Wetland systems in the Alexander watershed for the
improvement of water quality
In the past, natural wetland habitats were part of the
natural landscape in some areas in Israel, mostly the coastal plain. However,
most of these habitats have been lost due to anthropogenic development, mainly
draining to expand the lands available for agriculture use (Green M, 1996). In the Alexander watershed,
which used to be filled with natural wetlands (Sever, personal Communication),
very few natural wetland areas remain. (Juanico M and Friedler E, 1995).
Constructed wetlands have been very successful in the last
few decades for artificial treatment of wastewater and low quality water from
different sources. Technically, “constructed wetlands" have been
defined as "manmade complexes of saturated substrate, emergent and
submerged vegetation, animal life and water that simulate natural wetlands for
human use and benefits” (Green M,
1996). The
removal of different pollutants in the wetland system is based on natural
processes with the aid of either macrophytes, or different water plants,
floating or submerged (Ran N, 2004). The removal of pollutants is either biological,
chemical or physical and includes the following processes: physical
sedimentation, organic matter dissolution, adsorption, filtration, burial in
the sediments, nitrifaction/denitrfication and other microbial processes (Ran N, 2004; Avnon A and Laila Y, 2001).
Two main types of wetland systems exist. These are the free
water surface (FWS) and sub-surface flow (SSF). In FWS, high hydraulic efficiency
and good settling conditions contribute to high removal of organic matter and
suspended solids. In SSF, processes within the soil contribute to high removal
of phosphorus, nitrogen and heavy metals(Ran N, 2004).
While still relatively rare, wetland systems are currently
used in Israel for treating wastewater from different sources and
qualities. Examples of such
wetlands are a wetland in Noat-Smadar, designed to receive domestic and
agriculture effluents or the wetland at Kibbutz Lahav designed to bring
domestic and animal culture effluents to a level that enables irrigation (Gafni A, 1999). More recent examples are the
wetland planned in Kibbutz Lotan designed to supply water for a birdwatchers
park and wetlands already built in
gas stations around the country to treat the runoff. Another wetland, was built
adjacent the Seven Mill Dam in the Yarkon Stream has been purifying some of the
stream water since 2004 (Cohen 2006).
This chapter evaluates the function and the future
implications of such artificial wetlands for the restoration of the Alexander
stream in particular and for stream restoration in other Israeli streams.
Kibbutz Maabarot Experimental Wetland
System (based on:(Goren
A, 2006).
The wetland system is
located in the area of Kibbutz Maabarot, on an area of one and a half Donam,
between the Road Number 4 to the east and the Alexander stream in the
North. A water pump located in the
Alexander stream at Maabarot Park takes water from the stream every two hours
for a period of twenty minutes, so that the approximate amount of water
entering the wetland system amounts to about 534 liter/hour. The pump transfers
the water to the pre-treatment area. The pre-treatment stage in the wetland
comprises of a 6 meters diameter and 1.2 meter deep pool used as a
sedimentation pond following a treatment using the FWS system. After a residue
time of about 15 hours in this part of the system, the water flows out to the
next stage using a perforated hose. Hyacinth plants with intensive root systems
float in this pool, and take part in the water purification system.
From this system, the water continues to a SSF system
comprised of two sections : a smaller section with a surface area of 225 meters
and a residue time of 3.5 hours,
and the larger section , with a residue time of 3.5 days. In larger section, many plant species grow
including Willow, Purple Loosestrife, Common Reed, Nile Pypyrus and southern
Cat-Tail. These plant species were either taken from natural wetland in the
watershed or from the local plant nursery, and it includes both local and exotic
plant species. Species were planted arbitrarily within the system and without
any consideration of plant density. After this stage, the water is discharged
from the collection hose at approximately 410 liters/hour and is returned to
the stream.
Wetland sampling:
In order to assess the effectiveness of constructed
wetlands as a treatment option, sampling of the wetland was carried out three
times. The sampling program included measuring chemical concentrations from the
inlet to the system (Alexander Stream), the sedimentation pool with the
Hyacinth plants and the outlet of the system. The second sampling event was
carried out a month later and included two sample sets during the day (21:00
p.m. and 9:00 a.m.), taken from the inlet, outlet, sedimentation pool and a
point located at center of the SSF
system. A variety of water parameters were analyzed for the water samples
including: BOD, COD, TSS, NO2, NO3, NH4, TN, TP, PO4 and turbidity. Oxygen
measurements were also conducted in the stream and different sections of the
wetland during the evening hours (18:00), at night 22:00and early
morning (04:30).
Results and discussion:
As shown in graph 1, almost all water quality parameters
are lower in the outlet than in the inlet, indicating that processes within the
wetland resulted in this apparent reduction of pollutants. BOD and TSS levels
in the output were lower than <2 mg O2/l and <8 mg/l, respectively. All nitrogen species (NO2, NO3,
NH4) were lower than <1 mg/l in the output and TN had a value of a little
over 1 mg/l. results relating to TP and PO4 in output were not as low having
values of 2.79 mg/l and 8.83 mg/l respectively.
Graph 2, displays the percentage removal for these water
quality parameters. As indicated by the graph, very high percentages of removal
(>90%) is seen for the parameters of NO3, NH4, NO2, TSS, TN and turbidity.
High percentage of removal is also seen for organic load parameters of BOD and
COD with a removal of 85% and 73%, accordingly. A removal of only 45% is seen
for TP and almost no removal or even negative removal is seen for PO4.
Similar results have been found in other wetland pilot
studies established in Israel. For example, Ran et al, 2004 found that their
system had high treatment efficiency, with a high removal of TSS and organic
parameters and a very low almost negligible removal for Phosphorous. Moreover,
Green et al, experienced high removal of
suspended solids and organic load in their system while the phosphorous
removal levels changed from more than 90% to zero and even negative removal.
The decrease in Phosphorous removal is explained by the fact that most
Phosphorous removal is governed by sorption processes on the media used, and
once this sorption capacity is saturated this processes no longer takes place (Green M, 1996).
Graph 3 illustrates the different pollution concentrations
in the inlet, the sedimentation pool, the center of the SSF system and the
outlet. The objective of this graph is to show the relative contribution of
each system component to the purification of the water. As this graph reveals,
for most water quality parameters there is a reduction between the different
colored steps in the graph, meaning that each system component contributes to
the reduction in pollutant concentration.
Graph 4 displays the measured oxygen concentration in the
wetland system. These results show that oxygen concentration is very low inside
the wetland system (<0.5 mg/l). High oxygen concentrations are seen in the
stream at evening and night, with lower concentration in early morning. Higher
oxygen concentration was measured in the wetland outlet (4.4-6.5 mg/l), however
these oxygen levels are attributed to the waterfall created by the drop in
water level as the water from the hose reaches ground level. These results show
that in fact, the wetland system is suboxic(low-oxygen levels) with all
processes taking place under conditions of constant low concentrations. Very
high oxygen concentrations (above saturation) of oxygen in the stream during
the day and low oxygen levels during early morning suggest that the stream
faces eutrophic conditions.
Comparison between water
quality after wetland treatment and Inbar water quality standards
|
Parameter |
Inbar standards for rivers |
Wetland outlet |
Compliance of wetland quality with Inbar standard |
COD (mgO2/l)
|
70 |
15.5 |
Ö |
|
BOD (mgO2/l) |
10 |
1.7 |
Ö |
|
TSS (mg/l) |
10 |
7.1 |
Ö |
|
NH4 (mg/l) |
1.5 |
0.3 |
Ö |
|
TN (mg/l) |
10 |
1.2 |
Ö |
|
TP (mg/l) |
1 |
2.8 |
X |
Table 1: Selected water quality parameters from the
Proposed Inbar standards and their comparison to water quality parameters in
the output of the wetland system.
The
results of the water quality parameters examined for this wetland, indicate
that constructed wetlands of this kind can be used as an efficient means to
improve the Alexander stream water quality, and comply with most requirements
set forward by the Inbar water quality standards for discharge into rivers.
Nevertheless, Phosphorus requirements are not complied with, suggesting that
different measures should be taken for the compliance with Phosphorus
standards. Conclusions from other wetland pilot studies (Green 1996, Ran 2004)
strengthen the claim that wetlands are an efficient mean for water quality
improvement and could comply with Israeli standards for river discharge,
especially for suspended solids and organic matter removal.
Moreover,
the establishment of such wetland system have been indicated as one of the
possible alternatives for improving water quality in the Alexander stream(Juanico M and Friedler E, 1995). Gafny argued that the
integration of wetlands is the most essential component of the stream
restoration project(Gafny A, 1995)In fact, a plan for a wetland
project in the Alexander watershed is planned in the near future. This wetland
system will be based upon the pilot study established in the Alexander
watershed in 1995 (Green M, 1996). The planned wetland system
will be conducted on a small scale treating an amount of approximately 60
cm3/day. This wetland will consist
of two hundred meters pool units and will treat the water discharging from the
emergency WWTP in Yad-Hanna that treats the wastewater of Nablus and Tul-karem
(Almon, personal communication, 8.6.06). Nowadays, about six thousands CM are
discharged into the stream from the Yad-Hanna treatment plant, (treats the
wastewater coming both from Nablus and Tul-Karem). In order to treat this
amount of water, a larger wetland system with an area of about forty to sixty
Dunham will be required to bring the water quality to tertiary level. (Cohen,
personal communication, 25.7.06)
Since
reclaimed water seems to currently be the most available source for restoring
Israeli streams that have gone dry, some policy makers have adopted this
notion, arguing that it offers realistic solution both for stream restoration
and for efficient to removal
of contaminants from waste water effluents. (Juanico M and Friedler E, 1999; 2004).
Under
current water availability conditions, streams will continue to receive
effluents from WWTP. Under these conditions, if effluent quality fails to meet
water quality standards, than river system are not likely to improve in the
near future. Although the Nature and Parks Authority (2004) excess wastewater in the area
might be still discharged into the stream.
Thus,
it is obvious that if reclaimed wastewater will provide water for streams
undergoing restoration, than the water quality discharged from treatment plants
should improve. This improvement can be accomplished either through
conventional treatment or using wetland systems.
Wetland
systems present some advantages in comparison with conventional treatment
systems. In conventional treatment systems, nonrenewable fossil-fuel energy
provides the source of energy, where in wetland system the energy source is
renewable, mainly solar energy. Moreover, many conventional treating processes,
result in the formation of residues or sludge, which present a disposal
problem, one that those not exist in the wetland system. Wetland systems
require low maintenance efforts, resulting in lower operation and maintenance
costs compared to conventional systems {Kadlec R.H. and Knight R.L. 1996 #1340}
Thus,
given the water quality improvement demonstrated by wetland systems, such
systems might serve as an effective and environmental friendly solution for
water quality improvement before discharge of effluents into Mediterranean
streams
Graph 1: water quality
parameters in inlet and outlet of wetland system
Graph 2: % removal of water quality parameters in wetland
.
graph 3: Illustration of relative contribution of pollutant reduction
in selected parts of the wetland system.
Graph 4: Oxygen levels measured in different locations in wetland systems at 18:00, 21:00 and 04:30 in 27-28/6/20