Dirty process water is one of the biggest challenges faced by any industrial operation. Particulate matter and debris can clog pumps, nozzles, heat exchangers, cooling towers and other components, leading to reduced process efficiency and, in some cases, damaged equipment. Additionally, with water conservation becoming an increasing priority for many of today's facilities, plant managers and engineers are taking a careful look at how process water can be reused to its maximum potential without adversely affecting product quality.
Using adequate filtration is one way to achieve this goal. But with all the different filter types and process requirements, how can you be sure a given technique will perform as needed to keep your process running smoothly?
Identify the ProblemsThe first step in analyzing your filtration requirements should be to detail the specific aspects of the process that are affected by unwanted contaminants in the water. Separate corrosive/water quality issues from particle contamination issues wherever possible because these two sources might demand different methods for corrective action. Make sure that you understand the potential for contaminants to hinder effective production and interrupt operations for servicing/maintenance routines.
Pump Intakes. Surface and well waters often contain sand, and this sand can damage pumps and cause problems in the water recycling phase. Open water sources also can contain organic matter and other debris. Protecting the pump intake, the pump itself and the process from contaminants in the water source is important to ensuring the efficiency of the recycled water system (figure 1).
Nozzles and Small Orifices. The cleansing of processed products can involve either a deluge or a directed flow of water through nozzles or reduced-flow orifices. Unwanted contaminants must be controlled to keep these openings from becoming clogged or abraded by wear, which can affect proper operating characteristics.
Pits, Sumps and Basins. With rare exception, water at some point will be held in a manner that allows either settling of heavy contaminants or surface accumulation of floating debris. Techniques to reduce the accumulation of these contaminants and prevent them from flowing into other phases of the process are important to the overall operation of the process.
Existing Filters. Filters designed for ultrafine particle removal tend to clog quickly when larger particles are present. Water treatment typically performs better when large-particle contamination is removed first, thereby reducing the fouling of active media or treatment chemicals. When two or more contaminant types or sizes are present in the water, the best strategy for removal might be to combine filtration techniques.
Water Waste. Disposing of recycled water ultimately is inevitable. Over time, the water becomes contaminated beyond the capabilities of filtration and water treatment. However, effective filtration can extend the life of that water prior to disposal, in many cases clarifying it with minimal water loss.
Identify the ContaminantsAfter you have identified the areas in your process that are experiencing the biggest problems due to water contamination, you must next understand both the type and concentration of the contaminants before selecting the techniques that will provide effective performance. Even if only one type of contaminant is the source of a problem, other contaminants in the water should be identified to prevent additional problems downstream. (For example, a filter capable of removing sand can become clogged if undetected algae and other organic materials are in the water.)
Settled vs. Floating Contaminants. Particles that settle have characteristics that are different from particles that float or remain suspended in water, and different techniques are available for removing these dissimilar contaminants. Be aware that floaters might be more malleable and therefore able to pass through (or clog) some removal techniques.
Particle Count vs. Particle Volume. Today’s technology allows for sophisticated methods to determine both the numerical count of particles in a fluid stream and the total volume of contaminants. These are different numbers with different implications. For example, it is possible for 10 percent of the numerical count of particles to account for 90 percent of the accumulation of space (figure 2). If keeping a basin free of accumulation is an application goal, selecting filtration that removes the greater numerical count (90 percent, but all smaller particles) might be the wrong approach and could be more costly.
Protection vs. Performance. It is possible to remove nearly all contaminants from a fluid flow, but such an option might not be necessary or affordable. Instead, know the particle removal requirement to protect the equipment that needs to be protected. Think affordable protection, not absolute perfection.
Identify the Appropriate Application TechniqueBased on the overall contaminant load and the required level of protection, there are three basic approaches to particle removal. Each has its advantages and drawbacks.
Full-Stream Protection. This approach requires all fluid flow to pass through the filter before it moves through other parts of the process (such as spray nozzles). When the process downstream is sensitive to particle contamination, the full-stream approach provides the greatest level of protection. Because the filtration system will be expected to remove all troublesome contaminants, it might be logical to employ two or more filters with varying filtration capabilities to remove all of the substances that threaten the operation and efficiency of the process.
Side-Stream Protection. If diluting the particle concentration in an ongoing recirculation of flow is an acceptable technique, then side-stream protection might be adequate. This technique is designed to remove particulate matter at a rate greater than it is introduced into the system. (A range of 10 to 50 percent of side-stream flow is typical, depending on the contamination level and seasonal issues.) However, because this approach does allow a percentage of particulate matter to pass through the process, it might not be suitable for many applications. This technique also commonly requires a pump with the filter to re-introduce the side-stream flow back into the main-stream flow.
Basin Cleaning. With a focus on keeping unwanted contaminants from accumulating in a body of water, this technique does not directly protect downstream equipment, but it does reduce the potential carryover of contaminants into the process. Do not confuse this technique with simple turnover of the total fluid in the system. When designed properly, basin cleaning employs a flow pattern within the basin to control contaminant accumulation continuously throughout the basin floor (figure 3).
Compare the Contaminant Removal MethodsMany different products exist that can be used to remove particle contaminants from process water. In general, the different filtration types can be divided into barrier and separation techniques.
Barriers techniques include screens, cartridges, bags, membranes and other restrictions to flow that trap contaminants and allow the water to pass. Sand and media filters also qualify as barrier filters, given that the media bed is the barrier to trap the contaminants (figure 4).
Separation techniques can be as simple as a static body of fluid (a clarifier, for example) that allows some particulate matter to settle while other debris floats to the surface. Either or both contaminants are then removed by flushing at the bottom or by skimming at the surface. More sophisticated are techniques such as centrifugal separators, which employ a spinning action to remove settled particles; and dissolved air flotation, which encourages suspended particle matter and floating debris to move to the surface for skimming.
When evaluating the different contaminant removal methods, consider the following criteria.
Flow Range. Is the product capable of handling the flow rate of your application? Does it use a single unit or multiple units with manifolding? Does the product require a minimum flow rate for performance or backwashing for cleaning action?
Performance. What contaminants will the product remove from the fluid? What particles, if any, will pass through the product? What contaminants are not recommended for this product, and why?
Pressure Loss. What is the basic pressure loss? Does this parameter change as the contaminants accumulate on the product? What happens if excessive pressure loss builds up (figure 5)?
Replacement Parts. What parts, if any, will need replacement, and how often? How much do replacement parts cost? Are they readily available?
Downtime and Maintenance. What servicing routines are required with the product? How often are these routines needed, and how long do they take? Is duplicate equipment necessary during this process?
Automation. Is it available, and if so, at what cost? Is routine servicing still required?
Solids Handling. Does the product have options or accommodations for dealing with the collection, concentration and disposal of the contaminants (figure 6)? If so, what is the cost? How operator-intensive is the process?
Product Life. What is the expected life of the filtration system? What options are available (material changes, coatings, etc.) to prolong that life? (Consider corrosive water conditions and the components of the product.)
In conclusion, when evaluating filtration solutions, make sure your analysis is objective and not influenced by marketing literature or persuasive sales techniques. Consider, too, that these criteria might suggest a product that initially is more expensive but will cost less in the long run. By understanding how to evaluate filtration equipment based on your process requirements, you can control the factors that are most important to making a wise decision.