Bear Creek Watershed Association

Bear Creek Watershed Association wrote a great article about a lake effected by the flood of 2013. Check out how the flood changed the lake efficiency and how they turned it around.

The September 2013 flood event used Bear Creek Reservoir as a major flood control structure, which caused displacement and reduced efficiency of the in reservoir aeration system as installed by the City of Lakewood and monitored by the BCWA (BCWA Fact Sheet 6 Aeration BCR). A video survey was completed on the BCR aeration system on April 30, 2014 (BCWA TM2014.01 BCR Video Survey Aerators). The survey demonstrated air supply line damage (kinks and holes), aeration pan displacement, overturned aeration pans, reduced function, and some losses, which reduced the overall system efficiency by 40‐70%. The BCWA has established that an in‐reservoir aeration system is a necessary longterm smart management practice to protect and enhance the reservoir fishery (BCWA Policy 8 BCR Aeration).

image provided by Bear Creek Watershed Association

Since FEMA requires like‐kind replacement, Lakewood determined it would be more cost effective to upgrade and replace the aeration system using Lakewood funding. The BCWA assisted with new aeration configuration, system requirements and replacement options. BCWA and Lakewood staff removed most of the old aeration system and recycled these materials. The company Underwater Repairs Specialist installed 6 Quad Duraplate Diffusers (DDP9X4 Keeton Industries) and weighted line in November 2014 with assistance of Lakewood staff that corresponds to the pattern shown below.

image provided by Bear Creek Watershed Association

Duraplate Diffuser Placement

The diffusers are fine bubble (air supplied by a 7.5 hp compressor) and will increase the dissolved oxygen transfer into the reservoir water column. Lakewood and BCWA will conduct an evaluation on the effectiveness and efficiency of the new aeration system in the spring/ summer growing season of 2015. The diffusers are place on the bottom of the reservoir at an average depth is 9.4 m or 31 feet. The fact sheet shows the locations, placement depth and line length for each diffuser.

Since each diffuser module utilizes 4 of the 9” discs, there are a total of 36 individual 9” discs as per the installation. Consequently, the optimum air flow from the compressor should be 36 SCFM at pressure for the average installed depth of the diffuser modules at 9.4 m or 31 feet. There is about 1.4 pounds per square inch (psi) increased pressure per meter of depth in the water column. This average depth of the diffuser array increases the pressure needed by about 14 psi. As such, the compressor needs to deliver about 26 psi to each disc. The air pressure in the line to diffuser array needs to be about 105- 125 psi.

The Sullair ES6-10H, a 10 HP rotary screw compressor gives 36 SCFM at 125 PSIG (pound/square inch [gauge]). This is a good flow for the diffuser modules, which will emit bubbles in a size range as specified for the diffusers at 1-1.5 mm.

An increase in air flow above the recommended flow rate will result in a larger average bubble size, which is less efficient for aeration and mixing; and produces a resultant decrease in aeration efficiency relative to electrical consumption.

The recommendation is to replace the 7.5 HP compressor with either another 7.5 HP or a maximum 10 HP compressor (optimal), and retain the existing 7.5 HP compressor as an adequate backup unit. The city can then retire the 40 and 75 HP compressors. These larger compressors could cause the maximum flow to exceed the handling of the aeration piping and diffuser modules, and overpressure damage could occur. Since the aeration system effectiveness testing is schedule for this year and with the small potential that several additional diffuser arrays maybe required in the future, the optimal new compressor size should be 10 HP.

Results

From the results of the inspection after 24 hours of operation, it is the opinion of Keeton Industries that the system air flows are balanced correctly in the current configuration. With a properly designed aeration system, with correct air flows and a similar if not identical installed depth of each diffuser module, additional fine-tuning of the air flows is often not necessary. If it is necessary, the best indicator of correct balance is a visual verification of the bubble or “boil pattern and size” of each diffuser.