Description

Lake Rosemound is a freshwater lake located near St. Francisville, LA. It is a deep, irregular shaped lake that is divided into two basins, separated by a bridge on Lindsey Lane. Each basin is roughly 65-70 acres in size with multiple “fingers” or narrower bays and coves. An areal view can be seen in Fig. 1 below.

Issues

The lake has routinely had poor water quality and aquatic weed problems. The decision was made to install RobustAire™ diffused aeration systems into the main area of the lower basin in spring 2019.  If results are favorable, the upper basin will be addressed in 2020. The main area of the lower basin measures roughly 31 acres in size and has depths of 20-25 ft. throughout the entire area.

Project Goals

The goal of the project was to eliminate stratification creating uniform oxygen and temperature levels throughout the water column and to increase dissolved oxygen at the bottom of lake to improve biological function. The process of completely mixing the column of water using diffused aeration technology has proven effective to achieve both project goals but altered due to budgetary reasons.

Solution

RobustAire RA4-XL systems were installed in the lower basin’s main area (locations shown in Fig. 2). The estimated volume of water in the area addressed is 182 million gallons. Utilizing the 3/4HP compressor XL systems, each diffuser receives roughly 3.5 CFM of air at 15 psi.  Each diffuser was positioned in 15-20 ft. of water.

Assuming 3.5 CFM of air flow to each diffuser and an operating depth of 18’, each diffuser is creating 9,407,689 gallons of water flow per day. This creates a total system flow rate of 75,261,512 gallons per 24 hours. This gives the area addressed a turnover rate of 1 complete turn every hours (less aggressive mixing than recommended in most installations, but altered due to budgetary reasons).

Results

Data was collected on temperature and DO levels in 6 different locations before aeration was installed and 20 minutes after initial startup of the RA4XL systems.  Data is presented in Fig. 3.  As the data shows, there was a significant disparity in temperature and DO levels between the surface and the bottom at each test location before aeration start up.  Surface water was much warmer and had very high DO levels while bottom water was significantly cooler and had minimal DO, most at levels well below the threshold for a healthy aquatic environment (typically 5ppm).

After only 20 minutes of operation, the surface temperatures started to decrease, and the bottom temps started to increase slightly.  The most noticeable change, however, came in the DO levels.  The amount of dissolved oxygen at the bottom increased quickly while the surface DO decrease slightly.

The actual results were not surprising in that diffused bottom aeration has been proven an excellent choice in mixing deep, large bodies of water and creating a more uniform thermal and chemical water mix.  However, how quickly the bottom DO levels increased was an impressive result.  In Test Location 6, DO went from less than 1ppm to nearly 6ppm in only 20 minutes.  That is a significant achievement.

Next Steps

Later in 2019, tests will once again be taken around the aerated area in the lower basin as well as the un-aerated fingers in the lower basin and the un-aerated area in the upper basin to compare the effectiveness of the RobustAire Systems.  It is expected that DO and temperature levels in the aerated area will balance out with expected DO of over 5ppm throughout the water column.  The un-aerated areas of the lake will most likely have higher surface temps than and lower bottom temps than the aerated section due to poor mixing.  It is also expected that while surface DO levels may be above 5ppm in the un-aerated areas, bottom DO levels will most likely be less than 2ppm and possibly under 1ppm.

Contact us to learn more about our RobustAire systems and how they can increase DO in your application.