Air stones and diffusers, oh my!

Air stones and diffusers, oh my!

Picking the correct diffuser for your application will save both time, money, and possibly unnecessary mortalities in your system.

Over the years, we have heard many names for devices that diffuse air, oxygen, ozone, etc. The truth is that regardless of the term we call it, they all function similarly. The gas enters the device and is dispersed over its surface area and into the water.

Now that we have gotten through all the semantics, we can get into the mechanics and form, fit, and function of various diffusers available. Picking the correct diffuser for your application will save both time, money, and possibly unnecessary mortalities in your system.

Air diffusers, aka air stones, are used most with air in our industry but oxygen, ozone and even carbon dioxide are also options with certain styles of diffusers. Diffusers are usually described by their pore size.

The pore size directly affects the bubble size and pressure required to pass the gas through the diffuser. The size of the bubble translates to the surface area. The smaller the bubble, the greater the surface area that is available for the gas to transfer to the water.

Types of Diffusers

Air diffusers initially began as wooden blocks of limewood that were porous enough to allow air to flow through. They are still available in home aquariums but are not the most efficient and difficult to sanitize. Medium pore diffusers are often used with air blowers since they only require approximately 0.35 psi or 10” of equivalent water depth.

Air blowers are high-volume, low-pressure pumps, so they cannot be used with diffusers that require high pressures but are perfect for use with medium pore diffusers.

The most common medium pore diffusers are made from silica glass that has been bonded at super-high temperatures. Similar diffusers are made of alumina but are less common and, in our experience, are prone to breaking. Medium pore diffusers have an average bubble size of 1-3 mm. This larger bubble size is not ideal for long-term pure oxygen/ozone/CO2 applications since the surface is not maximized, and most of the gas is lost to the atmosphere.

Since pressurizing ambient air is inexpensive, the larger bubble is acceptable in that application. Fine pore or ultra-fine pore diffusers are used for oxygen, ozone, and carbon monoxide. These diffusers can require 25 to 50 psi to function properly.

These diffusers are available in an array of shapes and sizes, including those with a plastic base or aluminum base, barrel and flat, and even those that can be put together in a chain to create larger diffusers with smaller ceramic pieces.

Fine pore diffusers have an average bubble size of 0.5-2 mm, and the ultra-fine pore diffusers average 100-500 micron. The smaller bubble size makes it ideal for diffusing pure oxygen, carbon dioxide, or ozone since those are either generated on-site or brought into the farm in pressurized vessels.

This style of diffuser requires approximately 0.53 psi or 15” of equivalent water depth. Fine pore diffusers are a lower pressure alternative to ultra-fine pore diffusers.

Ultra-fine pore diffusers are made of resin-bonded ceramic plates or tubes that provide even smaller bubble sizes. Since they require 25-50 psi to function properly, they are often used in hauling trucks and smaller systems. The gas supply is either in pressurized bottles or even liquid oxygen.

They are maintenance intensive since the ceramic plates are easily fouled and not so easily cleaned. They are quite literally a lifesaver when needed. Most facilities employ low-head oxygenation systems such as speece cones or LHO’s for larger systems and longer-term use.

Both systems are very efficient at dissolving gases into water and have minimal gas losses when appropriately sized. This equipment will be covered in a future article.

Other types of diffusers include aeration tubing, drilled PVC pipe, and literally anything that air can flow through. The efficiencies of the diffusion system are directly related to the bubble size, so when choosing your preferred option, that must be part of the equation.

Of course, the smaller the bubble, the more expensive the diffuser, so a comparison of capital expenses and operating expenses is key to understanding the real cost of introducing gases into your system.