Here's How a Multimedia Filter Works

Aug 10

A multimedia filter is an effective water filtration method that provides clean drinking water. It’s a slower but useful method most often used in municipal water treatment plants.

Now, you probably have some questions – what is a multimedia filter, how does it work, and do you need one? What about choosing the correct filter? What components are there? And how much maintenance is required?

Those are all great questions to ask, and we’re answering them in today’s blog. Keep reading to learn everything you need to know about multimedia filters.

What Is A Multimedia Filter?

A multimedia filter reduces the Silt Density Index (SDI) and the Total Suspended Solids (TSS) in an incoming water feed. Multimedia filters are an excellent way to get rid of silt, clay, grit, organic matter, algae, microorganisms, and more. These substances naturally occur in many water sources but aren’t so great for drinking.

Because suspended solids can make the pressure drop, downstream filtration systems like membrane separation, reverse osmosis, and ion exchange beds might not be as effective forms of treatment.

But if you’re already using another filtration system, you can use a multimedia filter to pre-treat the water to filter it even further.

Do I Need A Multimedia Filter?

We recommend using a multimedia filter if the SDI is 3 or higher or the turbidity is over 0.2 NTU. It’s not an exact rule or science, but following these guidelines reduces the risk of the RO membranes fouling. RO membranes can foul from the substances that you want out of your water.

How to Choose a Multimedia Filter

There are several factors to consider when choosing your multimedia filter:

What is the maximum required flow rate?
Are the suspended solids or turbidity colloidal or non-colloidal?
What is the analysis of the water?
What quality of water do you need to achieve?
Is there enough water available for backwashing?

Once you have the answers to these questions, you can choose a filter that fulfills your water supply requirements.

How Does A Multimedia Filter Work?

Generally speaking, more modern multimedia filters are made of carbon compressed into a solid block. But some filters have three layers of media. Those layers are usually anthracite coal, sand, and garnet, along with a layer of gravel at the bottom of the filter. For these older filters, anthracite, sand, and garnet are the best choices due to their size and density.

The media are layered based on their weight, not their quantity. So, while garnet is smaller than anthracite, it’s heavier and will go on the bottom. This layering method allows the filter to go longer between backwashes because the largest particles are removed at the top, and the smallest particles are kept deep in the media.

Above those layers is a freeboard that takes up close to half of the filter’s space.

A well-maintained multimedia filter can get particulates down to 15 to 20 microns. If you use a coagulant with the filter, you’ll have water with five to 10 microns.

Components of A Multimedia Filter

Now that we know what a multimedia filter is and how it works, let’s look at the individual components that make up the filter.

Filter tank - Everything goes into the filter tank, which is usually made of stainless steel, FRP, or epoxy-coated steel. Steel is used since it can handle high temps and lots of pressure.
Media - The media is the carbon or coal, sand, and garnet we discussed in the last section.
Internal upper and bottom distributors - At the bottom are distributors that prevent the media from leaving the tank. At the top are distributors that distribute and maintain the flow as the filter does its job. The distributors are usually made of 80 PVC or stainless steel.
Valves - The valves operate uniquely according to the type of cycle the multimedia filter is going through. Common types of valves include automatic electric, pneumatic, or manual valves. However, some industries don’t accept electric valves in filtration systems. They require manual ones. Nonmetallic valves are most commonly used for seawater.
Controller - The controller regulates the automation of the filter. The controller is made of PLC, a digital stager, or an electromechanical timer. The kind of controller you pick is usually whatever works best with the main control in your facility.
Face piping - This connects all the valves, allowing the whole process to work seamlessly. Face piping is made of 80 PVC, stainless steel, or epoxy-coated carbon steel. You’ll determine what type of face piping to use based on whether your application is indoor or outdoor and what temperature and pressing your system operates at.
Flow controller - The flow controller goes onto the drain outlet. It controls the backwash flow rate and keeps the media inside the filter tank.

Maintaining A Multimedia Filter

Maintaining your multimedia filter involves a backwash that cleans out the bed of the filter. This becomes necessary when there’s a high-pressure drop or the turbidity levels increase. Generally speaking, backwashes are necessary when the pressure differential reaches 10 psi across the bed or when there’s a 10% increase in effluent turbidity.

You want the media in the filter to have jagged edges. If you notice them losing their edge and turning into a more rounded shape, you should replace them, so your filter continues to work optimally.

Get Your Multimedia Filter From J.Mark Systems.

Are you looking for a multimedia filter for your water filtration system? J.Mark Systems offers multimedia filtration with polishing filters for particle-free effluence. Our systems come with preassembled and pretested components, actuated valve trains for service and backwash, and finishes that are resistant to corrosion.

J.Mark Systems also provides wastewater treatment and other water filtration systems like reverse osmosis, membrane separation, and ion exchange. We bring decades of experience to the table and are well-equipped to help you find the right water filtration system for your industrial water supply.

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