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HEALTH AND SANITATION
We continue our series on rainwater harvesting, looking at improving the
quality of rainwater to minimise the observable contamination levels.
By Water Research Commission
Improving the quality of some aspects of rainwater harvesting
can minimise the contamination levels that could be observed
after catchment.
CATCHMENT AREAS
Rooftops
Microbial quality of rooftop harvested rainwater is often compromised
through bird droppings, poor collection and storage tank design, while
chemical contaminants may dissolve from the atmosphere during
precipitation and leach into the harvested rainwater, while roofing
material may leach and disintegrate into the rainwater before storage.
Contaminants can be introduced into runoff from the catchment surface
in two ways (Figure 1), either by the washing-off of contaminants that
have collected on the surface between rainfall events, or through the
leaching of chemicals and/or metals from the catchment material.
Ground surface
This harvesting method has been identified as a potential threat to
human and ecosystem health due to the high levels of chemical and
biological contaminants that have been directly linked to disease
outbreaks. When rainwater drops on ground surfaces, it picks up and
transports different chemicals, pesticides, metals, petroleum products,
sediment, human and animal faecal matter. A previous study on
ground surface harvesting reported that surface water collected using
conventional urban drainage techniques, such as gutters, pipes, and
channels, was contaminated by sewage.
STORAGE MATERIAL
As with the catchment surface, chemicals and/or metals can leach from
the rainwater storage tank material(s) or from the various components
located in the tank (Figure 1). The rainwater storage tank can also have
beneficial impacts on rainwater quality by providing a reservoir where
suspended dirt and debris can settle to the bottom of the tank.
Well-designed rainwater harvesting systems with clean catchments
June 2018 Volume 24 I Number 4
and storage tanks supported by good hygiene at point of use can offer
drinking water with very low health risk, whereas a poorly designed and
managed system can pose high health risks. Rainwater can be stored in
either above or underground tanks.
(a) Above-ground tanks
Above-ground tanks are the most commonly used in South Africa and offer
advantages such as easy installation, are cost effective, and they are also
easily accessible, which makes regular maintenance simple.
(b) Underground tanks
Underground tanks can be easily installed during the construction phase.
However, disadvantages of underground tanks include that it is difficult
to detect leaks and take corrective measures; possible contamination
by groundwater or floodwater in case of tank leakage; and water from
underground tanks cannot be drawn by gravity; therefore, power lifting
devices need to be used for drawing the water. Lastly, installing an
underground tank can be time-consuming and require special skills.
Advantages of underground tanks include being long-lasting, being in a
colder and sunlight-proof environment, which reduces algae and bacteria
growth, and they range from 700 to 10 000ℓ.
(c) Tank biofilm formation
The presence of biofilms influences the quality of rainwater. Research
has shown that the presence of biofilm includes negative effects, such
as biofouling in filter and biocorrosion and biocontamination in drinking
water distribution networks, but also positive effects such as biofilm
reactors for the degradation or production of chemical substances in
wastewater treatment processes. It has been suggested that biofilm
may have a function of self-cleaning of the tank and regulation of the
microbial quality in rainwater. Although biofilm might have a positive
impact on stored rainwater quality, only few studies investigated
bacterial composition and distribution, its development and role in this
particular environment.
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Factors affecting
rainwater quality