3.2 Media Filters, Membrane Filters & Tertiary Microscreens

3.2 Media Filters, Membrane Filters & Tertiary Microscreens

2025 WPI alignment: This section teaches tertiary filtration equipment: sand, anthracite, disk media, membrane filtration, and microscreens in the official Equipment Evaluation, Maintenance, and/or Operation content area.

Why this job task matters

Tertiary filters polish secondary effluent by retaining fine solids; the operator manages loading, headloss or transmembrane pressure, cleaning, integrity, solids return, and protection of downstream disinfection.

Core operating concepts

Operating and maintenance workflow

1. Record influent and effluent turbidity, flow, headloss or TMP, run time, and units in service.
2. Inspect media condition, valves, wash pumps, air scour, rotating drives, sprays, seals, and solids-return paths.
3. Initiate backwash at the plant trigger and confirm sequence, expansion or cleaning action, and waste routing.
4. For membranes, trend normalized permeability and follow manufacturer limits for relaxation, backpulse, maintenance clean, and recovery clean.
5. Investigate a sudden effluent-turbidity rise for overload, damaged media, torn cloth, seal failure, or membrane integrity loss.
6. Return a cleaned unit gradually and verify filtrate quality before sending it to downstream disinfection or reuse.

Diagnostic evidence

Calculation, control, or records connection

For a granular filter, hydraulic loading rate = total flow applied / filter area. Use only the area in service. WPI’s formula table also gives filter backwash rate = backwash flow / filter area. For membranes, flux is filtrate flow divided by membrane area; compare at similar temperature and operating conditions. A raw TMP comparison can mislead if flux changed at the same time.

Worked operator scenario

Two of four equal filters are backwashing while plant flow remains constant. The active-area loading doubles, and the remaining units show fast headloss. The operator calculates loading with two units, confirms the backwash sequence and available storage, and coordinates flow or upstream solids control. Returning a filter before rinse water clears could protect capacity but send a turbidity slug into UV or chlorine contact.

Common exam traps

• Installed filter area is not operating area when units are offline.
• A slow turbidity increase and a sudden integrity failure are not diagnosed the same way.
• More chemical cleaning is not harmless; incompatible concentration or exposure can damage membranes.
• Backwash waste and captured solids return to the plant mass balance and cannot be ignored.

Field-to-exam checklist

• Filter control uses loading, headloss or TMP, effluent quality, and cleaning response.
• Barrier breakthrough must be distinguished from routine fouling.
• Backwash rates and filtration rates use the area actually in service.
• Tertiary solids performance directly affects disinfection reliability.

Normalizing filter trends

Compare like conditions when judging a filter. Differential pressure at one flow cannot be compared directly with differential pressure at a very different flow, and total plant flow must be divided by the area of filters actually in service. Track run time, terminal headloss, backwash volume, post-backwash baseline, turbidity, and integrity indicators together. A normal clean baseline followed by short runs suggests higher incoming solids; a high baseline immediately after cleaning suggests incomplete cleaning, media condition, or an instrument problem.

Media specification and backwash mechanics

Granular filter media is specified by effective size, the sieve opening passing 10 percent of the media by weight, and uniformity coefficient, the ratio of the 60-percent size to the 10-percent size. A lower uniformity coefficient means a more consistent grain size and a more predictable bed. Dual-media beds place coarser, lighter anthracite above finer, denser sand so that solids penetrate into the depth of the bed before the surface blinds; because the anthracite is less dense, it re-stratifies on top after each backwash instead of mixing permanently.

Backwash has to do two things: fluidize and expand the bed enough to release captured solids, and carry them out through the wash troughs. Too little rate leaves compacted deposits that grow into mudballs and cause channelling; too much rate carries media over the troughs and is lost permanently. Air scour or a surface-wash sweep breaks the surface mat that hydraulic wash alone cannot lift.

Worked backwash rate. A filter measuring 20 ft x 20 ft has 400 ft² of area. Backwashing at 6,000 gpm gives 6,000 / 400 = 15 gpm/ft², a conventional granular-media wash rate. The same 6,000 gpm applied to a 900 ft² filter would be only 6.7 gpm/ft² and would probably fail to expand the bed.

Cloth and disk filters have no fluidized wash at all; the captured cake is removed by suction shoes or sprays and leaves as a liquid backwash stream returned upstream. Membranes need a different correction entirely: normalize permeability to temperature, because cold water is more viscous and raises transmembrane pressure seasonally without any additional fouling.