A big part of designing an efficient lagoon aeration system involves how aeration infrastructure fits into the project site. Length of header, location of power, and number and type of aerators are all examples of important considerations that can affect the efficiency of your lagoon system. In this third and final part in our Efficient Lagoon Aeration blog series we dive in to what to pay attention to when designing an aeration system layout to ensure that maximum energy aeration efficiency is achieved.
In the first article of this blog series, Efficient Lagoon Aeration: Part I Design Conditions, we looked at the design factors that go into the aeration calculations themselves. In the second article, Efficient Lagoon Aeration: Part II Field Conditions, we considered how to take the design calculations and apply them to the field.
For diffused air systems, power consumption and subsequently lagoon aeration efficiency is tied directly to flow rate and operating pressure.
1. Lagoon Depth: Operating pressure is largely determined by the depth of the lagoon; for example, for every 1 foot of diffuser submergence, there is 0.43 pounds per square inch (psi) of operating pressure.
- In deeper lagoons, higher overall oxygen transfer efficiency can be achieved, requiring lower flow rates and fewer aerators. However, deeper lagoons also require higher operational pressure due to being deeper, thereby leading to more energy consumption. This tradeoff generally favors a deeper lagoon.
- With shallow lagoons, aeration is extremely efficient per foot of depth due to the lower operating pressure; however, you need a lot of aeration diffusers and air to transfer the necessary oxygen.
2. Diffuser Type: Simply pushing air through a diffuser increases the operating pressure. The amount of operating pressure increase depends in part on the type of diffuser you use. For instance:
- Fine bubble diffusers generally have a pressure loss of about 0.5–0.75 psi
- Coarse bubble diffusers generally have a pressure loss of 0.1–0.25 psi depending on how much air you are trying to push through each diffuser.
This can moderately increase the power consumption of a lagoon fine bubble system. However, because lagoon fine bubble aeration is more efficient at transferring oxygen to water than the lagoon coarse bubble, it uses less air, resulting in less pressure drop in the air lines (or smaller, less expensive, air supply piping). This tradeoff, from an overall energy perspective, overwhelmingly favors fine bubble over lagoon coarse bubble diffusers.
3. Airflow Per Diffuser: The airflow per diffuser is correlated with operating pressure: the higher the airflow, the higher the backpressure. In the case of fine bubble diffusers, the more air you push through each, the lower the efficiency, as they will produce larger bubbles. With a lower lagoon aeration efficiency, more air is needed to provide the necessary oxygen. There is a tradeoff here because if you run diffusers at a higher airflow, you will need fewer of them, therefore lowering your capital costs. However, you will increase your backpressure, which is likely to lead to higher operating costs. Generally, from a lagoon aeration efficiency perspective, it is better to operate a diffuser at a lower airflow per unit.
4. Header & Lateral Design: Pushing air from the blower through the header and laterals to the diffusers increases operating pressure further. The amount of backpressure through a header and lateral system depends on:
- The total cubic feet of air per minute (cfm) required for aeration. Less air generally means the backpressure can be lower depending on the pipe size and length.
- Pipe inner diameter (ID) determines how much volume the air has to travel through the pipe itself.
- The length the air header pipe: The longer the pipe, the more friction the air will have as it travels through it and the higher the backpressure.
- Finally, the number of fittings and valves that the air has to travel through also adds friction and subsequent backpressure.
There are often a number of tradeoffs when designing a header system. For example, if the location of the aerated lagoon is not close to where a sufficient power source is, then you have to decide whether the cost of pumping the air a longer distance at a higher backpressure over 20 years is less expensive than installing a new electrical service closer to the lagoon. Generally, an efficient lagoon aeration system is designed so that the total backpressure between the blower and the header is a maximum of 0.5 PSI.
When designing an efficient lagoon aeration system, mixing has to be a critical part of the equation. Ultimately, you can design an extremely efficient aeration system, but if it fails to adequately mix and keep solids in suspension, then eventually sludge will build up and lead to diffuser clogging. Our article Wastewater Lagoon Mixing Alleviates Odor & Sludge Issues is a case study of the importance of wastewater lagoon mixing. When diffusers clog and foul, invariably backpressure will increase at the blower and cause motors to consume more power making your once efficient design turn into an energy hog or fail completely. For more information on what can happen when there is inadequate mixing, see our blog Causes and Effects of Wastewater Lagoon Sludge Explained.
There are a number of factors to keep in mind when designing for effective lagoon mixing:
1. Partial Mix vs. Complete Mix: A complete mix lagoon design generally means that the aeration system is designed with 10–15 cfm per 1,000 cuft of water—for most systems this means that higher airflow is needed for mixing purposes than that needed for oxygen transfer. Most lagoons, on the other hand, tend to be partial mix, meaning that there is no set airflow requirement and fewer diffusers are needed.
2. Area of influence: In a partial mix design, it is a good idea to position aerators so that they cover the lagoon adequately enough to minimally avoid dead zones. As a rule of thumb, the maximum area of influence of each aerator should generally be touching each other. Different types of diffusers have different areas of influence. The MARS Lagoon Aerator, for example, can have areas of influence up to 125’ in diameter around the diffuser. Purely fine bubble diffusers can have areas of influence between 20–25’ in diameter because they are unable to move as much water per assembly. This generally means that more fine bubble diffusers are needed for mixing—this drives up capital and long run maintenance costs.
Almost every decision when developing a layout design will affect how efficient your lagoon aeration system ends up being. Whether considering factors such as lagoon depth, diffuser backpressure, or header and lateral design, to name a few, understanding the tradeoffs will help you be smarter about each decision. Don’t forget, at the end of the day, lagoon mixing is the linchpin of any successful lagoon aeration system. If you spread your aeration too thin you may end up with an efficient system, but one that could prematurely fail.
Contact Triplepoint for Assistance Designing an Efficient Lagoon Aeration System—we can provide you with design calculations, budgetary costs, preliminary layouts, and lifecycle cost analysis.