9.6 Tertiary Filtration, Reclaimed Water & Reuse Under 15A NCAC 02U
Key Takeaways
- Tertiary filtration — deep bed sand, continuous backwash upflow, cloth or disc media, and membrane bioreactors — polishes secondary effluent to the low TSS and turbidity values that stringent permits and reuse rules require.
- Membrane bioreactors combine biological treatment with membrane separation, eliminating the secondary clarifier and allowing very high MLSS, but they require fine screening upstream and disciplined cleaning and integrity practice.
- North Carolina's reclaimed water rules are 15A NCAC 02U, which establishes conjunctive and non-conjunctive reuse, treatment and reliability requirements, continuous online monitoring, storage, setbacks, and permitted end uses.
- Reclaimed water distribution must be physically separated from potable systems, identified with purple piping and signage, and protected by cross-connection control — an illegal connection between the two is a high-health hazard.
- Filter performance is controlled by hydraulic loading rate, solids loading, backwash or cleaning frequency, and upstream process stability; a tertiary filter cannot compensate for a failing secondary process.
9.6 Tertiary Filtration, Reclaimed Water & Reuse Under 15A NCAC 02U
The Grade III/IV needs-to-know lists Reclaimed Water / Reuse as a required topic, and Grade 2 covers Advanced Treatment. Both point at the same operational reality: when a permit requires 5 mg/L TSS or a facility sells effluent for irrigation, secondary treatment alone will not get there.
1. Tertiary filtration technologies
| Technology | How it works | Typical effluent TSS | Operator focus |
|---|---|---|---|
| Deep bed granular (sand/anthracite) | Gravity or pressure filtration of secondary effluent | < 5–10 mg/L | Head loss, backwash volume and rate, media loss |
| Continuous backwash upflow sand | Sand is airlifted, washed, and returned while filtering continues | < 5–10 mg/L | Airlift function, reject flow, sand inventory |
| Cloth / disc media | Pile cloth on rotating discs or drums; backwashed by suction | < 5 mg/L | Cloth condition, backwash suction, solids loading |
| Membrane bioreactor (MBR) | Micro/ultrafiltration membranes immersed in or external to the bioreactor | < 1–2 mg/L, near-zero turbidity | Fine screening, permeability trends, cleaning, integrity |
| Chemical addition + filtration | Metal salt and polymer ahead of the filter for phosphorus | TP < 0.1–0.5 mg/L | Chemical dose control, filter loading from chemical floc |
Loading and control. Tertiary filters are rated by hydraulic loading (gpm/ft²) and solids loading (lb TSS/ft²/day). When secondary clarifier performance slips, the filter sees a solids load it was never designed for, backwash frequency climbs, and effluent quality follows the clarifier down. The filter is a polisher, not a rescue device.
2. Membrane bioreactors
An MBR replaces the secondary clarifier with membranes, so solids separation no longer depends on settleability. That allows MLSS of 8,000–12,000 mg/L and a much smaller footprint, and it produces effluent essentially free of suspended solids.
What MBR operation demands:
- Fine screening upstream — typically 1 to 3 mm — because hair and fibers braid onto membrane fibers and shear them.
- Permeability trending (flux normalized for temperature and transmembrane pressure). Falling permeability means fouling.
- Air scour and relaxation/backpulse cycles to keep the membrane surface clear.
- Maintenance and recovery cleans with hypochlorite (organics and biofouling) and citric or oxalic acid (inorganic scale), on a schedule rather than after a crisis.
- Integrity monitoring — a broken fiber shows up as turbidity in the permeate.
- Foaming and viscosity control: high MLSS means high viscosity, which reduces oxygen transfer efficiency; aeration energy per pound of BOD is higher than in conventional plants.
3. Reclaimed water in North Carolina — 15A NCAC 02U
North Carolina regulates the reuse of treated effluent under Subchapter 02U, Reclaimed Water. The framework distinguishes:
- Conjunctive reuse — reclaimed water used within a system that also has another permitted disposal option.
- Non-conjunctive reuse — reuse is the sole disposal route, which raises the reliability requirements substantially.
Core requirements an operator must know:
- Treatment and reliability. Reclaimed water systems must meet defined effluent standards with redundancy in critical unit processes, standby power, and automatic diversion of off-specification water to storage or an alternative disposal point.
- Continuous online monitoring of key parameters — typically turbidity and disinfectant residual — with automatic diversion when limits are exceeded.
- Storage sized for periods when irrigation or other uses cannot accept water (wet weather, winter).
- Setbacks from wells, property lines, surface waters, and habitable structures depending on the reuse type and application method.
- Permitted uses ranging from landscape and agricultural irrigation to industrial process water, cooling, dust control, and toilet flushing, each with its own conditions.
- Operator certification. Reclaimed water and surface irrigation systems are classified under 15A NCAC 08G, and the ORC must hold the certification matching the system type — for surface irrigation, the Surface Irrigation certification with weekly visitation under 08G .0204(2)(c).
4. Keeping reclaimed water out of the potable system
This is the single largest public health risk in reuse, and the controls are prescriptive:
- Physical separation between reclaimed and potable piping, with specified horizontal and vertical separation, as with water and sewer lines.
- Purple pipe, purple markings, purple valve boxes and hydrant caps, and signage at every point of use stating the water is non-potable.
- No cross-connections. Any potable makeup to a reclaimed water system is protected by an air gap or reduced pressure principle assembly, and the entire site is treated as a high health hazard for cross-connection purposes under 18C .0406.
- Public education and site controls, including restricting hose bibs and quick-connect fittings that could be misused.
5. Fitting tertiary treatment into the plant
- Order of operations. Filtration normally precedes disinfection, because removing solids removes the particles that shield organisms from chlorine or UV and raises UV transmittance.
- Filter backwash return. Backwash water carries concentrated solids back to the head of the plant; equalize it rather than slug loading the process.
- Chemical phosphorus polish. Metal salt added ahead of a tertiary filter can drive TP below 0.1 mg/L, but it increases sludge production and consumes alkalinity — watch nitrification if the plant has an ammonia limit.
- Instrumentation. Continuous turbidity on the filter effluent is both a process control tool and, for reuse, a compliance instrument.
[!WARNING] Do not chase a filter problem upstream of the real cause. Rising backwash frequency, rising head loss, and rising effluent turbidity on a tertiary filter almost always begin as a secondary process problem — bulking sludge, a high clarifier blanket, or solids washout. Diagnose the biology before rebuilding the filter.
What distinguishes a membrane bioreactor from a conventional activated sludge plant?
Under North Carolina's reclaimed water rules, which requirement protects the public from reclaimed water entering a potable system?
A tertiary cloth media filter has begun backwashing far more frequently and effluent turbidity is rising. Where should the operator look first?