Settling Tanks for Glass Filtration vs. CentraSep®

Gravity Settling Tanks, also called Sedimentation Tanks, have been used for many years in a multitude of industries to help separate solids from liquids. Municipal waste water treatment plants often use large settling tanks due to the vast amount of waste water they need to process. It is perhaps this widespread usage in these types of treatment facilities that led to many industrial and commercial applications also using settling tanks, but is it the best method for separating solids from liquids in those environments? Are buyers truly aware of the flocculation chemistry required by settling tanks and how difficult and critical it is to maintain the optimal level? Are buyers aware of the low maintenance of a CentraSep centrifuge system as well as its flexibility to easily adjust to production and flow fluctuations? This article will explore these questions, and more, in depth.

See How CentraSep Compares to Settling Tank Systems (PDF file)

First, let’s look at gravity settling tanks. A common misconception in the industry is that settling tanks can get processing fluids cleaner when compared to a centrifuge or other types of filtration systems. This statement is simply not true. A CentraSep centrifuge system can easily match the fluid clarity levels of a gravity settling tank if needed or desired. To understand this more, let’s take a closer look at each of these filtration methods.

The basic premise of a gravity settling tank is quite simple… Over time, heavier solid particles will naturally separate and will eventually drop to the bottom of the tank. Thus, the liquid at the top of the tank is cleaner and can be pulled off for re-use or recycling. In practice however, this process is not quite this simple. The natural separation process is only using standard gravity (i.e. 1 G), so it can take a very long time to occur. To make this gravity settling process happen faster, flocculation chemicals must be used, but this creates a much bigger and more complex problem of continually maintaining and balancing those chemicals.

These systems also require a lot of physical space for the high volume tanks that are needed. On a regular basis, those large tanks need to be emptied and cleaned, which requires the replacement of a huge volume of water, coolant or other processing fluid, at a great expense. In addition to the high cost of this effort, it also requires a lot of manual labor, and the plant’s production or processing operations are typically shut down for a lengthy period of time during these cleaning periods. These shutdowns can cost the company a lot of money in the form of lost productivity/lost revenues.

Tanks need to be cleaned on a regular basis for a number of reasons. One primary reason for needing to empty, clean and replace the fluid in a settling tank system is that the separated solids will accumulate to a point where they are filling the tank and no longer allowing the processing fluid to clarify. Another issue that could necessitate a tank cleaning is if a bacterial or fungal infection contaminates the processing fluid. In industrial applications, coolants and other processing fluids typically contain biocides to prevent these types of contaminations, but settling tank systems commonly use flocculation chemicals (discussed more below) that disrupt the fluid chemistry and the performance of the biocides, allowing these contaminations to take place.

Advocates for settling tank filtration praise the large tanks that are used, because a single system may be able to process the fluid for an entire plant. While this may seem like a positive selling point to some companies, they must consider that when the system is down for routine cleanings, or any type of equipment failures, their entire plant’s production or processing capability is also down. Of course, this would be the case for any type of filtration system that is handling the processing fluid for an entire plant, which is why most filtration experts recommend having more than one system for a large plant. These types of plants can’t risk having all their production or processing halted due to their filtration system being down. However, having multiple filtration systems is often not practical or possible with settling tanks due to the physical space and tank volumes that they require.

As solids sediment builds up at the bottom of settling tanks, it can compact into a very hard, concrete-like material, so to prevent this from happening and to get the solids out of the tank, on a regular interval, pumps and mixers are used to agitate the solids and get them back up into suspension. Of course, this action is in direct opposition to the goals of the settling process, but it is a necessary process to keep a settling tank system clean, in operation and functioning properly.

The sediment pulled out of these tanks is not yet solid matter. It is still a heavy, dense, dirty liquid, that would be very costly to dispose of, so settling tank users must do something else to further compact and “dewater” this waste, and to recycle the fluid (if possible). Various equipment and processes exist for performing this dewatering process, including centrifuges, filter presses, vacuum beds and others. However, most settling tank owners tend to use dewatering bags. These are very large porous bags filled with sediment that are hung up and allowed to drip-dry. Again, this process is only using 1 G of force (natural gravity), so this also takes a long time to get the sediment to a nearly dry state and ready for disposal. There are some additional issues with this process as well. These large dewatering bags consume a lot of space, and the process is very messy, labor intensive and expensive. These bags are expensive, and they are only used one time before being disposed of. In addition, the flocculation chemicals mentioned previously work against this process, because the flocculant clogs the pores in the dewatering bags. This reduces or eliminates the ability for the processing fluid to drain out.

As described above, the flocculation chemicals create multiple problems for settling tank systems, but they are necessary for proper operation. Since settling tanks rely solely on natural gravity (1 G of force) to separate the solid particles from the liquid, and since that is a very slow process, most settling tank operators add processes and/or equipment to try to speed up the separation process. In addition to more advanced tank designs that incorporate conical bottoms or other designs to try to collect and discard the sediment, and agitators (like air mixers and blowers), designed to move the sediment out of the tank, flocculation chemicals are added to help bind the smaller solid particles together, so they become larger, get heavier and drop out faster.

However, using flocculant to speed up the settling tank’s process is not a simple, straightforward task. Using flocculant not only creates the problems stated above (disrupting the function of biocides and clogging the dewatering bags), but it complicates system operation and maintenance, which leads to more manual labor, expenses and potential issues. Achieving the proper and most ideal balance of flocculant chemistry is an on-going, continual challenge, plus flocculation chemicals can also cause problems in certain production and processing environments (i.e. the chemistry can interfere with the production of coated glass, mirrors, solar panels, etc.).

Truly understanding how flocculants work and how to keep a settling tank system balanced can get very complicated, but the general concept is fairly easy to understand... The small solid particles in the process fluid, often called “fines” in industrial processes, have a negative, or ionic, electrical charge. The flocculant has a strong positive, or cationic, charge. Thus, these differently charged particles naturally attract each other and clump together to form larger particles that settle out faster. This seems easy enough to understand, and it is… until it isn’t.

The problem is that no fluid processing or filtration system is static (i.e. staying exactly the same). The flow rate of fluid and the amount of solids in the fluid changes depending on the time of day, the day of the week, the work shift, production demands, the amount of equipment in use, etc. And depending on the amount of solids in the process fluid, the amount of flocculant needs to change to meet those variables. In addition, the “size” of the solid particles are also ever-changing and this also has an impact on the balance trying to be achieved. Smaller fines are harder to capture and require more flocculant to get them to attract and clump together. Larger particles that are not immediately captured will continue to recirculate and as they do this, they get broken down into smaller and smaller particles, including the hardest particles to remove, the super-fines.

Thus, as flow rates, the amount of solids, the size of the particles, the amount of super-fines, etc. vary, the amount of flocculant needed must constantly be adjusted in order to stay in a perfect balance. If not enough flocculant is used, only the largest particles are separated out and the smaller particles build up. If too much flocculant is used, the entire process fluid system will shift from its desired negative (ionic) electrical charge, to a positive (cationic) charge, in which case the solids are no longer attracted to the flocculant, but rather, they actually repel each other (like trying to push two magnets together). And this is only what happens inside the tank. Outside the tank, in other parts of the processing fluid system, another problem is created. In the system’s piping, and at the machine head if in an industrial process, the proper negative charge is developed, which means the solids and flocculant are binding at points in the system where they shouldn’t be and clogging pipes, generating quality issues in machining operations, etc. This is a very destructive and costly situation that needs to be avoided by all means.

This chemistry and physics balancing act is trying to manage something called the Zeta Potential. Let’s look at an example… Before adding the flocculant, the negatively charged solid particles in the process fluid might make the solution a (-25) Zeta Potential (just as an example). Each filtration system is different, but in this example, maybe the desired, optimal Zeta Potential for ideal solids separation is (-10). Thus, the positively charged flocculant is added in the specific amount required to reach that desired Zeta Potential. Once the system is there, then the operator must maintain this perfect balance forever, which of course is easier said than done. As all the variables described above change, sometimes rapidly, the dosing level for the flocculant needs to be continually adjusted to keep this example system balanced at exactly (-10) Zeta Potential.

If the system doesn’t get reduced to this Zeta Potential, it won’t work effectively. Smaller particles won’t be captured and they will build up in the fluid, requiring more flocculant to get them out. But adding too much flocculant can cause the solution to reach a point called the Isoelectric Point, which is (0) Zeta Potential, and the point where the entire separation process stops working completely, because the solid particles and the flocculant no longer attract each other (and actually repel each other instead). When this situation occurs, additional chemicals are needed to try to reverse it, or the system must be shut down and the entire process fluid must be discarded and replaced (to start over).

If all of this is starting to seem complicated to manage, then you are correctly understanding how a settling tank filtration system actually works. At this point, you may be thinking that these chemical-dependent systems require a well-trained and highly experienced operator to maintain the perfect balance. And that would be the proper assumption and best approach to take, but even then, the most skilled operators are still just guessing as they attempt to monitor and regulate the amount of flocculant used. The problem is that they don’t know what the Zeta Potential is and they can only guesstimate it. A good Zeta Potential tester costs well over $60,000, so most companies don’t have one and won’t get one. And even if they did, it is not going to measure the Zeta Potential “continuously” and then instruct the operator to make the needed changes on the fly. Some companies have tried using less expensive opacity meters to measure the clarity of the process fluid, but this isn’t the same as measuring the Zeta Potential. Thus, a good, trained and experienced operator, combined with some good guesswork is how most settling tank filtration systems are operated today.

Despite all these problems with settling tanks, they continue to be purchased, typically based on a lower initial cost and a perceived simplicity to operate and maintain them. And while operating and maintaining these systems may be easier in some large waste water treatment facilities, for commercial and industrial applications, settling tanks have a very high cost of operation. They require a lot of manual labor, they use a lot of consumables, and they cause more production downtime than highly efficient alternatives like a CentraSep centrifuge.

CentraSep centrifuges operate based on a simple physics premise… Rapidly spinning fluid that contains solid particles, causes those particles to separate from the fluid. Rather than relying on only 1 G of gravity to do the work, a CentraSep centrifuge applies 2,012 G’s to separate even very small fines from the process fluid. It also uses no chemical flocculants, and no consumables like dewatering bags. The separated solids are automatically ejected from the centrifuge as a nearly dry cake (depending on the material), making disposal fees minimal.

CentraSep centrifuges run by themselves, with no operator and they are robust and flexible enough to easily handle any changes in the level of solids, flow rates, particle sizes, etc. In addition, only minimal routine maintenance is required to keep these systems running for many years. Production downtime is minimal, because the tanks may not need to be cleaned as much. Sediment will still build up, but typically not as quickly, and fungal and bacterial infections are not as frequent since chemical flocculants are not being used and the biocides in the processing fluid are not being adversely affected. Also, when tank cleanings are required, since the tanks are much smaller, the downtime is much less. With a CentraSep centrifuge, the fluid is continually cleaned and maintained well below most company’s goal ppm’s (parts per million) of solids, so production efficiency and quality are never adversely affected. Lastly, a CentraSep centrifuge system requires only a small fraction of the physical space needed for a typical settling tank system.

A CentraSep centrifuge is capable of this significant level of solids reduction and offers many other benefits, due to its unique, proprietary and patented design. The CentraSep unit is vastly different from all other industrial centrifuges. It is the only single-motor, vertical, self-discharging, automatic centrifuge on the market. It combines an innovative bowl/blade clutch design with a single AC motor and variable frequency drive (see cutaway illustration) to automatically extract solids from virtually any liquid at a wide range of processing flow rates. CentraSep centrifuges are able to remove a higher volume of solids / fines compared to traditional centrifuges. Also, every component of the CentraSep system that comes into contact with the process fluid is 316 stainless steel, so harsh acidic and caustic fluids, as well as hard and extremely abrasive materials like glass fines, can be processed safely and effectively. For more details about the CentraSep centrifuge's technology and patented design features, visit our Technology page.

Settling tank advocates are quick to focus on the initial price differences between a relatively simple settling tank system and an advanced state-of-the-art, patented centrifuge system, but those price differences really aren’t that dramatic, because settling tank systems are not as simple as they might appear. They require large tanks, plumbing, pumps, valves, sophisticated chemical dosing systems, electrical control panels, etc. Thus, a complete settling tank system will cost nearly as much as a centrifuge system, and when the cost of operation, including consumables, extensive labor and increased maintenance costs are considered, the cost comparison gets even closer. A large company will spend thousands of dollars per year just purchasing the disposable dewatering bags, then adding in flocculation chemicals, increased disposal costs, additional labor, etc. and the operating costs add substantially to the initial price of a settling tank system.

Another point that some settling tank advocates may focus on, is a misguided belief that settling tanks can get the process fluid cleaner than a centrifuge system, including the removal of the super-fines, but a CentraSep centrifuge system can easily match any level of clarity needed or desired. The key to this statement is asking what level of clarity is truly required to meet a company’s needs or goals. For a settling tank to achieve an extremely high level of filtration, the system must be in perfect balance and stay that way forever. As explained above, this perfect balance of solids, flow rates, particle sizes and flocculation chemicals is rarely achieved or maintained.

While it typically isn’t necessary, if a company wanted to achieve an extremely high level of filtration with a CentraSep centrifuge system, a small amount of flocculant could be used to bind up the super-fines. However, removing all of the super-fines is rarely necessary to meet the filtration ppm goals and needs of the vast majority of companies. Regardless, if that was the desired goal, using flocculant with a CentraSep centrifuge would not only meet this demand, but it could do so easier than a settling tank. Remember that the settling tank is only operating at the force of gravity (1 G), but a CentraSep centrifuge is operating at 2012 G’s. Everything but the finest of particles is already being separated out without the use of a flocculant, so if using a flocculant to remove the super-fines, very little chemical would be needed, making the management of the system easier and less costly.

Part of the problem with settling tanks is that many of the settling tank manufacturers are not filtration experts. They may be tank builders, or plastic or metal fabricators, but often they do not understand filtration and separation at an expert level. They just provide the tank and then rely on the coolant and/or flocculant provider(s) to consult on the actual filtration process. At CentraSep centrifuges though, the staff consists of qualified and experienced Filtration Engineers. Jeff Beattey, CentraSep’ President, stated, “Solving challenging separation and filtration applications is all we do. We have hundreds of centrifuge systems installed all around the world, in a multitude of harsh environments, separating all types of very fine materials, and many of those systems have been running for over 2 decades. We have a great understanding of filtration problems, and how our systems perform and what they can do. Our systems have replaced many settling tanks, filter presses, shaker screens, skimmers, and other types of filtration systems. We have studied why those other systems didn’t work, and we know when a centrifuge system will work best. We won’t replace or install a CentraSep system unless we know it will be the best solution for the application.”

For anyone willing to understand the issues involved in operating and maintaining a chemical-dependent settling tank system, the differences should be clear. There are many benefits and differential advantages for using a CentraSep centrifuge system. To learn more about CentraSep, and/or to discuss your application and needs, please explore our website or call +1.317.660.6670.

Summary of Issues Regarding Settling Tank Filtration Systems

Expenses & Labor Issues

• Labor for Routine Tank Cleanings, Handling the Dewatering Process, System Maintenance and General Operation
• Replacement of Entire Process Fluid/Water/Coolant After Tank Cleanings (typically uses 3x more water than a CentraSep centrifuge system)
• Loss of Production and Revenue During Tank Cleanings
• High Disposal Fees for Large Volumes of Wet Sediment
• Replacement of Dewatering Bags
• Power Consumption from Mixers, Agitators, Pumps, etc.
• Replacement of Flocculation Chemicals (regular usage and full replacement after tank cleanings)

Additional Disadvantages

• Working at Only 1 G of Gravity / Sedimentation Force (vs 2012 Gs)
• Slow Process / Takes a Long Time for Separation
• Requires a Lot of Space for Tanks and the Dewatering Process
• Requires a Lot of Manual Labor for Operation and Maintenance
• Requires a Well-Trained and Highly Experienced Operator
• Increased Production Downtime for Tank Cleanings (more frequent and higher tank volumes)
• Must Keep Agitating the Solids to Keep Them From Settling Too Much / Compacting, and to Get Them Out of the Tank (so this process is working against itself and not allowing solids to fully drop out)
• Dewatering Process is Messy and Expensive (bag replacement)
• Chemical-Dependency on Flocculant (forever)
• Balancing the System for Proper Flocculant Amounts and Flow Levels is a Constant Challenge
• Flocculant Disrupts the Processing Fluid’s Chemistry and any Biocides in it, which allows bacterial and/or fungal contaminations to occur.
• If used to process the fluid for an entire plant, when it is down, the entire plant’s production is also down.
• Flocculation chemicals can interfere with the production of coated glass, mirrors, solar panels and other processes that involve chemistry and coatings.

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