2.9 Direct, In-Line, Pressure, Cartridge & Diatomaceous Earth Filtration

Filtration alternatives to the conventional train

Conventional treatment — coagulation, flocculation, sedimentation, filtration — is the general-purpose answer and is covered elsewhere in this guide. But Colorado has hundreds of small systems drawing from clear mountain streams, springs, and reservoirs where a full conventional plant is neither necessary nor affordable. Regulation 100's classification table lists these alternatives separately for exactly that reason, and the WPI outline expects an operator to know which one fits a source.

The family tree

                 Coagulant fed
                       |
     +-----------------+------------------+------------------+
     |                 |                  |                  |
 CONVENTIONAL       DIRECT             IN-LINE           NO COAGULANT
 rapid mix          rapid mix          rapid mix         (slow sand, DE,
 flocculation       flocculation       (no floc)          bag/cartridge,
 sedimentation      (NO sed.)          (no sed.)          membranes)
 filtration         filtration         filtration

Each step to the right removes a unit process and therefore removes a buffer. The raw water must be correspondingly better.

Direct filtration

Direct filtration feeds coagulant, mixes, flocculates, and sends the water straight to the filters with no sedimentation basin. All the solids removal happens in the media.

Suitability limits generally cited:

Operating differences that matter:

• Coagulant doses are lower than conventional practice, because the goal is a small, filterable pinpoint floc rather than a large settleable one. Overdosing a direct filtration plant blinds the filter surface and collapses run length.
• Filter run lengths are shorter and backwash frequency higher, because everything the coagulant captures ends up in the bed.
• Filter aid polymer, fed just ahead of the filters at fractions of a milligram per liter, strengthens floc so it penetrates the bed rather than mudballing at the surface.
• There is no settled-water turbidity to watch, so the operator's only warning of a coagulation problem is the filter effluent itself. Continuous filter effluent turbidity monitoring and streaming current or jar testing become the primary controls.

In-line filtration removes flocculation as well; coagulant is injected in the pipe just ahead of the filters and floc forms in the media. It requires the cleanest and steadiest raw water of any coagulant-based process and leaves the operator no reaction time at all.

Pressure filters

A pressure filter is the same granular media bed, but installed inside a closed cylindrical steel vessel — vertical or horizontal — operated under system pressure. Because water enters and leaves under pressure, no repumping is needed after filtration, which is why pressure filters are so common on small groundwater iron and manganese systems and on pool and industrial applications.

Advantages: compact, skid mounted, low capital cost, no intermediate pumping.

Disadvantages, and every one of them is an exam point:

• You cannot see the media. There is no visual inspection of the bed surface, no watching the backwash, no checking for mudballs or media loss without opening the vessel.
• Air binding and short-circuiting are hard to detect except through headloss trends.
• Backwash is limited by vessel geometry and available flow, and inadequate backwash leads to cemented media.
• Higher differential pressure can be forced across a fouled bed, which pushes floc and turbidity through rather than triggering a backwash the way a gravity filter's declining rate would.

For that last reason, pressure filters used for Surface Water Treatment Rule compliance are viewed cautiously by regulators, and continuous effluent turbidity monitoring plus a hard differential pressure setpoint substitute for the operator's eyes.

Diatomaceous earth filtration

Diatomaceous earth (DE) filtration uses the fossilized skeletons of diatoms as a filter medium built up as a precoat cake on a septum — a rigid porous element of wire cloth, porous stone, or synthetic fabric.

The operating cycle:

1. Precoat. A slurry of DE is recirculated until roughly 0.1 to 0.2 pounds per square foot of septum area has deposited as a uniform cake, typically 1/8 inch thick.
2. Filtration with body feed. DE is continuously metered into the influent so fresh medium deposits along with the captured solids. Without body feed the cake blinds almost immediately. Body feed rate is proportional to raw water solids.
3. Backwash and recoat. When terminal headloss is reached, flow stops, the cake sloughs off, and the cycle restarts with a fresh precoat.

DE filtration removes Giardia cysts very effectively and is credited under the Surface Water Treatment Rules, but it is limited to low-turbidity raw water, generally below about 10 NTU, with low algae and low color, and it is essentially a batch process requiring careful operator attention. Regulation 100 classifies slow sand and diatomaceous earth plants as Class C below 350 gpm.

Safety: DE is a crystalline silica dust hazard. Handling requires respiratory protection, dust control, and adherence to the silica standard.

Bag and cartridge filtration

Cartridge filters are disposable pleated or wound elements in a housing; bag filters are fabric bags in a basket. Both are widely used on small systems and as pretreatment ahead of membranes and UV.

Key operating principles:

• Absolute versus nominal rating. A nominal 5 micron cartridge removes "most" particles at that size; an absolute rating is a verified removal efficiency and is what regulators require for pathogen credit.
• For Surface Water Treatment Rule compliance, bag and cartridge filters must be challenge tested by the manufacturer to establish a Cryptosporidium log removal credit under LT2, and the system must operate them within the tested conditions. Regulation 100 classifies compliance bag or cartridge filtration as Class C or B depending on flow, while non-SWTR use is treated as sediment pre-filtration at Class D or C.
• Change-out is triggered by differential pressure, not by a calendar. A typical terminal differential is 15 to 25 psid, with the manufacturer's maximum being the hard limit — exceeding it can rupture the element and dump captured solids downstream.
• Elements are not cleaned and reused. Backwashing or washing a cartridge destroys the pore structure and voids any removal credit.
• Pretreatment matters. Cartridges plug quickly on water with appreciable turbidity or iron, which is why bags are often installed upstream as a roughing stage.
• Sanitary handling. Change-outs open the treated water side of the barrier. Disinfect the housing, wear clean gloves, and follow with a chlorine flush and, where required, bacteriological clearance.

Choosing between them

The unifying principle is simple: the fewer barriers upstream of the filter, the better and steadier the raw water must be, and the more the operator must rely on continuous instrumentation rather than observation.