Surface Aeration or Diffused Aeration
When designing lagoon aeration systems selection of optimum aeration components is critical to maximum performance, long-term operation and easy maintenance. Lagoon aeration is generally broken down into two major categories surface aeration and diffused aeration.
Lagoon aeration systems dating back to 1960’s were almost all surface aerators. Surface aerators are still used today, but in fewer applications. Diffuser technology has matured and offers many processes, or energy benefits vs. surface aerators for MOST applications. A comparison of diffusers vs surface aerators is shown in table I enclosed. Surface aerators are still vital for special applications; however, diffused lagoon aeration systems are usually the optimum choice for lagoon aeration technology.
Diffused aeration technology for lagoons offers many benefits and features when properly applied. Proper lagoon diffused aeration design does involve implementation of several engineering decisions for optimum results. The decisions include:Fine Bubble or Coarse Bubble? Floating Lateral or Floor Mounted Diffuser Systems?
Clearly, any of the diffuser types can be successfully applied to most lagoon aeration applications. While all these systems are functional, there are key design benefits from an engineering analysis and selection of optimum lagoon aeration technologies or systems.Fine Bubble or Coarse Bubble Diffusers?
The selection of fine vs coarse bubble aeration requires a recognition of benefits or liabilities of each. Generally, the following items are driving factors in coarse/fine selection:
Maintenance – Coarse bubble diffusers generally win this comparison. Coarse bubble includes static tubes and coarse bubble diffuser units like MaxAir™ units.
Mixing - Fine Bubble diffusers are superior mixers vs coarse bubble in lagoons or any reactor pumping less than 3% solids. Coarse bubbles are better mixers when very thixotropic mixtures (3% solids or more) or when energy savings of fine bubble reduces mixing energy in the basins. Fine bubble at same energy levels pumps more water than coarse bubble at the same energy input, when operating at ½ energy savings with fine bubble, coarse bubble probably has treater pumpage (mixing).
Energy - Fine bubble truly dominates this variable with oxygenation energy savings of 40-50% vs coarse bubble. This aeration process energy savings is so significant that most systems today are designed with fine bubble to reduce operating costs; even with the knowledge there may be some maintenance required.
Since most lagoon aeration systems choose to employ high efficiency diffusers (Fine Pore) technology, we can address how to best deploy Fine Bubble Diffusers.Floor Mounted Diffusers
The first diffusers, coarse or fine, were all designed with piping on the floor and diffusers floor mounted. Typical floor mounted diffusers are shown in Table 2. Fine bubble lagoon diffusers were developed by EDI in early 1980’s and were very successful reducing energy and allowing patented access to each unit (hose plus float & rope to allow access). While these floor mounted systems were major improvements the floor mount had limitations:
- Level floor required to level unit and allow uniform air distribution.
- Sludge deposits were a problem in leveling.
- Sludge deposits could foul diffuser operation.
- Hard to lift unit for maintenance then replace into lagoon at original or proper location.
- Required maximum ballast to hold air pipes and diffusers under water.
- Piping not serviceable without draining the basin.
Floor mounted units were successful but had limitations from the floor mount that had to be recognized and engineered. Lagoon technology continued to progress, and floating air supply laterals were introduced for EDI systems in early 1990’s. (See examples on Table 3). With this innovation, major benefits were available:
- Laterals float on lagoon so no ballast to hold pipe under water.
- All piping and the diffuser components available to service without shutdown or draining lagoon.
- Diffuser units hang ABOVE floor.
- Floor elevation not critical as units register from water surface.
- Units suspended above sludge.
- Units always go back into proper location under the air lateral.
- No damage to liners.
- Allows major water level variation with units always at constant design pressure (constant submergence below floating laterals).
- Allows easy application of side slope mixing.
- Suitable for use with floating lagoon covers.
- Lower aerating pressure as lower losses since no need to overcome floor variations.
- Allows application of Bio-Sweep™ technology.
- Allows easiest deployment with wet installation and no need to dewater basin.