Designing Ecological Habitats - Gaia Education

wetlanDs for tHe ecovillage 159
treatment of the sewage, i.e. without oxygen. The concentration of suspended
solids (TSS) is reduced as well as pollution measured as Biochemical Oxygen
Demand (BOD). BOD is an index of the amount of organic pollution in the
influent water from the ecovillage.
There are two types of constructed wetland – surface flow and sub-surface
flow. The former allows wastewater to flow horizontally past and through
plant stems growing in soil, flooded with wastewater. While less expensive
to build, there are disadvantages associated with open water, such as danger
to children and animals and breeding of mosquitoes. For sewage treatment
in ecovillages, sub-surface flow constructed wetlands are recommended. In
this type of wetland, plant roots and gravel provide substrate for bacterial
films and the water flows about 10 centimetres below the gravel surface,
which provides a safer system with no odours from the wetland and no
mosquitoes, as there is no surface water.
The main advantages of constructed wetlands are that they provide
passive treatment; construction is relatively simple and operations minimal.
However they take significant amounts of land and need a level site.
Wastewater treatment is often a trade-off between energy and land. Operation
and maintenance is principally around maintaining the planted ecology in
good condition and ensuring ancillary mechanical equipment performs.
Wetlands have among the lowest wastewater treatment carbon footprints
for operations. Embedded energy and construction carbon footprint can
also be low where bulk materials, such as gravel, can be sourced locally. In
Ecovillage Design Education trainings, this technology is often used as the
basic system for the design and construction of a natural systems sewage
treatment plant.
Designing and Building a Constructed Wetland
The size of the wetland will depend on the number of people it serves plus
any extra wastewater from other ecovillage sources, such as food processing.
Water for washing dishes and food preparation, baths and showers and toilet
flushing is included in the person count. The low winter water temperature
is relevant as the warmer the water up to 30ºC, the smaller the size of the
wetland. Although the treatment slows down at cold-water temperatures,
wetlands can continue to operate at air temperatures below freezing, provided
there is some influent each day of water from the built environment. The
plants die back and fall on the wetland surface as insulating mulch. A blanket
of snow adds to the thermal insulation.
The following is a typical chart for sizing a constructed wetland for
between 100 and 1,000 people for effluent BOD of 15 mg/l at various water
temperatures.
The wetland is sized for winter conditions, when microbial life is at
its slowest. The size is the treatment area at full depth within the wetland.
Allow 30% extra land for sloping sides in the wetland, berming and access.
The length should be no more than twice the width. The reason for this