Tag Archives: Walnut Shell Filter

Advantages Of Walnut Shell Media

Our Walnut Shell Media Filter has a variety of advantages over other options in the industrial filtration space, especially when filtering oil and total suspended solids (TSS).  Because it is a backwashing filter, Walnut Shell Filters not only are much better filters, but are more efficient in many processes.

  • Hydrocyclones are often used for removing oil from water, but will not fully polish the water.  For example, oil droplets smaller than 20 micron are typically not removed by hydrocyclones.
  • Cartridges and bags are also frequently used, but must be disposed of.  Maintenance personnel are also required to change the filter media when it becomes plugged.  Increasing restrictions on disposal in certain industries have also led to skyrocketing disposal costs.

How Backwashing Filters Work

When compared to hydrocyclones and disposable media filters, backwashing deep bed filters have many distinct advantages.  Contaminants are captured by a granular media bed and then removed by an efficient tortuous path through the media bed.  After the bed becomes full of contaminants, it is then backwashed, which cleans the bed without incurring media disposal costs.  For the filter to operate at a high efficiency over a long period of time, all contaminants from the bed must be removed during the backwash.

Better Filtration

One of the advantages of walnut shell media is its superior filtration of wastewater.  For example, Walnut Shell Filters will typically remove 95% of solids at 5 micron, and 90% of suspended oil.  Walnut shells have a natural affinity for oil, causing it to bond to the surface of the media.  Unlike in sand filters, whose captured oil films over the surface, captured oil remains as droplets.  These absorbed droplets will then contact smaller droplets, which coalesce onto the larger droplets and increase the removal rate of small particles over time.  That’s why black walnut shells have an oil absorption capacity that is 2-3 times that of sand.

Improved Efficiency

One of Filtra-Systems’ goals is to provide industrial filtration solutions that can improve a company’s operations in multiple domains.  Systems that require less maintenance not only help reduce upkeep costs, they also help to ensure that needless interruptions of operations that can have a negative impact on a company’s bottom line do not occur.  That’s why our Walnut Shell Filters have beds that don’t require replacement for the entirety of the product’s life.  Because walnut shells are preferentially wetted by water, oil is easily scrubbed from the shells during a backwash.  Black walnut shells (as opposed to English) have a high modulus of elasticity, which explains why the beds don’t need to be replaced.  The expected yearly attrition rate is 2%, which is much lower than English walnut shells, pecan shells, and other types of media.

Likewise, these filtering systems need less floor space in the factory to operate, freeing up room for different machinery.  Walnut shell filters are typically sized at 10-12 gpm/ft2, which is a greater flux than completing media filters.  The higher flux means smaller filter housings can be used, which is what specifically translates to more floor space for customers.

Less water required to regenerate the bed is another benefit customers enjoy.  The flux required for fluidization of the bed is only 4.5 gpm/ft2 (based on a clean media bed, oil and solid saturation will increase this requirement), which is significantly lower than competing technologies.

Known Issues Of Sand Filters

To those who have had the misfortune of operating a sand filter, no explanation is needed for the colloquial term “mudball.”  To the uninitiated, mudballs form when sand filters are exposed to oil that results in an oil film across the sand.  this film prematurely clogs the filter, drastically reducing the system’s efficiency.  The plugged bed has a decreased filtration area, and will backwash more frequently.  Even after a backwash, the bed remains unclean because the backwash water does not penetrate the film, and channels around the mudballs.  As the problem persists the bed will inevitably need to be replaced, a manual operation that requires personnel to dig out the clogged bed.  This adds to unnecessary costs, both in paying trained personnel, as well as system downtime.

Which leads us to pricey filter replacement and disposal.  Generally, sand filter beds need to be replaced every five years.  However, with regular exposure to oil, replacement time can be reduced to as frequently as every six months.  An alternative to filter replacement is to clean the filter bed, a procedure that involves soaking the bed with a condensate, or a light oil.  The time consuming nature of this process means that sand filter users typically choose to replace the bed instead.  Sand is also an OSHA dusting hazard, and handling sand to replace the bed can result in exposure to plant personnel.

Sand filters typically use an air scour to remove oil and solids from the media bed.  Drawbacks to this technology include:

  • Incomplete fluidization of the media bed
  • Extra piping and valves required upon installation
  • Larger compressor and air regulator are needed
  • Possible installation of a vapor recovery and processing unit (VRU)

The compressed gas needed for this operation may also change the electrical classification of the area to Class 1, Div 1, increasing electrical costs for all process equipment nearby.

The backwash volume required for sand filters can be up to 5 times more than walnut shell filters.  Sand filters are designed at 6 gpm/ft2, and the flux required for fluidization of the bed is 12 gpm/ft2 (based on a clean media bed, oil and solid saturation will increase this requirement).

Known Issues Of Anthracite

Another common media filter is anthracite.  Anthracite filters usually have three layers of differently sized media: a top layer collects coarse contaminants, the second layer collects smaller particulate, and the third layer is the final polish.  Unfortunately, substantially more water is needed to clean anthracite filters.  This is because the backwashing process requires a subsurface wash to break up any waste before the standard backwash.  In fact, an anthracite filter may require seven times more water to backwash than a comparable walnut shell filter!

Additionally, upset conditions can cause the media to mudball.  The attrition rate for anthracite is much higher than walnut shells, and the media generally needs to be replaced every year.  Anthracite filters are designed with a flux of 4-4.5 gpm/ft2.

Filtration Design Flux

Backwash Fluidization Flux

Backwash Volume (compared to WSF)

Walnut Shell Filter

12 gpm/sqft

4.5 gpm/sqft

1X

Sand Filter

6 gpm/sqft

12 gpm/sqft

5X

Anthracite Filter

4 gpm/sqft

N/A

7X