Gathering water samples.
The lake was regularly monitored to determine
water quality.
Increased Oxygen to Promote Stratification Reduction
Vero Beach, Florida
Heron Cay, a high-end gated residential development in South Florida, was experiencing a number of problems in their 21-acre lake which is central to the community.
With maximum water depths of over 20 feet, stratification had resulted in a lake with severely low oxygen levels at the bottom. Having no beneficial bacteria to break down organics, heavy muck accumulation and foul odors from hydrogen sulfide gases were present. The lake was consuming what little available oxygen there was faster than it could be replenished, and excessive nutrient levels from fertilizer runoff only made conditions worse. The lake's Biochemical Oxygen Demand (BOD) was extremely high.
With the brunt of summer approaching, Heron Cay had a very real potential for a serious fish kill. Action needed to be taken, so the residents looked to Vertex to provide them with an efficient and cost effective method of bringing their lake system back to a healthy state.
Results
After the design and installation of a Vertex aeration system consisting of 11 diffuser stations being fed by compressors totaling only 2-1/4 horsepower, Heron Cay was set up on a monitoring schedule to determine how lake dynamics were being affected. The results over the four month monitoring period were dramatic.
Within days of initial start-up, the systems four main objectives were beginning to be realized:
The breakdown of temperature and oxygen stratification - the lake's water was being circulated.
Increased oxygen levels throughout the entire water column - creating a healthy environment for both fish population and beneficial bacteria.
Decreased the lake's Biochemical Oxygen Demand (BOD) - oxygen was no longer being consumed faster than it was being replenished.
The residents of Heron Cay are no longer in danger of oxygen-related fish kills, odors have been eliminated and the lake is returning to natural, healthy conditions.
"I've never seen the lake look better"
- Mark Sanderson, President, Heron Cay HOA
AERATORS STATISTICS
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Average Biological
Oxygen
Demand
Average Dissolved
Oxygen Level
at 12 ft
Average Oxydation Reduction Potential
Average
Nitrogen
Levels
Average Total Phosphate Levels
Average
Seechi Disc Levels
Winston Park
Nutrient reduction and oxygen transfer efficiency
in a South Florida retention pond using artificial destratification aeration
- Amanda Quillen, Ph.D.
Introduction
Various sources have quoted the efficiency of oxygen transfer from the diffuser bubbles to be around 5%, but after observing the flow of water and change in oxygen profiles after turning on aeration systems, it is obvious that the vast majority of oxygen transfer takes place at the water-atmosphere interface. Aeration allows a greater volume of water to come in contact with this interface. In order to estimate the total transfer of oxygen brought about by aeration, I monitored lake oxygen levels through startup of the system. I followed a variety of other water chemistry parameters, including nitrogen and phosphorus levels, to investigate other positive changes in the lake.
TURNOVER: refers to the number of times a day an aeration system pumps the total lake water volume from the bottom to the surface. For example, a 0.8 “turnover” means that the volume of water pumped by an aeration system to the surface in 24 hours is equal to 80 percent of the total lake water volume. A 1.0 “turnover” would therefore equal a daily pumping rate of 100 % of the total lake water volume.
By June, the lake had restratified, and oxygen levels were not high enough to sustain fish below a depth of 6 feet (1.8 meters). Oxygen levels were so depleted that despite careful reinstatement of the aeration system, a small fish kill occurred on June 24, 2009, just as the system began running full-time.
Results Nutrients: The highest orthophosphate levels were observed at the sediment-water interface. These levels were reduced from 0.34 mg/L on July 15, 2009 to 0.01 mg/L on April 7, 2010, a 97% decline. Ammonia levels were highest in the bottom waters, and were reduced 55% from 0.60 mg/L in July to 0.27 in April. Biological oxygen demand (BOD) improved steadily, decreasing to the background detection limit (2 mg/L) by October 23, 2009. This represents a 60% decline in BOD since the start of the study.
Nutrient reductions may be attributed to the increase in ORP brought about by aeration. It is interesting to note, however, that the ORP in the hypolimnion increased prior to increases in oxygen. The proximate cause of ORP increase is unknown, and warrants further investigation.
Oxygen transfer: Stratification was most enhanced at the beginning of and during the startup period rather than at the beginning of the sampling period due to hot weather between the June 3 and June 15 sampling dates. Rainy, cloudy, stormy weather leading up to the end of the startup period likely led to more rapid destratification of both temperature and oxygen profiles than may have occurred with aeration alone.
On the morning of June 24, the day after the aerators were turned on full-time, oxygen concentrations averaged only 0.7 mg/L (225 kg of oxygen in the entire lake), with surface values topping out at only 1.75 mg/L. These conditions resulted in a small fish kill involving mostly shad. Conditions likely improved rapidly, as no further fish kills were reported by the residents.
Near complete destratification of temperature was achieved by July 8, after 2 weeks of running the aerators fulltime. At this time, average oxygen levels plateaued at 5.4 mg/L (1682 kg total oxygen). This works out to approximately 4 kg oxygen transferred to the lake per hour (about 0.2 mg/L per day). If calculating efficiency of the system based on this and the amount of oxygen pumped into the lake, you get about 65% of what you pump in.
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