HEALTH AND SANITATION
The purpose of pre-filtration is to significantly
cut down on maintenance by preventing
organic build-up in the tank, thereby decreasing
microbial food sources. If rainwater is not pre-
filtered, a large amount of organic matter in the
form of leaves and dirt can enter the storage
tank. Aerobic bacteria begin to consume the
organic matter and use up all the dissolved
oxygen. This sets up anaerobic conditions
that allow anaerobic bacteria to predominate,
resulting in odour. Other benefits of pre-filtering
rainwater are reduced sediment build-up at
the bottom of the storage tank and less tank
maintenance.
• Leaf screens
Leaf screens are installed over either the gutter
or downspout to separate leaves and other
large debris from rooftop runoff. Leaf screens
must be regularly cleaned to be effective; if
not maintained, they can become clogged and
prevent rainwater from flowing into the storage
tanks. Built-up debris can also harbour bacteria
and enhance their growth within gutters or
downspouts.
• Gutter guards or screens
Gutter screens can be installed on the gutter
to filter debris before they enter the gutter.
One advantage of gutter screening is the
large filtering surface area, which can reduce
maintenance of the filter surface. Some
higher-quality gutter screening is nearly self-
cleaning and requires very little maintenance.
The micro-mesh screen can filter debris in the
80–100-micron size, which is beneficial for
potable or indoor fixture systems that require
superior filtration.
• Downspout filters (leaf catchers)
These filters are placed either at the top of
the downspout where it meets the gutter, or
somewhere along the length of the downspout.
These filters generally only provide coarse
filtration of 3175–1587 microns and should
only be used for rain barrel systems. A few
models can filter to 280 microns and make
good pre-filters for irrigation systems and
indoor non-potable uses.
• First-flush diverters
First-flush diverters direct the initial pulse of
rainfall away from the storage tank. While leaf
screens effectively remove larger debris such
as leaves, twigs, and blooms from harvested
rainwater, first-flush diverters can be used to
remove smaller contaminants such as dust,
pollen, and bird and rodent faeces. Simple
first-flush diverters require active management,
by draining the first-flush water volume to
a pervious area following each rainstorm.
First-flush diverters may be the preferred pre-
treatment method if the water is to be used for
indoor purposes.
• Insect screens
The inlet of the tank should incorporate a mesh
cover and a strainer to keep leaves from entering
the tank and to prevent access of mosquitoes
and other insects. The overflow should also
be covered with an insect-proof cover such as
plastic insect mesh wired around the pipe.
POST-STORAGE TREATMENT DEVICES
The fundamental difference between
centralised water treatment works and HWT is
not the underlying mechanism for treating the
water, but the point where such treatment is
implemented. While the former is a combination
of treatment methods, the latter (HWT) tends
to rely heavily on a single approach. These
PRE-STORAGE TREATMENT DEVICES
Pre-storage treatment methods are required to
keep sediment, leaves, and other debris from
entering the RWH system. Leaf screens and
gutter guards meet the minimal requirements
for pre-filtration of small systems, although
direct water filtration is preferred. All pre-
filtration devices should be low-maintenance or
maintenance-free.
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www.plumbingafrica.co.za
August 2018 Volume 24 I Number 6