12.4 Cartridge & Bag Filtration (Subclass 5)

The Simplest Filtration Subclass, and Its Traps

Pennsylvania defines Subclassification 5 as a process for substantial particulate removal by straining with bag or cartridge filters manufactured of various materials and pore sizes. There is no media bed, no coagulation chemistry, no backwash and no cake. Water passes through a disposable element, particles larger than the pore rating are retained, and when the element loads up it is thrown away and replaced.

That simplicity is why many very small Pennsylvania systems use it, and it is also why the examination concentrates on the few things that can go badly wrong.

Absolute Versus Nominal Ratings

This is the most commonly missed distinction on the Subclass 5 examination.

• An absolute rating states that the element removes particles of a given size at a defined efficiency, verified by a standardized challenge test. Regulatory removal credit depends on absolute-rated elements with documented challenge testing.
• A nominal rating is the manufacturer's approximate description of the particle size at which some fraction of particles is retained. It is not verified and provides no basis for a pathogen removal credit.

For a system relying on cartridge filtration as a Cryptosporidium barrier, only the absolute rating, applied within the manufacturer's tested flow and pressure envelope, supports the credit. Operating an element above its rated flow can force retained particles through the structure, a phenomenon called unloading.

Change-Out Criteria

Three independent triggers govern element replacement, and whichever comes first controls:

1. Differential pressure. A clean housing typically shows 2 to 5 psi across the elements. As solids accumulate, the differential rises, and DEP's rule of thumb is that cartridge filters should be changed when the pressure drop reaches 15 psi, or at whatever lower limit the manufacturer sets. Running past that limit deforms the element; DEP is explicit that a filter deformed by differential pressure must be replaced, because it can no longer seal or filter properly.
2. Elapsed time or volume produced. Even a clean-looking element degrades and can support bacterial growth, so a maximum service interval is set regardless of pressure.
3. Water quality. Rising filtrate turbidity or any particle count excursion ends the run immediately.

Operators trend the differential pressure daily. A sudden drop in differential is not good news: it usually indicates a torn bag, a blown-out cartridge seal or bypass around the element, and it is a finished water quality emergency.

What Pennsylvania Requires of the Barrier

Note that this is the tighter 0.3 NTU standard, not the 1.0 NTU allowance given to diatomaceous earth and slow sand filtration.

Housings, Ratings and Daily Checks

DEP treats the housing as part of the barrier, not just a container. It delivers water to the element, collects the filtrate, holds the element in position to prevent short-circuiting around it, and maintains system pressure. Housings are built of plastic or stainless steel and must be equipped with inlet and outlet pressure gauges so the differential can actually be read. Cartridges themselves are rated by tensile strength, and the smaller the pore size, the higher the inlet pressure requirement — which is why dropping to a finer cartridge without checking available pressure is a common design mistake.

One counterintuitive rule: coagulants and precoat are not recommended ahead of bag filters. Removal depends on the absolute pore size of the element rather than on building a cake, so upstream chemical only adds pressure loss and forces more frequent, more expensive change-outs.

The daily routine is short and fixed. Verify that flow, differential pressure, raw turbidity, filtered turbidity and chlorine residual are all inside their established ranges, watch chemical usage for day-to-day swings that signal a feed problem, and flush the cartridge filter vessel drains daily.

Prefiltration and Loading

Because there is no backwash and no solids storage, the operating cost is directly proportional to the solids the elements must capture. Nearly every practical installation uses staged filtration:

• A coarse bag prefilter (for example 25 to 50 micron) removes bulk solids cheaply.
• A finer cartridge (for example 1 micron absolute) performs the pathogen barrier duty.
• Sacrificial coarse elements are changed frequently; expensive fine elements last far longer.

Sources with sustained turbidity above a few NTU, seasonal algae, or appreciable iron and manganese exhaust elements so quickly that the technology becomes uneconomic. Cartridge filtration is a clean-source technology.

Sanitary Change-Out and Return to Service

Every change-out opens the treated water side of the barrier to the room. The disciplined sequence is:

1. Isolate and depressurize the housing; note and record final differential pressure.
2. Drain the housing and remove spent elements, inspecting the sealing surfaces and O-rings for damage or gasket cuts.
3. Clean the housing interior and disinfect it, along with the new elements where the manufacturer permits, using an approved chlorine solution.
4. Install new elements, verifying correct seating, knife-edge seal contact and hold-down hardware. A misseated element allows unfiltered bypass with no visible symptom other than an abnormally low differential pressure.
5. Refill, vent trapped air, and flush to waste until turbidity and any residual disinfectant flush are acceptable before returning to service.
6. Record element type, absolute rating, lot number, install date and differential pressure so that performance can be trended.

Exam tactic: if a scenario describes a cartridge system with normal turbidity but an unexpectedly low differential pressure, the answer involves bypass around a failed or misseated element, not improved water quality.