Containment of Slurries in Continuous and Batch Operations

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In the 1970s, the chemical operations used acetone and benzene for the main slurry solid-liquid separation process. Next, there was a push to minimize solvent use. We looked to use water as the process liquid, but still had open filter presses and rotary drum filters; the entire plant was white from titanium dioxide or pharma stearates.

Today, we all know that processes remain open with filter presses, vacuum filters, and centrifuges. Our job is finding solid-liquid separation process solutions that can be contained for high solids slurries (greater than 10% solids) during filtration, cake washing, and dewatering/drying. This discussion considers your options for both batch and continuous operations.

Batch operations

When it comes to batch operations there are many possible ways to go.

Nutsche filter-dryers. Sized to take the complete batch from the reactor and process it to completion (final dryness). The nutsche filter contains an agitator, normally three blades, sealed to the vessel and moving up and down, clockwise and counter-clockwise. The agitated nutsche filter can conduct pressure filtration, cake smoothing, cake washing (displacement and reslurry washing), vacuum and pressure drying, and then automatic cake discharge.

The agitated nutsche filter-dryer is based upon thick cakes from 5–7 cm up to 30 cm and higher. For this type of filter to be successful, the cake permeability must be able to accept a deep cake without compression. Circular or rectangular filter media with a drainage layer is installed on a perforated filter plate.

Contained filter-presses. A contained unit does not require a process change and can operate at a cake thickness down to 2.5 cm, which is not possible in a nutsche filter-dryer. In a typical contained filter-press design a housing seals the plates. Improved designs include pressure filtration up to 1m Pa, cake washing in the forward and reverse direction, cake drying in the forward and reverse direction using pressure blowing and vacuum, as well as automatic cake discharge.

Contained centrifuges. These vary in design depending upon the operation and the type of centrifuge (such as horizontal peeler, inverting basket, and disk centrifuges). Centrifuges can be blanked or inerted for operation as well as sealed designs.

Continuous Operations

In continuous operations with slurries new options surface.

Rotary pressure filters. A continuous pressure filter designed for thin cake to deep cake filtration with cake depths from 6–150 mm. A slowly rotating drum (6–60 rph) is divided into segments (called cells) each with their own filter media (synthetic cloth or single or multilayer metal) and outlet for filtrate or gas.

The outlets are manifolded internally to a service/control head where each stream can be directed to a specific plant piping scheme or collection tank. In this way, the mother liquor can be kept separate from the subsequent washing filtrates and drying gases. This allows for better process control as well as reuse and recovery of solvents and the gases. 

Pressurized vacuum drum filters. A rotating drum inside a pressure vessel. The unit consists of a filter drum, slurry trough, agitator, wash bars, and a pressure let-down rotary valve. The process begins by closing the pressure vessel, pressurizing the vessel with compressed gas. The rotary valve is also pressurized for sealing, and the filter trough is filled via the suspension feed pipe. The agitator is started to keep the solids in suspension. Filtration, cake washing, and drying are by vacuum operation.

Indexing vacuum belt filters. Provides for vacuum filtration, cake washing, pressing, and drying of high solids slurries. The technology is based upon fixed vacuum trays, a continuously-feeding slurry system and indexing or step-wise movement of the filter media. In practical terms, the operational features of the belt filter can be viewed as a series of Buchner funnels.

For the process operation, due to the stepwise operation of the belt, washing and drying efficiencies are maximized with the stopped belt and a plug-flow mechanism for gases and liquids. Cake pressing and squeezing further enhances drying. Finally, the fixed trays allow for the mother liquor and the wash filtrates to be recovered individually and recirculated, recovered, or reused for a more efficient operation. 

Final Thoughts

Process engineers have many choices to contain an operation. The decision is not easy:

  • Is the process batch or continuous?
  • Is it a thin-cake or thick-cake operation?
  • What is the filter media (synthetic or metal)?
  • What are the critical process steps?
  • What about maintenance and other parameters?

The design questions go on and on. In the end, whatever you choose, involve process, production, operations, and maintenance in your decisions.

This blog is an adapted version of my article for The Chemical Engineer. Read the full article here!

Clear thinking about automated clarification technologies

automated clarification technologies
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This is an edited version of an article I wrote for The Chemical Engineer. I hope you enjoy this take on automated clarification technologies.

Throughout my career in the solid-liquid separation market space, I have seen some interesting solid-liquid separation solutions. At one melamine resin facility, the slurry was in a formaldehyde process. The operators were wearing masks, opening up a manual plate filter in a room with residential floor fans, to dig out the cake from the paper filter media. 

In another case for zeolites, the client had multiple bag filters to clarify the filtrates following a vacuum belt filter. When the filtrates, the final product, remained cloudy, to my surprise, the client decided to add another set of bag filters!

A clarification system is employed after coarse-particle filtration or as a stand-alone system to remove fine particles at low concentrations. These particles are typically less than 5 µm and are in concentrations less than 5% solids down to ppm levels. 

Process engineers struggle to clarify process liquids. But there are ways to automate the clarification processes to improve filtration and minimize operator exposure. The cake solids structure and the nature of the process will determine which types of pressure-filtration, automated clarification technologies are best for you.

Candle Filters

A candle filter is a pressure vessel filled with tubular filters (called filter candles). A typical filter candle comprises a dip pipe to flow the filtrate and pressurized gas, a perforated core with supporting tie rods, and a filter sock. 

As the cake builds during operation, the candle filter’s removal efficiency increases, enabling removal of particles as small as approximately 0.5 μm. 

During operation, a feed pump or pressure from the reactor or feed tank forces the slurry into the bottom of the pressure vessel. The solids build up on the outside of the filter sock, while the liquid filtrate flows into the candle, through the registers, and out of the vessel. This process continues until the maximum pressure drop, design cake thickness, minimum flow, or maximum filtration time is reached. The cake is washed to remove impurities and residual mother liquor, and then dried by blowing gas through the cake. Next, low-pressure gas enters the individual candles and expands the filter socks.

This process breaks apart the dry cake, which detaches from the filter sock and falls into the vessel cone. Candle filters are used for thin-cake (5–20 mm) pressure filtration applications. They are best suited for filter cakes that are vertically stable.

Pressure Plate Filters

Like the candle filter, pressure plate filters comprise filter elements contained within a pressure vessel. However, instead of vertical filter candles, the vessel containshorizontal filter plates. These elements are slightly sloped, conical-shaped metal plates that support a coarse-mesh backing screen covered with filter cloth. 

An opening in the centre of the plate allows the filtrate to travel between plates and throughout the vessel. The filter cloth can be synthetic, as in the candle filter, or metallic as the cake discharge is by vibration or spinning. 

Operation is similar to that of a candle filter. For cake discharge, there are two main designs. In one, two unbalanced motors vibrate the filter plates to dislodge the cake from the filter cloth. In a second design, the plates spin so that the cake can be ‘thrown’ off the plates. Pressure plate filters are used for filtration of cakes up to 75 mm thick. 

Sintered Porous Metal Cartridges 

Another type of automated clarification technology is based upon sintered porous metal cartridges. These can be used in a variety of process flows such as inside-out filtration. After each cycle, solids are backwashed off the inside of the elements and discharged as a concentrated slurry or wet cake. They can also operate in a conventional outside-in filtration. Porous metal cartridges are used for high temperature applications greater than 200oC where the solids are well-defined hard crystalline shaped. 

Filter Aids 

Filter aids are generally the last resort. Often in clarification applications, the solids are very fine or amorphous, so can be difficult to filter. When filtered, the solids will create a thin, impermeable coating over the filter media and immediately reduce the filtration rate to an unacceptable level. In these difficult cases, filter aid pretreatment can be used to improve filtration properties and efficiently remove the fine solids from the process liquids. The types of filter aid include diatomite, perlite, and cellulose. 

In the precoat method, filter aid is used to generate a thin layer of solids on top of the filter media. Once formed, the filter aid cake functions as the primary filter media. Therefore, the filter cloth is no longer the real filter when precoat is used. For that reason, the filter cloth should be as open as possible while still retaining the filter aid material. 

The precoat process is achieved by mixing the filter aid into clear liquid or mother filtrate in a precoat tank. This slurry is then recirculated through the filter where the solids are captured by the filter media. The clean filtrate is recirculated back into the precoat tank. The precoat should be thick enough to ensure that the entire media surface is coated but thin enough so that it does not provide significant resistance to filtration. 

In body feed filtration applications, the filter aid is blended with the slurry feed either by dosing the concentrated filter aid suspension into the slurry feed with in-line mixing or by mixing the filter aid into the entire slurry batch and maintaining agitation. By adding filter aid into the slurry feed, the resulting filter cake is more porous, allowing higher and longer sustained flow rates. Body feed also helps to restrict solids movement which improves filtrate clarity. 

Needless to say, the use of filter aid improves filtration but requires more equipment, more process control, and results in more solids for disposal. 

Final Thoughts on Automated Clarification Technologies

How can the process engineer be successful? When confronted with a clarification process, don’t simple throw more bag filters at the problem. Conduct lab testing to analyze the cake structure, filter media, filtration pressure and cake thickness. With the data in hand, you can evaluate the different technologies and design a more reliable and cost-effective clarification process. Find a different approach!



Real World Examples of Particle and Cake Formation Influences

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Process engineers might love it if all of the filtration technology solutions they developed ran flawlessly, at all times, under all conditions. But, this isn’t realistic. Something might go wrong with the filtration mechanism itself. A change in the environment — upstream or downstream — could cause problems with particle or cake formation. Even the smallest shift in the operation process or procedure can prompt the dreaded phone call to the engineer: “the filtration system isn’t working.”

In my work at BHS-Sonthofen Inc., I’ve seen filtration technology impacted by particles and cake formation that weren’t predicted in designing the solid-liquid separation solutions. 

Particle Sizes Changes from Lab to Production

The existing process was a batch crystallizer operating at 0 – 5 degrees C with 13- 20% solids  to a batch vacuum filtration operation. The filter was designed for a five inch cake height. The objective of the process optimization was to move to a continuous process of continuous reaction to continuous filtration, cake washing and drying.

The BHS rotary pressure filter was installed for continuous pressure filtration.  What did the client find out?  Only the particle size has changed from lab to production!  As you can imagine, this was not a small change.

cake formation

Going back to the drawing board and testing processes again, we made the following changes to the filtration system:  new filter media, increased cloth wash pressure with a new solvent and finally a reduced cake thickness.  Yes, this trouble shooting required about 6 months of work, but problem solved!

Troubleshooting Filtration Technology

In another instance with grey water treatment units, a clarification application for the purge water treatment unit (PWTU) was installed and started up for a year of successful running. Then, inexplicably, the performance changed and the filter began plugging quickly during cycles.

cake formation


Troubleshooting the system we had to re-examine the filtration system under different conditions:

  • Clarifier overflow with no coagulant / no flocculants 
  • Clarifier overflow with only coagulant / no flocculants
  • Clarifier overflow with both coagulant and flocculants
  • Clarifier overflow with only flocculants / no coagulant

Taking a holistic approach to the system, we were able to determine chemical changes caused the larger particles to settle out. Only the smaller particles were reaching the filtration system, which was blinding the filter media.  By eliminating the flocculants  and reducing coagulant usage (even though this was better for the client, and not necessarily BHS as the chemical supplier, we were able to improve filtration rates and once again offer a consistent PSD.

Ultimately, with the right approach to troubleshooting, and by embracing the idea that we do on a daily basis is an art coupled with science, we can enjoy a strong sense of satisfaction when we get that filtration technology up and running again.

This blog is based on a presentation I made to the  8th World Congress on Particle Technology. View the presentation slides in full!

Selecting and Designing Combination Filtration for Solid-Liquid Separation

SLS equipment
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Filtration experts, over the years, have discussed combination filtration and debated its definition.

  • In the realm of cartridge filtration, simply defined, a combination filter is one that does at least one other processing job at the same time as filtering a suspension.  For example, this could be carbon canister which removes both suspended and dissolved components.
  • In water applications, a combination filter removes bacteria, sediment, chlorine taste and odor, and scale.
  • In lubrication oil filtration, combination filtration refers to full-flow and by-pass flow filtration.
  • For small scale process filtration, combination filtration is installing bag and cartridge filtration systems in series.

There is, however, a new definition of combination filtration that transcends the standard approach and will assist process engineers with trouble shooting and idea-generation.  The approach relies upon the slurry analysis and testing to uncover the “process symptom” and then develop a process solution called “combination mechanical slurry conditioning and filtration.”

Filtration Technology in Combination

There are, without doubt, there is a lot of SLS equipment already existing in the marketplace that can be applied in combination, including the use of chemicals such as flocculants and coagulants.  However, from a practical viewpoint, let me review general operating conditions at chemical plants and illustrate creative idea-generation when examining a process problem.

In this first case, we have a high solids slurry with a wide particle size distribution.  What should you do?  My idea is to provide filtration for the slurry with a continuous technology and let the fines bleed through; capture theses fines with clarification.  Yes, more filtration but a much more reliable system.


filtration procedure

This new definition of combination filtration will provide process engineers a framework for idea generation when analyzing an operating bottleneck.  Complete my application data sheets for new and existing application data for filtration for solid-liquid separation. Let us start the process of finding the right SLS equipment for your business.

Best Practices for Filtration Testing for Solid-Liquid Separation

filtration testing
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Testing in school has a negative connotation. Students dread tests. Parents bemoan “teaching to the test.” Teachers chafe against the curriculum parameters defined by testing expectations. Yet, the word “testing” should resonate much more positively with process engineers. After all, testing is the key for selecting the most suitable filtration tech for any individual solid-liquid separation task.

Although there is only limited theoretical background available, and even specialized engineering education at universities leaves many theoretical questions open, it is beneficial to have a minimum understanding of the theory of filtration itself. By identifying the role of each influencing part, the process engineer gains a potential tool to work with when it comes to understanding testing findings and developing a path forward in determining the best filtration procedure.

Just from experience, and for the benefit of engineers, some overview observations are necessary:

  • Don’t stop testing just because the first results suit your target
  • Don’t accept samples without verifying the parameters in the description
  • Never change more than one parameter at a time
  • One result is no result => verification is a must
  • Take a break and check the conformity of the results before you call it a day

Filtration Testing Requires Decision Making

In testing it’s essential to train yourself to stop and repeat. Don’t succumb to perceived certainty. After all, many parameters of the liquid and the solids have an influence on the filtration process.

  • Form and size of particles
  • Particle size distribution (PSD)
  • Agglomerate building behavior
  • Deformability
  • Compressibility
  • Liqiud viscosity
  • Solid content
  • Zeta-potential

While all of the above may not be known for all filtration applications, the final target is to find a theoretical approach together with a practical method of testing.

Sampling in Filtration Testing

Filtration tests need to be done with a “representative sample” defined as a sample “as close as possible” to the real production product.  Yet the specific characteristics of a slurry from the point of filtration are not obvious to everyone. That’s where testing comes in: the list of parameters is quite extensive and in many cases only a few are available.

Still, the more you can get the better. Although for the first tests, the ph-value, temperature, particle shape, size distribution, etc. are not necessary right from the beginning, these parameters are normally quickly measured and complete the picture of the suspension. It is obvious that solid content and viscosity do have an impact on the filterability.

“Suspending” Judgment in Filtration Testing

The characteristics of suspensions are not only caused by the liquid phase but also by the particles, the other half of a slurry. The solids can be of crystalline nature or amorphous, which means their structure is not really defined. They can also be organic (i.e. cell debris), fibrous, in-organic, compressible or incompressible, generate agglomerates or not, may have a zeta potential or not…. there are many possibilities.

An easy way to verify the type of solids is a sample check. If possible, the original suspension should be checked under the microscope. Then, the behavior of the solids can also be seen:

  • Do they tend to build agglomerates or stay on their own?
  • How is the distribution of the solids?
  • Is the structure of the solids needle-shape, potato shape, snow crystal or even fibrous?

The best practice in filtration testing is to consider all of these angles thoroughly before deciding on a filtration procedure.

I am a big fan of Sherlock Holmes who always warns “don’t jump to conclusions.”  This is one of the biggest risks we face during tests in the daily work of process engineering.  Let me know if you need help!

Selecting the Right Types of Filtration for Solid-Liquid Separation

types of filtration
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Filtration selection, if we think back to Sherlock Holmes, means “not jumping to conclusions.”  There is no “one size fits all” process solution.  Selecting a filtration technology requires a systems approach incorporated with other solids processing such as reactors, dryers, solids handling, etc.  You could gain an objective overview by filling out an application data sheet (like the ones I use for new or existing applications) that can help identify what’s involved in the specific solid-liquid separation.

Ultimately, the process has three components:

  • Material properties, which I’ll describe in more detail below
  • Separation performance objectives including, for example, filtrate quality (conductivity or residual solids) cake dryness, flowability of the cake, crystal breakage /fines generation and conditioning of the cake for further processing
  • Mechanical properties — The specification must be clear in terms of material of construction, temperatures/pressures, FDA validation, cleaning procedures, manufacturing codes, etc.  Each equipment type will have its own mechanical specifications that must be satisfied.

These three considerations are combined and ranked choices are then evaluated for operational, economic, and plant (internal and external) objectives.

Finding the Best Filtration Procedure

Your examination of material properties considers the solids and the liquids.  For solids, the engineer needs to know the total suspended solids (TSS) and solids concentration, particle size distribution (PSD), and particle shape.  The PSD should be based upon particle counts at different sizes rather than by weight or volume.

The particle shapes can vary:  spheres, rounded, angular, flaky, or thinly-flaked are among the examples.  Shape will influence the filtration rates for the process and also impact the PSD due to the nature of particle size measuring equipment.

Knowing this, the solid-liquid filtration system further requires a systems approach to incorporate other solids processing such as reactors, dryers, and solids handling, etc.  The full scope should include the actual upstream and downstream.

Consider this typical example of a chemical process including all of the associated processing steps:

  • Chemical synthesis and Crystallization:
    • Types of catalysts
    • Solvents
    • Continuous or batch
    • Temperature
    • Flashing
    • Inerting
  • Filtration
  • Drying
  • Dissolution
  • Hydrogenation
  • Secondary crystallization
  • Filtration
  • Final drying
  • Solids and slurry handling in all steps

General Guidelines to Selection

So, the question is where to begin to make the preliminary filtration technology choices for solid-liquid separation?  Here are some general guidelines for selecting among types of filtration:

Filter Press Continuous Vacuum and Pressure Nutsche  Filter & Filter-Dryer Clarification
Solid content of the suspension (%) 5 to 30 10 to 40 10 – 40 < 5
Maximum Pressure Difference 100 bar -1 to 6 bar 6 bar 10 bar
Cake Thickness (mm) 5 to 50 5 to 150 5 to 300 20
Average Particle Size 1 to 100 micron 1 to 100 micron 5 to 200 micron 1 to 50 micron
Type of Operation Batch Continuous Batch Batch
Comments Good for slow filtration and can produce dry filter cakes; Excellent cake washing and pre-drying Good when reactor batch times equal to total cycle times Disposable for low flows; candle and plate filters for large flows

Let me know if this is helpful to you.  My idea is to do a series of types of filtration systems for solid-liquid separation for various applications.  What is troubling you?

Innovating Zero Gravity Espresso

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Ambition to improve is everywhere, and not only limited to corporate labs and academia. I love to see examples of innovation in any field — from wind turbines to cloud seeding or surfing to strawberries. Those who are loyal readers of this blog know that I’ve already written about each of those. Today though I am turning to some new tech that relates to what we do every day to solve chemical engineering problems.

Despite what contrarians might say, humans long ago solved the mystery of making perfect coffee on earth. Yet building an espresso machine for the international space station was a much bigger challenge. Bloomberg last year featured an Italian engineering firm, Argotec, spent over 18 months with 11 engineers working to develop a microgravity brewing process that could meet NASA’s rigorous safety standards.

In Earth gravity, we force almost boiling water through finely ground coffee beans. The water boils, becomes less dense and the steam pushes into the air above. But hot water behaves differently in near-zero gravity. The steam does not rise; staying put it can create a dangerous vapor bubble suspended in a ball of water. So, to avoid bubbles, Argotec used a special thin-steel pipe to ensure the water couldn’t build up heat bubbles.

The next problem was pressure. NASA safety rules anything over 60 psig a concern. Espresso brewing requires at least twice as much pressure. Argotec engineers determined that they could address this by eliminating a traditional rotary pump and instead using an electric motor to drive the plunger.

What about leaks and pressure relief? This is a common problem for all chemical plants, but even more difficult in zero gravity. Argotec made it so that every piece of pipe had relief valves with piping back to a central pressure containment chamber.


That left the question of how to drink an espresso in outer space? Think, after all, of the Tang crystals that are so famous as a space drink. The espresso-loving engineers designed a mechanism to blow air through the coffee capsule into a zip-lock coffee cup.

chemical engineering problems

We all know how nice it is to have a hot espresso when trying to solve a process-related chemical engineering problem. Now our astronauts have the same opportunity to get the brain cells jumping in space. This is yet another exciting example of how process engineers have to brainstorm new approaches to separation problems.

Sleuthing CIP Process Solutions

CIP Process

For years, my focus as a process engineer has been thin-cake filtration, cake washing and drying technologies. I am continually engaged by the people I work with and the ongoing need to find new process solutions to problems.

There are always new challenges. Our latest BHS Sonthofen newsletter focused on practical solutions for solid-liquid separations in chemical and pharmaceutical applications. Particularly, this issue of Art and Science of Filtration, looked at new clean-in-place (CIP) challenges.

Our friends at CH2M and EI Associates collaborated to examine large-scale fermentation systems in biochemical and biopharmaceutical settings. In discussing designing new organisms to target desired chemical products, they addressed several challenges with genetically modified microorganisms (GMM):

  • GMM are not, typically, designed to be robust and can find competition with natural microorganisms difficult.
  • GMM are a new creation and can exhibit unforeseen and undesirable traits.
  • Incorporating extensive CIP and sterilize-in-place (SIP) systems to prevent contamination is critical.

In discussing considerations for CIP process, the authors focused on sequence, system configuration, equipment sizing, tank and piping design considerations, and more. That’s before they even examined the SIP considerations. It’s not for everyone, but I love trying to fit together the many puzzle pieces to make the process work for our clients.

Also in our newsletter, we shared a CIP presentation from my colleague Tim Ochel. He addresses influencing factors in CIP (velocity, temperature, chemicals, time and technology) and how BHS Filtration meets the need. For instance, BHS’ Rotary Pressure Filter (RPF) is menacing in transforming materials into value. It can handle filtration and washing, multi-step counter-current cake washing, and drying too. He demonstrates how our RPF is successful in applications where optimum cleaning effectiveness, cGMP compliance and contained product handling are required.

CIP process

There’s a lot to think about when it comes to CIP process in pharma. Another BHS article has talked about the advantages of continuous filtration for pharmaceutical manufacturing. We’re determined at BHS to keep abreast of process design strategy trends to make sure our clients are safe and streamlined while they work to save their customers from whatever ails them.


Elementary, My Dear Process Engineers!

Back in 2013, I encountered an article on Maria Konnikova’s book Mastermind: How to Think Like Sherlock Holmes. Businessweek had it in their MasterClass section in which they scan the book so we “don’t have to.” Yet I was intrigued enough to want to read the entire book (and you should too). Solid-liquid separation (SLS) may not have the glamor of investigating blackmail and burglary, extortion and espionage or murder and mayhem, yet I was struck by the similarities between process engineers and Sherlock Holmes and Dr. John Watson.

I have spoken widely and published articles purusing my idea that mindfulness, astute observation, and logical deduction are among the attributes we share with the famous fictional duo.

Cover image from
Cover image from

In my newly released practical guide to Solid-Liquid Filtration, I argue SLS process engineers have several similar attributes to Sherlock Holmes and Dr. John Watson. For example, we:

  • Know there is no benefit to jumping to conclusions.
  • Benefit from working with others to recreate events.
  • Apply problem-solving skills such as occasional silence, employing distancing and learning to discern the crucial from the incidental.

Since most university curricula don’t cover solid-liquid filtration, many engineers are left clueless as to where or how to begin. With this blog, I’ll be drawing on my over 30 years in the industry, to help provide engineers with a framework to analyze and think about process filtration problems. I’ll also be sharing my thoughts on other interesting reads and my experiences working with BHS Filtration and our European counterpart BHS-Sonthofen GmbH I’ll be focusing on the accurate and unbiased facts — just as Sherlock himself would want me to do!

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