Choosing the best filtration method

Selecting filtration equipment is the combined result of many considerations. In addition to removing the undesirable material from a liquid stream, the filtration method selected must also satisfy other requirements. Installed costs must be weighed against operating costs. Waste disposal costs must be considered.

Is continuous flow a requirement of the application, or can the filtration equipment be operated intermittently? Is working exposure to the process liquid during filter cleaning or replacement a problem? These and other factors must be weighed when choosing the right filtration method for a particular application.

The products being removed from a liquid process stream are as widely variable as the types of filters designed to remove them. Products being removed in potable water treatment applications are often molecular in size. However, many other filtration applications are concerned with the removal of specific trace solids from a liquid process stream. Surface-type filters using fibrous cartridges of mesh screens made of woven wire or fabric are often used in these applications.

The following analysis will focus on the removal of these trace solids, rather than the entire scope of liquid filtration.

Filtration methods

There are three basic filter types to consider when specifying a liquid processing filter:

• bag filters,
• cartridge filters,
• self-cleaning/clean-in-place (CIP) filters.

Each one has advantages and disadvantages when compared with the others, and the different types are appropriate for different applications. Self-cleaning/CIP filters are typically most appropriate for applications where flow rates are high, filter media replacement costs are high or exposure of the process liquid to workers or the environment is undesirable. They can be used to remove suspended solids of approximately one micron and larger from all types of liquid process streams.

Bag and cartridge filters can both remove suspended solids for applications with lower flow rates, where exposure to the process liquid is not a problem and lower volumes of solids must be removed.

Bag and cartridge filters are roughly equivalent in price, although self-cleaning/CIP systems tend to cost more initially. However, users should consider the total operating costs of the filtration system, not merely the initial purchase price.

Media replacement, disposal costs, labour costs, and down time should all be included when evaluating filtration systems.

Factors to consider

When selecting a filter for a particular application, the following criteria should be considered: How large is the process volume? What is the flow rate? Is it a continuous or batch process?

What are the material characteristics of the solids being removed? Such as:

• How large are the particles?
• Is the material hazardous?
• Can the material being removed be recycled back into the process stream at another point?

What are the waste disposal costs? Such as:

• How often do bags or cartridges need to be replaced?
• Can the waste volume be reduced or eliminated by switching to a different filtration method?

What are the labour and down time costs for filter or cartridge replacement? Such as:

• Can down time be reduced or eliminated by switching to a different filtration method?

If you do your homework prior to purchasing you will be able to choose the system that best fits your needs and budget.