from rrlations

P, Cooper, MSc, CEng, MlChemE (Fellow)*

Abst rac t

This paper reviews the design and performance of constructed wetlands for the treatment of domestic sewage. Horizontal-flow systems have now become accepted for secondary treatment where only BOD and SS consents are required. However, in recent years there has been increasing interest in systems such as vertical- flow and hybrid systems which are capable of achieving good nitrification. These systems have a greater and more reliable capability for oxygen transfer. As yet, there are only a few systems of this type in the UK, but the number is likely to increase. The paper describes the performance of (a) secondary and tertiary treatment systems, and (b) sludge-drying reed-beds.

Key words: BOD removal; constructed wetlands; nitrification; reed- beds; tertiary treatment; sludge-drying reed-bed.

This paper was presented at a workshop on Sewage-Treatment Strategies for Coastal Areas and Small Communifies, held in Edinburgh on €-7 December 1999

‘Consultant. (Formerly, Principal Technical Specialist, Water Research Centre, Swindon, UK.)

0 J .CIWEM I 2001 1 15 I May

I n t r o d u c t i o n

The reed-bed treatment system was introduced into the UK in 1985 by staff of the Water Research Centre(’). During the previous years, the system had been used in Germany and Denmark for treating sewage from village communities, but the results were variable; some systems had worked satisfactorily, but there were a number of plants which had failed because of problems relating to surface-flow and by-passing. There then followed a period of five years when the water companies in England and Wales and the water utilities in Scotland and DOE Northern Ireland cooperated in an evaluation of the different options. This research and development was successful and resulted in a set of UK guidelines which were then incorporated into European design and operation guidelines(*) which were introduced in 1990(3). By this time the use of reed-beds was well- established and they were starting to be regarded as standard practicelconventional technology for secondary treatment. It is estimated that there are now more than 525 reed-bed treatment plants of various typesc4). During the 1980s the main aim was compliance with standards for BOD and SS, and therefore the main interest was in horizontal-flow systems because they were simple and promised low construction and operating costs. There are now many examples of horizontal-flow systems for secondary treatment@); however, during the past ten years there has been a growing interest in developing systems which are capable of nitrification in both secondary and tertiary treatment situations.

‘Constructed wetland’ is the internationally-accepted term, but reed-bed or ‘reed-bed treatment system’ (RBTS) is more often used in the UK for historical reasons.

Reed-beds are particularly useful in treating sewage flows from remote locations - often where there is no sewerage system and sometimes no electricity. They are now being used to treat sewage from single houses, farms, visitor centres, golf courses, caravan sites, hotels, airports, lighthouses, in addition to domestic sewage from villages and industrial processes such as landfill leachate and mining wastes@).

H o r i z o n t a l - F l o w Systems

Secondary Systems The early systems were all horizontal-flow, the design, construc- tion and performance of which are detailed In 1986, a change from surface-sloped soil beds to flat-topped gravel beds was made, which effectively solved the problem of surface flow and the subsequent problem of inadequate per- formance due to by-passing.

The performance of individual systems has been well- documented(7), but the following two examples (in the Severn Trent Water region) illustrate what is possible:

Little Stretton (Leicestershire). This plant (Fig. 1) was designed in 1987 by staff at the Water Research Centre (WRc) and constructed by Severn Trent Water(@. The system serves a village of 40 population equivalent (p.e.), and an allowance of 20 p.e. was made for the anticipated flow from a nearby dairy.

The plant comprises eight small beds placed in series in a drainage channel down the side of a hill leading to a stream. The area provided per person is lower than normal at only 3.0 m2/ p.e. The system was the first to be planted with seedlings (rather than the rhizomes used previously with variable success) and only the second to use gravel rather than soil. The data from nine

79

P. Cooper o n

1992 Parallel operation

1993 Series operation

Fig. 1. Secondary-treatment horizontal-flow system at Little Stretton, Leicestershire

BOD Total SS COD Amm. N Kjeldahl N Total oxidised nitrogen (ms/l) (mg/l) (ms/l) (ms/l) (ms4 (msN

Feed Eff. Feed Eff. Feed Eff. Feed Eff. Feed Eff. Feed Eff.

306 46 105 25 774 154 61.1 43.3 81.4 47.5 0.5 0.11

Feed Bed 1 Bed 2 Feed Bed 1 Bed 2 Feed Bed 1 Bed 2 Feed Bed 1 Bed 2

333 109 22 109 43 15 - - 64.9 58.9 39.7 - - 0.4 0.13 3.4

years are shown in Table 1. Flows from the surrounding fields and the farmyard were brought under better control in 1990, and there was a significant improvement in performance which is reflected in the improvement in BOD, TSS and significantly in the reduced concentration in amm. N as a result of improved nitrification.

Middleton (Shropshire). This is a two-bed system which treats the flow from 30 p.e. and was built in September 1991, with an area allowance of 5.6 m*/p.e. The plant is operated as two beds in parallel and Table 2 shows the annual average performance data. There is a significant improvement in effluent quality with respect to the lower BOD, TSS and amm. N values,

and this might be as a result a. !he improved oxygen transfer resulting from the redistribution after the first bed at the inlet of the second bed. There is also evidence for denitrification of the nitrate produced in the system.

Tertiary Systems Most constructed wetlands which have been built in recent

years are probably tertiary-treatment systems. They are used to improve the quality of secondary effluents from rotating bio- logical contactors, biological filters and occasionally activated- sludge plants, and provide security of treatment now that effluent consents are more strictly monitored. Green and U p t ~ n ( ~ ) reported on the effluent quality from twenty-nine sites in Severn Trent Water using one year's data; another set of data showed forty-three effluents all containing less than 5 mg/l BOD. Green and U p t ~ n ( ~ ) also reported on another group of seventeen sites where constructed wetlands were installed to improve effluents which were in danger of failing more stringent consents. None of the effluents from these sites exceeded 6 mg/l BOD. Another group of nineteen sites had combined storm and tertiary reed-beds, none of which produced effluents exceeding 6 mg/l B0D(7,9r10).

By 1996 it was clear that, by designing a tertiary-treatment wetlands system at 1 m2/p.e., it was possible to achieve an effluent containing less than 5 mg/l BOD, less than 10 mg/l TSS and, in many cases, it was also possible to achieve substantial nitrification. In Severn Trent it has become standard practice to use 0.7 m2/p.e. for tertiary treatments, and smaller areas/p.e. are used for some short-term or remedial applications.

Table 1. Average performance data for Little Stretton horizontal- flow reed-beds

1987' 147 1988 112 1989 162 1990 112 1991 55 1992 26 1993 35 1994 58 1995 78

127 3.9 4.1 70 1.7 41 1.7 30 2.4 62 7.3 65

out

19 24 43 28 28 22 8

16 16

Amm. N (mg/l)

14.7

15.8 0.8 19.5 3.6

Total oxidised nitrogen (ms/l)

In

15.0 12.2 9.1 2.2 9.0

22.2 16.5 4.9 6.1

'July to December 1987

Table 2. Average performance data for Middleton (Shropshire) horizontal-flow reed-beds

Out

1 .o 3.4 3.0 6.2 6.2

16.6 11.4 8.8 5.1

0 J.CIWEM 1 2001 I 15 I May i

Constructed Wetlands and Reed-Beds: Mature Technology fo r the Treatment o f Wastewater f r o m Smal l Populat ions

Storm-Sewage Overflows During the last eight years, the use of reed-beds fortreating

storm-sewage overflows has been one of the major growth areas in Severn Trent Water(ll). Beds which are used just for storm-sewage overflow treatment are designed at 0.5 m2/p.e., but those which act as combined storm-sewage and tertiary- treatment beds are sized at 1 .O m2/p.e.(11). These combined beds have been very successful, and it is likely that they will pre- dominate in the future.

V e r t i c a l - F l o w S y s t e m s

Vertical-flow systems were part of the early Seidel systems(12) but have undergone a recent revival because they have much higher potential for oxygen transfer than horizontal-flow systems and therefore can achieve nitrification. Vertical-flow systems are intermittently dosed - the time between doses being 1 h or more. The surface tends to be flooded for a period of 5-10 mins, which means that the air in the voids of the bed is trapped, and this leads to enhanced oxygen transfer. Several beds are used in rotation to allow the organic solids on the surface to dry out and therefore avoid clogging of the surface. Beds are often made of layers of graded gravel topped with carefully selected sand.

In 1993, staff atthe WRc (Medmenham) installed avertical- flow system to act as a tertiary nitrifying system to improve the effluent from a 40-year old biological filter which was in need of refurbishment. The idea was to install the two-stage (series) reed-bed plant whilst the biological filter was being refurbished, so that there was continuity of treatment. Table 3 shows the performance during the first six months of operation. Plant design is given in greater detail eIsewhere(l3), together with an analysis of its performance. The system proved to comply (100%) with a consent of 20 mg/l BOD, 30 mg/l TSS and 10 mg/l amm. N. By 1996, the average effluent qualityfrom the reed-beds had improved to 1.8 mg/l BOD, 2.3 mg/l TSS and 0.8 mg/l amm. N.

Table 3. Performance of WRc Medmenham vertical-flow system in first six months of operation (May to October 1993)

Biofilter effluent (11 samples) Reed-bed 1 effluent (23 samples) Reed-bed 2 effluent (23 samples)

6.6 1.7 1.8

H y b r i d S y s t e m s

Tertiary-treatment horizontal-flow systems produce well- nitrified eff l ~ e n t s ( ~ ~ ’ ~ ) , but secondary treatment horizontal-flow systems cannot do this because of their limited oxygen-transfer capacity. As a result, during the last ten years there has been a growing interest in vertical-flow systems because (a) they have a much greater oxygen-transfer capacity, and (b) they are con- siderably smaller (1-2 m2/p.e.) than horizontal-flow systems (which need 5-1 0 m2/p.e. for secondary treatment). However, recently there has been a growing interest in hybrid systems, many of which are derived from the original hybrid systems of Seidel(12). In these systems the advantages and disadvantages of the horizontal-flow and vertical-flow systems can be combined

0 J.CIWEM 1 2001 1 15 1 May

to complement each other. It is possible to produce an effluent containing a low BOD, which is fully nitrified and partly denitri- fied and therefore has a much lower total N concentration.

Advantages and Disadvantages of Horizontal-Flow and VefljCal-FlDW Systems

goodfor (a) SS removal and bacteria removal because of their ability to filter, (b) BOD removal up to a set oxygen-transfer capacity, and (c) denitrification (because it provides oxygen as part of the nitrate). poor for nitrification because of limited oxygen-transfer capability.

good for nitrification because of their high oxygen-transfer capability, which also leads to good removal of BOD and COD. They can also remove some bacteria. reasonablefor SS removal, and can become clogged if the sand selection is not correct.

Basically, there are two types of hybrid system, depending upon whether the horizontal-flow stages or vertical-flow stages are placed at the front of the plant.

The first system is that described by Johansen and Brixcis) and illustrated in Fig. 2. The aim of the system is that the BOD is removed in the horizontal-flow bed to prevent interference with nitrification in the vertical-flow bed. The design recommends limiting the loading rate to the equivalent of a maximum oxygen- transfer rate of 30 g 02/m2. d for vertical-flow beds and 15 g 02/m2. d for horizontal-flow beds(l5,l6). The first plant which incorporated this system was built in Poland by C i u ~ a ( ~ ~ . l ~ ) , the performance data for which are given in Table 4. It is clear from this Table that the system achieves satisfactory BOD and TSS removal but that nitrification is incomplete. Significant total N reduction is taking place, presumably by denitrification.

The alternative arrangement with the vertical-flow stage placed first has been used in France at St Bohaire(19) and in the UK at Oaklands Park(20).

This plant (Figs. 3 and 4), which serves a p.e. of 65, contains two intermittently-loaded vertical-flow stages in series followed by two horizontal-flow stages. The two vertical-flow stages have a total area of 63 m2 and the two horizontal-flow stages have a total area of 28 m2. A two-year study was carried out by staff of the WRc, and the mean results (of 48 samples) are shown in Table 5.

Horizontal-flow systems are:

Vertical-flow systems are:

HF = horizontal flow VF = vertical flow

I

8.8m1/pe HF @ I I I I I I I I

! Recycle for 4 denitrification I if needed

VF

_ _ _ - _ _ _ _ _ _ _ I

0.76m2/pe

7g. 2. System used by Johansen and B r i ~ f ‘ ~ ) and : i ~ p a ( ’ ~ )

P. C o o p e r on

Influent

Table 4. Average performance of hybrid system at Sobiechy, Poland"z 18)

Effluent

Period March 7995 BOD (mg/l) TSS (mg/l) Amm. N (mgil) Total N (mg/l) Total P (mg/l) Air temperature ("C) Water temperature ("C)

Period June 1995 BOD (mg/l) TSS (mg/I) Amm. N (mg/l) Total N (mg/l) Total P (mgfl) Air temperature ( "C) Water temperature ("C)

Stage 1

Influent

Stage 2 Stage 3 Stage 4

110 64 48.5 56.5 11.2 2 9

370 132 44.3

16.2

12

81.4

i a

BOD TSS Amm. N Total oxidised N Ortho P

Effluent

285 57 169 53 50.5 29.2

1.7 10.2 22.7 18.3

17 21 6.6

24.2 0.9 2 6

21 20 4.7

27.5 4.2

i a 13

I I I I

14 17 14.0 22.5 16.9

10.0

All results expressed in mg/l

The data in Table 5 show that:

(i) BOD and TSS removal was satisfactory in the vertical-flow stages and nitrification proceeded in parallel with the BOD removal. There was significant nitrification taking place in the heavily loaded first vertical-flow stage; this is shown by the reduction in amm. N and the corresponding increase in total oxidised nitrogen;

(ii) The vertical-flow stages (at 1 m2/p.e.) were too small to achieve full nitrification;

(iii) Significant denitrification was taking place in the third and fourth (horizontal-flow) stages, despite the relatively low BOD, but perhaps aided by the long retention times - presumably by endogenous denitrification; and

(iv) There was some denitrification taking place in the vertical- flow stages, because the addition of the total oxidised nitrogen and amm. N concentrations in the effluent (36.5 mg N/I) does not add up to the amm. N concen- tration in the feed (50.5 mg N/I). In fact the concentration of amm. N in the feed probably under-estimates the total ammonical load on the vertical-flow beds. The concen- tration of Kjeldahl N would probably be in the range 70- 100 mg/l.

82

Fig. 3. Hybrid vertical-flow and horizontal-flow system at Oaklands Park, Newnham-on-Severn, Gloucestershire

Courtesy of Camphill village Trust

HF = horizontal flow VF = vertical flow

4 Fig. 4. System (used by Burka) at Oaklands Parkf**)

An alternative to the combination of vertical-flow and horizontal-flow stages has been used by Perfler and HarberW and Laber et a/(22) who used the intermittently loaded vertical- flow stages but incorporated a recycle to improve nitrification and denitrification. Two systems were used, one with recircu- lation (presumably allowing denitrification in the feed tank as well as the bed) and one with a dose of methanol. The two systems (A and 6) had approximately the same size (5 m*/p.e.), but system B was split into two equal stages with the system operated in series. The aim was to use an external carbon source and achieve more rapid exogenous denitrification. The systems achieved 72% removal of total N in the recirculation system (A) and 78% removal of total N for the methanol dosed system. Table 6 shows some results from their study.

0 J.CIWEM I 2001 I 15 I May

Constructed Wetlands and Reed-Beds: Mature Technology f o r the Treatment of Wastewater from Small Populations

Parameter

BOD coo TOC Amm. N Total N Total P

Table 6. Mean influent and effluent composition of System A (single bed with recycle)(21)

Influent Effluent

109 4 298 25

82 9 77 6 89 55 10 3

Author( s)

Burka and Lawrence(2D) Ci~pa(17,W Johansen and Br i~ ( '~ ) Cooper eta/(7J3) Perfler and Haberl(2i)

Platzer(26)

D e s i g n a n d O p e r a t i o n

System Calculated oxygen-transfer Overall area Comments configuration rate (g 02/m2) ( m2/p. e . )

2VF + 2HF 65 --+ 94 (VF beds only) 1.3 Long-term averages 1HF + 1VF 5.4 (combined beds) 10.3 Overall average for HF + VF beds 1HF t 1VF 15 (HF) 30 (VF) 9.6 Design recommendations

2VF 40-50 (VF only) 0.6 (tertiary) Tertiary nitrification only VF t Recycle 9.7(B) --+ 13.6(A) 5 2 systems. System A

with 1 bed. System B with 2 beds in series. Values for VF beds only

(VF only)

23 + 64 (VF)

(A) 2VF (6) VF t HF

Sizing In the UK, the following equation has been used for sizing the area of horizontal-flow beds:

0 J.CIWEM I 2001 I 15 I May

(1) Qd (In C, - In C,)

b o o Ah =

where: Ah = surface area of bed (m2) Qd = average daily flowrate of sewage (mVd) C, = daily average BOD of feed (mg/l) C, = required daily average BOD of effluent

keoo = rate constant (mld). ( m g 4

83

The factor kBoo has been measured as:

(a) 0.083 f 0.017 for 49 systems in Denrnarkcz3); (b) 0.067 to 0.10 in the UK(237,24); (c) 0.06 for secondary systems and 0.31 for tertiary systems

derived from data in the UK Performance Database(lo).

If the horizontal-flow bed is used for secondary treatment (fed with settled sewage), this tends to produce an area of 5 m2/p.e. and reduces the BOD and TSS to about 20 mg/l, but removes minimal amm. N.

If the horizontal-flow bed is placed in the tertiary position, the beds are sized at 0.5-0.7 m2/p.e. and can sometimes achieve complete nitrification.

For vertical-flow beds the information is less precise. In 1996, the author speculated that it would be reasonable to

Table 7. Comparison of oxygen-transfer rates

recommend: 1 mz/p.e. for BOD removal only, and 2 m*/p.e. for BOD removal and nitrification.

The systems at St Bohaire (France)(lg) and Oaklands Park (UK)(20) have the following areas:

St Bohaire Oakfands Park 1 st Stage 0.8 mz/p.e. 0.74 m*/p.e. 2nd Stage 0.4 m2/p.e. 0.23 m2/p.e.

In neither case is nitrification complete, but BOD removal is satisfactory.

For small systems ( 4 0 0 p.e.) Grant(25) recommends that the vertical-flow system is sized by the following equation:

A, = 3.6P0.35 t 0.6P (2)

where: Ai = area of first vertical-flow bed (m2) P = population equivalent.

A2, the area of the second vertical-flow bed, should be 50% of Al if the sewage is from a septic tank, and 60% of the area of A, if no septic tank is used. This results in Ai ranging from 2 m2/p.e. for 4 p.e. to 0.78 mz/p.e. for 100 p.e.

Distribution Most vertical-flow systems are designed with layers of

graded gravel as the media. This is usually topped up with coarse sand to allow the liquid to be distributed over the whole surface area (ensuring even distribution) and then to let it pass through the bed. The selection of this layer (so that it does not clog) is essential for good operation of vertical-flow sys- tem~(7~13.251,

Oxygen transfer The sizing of the beds is closely linked to the oxygen-

transfer capacity of the system which is linked to the intermittent dosing system and the hydraulic loading rate.

P l a t ~ e r ( ~ ~ ~ ~ ~ ) and Cooperc2*) have made estimates of oxygen- transfer rates in a range of vertical-flow and hybrid systems using calculations based on the removal of COD, amm. N, and BOD. A comparison of the oxygen-transfer rates calculated for these vertical-flow and hybrid systems is shown in Table 7. The values found by P l a t ~ e r ( ~ ~ ~ ~ ~ ) and CoopercZ8) are similar and indicate that the value of 30 for vertical-flow systems shown by Johansen and Brix(I5) might be conservative. It is perhaps relevant to point out that the area needed for secondary treatment of settled sewage is typically 5 m2/p.e., which can be back-calculated to give an oxygen transfer of about 8 g Oz/m2. d.

P. C o o p e r on

In a study of agricultural wastewater treatment, Job(29) reports even higher rates of oxygen transfer in the vertical-flow stage.

Sand/Soil Clogging

beds is sand or soil clogging, and this can be prevented by: Another important design consideration for vertical-flow

(a) Correct selection of the and(^,'^); (b) Use of four or more beds and rotating them; and (c) Limiting the organic loading rate to 25 g COD/m2. d(30).

Sludge-Drying Reed-Beds

Sludge-drying reed-beds (SDRBs)have been used for the past fifteen years but only in small numbers. The principle behind their use is that the rhizomes and roots of the reed-plants enhance the hydraulic conductivity of the sludge and speed up the rate of drainage. Wind rock also causes the stems of the Phragmites australis to move, creating holes at the surface around the stems and enhancing drainage.

SDRBs have become very popular in Denmark(31) and are also widely used in France(32) and the USA(33), but very few have been installed in UK. The following might be contributory factors to the low use in this country:

(i) Most standard sludge-drying beds were abandoned in the 1960s and 1970s because, in the UK, the drying rate is not high enough to achieve the required degree of drying; and

(ii) The first SDRBs (built in the UK in the late 1980s) failed because of design errors which caused the reeds to die.

The design loading rates are usually in the range 30-60 g DS/m2. year. The target is a reduction of 90-95% in volume with a typical final dry-solids concentration of l5-3O0h.

The author designed two systems for West of Scotland Water sites at Ballygrant and Kiells on the Isle of Islay. The waste activated sludge from these two sites had to be transported by trailer across two ferry links to the mainland for disposal at larger works. The first of the sites at Ballygrant was constructed in 1996 and for three years the waste activated sludge from Keills was transported there for treatment. Following the success of the Ballygrant SDRB, the second system at Kiells was constructed during the summer of 1999.

c o s t s

Capital Costs The construction cost of reed beds/constructed wetlands will depend upon several factors including:

(i) The ease or difficulty of construction on the site; (ii) Whether or not a liner is needed. Some sites will have clay

on site which may be ‘puddled’ to seal the bed; (iii) The sophistication of the design. VF systems need more

complicated flow-distribution arrangements and might need pumping at two or three different stages. They are more costly than HF systems;

(iv) In some cases, roads, paths and fences might have to be included in the design;

(v) The construction standards which are required by the client; and

(vi) The size of the unit. (Large systems will cost less/m2 to build than the very small systems.)

04

The costs vary from less than f50/m2 for simple con- structions to more than f100/m2 for HF systems built to very tight construction standards. HF systems built to the stringent standards of Severn Trent Water cost f 1 00/m2. These costs are for the area of bed which has to be constructed, but will also include items such as pumps, collection sumps and the control- unit housing, if needed. There is very little information on the cost of VF systems because so few have been built, but they are likely to cost more than f l 00/m2.

Operating Costs Secondary-treatment HF systems are normally maintained

by water-company mobile gangs and will need about 1-2 days/ month for maintenance (mainly the inlet distribution system). Tertiary treatment systems are also maintained by mobile gangs but need far less maintenance -typically 1-2 daydyear.

Vertical flow systems will need more operational input unless they can be automated, because of the need to change over the bed in use, in rotation, on a daily or two-day cycle.

Conclusions

During the last fifteen years, the design and performance of reed-beds has advanced considerably. This has been helped by national and international cooperation in exchange for experience. The main changes and improvements are as follows:

(i) Reed-beds are now accepted as being appropriate for treating sewage from villages serving up to 2000 p.e., and most of the systems are in the 20- 500 p.e. range;

(ii) Gravel has become the favoured medium because it provides a safety factor with respect to hydraulic conductivity. Flat surfaces are also preferred to help avoid surface-flow;

(iii) Seedlings are now the preferred planting option to provide rapid covering of reeds. This has had some impact upon perception of reed-beds in that, in the UK, the appearance is important;

(iv) During the last ten years, the use of reed-beds for tertiary treatment has become established because it provides a low-cost method of secure treatment;

(v) The use for treatment of storm-sewage overflows has developed in the last six years and is increasing;

(vi) The use of reed-bed treatment for remote unsewered locations is well-established and increasing;

(vii) Horizontal-flow systems for secondary treatment provide adequate removal of BOD and TSS if properly designed but, because of oxygen-transfer limitations, they cannot achieve significant nitrification;

(viii) Vertical-flow systems have been developed and can achieve complete nitrification;

(ix) Hybrid systems are at an early stage of development but can achieve both nitrification and some denitrifi- cation;

(x) So far, phosphorus removal has not been achieved in many systems, but those which contain iron in the media will remove P until the capacity is exhausted; and

(xi) Sludge-drying reed-beds have been widely used in some other EU countries but less so in the UK. There is potential for use on remote sites.

0 J.CIWEM I 2001 I 15 I May

Constructed Wetlands and Reed-Beds: Mature Technology fo r the Treatment of Wastewater f rom Smai i Populat ions

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( l i

0 J.CIWEM I 2001 I 15 I May

(18) CIUPA, F. The experience in the operation of constructed wetlands in North- Eastern Poland. Paper presented at 5th lnt. Con[ on Constructed Wetlands SVslemS for Water Pff l l~t jon Control. Vienna, Austria. September, 1996.

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