4.3 Direct, Pressure, and Cartridge Filtration

Three labels that answer different questions

The WPI Class I outline lists direct filtration, with pressure direct and cartridge filtration as examples. These terms are easy to misread as three mutually exclusive treatment trains. They are better separated by the question each answers:

• Direct filtration asks which pretreatment steps precede the filter: coagulated or flocculated water goes to filtration without a sedimentation or flotation step.
• Pressure filtration asks how water is driven through media: a pump or upstream pressure moves it through an enclosed vessel rather than an open gravity box.
• Cartridge filtration asks what captures particles: water passes through a replaceable porous element.

A direct-filtration plant can therefore use an open gravity filter or an enclosed pressure filter, and a cartridge unit can appear in a direct or another approved treatment arrangement. Identify the process sequence, driving force, and media separately before diagnosing a problem.

Direct filtration places the solids burden on the filter

Conventional treatment removes floc in sedimentation before filtration. Direct filtration omits that separation step, so the filter receives the conditioned particles. Coagulation is essential in typical direct treatment; a distinct flocculation stage may be used, while an engineered in-line arrangement may provide little separate flocculation. Exact configuration matters. Do not describe raw, unconditioned filtration as direct filtration merely because no clarifier is visible.

Without a clarifier as a buffer, a change in raw turbidity, algae, color, temperature, pH, flow, coagulant delivery, or mixing can reach the filter quickly. Direct treatment is generally applied where source quality and approved design make that loading manageable. That does not create a universal source-turbidity cutoff for the WPI exam. The applicable authority, design basis, and plant procedure determine the operating envelope.

The operator trends raw and conditioned water, chemical feed, individual-filter effluent turbidity, head loss, flow, and run length. Shorter runs across all units after a source event point toward increased incoming solids or poorer conditioning. Deterioration in one unit with stable shared influent suggests a local filter or instrument problem. Bench results guide; full-scale effluent confirms.

Pressure filtration encloses the process

A pressure filter commonly contains granular media in a rated vessel. Pump pressure supplies the driving force, and differential pressure develops as the bed and hydraulic path resist flow. The vessel saves open hydraulic head and may fit compact systems, but pressure does not itself improve coagulation or guarantee particle removal. The operator cannot see the bed during service, so reliable inlet and outlet sampling, turbidimeters or other approved quality measures, flow, valve position, and pressure readings are critical.

Use differential pressure as a comparison between inlet and outlet pressure, not as a stand-alone water-quality result. A gradual increase with acceptable effluent commonly indicates loading. A rapid increase may reflect a high solids event, valve restriction, fouled media, or an incomplete previous cleaning. Poor effluent with unexpectedly low differential pressure may indicate ineffective conditioning, a damaged internal, media loss, channeling, bypass, or a faulty reading. Follow the manufacturer's and plant's approved terminal conditions; no single pressure loss applies to every vessel.

Pressure vessels store hazardous energy. Before opening a manway, housing, drain, or fitting, the unit must be removed from service, isolated, depressurized, drained, and controlled under the facility's lockout procedure. A gauge at zero is evidence to verify, not permission to ignore isolation.

Cartridge filtration uses replaceable elements

A cartridge filter directs water through one or more replaceable elements. Surface cartridges capture particles mainly near the upstream face; depth cartridges retain material through a thicker porous structure. Pleated, wound, spun, and other designs have different capacities and compatibility. A manufacturer's nominal retention rating and absolute rating are not interchangeable, and the exact meaning depends on a documented test method. Never invent a universal micron size or claim pathogen credit from a label alone.

Cartridges must match the approved material, size, rating, housing, seals, flow direction, and service. A torn element, missing or rolled gasket, loose end cap, wrong length, or poor housing closure can create bypass. Differential pressure may remain surprisingly low because water is not traveling through the intended media. On the other hand, rising differential pressure with stable effluent generally means the element is collecting solids; replace it at the approved condition rather than forcing excess flow or cleaning a disposable element not designed for reuse. Record the element identity, installation date, readings, water quality, and disposal.

Troubleshoot the combined system

Suppose a pressure direct-filtration skid develops steadily rising differential pressure while outlet turbidity remains stable. Confirm both pressure instruments and flow, review raw-water and chemical-feed trends, and follow the approved run-termination and cleaning sequence. The evidence indicates loading, not an automatic need to increase coagulant. After service, outlet turbidity suddenly rises while differential pressure stays very low. Protect finished water, verify the sample and analyzer, then inspect valve lineup, media or cartridge seating, seals, internals, and installation records under safe isolation.

The Class I pattern is to separate upstream conditioning, hydraulic resistance, and barrier quality. Never use pressure alone as proof of treatment. Verify the water-quality result, compare units, locate whether the change is common or local, remove an unsafe barrier from service under procedure, correct one supported cause, and confirm performance before return.