5.3 Slow Sand, Diatomaceous Earth, Cartridge and Membrane Filtration
Key Takeaways
OHA recognizes four standard filtration methods: conventional, direct, slow sand and diatomaceous earth; other technologies need demonstrated equal performance and an approved pilot study.
Slow sand filters run at about 0.03 to 0.10 gpm/sq ft without coagulant; the biological schmutzdecke does most of the work and is restored by scraping and a ripening period.
Diatomaceous earth filters need a precoat and continuous body feed added in proportion to influent flow; failing to add body feed is a sanitary survey significant deficiency.
Membrane flux is permeate flow divided by membrane area, recovery is permeate divided by feed, and rising transmembrane pressure at constant flux signals fouling.
Membrane systems receiving pathogen credit must run direct integrity tests such as pressure-decay tests at least daily, plus continuous indirect monitoring such as filtrate turbidity.
Why Alternative Filtration Matters in Oregon
Many small Oregon systems draw from clear Cascade or Coast Range streams where low turbidity favors simple filters. Salem's Geren Island facility on the North Santiam is a well-known example of large-scale slow sand filtration. OAR 333-061-0050(4)(c) names four standard filtration methods: conventional, direct, slow sand and diatomaceous earth. Other technologies, including cartridge, bag and membrane filters, are acceptable only if their removal of target organisms is shown to be equal or better. Every method other than conventional filtration requires an approved pilot study before full-scale use.
Performance Standards at a Glance
| Filter type | Typical rate | Turbidity standard | Federal Giardia / virus removal credit |
|---|---|---|---|
| Conventional | 2-6 gpm/sq ft | ≤ 0.3 NTU in 95% of 4-hour CFE readings; never > 1 NTU | 2.5-log / 2.0-log |
| Direct | 2-6 gpm/sq ft | Same as conventional | 2.0-log / 1.0-log |
| Slow sand | about 0.03-0.10 gpm/sq ft | ≤ 1 NTU in 95% of readings; never > 5 NTU | 2.0-log / 2.0-log |
| Diatomaceous earth | about 1 gpm/sq ft | ≤ 1 NTU in 95% of readings; never > 5 NTU | 2.0-log / 1.0-log |
| Cartridge, bag, membrane | product-specific | State-set limits based on demonstration | Based on challenge and pilot testing |
Disinfection must provide whatever inactivation the filtration credit does not, for a total of 3-log Giardia and 4-log virus control.
Slow Sand Filtration
A slow sand filter is a box of fine sand, often 2 to 4 feet deep, with an underdrain and no coagulant. Water passes so slowly (roughly 0.03-0.10 gpm/sq ft) that a sticky biological layer, the schmutzdecke, forms on top. It removes particles and pathogens by straining, adsorption and biological predation.
- Best fit: low-turbidity, low-color water (generally under about 10 NTU). Algae and high turbidity clog the surface quickly.
- Cleaning: when headloss reaches its limit, the filter is drained and the top inch or so of sand is scraped off, or the sand is harrowed in place. Sand is replaced (re-sanded) after many scrapings.
- Ripening: a freshly scraped filter performs poorly until the schmutzdecke regrows, often a day or more. Operators filter to waste during ripening and watch turbidity closely.
- Limits: slow sand cannot remove dissolved contaminants. That is why Salem's 2018 cyanotoxin event required added treatment for dissolved toxins.
Diatomaceous Earth (DE) Filtration
DE filters strain water through a cake of fossilized diatom skeletons supported on a septum.
- Precoat: a thin layer of DE is deposited on the septum before filtering, commonly around 0.1-0.2 lb per sq ft.
- Body feed: DE is added continuously with the influent flow so the cake stays porous instead of blinding.
- Run end: when headloss reaches its limit, the cake is sluiced off and a new precoat is applied.
Any interruption of flow can cause the cake to fall off the septum, so DE filters need careful start-up and shutdown. OAR 333-061-0076 lists "body feed not added with influent flow" as a significant deficiency.
Cartridge and Bag Filters
Pressure housings hold disposable cartridges or bags rated in microns. They suit small groundwater-under-influence or spring sources with low turbidity. Elements must be replaced when the pressure differential reaches the manufacturer's limit. Not doing so is another listed significant deficiency, because overloaded elements can rupture or bypass.
Membrane Filtration
| Membrane | Approximate pore size | Removes |
|---|---|---|
| Microfiltration (MF) | about 0.1 µm | Particles, protozoa, most bacteria |
| Ultrafiltration (UF) | about 0.01 µm | Adds some viruses and large organic molecules |
| Nanofiltration (NF) | molecular | Hardness, color, many organics |
| Reverse osmosis (RO) | molecular | Dissolved salts, most dissolved contaminants |
Operating terms
- Flux: permeate flow per unit membrane area, usually in gallons per square foot per day (gfd).
- Recovery: permeate flow ÷ feed flow × 100. RO recovery is limited by scaling, so the remainder leaves as concentrate (reject) that needs disposal.
- Transmembrane pressure (TMP): the pressure driving water through the membrane. At constant flux, a rising TMP means fouling: particulate, organic, biological or mineral scale.
- Cleaning: routine backwash or air scour, chemically enhanced backwashes, and periodic clean-in-place (CIP). CIP typically uses chlorine or caustic for organic and biological foulants and acid for mineral scale.
Worked example
A UF plant produces 1.2 MGD of permeate from 1.30 MGD of feed using 48,000 sq ft of membrane.
- Flux = 1,200,000 gpd ÷ 48,000 sq ft = 25 gfd
- Recovery = 1.2 ÷ 1.3 × 100 = 92.3 percent
Integrity testing
A membrane only removes pathogens if fibers and seals are intact. Systems receiving pathogen credit must:
- run a direct integrity test, typically a pressure-decay test, at least once each day of operation; and
- monitor indirectly and continuously, usually by filtrate turbidity.
Repairs such as pinning broken fibers follow any failed test. Failure to perform direct integrity testing is a listed significant deficiency in OAR 333-061-0076. RO plants also need pretreatment (cartridge filters, antiscalant, sometimes acid) and post-treatment, because permeate is aggressive and must be stabilized before distribution.
A slow sand filter has just been scraped, and its effluent turbidity is higher than normal. What is the correct operating response?
Add alum ahead of the filter so the fine particles coagulate and settle
Filter to waste while the schmutzdecke re-forms, then return it to service
Backwash the filter with air scour until the effluent turbidity drops
Raise the filtration rate to 3 gpm/sq ft to compact and tighten the sand bed
During a sanitary survey, the OHA inspector finds that a diatomaceous earth plant applies precoat but adds no body feed during the run. How is this classified?
A significant deficiency, because body feed must be added with flow
A violation of the cyanotoxin rules that requires a health advisory
Acceptable practice, because precoat alone provides complete removal
A minor recordkeeping issue that needs no corrective action at all
A microfiltration skid's transmembrane pressure has climbed steadily over two weeks while flux has stayed constant. What does this most likely indicate?
Normal break-in, because TMP always rises in brand-new membranes
Membrane fouling that calls for a chemically enhanced backwash or CIP
Excessive chlorine residual in the permeate leaving the membrane skid
A broken fiber that is letting unfiltered water bypass the membrane
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