HEALTH AND SANITATION
27
Constant monitoring of the rainwater harvesting system will ensure that it
performs better, thereby producing rainwater of an acceptable quality consistently.
By Water Research Commission
Different filtering or removal methods should be
used throughout the rainwater harvesting system to
ensure that contaminant concentration is minimised
or prevented altogether.
NON-POTABLE RAINWATER USE – STORAGE,
FILTRATION, AND DISINFECTION GUIDELINES
Rainwater for non-potable use should be stored in
leakproof containers with tightly covered lids to ensure
that mosquitoes and other contaminants do not enter the
tank. Sunlight should not be allowed to enter the tank to
prevent growth of algae.
Depending on the use of non-potable water, there are
varieties of filters that can be used to remove particulate
matter. These cartridge filters should be fixed on the side
of the pump where discharge occurs. The filters need to
be correctly sized to minimise any interruptions to water
flow and to maintain the pressure of the water supply. A
5 micron (um) has been shown to be sufficient for non-
potable indoor use of rainwater.
Chlorination and ultraviolet light (UV) disinfection are
largely sufficient to ensure that microorganisms are
killed, and microbial growth is limited in water for non-
potable use. Bleach (6% sodium hypochlorite) can be
directly added to storage tanks in regular intervals or
through the use of an injection pump that maintains the
chlorine dose at 0.2 parts per million.
POTABLE RAINWATER USE – STORAGE, FILTRATION,
AND DISINFECTION GUIDELINES
Through the use of first flush diverters, rainwater needs
to be prefiltered before it is stored in a leakproof storage
container. However, the storage tank for potable use
must be approved by a body that deals with legislature
governing food and water safety. Similarly, to tanks
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containing water for non-potable use, tanks must be
able to keep out contaminants and prevent algae growth.
The tanks should be covered tightly, properly ventilated,
and kept from sunlight. Depending on the use of potable
water, there are varieties of filters that can be used to
remove contaminant matter. These cartridge filters should
be fixed on the side of the pump where discharge occurs.
The filters need to be correctly sized to minimise any
interruptions to water flow and to maintain the pressure
of the water supply. A filter that is 3 microns or larger in
diameter and is capable of removing up to 99% of the
particulate matter present, is ideal for potable water use.
The occurrence of
contamination in rainwater
occurs when it is harvested
and subsequently stored.
Another important feature of the used filter will be the
ability to remove protozoan pathogens. To improve the
aesthetic quality of the water, an activated charcoal filter
can be added. Chlorination and UV light disinfection
are adequate disinfection methods for potable water
use. Sodium hypochlorite bleach can be used if readily
available. Optimum disinfection will take place in the
system if a residual chlorine concentration of at least
0.2mg/L is maintained at all times.
A UV dose of > 40mJ/cm 2 is usually sufficient to
inactivate most bacteria, parasites and viruses.
However, the recommended dose of UV for potable
water treatment is at > 186mJ/cm 2 . This dose is able
to effectively destroy viruses with double stranded
DNA, which have been known to cause waterborne
disease outbreaks.
COMBINING RAINWATER HARVESTING
MAINS WATER SUPPLY
In Australia, Asia, and Europe, rainwater is already
being used in conjunction with public water distribution
systems to reduce the demand on municipal supply.
The focus, however, is to use rainwater for non-potable
Continued on page 29 >>
May 2018 Volume 24 I Number 3
Rainwater –
post-harvesting treatment