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IMPORTANT DESIGN FACTORS
Climate and regional applicability
Since WWG systems rely primarely on green plants
and microbes, they perform more rapidly in warm, sunny conditions, the
approach is ideal for climates ranging from tropical to semi-Mediterranean-type
climates and some southern hemisphere semi-desert and desert climates.
In these conditions with higher temperatures and increased sunlight,
system productivity is high year-round. Applications for colder regions
however can also be very effective as has been shown in WWG projects
in New Mexico and Poland, located in high elevation sites with long
winters. However, in colder climates, the necessary surface the same amount of wastewater
must be larger (at least twice than for a warmer climate) to accomplish
similar treatment. Wastewater Gardens
are not only especially recommended for use in on-site systems, close
to the facilities they are to service, but have also demonstrated their
responsiveness in areas with groundwater close to the surface, for sites
with rocky or impermeable clay soils (that often prevent standard leachfields
from operating), as well as for sensitive areas close to rivers, lakes
and coastal waters (possible creation of buffer zones).
Greywater treatment
Greywater refers to wastewater other than that from the
toilet (including sometimes toilet lavabo) refered to as faecal or blackwater.
Greywater includes showers and bath water, laundry machines,
sinks and kitchen water (although in some applications kitchen water
can be joined to the faecal water and be treated by a WWG unit as
well, as it contains food particles, grease and oil). Some water issued
from industrial operations can also be classified as greywater
although the water content would be carefully analysed in order to
adapt the constructed wetland's design to purify ecologically harmful and sometimes synthetic compounds.
In numerous situations however, greywater doesn't call for
a treatment as intensive as what is provided by a constructed wetland
/ WWG unit and so the WWG unit is used just to treat the black
water. If land is not an issue though and that there is a large
quantity of graywater to be treated, then a constructed wetland /
WWG unit is also appropriate and will generally represent much smaller
surfaces than if it was treating blackwater.
In the case
where both types of water are separated, keeping WWG systems for blackwater
treatment only, greywater would only need to pass through
a sedimentation tank before going directly into subsurface irrigation
trenches. The advantages of separating greywater is that
more irrigation can be accomplished with the wastewater and overall
project costs will be lower, as the WWG unit will treat a smaller
quantity of water. However, we often work in situations where the
separation between black and grey waters is too difficult and/or expensive,
just as a "retrofit" to existing plumbing, and therefore design the
WWG system to treat both types of water.
Stormwater treatment
In urban settings and in regions subject to flooding during
storms, constructed wetlands are being employed to clean the water and
to slow down the movement of water which creates flood pulses. Stormwater
from urban and paved surfaces often contain pollutants like oil and
fuel residues which can be readily cleansed in a wetland. In addition,
in areas where freshwater supplies are limited or expensive, making
use of the stormwater enables the greening of the landscape using a
natural and renewable resource.
Space requirements
In the Western-type housing sector for example, we
assume normal wastewater generation of 125-200 litres per person per
day (European average) although this number can vary greatly according
to cultural norms, geographical location and type of infrastructures
and appliances (in the USA for example average amount of wastewater per day per person can be as high as 500 Liters); for example, a city dweller will have the tendency to
use more water than a countryside dweller and a hotel guest for example
can have a water consumption that is double or triple than a resident's.
For a generation of 150 Liters of both blackwater and greywater
mixed, depending on the climate and requirements of purification, we
would apply a surface of 2.5 to 4 m2
of WWG surface of horizontal flow design. This indication depends on many factors including the
climate (the warmer the climate, the smaller an area is needed since
the plants and microbes are more effective year-round), the kind of
infrastructure and water faucets used, the amount and nature of wastewater
generated, what standard of treatment is required or desired. In cold
climates, these numbers may be twice or three times as large, depending
on the level of treatment required during the cold periods of the year
when plants are dormant and bacterial activity is slower.
In the industrial sector, no pre-indication can be given as WWG surface
will depend on the nature of the industry and thus on the type of compounds
to be treated in the water.
Maintenance
Our representatives and designers provides detailed on-site training of indicated person or personnel
and a maintenance manual upon completion of the project. Proper functioning
of the systems is dependent on several simple but important principal
maintenance steps are:
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Primary treatment: septic tanks or similar
systems need normal maintenance, with a final filter at the end
of the primary treatment (before the water enters the WWG unit);
depending on the nature of the filter used (local natural fibers
or prefabricated) it will need checking every one to six months
and washing/rinsing/or change of material if necessary. In the case where the primary
treatment is a septic tank, itshould be pumped out when solids fill more than half
its depth (in the case of a septic tank, a standard requirement
for its maintenance).
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Gravel: the surface of the gravel should
be kept as clean as possible; if porosity of the original gravel
declines, new gravel can be substituted or the original gravel removed
and cleaned. The plants can then be transplanted back in, and the
system can continue effective treatment for decades more.
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WWG Water Level: Water levels in the wetland
cells need to be checked periodically via the control box, especially
during periods of low occupation when evapotranspiration may exceed
input into the treatment wetland. Until plants become well established
it is important that water levels not be allowed to drop below their
root zone.
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Plants: the wetland plants need normal
garden care - pruning for appearance and encouragement of new growth
and flowers. Heavy prunings of plants should be removed from the
WWG to prevent reduction of gravel porosity when that material decays.
The prunings can be used for mulch outside the system or added to
compost piles. Should the WWG be planted before being used and connected
to the primary treatment (septic tank or similar), in absence of
the sewage water's nutrients it may be necessary to fertilize the
wetland to help the plants get established (not forgetting an adequate
water levels).
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Drainage: ensuring that drainage is adequate
around the WWG so that runoff rainwater and soil do not wash into
the system is extremely important. Wastewater Gardens®
systems are built with a berm higher than surrounding ground level, but
one must check occasionally to make sure soil has not built up around
the WWG basin, which would allow rain runoff and soil to enter.
In the case of the drainage of treated water, if drainage into the
soil is chosen, flushing of the drain pipes may be necessary to
avoid clogging.
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