By: Amy Stone*
So you have a filter and you’re wondering what to use inside it. The answer depends on what results are needed with the filter. In the past, we have reviewed the differences between filters, now we will dive into the medias.
So you have a filter and you’re wondering what to use inside it. The answer depends on what results are needed with the filter. There are several options for media that can be used in a traditional sand filter. In the past, we have reviewed the differences between filters, now we will dive into the medias.
“The efficiency of each style of media depends on its chemical make-up, size and shape as well as the type of waste that is being captured. All of these characteristics help determine the void space, the backwash efficiencies and the way the waste is detained in the filter.”
It is also important to understand that the depth of the media plays an important role in filter efficiencies and how the media performs. The deeper the bed, the better the filtration. This is true of all medias that we have used over the years.
As discussed in previous articles, sand filters with sand were designed to be used with systems where the water is chlorinated. Most, if not all, of our systems contain live animals which makes it impossible to have chlorinated water. This is the main reason to look at alternative media choices when using these filters.
Another consideration for any of the media choices is the recommended filter rate and how that interacts with biologically active water. Most filter manufacturers recommend filter flow rates of up to 20 gpm/ft2 and backwash rates of 23-25 gpm/ft2, again with chlorinated water.
For our industry, filters should be derated by as much as half. Meaning a filter rated at 100 gpm for a pool should be considered for 50-60 gpm in an aquatic system.
Traditional sand is a silica-based sand. Multiple grades are available to use however, the most popular is the 20 sand. Sand is generally used in conjunction with a coarser grade below the laterals as a support material. Most applications that we see are pea gravel which measures 3mm to 6mm diameter.
The “#20” refers to the sieve size used to separate the sand grains. Standard measurement for this size sand is 0.45 mm to 0.55 mm. The rule of thumb for particle capture rate for sand is 20 micron. Of course, we have seen situations where sand can capture finer particles.
All that said, sand media, while inexpensive, is prone to clumping and clogging. Sand is also prone to colonization of bacteria and zooplankton. If it isn’t maintained often and properly, it can very quickly become difficult to work with and ineffective. In our experience, this isn’t the best solution but will work if it is all that is available.
Mixed media has been a magic bullet for years as an alternative to traditional sand. It usually has three or more types or sizes of media. Some media types include crushed garnet, pea gravel, anthracite, activated carbon, sand and more.
“The concept is that with the different types and sizes of media, it will be more efficient in capturing the waste stream than just one type of media. In some cases, the mixed media is meant to handle more than one process in the same vessel.”
In terms of efficiency, it is wholly dependent on the types of media chosen for the filter. Unfortunately, due to the many options out there, it is nearly impossible to describe the desired effect from using it in a system. It is also biologically active which allows for bacteria and zooplankton to colonize.
Plastic Bead Media
Plastic bead media can be used in sand filter vessels. It is available in both floating and sinking types. If the sand filter is to be used in a traditional manner with plastic beads, then sinking beads are the only option
Floating beads are used when the flow is reversed through the filter and requires a modification to the internal plumbing. Plastic beads can filter to about 30 microns depending on the size and shape of the bead.
They also allow for bacteria growth. They are commonly used in situations where finer particle filtration is not needed.
Crushed Glass Media
Crushed glass media is getting more popular. It is environmentally friendly in that it is usually made from recycled glass. There are several manufacturers that supply this media with various configurations.
Standard crushed glass would consist of all types of recycled glass. The efficacy depends on the purity of the glass that is included in the mix. It can be as rudimentary as all sizes of glass particles in the mix or more standardized where the glass is graded out to match a similar profile to traditional sand and gravel.
For instance, there is activated glass media which uses only green and brown glass. This style media is known to have a negative charge which attracts the waste particles to the surface of the glass.
“One manufacturer uses a proprietary process to enhance the negative charge which helps increase the efficiency of particle capture.”
This particular glass media can capture particles down to 4 micron and lower. They also have a hydrophobic version of the media which can filter to less than 1 micron in a single pass. This media can also be used to remove heavy metals.
The upside to this media is that it does not allow bacteria to colonize on its surface as long as it is maintained properly. For situations where bacterial removal is needed, this is an inexpensive option. It also uses less water in the backwash process.
“Typically, this media only needs one and a half to two filter volumes of water for 90% of the waste to be released. There are specific parameters for backwash and filter rates for this media.”
Non activated crushed glass is a less expensive alternative but due to the fact that all colors of glass are included, the media can allow bacterial colonization. No matter what media you choose, it is always good to look at the application and find what works best. Not all medias are created equal.
Amy Riedel Stone is President and Owner at Aquatic Equipment and Design, Inc. She was formerly a Manager at Pentair Aquatic Eco-Systems, and she studied Agriculture at Purdue University. She can be reached at firstname.lastname@example.org