5.1 Preliminary Treatment: Screening, Comminution & Grit Removal

5.1 Preliminary Treatment: Screening, Comminution & Grit Removal

Preliminary treatment is the initial physical processing stage in municipal wastewater facilities. Its primary purpose is protecting downstream mechanical infrastructure—including raw sewage pumps, piping, valves, flow meters, aeration diffusers, and anaerobic digesters—from clogging, abrasive wear, and inert solids accumulation. By conditioning influent wastewater upon arrival at the headworks, preliminary processes isolate coarse debris and heavy mineral particles before secondary biological treatment.

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1. Screening Technologies & Operational Parameters

Screening equipment removes buoyant, coarse, and fibrous solids (rags, wipes, plastics, timber, and sanitary items) before wastewater enters downstream unit processes.

Coarse Bar Racks (Trash Racks)

Coarse bar racks consist of parallel steel bars with clear openings of 1.5 to 2.5 inches (38 to 64 mm). Placed at a 30° to 45° angle to facilitate manual raking, they are typically installed in lift stations or bypass channels surrounding mechanical screens to intercept massive debris during power outages or extreme storm surges.

Mechanically Cleaned Bar Screens

Standard across modern treatment works, mechanically cleaned bar screens utilize clear bar spacing of 0.5 to 1.0 inch (12 to 25 mm) mounted at 60° to 80°:

• Front- vs. Back-Cleaned: Front-cleaned units position raking tines on the upstream face to lift captured solids directly into a discharge trough. Back-cleaned units operate mechanical chains behind the bars, reducing jam risks from large debris.
• Head Loss & Automation: Cleaning cycles are automated via differential head level sensors (ultrasonic or float switches) measuring water levels upstream and downstream of the rack. A differential of 2 to 6 inches (50 to 150 mm) triggers raking. Programmable interval timers provide redundant operation during low flows when debris accumulates slowly.
• Velocity Criteria: Channel approach velocity must be maintained at 1.25 to 1.5 ft/s to prevent grit deposition in the approach flume, but capped at 3.0 ft/s through the screen bars to prevent hydraulic extrusion of compressible rags.

Fine Screens

Fine screens utilize wedgewire slots, perforated drums, or band screens with openings of 0.01 to 0.25 inches (0.25 to 6.0 mm). Often installed upstream of membrane bioreactors (MBRs) to capture hair, seeds, and lint, they require high-pressure washwater sprays (40–80 psi) and hot-water defrost systems to prevent blinding from fats, oils, and grease (FOG).

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2. Screenings Management & Disposal

Captured screenings consist of putrescible, fecal-contaminated debris averaging 80% to 90% water content:

• Washing: Screenings washer-compactors flush raked solids with non-potable plant effluent. This returns soluble fecal organics back into the wastewater flow for downstream biological nutrition.
• Compaction and Dewatering: A heavy-duty screw auger compresses washed solids against a discharge cone, achieving a 50% to 80% volume reduction and raising cake dryness to 35% to 50% total solids.
• Disposal: Dewatered screenings drop into continuous bagging chutes or sealed dumpsters. Operators apply lime (calcium hydroxide) to suppress odors and control vectors (flies, rodents). Processed screenings must be hauled to a permitted municipal sanitary landfill (Subtitle D); under environmental regulations, screenings cannot be land applied.

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3. Comminutors, Macerators & Inline Grinders

Comminutors and inline sewage grinders shred solids into uniform particles (0.25 to 0.38 inches / 6 to 10 mm) directly within the flow stream rather than removing them:

• Operation: Motor-driven cutting cylinders or counter-rotating dual-shaft hex cutters rotate against a stationary hardened tool-steel comb.
• Rock Traps: Upstream depressed sumps (rock traps) intercept stones, masonry, and tramp metal that would shatter cutter teeth.
• Maintenance & Hazards: While comminution eliminates screenings handling, synthetic wipes and plastics re-weave downstream into dense "rag balls" that foul pumps and digesters. Cutter service requires strict Lock-Out/Tag-Out (LOTO) procedures due to severe amputation hazards, and atmospheric testing for hazardous gases ($H_2S$, $CH_4$, $CO$, $O_2$ deficiency) before entering confined wet wells.

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4. Grit Removal Principles & Basin Configurations

Grit consists of heavy non-biodegradable inorganics: silica sand, gravel, cinders, coffee grounds, and eggshells.

Physical Principles of Separation

Grit removal exploits the substantial differential in Specific Gravity ($SG$):

• Inorganic Grit: Specific gravity ranges from 2.0 to 2.65 (silica sand $\approx 2.65$).
• Organic Sewage Solids: Specific gravity ranges from 1.01 to 1.05 (buoyant fecal matter).

Under gravitational sedimentation, particles with an $SG$ of 2.65 settle an order of magnitude faster than organic matter. Basins are designed to capture particles $\ge 0.20\text{ mm}$ (65-mesh) while keeping organics suspended.

Velocity-Controlled Horizontal Flow Channels

Long rectangular flumes maintain a forward horizontal velocity of 1.0 ft/s (0.30 m/s) within an operating window of 0.75 to 1.25 ft/s:

• If velocity drops below 0.75 ft/s, putrescible organics settle with grit, causing foul odors.
• If velocity exceeds 1.25 ft/s, fine grit washes through to primary clarifiers.
• A downstream proportional flow Sutro weir or Parshall flume automatically maintains the 1.0 ft/s velocity as flow varies.

Aerated Grit Chambers

Aerated chambers introduce compressed air along one side of a rectangular basin, inducing a spiral, helical roll pattern:

• Air Delivery & Detention: Airflow rates (3 to 8 scfm per linear foot) are adjusted to maintain a bottom sweeping velocity of ~1.0 ft/s. Hydraulic detention time is 2 to 5 minutes at peak flow.
• Action: Heavy grit overcomes the rolling current and drops into a collection hopper, while lighter organics remain suspended. Diffused air also strips foul gases ($H_2S$) and freshens septic sewage.

Vortex Grit Separators (e.g., Pista Grit)

Vortex units feature a compact cylindrical chamber with tangential influent entry:

• Boundary Layer Effect: Tangential entry and a low-speed rotating paddle turbine create a controlled vortex. Fluid friction along the sloped floor induces a boundary layer current (the "teacup effect") that sweeps dense grit into a center collection cone, while organic solids stay suspended in the vortex core.
• Vortex separators provide high grit capture (>95% of 65-mesh grit) across wide flow turn-downs within a very compact footprint.

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5. Grit Classification, Washing & Disposal

Grit slurry collected in hoppers (1% to 5% solids) is lifted by vortex or air-lift pumps to classification equipment:

• Hydrocyclones: Slurry is injected tangentially under pressure. Centrifugal force flings dense grit outward to the cone walls, discharging through the bottom underflow, while water and lighter organics exit the top overflow back to the headworks.
• Screw Classifiers: An inclined spiral auger draws grit up an inclined trough, allowing washwater to drain backward into the process flow.
• Clean Grit Benchmark: Washed, dewatered grit must contain $\le 10%\text{ to }15%$ volatile (organic) solids, appear sandy, and be disposed of at an approved sanitary landfill or used as daily cover.