A lagoon aeration system will require some method to convey air from the onshore blower to each aeration unit. Two distribution methods are typical: individual air lines or lagoon laterals, either submerged and fixed to the bottom; or floating on the surface.
In this article we’ll compare the benefits and drawbacks of individual air lines and lagoon laterals to help you to determine which method is best for your application.
Lagoon Individual Air Lines
Individual air lines bring air from the centralized blower to an onshore header and from there to each diffuser unit. These air lines are ballasted so they rest on the bottom of the lagoon. Valves are often used on the header end of individual air lines so that each aerator can be throttled down or turned off for maintenance.
Benefits of Individual Air Lines:
Lines are submerged: Very little of the air lines is exposed at the surface, and the air lines are both weather protected and out of the way at the bottom of the lagoon.
More control: With individual air lines, the system can be balanced and rebalanced from the shore. If an operator wants more air at one end of the lagoon, he or she can quickly make that happen. This is most beneficial in systems with variable airflow: The valves on each air line will allow for quick adjustment.
Less maintenance: Since individual air lines are mostly continuous runs of tubing and they are not on the surface of the lagoon, there are fewer joints and points of failure, reducing the potential for leakage.
Drawbacks of Individual Air Lines
The “spaghetti” effect: In applications with many closely packed aerators, individual air lines can begin to overlap or become tangled with other aerators. This can lead to a “bowl of spaghetti” effect that makes aerator handling difficult.
Cost: In applications with many aerators or in larger ponds, individual air lines can be more expensive than laterals.
Backpressure: Backpressure is a function of pipe diameter, airflow, and length. Since our air line comes in one size, applications with very long runs (over two hundred feet) and/or higher airflows will see a higher backpressure. This will raise brake horsepower and may also require larger blower sizing.
Subject to lagoon bottom conditions: With individual air lines, the aerators rest directly on the lagoon bottom. Depending on the quality of the lagoon’s construction, the aerators’ installation, and sludge levels, you may see a variation in aerator heights and levelness. Airflow will take the path of least resistance, favoring higher aerators. This can lead to an imbalance in air distribution in the lagoon, which can be solved by throttling down the higher aerators via the onshore valves until all aerators show an even bubble pattern.
Laterals are pipe or trunk lines that bring bulk air out into the lagoon. It’s called a lateral because it is a side offshoot of the header. Generally there are two approaches: 1) Fixed Laterals, which are submerged and attached to the bottom of the lagoon with concrete anchors; 2) Floating Laterals, which float on the surface of the water and are tethered to each end with cable to prevent them from blowing with the wind. Laterals tend to be used in larger applications (50+ aeration units) as they can save on equipment cost and make the system more maintainable.
Fixed Lagoon Laterals
Fixed laterals branch off a main header coming from the blower and go into the lagoon, intermittently anchored to concrete blocks. They can be made of Ductile Iron Pipe (DIP) or High Density Polyethylene (HDPE) pipe. Saddles located intermittently at the bottom provide a branch to each aerator. Typically, after the saddle there is a heavy flex air line that connects to the aerator with enough slack to allow the aerator to be pulled up to the surface for maintenance.
Benefits of Fixed Lagoon Laterals
Weather resistant: Fixed laterals are out of sight and out of mind. Rain, wind, hurricanes, and cold temperatures have no impact on operation.
Low maintenance: Because they are fixed, there is no tensioning of tether cables and no visible maintenance points during normal operation.
Easier aerator maintenance: Because they are submerged, maintenance crews can float freely on the surface of the water without having to avoid floating laterals that would otherwise be there.
Drawbacks of Fixed Lagoon Laterals
Lagoon downtime: To anchor a lateral to the bottom, the lagoon needs to be drained and taken out of service. In some circumstances, this is not an option.
More expensive: Due to the need to pump down the lagoon, deal with existing sludge on the bottom (remove or push around), and then bring in concrete and stainless strapping to anchor the pipe, fixed laterals are almost always more expensive to install.
Difficult maintenance: If there is a problem with the lateral, it is much harder to fix when it’s submerged on the bottom. The lagoon must be drained or divers will need to be sent down to fix it. Both are inconvenient and expensive options.
Susceptible to water ingress: Water can penetrate any aeration system given enough time. After installation, as air is introduced, water can sit in the laterals and delay or prevent the even distribution of air if the water isn’t completely pushed out. This can lead to dead zones in the lagoon.
Challenges with air balancing: As lagoon bottoms can vary in elevation, some aerators can end up being lower then others and, as a result, will receive more or less air. With fixed laterals it can be more difficult to adjust for these differences as you have to pull up an aerator and either add an orifice/valve to it or raise its elevation.
Floating Lagoon Laterals
Floating laterals, as the name suggests, branch off the header and float on the lagoon’s surface. They are constructed of HDPE material and are tethered on at least the ends by stainless steel aircraft cables. A tensioning system allows the tether to be tightened during the natural expansion and contraction that occur during seasonal temperature fluctuations. Floating laterals are widely used by manufacturers of fine bubble lagoon aeration systems and can be found in all 50 states and Canada.
Benefits of Floating Lagoon Laterals
Even air distribution: This is one of the biggest benefits of laterals. If the aerators are suspended from the lateral and do not touch the bottom, because the lateral itself is situated on a level surface (the surface of the water), all the aerators are at the same elevation beneath them.
Lower pressure: Larger pipes produce less backpressure, allowing more air to reach farther out into the lagoon and taking less of a toll on brake horsepower.
More efficient and economical for larger applications: For very large lagoons where air needs to be delivered 500 feet or more from the main header, floating laterals can be upsized and run out relatively cost effectively, with only a few pipes. Compare this to running hundreds of feet of individual air line to dozens or hundreds of aerators.
No downtime: Floating lagoon laterals can be installed within a lagoon when it’s in service with no disruption.
Individual aerator adjustment: Each aerator can be installed with its own ball valve to balance airflow that can be adjusted from the surface.
Drawbacks of Floating Lagoon Laterals
Susceptible to weather issues: Inclement weather will affect the operation of the lateral. During high winds or hurricanes it can be severely damaged and make the repair or replacement of aerators difficult. During extreme cold, ice formation on the surface of the lagoon can interfere with the lateral and even cause breakage to the air line connections. Due to the lateral being exposed to the elements, wear and tear–including the tethering system and connections—is inevitable and will result in higher maintenance requirements.
Expansion and contraction concerns: HDPE expands and contracts substantially with changes in temperature, requiring the operator to adjust the tethers to maintain the proper tension. Laterals that get too loose will snake around the pond, leaving dead zones as the snaking brings suspended aerators with it, or dragging bottom-sitting aerators around with wind movement. Laterals with too much tension will increase the potential for lateral damage.
Not suitable for variable depth: Surface laterals with hanging aerators are not suitable for lagoons with a changing depth. As depths increase, the aeration depth stays the same, leaving the lower portions of the lagoon unoxygenated and undermixed. This can lead to both lack of treatment and sludge accumulation. If depths get too shallow, the aerator will run into accumulated sludge. In addition, the lateral would have to be properly tensioned or loosened as the lagoon depth changes.
Difficult individual aerator control: Although an aerator’s airflow is technically adjustable with a ball valve, it’s a difficult undertaking. It would require the operator to get on a boat and navigate around the laterals and support cables, then hand adjust each aerator. This lack of individual aerator control can lead to air distribution anomalies.
Choosing between Individual Air Lines and Lagoon Laterals
The best method for conveying air from the blower to the aerators is site- and installation-specific. Factors to consider are whether the lagoon bottom is accessible, the number and density of aeration units, the distance from the header to the lagoon, climate, and budget.
Triplepoint generally biases towards individual air lines on most applications, as operators seem to gravitate to their clean look, easy on-shore controls, ease of installation, low maintenance requirement, and protection from the elements. On applications with high pressure/airflow, lots of units, or significant budget constraints, we consider the implementation of laterals.
Need assistance with your lagoon aeration system design? Contact Triplepoint, the lagoon experts. We can help you design an efficient, cost-effective lagoon aeration system, and provide design calculations, budgetary and lifecycle costs, and preliminary layouts. Request a free, no obligation quote.