The Benefits of Bubbles – The dynamics of subsurface aeration

aeration There’s some good general information
out there about aeration — including
a great article by Rick Weidman in the
March/April issue of POND Trade Magazine
on how profitable lake aeration and maintenance
programs can be — but there’s very little readily
available information about the mechanics of aeration.
A good working foundation in the dynamics
of subsurface aeration will help you and your
customers achieve cleaner, clearer ponds and lakes
that require fewer cleanups, cost less to maintain
and provide a great source of additional income.

The Importance of Proper Oxygen Levels Year-Round

First, let’s go over why proper oxygen levels
are critical, in both summer and winter, to keep
your ponds and lakes clean and clear. Of course,
all fish, animals and plants require oxygen to live,
so keeping oxygen levels as high as possible keeps
fish, frogs, plants and other life happy and healthy.

The amount of oxygen that water can carry
goes down as temperatures go up, so the heat of
summer can be stressful, especially when there’s
no breeze to help mix the air into the water. The
lowest oxygen levels occur on still summer nights
just before dawn, when photosynthesis reverses
and plants pull oxygen (O2) out of the water and
exhale carbon dioxide (CO2) into it. The warmth
of the water also increases metabolic rates of plants
and animals, so fish require more oxygen just as
levels decline.

The major algae blooms that occur in summer
threaten O2 levels at night, and they can remove
all the oxygen if there’s a mass die-off, as the algae
decompose. To make things worse, the warm blanket of low-oxygen surface water
can create a thermocline — a boundary
between the warm upper and cooler
deeper water — from wind and wave
action and keeps oxygen from mixing into
the water below. Fish retreating below
the thermocline to deeper, cooler water
consume all the oxygen and can suffocate.
An air diffuser at the bottom of the pond
in the summer is a simple, cost-effective
solution that mixes the layers of water,
distributing life-giving oxygen throughout,
night and day.

Winter

Winter waters hold more oxygen, but
when water gets too cold and freezes, ice
can seal off the surface, cutting off the
supply of oxygen available to fish and
other aquatic organisms and allowing
carbon dioxide to build up to lethal levels.
Snow cover makes things worse, reducing
the amount of light so aquatic plants can’t
produce O2 and consume the CO2.

The bubbles of an air diffuser set just
below the surface will circulate warmer
water upwards to melt a small hole in
the ice, even under extreme conditions.
The photo below shows an ice tube that
formed as the bursting bubbles re-froze
immediately in temperatures of 20 degrees
below zero Fahrenheit, but the aerator was
still able to keep oxygen levels up and fish
safe. It’s important to keep the air diffuser
shallow in the winter, so the deepest waters,
which are the warmest in wintertime,
aren’t disturbed. My friend Warren Franz
tells of intentionally setting a diffuser at the
bottom in a small, deep pond to test what
would happen in a Wisconsin winter. The
rising bubbles kept the water in motion
while the water temperature dropped
below 32 degrees, until finally, the supercooled
pond froze all at once — solid ice
to the bottom, 13 feet down!
demi_GoofyBacteriaCycle
## Bacteria ##
But aeration isn’t just important for
animals and plants — they’re only two
thirds of the story. Bacteria, the third
group of organisms in the aquatic cycle,
convert wastes and toxins to nutrients that
feed plants and animals alike at the very
base of the food chain … and once again,
oxygen plays a critical role.

Bacteria such as the nitrogen-converting
bacteria that turn toxic ammonia to
plant-feeding nitrates are aerobic; they
require oxygen. Other bacteria have the
faculty of digesting wastes either way, with
or without oxygen, which makes them facultative
heterotrophs … but even if they can
work anaerobically, they metabolize wastes
far better with a ready supply of oxygen.

Think of bacteria as engines that work
tirelessly converting ammonia and organic
wastes to nutrients for higher organisms.
Oxygen is just as essential to their performance
as it is in an internal combustion
engine, and for the same reasons. A
gasoline or diesel motor may still fire up
with a clogged air filter, but it will run
more slowly and inefficiently, smoking
and stinking — and the same goes for
bacteria. Without oxygen, aerobic bacteria
can’t “burn” ammonia, and heterotrophic
bacteria consume wastes more slowly
and produce smelly, toxic methane and
hydrogen sulfide.

On the other hand, given enough
oxygen, aerobic bacteria will convert
ammonia from animal wastes into
nitrates that then get consumed by plants.
Heterotrophic bacteria break down the
sludge at the bottom of the pond or lake,
removing nutrients that would otherwise
fuel algae blooms. Aeration supercharges
these reactions, increasing both ammonia
conversion and sludge digestion exponentially.
The rising bubbles of a well-designed
and well-placed aeration system can even
create powerful currents that bring the
anaerobic sludge at the bottom up into the
water column, allowing half a foot of muck
per year to be converted to carbon dioxide and just bubble away! Sure beats shoveling
that sh— … uh, stuff!

One caveat: mixing in too much low-oxygen
bottom water or anaerobic sludge all
at once could drop the total dissolved oxygen
levels too quickly, so aeration and the addition
of aerobic bacteria should be started slowly to
avoid fish kills when more than one-sixth of
the volume of the pond or lake lacks oxygen.

Using Aeration Effectively

Now that we understand why we want to
use an aerator year-round, let’s look at how
to use subsurface aeration most effectively to
get the most oxygen in and the most carbon
dioxide out.

The dynamics of aeration depend as
much on surface area and circulation as air
volume, and the depth of the water makes
a big difference. The deeper the water, the
greater the fetch; that is, the more water each
rising bubble will displace upward as it makes
its way to the surface. The greater the total
volume of water that gets pushed upward, the
wider the water will spread out when it reaches
the surface. And all that water being lifted
upwards needs to be replaced, by a counter
current that sweeps along the bottom inward
to the diffuser, to be lifted upward in its turn.

A relatively modest amount of air can move
tremendous volumes of water under ideal
conditions. The best-case scenario is a fine
bubble air diffuser set just off the bottom in
a deep, bowl-shaped depression with smooth
sides. The rising column of air bubbles carries
deeper, colder, denser water from the bottom
up toward the surface, moving up to 10 times
its own volume of water — especially remarkable
because of how much more water weighs
than air. It’s not a perfect ratio, because the
heavier bottom water tends to slip off to the
sides of the column as it’s moved into warmer,
lighter surface waters, decreasing the efficiency
somewhat. And wind and thermoclines can increase that slippage, but the bubbles
keep moving water upward regardless. The
“boil” of rising water at the surface lifts
water a few inches, and the water spreads
out over the surface, as much as 100 feet
outward. Manufacturers reliably claim that
up to 10 acres can be aerated with a single
three-quarter horsepower air pump and
properly placed diffusers, as long as the
lake is deep enough.

Shallow bodies of water (under four
feet in depth) are actually harder to aerate.
Although they aren’t usually as stratified
and rarely suffer from thermoclines,
shallow ponds have little vertical room for
bubbles to rise. Diffusers don’t get a chance
to move as much water on the way up, and
they won’t affect as much water laterally
either. Aerating shallow lakes and ponds
effectively usually requires more diffusers
spaced much more closely together, which
increases the cost per unit area somewhat.
But low-pressure air pumps and blowers
suitable for shallow water cost less to run
per cubic foot per minute (CFM) of air
than high-pressure, deep-water compressors,
so the choice of the proper source
of air is important. Surface units such
as fountains and paddle-wheel aerators
can be effective in shallow water, but the
paddle wheels are noisy and generally less
attractive, while the surface fountain types
look wonderful but don’t do much mixing
beyond a very limited area, offering more
show than D.O. (dissolved oxygen).

Nutrients raised by an air diffuser at the far side of this pond feed an algae bloom; the water cleared after excess nutrients were consumed
Nutrients raised by an air diffuser at the far side of this pond feed an algae bloom; the water cleared after excess nutrients were consumed

Diffusers

Diffusers generally fall into two types.
Needle-punched EPDM diaphragms from
the wastewater treatment industry are high-pressure units that put out relatively
large bubbles and resist clogging. Fine
bubble ceramic and extruded diffusers
are designed to de-stratify and oxygenate
ponds and lakes. Bubble size and restriction
are the key factors. The smaller the
bubbles, the greater the surface area in
contact with the water, facilitating both
the movement of water as more friction is
generated and gas exchange of both oxygen
and carbon dioxide.

To illustrate, let’s analyze what happens
to surface area as bubble size shrinks. A
bubble 6mm in diameter, about one-quarter
inch, has a volume of 36π cubic
millimeters and a surface area of 36π mm2,
while a 2mm bubble, the diameter of a
pencil lead, has a volume of 4/3π mm3 and
a surface area of 4π mm2. It takes 27 of the
2mm bubbles to hold the same volume of
air, 27 x 4/3π =36π, but the surface area of
the 27 little bubbles is 27 x 4π = 108π mm2
– three times as much as the big bubble!
Three times as much surface area means
three times the gas exchange and much
more friction to help carry water upwards
with the same volume of air — and the
tinier the bubbles, the better it gets.

The next most important factor is the
resistance or pressure it takes to force water
through the diffuser, expressed in inches of
water. Adding the friction of the diffusers
and the friction in the piping to the actual
depth of the pond or lake will give you the
actual depth or pressure the air pump will
have to overcome. IMPORTANT: The
relationship between depth and pressure is
simple, and you probably already know it
if you spec pumps for water gardening. It
takes the same 1 psi to push air down 28
inches or 2.3 feet as it takes to lift water 2.3
inces, so 5 psi is the pressure at 11.5 feet.
The table above will let you calculate how
many additional inches of water the pump
will have to handle to compensate for friction
in 100 feet of pipe.

For example, let’s imagine you want to set a diffuser that handles 5 CFM and
adds 12 inches of resistance, at the end of a
200-foot pipe run so the air source can be
located under cover in the barn, and your
pond is seven feet deep. What pressure
would you need to get that volume of air
through 3/4 inches of pipe? 1’ + (2 x 5.0’) +
7’ = 18’, or divide by 2.3 for the pressure in
psi, about 8 psi. You might consider changing
to one-inch pipe to lower the pressure
required – 1’+ (2 x 1.7’) + 7’ = 11.4’ or 5
psi, and now you can use a smaller pump
that costs less to buy and to run.
Demi_Chart

Air Pumps

Diaphragm air pumps are perfectly
suited for ponds down to about nine feet
deep, 4 psi and under 10,000 gallons.
These are aquarium pumps on steroids,
using rubber or silicone diaphragms
attached to magnets that oscillate back
and forth between two cavities fitted with
flexible check valves to alternately draw air
in and pump air out. Typically producing
one to two cubic feet per minute, these
pumps cost little to run, often under 50
Watts. Inexpensive with few moving
parts, replaceable diaphragms and check
valves — and relatively quiet — they
have become the standard for backyard
ponds. Keep heat in check with generous,
non-restrictive air lines and
change out the rubber or
silicone when needed, and
they will last for years.

Linear piston air
pumps trade double-sided
diaphragms for a single air
chamber with a single
large, magnetically
driven piston sliding back
and forth on a cushion of
air. When the electromagnet
is energized the piston is drawn back against
a return spring, drawing in air, which is
pushed out as the current is cut and the
spring drives the piston forward. Linear
piston pumps are more expensive, typically
quieter and perform longer before needing
servicing than diaphragm pumps, and
perform in the same general range: up to 4
psi, 9 feet and about 5 CFM max.

Linear diaphragm pumps combine the
linear motion of the piston pumps with
heavy-duty diaphragms for a more powerful
pump capable of up to 9 CFM and 7
psi, or 15 feet.

Rotary vane compressors work like
a Wankel engine, with rotating vanes
on an off-center rotor that draws in air,
compresses it and exhausts it as the rotor
spins inside a chamber fitted with intake
and exhaust ports. Mostly for commercial
and lake use, rotary vane compressors last
for years with minimal maintenance and
produce less heat than other air sources,
but the vanes are more expensive to replace
when worn. Volumes up to 50 CFM and
pressure of 15 psi, equivalent to around 35
feet of depth, are typical.

Finally, regenerative blowers offer
extremely high volumes of air at very low
pressure — up to 650 CFM at 4 psi — for
large, shallow lakes, minimally restrictive
piping and non-restrictive diffusers. They are
typically used in aquaculture, where tremendous
volumes of water are required in the
shallow ponds where fish are raised commercially, and noise isn’t a major consideration.

System Sizing

The total volume of air
required is a function of many
factors, including how much
of the pond or lake is anaerobic,
its temperature, whether
it’s stratified, how deep it
is and how it’s shaped. But
there are a couple of guidelines
to follow. Moving the
full volume of the pond every
24 hours is an ideal situation
(that may not be possible, but
it’s a good starting point).
The most efficient systems
can move about 2000 gpm
for three-quarter hp under
ideal conditions.

Next, figure the pressure
of your system. Add the resistance
of the diffuser(s) you’ll
be using, to the pressure
required to overcome the friction
in the pipe, to the depth
the diffusers will be set.
A 90’ x 75’ x 7’ deep pond
= 53,000 cubic feet, so we’d
want to move 2,200 ft. per
hour or 37 CFM. A good fine-bubble
diffuser in deep water
will move six times as much
water as its rated air volume,
so we want 37/6 or about
6 CFM. For a two-diffuser
system where each is rated at
3.0 CFM and 12 inches of
resistance, with 150 feet of
low restriction 1.” tubing in
7 feet of water, we’ll find we
need a pump that provides
6 CFM at (2 x12″ ) + (0.0’)
+ 7’ = 9’ or about 4 psi. (
=
Negligible resistance.)

Two diffusers each rated
at 3 CFM driven by a linear
diaphragm pump that will
handle 6 CFM at 4 psi would
do nicely.

For further assistance with
your calculations, and for
assistance with large lakes,
visit a couple of different
manufacturers’ websites and
ask for help for the tough
ones. They have folks on staff
who are real experts, and will
be able to give you a much
better idea than this simple
overview. Remember, proper
aeration in conjunction with
a maintenance program that
includes regular applications
of heterotrophic bacteria can:

  • Cut your customer’s
    long term maintenance costs
    drastically, by eliminating
    drain-down cleanups.
  • Maintain excellent water
    quality and clarity, eliminating
    odors and increasing
    customer satisfaction.
  • Pay off for you and your
    company, with “bread-and-butter”
    money that comes in
    regularly.
  • 2 Responses to The Benefits of Bubbles – The dynamics of subsurface aeration

    1. Ruvane Richman July 4, 2019 at 6:10 PM #

      Do the bubbles need to rise to the surface for my submersible aeration system to be effective?

      • Lora Lee Gelles July 8, 2019 at 1:30 PM #

        You’re absolutely right. If you don’t see the bubbles, then your system isn’t producing enough pressure to force the air down to the diffuser. Its limit, or shut-off, is somewhere between 8’ and 14’ mark. You will want a compressor that is delivering air at 14’ of depth. In our product line, that would be one of our Deep Water systems, which actually create enough pressure to aerate at depths of up to 45’. 14’ is right in their ‘happy zone’. What size water feature is it? (Btw, it would be “Mr” if I liked prefixes. I’m older than all those ladies who usurped my name. Most of them aren’t even really named “Demi”, they’re just lazy Demetrias and the like. You’re talking to the real one.

    Leave a Reply