Question 1

When should I use a bag filter instead of a cartridge filter?

Accepted Answer

The primary decision factors are flow rate and suspended solids loading. Bag filters are preferred when the suspended solids concentration in the feed is high — typically above 50–200 mg/L — because a single No. 2 filter bag can hold several hundred grams of solids before the pressure drop triggers a change, whereas a cartridge element becomes blinded much sooner and at significantly higher cost per change. For flow rates above 5–10 m³/hr, bag filters are also more economical: a single-bag housing handles up to around 30 m³/hr, and multi-bag manifolds scale easily to 200 m³/hr or beyond without the large number of cartridge elements that would be required. Cartridge filters are preferred when the target micron rating is below 1 µm, when a very high effluent quality (low NTU) is required, or when solids loading is very low and infrequent element changes are acceptable at that loading. For bulk suspended solids removal ahead of RO membranes or process equipment, a bag filter is typically the lower-cost first stage.

Question 2

How do I select the correct micron rating for a bag filter protecting a reverse osmosis system?

Accepted Answer

RO membranes require a feed SDI (Silt Density Index, 15-minute test) below 5, and ideally below 3 for extended membrane life. A bag filter alone does not reduce SDI to these levels — it removes bulk suspended particles but not the fine colloids that drive SDI. The correct approach is to use a bag filter as a pre-filter to a multimedia filter or cartridge filter, and then rely on those downstream stages for SDI reduction. For the bag filter stage protecting pre-RO equipment, a 25–50 µm rating is common to remove sand, rust flakes, and large suspended particles that would blind the downstream filter rapidly. If the upstream source is relatively clean settled or tank water, a 10–25 µm bag filter ahead of a 5 µm cartridge filter is a practical pre-RO protection train. Never size the bag filter finer than necessary for its role: the coarser the bag, the longer its service life between changes and the lower the total operating cost.

Question 3

How often do bag filter elements need to be replaced?

Accepted Answer

Replacement frequency depends entirely on the suspended solids loading in the feed water — there is no fixed calendar interval. The correct trigger is differential pressure: when the pressure drop across the bag housing reaches approximately 0.8–1.0 bar (or the manufacturer's recommended limit), the bag should be changed regardless of how much time has elapsed. In lightly loaded applications such as tank water or RO permeate polishing, a bag may last several weeks to months. In heavily loaded applications such as post-DAF effluent or cooling tower blowdown, bags may need daily or even shift-based replacement. Installing a differential pressure gauge or transmitter on the housing is the most reliable way to determine change intervals. Running a bag beyond its rated differential pressure risks bag bypass — particles push through or around the bag — which defeats the purpose of filtration. For high-solids-loading duties, consider upgrading to a duplex manifold so one housing can be changed while the other remains in service.

Question 4

What is the difference between felt and monofilament bag filter media?

Accepted Answer

Felt bag filter media (polyester felt or polypropylene felt) is a random-fibre structure that provides depth filtration — particles are trapped throughout the thickness of the felt rather than only on the surface. Felt bags have a high solids-holding capacity and are well suited to applications with significant suspended solids loads, such as pre-RO, cooling water, and process water protection. The random fibre structure means absolute micron ratings are approximate; felt bags are rated nominally (e.g., a 25 µm felt bag passes some particles smaller and larger than 25 µm). Monofilament bags are woven from a single continuous polymer filament in a precise weave pattern, giving them an absolute or near-absolute micron rating — the opening size is consistent and controlled. They are used when tighter, more consistent particle removal is required, or when the filtered particles need to be recovered (e.g., in paint or resin filtration where the solids have value). Monofilament bags are also easier to clean and can sometimes be washed and re-used, whereas felt bags are typically single-use disposables. Polyester felt is the most common choice for industrial water and effluent filtration; PP felt is preferred for acidic or alkaline chemical service.

Question 5

How do I size a multi-bag filter manifold for a high flow rate application?

Accepted Answer

Sizing a multi-bag manifold starts with the flow capacity of a single No. 2 bag (7" × 32"): at a differential pressure of 1 bar, a No. 2 bag handles approximately 25–30 m³/hr of water. For other fluids or finer micron ratings, reduce this figure proportionally. Divide the total required flow rate by the per-bag capacity to determine the minimum number of bags needed: for example, a 100 m³/hr requirement calls for at least 4 No. 2 bags (4 × 25 m³/hr). Select the next standard manifold configuration above this minimum — 2, 4, 8, or 12 bags are typical off-the-shelf options; custom configurations can be fabricated. For continuous-operation applications where downtime for bag changes is unacceptable, specify a duplex arrangement: two identical manifolds (or two halves of a manifold) in duty/standby configuration, with isolation valves allowing one side to be changed while the other carries full flow. Consider whether SS 304, SS 316, or polypropylene housing construction is appropriate for the fluid chemistry and temperature. SS 316 is preferred for saline, acidic, or pharmaceutical applications; PP is cost-effective for dilute acids and alkalis up to 60°C.

Standard bag sizes	No. 1: 7" × 16"; No. 2: 7" × 32" (No. 2 most common for industrial duty)
Micron ratings available	1, 5, 10, 25, 50, 100, 200 µm
Filter media	Polyester felt, polypropylene felt, nylon monofilament
Housing materials	Stainless steel 304/316, polypropylene, HDPE
Design pressure	Up to 10 bar
Max operating temperature	SS housings: 120°C; PP housings: 60°C
Flow per No. 2 bag — water	Up to ~30 m³/hr at 1 bar differential pressure
Multi-bag configurations	2, 4, 8, 12 bags in parallel; duplex (duty/standby) available

Bag Filter System

About Bag Filter System

Technical Specifications

How a Bag Filter System Works

Inlet Flow Distribution

Inside-Out Depth Filtration

Particle Retention on Bag Surface

Differential Pressure Monitoring

Bag Replacement (No Backwash)

Key Advantages

Industries & Use Cases

Supply & Support

Related Equipment & Solutions

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