Introducing air to a wastewater treatment system or, lagoon or pond is vital to maintaining proper dissolved oxygen (DO) levels for sewage treatment. The process of introducing air is called “aeration”. Aeration brings the wastewater being treated into contact with air with some of the air being absorbed by the wastewater. The aeration process typically performs the additional function of mixing in a wastewater treatment system as well. Proper mixing is important because it ensures that all of the wastewater in the tank or lagoon has the same amount of DO, solids, microbes and incoming sewage.
There are two main means for performing aeration: diffused air and mechanical. This blog post will delve into the details of these two means and look at some of the advantages and disadvantages to each.
First up, let’s look at diffused air aeration systems
What is a diffused air aeration system?
A diffused aeration system uses an air compressor, air tubing or pipe and air diffusers typically located at the bottom of the tank or lagoon. Air is delivered from the compressor to the diffusers using the pipe or tubing. Air is passed through the diffuser which creates bubbles that come into contact with the water.
How a diffused air aeration system works
A diffused air aeration system moves air to come into contact with water. The bubbles rise to the surface of the body of water. While they rise, a percentage of the air is absorbed into the water (called oxygen transfer) and used by the microbes for respiration. The rising action of the bubbles also creates a column above the diffuser which displaces the water causing an upward current and laminar flow pattern. This flow pattern causes the mixing action created by the diffuser. The size of the diffused air system and number and location of the diffusers is determined by the calculated air requirements for the incoming sewage and configuration of the treatment tank or lagoon.
Main diffused air aeration system components
A diffused air aeration system typically consists of an air compressor or “blower”, control valves and gauges, air distribution pipe or tubing and diffusers.
Variations of diffused air aeration systems
Diffused air aeration system equipment and design can emphasize aeration or mixing. Diffusers come in various types such as;
- Fine bubble diffusers (more aeration action)
- Coarse bubble diffusers (more mixing action)
Each one of these diffuser types has its own air flow requirement so the air compressor and delivery pipe or tubing size needs to be selected accordingly. Valves can be configured to control the amount of air going to each diffuser or zone in the tank or lagoon as required. Diffusers typically come in a disc or tube form.
Advantages & Disadvantages of diffused air aeration systems
Diffused air aeration systems offer a lot of advantages for operators and owners of wastewater treatment systems and lagoons such as:
- Greater level of adjustment and control due to the air control valves
- More complete mixing from the bottom of the tank or lagoon to the top of the water surface
- Superior Standard Aeration Efficiency (SAE) or pounds of air delivered to water per unit of energy required to run the system
- Low maintenance requirements
- Flexible design can be implemented and sized based on size and configuration of tank or lagoon
- Only mechanical components are the air compressors, valves and gauges which are located on the ground in an easy to access area.
- More efficient for deeper applications
While diffused air aeration systems provide great advantages, there are some disadvantages or considerations involved such as:
- More complex to implement. All of the components need to be sized based on the amount of air and diffusers required by the tank or lagoon
- Higher up-front costs because of complexity, amount of equipment and installation
- Potential noise issue from air compressors (can be mitigated)
- Diffusers can become fouled by sludge (diffusers should be hosed off every 1-2 years or when they begin to fail)
- Less efficient for shallower applications < 8’
Diffused air aeration system summary
As you can see, diffused air aeration systems are a flexible, cost efficient solution that provides complete mixing and the maximum bang for the buck with regards to operation. Before deciding on installing a diffused air aeration system however, one should carefully consider the upfront costs, and the design and installation requirements.
Mechanical Aeration Systems
What is a mechanical aeration system?
A mechanical aeration system moves the water to come into contact with the surrounding air above the surface. The mechanism constantly draws water into it and pushes it out. The movement of the water creates a current that causes a mixing action in the tank or pond.
How a mechanical aeration system works
As previously described, a mechanical aeration system moves water to come into contact with air. As the water is moved, air is introduced and absorbed by the water. The movement of the mechanism brings new water into contact with the air and mixes the aerated water with the rest of the tank or lagoon.
Main mechanical aeration system components
A mechanical aeration system consists of a mechanism that moves the water such as a pump and tubing or an electric motor that turns a propeller. Surface systems are enclosed in a housing with pontoons to keep it buoyant. Mechanical aeration systems are typically modular in nature with everything being included in a single piece of equipment. In a typical situation, multiple systems are deployed in a tank or pond to create uniform coverage.
Variations of mechanical aeration systems
A mechanical aeration system can take many forms such as:
- A fountain that sprays the water into the air
- Pontoon mixers that pull water from near the surface and spray it out in jets
- Propellers that churn the water
Advantages & Disadvantages of mechanical aeration systems
Mechanical aeration systems definitely offer some advantages such as:
- A very high level of mixing and oxygen transfer with the water that is moved is accomplished
- Very simple and cost effective to implement. No system to design or install
- Equipment is simple to implement
- Flexibility because existing units can be moved around (not waterfalls), and additional units can be added
- Waterfalls and fountains make nice features to ponds or lagoons
- More efficient for shallower applications
Some of the disadvantages are:
- Mixing and oxygen transfer are highly localized and often limited to the area immediately around the unit
- While mixing on the surface is accomplished well, there is little to no mixing on the bottom of the tank or lagoon. This can create zones of little to no air or mixing and cause sludge accumulation and septic conditions on the bottom
- Lower Standard Aeration Efficiency (SAE) or pounds of air delivered to water per unit of energy required to run the system when compared to diffused air aeration systems
- More mechanical equipment is required which can become fouled and can also require more maintenance
- Mechanical equipment is located in or on the water and must be retrieved in order to be serviced
- Systems are exposed to the elements effecting equipment lifespans
- Noise from water splashing (this can be more pleasant than the sound of machinery)
- Fountains can spray a mist that carries outside the boundaries of the lagoon or pond in high winds which may be undesirable
- Surface mixing is less efficient for deeper tanks and ponds
Mechanical aeration system summary
If you’re looking for a quick, easy to implement solution to raising the DO levels and getting some mixing in your shallow lagoon or pond then a mechanical aeration system might be the right solution for you. If your lagoon is deeper than 8 feet, you might have some buildup of sludge on the bottom and you won’t be able to mix the water at greater depths.
Which is better? Diffused air or mechanical aeration?
If you have a small shallow pond then a mechanical mixer might be your best option. For top to bottom mixing, the diffused air route is definitely best. Also keeping solids in suspension means they won’t settle on the bottom as much and that can save money in the long run when it comes to having to dredge sludge out.
For increasing to DO levels
Diffused air aeration is the way to go here. It has a better SAE than mechanical systems, you can adjust how much air is delivered and you will have more uniform DO levels at all depths of your lagoon. For wastewater systems, having uniform DO levels in your pond or lagoon is a real advantage. Your microbes will work better at not only treating the wastewater but any sludge that tries to accumulate on the bottom as well. If you dose with sludge reducer, there will be plenty of air for the microbes to do their job!
We are happy to explore all of these advantages and requirements and discuss the implementation of a diffused air aeration system with you! Call Randy Sorenson at 303-520-4961 or send us an email.