Media Bed Filtration

For industrial filtration applications, pressure vessels with loose media or sand are widely utilized. The backwash is fluidized in order to shake and loosen filter media as a means to release the trapped dirt, which is then removed in the flow of backwash. Once the backwash cycle is complete, the bed is capable of settling before the filter resumes service, including or excluding normal flow. It’s here where a “filter-to-waste” cycle is utilized (after they settle for assurance) and the filtration media is re-stratified. In addition to this, any loose dirt is pulled from the collectors/underdrain.

What’s Multimedia Filtration?

Multimedia filtration is known as the pressure filter vessel that handles three or more types of media, as opposed to a “sand filter” which oftentimes only utilizes one grade of sand for filtration media. For single media filters (on the setting cycle), the smallest or finest media particles will remain on top of the bed as the heavier, larger particles tend to stratify (in proportion to their mass) lower in the filter. At Newark Wire Cloth we manufacture Hub and Header Lateral Systems to provide uniform distribution and collection within the media bed. 

Given what these “sand filters” do, there’s only limited use of media depth since almost all particles that have been filtered are trapped at the highest point of the filter bed. They can also be trapped with only 1-2 inches of space in the filter with the least amount of space between them. It is crucial to have the correct amount of space between the Lateral system and Filter Nozzle to assure flow equalization. So given this, these filter run times tend to be short due to the filter “blinding” and stopping the flow altogether.

How They Filter

A multimedia water filter will typically utilize at least three layers of media for filtration, which are garnet, sand and anthracite. These certain media are often selected due to how drastically different their densities happen to be. Anthracite happens to be the lightest media for filtration in units per volume, which is then followed by sand and garnet respectively.

The main concept behind the utilization of media with considerable differences is that the backwash cycle places the lightest media, that has the largest particles, at the top of the filter. Intermediate sized particles such as sand will rest in the middle, as heavier media (with the smallest particles) such as garnet will rest at the bottom of the filter.

Filtration Bed Layering

In the bed layering filtration, it’s the smaller particles that rest into the lower layers, as the larger contaminants stay stuck in the first filter layer. The trapping of such contaminants (in this manner) allows more efficiency for removal and longer run times when the project is underway. Simple sand filters are expected to eliminate any particle to at least 25-50 microns, which is in comparison to a multimedia filter that can remove them to 10-25 microns. This is where the filter leaves are crucial. If the filter leaf is properly designed it results in even cake buildup, avoidance of “bridging,” and maximum filter cycle flow. 

In events where operations are working at a higher pressure, particles can be driven so far into the media bed that a simple backwash cycle isn’t capable of removing all of it. Given this scenario, the dirt goes deep into the filter and it only continues to build up, which leads to high differential pressures and shortened filter runs. To help loosen the packed dirt, filter backwash might involve air scour to the media bed. In the event this step is utilized, it is followed by a “drain down” moment so that the water can be bled from the filter.

Key Takeaways:

Given the information above, we’ve learned that:

– The backwash step is what cleans out the filter.

– Multimedia filtration is known as the pressure filter that handles three or more media.

– Filtration bed layering is the order in which particles are organized.

If you have any other questions regarding your filtration needs and wants, contact Newark Wire today!