12.1 Tertiary Filtration, Membrane Bioreactors and Oregon Recycled Water Classes
Key Takeaways
Tertiary filters (sand, dual media, continuous-backwash upflow sand, cloth disks) polish secondary effluent solids and are often paired with metal-salt dosing for low phosphorus limits.
MBRs run submerged micro- or ultrafiltration membranes at high MLSS with air scour, relax and backwash cycles; fine screening ahead of them protects the membranes from hair and rags.
Oregon Class A recycled water must be filtered to an average of 2 NTU in 24 hours (no more than 5 NTU over 5 percent of the time, never above 10) and disinfected to a 7-day median of 2.2 total coliform per 100 mL.
Class B also meets 2.2/23 total coliform but needs no filtration; Class C meets a median of 23 and 240 in two consecutive samples; Class D meets E. coli limits of 126 (30-day log mean) and 406 per 100 mL.
Every recycled water program needs DEQ approval of a recycled water use plan and must observe the class's allowed uses, setbacks, signs and harvest restrictions.
Tertiary Filtration
Secondary clarifier effluent usually carries 5 to 20 mg/L of suspended solids. Tertiary filters remove much of that, along with the phosphorus and BOD bound to those particles.
| Filter | How it works | Operating notes |
|---|---|---|
| Granular (sand or dual media) gravity filters | Depth filtration through graded media; periodic backwash | Watch headloss, effluent turbidity and backwash frequency; solids spikes from the clarifier shorten runs |
| Continuous-backwash upflow sand | Influent rises through sand while an airlift continuously cleans and returns sand | No shutdowns for backwash; check airlift operation and reject flow |
| Cloth disk (pile cloth) filters | Water passes outside-in through cloth on rotating disks; vacuum shoes backwash sections | Small footprint; watch cloth fouling, grease and solids carryover |
| Membrane filtration | MF or UF of secondary effluent | Highest solids removal; foundation for advanced reuse |
Chemical phosphorus polishing: alum or ferric salts are dosed ahead of the filter, sometimes with a small flocculation zone, to precipitate soluble phosphorus. Dose by molar ratio and jar test, monitor alkalinity and pH, and expect more filter solids. DEQ's Table A rates activated sludge with tertiary filtration, including chemical phosphorus removal, as Class III up to 5 MGD.
Membrane Bioreactors (MBR)
An MBR replaces the secondary clarifier with membranes, usually submerged hollow fibers or flat sheets with micro- or ultrafiltration pores, drawing permeate under a slight vacuum.
- High MLSS, often about 8,000 to 12,000 mg/L, in a small footprint.
- Fine screening (typically 2 mm or finer) ahead of the MBR is essential. Hair and rags braid around fibers and destroy membranes.
- Air scour continuously shakes solids off the membrane surface. Permeation cycles include short relax periods and backpulses.
- Maintenance cleans use dilute sodium hypochlorite for organics and citric or other acids for scale. Periodic recovery cleans restore permeability.
- Track transmembrane pressure (TMP) and permeability (flux per unit TMP). Falling permeability calls for cleaning or reduced flux.
- Effluent turbidity is usually well under 1 NTU, an excellent base for Class A recycled water.
Oregon Recycled Water: OAR 340-055
Treated effluent that is suitable for a direct beneficial purpose is recycled water. DEQ permits its use through NPDES or WPCF permits and a DEQ-approved recycled water use plan. The producer must make sure users follow the rules, and users must use it only for the purposes allowed for its class.
| Class | Treatment | Bacteria criteria | Minimum monitoring | Example uses (each class also allows the uses of the classes below it) |
|---|---|---|---|---|
| Nondisinfected | Oxidized | None | Per permit | Irrigation of fodder, fiber, seed crops not for human ingestion, commercial timber |
| Class D | Oxidized and disinfected | E. coli 30-day log mean ≤ 126 per 100 mL; no sample over 406 | Weekly | Firewood, ornamental nursery stock, Christmas trees, sod, pasture for animals |
| Class C | Oxidized and disinfected | Total coliform 7-day median ≤ 23 per 100 mL; ≤ 240 in any two consecutive samples | Weekly | Processed food crops; orchards and vineyards with direct-to-soil irrigation; golf courses, cemeteries, highway medians, campuses; listed industrial and construction uses; landscape impoundments |
| Class B | Oxidized and disinfected | Total coliform 7-day median ≤ 2.2 per 100 mL; no sample over 23 | 3 times per week | Stand-alone fire suppression; non-residential toilet and urinal flushing; floor drain trap priming; restricted recreational impoundments |
| Class A | Oxidized, filtered and disinfected | Before disinfection: turbidity average ≤ 2 NTU in 24 hours, ≤ 5 NTU more than 5% of the time, never over 10 NTU. After: total coliform 7-day median ≤ 2.2, no sample over 23 | Total coliform daily; turbidity hourly | Any agricultural or horticultural irrigation; parks, playgrounds, school yards, residential landscapes; car washes and fountains; nonrestricted recreational impoundments; groundwater recharge under OAR 340-044 |
Setbacks and site management examples
| Requirement | Nondisinfected | Class D | Class C | Class B |
|---|---|---|---|---|
| To a drinking water supply source | 150 ft | 100 ft | 100 ft | 50 ft |
| Sprinkler irrigation to property line | Sprinklers prohibited without DEQ approval | 100 ft | 70 ft | 10 ft |
| Spray near food service or drinking fountains | n/a | 70 ft | 70 ft | 10 ft |
- Nondisinfected water: no public access, and no irrigation for 30 days before harvest.
- Class D: post signs around the irrigation site, keep dairy animals from direct contact, and stop irrigating fodder, sod, nursery stock and similar crops 3 days before harvest.
- Class C and Class B: post signs at golf courses, cemeteries and impoundments, irrigate processed food crops no closer than 3 days before harvest, and keep edible orchard and vineyard crops off the ground.
- Class A: no spray onto areas where food is prepared or served, or onto drinking fountains.
Recycled water pipes and appurtenances are color-coded purple and must never be cross-connected to potable water.
An Oregon plant wants to sell recycled water for irrigating a public school playground. What class is required, and what treatment step distinguishes it from Class B?
Nondisinfected water with a 150-foot setback
Class A, which adds filtration before disinfection
Class C, which requires a weekly E. coli test
Class D, which requires warning signs only
Which numeric criteria apply to Oregon Class C recycled water?
Total coliform 7-day median of 2.2 per 100 mL and no sample over 23
E. coli 30-day log mean of 126 per 100 mL and no sample over 406
Total coliform 7-day median of 23 per 100 mL; no 2 straight samples over 240
No bacteria criteria, only a turbidity limit of 2 NTU as a daily average
An MBR plant's membrane permeability has dropped quickly since a period of heavy rag loading at the headworks. What should the operator check first?
The anaerobic digester temperature and mixing
The chlorine contact basin and its residual analyzer
The fine screens ahead of the membrane tanks
The UV lamp intensity readings and sleeve cleaning
Sections you finish are checked off in the contents.