In 2019, Louisiana Pond Management was contacted to design and install an effective aeration system for Lake Rosemound in St. Francisville, LA. The lake is 183 acres, consisting of two recreational lakes that are conjoined.
Lake Rosemound sits in the Tunica Hills, approximately 14 miles north of historic St. Francisville, Louisiana. It is a tight knit community whose members are enthusiastic about their lake.
(If you are the impatient type, just skip to the end for results. Ken)
The deep central body of each lake was targeted as the area to be aerated to have the most positive effect. Near the dam, the water is between 16 and 30 feet deep. The deeper you placed a diffuser in an installation, the more efficient it will operate to induce water flow. The depth in the targeted area was going to work well with respect to efficiency. Power was located near the swimming beach and would be made available to the equipment site chosen. The swimming beach is near the road that separates the two lakes and also near the bridge that spans the connection between the two lakes. The electrical equipment for diffused aeration is safely locked away on shore in a weatherproof cabinet. Swimming in a lake or pond with diffused aeration is not a concern, since there is no electrical service in the water. Swimming, boating, and fishing are not restricted by diffused aeration.
Kasco Marine Robust-Aire was the brand of equipment chosen for the project. Ken Rust, the owner of Louisiana Pond Management, worked at Kasco Marine for over 5 years as the project manager for Robust-Aire diffused aeration, developing and marketing Robust-Aire diffused aeration systems throughout the United States.
The Robust-Aire XL models deliver a higher volume of air to each diffuser, using ¾ HP compressors as an upgrade from the standard ½ HP compressors. Since the water was deep, injecting air in a higher volume (3.3 CFM verses 1.5 CFM on standard systems) to fewer locations was an approach that would help make the project more efficient and affordable as this uses less diffusers and tubing to accomplish the same work. The body of water was also open and connected, without islands, so we were not limiting the massive amount of induced water flow by pinching it in an inlet. Each large cabinet can contain two of the high volume compressors and each compressor can deliver air to two diffusers at the accelerated rate. We chose two RA4XL systems for a total of eight diffusers in the South Lake in March 2019. We installed the same systems in the North Lake in February 2020.
We coordinated with the lake owners association for contracting and specifications of the equipment site and electrical circuits to supply the systems. A cement pad was poured and the area fenced. They constructed it in 2019 with forethought to accommodate the 2020 installation as well. The systems operate on 120 volt power and weatherproof 120 volt outlets were installed with bubble type covers. This allows the power cables to be plugged in and remain protected from rainy weather. A flexible drainage pipe was used as a conduit from the compressors to the lake edge in order to protect the self-weighted air tubing from lawn maintenance and the curious.
A total of 5,500 feet of self-weighted, ⅝” diameter tubing was utilized to install diffusers in the South Lake, and a total of 7,100 feet of the tubing was utilized for the installation in the North Lake. Just for reference, a reel of 500 feet of ⅝” tubing weighs approximately 350 lbs.
Each diffuser location was marked with GPS and a small commercial grade float was attached with a retrieval line. The float is approximately 5 feet under water to avoid issues with boat traffic.
Dissolved oxygen and temperature measurements were taken with a YSI dissolved oxygen meter before and after aeration was operational.
After starting the systems, the air distribution was balanced between the diffusers. Compressed air seeks the path of least resistance/lowest pressure, so it prefers the shallowest water with the shortest tubing lengths to the diffuser. The compressor systems include a valve for each diffuser line and each compressor has two diffusers to balance the air distribution between. So if a diffuser was located in 20 feet of water and had 500 feet of tubing and was paired with a diffuser in 30 feet of water with a 1,000 ft length of tubing, the pressure would be greatest in the second diffuser with an additional 10 feet of water pressing down on it and the additional friction of 3.3 cubic feet of air traveling under pressure for 500 additional feet in a ⅝” diameter tube. This is the reason for the valves. By closing the valve on the first and slowly monitoring the change, you can equalize it to the pressure that the second diffuser is experiencing. Once equalized, the flow is balanced and they only need seasonal monitoring and adjustment.
A time-out here to sing the praises of a rocking piston compressor that can actually perform this kind of work: pushing air under pressure, 24 hours a day and 7 days a week. All they ask for is a cooling fan, an air filter change (clean once per month and change once a quarter), and a piston rebuild about every 2 years. Each compressor costs about $27 per month to operate full time. So the entire system for both lakes costs about $216 per month.
We agreed on a quarterly maintenance plan to clean out the cabinets (bugs, spiders, dust and debris), change the air filters, balance the diffusers, and generally inspect the equipment. We also have built in the cost of rebuilding the compressor pistons every 2 years. This keeps the systems in good shape. It is also a good idea to remove vegetation from close contact with the compressors; and to avoid lots of dust and debris exposure (weed trimmers, blowers, and dusty roads). Ant bait application is generally a good idea as well, since fire ants can rapidly invade electrical outlets and equipment and cause damage.
Here’s what you have all been waiting for….
This first thing noted was the enthusiasm for the installation by the homeowners and specifically the fishing population. They were curious and supportive, visiting with us during the South Lake installation. After installation, they were happy to experience increased fishing activity specifically in the general area of the diffusers.
The second part is measurements… real measurements. In 2009, Kasco invested in third party testing of their diffuser flow rates: the ability of the compressed air bubbles to induce mixing water flow. A single Robust-Aire diffuser with 3.3 CFM (cubic feet per minute) of air flow in 30 feet of depth is creating approximately 27 million gallons of induced mixing per 24 hour period (see table below).
This mixing is dynamic because it has the temperatures of the different water layers working for and against it. Water bodies stratify into temperature layers, with solar radiation from the sun warming the top layer. Penetration is limited by the ability of the light to penetrate the water. Additionally, the sun will generally encourage single-cell algae growth and darken the upper water, further limiting the ability of light to penetrate the water depths. The thermocline is the name of the zone between them that durably separates the two layers.
The aeration system’s job is to break this separation. As air is injected and broken up into bubbles (about 2 millimeters or twice the size of window screen openings for reference), the bubbles rise to the surface at 1 foot per second and drag the cooler, bottom water to the surface, rolling it in contact with the atmosphere, allowing it to literally breathe and exchange gases, and mixing it with the warmer surface water. It is still a mass of cooler water and heavier than the surrounding warm surface water, so it sinks again through the water column, creating additional mixing action. This is the key to the efficiency of diffused aeration and why it is the most effective tool for aerating and mixing large bodies of water.
So cooler water is at the bottom of the lake separated from the upper water, and contacts the atmosphere for gas exchange. It is also separated from the oxygen production of surface algae and oxygen- creating photosynthesis. The cooler, lower layer of water is low in oxygen and the upper, warmer layer of water is higher in oxygen (especially on sunny days). Mixing the water mixes the temperatures and the oxygen content.
Results for the lakes were positive and predictable (see table below). Lower oxygen levels on the bottom of the lakes were increased dramatically. Temperatures were much more consistent from top to bottom in the lakes. Also, there was a reduction in oxygen levels in the surface measurements. This is natural. As mixing occurs, some of the surface oxygen production is diluted with the bottom water circulation and some of the oxygen is lost to the atmosphere with the energy of mixing.
With aeration, fish enjoy consistent and higher levels of oxygen, and more consistent water conditions with respect to temperature and pH (which is also mixed). Consistent water conditions encourage fish health, growth, and vigor. Fish’s blood chemistry is in direct contact with the water chemistry and their metabolism must change to accommodate constantly changing environmental conditions. This load on their metabolism is reduced with consistent water conditions. Fish are also protected from seasonal turnovers which can cause lethal drops in oxygen. Fish populations enjoy expanded habitat availability in deeper waters that would otherwise be located in areas with oxygen too low to allow for fish to inhabit.
The Lake Rosemound Association has been a pleasure to work with throughout the project. They are invested in their community and the lake. They are eager to learn about aquatic biology and preserve this natural resource.