Figure 3: A constructed wetland system behind a greenhouse facility.
gens is therefore a sustainable and relatively inexpensive method of water treatment for the greenhouse
industry. It is not only the populations of plant pathogens that are controlled but studies have shown that
even populations of human pathogens such as Escherichia coli and Salmonella spp can be significantly reduced through the use of constructed wetlands. It also
lowers the levels of certain nutrients that may have accumulated in the re-circulated nutrient solution and can
also eliminate pesticides and other organic contaminants in the wastewater.
In the past natural wetlands were used to treat
wastewater but today specially constructed wetlands
can be used almost anywhere to take advantage of the
same processes that occur in natural wetlands. Constructed wetlands keep beneficial micro-organisms alive
and use natural processes including macrophytes and
other microbial communities to reduce populations of
pathogens. Different design types exists with the most
popular for the treatment of greenhouse wastewater
being the subsurface flow constructed wetland system.
In this horizontal flow system the wastewater enters at
one end of a plastic lined channel filled with gravel as a
growing and filtering medium and the vegetation rooted in the gravel bed. The liner retains the wastewater
and prevents groundwater from entering the system.
The depth of the gravel bed will be determined by the
vegetation type but is normally between 50 and 75cm.
The water level is normally maintained below the surface of the gravel through a stand-pipe structure at the
discharge end of the filter bed. The filtered water is
then drained out of the wetland system at the end of
the bed through an outlet system below the gravel bed.
The processes involved in pathogen removal include
physical, biological and chemical actions. The physical
processes consist of filtration, sedimentation, soil and
biofilm adsorption and aggregation. Solids are removed
through the physical filtration within the gravel. The
biological processes are made up of predation, bacteriophage activity and natural death while the chemical
processes are a result of the antimicrobial metabolite
release from the plants in the system. Microflora such
as Bacillus spp and Pseudomonas spp. can suppress
plant pathogens such as Fusarium spp. and Pythium
spp. through antagonistic actions. Pseudomonas spp
have been shown to control Pythium spp populations
through competition for the same food source. Bacillus
spp on the other hand produce a variety of antimicrobial substances that help with the control of several potentially harmful bacteria and fungi. The efficiency of
the constructed wetland in removing pathogen will be
determined by several factors some of which can be
manipulated. Important environmental factors include
the pH and temperature. The type of vegetation as well
as the planting density will also have a significant effect
on the efficiency of the wetland system. The organic
carbon content in the wetland system can also affect
the function of the wetland system. A higher carbon
content in the system, added in the form of compost to
the growing substrate will result in a larger biodiversity