HEALTH AND SANITATION
25
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RECOMMENDED MINIMUM WATER GUIDELINES
FOR RAINWATER HARVESTING
In order to maintain the quality of harvested rainwater,
the minimum water guidelines developed should address
the six basic components that make up a rainwater
harvesting system:
• Catchment surface, environment, atmospheric
conditions.
• Gutters and downspouts through which water
moves from the roof into the tank.
• First flush diverters, leaf screens and roof washers
— remove large pieces of debris and dust before
the harvested water goes into the tank.
• Storage tank(s).
• Controls and pumps — determine the level of
water in the tank, minimise air gaps, and prevent
backflow.
• Filtration, treatment, and disinfection — depends
on the end use of the water. For potable use,
filtration, treatment, and disinfection need to be
used. While for non-potable use, filtration and
treatment might be sufficient.
GUIDELINES FOR CATCHMENT SURFACES,
GUTTERS, DOWNSPOUTS, STORAGE TANKS, AND
FIRST FLUSH DIVERTERS
Improving the roof run-off quality of rainwater
minimises the contamination levels that could be
subsequently observed in stored rainwater. Removal
of vegetation from around the catchment area and the
installation of leaf guards prevent large debris from
entering the storage tank.
Birds, rodents, and small animals should also be
monitored so that they do not wander around the
catchment area, thereby introducing microorganisms
into the run-off rainwater, primarily through their
faecal droppings.
The material the roof or catchment area is made of, has
an influence on the quality of the water harvested. A
metal roof made out of powder-coated steel is a smooth
surface that allows efficient water run-off from the roof
and furthermore, it is highly resistant to corrosion. Slate
is also ideal as a catchment surface; however, compared
to metal, it is costly. Irrespective of the type of roofing
material used, testing of the run-off must be conducted
to determine the microbial and chemical quality of water
obtained from the various roofing materials.
Guttering systems are often designed to maximise
the amount of water that can be harvested. Guttering
is usually made of polyvinylchloride (PVC), vinyl, and
seamless aluminium. These systems can also be critical
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control points when it comes to the introduction of
contamination into stored rainwater.
Some of the designs of guttering systems that minimise
contamination include the use of a primary filter such
as coarse screen or fine screen. A leaf screen is a
type of coarse screen that is able to prevent larger
particulate matter from entering the tank. For rainwater
harvesting systems installed in areas with dense
vegetation, it should be a requirement to have one of
these types of screens.
The fine screen, such as a strainer bucket, sits between
the delivery pipe and the tank, preventing small
insects and possibly rodents from having access to the
storage tank. The sizes of these filters can be regulated
depending on what type of particulate matter needs to be
removed. The first litres of rainwater that are harvested
should be drained off through the use of first flush
diverters. This reduces the contamination that can arise
from a dirty roof or catchment surface area. The first
flush of water need not be wasted and can be diverted
to an area with vegetation and plant life that would
require watering, with the exclusion of small gardens and
anything grown for ingestion.
There are various types of first flush diverters that can
be used, with the simplest ones being made out of a
PVC standpipe. Various factors need to be taken into
consideration when determining how much water must
be diverted initially. These factors include the duration
between rainfall events, the slope and smoothness of the
catchment area, as well as the intensity of the rainfall
event. In general, first flush diverters should be able to
divert at least 38ℓ of water per 92m 2 of roof area.
The storage tank is another critical control point in the
design of a rainwater harvesting system. Although there
are a variety of options when it comes to choosing
a storage tank, there are certain characteristics that
all tanks must have in order to optimise the quality
of rainwater. All tanks must have lids and be covered
at all times, and preferably with vents, which hamper
mosquito breeding.
To block sunlight, thereby preventing algal growth, tanks
must be opaque. If the tank is not opaque at the time of
purchase, it should be painted. With continual use of the
tank, sediment build up and biofilm formation might occur
in the tank. It is necessary to have easy access to the tank
in order to clean and perform the necessary maintenance,
thereby maintaining the quality of stored rainwater. The
location of the tank can have negative impacts on the
quality of stored water, especially if the tank is stored
Continued on page 27 >>
March 2018 Volume 24 I Number 1