Gábor Horváth
Bachelor’s thesis
May 2012
Environmental Engineering
Tampereen ammattikorkeakoulu
Tampere University of Applied Sciences
Environmental Engineering
Review of Subsurface Flow Treatment Wetland Feasibility in Finland
Bachelor's thesis 44 pages,
May 2012
Constructed wetlands are engineered systems for treating wastewater.
They have
normally been used only as secondary treatment systems. However, over the last five
decades, they have started to be utilized more extensively, as problems with operation
and maintenance are gradually being solved. In Finland, treatment wetland is still
ignored as a way to replace expensive chemicals for wastewater treatment purposes.
The wetland system is rejected partly because the biological and chemical processes are
temperature dependent, and secondly, there are concerns about ice formation and its
effect on hydraulic flow, hydrology and hydraulics.
Thermal consequences for
biologically or microbiologically mediated treatment processes are the main constraints.
Constructed wetland systems in Finland have commonly failed because the temperature
coefficient has not been designed carefully, and clogging by organic matter has
occurred in the inlet of the pool. Therefore, energy and water balance calculations as
well as thermal modeling are useful tools to prevent design, operation and maintenance
Studies of constructed wetlands have shown less sensitivity to temperature swings in
full-scale experiments than laboratory-scale ones. The lab-scale results should not
prevent a full-scale trial because biological living beings in the nature interact with and
affect the environment in ways which cannot be predicted in laboratory-scale testing.
The wetland treatment method relies on anaerobic and partly aerobic conditions, which
are essential for the transformation of nutrients and organic pollution to take place.
A common problem with treating wastewater with an SSF wetland system is clogging
failure. Also, oxygen transfer is reduced significantly by the need to use an insulating
mulch layer, compared with situations where a mulch layer is unnecessary. Nitrogen
removal is low due to the lack of oxygen availability, but this can be increased by
artificial aeration.
Key words: cold climate, temperature, horizontal subsurface, clogging, aeration
1 INTRODUCTION ....................................................................................................... 6
1.1 Objective .............................................................................................................. 6
1.2 Existing Legislation in Finland ............................................................................ 6
1.2.1 Treating Wastewater in Areas Outside Sewer Network ........................... 7
2 Constructed Wetlands................................................................................................ 10
2.1 Free Water Surface Constructed Wetlands ........................................................ 11
2.2 Horizontal Subsurface Flow Treatment Wetlands ............................................. 12
2.3 Vertical Subsurface Flow Treatment Wetlands ................................................. 13
2.4 Hybrids............................................................................................................... 14
2.5 Water Purification Processes in Treatment Wetlands........................................ 15
2.5.1 Sedimentation.......................................................................................... 17
2.5.2 Phosphorus sorption ................................................................................ 17
2.5.3 Plant uptake ............................................................................................. 18
2.5.4 Nitrogen processes .................................................................................. 19
3.1 Horizontal Subsurface Flow Wetlands Hydraulics ............................................ 22
3.1.1 Cold Climate Effects on Hydrological and Hydraulic Conditions.......... 23
3.2 Ice Formation ..................................................................................................... 24
3.3 Temperature Dependence of Treatment Performance ....................................... 25
3.3.1 Thermal Conductivity of Various Materials and Insulation (Mulch) ..... 25
3.4 Wetland Energy Flows ....................................................................................... 27
3.4.1 Modified Energy Balance Term for Cold Climate.................................. 28
3.5 Cold Climate Solutions ...................................................................................... 29
3.5.1 Free Water Surface Treatment Wetland .................................................. 29
3.5.2 Insulation of Horizontal Subsurface Flow Wetlands .............................. 30
3.6 Wetland Plant Species Selection........................................................................ 31
3.7 Treatment Performance...................................................................................... 32
3.7.1 Clogging .................................................................................................. 33
3.7.2 Aeration ................................................................................................... 34
3.7.3 Monitoring .............................................................................................. 35
3.7.4 Carbonaceous Biochemical Oxygen Demand ......................................... 35
3.7.5 Total Nitrogen ......................................................................................... 36
4.1 Capital Cost........................................................................................................ 37
4.2 Operation and Maintenance Costs ..................................................................... 38
5 CONCLUSIONS AND RECOMMENDATIONS .................................................... 39
REFERENCES................................................................................................................ 42
Best Available Technology
Biological Oxygen Demand
Carbonaceous Biochemical Oxygen Demand
Chemical Oxygen Demand
Cold climate
“cold climate” means annual average temperatures lower
than 10°C, as well at least one month snow cover and natural
vegetation equals to boreal species
Constructed Wetland
to avoid confusion, Constructed Wetland and Treatment
Wetland used by turns but it refers to the same meaning in
the context
Free Water Surface
Hydrogen Peroxide
Horizontal Flow
Horizontal Subsurface Flow
Horizontal Subsurface Flow Treatment Wetland
Suspended Solid
Subsurface Flow
Finnish Environmental Institute
Tampere University of Applied Sciences
Treatment Wetland
Vertical Subsurface Flow
Vertical Flow
As the human population grows, there are corresponding increases in the amount of
anthropogenic waste generated. Conventional wastewater treatment development
projects cannot keep up with the continuously increasing amount of waste created in the
inbound side by urban and industrial developments, and inevitably partly or completely
untreated waste must be released to the nature. The discharged solid and liquid waste is
highly concentrated in nutrition and harmful substance to the nature, and must be
assimilated by the ecosystems of the environment.
The release of the effluent contamination into the environment without appropriate
treatment may overwhelm receiving water bodies and widen the effects of pollution,
endangering ecosystems health (Gray 2008). Our social, economic and environmental
coherence is sought in the management to ease these rising dangers. One of the major
problems which need to be solved is the protection of water resources, especially
surface and ground waters. Wetlands are one of the many possible ways to treat our
water resources. Swamp or marsh areas where the water table is at/near the surface at
least part of the year and is generally characterized by the presence of specially adapted
vegetation types and soil characteristics that developed a response to the wet and
saturated conditions (Kadlec et al. 2009).
The overall goal of this thesis is to revision the feasibility of constructed wetlands for
wastewater treatment under cold climate. The thesis intends to be a summarized work to
present up-to-date methods about horizontal subsurface flow constructed treatment
wetlands (also named constructed wetlands in this thesis) and their removal
effectiveness in cold climate. The objective of this study is to analyze the criteria steps
for a successfully operating HSSF wetland. The thesis also shortly discusses the current
wastewater treatment and management legislation permits.
Existing Legislation in Finland
In Finland, about one million residents and over one million vacationers are located
outside the municipal sewer systems. There are about 350 000 onsite wastewater
systems serving permanent dwellings. The estimated amount of untreated discharge of
phosphorus to waterbodies is 50% higher than in urban areas (Ruokojärvi A. 2007).
Therefore, rural wastewater treatment is tightly connected to eutrophication and needs
to be considered in planning further water management and restoration.
Treating Wastewater in Areas Outside Sewer Network
The Decree, also called Onsite Wastewater (542/2003) System Decree (Wastewater
treatment at...2012), came into force on 1.1.2004. The Decree sets minimum standards
concerning wastewater treatment for planning, construction, use and maintenance of
treatment systems. The relevant information to thesis is the building act which states
that new building after 1.1.2004 should fulfil the required treatment obligation
immediately. The same obligation stands for old buildings built before 1.1.2004, and
must reach the standard treatment releasing treated water to the nature, before 1.1.2014.
The Decree redrafted the permit to ease the public stress due to community demand of
strict due date as well tight nutrient restriction.
The Decree states primary criteria to analyse the removable organic matter, and the
amount of processed phosphorus and nitrogen in wastewater treatment systems before
allows being available as a treatment technology for the public. In table one, the
minimum criteria limits can be seen for passing the standard for dwellings at a
discharged load of 50 g/d for BOD, 2.2 g/d for total P and 14 g/d for total N.
TABLE 1 Old reduction limits for treating wastewater in areas outside sewer network
(Santala and Jorma from Finnish regulations...)
Total P
Total N
The updated version of Onsite Wastewater Treating (209/2011) law entered into force
on March 15th, 2011, and gave time for premises to install proper treatment system on
those building sites which were completed before the entry came into force from
1.1.2014 to 15.3.2016.
The Decree (209/2011) announces minimum treatment requirements for onsite
wastewater systems (see table 2) and give guidance for existing systems whom must
have a specified wastewater system report to apply for a new building permit which
will include a wastewater system plan for the premises. The wastewater system must be
built according to the Plan and Maintenance instruction, and it is necessary to provide
help to implement the manual according to the regulation by municipality.
TABLE 2 Requirements for the reduction of wastewater loads as specified by treating
wastewater in areas outside sewer network degree (Wastewater management at..., 2012)
Total P
Total N
The Decree emphasise that “no treatment method or device is automatically better than
another.” The natural preservation is important and very strict in terms of talking about
tight reduction demands, which do not take area differences into account. The list which
specifies all the best available up-to-date sources for treating wastewater in rural areas
does not include many biological treatment methods, which e.g. would be suitable for
small waste water uptake in dwellings. The author´s only concern that Finnish
Environmental Institution (SYKE) does not favour CW (also named “root zoned”)
method as an ideal treatment due to Finland located CW method experiments failures
because of inadequate fundamental design errors. In this thesis, the basic failures will be
discussed to prevent fault and to make CW methods preferable to treat wastewater in
rural areas in Finland.
The nature and water resources protection in rural municipalities with low population
densities do not have the financial capital for conventional treatment plant, even if the
community has the money, it is still difficult to attract qualified technical expert for
overseeing large conventional treatment facilities. In such rural areas, alternative
treatment methods are available with high removal capacity. Subsidies are available for
those residents whom have financial difficulties in a form as governmental funds on
social grounds (max 35% of costs); tax claim deductions for the work done etc.
By 2014, 95% of all properties outside sewerage networks should be equipped with
facilities corresponding to best available wastewater treatment techniques; and by 2018
all properties should be duly equipped. In the year 2000 the phosphorus loads entering
watercourses totalled some 350 - 400 tonnes annually, and this figure should be reduced
by 2015 to 100 - 150 tonnes.
Constructed Wetlands
Constructed wetlands are engineered systems which provide secondary treatment and
designed to cleanse and detoxify surface water by mimicking many of procedures
and/or process that naturally-occur (Knowles P. et al 2011). They are designed to take
advantages of many unique conditions such as sedimentation, filtration, chemical
precipitation, microbial interaction, plant assimilation and adsorption to soil particles.
The several simple beneficial attributes in constructed wetlands are the capacity to
operate on energy provided by the sun, fix itself up and create productive treatment
capacity over time, provide natural space for animals, increase oxygen levels in
subjected water bodies and decrease levels of carbon dioxide, and reach high levels of
treatment with small levels of maintenance (Wallace, 2000, Vymazal et al. 2008) give
wetlands the ability to remove several substances like nitrogen, carbon, sulphur,
potassium, and phosphorus, what result in an increase in water quality (Gray 2008).
Treatment wetlands employ these processes in order to provide effective environmental
and sustainable systems for wastewater purification improvement in small and rural
communities which ensure researchers to expand the applicability territories of such
systems (Gray 2008).
Apart from this great opportunity, the ability of treatment wetlands to maintain
sufficient removal levels under boreal climate has been questioned. Wetlands are
avoided to treat wastewater due to questions of effectiveness, understanding and
sufficient reference studies in the Nordic regions. The biological and chemical processes
are temperature depended which requires careful design and maintenance and some
energy input for good level of removal efficiency in such climate. Systems mostly not
succeed because of the temperature coefficient not designed carefully and systems get
clogged by organic matter (Kunihiko Kato 2012). One of the necessary implementation,
when designing subsurface flow wetland to overcame the above mentioned problems, is
the mulch (blanket) layer which prevents the system from dropping below 0 ºC.
Studies showed that the blanket cover will have a positive effect on rates of oxygen
transfer, plant establishment and performance level with which pollutants are removed
negatively therefore new ways of wetland method must be employed for good results.
This study aims to investigate this subject because constructed wetlands remain one of
the least expensive treatment systems based on their low annual costs with minor energy
and chemical consumption (Kadlec et al. 2009, Wallace et al. 2000).
Subsurface flow wetland has the primary benefit to not be exposed to the atmosphere
which secures its homogenized environment during the treatment procedure and
minimize the energy loss through evaporation and convection. It was first introduced in
Germany (Wallace, 2000). The two most efficient applicable solutions for winter
conditions are horizontal subsurface flow (HSSF) and vertical flow (VF). However
vertical flow requires more attention in installation as well during maintenance.
Therefore it is not advised for small application areas.
Free Water Surface Constructed Wetlands
Surface or horizontal flow wetland is fed in an inlet where the fluid slowly run through
the soil or another medium which able to sustain rooted vegetation (if present) in a
shallow unit. One of their primary design purposes during the passage is that the
wastewater will come into contact with a network of aerobic, anoxic and anaerobic
zones. Aerobic process occurs in the roots and rhizomes of the wetland vegetation that
leak oxygen into the substrate (Vymazal 2000). Wastewater purifies by microbiological
degradation and by physical and chemical processes through the rhizomes transfer.
FWS effectively remove organic compounds (TSS BOD and COD) from the water.
PICTURE 1 Basic draft of a Horizontal Flow Wetland (adapted from Kadlec and
Wallace(2009) Treatment Wetlands Second edition.
Horizontal Subsurface Flow Treatment Wetlands
Horizontal subsurface flow treatment wetlands are planted with gravel filters which
means subsurface flow wetland designs have no natural wetland analog (Kadlec et al.
2009). This system has promising treatment solution for territories such as Finland
where innovative answer have to been found as a response to cold climate. The concept
was developed in the 1960s in Germany (Vymazal et al 2008).
Yet, Denmark is a word-pioneer in promoting root-zone method for cold climate zones.
Therefore the most reliable data comes from their projects. According to studies, the
nitrogen removal and filtering of pre-treated wastewater are the major goals in the most
of the cases. If the winter extreme cold, it is a common procedure to store the
discharged wastewater in a tank during extreme winter and reflow it again during spring
time onto the site.
The HSSF reed bed is the most widely used concept of TW. Typically rectangular in
design, the subject water flows into the pool from the inlet pipe and it slowly passes
through the media below the surface in a horizontal path before discharging at the
outflow pipe. Over time, the subsurface will become clogged as a result of the physical,
biological, and chemical processes that occur through wastewater treatment
(Ouellet-Plamondon et al 2006, Kato K et al 2011). The hydraulic performance of the
subsurface will eventually deteriorate to the point where the TW will no longer function
as required. Remediating clogged TWs is the major cost associated with utilizing this
technology, and therefore it is desirable to obtain a greater understanding of the
clogging process so the longevity may be improved (Wallace et al 2000,
Ouellet-Plamondon et al 2006, Kadlec et al 2009, Vymazal et al 2008).
PICTURE 2 HSSF wetland schematic. (adapted from Wallace and Knight (2009)
Small-scale constructed wetland treatment systems)
Vertical Subsurface Flow Treatment Wetlands
The earliest vertical-downflow systems were used in Germany in the 1970s; similar
systems were used in the Netherlands as well. HF beds have low ability to oxidize
ammonia to nitrate mainly due to insufficient amount of oxygen transferred by
macrophytes to the rhizosphere (Vymazal et al 2008).
VF wetlands consist of a bed of sand topped with sand/gravel and vegetation.
Wastewater is led from the top and gradually penetrates downward through the media
and collected by drainage at the bottom. The size fraction distribution of gravel is larger
in the bottom layer (e.g., 30-60 cm) and smaller in the top layer (e.g., 6 mm).
Wastewater gradually infiltrates down through the bed and is collected by a drainage
network at the base. The bed drains completely free and it allows air to refill the bed.
This leads to good oxygen transfer and hence the ability to nitrify. The main purpose of
macrophytes presence in VF CWs is to help maintain the hydraulic conductivity of the
bed. It has a much greater oxygen transfer capacity resulting in good nitrification, and
effectively removes BOD5, COD and pathogens but it is difficult to maintain such
systems in small communities. It has also the advantage in sizing because it requires a
considerably smaller area than a HF system.
PICTURE 3 Typical arrangement of a downflow vertical-flow constructed wetland
(adapted from Vymazal et al. 2008)
Various types of constructed wetlands may be combined in order to achieve higher
treatment effect, especially the nitrogen removal is surpassing. There has been growing
demand to achieve full-nitrification effluents but secondary treatment HF systems
cannot do this because of their limited oxygen transfer capacity (Vymazal et al 2008).
VF systems have a much greater oxygen transport capacity and therefore provide a
better condition for nitrification. The problem that very limited or no denitrification
occurs in VF systems. Therefore, there has been a growing interest in hybrid systems.
Hybrid systems most often a combination of VF and HF systems and this way the
engineers gain the individual systems advantages to eliminate the other systems
disadvantages. Depending on the purpose, hybrid wetlands could be either HF which
followed by VF or VF followed by HF wetland (UN-HABITAT, Vymazal et al. 2008).
PICTURE 4 Two-stage Horizontal and Vertical flow hybrid wetland system (adapted
from Vymazal et al. 2008)
Water Purification Processes in Treatment Wetlands
Wetlands, general are considered as a negative agent in the combating the climate
change because of their emission of greenhouse gasses e. g. methane and their low
albedo which is supposed to result in temperature increase in the surroundings (Brom et
al, 2008) yet constructed wetland (CW) system gain wider practice thanks to their
treatment and buffering of effluent and runoff water.
In Picture five, the general removal mechanism can be seen. The different kinds of
removal mechanism effectiveness differ from one to another; depend on the wetland
type applied in practise (e.g. HF has efficient volatilization removal while HSF has very
limited or negligible).
PICTURE 5 Pollutant removal mechanism (adapted Kadlec et al. 2009)
Treatment wetlands are complex collection of wastewater, substrate, vegetation and
microorganisms systems (reed zoned method is the most used in Europe), designed to
achieve a specific water treatment function, and have a specific, different and more
accurate meaning than the wider term of “Constructed Wetlands” which often applied to
wetlands created for water quality improvement purposes (Fonder et al). The system
purifies water by organic matter (BOD, oxidizing ammonia, reducing nitrogen and
phosphorus quantity) (Vymazal et al. 2008, Kadlec et al. 2009).
TABLE 3 Formulated pollutant removal mechanism of Picture 5 (adapted from
Constructed wetlands manual…2008)
Wastewater component
Removal process
Suspended solids
Sedimentation, Filtration
Soluble Organic
Aerobic& anaerobic microbial degradation
Matrix sorption, Plant uptake
Ammonification followed by microbial nitri-
fication, denitrification, plant uptake, matrix
2.5.1 Sedimentation
Primary sedimentation is used to reduce the high concentration of total suspended solids
(TSS). The operation is accomplished with a constant flow within a deep (typically 3 to
5m) pond or septic tank. Settled solids are removed and can be further treated or
disposed. SS also removed by bacterial metabolism and physical processes and may
also be removed by particulate nitrogen from the water, either as a structural component
or as sorbed ammonia.(Vymazal et al 2008, Wittgren et al 1997)
In SSF wetlands, the main concern is the method of the sedimentation and there is no
evidence that inlet zone clogging can be avoided, as the process is a cumulative practice
and the majority of the particulate matter is settled in the inlet area (Kadlec et al 2009).
This can be explained by physical processes as sedimentation and decantation which are
important in particulate organic matter removal are unaffected during colder season.
Studies (e.g. Jenssen et al 2008, Wallace et al. 2000) suggest a ten years maintenance
circle to avoid overflow problems occurs by clogged in the inflow.
2.5.2 Phosphorus sorption
Sorption is important for phosphorous retention during the start-up period; for ammonia
and nitrogen removal. The bed material represents a potential adsorption for removal of
phosphorus if the soil has the capacity to bond phosphorus substances. The amount
which will be stored it depends on the volume of the inflow wastewater and the soil
retention capacity.If the soil and sediment reach equilibrium with the surrounding water
the substance will be released to the water and will be flow into the water body
untreated. Consequently, it is important to solve constant uptake by plant of phosphorus
or other chemical and biological reaction.
There are certain equations to be used to calculate bed media total sorption capacity.
Phosphorus retention also influenced by the amount of iron, aluminium, calcium and
organic matter present in the soil. As phosphorus retention is endothermic, colder water
temperature will decrease the sorption capacity if the bed aggregate (Kadlec et al 2008).
2.5.3 Plant uptake
Plant takes nutrient to sustain their metabolism. They might also capture chemicals
found in the root zone, which stored or transferred into gases. The uptake occurs in the
root, which locates in the wetlands soil.
Nutrient uptake by plants and microbial transformations of wastewater components and
plant litter in wetlands are both directly and indirectly affected by climatic conditions.
Directly in the sense that plant physiology is governed by solar radiation and
temperature, and microbial processes by temperature alone. Indirect influences include
the dependence of biological and biochemical processes on physical conditions, which
in turn are affected by the climatic conditions (Wallace et al. 2000).
The annual plant uptake of nutrients, and the potential for harvest of nutrients, varies
with the affect of the climatic coefficient. This is illustrated with nitrogen and
phosphorus uptake in treatment wetland plants from different climatic regions (Wittgren
H. et al 1997).
In table four, Wittgren (1997) and his colleagues collected years of data from different
locations, mostly from Nordic countries, and calculated the nitrogen and phosphorus
uptake by plants species. It is extremely useful at wetland design when plant species
choose is in question. In order to let continuous nutrient uptake by plant requires several
harvests per year. Without harvesting, the plants reach saturated level in nutrient load
where plants cannot uptake anymore. Harvesting is not used in dormant time, and
usually plants are not harvested before winter period so the dead litters may provide
insulation for the system which might protects it from freeze (Wittgren et al 1997,
Jenssen et al 2008).
TABLE 4 Annual nutrient uptake for selected plants in wastewater treatment wetlands
(examples from Wittgren H. et al 1997)
(kg ha-1 yr-
(kg ha-1 yr-
Cladophora glomerata+ Elodea canadensis (Sweden)
Glyceria maxima (Sweden)
Phalaris arundinacea (Alberta, Canada)
Salix spp- stems (Sweden)
Salix spp- stems+leaves (Sweden)
Plant species(location)
Cold temperature
Oxidation of organic matter and nitrogen transformation are the most important
microbiogically treatment processes affected directly by temperature. These processes
are also sensitive to the level of available oxygen. Phosphorus removal, being largely a
physical (sedimentation) and chemical (adsorption) process, is less directly sensitive to
temperature, but may be influenced by the oxygen availability due to the sometimes
large role played by redox sensitive adsorption to ferrous/ferric oxides. Oxygen
presence or lack of presence plays a major role in the efficiency during the treatment
procedure. As a result, supplement oxygen must be available to the system either
naturally or artificially for the best removal capacity in winter conditions (Kadlec et al
2008, Wallace et al 2000, Grey 2008).
2.5.4 Nitrogen processes
Nitrogen (N) exists in various forms and compounds, and is continuously involved in
chemical processes that change it from one for to another through reversible reactions
(Grey 2008). There are several chemical processes which transform N such as
nitrificitation, denitrification, ammonification, N fixation and N assimilation. These
transformations create a complex nitrogen cycle in wetland systems (Grey 2008,
Jenssen et al 2008).
Ammonia can be picked up by plant uptake. This means that nitrification is the
dominant mechanism for ammonia reduction in HSSF wetlands. But some cases this
method is not so effective. Because all mechanism of ammonia reduction require
oxygen, it is useful to speculate that the amount consumed in the HSSF wetland has
other chemical contribution as well.
In this paragraph tries a clear overview about the important factors implementing a
successful system is given. The best practice to implement a successful treatment
wetland should be in warmer regions due to effectiveness and additional attention on
maintenance, however numerous studies suggest the effective applicability such
systems in colder climate (Wittgren et al. 1997).
The two most important aspects when designing HSSF TW are the avoidance of
clogging and hydraulic failure (Kato K et al. 2011). If the system design based on
inadequate background knowledge, it most probably will lead to flood on the surface,
especially wetlands that used soil for bed medium. Therefore, studies suggest the use of
FWS wetland for small summer dwellings, permanent dwellings with low contaminant
impact (e.g. dry toilet deployed) which is not recommended all year maintenance in
Finland. In other cases subsurface flow wetland is the reasonable answer to residents.
In cold climate, biological processes are more sensitive to temperature fluctuations,
especially at low temperature conditions. Lower winter temperature coupled with low
oxygen accessibility, although there is more soluble oxygen still the gas exchange may
be reduced by the reason of dormant plants and insulation layer or accumulated dead
litter (depends on the used method). Low soluble oxygen results in low aerobic organic
matter decomposition. Consequently, it is important to pay attention on the maintenance
during winter time to ensure the system continuous optimal procedure (Gray 2008).
The nitrification process considered the main limitation factor when wetlands
considered secondary treatment method in cold climate. Some studies suggest oxygen
enhance in the system through macrophytes but the exact contribution remains a debate
(Ouellet-Plamondon et al. 2006, Wittgren et al. 1997). There are some alternative
solutions to improve different ways the oxygen presence in HSSF e.g. injection of
compressed air in the bed matrix (Wallace et al. 2000) which favor nitrification and
improve total N removal (approx 22-24%). Artificial aeration demands extra energy
input which comes with additional cost, but some instances, it may be still profitable in
long term.
One of the great feasibility of HSSF is that it can be easily adapted to cold climate
environment. Experience suggests that most of the removal occurs within the first few
meters of a HSSF reed bed length. As a result, the distance between the inlet and the
outlet is usually between 11-15m. Studies have also showed that horizontal excess in
length has minimal impact on the overall treatment efficiency. The width
recommendation has increased over time as more practical projects were published.
Solids drop out in the first few meters of reed bed, resulting in clogging and can thus
compromise even distribution of flow. Spreading the distribution over a wider flow path
helps minimize the extent of clogging and increase the longevity of the wetland
(Ouellet-Plamondon et al. 2006). Additional protection to prevent freezing can be
elevated by specific design such as larger and deeper bed or applying a natural/artificial
insulation on the structure (Kadlec et al. 2009, Wittgren et al.1997, Wallace et al. 2000).
Horizontal Subsurface Flow Wetlands Hydraulics
Hydraulics of HSSF can be described simple as water flows through in a planted bed of
porous medium. The design is not that simple, and there are numerous problems arise in
practise. Problems may trace back to design failure or wetland characteristic changes in
the bed structure. Hydraulic design is crucial if the failure wants to be avoided e.g.
clogging. Most of the SSF wetlands are rectangular as can be seen in Picture six. The
picture includes all the physical factors which alter the hydraulic flow. A good
meteorological, topographic and physics background needed to design a HFFS wetland.
PICTURE 6 Notation for HSSF bed hydraulic calculation for the simplest case. The
actual velocity of water is v=u/ε. The subscripts i and o stand for inlet and outlet,
respectively. (adapted from Kadlec and Wallace (2009) Treatments Wetlands 2nd
Note for the figure:
=elevation of bed bottom, m
=elevation of bed surface, m
=elevation of water surface, m
= precipitation, m/d
= distance from inlet, m
= evapotranspiration, m/d
= water depth, m
= bed length, m
= volumetric flow rate, m3/d
= bed depth, m
3.1.1 Cold Climate Effects on Hydrological and Hydraulic Conditions
Cold winter climate with snow will influence the wetland water balance as a result of
dry winter periods with snow accumulation and little run-off, followed by snow melt
and flooding during spring. The magnitude of the influence will largely depend on the
wetland-to-catchment ratio. Hydraulic residence time may decrease significantly during
snow melt. If this is the case, it is advised to provide storage for the melting water
avoiding flooding in the system (Wittgren et al 1997). Many households in rural areas
have shallow piping, and these systems need constant flow (bleeding) in the pipes to
prevent freezing of the water supply during cold periods. This will decrease the effluent
water temperature and decrease the hydraulic residence time in on-site natural treatment
systems. The hydraulic loading will be several times higher than design values and may
lead to surface flow in SSF systems, with freezing and hydraulic failure as a
consequence. Therefore proper modelling is the key for successful long term operation
if minimum maintenance care is aimed.
The Hydraulic loading rate (q) depends on flow and wetland area as follow:
q = hydraulic loading rate (HLR), m/d
A = wetland area (wetted land area), m2
Q = water flow rate, m3/d
Hydraulic loading rate can be extremely useful in a well deigned constructed wetland if
common risk of problems need to be lowered e.g. clogging, oxygen, freeze.
Ice Formation
The formation of ice greatly determines the feasibility of wetlands. Studies have shown
that ice will begin to form on a water surface when the water temperature reaches as low
as 3°C, because of density differences and convective losses. The insulation layer of
snow and the dead vegetation acts as an effective way to prevent freezing in the system.
If snow accumulates before a significant ice layer happens to form, subsequent freezing
is strongly inhibited (Wittgren B et al. 1997).
The presence of ice layer might be beneficial if the ice layer provide insulation and
decline the cooling of the underlying water but if the flow beneath ice fall shorts it leads
to subsequent flooding, freezing and hydraulic failure. It might be beneficial to operate
the HF wetland with a higher water level at the time of freezing and thus create space
for both water and air beneath the ice. In SSF wetlands, it is suggested to dig a deeper
hole to avoid; or applying a mulch layer to prevent the system from freezing. The
prediction of ice formation and thickness requires calculation of energy balances and
water temperatures in treatment wetlands. Major factors for the calculation are wetland
temperature, wind speed, wetland dimension, and loading rate of wastewater, the
ambient air temperature depth and thermal conductivity of different layers (snow, ice,
water, plant litter and another porous media)
Temperature Dependence of Treatment Performance
The temperature dependence of reaction rate constants is commonly expressed from
van´t Hoff-Arrheniusas equation:
𝑘𝑇1 = 𝑘𝑇2 ∗ 𝜃 (𝑇1−𝑇2)
kT1 and kT2 are first-order rate constants at temperatures T1 and T2, respectively, and θ is
the temperature coefficient.
Small-scale, well controlled experiments often show clear temperature dependence for
microbiologically mediated process, such as BOD removal and nitrogen transformation.
In full-scale treatment wetlands the temperature dependence is not as strongly correlated
as in small-scale. Studies showed rare correlation between BOD removal and
temperature in full scale. The same connection was noticed in COD as well. Phosphorus
usually not affected by temperature; however Nitrogen shows significant temperature
dependence with temperature coefficient. Ammonium removal occurs by cation
exchange in SSF wetlands during fall and winter. Nitrification and plant uptake might
than “strip” the soil media of ammonium during the warm season renewing the bed for
the next cool season (Jenssen D et al. 2008).
Studies suggest temperature adoption of the microbial community to the climatic
conditions and might explain the tendency of cold climate wetlands to show less
temperature dependence than in laboratory circumstances. The developments presume
that the wetland is in a steady state, but it is rarely the case. The aim is to achieve good
performance results for long-term. It is presumed that the porous medium is isotopic.
This is probably not true, due to the presence of plan roots and other introduced
particulates (Jenssen D. et al. 2008, Wittgren et al. 1997).
Thermal Conductivity of Various Materials and Insulation (Mulch)
Good insulation design for wetland treatment in cold climate eliminates hydraulic
failure e.g. due to freezing. It is a good plan to discover what effects of mulch and plant
species will have on constructed wetlands to eliminate heat transfer issues (Wallace et
al, 2000). A proper insulation is necessary because cooling water flow in the soil will
reduce the temperature in the system and could lead to malfunctioning e.g. clogged,
freeze (Kadlec et al. 2008). In general for HF wetland, the increased depth, an ice cap
and an average of 10 cm water level under the surface equally elevates the frost
mitigation (Kato K et al 2011).
Conductive studies suggested using e.g. wood chips, pine straw for mulch material.
After a variety of mulch types have been tested Wallace is (2000) listed various kind of
insulation material for colder climate. The material must be well processed and
decomposed, therefore it will not let additional nutrient intake on the system, and must
have a good moisture retention capacity so seeding are not threatened to drought stress.
The nutrient composition of the layer needs to be homogenised, and the structure of the
fibre have to be soft enough to provide a good thermal conductivity, and the mulch
formation must have a good contact between the seeding layer and insulation to not be
an obstacle for fertilization ( Kadlec et al. 2009).
Table five represents the different kind of mulch materials nutrient effluent excess on
the wetland system in the first two years. Wallace (2000, 2005) calculated (using
carbonaceous biochemical oxygen demand (CBOD)) the nutrient soaking by physical,
biological processes.
TABLE 5 CBOD5 Values for Different Blanket covers Calculated Emissions (examples
from Wallace et al., 2000)
Year 1
Year 2
Wood Chips
40 mg/L
20 mg/L
Poplar Bark(“hog fuel”)
60 mg/L
20 mg/L
Wood Chips buried under Sand
120 mg/L
80 mg/L
Reed-Sedge Peat
5 mg/L
3 mg/L
High Quality Yard Waste Compost
5 mg/L
5 mg/L
Choosing the right mulch material is very important at the establishment process. It not
only maintains the process but also helps to prevent additional nutrition intake from the
mulch into the system. Applying the wrong insulation material will highly affect the
constructed wetlands operability and most likely will appear in the maintenance
In table six, another example can be found where the author lists different kind of
materials used as mulch and their parameters according to physical conditions e.g.
thermal conductivity. The point of reference material was the Styrofoam in the study of
Jenssen (2008). This is a good reference data which helps what kind of physical
conditions needs analysing to find the best available insulation layer.
TABLE 6 Thermal properties of various materials and insulation equivalent to the
insulation provided by 10 cm of Styrofoam (data used from Jenssen et al. 2008)
Thermal conductivity (W/mK)
Eq. thickness to
10cm Styrofoam
Peat dry
Peat saturated
Straw dry
Sand Haugstein
Leca (0-4mm)
Leca(0-4 mm)
Wetland Energy Flows
Water temperatures in treatment wetlands are driven by energy flows (gains and losses)
that act on the system. During warm conditions, the largest energy gain is solar
radiation, and the largest energy loss is evapotranspiration (Kadlec et al., 2009).
Because temperature exerts a strong influence in some chemical and biological
processes, it is important to wetland design. In cold climates, freezing of the wetland
may be an operational concern. It is important to understand the energy flows within
treatment wetlands to ensure systems will remain functional in subfreezing conditions.
Modified Energy Balance Term for Cold Climate
When the water surface is below ground, a key assumption in the energy balance
approach is no longer valid: the transfers of water vapour and sensible heat are no
longer similar. Water vapour must first diffuse through the dry layer of gravel, and than
be transferred by swirls and eddies up through the vegetation to the air above the
ecosystem. Energy balance equation can be simplified for winter (Wallace et al, 2000,
Kadlec et al 2009) by the energy lost to the atmosphere equals to the conductive transfer
from ground, plus the difference from the energy entering and leaving from the water.
Eloss = G + (Ui –Uo)
MJ/m2/d= MJ/m2/d+ (MJ/m2/d-MJ/m2/d)
A constructed wetland which was designed for cold climate requires that energy loss be
“throttled down” so the energy inputs, G + (Ui –Uo) can equal to what was lost. Do not
design a system to depend only on ice layer and snow cower if the system was planned
to operate successfully by minimising open heat loss through evapotranspiration.
Studies (Wallace et al 2005, Jenssen et al 2008) showed that ice cannot be expected to
work as an efficient insulator due to high thermal conductivity (e.g. 0.19 MJ/m/d/ºC)
which is about four fold to that of water in liquid state (e.g. 0.05 MJ/m/d/ºC). However,
ice can be used to minimize evapotranspiration. It is usable together with air interstice
or blanket (mulch) to increase the thermal resistance.
Wallace (2000) study analyzed 28 systems with a hydraulic loading rate of 2cm/day
with 15cm of blanket cover filling showed that hydraulic consummation of subjected
treatment wetlands could not be compromised. Mulch insulation provides an adequate
layer when there is severely cold, even extreme temperature such as 20-30º C below
freezing point, but no adequate snow cover. Wallace study showed that mulch
insulation provides a good thermal loss of 0.31 MJ/m2/d compare to a 5cm ice cap and
5 cm air gap would result 1.22 MJ/m2/d, almost 4 times greater (Wallace et al., 2000,
The temperature of the influent flow does not affect the heat linearly of the constructed
wetland system due to the dissipation thermal gradient. According to field monitoring
the heat scatters within the first 25% of the bed length (Wallace, 2000). The aim is to
find a balance point temperature to keep up the system from freeze, and the effluent
temperature considered heavily in the equation. If the system is economic and properly
designed than the energy loss (Eloss ≤ G) in the atmosphere must be equal or lower than
what the ground conductively transfers from the system.
Cold Climate Solutions
The author felt important to mention FWS wetland because of the rather cheap overall
deploy cost which makes it attractable for small dwelling house owners whom have
little effluent impact on the nature.
In the cold climates, the limited oxygen-transfer capability of conventional
subsurface-flow treatment wetlands has lead to the development of alternative design
configuration that improves subsurface oxygen availability. Subsurface-flow (SSF)
wetlands equipped with mechanical aeration have been widely used in North America to
increase oxygen transfer rates and sustain aerobic conditions in the substratum (Kadlec
et al 2008, Jenssen et al 2008).
Free Water Surface Treatment Wetland
FWS treatment wetlands may be the perfect option to replace other existing methods
offered from the database in Finland. A full year operational wetland system allows ice
formation, and by raising the amount of inflow water an ice-air-water layers gap can be
modelled. This is accomplished by raising water levels slightly above the media at the
time of freeze-up. After the surface water freezes, the water level is dropped below the
media surface, creating a dry media gap sealed by ice. Seasonally operated FWS offers
the solution to keep discharged water in a pond or tank during winter time, and reflow it
in the system in good weather conditions. This might be a good solution to cottages
which is accommodated rarely during winter times and preferably dry toilet is installed
on the premise.
Treatment wetlands that operate in cold (subfreezing) environment face several unique
design challenges. During periods below freezing, the water temperature can no longer
be approximated by air temperatures once an ice layer forms on the wetland. Effluent
water temperatures will be in 1 to 2ºC, and the thickness of the ice layer becomes a
design consideration. The formation of an ice layer will reduce the depth of the water
column, reducing detention times, unless the water level is increased in the fall to
accommodate the anticipated thickness of the ice layer. Energy balance calculations are
required to determine the extent of ice formation (Kadlec at al. 2009).
Insulation of Horizontal Subsurface Flow Wetlands
Because HSSF systems can be insulated by the addition of dry gravel and mulch layers,
the balance condition energy fluxes can be modified to prevent ice formation. These
layers add heat flow resistance over and above that which occurs naturally in the
wetland (standing dead, litter, and the snow trapped in the senesced vegetation). These
natural insulation effects can be very important, and may in fact be one of the most
important thermal functions of the vegetation during the winter months.
PICTURE 7 Cross section of a SSF wetland in winter (adapted from Kadlec and
Wallace 2009 Treatment Wetlands, 2nd edition)
The ice thickness can vary significantly from year to year due to variations in snowfall
and temperature. The principal factor is the insulation provided by the snow layer.
Areas of emergent wetland vegetation are much more effective in trapping snow than
non vegetated areas.
In a HSSF wetland, the added insulation provides a better support with an ice layer on
the top of dry media if the bed soil or standing dead plants kept dry which means
lowering water level in the system during winter times (Jenssen et al 2008) or using
specific mulch which keeps the area wetness (Wallace et al., 2000). Often blankets,
supported by standing dead litter are the most effective solution
Wetland Plant Species Selection
Selection of the plants for TWs is important (Wallace et al, 2000, Vymazal et al 2008).
Therefore previous knowledge of geography and wildlife are necessary for successfully
choice of plant species and for ensuring them to be well propagated rapidly spread, and
resistant to grazing pressure. One step is to prevent graze time by deploying fences
around the TWs area but that cause additional costs which might not be affordable for
areas where people have low-budget. The other step would be finding plants which are
less favoured by animals.
TABLE 7 Recommended Wetland Plant Species (data used from Wallace et al 2000)
Common name
Scientific Name
Duck Potato
Sagittaria latifolia
Green Bulrush
Scirpus atrovirens
Blue Flag Iris
Iris versicolor
Cup Plant
Silphium perfoliatum
Stiff Goldenrod
Solidago rigida
Swamp Milkweed
Asclepias incarnata
Sandbar Willow
Salix exigua
Mulch layer establishment is strongly influenced by the material used. Systems will
have poor seed germination and place large drought stresses on seedling if the mulch
layer is implemented poorly or non-existent in cold climate. Consequently, only sprouts
plants can be established due to boreal climate. It needs to be counted to the
implementation plan and species must be purchased minimum three growing seasons
before the planting. Better mulch design results a more hospitable conditions to plant
seedlings and allows for seed germination. Studies have showed that as little as one
growing season plants can be established under these circumstances (Wallace et al,
2000). In the first growing season, blanket cover should be irrigated by water because it
helps the seed growing by dissolving nutrients from the mulch layer and ensuring that
the layer will be more homogenous with the system.
Treatment Performance
Kadlec and Wallace demonstrated (2009) that there are still flow related problems (e.g.
clogging) in the treatment systems and there is not a good existing design for
constructed wetland to predict accurate performance outcome yet.
Sedimentation of suspended solids within SSF systems can cause potential clogging of
the system, especially near the inlet (Kato K 2011). Degradation products from the
volatile portion of the TSS can accumulate within the filter together with the mineral
fraction of TSS. These deposits can block the pores and decrease the hydraulic
conductivity (Wittgren et al 1997).
The suggested procedure is to remove the clogged bed media in question, and replace it
with new soil. This might raise greatly the maintenance cost if the bed clogged and
signs of flood appear on the surface. If additional cost is a problem or difficult to
implement Kedlec(2009) mentions a washing process after the clogged bed soil was
removed from the system. If none of the above mentioned options can be carried out
than a hydrogen peroxide (H2O2) chemical oxidising may help in situ to eliminate the
clogging problem.
Bed clogging comes from the hydraulic failure of full-scale systems (often at a high
price) because clogging phenomena takes longer to develop than a typical graduate
student would work on it. The consequence is the long term viability, and maintenance
requirements, of HSSF wetlands is still unknown, despite the fact that thousands of
systems have been constructed worldwide (Kadlec et al. 2009)
A new method was developed to describe how the hydrological behaviour of a HSSF
CW changes over its lifetime. The model validates measurements of permeability,
hydraulic gradient and total solids accumulation. The governing equation for the
movement of water through this system is the Groundwater Flow Equation (GFE),
= ∇(𝐾(𝜃)ℎ∇ℎ) + 𝑅
where h is the hydraulic head, equal to the elevation (D) plus the metric pressure head
(Hp), K is the hydraulic conductivity, a function of the water content (θ), R is the
vertical recharge, and Sy is the specific yield, a property of the porous material
describing its ability to retain water for a change of h.
In boreal area, applied constructed wetland is a sensitive setup and requires meticulous
implementing in order to achieve good efficiency, avoid failure and minimize error
potentiality so it is crucial to take the clogged effect into consideration as well during
establishment process.
Previous project works revealed that aerated subsurface-flow TWs are a viable
technology selection for effective removal of ammonia-nitrogen of landfill leachate,
especially under boreal conditions. Saturated wetlands with mechanical aeration are
extremely efficient in removal of ammonia-nitrogen (Jenssen et al 2008, Kato K et al.
2011, Kadlec et al. 2009).
Nitrogen is a temperature sensitive substance or it was believed until research results of
studies (Wallace et al, 2000, Jenssen et al. 2008) presented that nitrification can be still
removed from the system lower than 4ºC. The same studies showed that ammonia at
temperatures as low as 2.8 to 4.4ºC can oxidize in HSSF treatment wetlands. A VF
constructed wetland reached nitrification at temperature of 2 - 5ºC in Northern America
(Wallace et al., 2000). The reason why these studies achieved such a significant results
is because biological processes are more oxygen than temperature sensitive. Therefore
artificially aerated constructed wetlands are the answer for achieving good removal
A Forced Bed AerationTM which is a patent-pending design from Wallace (2000) has
shown great results helping nitrogen removal but any other kind of manufacture
designed aeration method will have the same positive effect on the wetland removal
efficiency. A pilot treatment wetland was carried out at the Jones Country, Iowa landfill
to show feasibility of an aeration system. After the initial time an overall CBOD
removal was in excess of 93 % and ammonia reduction was in excess of 90 % at very
low temperatures (3ºC inlet and 0ºC outlet). A family home conducted study resulted in
89 % or greater Total Nitrogen removal by nitrification and denitrification in cold
climate condition (Wallace et al., 2000). Aeration within the system also showed
positive correlation of redox potential in the root morphology of wetland vegetation.
The monitor process must meet the municipality operating permits to avoid confliction.
Monitoring requirements are generally quarterly or monthly, depending on the permit.
Data collected from influent and effluent sources.
Carbonaceous Biochemical Oxygen Demand
Carbonaceous biochemical oxygen demand measures the depletion of dissolved oxygen
by biological organisms in a body of water in which the contribution from nitrogenous
bacteria has been suppressed. CBOD method is widely used as an indication of the
pollutant removal from wastewater in US (Nivala et al 2007).
Biochemical Oxygen demand (BOD) is the amount of oxygen required by microbes or
microorganisms to break down the organic material and oxidize the inorganic matter
present in the water sample. It is essential that BOD is reduced before release to the
waterbody to prevent harm to aquatic life. Too high BOD indicates the risk that,
microorganisms will consume the oxygen present in the water column as they degrade
organic matter, thus creating an anoxic environment and altering the aquatic ecosystem
(Gray 2008).
The five days carbonaceous biochemical oxygen demand (CBOD5) is linked to a
developed age, stable population of microbes in constructed wetland. According to
Wallace (2000) the subjected wetland should be timed to start operating at full
wastewater load in late fall “cold start” because vegetation and media of the system
needs adjustment time and winter is perfect to do that due to plants are dormant.
Constructed wetlands projects show that systems with good blanket layer under cold
climate, achieve a level of 75% reduction in CBOD5 values in the first year, and a
treatment resulting in 90 % in the following year (Wallace, 2000).
The systems
revealed a better insulation layer with reed-sedge versus wood chips which suggest
reed-sedge mulch use in Finland. Reed-sedge peat insulation may offer a low-cost
thanks to domestic peat production in Finland.
Total Nitrogen
Nitrogen removal in a subsurface flow TW is very sensitive of oxygen absence. In the
oxygen absence, ammonia is not well converted to nitrate. The limited nitrogen removal
has been observed in many studies apart that the hydraulic loading rate should allow
nitrification to occur. (Wallace et al, 2000).
The economics of treatment wetlands consists of two major factors: capital costs and
operation costs. The capital cost for free water surface and subsurface flow wetlands are
more or less the same, the cost of difference comes from the gravel which used for
HSSF wetland (Kadlec et al. 2008). It is not possible to tell exact figures when planning
to estimate average cost of a wetland system, because conductors use local material and
labour during construction. The price is also determined by the place where the
manufactured parts are being made and shipped. Residents are advised to compare the
capital cost for the different best available systems (BAT) before implementation. There
is a tendency that alternative technologies replace wetland offered solutions due to high
land value in urban areas, however in rural areas it is still competitive solution by
reason of operation cost significantly lower than e.g. chemically treated. Alternative
solutions also have the disadvantage that requires energy input while constructed
wetlands require minimal or none.
Capital Cost
Treatment wetland systems could provide significant savings when installation costs are
conclusive. The sustainable nature of these systems requires little or no energy inputs,
and also makes them a green investment. Estimated cost savings could encourage
smaller communities and households to install such systems (Gray, 2008).
The sampling of the soil composition and the raw water characteristic is important to
study to reduce the problems which may cause additional greenfield investment. The
costs which need investing to prevent groundwater contamination are land area, soil
construction, and constructed wetland bed liners. The bed media selection is crucial for
good hydraulic flow and plants grow.
Site investigation is the first step before
designing the wetland and piping system. It is advised to find the most resistant plant
against extreme winter conditions to keep some level of removal by plant uptake during
winter times. Additional cost are site work (e.g. fencing, access roads etc.), and human
use facilities.
The construction site must be evaluated and soil characteristics need well
understanding, including soil composition, groundwater elevations, and site topography.
Soil identification on the site is done carefully to avoid additional cost e.g. transfer soil
from other region. Small scale soil transformation may be done by (0.05ha) human
power. Greater soil transformation needs to be involved with machine power.
wetland bed liners means additional cost if the soil topography and
characteristic of the ground permeability (e.g. threaten of contamination of ground basin
water) are too high.
Media means the main cost because meanwhile HF wetland use only 30-40 cm soil, the
HSSF wetland needs 50-80 cm of soil which needs to be shipped to the construction if
the soils structure is not adequate on site. It is important to pick the right media due to
high risk of clogging. The better the composition of the soil the longer the system will
operate without human interfere.
In cold climate region, insulation might be necessary for HSSF systems if natural
processes are not sufficient e.g. snow cover, dead litter. It might be done with straw, or
some blanket cover for cottages. Peat may be the best economic solution in a reason of
constant availability in Finland. Hydraulic control of wetlands must provide a fluent
piping and water level distribution, especially in Finland where cold weather means a
constant threat for HSSF wetlands failure.
Operation and Maintenance Costs
Wetland systems have very low Operation and Maintenance costs e.g. energy input by
pumping, compliance monitoring, maintenance of access roads, mechanical repair. One
primary operating cost of a dwelling HSSF system is the cost associated with pumping
the septic tank. Another cost may arise from piping system does not run deep enough in
the ground at dwelling houses, and it requires constant circulation in the piping to avoid
freeze (Kadlec et al. 2009). Other costs are determined by the local market. In an
example, a constructed wetland development providing wastewater treatment for
46-home residential was calculated an annual operational cost of a 25 085 USD (from
2006 inflation) in Minnesota. It costs approximately 545 USD for a single home
annually ( Kadlec et al 2009).
Sub-surface flow treatment wetlands provide with unsaturated surface layer provides a
better thermal protection than surface-flow constructed wetland in cold regions.
Therefore such systems enjoy greater use in colder climate zones. Aerobic pre-treatment
(e.g. sandfilter, aerated pond or tank) is an important key design if the system has to
meet a strict regulation limit. Energy loss prior to discharge to the wetland can be
minimized if the supply piping system and the pre-treatment units are also insulated. If
the plant material remains intact for winter period, it may provide some insulation. It is
still advised to not harvest the plant in the first growing semester due to dead litter is
thin and may not provide enough protection against cold if mulch layer is not used.
It is recommended to apply mulch layer instead of dead litter with litter harvest which
helps greater nutrients removal from the plant site. SSF surface can be covered with a
porous media with low thermal conductivity which is to be kept unsaturated during the
winter. SSF systems depth should not exceed 60 cm. One possibility to add an extra
10-30cm soil layer which calculated into the level of freeze into the soil which allows
the operating system to not froze and still has hydraulic capacity to conduct the applied
water. This is a disadvantage due to the level of the flow is below of the root system.
According to Jenssen (2008), locations where extensive periods of air temperature
<-20 °C experienced a seasonal storage for the winter waste load might be necessary.
It is interesting that subsurface flow constructed wetlands impose several unique design
elements which are widely used in Nordic countries but it is scarcely in Finland. The
unique designs elements (e.g. artificial aeration, blanket cover) ensure the usability of
treatment systems in cold climate. The Surface Flow and Sub-surface Flow wetlands
bring unique devise to Finland. Surface Flow wetland may be impulsive to residents
whom use their dwellings for temporary periods in summer time and have low nutrition
load in their wastewater. Sub-surface wetlands are advised to residents whom use their
summer houses as permanent home or frequently accommodated. However, improper
installation design, negligent maintenance and unacceptable operational use may lead to
fatal failure therefore extra attention may be paid on these criteria to eliminate
additional costs and ensure whole year operation without extra time spent on
A properly designed insulated wetland is able to achieve high levels of BOD removal,
by preventing the system from hydraulic failure and freeze. Treatment performance will
improve after the first growing season. It is advised to use a well decomposed organic
material to eliminate the affect of additional nutrient load in treatment efficiency.
According to Kadlec (2009) a careful design needs to be implemented about which
plants will be used in the constructed wetlands because some plants do not favor some
mulch material. If the wrong material used for blanket cover, the plants may be
intolerant and die or do not develop properly. Jenssen (2008), Wallace (2000, 2005) and
Nivala (2007) have noticed in their studies that horizontal subsurface flow wetlands
without a design of artificial aeration do not transfer enough oxygen to the roots to
satisfy both phosphorus and nitrogenous removal under cold weather but systems
installed with artificial aeration pump achieved same retention results as in warmer
regions for nutrients. However, snow and ice used as insulation may be enough to reach
reduction limits if the wastewater is minimally loaded with nitrogen and phosphorus.
It seems that sub-freezing environments require some type of insulation strategy for
constructed wetlands depending on climate zone. Dead leafs of decayed vegetation
material has been found as one good method insulating the constructed wetlands.
problem with leaf litter application is that heat gets lost at the not evenly
distributed spots. Wallace study (2005) showed that even small breaches could result in
substantial heat loss. Mulch initially was used as a cover to reduce the odour and
sunscald affects but nowadays it is used to protect the system from freezing. According
to Kadlec and Wallace in the book of Treatment Wetlands (2009) the mulch was first
times used as an insulation media in 1996 on a subsurface flow constructed wetland.
The collected data showed that an additional blanket cover highly effective keeping the
system away from freeze. Later on, a computer modelling systems based on Wallace
work (2000) calculated that mulch can prevent the system from freeze as low as -20 ºC.
Constructed treatment wetland nowadays considered as reliable system and gained
wider use in different application areas e.g. wastewater treatment, agricultural run-off
reduction. Removal of phosphorus is low unless some special media with high sorption
capacity is used. Artificially aerated systems improve organic matter and nitrogen
removal, especially in winter times, the most anticipated times when satisfactory
reduction removals are expected. TSS removal was also noticed in aerated systems.
The following things like temperature, clogging, aeration need to be considered when
designing constructed wetlands. Cost related elements are need to be considered as well
as land area for construction, soil construction, liner, rooting medium, wetland plants,
and hydraulic control structures. The fee of these elements depends from the market
price value.
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