28-04-2014, 12:49 PM
Constructed wetlands for wastewater treatment
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Definition
A main part of the pollutants contained in wastewater are nutrients that can be removed in wastewater
treatment plants by reproducing natural self-purification processes. Conventional treatment plants like acti-
vated sludge plants enforce biological organisms with energy-intensive mechanical equipment to decompose
complex compounds, to incorporate the nutrients in biomass and finally to separate that biomass from the
purified water. Thus such plants are energy intensive reactors with relatively small area demand that are suit-
able for centralized wastewater treatment.
Constructed wetlands are principally using the same natural degradation processes and nutrient uptake but
they are acting as “extensive systems”. The high degree of biodiversity present in these systems allows mul-
tiple and various degradation mechanisms for several classes of compounds, and therefore higher perform-
ances in comparison with the technological treatment plants in which only few families of specialised bacteria
are grown. The purifying processes take place without input of “human produced” energy by, for instance, oxy-
genating pumps. Furthermore there is no excess sludge to be removed since there is a balance of biomass
growth and decomposition in the constructed wetland system. As a compensation to the low energy demand
there is a relatively large area demand. Accordingly constructed wetlands are usually suitable and cost effec-
tive for small and medium size wastewater treatment.
Domestic or municipal wastewater
Treatment of domestic or municipal wastewater is currently a conventional application. There are several
thousand of operating constructed wetlands worldwide, since more than 15-20 years, and the most used are
the subsurface flow systems. The most available sets of monitoring data (like as the North American Data-
base, the UK Wrc database, several European collections and so on) are related to this kind of application.
Industrial wastewater
There are numerous possibilities also for industrial wastewater like chemical industry, laboratory effluents,
landfills, acid mines... and agricultural or agro-food wastewaters like wineries, olive oil mills, dairy, and in gen-
eral all the high organic content characterised waters.
Application fields 2
A lately developed application of constructed wetland is related to the diffuse (or non-point) pollution treat-
ment. Several kinds of diffuse pollution, like agricultural or urban or infrastructures runoff can be faced using
extensive natural treatments, which effectiveness in the removal of nutrients (Nitrogen and Phosphorous) and
micropollutants, like persistent organic compounds (i.e. Polyciclic Aromatic Hydrocarbons generated by vehi-
cles fuel engines) makes this kind of techniques very suitable for watershed scale approaches wherever a
specific local treatment turns out to be inapplicable.
Due to the commonly very high amount of needed surface for this kind of applications, the most used sys-
tems are the Free Water Surface (FWS) constructed wetlands; their realisation and maintenance costs expens-
es are in fact observably lower than the subsurface systems, not considering the added values produced by
their high naturality and the acceptability and the enrichment of the environment as of the biodiversity.
Treatment process scheme
The most common treatment scheme consists of a primary treatment by a filtration-sedimention device,
like as septic or Imhoff thanks and often grids and degreasers, followed by a subsurface flow constructed
wetland as secondary stage and then by a FWS CW as polishing stage.
When denitrification has to be obtained with a high N removal rate, the third stage should preferably be an
horizontal flow reed bed.
The primary treatment should be sized with a volume per person of about 500-600 liters.
Common configurations
Some examples of common configurations are shown in this slide. The simplest one is a single HF reed
bed, advicable when the requested treatment goal are limited to the Organic Content and Total Suspended
Solids removal. This configuration has the simplest and cheapest management and for this reason is very
affordable in the long period.
When a high removal of Ammonia is needed, it’s necessary to adopt a VF system, that needs a pumping
system or an energy-free feeding system (like as syphons or floating valves or tipping buckets) to spread the
wastewater in an alternate way on the bed surface. It’s normally advised to divide the needed surface in paral-
lel beds, in order to permit a longer resting time after each load of the system.
Hybrid system, that are basically combinations of the three typologies, HF, VF and FWS, are the most
effective in obtaining a very complete purification.
A remarkable optimization of the treatment scheme can be obtained with the segregation of black and grey
wastewater, especially with the final aim of effluent reuse. In fact, greywater are more easily purified than the
black ones, and the effluents are usually less contaminated by pathogens.
Horizontal flow RBTS
This type of RBTS consists in a properly designed basin that contains gravel or sand as substrate, wetland
plants (normally Reeds) and microorganisms; the bed is fed with wastewater coming from a suitable primary
treatment by a simple inlet device.
The subsurface horizontal flow systems (commonly named reed beds when planted with Phragmites) are
most appropriate for treating primary wastewater, because there isn’t an atmosphere/water interface, and this
fact makes this technology particularly safe from the public health point of view.
Therefore these systems are actually useful for on-site treatment of septic tank effluents and grey water.
The HF systems are realized as gravel containing beds where the filling material is sized to offer an appropri-
ate hydraulic conductivity (the most used media are coarse gravel, fine gravel and coarse sand) and to furnish
a large available surface for the biofilm growing.
The beds are waterproofed by plastic membrane liners (HDPE or PVC) or clay. The water level remains
always under the surface of the bed; the wastewater flows horizontally by a slope (about 1%) obtained by
a sand layer under the membrane liner. The subsurface flow prevents odours and mosquitoes and permits
public access in the wetland area. This kind of CW is particularly efficient in Suspended Solids, Carbon and
Pathogens removal, as well as for Denitrification, while, due to its prevalently anoxic conditions, nitrification is
quite limited.
Vertical flow RBTS
Vertical flow reed beds (VF) differ from the horizontal ones for the feeding method, the direction of the
water flow and the filling media. In these systems the wastewater is applied through a distribution system on
the whole surface area and passes the filter in a more or less vertical path. The pre-treated wastewater is
dosed on the bed in a large batch (intermittent feeding), thus flooding the surface. During the time between the
feedings the pores within the filter media can fill up with air which is trapped by the next dose of liquid. Thus
oxygen requiring nitrifying bacteria are favoured and full nitrification can be achieved, but only a small part of
the formed nitrate is denitrified under aerobic conditions. The denitrification and thus total nitrogen elimination
can be increased by a partial recirculation of the nitrified effluent into the first chamber of the septic tank. The
treated water is collected in a bottom drainage system to be discharged. The beds are waterproofed by plas-
tic membrane liners (HDPE or PVC) or clay. The water level can be mantained with a height of about 5-10 cm
from the bottom of the bed, or otherwise the beds can be totally empty after each feeding pulse.
Role of plants
The main role of the aquatic plants (please be sure to select only aquatic plants because of the always
water saturated environment that is a fundamental aspect in constructed wetlands) is to act as catalyzers in
the purification process. This process, as seen before, is a combination of microbiological, chemical and phys-
ical processes. The plants haven’t a significant action as direct removal (for some substances, like N and P or
organic matter, we can talk of a contribution in the order of 10-20% during the vegetative season); they offer
instead a very efficient support for the growth of aerobic bacteria colonies on their rhizomes. Air is pumped
towards the root zone by several mechanisms, like convection.
Another important plant’s function is the maintenance and continuous re-estabilishment of the hydraulic
conductivity inside the beds (preventing that way hydraulic short-cuts that could produce unexpected and
undesired HRT decreases).
Amongst all macrophytes, Phragmites australis or communis is the most used worldwide for its optimal
performances, for its ability in developing deep roots (0.5-0.7 m), for its resistance to aggressive wastewaters
and to diseases.