HEALTH AND SANITATION
29
<< Continued from page 27
uses. For outdoor use, rainwater can be used for irrigation and landscape
watering. While for indoor use, rainwater can be used for laundry services
and flushing of toilets. In order to maintain water quality and contamination
of the public water distribution systems, there needs to be in place proper
cross-connection control, the rainwater harvesting system should be
clearly marked, and system maintenance performed regularly. Guidelines
that were mentioned in prior issues pertaining to catchment area,
collection, storage tanks, and water treatment are also applicable when
rainwater is harvested to be used in conjunction with the public water
distribution system.
Protection from cross-contamination, which is likely to occur during a drop
in pressure, is avoided by using a backflow prevention device. Alternatively,
an air gap can be used as well. The pump that is used for rainwater
harvesting should also have prior approval for use in a potable water
system. This is to ensure that the pump delivers t he relevant minimum
pressure required by the system. Labelling of piping and designating a
specific colour to pipes that carry rainwater, serves as warning to users
that water is not for potable use and should not be used for drinking. Apart
from labelling the pipes, the fixtures that use rainwater should be labelled,
namely, toilet, irrigation outlet, and so on. Personnel who manage and
operate public water distribution systems should have knowledge about
rainwater harvesting systems to ensure both systems function optimally.
When pathogens occur in rainwater they are usually in very low numbers
when compared to their numbers in unprotected surface water. Pathogen
contamination is likely to occur when faecal matter from human, animal
or bird droppings enters the rainwater collection and storage system.
Some of the pathogens that have been detected in rainwater include the
bacteria Pseudomonas sp., Shigella sp., Salmonella sp., Vibrio sp., and
Campylobacter sp. The protozoa Cryptosporidium sp. and Giardia sp.
have also been detected.
If storage tanks are not covered adequately, they present a breeding
ground for mosquitoes, which then can transmit dengue virus and the
malaria parasite (Plasmodium sp.). Algal growth in the tank will also
occur readily if sunlight is able to reach the tank.
Chemical
Like with microbiological contaminants, most chemicals found in
rainwater occur in the system when water is collected, treated, and
then distributed. The sources of chemical pollution include atmospheric
deposition, the material that makes up the catchment area (rooftop and
drainage pipes), the composition of storage tanks, as well as the rainfall
levels that occur in the area in which the rainwater harvesting tank is
situated. Studies have shown a higher level of chemical contamination to
be present in harvested rainwater during the dry season as compared to
during the wet season when there is an increase in rainfall events.
CONTAMINANTS IN RAINWATER
Microbiological
Unlike other rainwater contaminants, microbiological contaminants in
rainwater present a greater and health risk to the user, because they
can cause diseases rapidly. For infants, the elderly, and individuals
with compromised immune systems, waterborne illness acquired from
contaminated rainwater can pose a serious and sometimes deadly health
risk. Pathogens (disease causing organisms) do not usually alter the
taste, smell, or look of water and therefore one cannot tell without some
kind of water testing that the water is contaminated. Pathogens can
multiply very quickly and cause disease even after users have just had
one drink of contaminated water, and the affected user can then pass the
disease on from one person to the next. Although contamination can occur from absorption of airborne
chemicals, this type of contamination contributes very little to the overall
deterioration of the chemical quality of harvested rainwater. If chemical
contaminants are in high enough concentrations, they alter the aesthetic
quality of the water (taste and smell). The concentrations of chemical
contaminants usually remain constant in rainwater and therefore,
periodic testing is usually sufficient to monitor chemical levels. This is
in contrast with microbiological contaminants where continuous and
prolonged monitoring is required to ensure microbial-free rainwater. The
main chemical contaminants in water include metals, minerals, inorganic
chemicals, and volatile and synthetic organics.
Generally, rainwater does not contain any form of microbial contaminant.
The occurrence of microbial contamination in rainwater occurs when
it is harvested and subsequently stored. Household use may further
deteriorate the microbial quality of rainwater. There are two types of
microbial contaminants that occur in rainwater: pathogenic organisms
and non-pathogenic organisms (do not cause disease). Non-pathogenic
organisms are usually found in higher numbers and include Escherichia
coli, which is used as a general indicator of water quality. Metal contaminants that are found in rainwater originate from atmospheric
pollution and leaching from storage tanks and metallic roofs. There is
a slight acidity (pH ~ 5.6) that is sometimes found in rainwater and,
therefore, it takes time for metal to dissolve in rainwater. Nevertheless, this
slight acidity leads to the ability of rainwater to dissolve heavy metals such
as zinc and lead, which are usually found in roofing and storage tanks.
Metal ingestion may not pose a health risk immediately, but exposure over
time (months or years) could lead to deterioration in health. Lead ingestion
in young children is particularly dangerous as it could lead to neurological
effects, which then retard development.
Alternatively, thermotolerant coliforms can also be used as an indicator.
Even though non-pathogenic organisms do no not cause disease, they
affect the general aesthetic quality of the water and might hamper the
efficiency with which the rainwater harvesting system works. Algae have
been shown to make water slimy and also plug up the water filters, while
bacteria and fungi can line water pipes, leading to biofilm formation that, in
turn, supports the growth of pathogenic organisms if they are present.
Unlike non-pathogenic organisms, which are the more dominant
population in rainwater, pathogens are not normally found in rainwater.
www.plumbingafrica.co.za
General debris
Debris in harvested rainwater refers to contaminants that can be
physically seen. These would include insects, bird, and animal
droppings, as well as dust and leaves. Not only does debris have an
effect on the aesthetic quality of rainwater but they are also carriers
for microorganisms (bacteria, parasites, and viruses) especially in
the case of dust, leaves, and animal and bird droppings. The surfaces
of leaves and dust might also be contaminated with potentially
harmful chemicals. PA
May 2018 Volume 24 I Number 3