Diffused Air vs. Mechanical Wastewater Aeration Systems

By: Tom Frankel
Post Date: September 9th 2019

Post Tags: industrial wastewater, wastewater treatment,

Table Of Contents

  1. HOW TO PICK THE RIGHT AERATION SYSTEM FOR YOUR WASTEWATER TREATMENT PLANT
  2. HOW DOES A DIFFUSED AERATION SYSTEM WORK?
  3. DIFFUSED AERATION SYSTEMS — PROS
  4. DIFFUSED AERATION SYSTEMS — CONS
  5. HOW DOES A MECHANICAL AERATION SYSTEM WORK?
  6. MECHANICAL AERATION SYSTEMS — PROS
  7. MECHANICAL AERATION SYSTEMS — CONS
  8. WHY DIFFUSED AIR IS THE BEST AERATION SYSTEM

Have you ever wondered about the differences between types of aeration systems and what consequences they have for the operations of a wastewater treatment plant?
Two main types of aeration systems exist — mechanical aeration and diffused aeration. Mechanical aeration operates from the surface of a tank or lagoon, whereas diffused aeration typically operates from the bottom.
Older models of aeration systems are often mechanical systems that have served their plants reliably for years. The wastewater industry changes with new advancements, however, and more and more plants turn to diffused aeration.

How to Pick the Right Aeration System for Your Wastewater Treatment Plant

Different plants must prioritize different needs and goals throughout the wastewater treatment process. The choice of aeration system that is right for your wastewater treatment plant will depend on a few different factors:

Wastewater treatment plants can satisfy many of these needs at the same time with the choice of a high-quality, effective aeration system.

Diffused Wastewater Aeration Systems

Aeration forms a vital part of the wastewater treatment process, particularly the secondary treatment phase. It mixes wastewater with oxygen to allow aerobic bacteria to digest biosolid particles in the water. Aeration plays a particularly vital role in the activated sludge phase, during which bacteria feed on organic matter until flocs of matter and bacteria settle out and form sludge. The sludge recirculates, along with its bacteria, to enhance further breakdown of organic matter within the wastewater.
Diffused wastewater aeration systems provide a crucial tool for this process. They pump small, less concentrated amounts of air into the liquid to provide a highly efficient aeration process.

How Does a Diffused Aeration System Work?

A diffused aeration system typically operates vertically. It sends up concentrations of bubbles from near the floor of a wastewater facility’s lagoon or tank. Typically, the system comprises onshore blowers, pipes and diffusers. Compressed air pumps through pipes and filters into the water through the diffusers, which create tiny bubbles. The bubbles displace and mix the water and create a spiral flow pattern, transferring oxygen into the water column as they rise to help bacteria do their work.
Usually, the diffusers cover as much of the floor as possible while allowing some space for technicians to walk and do maintenance. This full coverage allows for an even distribution of oxygen. Plants can determine the number of diffusers and blowers necessary by considering the volume and type of sewage and the dimensions of the treatment tank.
Diffused aeration systems come in two main types — fine bubble and coarse bubble. Fine bubble aerators provide enhanced aeration and efficiency, while coarse bubble aerators excel at mixing and increasing the level of dissolved oxygen. Each type of bubble aerator has specific airflow requirements, so plant operators must choose blowers and pipes accordingly.

Fine Bubble Diffusers


Fine bubble diffusers are made of flexible membranes that contain thousands of minuscule holes and produce bubbles from 1 to 3 mm in diameter. These diffusers come in three primary types:

  1. Tube diffusers: These types of diffusers have more surface area to allow for the release of more bubbles per diffuser. They are longer and more neutrally buoyant than discs, and they provide the most air via the least amount of stainless steel tubing, so they are particularly well-suited to retrievable aeration systems that will lift out of a tank for cleaning.
  2. Disc diffusers: These all-purpose diffusers are the most popular for fixed aeration systems that are attached to the tank floor. Their somewhat lower maintenance and longevity make them natural choices.
  3. Needle-perforated panels: Unlike the membranes of tube and disc diffusers, the membranes of these diffusers usually do not flex. They contain fixed orifices whose bubble sizes remain constant with varied airflow. They also become much less efficient with deeper tanks and operate with high backpressure and headloss.

Fine bubble diffusers cannot operate with too much airflow, or the slits never quite close. They then produce coarse bubbles instead of fine bubbles. The tiny bubbles do not displace a large column of water, so these diffusers do not contribute extremely powerful mixing, but they provide enough mixing to keep typical biosolid particles in suspension when operated as intended.
The aggregate of small bubbles produced by fine bubble diffusers has a larger surface area than the fewer larger bubbles produced with coarse bubble systems, so they are prime for oxygen transfer and efficiency. The tiny bubbles also rise more slowly, leading to more contact time and higher oxygen transfer. They can generally operate with half the airflow of coarse bubble diffusers.

Coarse Bubble Diffusers

By contrast, coarse bubble diffusers displace more water with their larger bubbles and lend themselves to stronger mixing. They are also less susceptible to fouling than fine bubble diffusers because their larger pores do not clog as swiftly. Mixing and susceptibility to fouling are two important attributes in wastewater environments with high suspended solids content, so coarse bubble diffusers are superior to fine bubble diffusers in these situations and in other environments where other priorities take precedence over efficiency.
We outline the pros and cons of diffused aeration systems below.

Diffused Aeration Systems — Pros


The advantages of diffused aerators include the following:

Diffused Aeration Systems — Cons

The drawbacks of diffused aerators include the following:

Mechanical Wastewater Aeration Systems

Unlike diffused aeration systems, mechanical aeration systems use rapidly rotating shafts and propeller blades rather than flexible membranes and minuscule bubbles. They typically allow for stronger localized mixing and higher localized dissolved oxygen concentration but lack some of the benefits that diffused aeration provides.

How Does a Mechanical Aeration System Work?

A conventional mechanical aeration system operates horizontally. It consists of a pump and tubing or a motor that turns a propeller. The system scoops up water and pushes it out. This churning creates a current and provides mixing. As the water churns, it draws in and absorbs air. The equipment mixes this aerated water with the rest of the water in the tank or lagoon and constantly brings in more air to aerate and mix.
Most mechanical aeration systems are modular, with just one piece of equipment to install. Plants can install multiple systems in a tank for full coverage. These surface aeration systems typically require buoys to remain at the wastewater’s surface.
The following are three common types of mechanical aeration systems:

  1. Horizontal aspirators: These systems contain propellers that turn more or less horizontally. They push air down the vertical shaft and into the tank.
  2. Vertical aerators: These systems contain propellers that turn vertically to drive water into the air.
  3. Jet aerators: These systems function as a combination of both horizontal and vertical types, with injected air for further aeration.

We outline the pros and cons of mechanical aeration below.

Mechanical Aeration Systems — Pros

Mechanical Aeration Systems — Cons

Why Diffused Air Is the Best Aeration System

For lower system costs, durable, state-of-the-art equipment and a decreased burden of repairs and replacement for wastewater treatment plants and maintenance crews, diffused aeration systems are far and away the superior choice. They increase oxygen transfer and aeration efficiency, provide uniform mixing, and keep oxygen levels high so that helpful aerobic bacteria thrive and work effectively.
If your plant wishes to renovate its existing aeration and change to an efficient, high-tech, low-cost and easy-to-manage system, moving to diffused aeration would provide all those benefits and more.

Contact SSI Aeration About Diffused Aeration Systems


SSI Aeration is here for all your diffused air needs. We offer fine bubble diffusers, coarse bubble diffusers, disc and tube diffusers, a variety of premium, patented protective coatings, retrievable diffuser systems and a range of options to mesh with diverse budgetary requirements. Our diffused aeration systems are innovative and energy efficient, and we are a global leader in design, manufacture and installation. Contact us today.