4.4 Slow Sand, Direct, Pressure & Cartridge Filtration

The Filtration Family

Granular media filtration preceded by coagulation, flocculation, and sedimentation — conventional filtration — is the workhorse and the subject of the preceding sections. The Class II criteria also name conventional filtration (e.g., slow and rapid sand, upflow) and direct filtration (e.g., pressure direct filtration, cartridge) as distinct things an operator monitors, evaluates, and adjusts. Each alternative trades capital cost, footprint, or complexity against tolerance for raw water variability.

---

Slow Sand Filtration

Slow sand filtration is the oldest engineered drinking water process still in wide use, and it works on an entirely different principle from rapid sand filtration. It is a biological process, not a chemical one.

The Schmutzdecke

Over days to weeks of operation, a biologically active layer develops on the sand surface. This layer — the schmutzdecke, German for "dirt cover" — is a mat of algae, bacteria, protozoa, and accumulated particulate matter. It performs most of the filter's pathogen removal through straining, predation, and biological oxidation. Mature slow sand filters achieve excellent removal of Giardia cysts and bacteria and meaningful removal of viruses and dissolved organic carbon, all without coagulant.

Operating Characteristics

• Loading rate: approximately 0.03 to 0.10 gpm/ft², roughly fifty times slower than rapid sand filtration. This is the defining number and the reason the footprint is large.
• Media: fine sand, typically finer than rapid sand, in a deep bed with a supporting gravel layer and an underdrain.
• Raw water requirement: consistently low turbidity, generally under about 10 NTU. High turbidity plugs the schmutzdecke and forces impractically frequent cleaning. Algal blooms are also problematic.
• Filter run length: measured in weeks to months — commonly two to six months — with longer runs on better raw water.
• No coagulant is required, which is a major operating advantage for small systems that cannot staff a chemistry-intensive plant.

Cleaning and Ripening

A slow sand filter is not backwashed. When headloss reaches its limit, the filter is drained and the top 1 to 2 inches of sand — the schmutzdecke and the fouled surface layer — is scraped off and removed. After several scrapings the bed thins to a minimum depth and must be resanded.

The critical operating consequence: scraping removes the biological layer that does the treating. The filter must then ripen — be returned to service and operated, often to waste, while the schmutzdecke re-establishes over a period of days. A freshly scraped filter placed straight into full service is a filter operating without its principal pathogen barrier, and this is exactly the kind of judgment a Class II exam tests.

---

Direct Filtration

Direct filtration applies coagulant and flocculates, then sends the flocculated water straight to the filters. There is no sedimentation basin. All the solids that would have settled out in a clarifier instead go onto the filter.

Raw Water Limits

Because the filter absorbs the entire solids load, direct filtration is viable only on consistently good raw water. Widely cited screening criteria are:

• Turbidity averaging under roughly 10 NTU, with short excursions tolerable but sustained high turbidity disqualifying. More permissive guidance extends to 25 NTU; more conservative practice holds nearer 10.
• True color under roughly 20 to 25 color units.
• Low algae counts, since algae both plug media and are hard to coagulate.
• Stable quality, which matters as much as the absolute numbers — a source that is usually 5 NTU but spikes to 300 NTU is a poor direct filtration candidate.

Operating Consequences

• Much shorter filter runs than conventional treatment, because the filter is doing the clarifier's work as well as its own.
• Lower coagulant doses are typical, since the goal is a small, filterable floc rather than a large, settleable one. Overdosing blinds the filter almost immediately.
• Very little margin for error. A conventional plant's clarifier buffers a coagulation upset; a direct filtration plant's filters see it within minutes.
• Lower capital and chemical cost and a smaller footprint — the reasons it gets built.
• Less sludge, and no clarifier sludge handling, though backwash solids increase.

In-line filtration removes the flocculation basin as well, injecting coagulant immediately upstream of the filter and relying on the media itself to provide contact. It is the least forgiving arrangement of all and is usually restricted to very high quality, very stable sources — and is frequently paired with pressure filters.

---

Pressure Filters

A pressure filter is granular media contained in a closed steel vessel, operated under the pressure of the upstream system rather than by gravity. Vessels may be vertical or horizontal.

Advantages

• No repumping. Water enters under pressure and leaves under pressure, so the plant avoids an intermediate pumping stage and its energy cost. This is the principal reason pressure filters are chosen.
• Compact, skid-mounted, factory-built, which suits small systems and industrial applications.
• Well suited to iron and manganese removal on groundwater, often the most common application.

The Central Drawback

The operator cannot see the media. In a gravity filter, an operator watches the bed during backwash and can directly observe media boiling, uneven wash, mudballs, cracks in the surface, and media loss over the wash troughs. Inside a pressure vessel none of that is visible. Consequences:

• Mudballs and media cementing develop undetected until they show up as chronic headloss or short runs.
• Media loss to the backwash is not observed.
• Cracking and short-circuiting along the vessel wall are not observed.
• Backwash adequacy must be inferred from flow rate and pressure rather than confirmed visually.

Because of this, pressure filters demand disciplined instrumentation — effluent turbidimeters, differential pressure monitoring, verified backwash flow — and periodic internal inspection with the vessel opened and drained.

Regulators are correspondingly cautious. Many states restrict pressure filters on surface water or limit the log-removal credit granted, precisely because the process cannot be observed and a sudden pressure surge can drive accumulated solids straight through the bed.

---

Cartridge and Bag Filtration

Cartridge filters and bag filters are disposable or replaceable media elements, typically rated from about 1 to 5 micrometers nominal or absolute. They are used almost exclusively by small systems with very high quality raw water, and as polishing or pretreatment steps within larger trains.

Characteristics:

• No coagulation, no backwash. When the element plugs, it is replaced. Operating simplicity is the entire appeal.
• Very low tolerance for turbidity. Elements plug rapidly above a few NTU, and replacement cost becomes prohibitive.
• Absolute-rated cartridges provide a defined pore size and can earn Cryptosporidium removal credit under the Long Term 2 Enhanced Surface Water Treatment Rule toolbox when they are challenge-tested and verified; nominal-rated cartridges do not provide that assurance.
• Integrity matters. A bypassed, torn, or improperly seated element passes unfiltered water with no visible indication, so housing seals and differential pressure must be checked.

---

Turbidity Performance and Regulatory Credit

The Surface Water Treatment Rules set turbidity limits by filtration technology. For conventional and direct filtration, combined filter effluent turbidity must be ≤ 0.3 NTU in at least 95 percent of measurements taken each month, and must never exceed 1 NTU. Slow sand filtration and diatomaceous earth filtration carry their own, less stringent limits, and alternative technologies including cartridge and membrane systems are approved by the state on a demonstrated-performance basis.

Two operating points apply across every technology in this section:

1. Continuous monitoring of individual filter effluent is the tool that catches a failing filter before it contaminates the combined effluent. On pressure filters and cartridge systems it is essentially the only tool.
2. Filter-to-waste or ripening after any return to service matters more, not less, on these alternative technologies — a scraped slow sand filter has lost its biological barrier, and a restarted direct filtration plant has no clarifier upstream to catch what the unripened filter passes.