Have you seen the Yahoo group dedicated to pool conversions?
http://groups.yahoo.com/group/swimming-ponds/messages/1
They're pretty keen on still being able to swim in the pool/pond, but it seem's like they are more interested in water plants rather than growbeds+vegetables.
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ata@ata.org.au Issue 91 April-June 2005 ReNew 17
It’s 6:30 am and the rest of the family
is still asleep but Dave Keenan can’t
stay in bed. He can hear the pipes a
calling. Downpipes, drain pipes, ag
pipes, electrical conduit. According to
Dave, this is the stuff that energy-efficiency
revolutions are made of.
Not to mention gravel, builder’s plastic,
geofabric and trellis mesh. Bilge
pump motors with model-boat propellers,
ultraviolet lamps with 12 Volt inverters,
and living organisms including
fish, aquatic plants, aquatic invertebrates
and last but very far from least, zillions
of microorganisms.
These are the components Dave uses
to turn what he calls ‘those fake blue
toxic waste dumps’ into freshwater
aquatic habitats that are a joy to swim
in. In other words, to retrofit standard
swimming pools, above or below
ground, and turn them into low-energy,
biologically-filtered, chemical-free
‘swimming ponds’.
The Australian Greenhouse Office
(AGO) estimates that a typical unheated
50 kilolitre swimming pool (approximately
9m x 4.5m x 1.2m deep) will
use 2,200 kilowatt-hours of electricity
per year when operated in the recommended
manner. Most will use far
more, but even at 2,200kWh this will
generally be the single largest user of
electricity in the household, and therefore
the household’s single largest contributor
to global warming.
The AGO estimates that there is scope
to reduce the energy consumption of
swimming pools by 10 to 15%, mainly
by attention to the pump and its motor.
But Dave has proved it’s possible to do
much more than that. His ‘living’ swimming
pool uses about one quarter of the
energy of a typical ‘dead’ swimming
pool.
How is it possible?
‘It becomes possible when you look not
only at efficiency, but at efficacy, and
when you pay attention to where all that
energy is actually going,’ explains Dave.
Engineers define energy efficiency as
useful-energy-out divided by energyin.
This is an appropriate measure for a
component such as a pump, which converts
electrical energy to a different
form of useful energy, namely that of
the pressure and flow of water.
But how do you measure the energy
efficiency of a whole swimming pool
filtration system? You certainly put energy
into it; however, the useful product
of that energy is not another form of
energy, but the cleanliness of a certain
volume of water for a certain length of
time. So perhaps we should instead
speak of its efficacy in litres per watt.
A carefully operated conventional filtration
system has an efficacy of around
200 litres per watt. Dave’s living system
currently cleans about 800 litres per
watt.
So where is most of that 2,200kWh
going? That’s an average of 6kWh per day.
Most of it is not being lost in the pump,
or motor, or chlorinator. Most of it is
turned into unusable low-temperature
heat due to flow resistance in the filter.
Why is that? ‘Because the filter must have
pores that are small enough to mechanically
stop the particles it is intended to
filter out. It has a cross-sectional area of
only about one square metre and is about
a metre deep. It soon clogs and has to be
backwashed,’ says Dave.
The filter in a living system consists
Turn your swimming pool into
a swimming pond
Chemical-free swimming in your own backyard.
ReNew looks at a unique natural filtration system that can be retrofitted
to your existing swimming pool
18 ReNew I s sue 91 April-June 2005 email:
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of rounded pebbles between about 5 and
7mm in diameter, so the passages between
them—or the pore size—is about
1mm. This is at least 10 times larger than
the pore size of a conventional pool filter,
yet it can filter out single-celled algae
one thousandth of a millimetre (a
micrometre) in size, and effectively
clear the water in a single pass. And it
doesn’t clog or require backwashing.
How can this be?
‘It has at least two things going for it,’
says Dave, ‘gravity and evolution.’
‘It’s at the bottom of the pond so the
larger particles just settle there anyway,
and it is colonised by microorganisms
that have evolved for billions of years
to grab hold of anything that goes past
that looks remotely like food. You just
need to give them something to attach
themselves to and plenty of oxygen, and
take away the carbon dioxide, nitrate and
phosphate that results from the breakdown
of this unwanted organic matter.’
Once you have the flow resistance of
the filter reduced to negligible proportions
by going biological, the pipes start
calling. Flow resistance of pipework
can be reduced by increasing its diameter,
shortening it and reducing the
number of bends. Using an extra lowvoltage
submersible pump means the
whole system can actually be in the pool
and the pipework can be reduced to a
single short, straight, large-diameter
piece going from bottom to top.
There is often a sort of multiplier or
feedback effect that happens when you
start on these efficiency (or efficacy)
crusades. Amory Lovins’ group (from
Rocky Mountain Institute in the USA)
found that when you lighten the body
of a car and reduce its drag and rolling
resistance, you not only get to have a
smaller engine because you’ve reduced
these losses, you get to have an even smaller
engine because you’ve reduced the size
of the engine (and therefore the losses
due to its size and weight).
A similar thing happens here with the
pump. Standard pool pumps are of the
kind we call centrifugal. The water
comes into the centre of a rotating impeller
that has radial fins or vanes, and
is thrown outward by centrifugal force.
It is then collected and funnelled to the
outlet pipe. This in itself involves a
right-angle bend in the water path and
enormous turbulence, but for the pressures
required to overcome the filter
resistance in a standard pool filtration
system (typically 100kPa) these are the
most efficient kind of pump.
You’ve already heard how reducing
the filter resistance meant that Dave’s
pump became small enough to conveniently
power it from extra low-voltage
(12 or 24 Volts). This made it safe to put
it in the pond, thereby allowing him to
reduce the pipe resistance. Because
these modifications drastically reduced
the pressure requirement of the pump,
Dave found that he could then use an
entirely different kind of pump, called
an axial or propeller pump, which produces
two to four times the flow for
the same amount of power.
How does the living
system work?
The large surface area of the gravel provides
many sites for aerobic bacteria and
other useful microbes to attach themselves.
By spreading the filter over the
entire bottom of the pool, we can turn
over the whole volume of the pool in
two hours (that’s twice as fast as a typical
chlorinated pool), while keeping
the water velocity through the gravel at
less than a millimetre per second. This
low velocity allows the microbes to stay
attached to the gravel, and to grab their
food (algae and planktonic bacteria et
cetera) as it goes past.
The oxygen the microbes need is dissolved
at the water surface. The pump
draws the water down through the gravel
via a manifold of slotted pipes underneath
and returns it to the surface to
lose its carbon dioxide and regain oxygen.
Most of what is normally done by
highly corrosive chemicals such as
chlorine or ozone, the aerobic bacteria
do using ordinary oxygen.
It is important for the surface to be
agitated by the returning water, to get
rid of the invisible film of dust, oil and
microorganisms (mostly unicellular
algae) that would otherwise prevent gas
exchange.
The pump must run 24 hours a day to
keep the good microbes alive, but its
speed can be greatly reduced at night
and when no one is swimming, to save
energy.
An early model
pump shown
operating
inside the fish
and plant
refuge.
WWW:
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ata@ata.org.au Issue 91 April-June 2005 ReNew 19
What about leaves falling to the bottom
of the pond? What could look more
natural on a gravel bottom? And the good
microbes soon decompose them. At
present, Dave just removes any floating
debris with a net before swimming. He’s
also working on a 12 Volt powered floating
skimmer.
The nitrate and phosphate are removed
from the water by growing
plants, which must be periodically harvested
and thrown on the garden. At
present Dave uses a submerged, freefloating
plant called Foxtail (Ceratophyllum
demersum).
Sometimes the influx of nutrients is
too rapid for these plants to keep up,
such as when lightning produces nitrogen
oxides which come down as nitrate
in the rain. In that case, when the sun
comes out again, the filamentous
(stringy) algae, which normally exists
as green velvet on the walls, grows rapidly
until it has consumed the excess
nutrients. Unlike the unicellular or
planktonic algae, this filamentous algae
(sometimes called blanket weed) avoids
being drawn into the gravel filter by attaching
itself to the walls. It is quite
harmless—in fact you can eat it—but
but before the streamers start to interfere
with the enjoyment of swimming,
brush the walls to set them adrift and
remove them with a large net.
Because the water in a swimming pond
is free of toxic chemicals, it is possible
for mosquitoes to breed, so Dave also
adds mosquito-eating fish: native Firetail
Gudgeons (Hypseleotris galii). The fish
do not need feeding at the very low stocking
rates used: about 10 fish per square
metre of surface. A fence made of plastic
mesh holds the plants within a strip about
300mm wide along one side of the pool.
The fish can swim through the mesh and
this gives them a refuge when people are
swimming.
A layer of the same mesh is tied down
just under the surface of the gravel.
Dave calls this ‘boy-proofing’. It prevents
active human feet from churning
up the gravel and thereby
destroying its microbe population and
clouding the water.
Dave also employs a backup system
consisting of a germicidal ultraviolet
lamp. It uses a more energetic wavelength
than any that reach the Earth’s surface
from the Sun, called UV-C. This should
kill any remaining pathogens in the water,
although for very resistant organisms,
it may take several passes.
Maintenance
How does the amount of maintenance
compare with a standard pool? Dave
reckons it’s probably about the same.
The irregular jobs of removing filamentous
algae a week or so after rain, and
harvesting the plants, are offset by the
absence of regular maintenance. The ultraviolet
lamp and pump motor need
replacing about once a year. This costs
around $150, but you should save twice
that on electricity and pool chemicals.
How much does it cost?
Dave’s system is modular. He suggests
one lift-tube, with its own independent
under-gravel intake manifold, for
every 15 to 20 kilolitres. The materials
cost about $1000 per module. It’s fairly
labour intensive to build, but should
be no problem for a competent do-ityourselfer
with friends and family to
help out.
If you’re building a new pool, you
could save the cost of the standard filtration
system. But be aware that a flat
or stepped bottom is a lot easier than a
sloped bottom, for building and maintaining
the gravel filter.
Is it safe?
At this stage, nobody can say for sure
whether a swimming pond is as safe for
our health as a standard chemically treated
pool, but according to Dave, ‘there’s
no such thing as complete safety, only
various compromises.’
‘The pursuit of extreme safety, whether
in health or national defence, generally
results in less safety, not more,’ he says.
‘For example, both chlorine and ozone
react with organic matter to form substances
that are know to cause cancer.’
Waterborne diseases can be classified
as bacterial (such as cholera, typhoid and
dysentery), protozoan (such as giardia
and cryptosporidium) or viral (such as
hepatitis A and E, polio and various minor
gastrointestinal diseases). Most
waterborne diseases are transmitted faeco-
orally, which means an infected person
would have to leave some faecal
matter in the water and someone else
would have to swallow the water before
the infectious organisms were removed
or destroyed by the filtration system. Ear
infections can also occur. Some people
can continue to carry some of these diseases
without showing any symptoms.
Fortunately, for normal healthy people,
these diseases are either extremely rare,
not serious, or readily treated.
Chlorine takes less than an hour to
kill most pathogens, but cryptosporidium,
or ‘crypto’ for short, can survive for days.
Crypto causes watery diarrhoea and abdominal
cramps. A person with a normal,
healthy immune system can expect
symptoms to last for a week or two.
High-dosage-rate UV-C is more effective
than chlorine against crypto, because
the level of chlorine needed to
achieve the same kill-rate is unacceptable
to humans. However, Dave estimates
that the UV-C dosage rate of his
current system is about as effective as
standard chlorine levels. Dave is concerned
that excessive use of UV-C may
cause other problems, because in the
words of Robert Heinlein, ‘there ain’t
no such thing as a free lunch’.
He points out that the developed
world seems to be seeing more and more
health problems caused by exces20
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sive hygiene: ‘Polio itself was such a disease.
Infection normally occurred when
children were young and the symptoms
mild. Immunity followed. But with sanitation,
first contact started to occur later
in life when the disease can have serious
consequences. There is also increasing
evidence that occasional mild
infections of various kinds, may be
beneficial in preventing your immune
system from becoming overactive and
turning against you, as seems to happen
in the case of asthma, allergies, inflammatory
bowel disease and others.’
However, swimming ponds should
be avoided by pregnant women and people
who have weakened immune systems,
such as infants and those who are
very old, malnourished, have HIV/AIDS
or an inherited immune deficiency; and
cancer or transplant patients taking immunosuppressive
drugs. Of course
people who have any disease that involves
diarrhoea or vomiting should not
swim, so as to avoid infecting others. If
you’re worried, says Dave, the simplest
thing to remember is ‘don’t drink the
water’. Public health authorities issue
exactly the same warning for standard
chlorinated and ozonated pools.
For those seeking further assurance, a
simple test kit called ‘Bacteria Check’ is
available from EnviroEquip for $28. It
cannot tell you whether any human pathogens
are present—such tests are very
expensive and time consuming—but it
will reveal the presence or absence of
total coliform bacteria. These are not dangerous
in themselves—they occur naturally
in huge numbers in the gut of all
animals, including humans—but high
levels are an indicator of possible faecal
contamination or filtration failure, and
therefore possible risk of disease.
It is safer to swim in a bio/UV-filtered
swimming pond than in most
lakes, dams, rivers and creeks in which
humans swim, so if you’re happy to
swim in these, you shouldn’t have any
worries about a swimming pond.
Now that we’ve got all that heavy stuff
out of the way, we can tell you that swimming
ponds are fun! After swimming in
one, it’s hard to go back. Chlorinated
pools suddenly seem so offensive in the
way they affect your skin, hair, nose and
eyes. And they start to look so unnatural
you feel they might as well be painted
bright orange. Swimming in a
swimming pond is like swimming in a
creek—a particularly clean creek. The
gravel under your feet feels good and
there are things to see.
The Future
Dave Keenan isn’t the only one thinking
about swimming pool conversion.
Try google searches on “swimming
pond” and “natural swimming pool”
(with the quotes), and you’ll see there
are other ways of obtaining chemicalfree
swimming.
However, Dave’s achievement of extremely
low energy consumption and
the ability to retrofit existing pools appears
to be unique. Others have concentrated
on obtaining a natural look in
a purpose-built swimming pond.
There’s no reason why these things can’t
be combined. Dave has started a ‘swimming-
ponds’ Yahoo! group for exchanging
information. These are early days
for the field and there is plenty of scope
for you to make a contribution. ✲
Laying the gravel filter in the bottom of the pool.
For more information contact Dave
through his website:
www.users.bigpond.net.au/d.keenan,
or join the swimming ponds e-list by
visiting
www.groups.yahoo.com/
group/swimming-ponds/, or sending
an empty email to swimming-pondssubscribe@
yahoogroups.com
Dave would like to thank Robin
Holland and Sigi Gutjahr of Mudgeeraba
for getting him started on developing
this system, and the following
people for freely giving of their time
and ideas during the development of
the system: his wife Janelle, his father
and brother, his friends Brendan Lee,
Jan McNicol, Eddie Matejowsky, Ross
Pink, Clare Rudkin and family and
James Hill, Jim at Hobbyrama, Doug at
Radio Active Manufacturing and John at
Choice Electric Co.
Backyard Aquaponics
