Bag vs Cartridge Filtration: Which to Use and When

Bag filters and cartridge filters both remove particulate from liquid process streams — but they are not interchangeable. Choosing the wrong technology for your application raises operating costs, shortens filter life, and can push contaminants through that should have been captured. The decision comes down to five factors: flow rate, target particle size, dirt load, change-out frequency, and cost per unit of throughput. This guide gives you the technical basis to choose correctly the first time, with a side-by-side comparison table and application-specific guidance for Canadian industrial and environmental operations.

What Is the Difference Between Bag and Cartridge Filtration?

Both technologies hold a filter medium inside a pressure housing and force liquid through it under system pressure. The differences are in geometry, filtration mechanism, dirt-holding capacity, and achievable particle size removal.

Bag Filters: Volume and Dirt Load

A filter bag is a fabric sack — typically polypropylene felt, polyester felt, or nylon monofilament — that sits inside a bag filter housing. Liquid enters the housing, passes through the bag from inside to outside, and exits clean. Particulate accumulates on the inner surface and within the fabric depth.

The standard #2 filter bag (7″ × 32″) presents approximately 1.5 ft² of filtration area. The #1 bag (7″ × 16″) offers 0.8 ft². Multi-bag housings stack multiple bags in parallel to multiply effective area and flow capacity. A 4-bag housing running #2 bags handles 80–120 GPM at normal face velocities. Bag filters operate in the 1–200 micron range — suited for bulk particulate removal, not fine polishing.

Cartridge Filters: Precision and Fine Polishing

A cartridge filter is a cylinder — pleated membrane, wound string, melt-blown polypropylene, or activated carbon block — that provides a much larger surface area per unit volume than a bag. A standard 10″ × 2.5″ cartridge has 0.1–0.4 ft² of filter area; a 40″ high-flow cartridge can reach 6 ft² in the same housing footprint.

Cartridges operate from 0.2 to 100 micron nominal, and from 0.2 to 40 micron absolute. Absolute-rated cartridges (Beta ratio ≥ 1000 at the rated micron) guarantee that 99.9%+ of particles at or above the rated size are captured — a specification bag filters cannot match. Cartridges handle lower dirt loads per unit than bags; they are designed for high-purity final filtration, not bulk removal.

Depth vs. Surface Filtration

Felt filter bags are depth filters: particles penetrate the fabric and become entrapped throughout the thickness of the media, not just on the surface. This gives bags a higher dirt-holding capacity relative to their surface area, but produces a nominal (not absolute) efficiency rating — a 5-micron felt bag captures most particles above 5 microns, but not all.

Nylon monofilament (NMO) bags and pleated membrane cartridges are surface filters: particles are captured at the media surface, which produces a sharper, more precise cut-point and makes the medium washable and reusable. Absolute-rated pleated cartridges combine surface filtration with tight pore size distribution to deliver guaranteed removal efficiency.

When to Choose Bag Filtration

Bag filtration is the right choice when flow rate is high, dirt loading is moderate to heavy, and fine particle removal below 1 micron is not required. The economics are straightforward: a single #2 bag holds 200–500 grams of particulate at typical change-out pressure drops of 10–15 psi — far more than a comparably sized cartridge. At high dirt loads, cartridges plug rapidly and become prohibitively expensive to operate.

Applications Where Bags Win

• Groundwater treatment and pump-and-treat remediation — Iron, manganese, and sediment loads are often high enough to blind a cartridge within hours. A multi-bag housing running 10-micron polypropylene bags provides cost-effective bulk removal before secondary treatment.
• Industrial process water — Coolant systems, parts washing, spray rinse water, and metalworking fluids generate significant particulate. Bags handle the volume and dirt load that would make cartridge operation uneconomical.
• Chemical and paint manufacturing — High-flow transfer filtration, pigment removal, and batch processing benefit from the low capital cost and high throughput of bag systems.
• Food and beverage processing — Nylon monofilament bags offer washable reusability and FDA-compliant materials for product-contact applications where cartridges would generate too much waste.
• Mining and aggregate operations — Process water with high suspended solids (TSS > 100 mg/L) will blind cartridges immediately. Bag filters are the first-stage workhorse.

Rule of thumb: if you expect to change your filter more than twice per week under normal operating conditions, or if your flow rate exceeds 20 GPM, evaluate bag filtration first.

When to Choose Cartridge Filtration

Cartridge filtration is the right choice when you need fine particle removal (below 5 micron nominal or absolute-rated performance), low flow rates, or a compact system for high-purity applications. The absolute-retention capability of pleated membrane cartridges is the primary differentiator — no bag filter can guarantee particle removal to 0.2 micron.

Applications Where Cartridges Win

• Pharmaceutical and ultrapure water — Absolute-rated 0.2-micron membranes are required for bacterial and particulate control in water-for-injection (WFI), purified water systems, and process chemical make-up.
• Semiconductor and electronics manufacturing — DI water systems and chemical distribution require consistent sub-micron filtration with no fibre shedding — NMO bags cannot deliver this.
• Final polishing after bag pre-filtration — In two-stage systems, the cartridge sees only the fine particles that passed through the upstream bag, extending cartridge life to weeks or months rather than days.
• Low-flow laboratory and pilot systems — At flow rates below 5 GPM, bags are oversized. A small multi-cartridge housing is more appropriately scaled.
• Sterile and aseptic filtration — Sterilizing-grade 0.2-micron cartridges are the only option for process streams requiring a sterility assurance level (SAL).

Rule of thumb: if your target filtration rating is below 5 micron nominal, or if you need absolute-rated retention with a documented Beta ratio, use a cartridge. If your TSS or dirt load is high, pre-filter with bags first.

Side-by-Side Comparison: Bag Filter vs Cartridge Filter

Cost Considerations: Lifecycle, Not Unit Price

The upfront cost of a filter bag or cartridge is rarely the right metric. What matters is cost per cubic metre (or gallon) of liquid processed — which means accounting for change-out labour, disposal, and the throughput between change-outs.

Bag Filter Cost Economics

A Sampson PLATINUM #2 polypropylene felt bag handles roughly 10,000–80,000 litres at typical industrial contamination levels before reaching terminal differential pressure (15 psi). Labour for bag change-out is typically 5–10 minutes per housing. At CAD $10 per bag and 10,000 litres per bag, the media cost is $0.001/litre — before accounting for labour and disposal. For high-volume, high-dirt-load applications, this is far lower than any cartridge option.

Cartridge Filter Cost Economics

A 10″ × 2.5″ melt-blown cartridge at 5 micron in a moderate-quality water application handles roughly 500–5,000 litres before change-out. At CAD $5–$15 per cartridge, the media cost ranges from $0.001 to $0.030/litre — higher per unit of throughput than bags when dirt loads are comparable. Cartridges earn their cost premium when the application genuinely requires sub-5-micron performance or absolute retention, because no bag filter can substitute.

Two-Stage System Economics

The most cost-effective approach for applications requiring final filtration below 5 micron: run a bag filter first to remove the bulk contamination, then pass the pre-filtered liquid through a cartridge housing for final polishing. In practice, this arrangement extends cartridge life by 3–10× over cartridges used as the sole filtration stage. The capital cost of a pre-filter bag housing typically pays back within 3–6 months through reduced cartridge consumption.

Frequently Asked Questions

Can I replace a cartridge filter system with a bag filter to save money?

Only if your target micron rating is 1 micron or coarser and you do not need absolute-rated retention. Bag filters (felt) achieve nominal ratings down to approximately 1 micron; nylon monofilament bags achieve more precise surface filtration down to 25 micron. If your current cartridge system is running at 5 micron nominal or finer because your process genuinely requires it, switching to bags will degrade filtration quality. If the cartridge was oversized for the application — a common legacy situation — bags may perform adequately at lower cost. Contact ERE to review your application before switching.

What is the maximum flow rate for a bag filter housing?

A single #2 bag (7″ × 32″) handles 20–30 GPM at a face velocity that prevents bypass and maintains filtration efficiency. Multi-bag housings scale proportionally: a 4-bag housing handles 80–120 GPM, an 8-bag housing handles 160–240 GPM. Very large industrial applications use custom multi-bag manifolds rated to 1,000+ GPM. Cartridge housings can be paralleled similarly, but the pressure drop per cartridge is typically higher than per bag at equivalent flow rates, making bags more practical for high-volume applications.

Do bag filters and cartridge filters use the same housings?

No — bag filter housings and cartridge housings are purpose-built for their respective media. Bag housings use a basket or ring-seal configuration sized to standard bag dimensions (#1, #2, #3, #4). Cartridge housings use tie-rod or sump configurations with cartridge end-cap seals (DOE, SOE, or flat-cap). You cannot run a bag in a cartridge housing or vice versa. ERE supplies both; see bag filter housings and cartridge filter housings.

How do I know when to change a bag or cartridge filter?

The industry-standard trigger is differential pressure (ΔP) across the housing. Install pressure gauges on the inlet and outlet of any filter housing you plan to monitor. A bag filter is typically changed at 10–15 psi ΔP; cartridge housings at 15–25 psi ΔP depending on the housing and cartridge type. Time-based change-out (e.g., every 30 days) is a fallback for systems without pressure monitoring and frequently results in either premature changes (waste) or late changes (bypass when the filter media ruptures under excess pressure). Invest in pressure gauges — they pay for themselves quickly.

Related articles

• Bag Filter Housings: Sizing and Selection Guide
• Industrial Filter Bags: Complete Selection Guide for Canada
• Filter Bag Micron Ratings: How to Choose the Right Rating

Lire en français : Filtration par sac ou par cartouche : comment choisir