cells represents 29% of the total incoming nitrogen. This apparent conflict in settlement rates is due to algal growth occurring in the pond (inorganic nitrogen is converted to organic nitrogen), the algae eventually settle out, some are nitrified, the remainder are immobilised in the substrate, thus increasing the actual settlement rate. Net settlement and plant uptake (plants die back and enter litter fall storage each winter) account for 54.5% of the total nitrogen reduction rate and denitrification accounts for the remainder.
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80
20 19
70
18 17
60
16 15
50
13 12 11
30
10 20
9 8
10
7
Temperature (degrees C)
Denitrification % Reduction
14 40
6 0
5
Denitrification as % Nitrate Reduction Ambient Water Temperature
-10
4 3 2
-20
1 -30 6-Dec-99
26-Nov-99
6-Nov-99
16-Nov-99
27-Oct-99
7-Oct-99
17-Oct-99
27-Sep-99
7-Sep-99
17-Sep-99
28-Aug-99
8-Aug-99
18-Aug-99
29-Jul-99
9-Jul-99
19-Jul-99
29-Jun-99
9-Jun-99
19-Jun-99
30-May-99
20-May-99
30-Apr-99
10-May-99
20-Apr-99
10-Apr-99
31-Mar-99
21-Mar-99
11-Mar-99
0
Figure 2 Relationship between Denitrification Rate and Temperature in Cell 3 PHOSPHORUS REMOVAL The phosphorus removal capacity of Constructed Wetlands depends on the physical, chemical and microbiological processes, which influence phosphorus incorporation in both inorganic and organic forms in wetland sediments. These processes include sedimentation, sorption-precipitation and incorporation into plant and microbial biomass (Nguyen, 2000). There is no natural sink for phosphorus unlike nitrogen and removal is either by short-term storage as plant and microbial biomass or medium to long-term storage by immobilisation in the substrate. Table 5 Phosphorus Mass Balance
1st Reed Bed Area: 277m2 Retn. Time: 1.5 d Retn. Pond Area: 300 m2 Retn. Time: 4.4 d 2nd Reed Bed Area: 448m2 Retn. Time: 2.4 d
Tot. P PO4-P Organic-P Tot. P PO4-P Organic-P Tot. P PO4-P Organic-P
Influent mg/l g/d 5.1 283 4.17 232 0.93 51 4.6 259 4.6 259 nil nil 4.35 248 3.58 204 0.72 41
Effluent mg/l g/d 4.6 259 4.6 259 nil nil 4.35 245 3.58 204 .72 41 4.45 258 3.15 180 1.3 78
Reduction g/d Settlement Plant uptake
24 2
Settlement Plant uptake
14 nil
Settlement Plant uptake
-13 3
The reduction of phosphorus by the wetland system is poor having an average reduction rate of 13% for Total Phosphorus and 26% for Ortho-Phosphorus. The average summer and winter reductions are 27% and -1% respectively for total Phosphorus and 32% and 22% respectively for Ortho-Phosphorus. Materialisation, the degradation of organic to inorganic phosphorus, in the wetland is also limited. The average ratio of Ortho to Total Phosphorus at the inlet is approximately 90%; however, the ratio at the outlet averages 76%. This reduction is due to phytoplankton growth in the detention pond during the summer period, in which ortho-phosphorus is assimilated by the phytoplankton and is subsequently converted to organic phosphorus. Average phosphorus loading and reductions for the entire period of analysis (1998 & 1999) and for the growing season (15/4/99 to 21/10/99) and non-growing (28/10/99 to 29/3/00) are illustrated in Figure 3.
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Plant uptake The total phosphorus uptake by the wetland species is estimated to be 0.007g m-2 d-1. This is virtually insignificant when compared to a total phosphorus loading on the system of 0.39g m-2 d-1, 55 times smaller than the loading rate. On dieback a large proportion of nutrient (both nitrogen and phosphorus) held in the stem and leafs retreats down to the roots where it is stored for next years new growth. Johnston (1993) estimated that between 20 and 50% of the nutrient concentration is retained annually by the wetland vegetation. The remainder becomes litter fall and will gradually decompose and eventually either return to the water column or be retained in the substrate. Settlement The net particulate organic and ortho-phosphorus settlement rate is calculated from the residual of the mass balance, applied to each wetland cell (see Table 5). Long-term loss due to settlement of the particulate matter accounts for approximately 9% of the total incoming phosphorus, this is extremely low as over 20% of the total phosphorus entering the wetland is in particulate form. Removal/retention efficiency of the substrate is highly dependent on the loading rate; long-term phosphorus removal is achieved with loadings of less than 5g (P) m–2 year –1 (Richardson and Craft, 1993). White et al. (2000) reported a 60% phosphorus immobilisation with an average loading of 4.75g (P) m–2 year –1. In Williamstown the average yearly phosphorous loading is 101g (P) m–2 year –1 which is considerably higher than the above recommended rates (20 times).
DISCUSSION The final effluent from the Williamstown wetland is of acceptable quality for discharge to the groundwater via a percolation field, having typically a BOD of 9mg/l (st. dev. 6mg/l), total suspended solids (TSS) of 9mg/l (st. dev. 10mg/l), Total Nitrogen of 12.7 mg/l (st. dev. 3.2mg/l), Ammonia-N of 0.81 mg/l (st. dev. 1.79 mg/l), Nitrate-N of 9.22 mg/l (st. dev. 2.7 mg/l), Total Phosphorus of 4.5 mg/l (st. dev. 2.4) and Ortho-Phosphorus of 3.1mg/l (st. dev. 1.1 mg/l). The average percentage reduction in the wetland system over the two-year study period is 48% for BOD, 83% for TSS, 62% for Total Nitrogen and 12 % for Total Phosphorus. Pathogen removal in the wetland system is excellent having a faecal coliform removal efficiency of 99.77% (st. dev. 0.27%) and a final effluent mean concentration of 118 No./100ml for the monitored period September to March 2000. This removal efficiency is no surprise given the large hydraulic retention time of 8 days. A conservative T90 of 72hours (night time) would achieve this removal efficiency through die-off alone. This wetland system has presently 6.8m2 per P.E. (based on a present day P.E. of 147). This is significantly above the EPA (2000) recommended design standards of 1 m2/p.e. for tertiary treatment. It is very evident from this study that the primary treatment process in the wetland system for BOD, nitrogen, phosphorus and of course suspended solids is physical settlement of the particulates. A total hydraulic retention time of 8 days allows ample time for the finer particulates to settle out. However the formation of algal blooms in the retention pond during the growing season reduces the efficiency of this process significantly. The nutrient uptake by the emergent wetland vegetation does not play a significant role having annual uptake rates of 24g m-2 nitrogen and 3g m-2 phosphorus. These uptake rates represent only 3.7% of the total nitrogen load and 1.8% of the total phosphorus load. Kadlec (1989) cites annual uptake rates for wetland vegetation (Typha) treating secondary effluent in Michigan USA of 60g m-2 nitrogen and 12g m-2 phosphorous. He also cites that in natural wetlands, where there is a lesser availability of nutrients, uptake rates can be three to five times lower. Based on crop densities at Williamstown, vegetation development was 2/3 complete by the end of the study period, requiring one more growing season to achieve full crop density. Constructed wetlands take between three and four years to reach full vegetation density and several years longer to reach a state of stationary nutrient dynamics. Aside from nutrient uptake, the vegetative crop is necessary for summer shading to eliminate algal blooms,
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shelter against wind shear turbulence (resuspension of sediments) and to aid settlement by filtration. The treatment efficiencies for the individual cells clearly show that inclusion of an extra wetland cell in place of the detention pond, which promotes eutrophication in summer due to the lack of vegetative cover, would better serve the system. The difficulty with including floating vegetation cover in the pond is the exposure of the site to the prevailing wind direction, which has the tendency to pile floating vegetation to one side or to remove it completely. In regard to aeration of the substrate through plant rhizome oxygen release, in-situ redox measurements indicate that oxygen is limited and that conditions in the substrate and at the water interface are generally anaerobic favouring denitrification as opposed to nitrification. Under these conditions decomposition and mineralisation of settled organic matter will be very gradual and therefore litter fall and settled organic particulate matter will be practically immobilised in the substrate producing an organic peat. Overall the performance of the Williamstown wetland system is without doubt very encouraging in promoting the use of FWS wetlands as tertiary treatment system for protecting sensitive receiving waters. Ample hydraulic retention time will ensure good physical treatment through settlement and will also allow sufficient time for pathogen die-off. These systems perform well in nitrogen reduction through good summer denitrification rates and long-term/permanent storage of settled organic nitrogen as litter in the substrate. However, in terms of phosphorus reduction these systems are of limited value unless they are designed to be excessively large. Importantly, sufficient sizing of these systems will allow their safe use as coverage to breakdown/malfunction of the secondary treatment plant. These systems have the added advantage of smoothing out by virtue of their storage volume variations in the influent quality. ACKNOWLEDGEMENTS The authors wish to thank Mr. Paul Ridge, Senior Executive Engineer Galway Co. Council for initiating this project and providing the necessary funding. They would also like to thank Dr. Michael Flanagan, Regional Inspectorate, EPA, Castlebar and Mr. John Heneghan, Tuam Water Laboratory, Galway Co. Council for their invaluable assistance in the water quality measurements. REFERENCES Bachand, P.A.M., Horne, A.J., (2000).”Denitrification in Constructed Wetland Free-Water Surface Wetlands:II. Effects of Vegetation and Temperature” Ecological Engineering, 14, pp. 17-32. Daly D. (1985) “Groundwater in County Galway with Particular Reference to its Protection from Pollution”, Geological Survey of Ireland, Rep. 2.2.7/2.11.7, July 1985. EPA (2000) “Waste Water Treatment Manuals – treatment systems for small communities, Leisure Centres and Hotels”, EPA, Johnstown Castle. Johnson, C.A. (1993) “Mechanisms of Wetland-Water Quality Interaction” Constructed wetlands for water Quality improvement, Moshiri, G.A., Publ Lewis, pp293-299. Kadlec R. H. (1993) “Dynamics of Inorganic and Organic Materials in Wetlands Ecosystems” in Constructed wetlands for Water Quality Improvement, Moshiri, G.A., Publ Lewis, pp459-468. Knight, R.L., Ruble, R.W., Kadlec, R.H., and Reed S. (1993) “Wetlands for Wastewater Treatment Performance Database.” In Constructed Wetlands for Water Quality Improvement”, Moshiri, G.A., (ed). Lewis, pp 35-58. Nguyen, L.M., (2000) “Phosphate incorporation and transformation in surface sediments of a sewage-impacted wetland as influenced by sediment sites, sediment pH and added phosphate concentration.”, Ecological Engineering, 14, pp139-155. Richardson, C.J., Craft. C.B., (1993). “Effective Phosphorus Retention in Wetlands: Fact or Fiction.” Constructed Wetlands for Water Quality Improvement. Moshiri, G.A., (ed.). Lewis, pp. 271-282. White, J.S., Bayley, S.E., Curtis, P.J., (2000). “Sediment storage of Phosphorus in a Northern Prairie wetland receiving municipal and agro-industrial wastewater” Ecological Engineering, 14, pp. 127-138.
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Variation in Phorphorus throughout Williamstown Wetland Winter Season (28/10/99-29/2/00)
7
Organic P (mg/l) Particulate ortho-P (mg/l)
Phosphorus conc. (mg/l)
6
Soluble ortho-P (mg/l)
5 4 3 2 1 0 Inlet
Cell 1
Cell 2
Variation in Phosphorus throughout Williamstown Wetland Summer Season (15/4/99-21/10/99)
7
Organic P (mg/l) Particulate ortho-P (mg/l) Soluble ortho-P (mg/l)
6 Phosphorus conc. (mg/l)
Cell 3
5 4 3 2 1 0 Inlet
Cell 1
Cell 2
Variation in Phosphourus throughout Williamstown Wetland (Averaged throughout study)
7
Organic P (mg/l) Particulate ortho-P (mg/l)
6 Phosphorus conc. (mg/l)
Cell 3
Soluble ortho-P (mg/l)
5 4 3 2 1 0 Inlet
Cell 1
Cell 2
Cell 3
Figure 3 Variation in Phosphorus Concentration in Wetland Cells
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