Environmental Technology, Vol. 28. pp 621-628
© Selper Ltd., 2007
PERFORMANCE AND COST COMPARISON OF A FWS
AND A VSF CONSTRUCTED WETLAND SYSTEM
V. A. TSIHRINTZIS1*, C. S. AKRATOS1, G. D. GIKAS1, D. KARAMOUZIS2 AND A. N. ANGELAKIS3
1
Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering,
Democritus University of Thrace, 67100 Xanthi, Greece
2
Hydraulics, Soil Science and Agricultural Engineering Division, Department of Agriculture,
Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
3
Institute of Iraklio, National Foundation for Agricultural Research, P.O. Box 2229,
71307 Iraklio, Greece
(Received 1 February 2006; Accepted 10 January 2007)
ABSTRACT
Two constructed wetland systems, treating domestic wastewater, are compared in terms of performance and costs. One is a
free water surface (FWS) wetland system located in Pompia, Crete, south Greece, and the other one is a vertical subsurface
flow (VSF) wetland system located in Gomati, Chalkidiki, north Greece. The FWS system is designed for 1200 p.e. Its
construction cost was 305,000, and the capital, operation and maintenance cost was 22.07 p.e.-1 yr-1 or 0.50 m-3 of
influent. The VSF system is designed for 1000 p.e. Its construction cost was 410,850, and the capital, operation and
maintenance cost was 36.81 p.e.-1 yr-1 or 0.56 m-3 of influent. Both systems achieved high removal rates for BOD5, COD,
TSS, TKN, phosphorus, TC, and FC, which makes them ideal for small communities in the Mediterranean region.
Keywords: Free-water surface constructed wetland, vertical subsurface flow constructed wetland, treatment performance,
construction cost; operation cost
are now installed for single family use (e.g., [19]).
INTRODUCTION
The aim of this paper is to provide a perspective for
Constructed wetland (CW) wastewater treatment
applying
constructed
wetland
technology
in
the
systems are considered more reliable compared to
Mediterranean
conventional systems [1], and are ideal technologies for small
Descriptions,
communities, due to their low construction, operation and
constituent
maintenance costs, easy adaptation to the environment and
maintenance (O&M) costs of two constructed wetland
limited generation of by-products [2,3].
systems (a FWS and a VSF) are presented. Both systems treat
One question however, is which is the optimum CW
regions
design
removal
and
specifically
considerations,
performance,
in
Greece.
construction
and
operation
cost,
and
domestic wastewater and were designed for comparable
type (i.e., free-water surface (FWS), horizontal subsurface flow
treatment capacities.
(HSF) or vertical subsurface flow (VSF) system) to use in a
specific region, in terms of performance, costs, area
requirements, and other factors. Most studies in the literature
METHODS AND MATERIALS
emphasize specific systems in terms of general performance
System Description
[4-9]. Other studies examine the effect of various design
parameters [10-13]. Comparisons of various CW types in the
same region are limited (e.g., [14,15]). Construction and other
Two constructed wetland systems treating domestic
cost data for CW systems are also limited (e.g., [16]). The
wastewater are compared in terms of costs and performance.
necessity of pretreatment is an issue for discussion, since
One is a FWS wetland system located in Pompia, Crete, South
modified VSF designs in France operate successfully without
Greece, and the other is a VSF wetland system located in
pretreatment [17, 18]. Finally, small-scale on-site CW systems
Gomati, Chalkidiki, Macedonia, North Greece.
621
maximum hourly flow rate 27.7 m3 h-1; influent biochemical
FWS system
The main components of the FWS system are [20]: (a) a
oxygen demand (BOD5) 400 mg l-1; septic tank effluent BOD5
septic tank (up-flow reactor simulation) equipped with three
250 mg l-1; wetland effluent BOD5 10 mg l-1 and (COD) <50 mg
screen vault filters [3]; (b) a FWS constructed wetland
l-1; retention time 5-14 d (depending on the season of the
consisting of two basins in series, with surface areas of 4300
year); sewage average temperature in the winter 10 °C and in
m2 and 1200 m2 (the inflow is uniformly distributed across at
the summer 22 °C.
the inlet of each basin using manifolds); (c) two chambers, one
in each basin, for regulating the water level; (d) small pumps
VSF system
and a pipeline for the recirculation of the effluent back to the
The main components of the facility are: (a) inflow
inlet of the first basin; (e) a compost filter for odor control in
structure; (b) a rotating disk screen with 1 mm openings; (c) a
the septic tank.
Vegetation selection included two species of reeds
closed twin settling tank (48 m3 each chamber; dimensions 4m
(Phragmites australis and Arundo donax). The facility was
x 6m x 2.5 m); (d) a closed twin sludge digestion-stabilization
constructed in the early months of 1999. The vegetation was
tank (48 m3 each chamber; dimensions 4m x 6m x 2.5 m); (e)
planted late in the winter of the same year, and due to the
an open siphon tank (3.2 m3; dimensions 1.0 m x 4.2 m x 0.8
favorable climatic conditions prevailing in the area, it
m) for intermittent wastewater feeding (Figure 1); (i) a stage I
established well very rapidly. By the end of the year, the
VSF circular basin (4 cells, 640 m2, sand and gravel fill, 1 m
vegetation was very dense and more than two meters in
deep); (g) a stage II VSF circular basin (4 cells, 360 m2, sand
height.
and gravel fill, 1 m deep); (h) a stage III rectangular HSF cell
The basic parameters used in the design of the FWS
(800 m2, sand and gravel fill, 0.5 m deep); (f) a VSF circular
facility are [20]: population served 1200 p.e.; mean daily flow
basin (4 cells, 240 m2) which receives digested-stabilized
3
rate 144 m
-1
3
d ; maximum daily flow rate 216 m
Figure 1.
-1
d ;
sludge for drying and storage. All wetland basins are planted
View of the siphon tank of the VSF system.
622
with Phragmites australis. The HSF cell was not planted at the
settling tank outflow, siphon, stage I VSF outflow and stage II
time of the study and was out of the wastewater stream. The
VSF outflow over a monitoring period from July 2003 to
VSF system was planted in May 2003 and was immediately
August 2004. The samples were analyzed in the laboratory
put to operation. The plants were dense and nearly fully-
following APHA standard methods for BOD5, COD, TSS,
grown by October 2003.
Total Kjeldhal Nitrogen (TKN), Total Phosphorus (TP) or PO4,
Total Coliforms (TC) and Faecal Colifoms (FC). Calculations
The route of the wastewater through the system is the
following: from the inflow structure to the rotating disk
of
screen and to the settling tanks. The sludge is collected at the
concentrations were performed in a spreadsheet.
statistical
parameters
of
measured
constituent
bottom of the settling tank (estimated volume 4.8 m3 week-1),
it is then pumped to the sludge digestion-stabilization tanks,
Cost Evaluation Method
and then to the VSF sludge basins. The leachate collected at
the bottom of the VSF sludge basins is pumped back to the
The actual construction cost of the two systems was
siphon, and together with the wastewater from the settling
used. Since the FWS system was constructed in 1999 and the
basins feed the stage I VSF basin (2 cells operate at a time).
VSF in 2003, for comparison the cost of the FWS was
Then, the wastewater is led to the stage II VSF basins (2 cells
expressed in 2003 prices using a reported inflation rate of
operate at a time). In the future, the flow will continue to the
approximately 3.1%. An economical life of 30 years and a
stage III HSF basin (not in operation now). The effluent of the
capital discount factor of 6% were assumed to calculate net-
basin discharges into a nearby stream, approximately 5 km
present-value cost [21]. Operation and maintenance costs
from the coast.
were obtained for the operation time periods from the records
of the local authorities operating the two facilities, and were
The main design parameters for this system are
summarized as follows: the design population is 1000 p.e. The
expressed in 2003 prices for the FWS system.
design mean daily flow of the system is 180 m3 d-1. The
maximum hourly flow is 28.5 m3 h-1. The design hydraulic
RESULTS
loading rate is about 36 m yr-1, and the organic loading rate
196 kg ha-1 d-1. Design influent and effluent concentrations are
System Performance
as follows: for BOD, influent 330 mg l-1, settling tank effluent
196 mg l-1, VSF effluent 12 mg l-1, HSF effluent < 10 mg l-1. For
FWS system
total suspended solids(TSS), influent 380 mg l-1, settling tank
The results during the 3-year period of the FWS facility
effluent 80 mg l-1, VSF effluent 12 mg l-1, HSF effluent < 10 mg
monitoring could be summarized as follows: mean BOD5,
l-1.
COD and TSS removals about 95%, mean TKN and TP
removals about 53%, and TC and FC removals >97% (without
System Monitoring, Sample Analysis and Statistics
any disinfection). Removal efficiencies of BOD5, COD, TSS,
TKN and TP in the final effluent for the monitoring period are
Grab samples were collected regularly at various points
presented in Table 1. Very high removal rates of BOD5, COD
along the FWS system (i.e., inlet, settling tank outflow and
and TSS (94.4%, 96.1%, and 95.6%, respectively) have been
system outflow) over a 3-year monitoring period from August
observed in the septic tank. On the other hand, low removal
1999 to August 2003, and along the VSF system (i.e., inlet,
rates of TKN and TP of 52.5% and 53.1%, respectively, have
Table 1.
Measured concentrations of BOD (mg l-1), COD (mg l-1), TSS (mg l-1), TKN (mg l-1), and TP (mg l-1) in the influent
(IN), the septic tank effluent (SE) and the final effluent (FE), and overall efficiency (TE, %) for the FWS system in
Pompia, Crete, Greece.
Parameter
BOD
IN
SE FE
COD
TE
mg l-1
IN
SE
FE
mg l-1
%
TKN
TSS
TE
IN
SE
FE
mg l-1
%
TE
IN
SE
FE
mg l-1
%
TP
TE
IN
SE
FE
mg l-1
%
TE
%
Average
165
39 7.7
94.4
455
100
18
96.1 191
36
5.6
95.5 38
25
18
52.5 13
9.1
6.2
53.1
Std. Error
31
4.0 1.3
1.0
31
9.8
2.7
0.5
40
5.4
0.8
0.9
3.4
1.7
1.7
4.8
1.5
1.3
1.1
4.7
Min
52
11 2.0
86.5
280
44
2.0
92.7 38
4.0
1.0
86.8 17
8.0
4.0
23.1 4.8
2.3
1.6
10.6
Max
540
60 16
99.1
798
180
40
99.6 720
90
12
99.3 62
36
27
83.1 24
22
21
78.5
# of Data
14
14 15
14
17
18
18
17
18
18
17
18
18
17
18
18
17
17
623
17
17
been obtained in the tank. TN and TP removed in the septic
hydraulic path of the system. It is obvious that to improve
tank were probably in organic form as particulate organic
nitrogen and phosphorus removals the last stage of the
matter. Lower removal rates of various constituents in similar
system should also be planted and be put soon in operation.
septic tanks have been reported [3].
System Costs
VSF system
The monitoring results of the VSF system during the 13-
FWS system
month period of operation could be summarized as follows:
System cost calculations are presented in Table 3. The
BOD5 removals >92%, TKN removals >89%, and TC removal
actual capital cost for the FWS system was 305,000 (prices of
>99% (without any disinfection). Removal efficiencies of
1999, including 18% VAT). To compare this cost with that of
BOD5, COD, TSS, TKN and TP in the final effluent for the
the VSF system, it was expressed as 344,615 in 2003 prices
monitoring period are shown in Table 2. Removals are
(287.18 p.e.-1) using the 3.1 % inflation rate. This cost also
satisfactory, considering that the facility was still new and the
includes 115,000 for access road and administration room
plant root system was probably not fully developed yet.
construction and other works. Some of this work was not
Relatively, high removal rates of BOD5, COD and TSS have
actually necessary, such as extra roads outside of the facility.
been measured in the settling tanks. On the other hand, lower
In addition, the soil used for planting in the treatment cells
removal rates of TKN (77%) were observed, while TP removal
was transported from a distance of more than 10 km with a
showed fluctuation and some times increased along the
relatively high cost. This work was also unnecessary. The net-
Table 2.
Measured concentrations of BOD (mg l-1), COD (mg l-1), TSS (mg l-1), TKN (mg l-1) and TP(mg l-1) in the influent (IN),
the settling tank effluent (STE), the VSF effluent (VSF) and overall efficiency (TE, %) for the VSF system in Gomati,
Chalkidiki, Greece.
Parameter
BOD
IN
COD
STE VSF
TE
mg l-1
%
IN
TSS
STE VSF
mg l-1
TE
IN
TKN
STE VSF
mg l-1
%
TE
IN
TP
STE VSF TE
mg l-1
%
IN
STE VSF TE
mg l-1
%
%
Average
485 193
39
92
626
243
62
89
1077
208
9
95
77
51
14
77 17.5
8.2
5.6
62
Std. Error
246 111
29
6
260
119
31
6
1784
474
13
8
47
50
6
20
9.0
3.9
3.1
22
Min
62
10
4
78
238
96
0
81
26
23
0
75
0
8
0
31
7.5
4.3
2.4
24
Max
819 355
92
100
1171
465
106
100
7060
2158
47
100
187
251
27 100 29.3 14.9 11.9 89
# of Data
20
19
20
20
20
19
20
20
20
19
20
20
20
19
20
Table 3.
Capital and operating costs () for the two facilities.
Cost ()
Cost category
FWS System
VSF System
Capital, including VAT (construction cost)
344,615
410,850
Construction cost per p.e.
287.18
410.85
Net-present-value cost
25,036
29,848
Annual average O&M cost
1,445
6,960
O&M cost per p.e. per year
1.20
6.96
O&M cost per m per year
0.03
0.11
Total annual cost (capital and O & M)
26,481
36,808
Total annual cost per p.e.
22.07
36.81
0.50
0.56
3
3
Total annual cost per m of influent
624
20
8
8
8
8
present-value cost was estimated at 25,036 yr-1 using a 6%
p.e.-1yr-1 or 0.56 m-3 of influent.
discount factor. The total mean operation and maintenance
Design, Construction and Operation Problems
(O&M) cost of the FWS system in the first three years of
operation was estimated at 1,445 yr-1 (i.e., 1045 for energy
used, 300 for works, and 100, for miscellaneous expenses)
No major problems were observed in the FWS
or 1.20 p.e.-1 yr-1 or 0.03 m-3 of influent. Net-present-value
constructed wetland. It seems that this CW has been
cost and O&M cost are added to a total annual cost of 26,481,
designed, constructed and is operated very successfully. The
and the mean figures become 22.07 p.e.-1 yr-1 or 0.50 m-3 of
VSF constructed wetland achieves a high removal efficiency
influent.
for all pollutants. Nevertheless, it is believed that its
performance could be even better if some design, construction
and operation problems were resolved. These can be
VSF system
summarized as follows.
The total construction cost of the VSF system was
410,850 (prices of 2003, including 18% VAT) or 410.85 p.e.-1
Design problems
This construction cost also included costs for access road (250
A major design problem is the sizing of the siphon that
m paved road), construction of 550 m sewer line to bring
feeds the first stage of the VSF cells (Fig. 1). The dimensions of
wastewater to the facility and 400 m sewer line for effluent
this siphon are 10x4.2x0.8m or 3.2m3 of flooding volume. The
disposal to the final receiver, fencing, landscaping and other
siphon floods two cells at a time, i.e., 320 m2, therefore, the
works. These extra works are estimated at about 100,000.
average flooding depth is 1.0 cm. If one considers surface
The net-present-value cost was estimated at 29,848 yr-1. The
irregularities of the planted cells, it is obvious that this depth
operation cost for the VSF system (for the first 10 months of
is small. Usually, 4 to 5 cm of flooding depth are
operation) comprises electricity (lighting and operation of 9
recommended. This problem was obvious in this facility. The
pumps, estimated at 67.50 month-1 on the average), salaries
flooding was limited to about a 1 m wide area around the
for the operator and maintenance works (500 month-1 on the
perforated feeding pipes (Fig. 2), something seen by denser
average) and miscellaneous other expenses (12.50 month-1).
plant growth in this area. Therefore, a major part of the
Therefore, the total operation cost is approximately 580
available facility area was not used, reducing active treatment
month-1 or 6,960 yr-1 or 6.96 p.e.-1 yr-1 or 0.11 m-3
area and performance. To fix this problem, it is recommended
of influent. Net-present-value cost and O&M cost are added to
that the siphon is replaced to one of a larger size that would
a total cost of 36,808 and the mean figures become 36.81
provide at least 4 cm of flooding.
Figure 2.
View of the stage I distribution pipe.
625
Constructions problems
It is recommended that these problems are addressed to
A construction problem was the proper placing of the
improve the system treatment performance.
porous media and the installation of filtering material. Some
porous media was washed out from the drainage pipe at the
DISCUSSION AND CONCLUSIONS
bottom of the wetland cells. Obviously, this was a result of not
placing proper filtering material. As a result, seepage holes
In general, selection of the appropriate constructed
developed at areas of the cells from where wastewater could
wetland system depends on wastewater characteristics,
seep out untreated. Another construction problem was in the
experience gained, local conditions and site constraints. FWS
last stage (HSF), which was not perfectly level in the lateral to
systems are less expensive to construct, to operate and to
the flow direction, resulting in preferential flow (and plant
maintain, are less sensitive and susceptible to problems, and
growth) on one side (Fig. 3). Again, this resulted in reduction
have greater potential for wildlife support. VSF systems
in total active treatment area and perfomance.
generally require less land area, are less susceptible to
freezing, mosquitoes and odor problems, and do not have
wastewater exposed at the surface, thus providing minimal
Operation and maintenance problems
Operation and maintenance problems were also
human contact and health risks. These systems are considered
observed in some of our visits. For example, plants were
more susceptible to clogging of the media. However,
grown (and not removed) inside the outlet overflow pipe of
neither odor nor clogging problem in either system has
the last HSF stage, obstructing outflow and resulting in
been observed so far. It is noted that a possible problem
flooding of the system.
of mosquitoes in the FWS project was faced effectively by
Figure 3.
View of the HSF constructed wetland cell.
626
and storage) or 2.04 m2 p.e.-1 Thus, as expected, the VSF
planting from the start Gambusia spp. fish.
In terms of performance, the organic loading rates were
system provides comparable treatment at significantly
slightly higher in the VSF than in the FWS system.
less area (less than half), for slightly higher average
Furthermore, ambient temperatures in the VSF system,
design flow rate (180 m3 d-1 vs. 144 m3 d-1), and at lower
o
located in Northern Greece, are 5 to 10 C lower. Nevertheless,
operational temperatures (north vs. south Greece).
the high efficiency of both systems has been observed. The
d.
When comparing the construction costs of the two
cost analysis, incorporating both capital and operation and
systems, it seems that the VSF is slightly more
maintenance costs, also suggested a low cost for both systems.
expensive, probably due to the fact that this system
The FWS system was less expensive to construct and to
contains more concrete and several pumps. Generally,
operate. However, the VSF system required considerably less
the FWS system construction is much simpler. In terms
land area (in this economic analysis the price of land was not
of the capital and operation cost, it also seems that the
considered). In terms of construction and O&M problems, the
FWS system is less expensive. Both systems are
VSF system, which is more complex in design, construction
considered less expensive, both in construction and
and operation, showed most problems, which, however, could
operation, when compared to equivalent conventional
have been predicted and avoided from the beginning. Finally,
treatment systems operating in the same areas.
the FWS system may freeze for a few days in the winter, if
e.
When comparing design, construction operation and
installed in areas where the temperature drops below 0oC
maintenance problems it seems that, the VSF was more
(e.g., North Greece).
susceptible to problems since it is a more complex
More specifically, the following can be drawn from the
system. For both systems, careful design and
comparison of the FWS and VSF systems:
construction, and proper maintenance are very
a.
important.
b.
Constructed wetlands are considered appropriate
wastewater treatment systems for the Mediterranean
In conclusion, the treatment efficiencies of the two
environment, generating excellent quality of effluent at
systems are comparable (except for TKN and TP where the
the secondary treatment level. In this comparative
VSF system had higher removal efficiencies), costs seem to be
study, BOD5, COD and TSS reductions of about 95%
less for the FWS system, and land requirements are quite
were observed for the FWS CW. BOD5 and TSS
lower for the VSF system. Thus, one can select either system
reductions were similar for the VSF system, while COD
in terms of treatment efficiency. When land is available, the
reduction was about 89% for this system. In addition,
FWS system would be preferable because of its simplicity, less
for the FWS system reductions of TKN and TP of about
expensive construction, and more reliable and problem-free
53% were measured, and removal rates of TC and FC of
operation. If land availability is a problem or land value is
98.7% and 97.1%, respectively. For the VSF system, TKN
high, then the VSF system would be more preferable. A
removal was 77% on average, while mean phosphorus
careful design and construction, and proper maintenance are
removal efficiency was 62%.
necessary in any case to avoid operational problems.
Reasons for the lower efficiency of the VSF system in
COD removal may be that it was new at the time of the
ACKNOWLEDGEMENTS
study and the plant roots were probably not fully
c.
developed. Furthermore, the HSF basin of the system
We thank G. Dialynas, N. Kefalakis and K. Tsagarakis
was not in operation during the study. Nevertheless, the
for providing information on the study. Sample collection and
two systems seem to be very promising in producing a
analyses for the VSF constructed wetland system were
high effluent quality.
performed by A. Paltsoglou, K. Vragalas and J.N.E.
The total wetland area (after pre-treatment) of the FWS
Papaspyros. The evaluation of the VSF system was co-funded
system is 5500 m2 (4.58 m2 p.e.-1), while that of the VSF
by the European Social Fund & National Resources – EPEAEK
system is 2040 m 2 (including 240 m2 for sludge drying
II – PYTHAGORAS II.
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