What Is a Bag Filter and How Does It Work?

Bag filters are one of the most widely used filtration solutions in industrial and commercial water treatment -- valued for their ability to handle high flow rates, capture large sediment loads, and maintain consistent pressure without the rapid clogging that limits cartridge-based systems at scale. If you are evaluating filtration options for a high-volume application, understanding how bag filters work and where they fit relative to other filter types is the most practical starting point.

What Is a Bag Filter?

A bag filter is exactly what the name suggests -- a porous bag made from a filtering material, secured at the opening by a rigid ring that seats inside a dedicated filter housing. Liquid enters the housing, fills the bag, and passes through the bag's wall under pressure, leaving suspended particles and contaminants trapped inside. The filtered liquid exits through the housing outlet, and the retained particles remain in the bag until it is removed and replaced or cleaned.

The housing itself is typically constructed from stainless steel, carbon steel, or polypropylene, sized to accommodate one or multiple bags depending on the flow volume required. Standard bag sizes -- Size 1 (7" x 16") and Size 2 (7" x 32") -- fit the most common industrial housings, though other sizes are available for specialty applications.

How a Bag Filter Works

The filtration mechanism in a bag filter is surface filtration combined with depth filtration depending on the media type. As liquid flows in from the top or side of the housing and fills the bag, pressure builds across the bag wall. Particles above the filter's rated micron size are physically prevented from passing through the media pores and accumulate on the inside surface of the bag.

Unlike a cartridge filter where a clogged surface section blocks the entire cross-section, a bag filter has a large surface area relative to its rated capacity. When a section of the bag wall becomes partially loaded with captured particles, the remaining surface area continues to pass liquid freely. This is why bag filters maintain lower differential pressure and higher flow rates than equivalent cartridge systems when processing high sediment loads -- a partially loaded bag does not starve the system the way a partially clogged cartridge can.

When the pressure differential across the housing reaches a threshold indicating the bag is loaded, the housing is taken offline, the bag is removed and either discarded or manually cleaned, and a new bag is installed before the system is returned to service.

Bag Filters vs. Cartridge Filters

The choice between a bag filter and a cartridge filter hinges on flow rate, sediment load, and application type. The table below summarizes the key differences.

Bag filters are rarely the right choice for residential drinking water filtration. The residential applications where sediment removal is needed are better addressed by cartridge-based point-of-entry systems or Rusco spin-down pre-filters, which handle high sediment loads in residential scale without the industrial housing infrastructure bag filters require.

Bag Filter Media Types

The filter media determines what a bag filter captures, how it handles temperature and pH, and whether it can be cleaned and reused. The table below covers the most common materials.

Felt media provides depth filtration -- particles are captured throughout the thickness of the material, not just on the surface -- which extends capacity between bag changes under moderate sediment loads. Mesh media provides surface filtration with defined, consistent pore openings, which makes it more appropriate for applications requiring tight particle size control and allows physical cleaning to restore the media.

Key Selection Factors

The DFS bag filter overview page outlines the four questions that narrow the selection to the right bag filter for a given application:

What type of sediment or particles are being filtered? Particle type, shape, and density all affect media selection. Fine silts behave differently from paint pigments, and aqueous solutions behave differently from hydrocarbon solvents.

What is the pH of the liquid? Housing material compatibility is pH-dependent. Aluminum housings are appropriate for liquid in the 6.5 to 8.5 pH range. Outside that range, polypropylene or stainless steel housings are the appropriate choice. Media compatibility with pH should also be confirmed -- polypropylene felt handles a wider pH range than polyester in some aggressive chemical environments.

What is the required flow rate? Flow rate determines housing size and whether a single-bag or multi-bag housing is needed. Micron rating and flow rate are directly linked -- a finer micron rating increases pressure drop at any given flow rate. Balancing particle capture requirements against acceptable pressure drop is the core sizing calculation for any bag filter installation.

What are the temperature limits? Felt and polypropylene media have upper temperature limits that vary by material. High-temperature applications may require stainless steel mesh or specialty media designed for elevated operating temperatures.

Applications Where Bag Filters Are the Right Tool

Bag filters are the standard solution in industrial and commercial settings where high flow rates and high sediment loads make cartridge filtration impractical. Common applications include paint and coatings manufacturing, metalworking coolant filtration, food and beverage processing, chemical process fluid filtration, ink and resin production, and pool and water feature recirculation at commercial scale.

In water treatment specifically, bag filters are used as primary sediment removal stages upstream of more sensitive equipment such as membranes, ion exchange systems, and UV disinfection systems in municipal and industrial contexts where incoming water carries heavy suspended solids.

Finding Bag Filters at DFS

The DFS bag filtration products collection covers bag filter housings and replacement bags for a range of industrial applications. The replacement bag filters collection includes nylon, felt, polyester, and polypropylene media from Pentek, American Plumber, and other leading bag filter manufacturers, available in standard Size 1 and Size 2 configurations across micron ratings from less than 1 micron to several hundred microns. The bag filters overview at discountfilterstore.com/pages/bag-filters includes additional guidance on matching media type and housing material to specific liquid and application requirements.

For applications where a residential or light commercial sediment solution is needed rather than industrial bag filtration, the whole house filter systems collection and the sediment replacement whole house filters collection cover cartridge-based options across a range of micron ratings and housing sizes.

Questions about which bag filter configuration fits your application? Call the DFS team at 1-800-277-3458.

Frequently Asked Questions

What is a bag filter used for?

Bag filters are used in industrial and commercial applications where high flow rates and high sediment loads make cartridge filtration impractical. Common uses include paint and coatings manufacturing, metalworking coolant filtration, food and beverage processing, chemical process fluid filtration, and upstream sediment removal in larger water treatment systems. They are not typically recommended for residential drinking water filtration.

How does a bag filter work?

Liquid enters the bag filter housing, fills the bag under pressure, and passes through the bag wall. Particles above the filter's rated micron size are captured inside the bag and retained as the filtered liquid exits through the housing outlet. Because the bag has a large surface area, partial loading by captured particles does not sharply restrict flow the way cartridge filter clogging does.

What is the difference between a bag filter and a cartridge filter?

Bag filters handle higher flow rates and heavier sediment loads than cartridge filters, maintain lower pressure drop under load, and some media types are cleanable and reusable. Cartridge filters are better suited for residential and light commercial applications, offer finer micron ratings down to 0.5 microns or below, and are the standard choice for drinking water treatment. Each filter type has applications where it is clearly superior to the other.

What micron ratings are available in bag filters?

Bag filters are available from less than 1 micron to over 1,500 microns depending on the media type and application. Felt media is available in fine ratings down to 0.5 microns for high-clarity industrial applications. Nylon mesh bags are available in coarser ratings suited for particle classification and pre-filtration. The right micron rating is determined by the particle size you need to capture and the acceptable pressure drop at the required flow rate.

Can bag filters be cleaned and reused?

It depends on the media type. Nylon mesh and stainless steel mesh bag filters can be cleaned -- typically by backwashing, ultrasonic cleaning, or mechanical agitation -- and reused multiple times before replacement is needed. Felt media (polypropylene and polyester) is generally a single-use disposable material. The depth filtration mechanism of felt captures particles throughout the media thickness in a way that is difficult to reverse by cleaning.

How do I know when a bag filter needs to be replaced?

The primary indicator is pressure differential across the housing -- a differential pressure gauge installed on the inlet and outlet lines shows the pressure drop across the bag. As the bag loads with captured particles, pressure differential increases. Most applications establish a maximum acceptable differential pressure that triggers a bag change. Visually inspecting a removed bag also reveals loading level, though differential pressure monitoring is more objective and operationally reliable.

What housing material should I use with my bag filter?

Housing material selection depends primarily on the pH and chemical composition of the liquid being filtered and the operating temperature. Polypropylene housings are a versatile choice for aqueous solutions across a wide pH range and moderate temperatures. Stainless steel housings handle high temperatures, abrasive fluids, and aggressive chemicals. Aluminum housings are suitable for neutral-pH applications in the 6.5 to 8.5 range. Confirm compatibility between the housing material and the specific fluid chemistry before installation.