One of the key aspects of designing an efficient lagoon aeration system is properly taking into account the conditions in the field. Factors such as wastewater composition, barometric pressure, and ambient temperature all play a role in the end design efficiency. The truth is that there is a big difference between the efficiency of an aerator under clean water lab testing conditions and the efficiency in actual wastewater.
Following Part I of our Efficient Lagoon Aeration blog series, Part II focuses on how to take the theoretical design conditions and apply them to the field. In the final installment, Part III, we will be looking at the how to best design the layout, header, and piping of the aeration system for maximum lagoon aeration efficiency.
Aeration System Lab Results
Clean water oxygen testing is generally performed under standard American Society of Civil Engineers (a.k.a. ASCE) lab testing conditions. It is intended to normalize results between aeration systems and manufacturers. It is not intended to predict oxygen transfer efficiency in wastewater field conditions. To get a better feel for what the actual field conditions and results will be, adjustments must be made to the lab results. Failure to properly adjust the lab results for the field conditions during design can lead to system failure in a variety of forms, such as:
High lagoon BOD, TSS and Ammonia effluent
- Lagoon odors
- Lagoon algal blooms
- Low lagoon DO
- Undersized aeration system
- System failure
- Rapid equipment degradation
- High lagoon maintenance costs
Field Condition Adjustments
There are a variety of field conditions that you need to adjust for when choosing an aeration system for your wastewater lagoon. The most important ones are outlined below.
1. Dissolved Oxygen (DO)
DO is one of the key concepts in aeration efficiency. A wastewater lagoon with no aeration will have all of its natural dissolved oxygen consumed by Biochemical Oxygen Demand (BOD). This will result in anaerobic/anoxic lagoon conditions. At this point the wastewater lagoon will have a DO reading of 0.0 or close to it. If you were to entirely saturate the water with oxygen, the DO level would increase to a maximum of 10.71 mg/L (at sea level). However, as you approach saturation it becomes more difficult to get the oxygen to dissolve, similar to the way a wet sponge doesn’t soak up water as well as a dry one.
Many aerated wastewater lagoon designers target a DO level around 2mg/L. This strikes a nice balance between having too little oxygen (anoxic conditions) and too much, which generally results in a waste of power. The higher a designer sets the lagoon DO target, the more aeration that is needed. This is an important thing to look at when comparing different lagoon aeration alternatives, as the DO field design point tends to vary based on each manufacturer’s design preference. If you are looking to compare aeration systems on an apples-to-apples basis, make sure they’re designed with the same DO level in mind. For more information on why DO is important, see our article on Wastewater Lagoon DO: A Case Study.
2. Elevation and Air Pressure
As elevation goes up, air pressure goes down, as does the amount of oxygen in the air and the saturation level of DO in the water. For example, a wastewater lagoon at 500 feet above sea level may require 2% more air than the same lagoon at sea level. At 1,000 feet it may require 7% more, and at 5,000 feet it could be up to 50%. This is another important design factor to keep in mind, especially when comparing different aeration systems side by side—for a accurate comparison, make sure that they are both designed with the same elevation.
3. Temperature and Theta Factor
As temperature rises, the equilibrium concentration level decreases substantially. Maintaining a high DO level when temperatures are low is much easier than when they are high. However, with temperature there is also an offsetting factor (theta), which is generally accepted to be 1.024. This indicates the increased transfer efficiency with increased temperature. At low DO levels, the theta factor dominates, causing an increase in efficiency with increased temperatures. At medium to high DO levels, the saturation correction factor dominates and higher temperatures lead to lower efficiencies. (I realize this might be a little more complicated. Don’t hesitate to get in touch and we can have a more in-depth discussion about this).
4. Beta Factor
The beta factor is generally assumed to be between 0.9 and 1.0. It represents an adjustment to the saturated dissolved oxygen level at the site of the wastewater lagoon, based on total suspended solids. For most wastewater lagoons, this number is assumed at approximately 0.95, however, it may vary depending on the level of TDS (Total Dissolved Solids).
5. Alpha Factor
This might be the most elusive, overlooked, and abused element in the design of wastewater lagoon aeration systems. It is a catch-all factor used in the wastewater industry to correct for the difference between clean water use in the labs, and “dirty” water use in the field. It also corrects for aeration type, bubble size, and wastewater contaminants. Designers have been known to either ignore it altogether, or apply a generic number (e.g. 0.8) to all applications. Both of these tactics are misleading and can result in lagoon aeration systems that are designed incorrectly for the given field conditions. Particularly in deep lagoons and with fine bubble applications, the alpha factor will have a significant effect. Here are a few benchmarks of the type of alpha factor that can be utilized in different lagoons and applications:
|Deep Lagoons with Fine bubble||0.4–0.5|
|Shallow Lagoons with Fine bubble||0.6–0.65|
|Coarse Bubble Diffusers||0.8|
|Surface Aerators||1–1.2 or 0.75–0.9|
The alpha factor largely depends on the composition of the wastewater you are trying to aerate. Normal municipal wastewater is understood to be fairly consistent from location to location and, therefore, alpha factors can be safely assumed. For industrial wastewater, on the other hand, it is much harder to know what alpha should be used. This is mainly because the composition of industrial wastewater is not consistent from location to location. Ultimately, the only way of knowing what the alpha factor should be for a particular type of wastewater is to actually test for it. In most cases this is not a feasible proposition; as a result, engineers tend to make educated guesses.
Be wary of manufactures that use alpha factors that fall outside of the given ranges outlined above. Utilizing a liberal alpha factor is one of the classic “smoke and mirrors” tactics for manipulating aeration calculations to get a lower power requirement for one system versus another. In the end, each proposed aeration system is going to be aerating in the same wastewater and utilizing the same basic method of creating bubbles in water; whether it’s one type of fine bubble diffuser versus another, when it comes to aeration efficiency, one fine bubble is likely to be the same as another.
In conclusion, in order to design an efficient lagoon aeration system one has to take into account the field conditions that ultimately determine how much energy is actually used. Factors including dissolved oxygen target, site elevation, and wastewater composition all play a critical role in making the adjustments from lab efficiency to field efficiency. By being aware of the industry best practices and standard assumptions, you will better be able to compare different aeration systems side by side knowing that each manufacturer is utilizing a common basis of design. Nonetheless, field transfer efficiency is only one factor that goes into an efficient lagoon aeration system—layout design is also critical. Stay tuned as we will be dealing with that in our third and final article of this three part series.
Contact Triplepoint for Assistance Designing an Efficient Lagoon Aeration System – we can provide you with design calculations, budgetary costs, preliminary layouts and lifecycle costs analysis.