The Benefits of Bubbles – The dynamics of subsurface 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!

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).

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 in., 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.

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.