BOTTOM AERATION VS. SURFACE AERATION

Bottom diffusers are superior to surface aeration in all full-scale applications.  By full-scale, we mean anything other than a backyard pond or lake that has been so severely sedimented that it is less than 4’ deep.  At that point, there are few options other than to dredge mechanically.  

At Clean-Flo, we refer to the function of our systems not as aeration, but as inversion and oxygenation.  This is more precise in that it describes the actual function of these systems and distinguishes them from aeration systems that may actually do little to oxygenate a body of water.  Here are a few reasons why inversion oxygenation with bottom diffusers is a better option than surface aeration.  If you are seeking to understand why this approach technically accomplishes the task more effectively than a fountain or other method of surface aeration, these are the reasons:

Surface aeration doesn’t alter conditions on the bottom of the lake.  In stratified lakes, most of the microbial activity on the bottom is anaerobic and produces foul-smelling and potentially toxic gases such as methane and hydrogen sulfide.  In a short period after the initial startup, bottom aeration systems raise these gases from the bottom and expel them to the atmosphere.  Afterward, the altered microbial conditions on the bottom stop producing these gases in the sediment.

Oxygenation of the bottom prevents phosphorous from dissolving into the water column from organic bottom sediment.  This locks it away from uptake by aquatic vegetation and algae, which otherwise can lead to explosive growth.

Bottom aeration achieves full oxygenation of the water column.  First, it introduces oxygen by direct diffusion between the water and microscopic bubbles.  Second, tremendous volumes of water are circulated from the bottom to the surface by the surface tension between the bubbles and water.  A single diffuser in 10 feet of water can circulate 5.5 million gallons of water a day!  Once at the surface, oxygen dissolves into the water from the atmosphere to equalize its concentration in water with its concentration in the air.  

By oxygenating the entire water column, bottom aeration transforms the bottom from an environment of oxygen depletion to an environment that is oxygen rich.  This stops the production of noxious gases and accelerates the decomposition of organic muck.  It also makes the bottom strata habitable to vital components of the food chain such as zooplankton, crustaceans, and fish. 

Bottom aeration promotes complete mixing, and destratifies lakes.  Surface aeration does very little in this respect.  There are numerous reasons why mixing is essential.   Sharply contrasting temperature, pH, oxygen, nutrients, and biomass strata impair the ecological health of lakes.  Thermal stratification discourages their mixing and the biota in each stratum adjust accordingly.  The bottom becomes a dead zone, inhabitable only by anaerobic bacteria.  The top may become a nutrient-rich broth in which algae thrive.  The lack of mixing allows more dangerous and motile cyanobacteria to travel throughout the water column and take advantage of higher nutrients lower strata, and of sunlight in upper strata.

With bottom aeration, no machinery is placed in the water.  There are no moving parts in the water to corrode, no electrical connections to short, and no potential exposure of people using the water to hazardous electricity.

Surface aeration is sometimes used to prevent water from freezing.  Because parts are often exposed to cold air – especially with the use of fountains – this always carries the risk of parts freezing and breaking.  Bottom diffusion is a far more effective method for freeze prevention, which can be used to protect docks and other structures from being destroyed by ice.  

The comparison of the cost of oxygenation by bottom aeration vs. surface aeration is pennies-on-the-dollar.  For example, a bottom diffuser in 10’ of water requires compressed air supplied by 0.25 hp of energy (0.18kW).  Daily consumption equals 4.5 kWhr of energy to circulate 5.5 million gallons per day.  At an average cost of $0.10/kWhr, this equals $0.45/day.  On the other hand, a liquid pump moving the same volume of water will require a 30 hp pump, and 540 kWhr of energy.  At an average cost of $0.10/kWhr, this is $54/day.  In essence, bottom aeration is approximately 100 times more efficient at circulating water.

• Apart from the cost of energy, surface aerators are more difficult to maintain than bottom aeration systems.  The components of bottom diffusion have fewer moving parts, are less subject to corrosion, and with routine maintenance have long lifecycles.  On the contrary, surface aeration systems are more mechanically complex, require more costly equipment maintenance, and are typically far more costly to repair or replace.  

A brief internet survey of both commercial and academic articles confirms these conclusions.  There is near-universal consent as to the greater effectiveness of bottom diffusion than of surface aeration.