8.1 Tertiary Filtration Process Evaluation

8.1 Tertiary Filtration Process Evaluation

2025 WPI alignment: This section teaches tertiary filtration through media such as sand, anthracite, or disk filters in Treatment Process Evaluation and Adjustment, the 40-question area containing 6 recall, 34 application, and 5 calculation items.

Why this process task matters

Tertiary filtration reduces remaining suspended solids and turbidity after secondary treatment, supporting tighter effluent limits and more reliable disinfection. Process evaluation distinguishes normal solids loading from breakthrough, poor coagulation, or equipment failure.

Process-control model

Evaluation and adjustment sequence

1. Trend flow, operating area, influent/effluent turbidity or TSS, headloss, run length, and backwash frequency.
2. Observe distribution, surface condition, air/water wash, panels, seals, drains, and waste routing.
3. Calculate loading whenever flow or units in service change.
4. Investigate upstream clarifier or chemical changes before treating short runs as a filter-only problem.
5. Backwash at the approved trigger and confirm sequence, rate, expansion/cleaning, rinse, and return-to-service quality.
6. Use effluent quality and restored headloss/run length to verify the correction.

Diagnostic evidence

Calculation and mass-balance connection

Use hydraulic loading rate = total flow applied / area in service and WPI’s filter backwash rate = backwash flow / area. If one of four equal filters is out, each remaining filter receives one third of total flow, not one quarter. Compare headloss at similar flow because a higher flow can raise clean-bed headloss even without more fouling.

Worked operating scenario

Filter runs shorten immediately after a secondary clarifier upset, but post-backwash headloss returns to normal. The operator treats increased influent solids as the primary driver, restores clarification, balances active filter loading, and avoids unnecessary chemical cleaning. The normal clean baseline argues against permanent media fouling.

Common exam traps

• Run length depends on upstream solids and flow, not just filter condition.
• Use area in service, not total area.
• A backwash command does not prove the correct valves, flow, or air scour occurred.
• Return-to-service water must meet the plant’s quality or recycle requirement before normal routing.

Field-to-exam checklist

• Evaluate filters with influent load, active-area loading, headloss, effluent quality, and wash recovery.
• Distinguish gradual fouling from sudden integrity or bypass events.
• Tertiary filtration supports downstream disinfection by reducing solids and turbidity.
• Backwash performance is part of treatment, not merely maintenance.

Separating hydraulic and solids headloss

Headloss should be compared at a common flow whenever possible. A clean filter operated at higher flow can show more headloss even before depositing solids, while a fouled filter may show the same headloss only because flow declined. Plot headloss with flow, turbidity, and elapsed run volume. This avoids backwashing a hydraulically loaded but clean unit or leaving a restricted unit online because its flow—and therefore its displayed differential—has quietly fallen.

Compare filter runs on an equal basis

A short run is meaningful only after accounting for influent solids, flow per active area, terminal criterion, and the post-backwash starting condition. If loading increased, shorter runs may show normal solids capture rather than equipment failure. If the clean-bed headloss remains high, review backwash expansion, air scour, surface wash, valve sequencing, media condition, and waste routing. Confirm turbidity and barrier integrity before returning filtrate to service; a restored pressure profile alone does not prove acceptable effluent.

Turbidity, ripening, and the link to disinfection

Filtered turbidity is the practical surrogate for particle carryover, and its importance is downstream rather than aesthetic. Particles shield organisms from both ultraviolet light and chlorine, and they exert chlorine demand. A filter that lets turbidity climb is therefore quietly raising the disinfectant dose the plant must feed, or quietly reducing the ultraviolet dose actually delivered, whether or not the effluent solids limit is threatened.

Ripening is a real and predictable event. Immediately after a backwash, a granular bed has lost the deposited material that helped it capture fine particles, so filtrate turbidity is elevated for a short period until the bed re-forms. Plants manage this with filter-to-waste, a delayed return to service, or a slow-start ramp on the returning unit. Sending an unripened filter's first output straight into ultraviolet contact is one of the more common self-inflicted disinfection failures.

Worked loading with a unit offline. Four filters each measure 15 ft x 20 ft, giving 300 ft² each and 1,200 ft² total. At 6.0 MGD — that is 6,000,000 / 1,440 = 4,167 gpm — all four in service run at 4,167 / 1,200 = 3.5 gpm/ft². With one filter offline the remaining 900 ft² carries 4,167 / 900 = 4.6 gpm/ft², a 33 percent increase in loading that will shorten runs and can push a marginal bed into early breakthrough.

Chemical addition ahead of the filter changes the trade-off. A small coagulant or filter-aid polymer dose markedly improves capture of fine particles and of particulate phosphorus, but it also builds headloss faster and shortens runs. The correct comparison is filtrate quality and total backwash water used together, not run length alone.