Are you not getting the aeration performance you expect? Perhaps you’re not even aware that your system is lagging. In this age of doing more with less — practically a mandate considering high water-quality demands coupled with tight municipal budgets — operational optimization is essential. Since mechanical aeration is a cornerstone of successful wastewater treatment, it’s a natural place to focus your optimization and troubleshooting efforts.
Consider the following five tips as part of your aeration system assessment.
The performance of mechanical aerators should be evaluated in terms of oxygen transfer efficiency, but not necessarily in pumping rate or air volume flow (SCFM - standard cubic feet per minute) measures.
The performance of mechanical aerators is rated in terms of their oxygen transfer efficiency usually expressed as pound of oxygen per horsepower per hour (or kilograms of oxygen per kilowatt hour) at standard conditions. Standard conditions exist when the temperature is 20 degrees C, the dissolved oxygen (DO) is at 0.0 mg/L, and the test liquid is tap water. Efficiency claims for aerator performance should be accepted by the design engineer only when they are supported by actual test data.
The performance measures of oxygen transfer efficiency should be conducted per standard conditions as outlined in the American Society of Civil Engineers (ASCE) standard.
The performance of mechanical aerators is evaluated based on a standard aeration efficiency (SAE) or oxygen transfer efficiency (OTE), expressed as mass oxygen transferred per unit wire power per time(kg/KWh or lbs per horsepower-hour) under standard conditions of temperature, pressure, and DO concentrations.
The standard conditions are outlined by ASCE, whose standard test to develop a comparison of mechanical aeration equipment is known as standard oxygen transfer efficiency (SOTE) or standard aeration efficiency (SAE). This data provides a level and equal platform comparison for all equipment types. Testing and reporting oxygen transfer results other than that outlined by ASCE may provide false data, which in turn will provide inaccuracies in process design calculations. SOTEs are tested in clean water, and then formulas have been developed to calculate the actual oxygen transfer rate (AOR) — or what you will get under field conditions — determined by the properties of your wastewater.
Know what to (really) expect. In order to design an aeration system, the SOTE must be corrected to obtain a proper AOR value.
AOR is the actual oxygen transfer rate of the device under actual conditions and must equal or exceed the theoretical oxygen or actual oxygen requirements of the biomass in the system. This value must be adjusted for the efficiency of the aeration device, temperature, and actual site conditions using correction factors alpha and beta. The actual amount of oxygen required must be obtained by applying factors to a standard oxygen transfer efficiency that reflect the effects of salinity-surface tension (beta), temperature, elevation, the desired operating oxygen level, and the effect of mixing intensity and basin configuration.
Mixing is important.
In the design of aeration basin or lagoons, it is extremely important to check the mixing power requirements, as it may be the controlling factor in many cases. Utilizing the directional aspirating mixers can be the best solution to provide efficient mixing in the basin. Proper mixing assures good contact between bacteria and wastewater, helps to provide a uniform oxygen distribution, and avoids stratification and sedimentation.
Consider energy savings and efficiency.
Utilize premium efficiency motors for your mechanical aerators.
Check that the submerged depth of the mechanical aerator is set to produce the maximum mixing and aeration at a lowest amperage draw.
Utilize the DO control sensors and stage unit operation to match the DO demand. Use timers to turn units ON/OFF or variable frequency drives (VFDs) to change speed.
Provide proper maintenance by monitoring units for excessive vibration and amp draw to detect fouling.