THE CATCHMENT AREA
When rain falls on a roof it usually starts gently and then
increases in intensity. This rainfall will wash adhering pollutants
such as soot and bird manure into the storage cistern causing
faecal contamination. With leaf debris present, "ponding"
of stagnant water in gutters can occur making matters worse.
It is for this reason many authorities discourage or forbid
the use of urban collected rainwater for human consumption.
However, due to water shortages, rainwater collection for
non-potable uses such as toilet flushing etc is beginning
to be encouraged.
Where potable rainwater systems are allowed, the order in
which components are installed are:
(b) gutters (screened);
(c) in-line basic type filter (optional);
(d) roof-washer/first flush diverter;
(f) UV or O3 sterilizer;
(g) micron filter.
In some cases disinfectant (usually chlorine) is also used.
For serious researchers on the topic "Rainwater Harvesting",
how many times have you read "It is well known/generally
accepted that the first volume of rain after a dry spell if
allowed into the cistern can cause pollution." And so
it can. For this reason roof washers were developed and manufactured
with the premise that … "A rule of thumb is that
1 gallon of water is sufficient to trap for every 100 sq.ft.
of foot area. "
"Studies have shown that the first mm of rain will remove
most contaminants from a roof. " What studies? What's
"most"? It would depend on what is on the roof.
If the roof has salt deposits from being close to the sea
then the above statements might well be true, but what about
creosote deposited by a fireplace? What about dust and leaf
debris that is furthermost from the downspout? If your house
has just been crop dusted inadvertently or otherwise, do you
think a gallon per 100 sq.ft. would be enough to stop the
pollutants from entering the cistern?
These roof washers, whether they are first flush pits, holding
tanks, settling tanks, roof washer boxes etc can only be described
as containers. All containers work the same way; they collect
the first volume of water and presume that the rest of the
water going to the cistern will be free from any pollution.
What can happen instead is that the contained water can mix
with the cistern water causing the cistern water to become
a biological hazard.
When I speak of container and ball diverters I refer to those
units that isolate the first flush of water by means of a
float ball. Instead of using a float ball why not use a sink
ball? Instead of containing the initial water why not dump
it either to the storm drain or to the cistern overflow? Instead
of having a large container that's awkward to install and
heavy, why not have one that's just 7 lbs when operating?
Instead of having a device where you have to unscrew an access
to clean out the sludge and debris deposited why not one which
self-cleans for all debris under ½ inch? And finally
instead of a device that diverts a fixed volume why not one
which has adjustable automatic diversion?
FLOW-RATE DIVERSION - AN ALTERNATIVE
TO VOLUME DIVERSION
In 1993 SafeRain was invented. SafeRain is a first flush water
diverter which combines what I call a “differential
volume” action with an extensible member. There have
been many “differential volume” first flush diverters
but none appear to have had much success. Briefly “differential
volume” device intercepts a portion of the rain inside
a body (sphere or cylinder) which is balanced by a counter-weight.
When the combined weight of the intercepted water and the
valve body exceed the counter-weight then the body will move
in one motion to seal a valve seat causing diversion sometimes
called the “pilot” principle Instead of a counter-weight,
SafeRain suspends the valve body (ball) from a shaft with
a length of special water-proof elastic.
The shaft enables the ball position relative to the seat to
be adjusted. This changes the flow-rate at which the ball
closes the seat. SafeRain was the first to employ “an
extendible member” (elastic). Above this shaft is a
flat plate with holes in it (baffle plate). This attenuates
the flow of incoming water into the ball. It also prevents
premature closure when there is a recent surge of water.
Recently, there has been a copy of SafeRain released into
the US market. This version uses a ball (looking like an exact
replica of a SafeRain ball) attached to a shaft by a spring.
A spring cannot be wound up so no adjustment is possible.
Also no baffle plate directly above the ball is provided so
that in the event of a sudden surge the ball is more likely
to close prematurely.
HOW SAFERAIN WORKS
When rain starts it will either go around the sides of the
baffle or hit it (Figure 1). Some of this rain will go through
the holes in the baffle and some of this will enter the ball.
Typically about 0.0005 of the rain enters the ball and if
this entry rate into the ball is faster than the drain rate
then the ball will begin to descend eventually closing the
valve seat. The water will then divert to the cistern (Figure
When rain ceases the water
in the unit will slowly drain out of the ball (Figure 3).
When it is empty the elastic will pull it back to its original
position (Figure 4).
By winding the ball up or down
the flow rate at which the ball closes can be adjusted. The
standard model has a closing range between 5 litres/minute
and 35 litres/minute. The "normal" setting is about
10 litres/minute - 15 litres/minute. Of course it can be closed
or opened manually. The leakage rate for the ball when closed
is about 50-100 ml/minute.
In drizzle conditions the unit
will not work automatically. This is desirable since the roof
would not be swept of adhering materials. A sudden rain-burst
would not fill the ball immediately since the ball floats
and the baffle plate attenuates the immediate impact of the
incoming water. This enables the furthermost debris etc to
reach the downspout before closure.
The above description is schematic only. There are other design
features not mentioned that are included for smooth operation.
The design can be adopted for inflow from any direction. The
lifespan of the elastic is 7+ years. The pressure drop across
the unit is minimal. This is important in tropical and monsoonal
areas. SafeRain is not a leaf filter although it does have
an internal ½ inch mesh stainless steel screen.
For urban use rain must fall firstly to clear the air of smog
etc, and then go to clean the roof. The efficiency with which
it does the former depends on the intensity while the latter
depends on the momentum of the moving rainwater across a surface
(roof). Ignoring roof pitch we can equate this with flow rate.
The use of small roof washer boxes should be discouraged.
They are ideal breeding grounds for microorganisms. The use
of large settling tanks, holding tanks or any other container
that allows for suspended solids to settle, although an improvement
on the small roof washer boxes, should be avoided. To say
that "if you can see it, it's pollution" and "if
you can't see it, it's not there", represents a triumph
of hope over logic.
The realisation that the first flush of water needs to be
separated from a cistern if that cistern is being used for
consumption has been acknowledged since the 1920's. The technology
of these first flush systems since then has never strayed
from the container/volume idea. SafeRain has designed a flow-rate
first flush diverter that has taken rainwater harvesting out
of the 20th century and into the 21st.
Recurrence Interval/Rainfall Intensity
A Sensible Alternative to the "First Flush" as a
Authored by: Thomas R. Adams, P.E. and Robert A. Strong. Jr.
Vortechnics Inc., www.vortechnics.com
1997 Vortechnics, Portland, Marine
Web link: www.vortechnics.com/techbulletins/02%20Recurrence%20Interval%20Rainfall%20Intensity.pdf