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The
Secret to Successful Rainwater Harvesting:
Floating Filter Intakes
by Doug Pushard
Why
worry about where the water in the tank is pulled from? Isn't
it all the same? The simple answer is NO. No matter how good your
first flush system, if you have one at all, some particulates
are going to enter the tank. Eventually, this material will settle
on the bottom of the tank. So taking water off the bottom picks
up these particulates. Some may be caught by an outgoing filter
but this water may also contain heavy metals or chemicals which,
at worst, may not always be filtered out and at best, will clog
the intake filter faster.
Some
of these incoming materials will float when they first enter,
the tank so taking water straight off the top is not a good idea
either. This is where a device commonly called either a "floating
extractor" or a "floating intaker" comes in.
 It
is an often overlooked part of most rainwater catchment systems.
With all rainwater systems, some particulates will enter the storage
tank and either settle to the bottom or float.
As
the name implies, a floating extractor floats in the water in
the tank and its sole purpose is to intake water from the calm,
clean water that is in the middle of the tank. It connects to
the pipe outlet, typically near the very bottom of the cistern,
where water is drawn for household and irrigation use. On most
systems, this intake pipe is normally left uncovered.
To
illustrate the difference in water quality, water samples were
drawn from the same cistern at the same time. The results of hese
tests are shown in the chart.
 As
is visible from the tests, the cleanest water is from the middle
of the tank. These water quality tests were performed at a nationally
certified test laboratory ( i.e. Related Topics) with the top
water (T) being sampled first, followed by the center (C) and
then the very bottom (B). The bottom of the tank was not scraped,
which would have given a more extreme result.
Water
from the top of the tank has significantly more particulates than
the other two tests.
| Tests
Explained
pH
- The measure of the water's relative acidity or alkalinity.
A pH of 7 is neutral while higher numbers indicate acidity
and lower alkalinity.
Turbidity
- Measures the impedance of light through water. The particles
which cause turbidity can interfere with disinfection by
sheltering microbes.
Hardness
- inhibits the cleaning action of soaps and detergents.
It can also cause deposits of scale on the inside of hot
water pipes and cooking utensils. Hardness is caused by
calcium and or high levels of magnesium salts.
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Additionally,
either the top or bottom had the higher mineral and metal content
(i.e. Calcium 16% higher,
Copper
7.1% higher, Iron 11.6% higher, Magnesium 7.1% higher, and Manganese
16% higher) than the water in the middle. Water from the very
bottom and the very top of the tank are clearly the worst. The
tested cistern has only been in use for 2 years and it is reasonable
to expect that over time the metals, minerals and solids will
continue to build up, and as this "stuff" builds up,
will get closer and closer to the intake pipe and will become
part of the everyday incoming water supply.
It
is critical to note that particulates in the water can also decrease
the effectiveness
of UV lamps by potentially blocking harmful bacteria from sterilization.
The
pH and hardness do not vary much with the depth of the water;
however, these readings are lower, and in some cases vastly lower,
than nearby well and city water.
So
taking water from the middle of the tank is clearly the best solution.
Floating extractors are a quick and inexpensive way of getting
to this water; providing access to the best water, filters will
last longer due to having less to filter, and best of all, they
are simple and inexpensive.
You
can purchase one for $75-$100 or build a simple one yourself for around
$30. Either way they are a great investment and should be included
in every system.
Related Links:
EPA
published water standards
Article
on simple sand filter for water purification
National Testing Laboratory
Instructions on How To Build and Other Information Products
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