6.1 Preliminary Treatment: Screening & Grit Removal

Purpose & Role of Preliminary Treatment

Preliminary treatment is the initial stage of wastewater processing at the treatment facility headworks. Its primary objective is not significant biochemical transformation, but rather the physical removal or conditioning of coarse debris, rags, floatables, plastics, and heavy abrasive inorganic grit present in raw influent wastewater.

Effective preliminary treatment accomplishes several vital operational objectives:

1. Equipment Protection: Shields downstream raw sewage pumps, reciprocating sludge pumps, progressive cavity stators, and centrifuge bowls from severe abrasive wear and catastrophic mechanical jamming.
2. Pipeline & Basin Maintenance: Prevents heavy inert grit (sand, gravel, glass) from accumulating in primary clarifiers, aeration basins, anaerobic digesters, and gravity pipelines where removal requires expensive manual dewatering and confined space entry.
3. Process Optimization: Eliminates synthetic rags, wet wipes, and plastics that wrap around mixer impellers, foul dissolved oxygen (DO) probes, blind membrane bioreactor (MBR) cassettes, and form unsightly floating scum mats.

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Raw Wastewater Characterization & Typical Municipal Concentrations

Municipal wastewater consists of liquid wastes discharged from domestic residences, commercial establishments, light industrial connections, and institutional facilities, combined with varying fractions of storm runoff and groundwater infiltration/inflow (I&I).

Domestic vs. Commercial / Industrial Influences

• Diurnal Variations: Domestic wastewater exhibits classic diurnal peaking patterns, with peak morning (07:00–10:00) and evening (18:00–21:00) flows and organic loads, dropping to minimal flow and dilute loading during early morning hours (02:00–05:00).
• Industrial Loads: Industrial inputs (food processing, bakeries, metal finishing, breweries) introduce shock organic loadings, extreme pH swings, elevated temperatures, high chemical oxygen demand (COD), or high concentrations of fats, oils, and grease (FOG).

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Screening Equipment: Bar Racks, Mechanical Screens & Fine Screens

Screening is the first unit operation encountered by raw wastewater. Screens are classified according to the clear spacing between screening elements.

                                  RAW INFLUENT WASTEWATER
                                             │
                                             ▼
                             ┌───────────────────────────────┐
                             │     COARSE BAR SCREENING      │
                             │  (Manual 1-2" / Mech 0.25-1") │
                             └──────────────┬────────────────┘
                                            │
                                            ▼
                             ┌───────────────────────────────┐
                             │   FINE SCREENING (1 - 6 mm)   │
                             │  (Perforated Drum / Belt MBR) │
                             └──────────────┬────────────────┘
                                            │
                                            ▼
                             ┌───────────────────────────────┐
                             │      GRIT REMOVAL SYSTEM      │
                             │  (Velocity / Aerated / Vortex)│
                             └──────────────┬────────────────┘
                                            │
                                            ▼
                             To Primary Clarifiers / Pumps

1. Coarse Manual Bar Racks

• Clear Bar Spacing: 1.0 to 2.0 inches (25 to 50 mm).
• Inclination Angle: Positioned at 30° to 45° from horizontal to facilitate manual raking by plant operators using handheld tined rakes.
• Application: Typically installed in emergency overflow bypass channels, small packaged facilities (<0.5 MGD), or ahead of mechanical screens during power outages.
• Operator Considerations: Manual racks are labor-intensive, create pathogen and splash exposure risks, and require frequent cleaning during rainstorms to prevent upstream sewer surcharging.

2. Mechanically Raked Bar Screens

• Clear Bar Spacing: 0.25 to 1.0 inch (6 to 25 mm).
• Inclination Angle: Positioned at 60° to 85° from horizontal (or vertical).
• Design Types:
  • Front-Cleaned, Front-Return: The rake teeth enter the screen bars on the upstream face and pull solids upward to the discharge chute. This prevents solids trapped on the upstream face from being pulled through or jamming the bottom inversion chain.
  • Back-Cleaned: The cleaning mechanism operates from the downstream clean face, penetrating through the bars to push debris off the upstream side.
• Automation & Control:
  • Differential Head Level Sensor Control: Ultrasonic or bubbler level sensors measure the liquid depth upstream and downstream of the screen. When debris blinds the screen, head loss ($\Delta h$) increases. When $\Delta h$ reaches 2 to 6 inches (50 to 150 mm), the PLC automatically initiates a cleaning rake cycle.
  • Timer Backup: An adjustable cycle timer (e.g., 15 to 30 minutes) initiates periodic raking during low-flow periods to prevent organic solids from settling, turning septic, and producing odors.
  • High-Level Alarm: Triggers emergency alarms or opens automated bypass gates if head loss exceeds safe channel freeboard.

3. Fine Screens

• Clear Openings: 1 to 6 mm (0.04 to 0.25 inches) using perforated plates, continuous travelling mesh belts, or wedge-wire rotary drums (internally or externally fed).
• Application: Widely deployed in advanced facilities, particularly upstream of Membrane Bioreactors (MBRs) to prevent hair, lint, and synthetic fibers from braiding around hollow-fiber membranes.

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Screenings Characteristics & Washer-Compactor Processing

Generation Rates

Screenings volume varies widely based on sewer system type (separate vs. combined), community demographics, and precipitation:

• Typical Volume: 0.5 to 5.0 cubic feet per million gallons (cu ft/MG) of wastewater treated ($3.7\text{ to }37\text{ L/1,000 m}^3$).
• Storm Events: First-flush wet weather events can surge screenings generation up to 10 to 20 cu ft/MG within hours.

Screenings Washer-Compactor (Wash Press)

Unprocessed raw screenings consist of feces, rags, paper, food particles, and hygiene products containing 75% to 90% water and high concentrations of putrescible organic matter.

Raw Screenings Infeed ──► [ Washing Zone: High-Pressure Spray ] ──► Washwater + Organics Return
                                        │                               (Back to Plant Headworks)
                                        ▼
                             [ Screw Compactor Press ]
                                        │
                                        ▼
                           [ Plug Dewatering Cylinder ] ──► Clean, Dry Screenings (30-50% DS)
                                                               (To Disposal Dumpster)

1. Washing Cycle: Screenings enter a washing hopper where high-pressure wash water and mechanical agitation break up and detach fecal matter and soluble biochemical oxygen demand (BOD), returning these organic nutrients to the wastewater stream for downstream biological treatment.
2. Compaction & Dewatering: A heavy-duty stainless steel screw auger conveys the washed screenings through an adjustable friction restriction plug (cone).
3. Performance Metrics:
   • Volume Reduction: 50% to 70% reduction in total screenings volume.
   • Weight Reduction: 60% to 80% reduction in hauled weight.
   • Dewatered Dry Solids: Produces clean, non-putrescible screenings with 30% to 50% Dry Solids (DS).
4. Disposal Mandate: Processed screenings must pass the EPA Paint Filter Liquids Test (SW-846 Method 9095B—no free liquid passing through a 60-mesh filter over a 5-minute test period) prior to transport to a permitted municipal solid waste landfill.

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Comminutors & In-Line Macerators

Comminutors and macerators (grinders) are motor-driven rotary cutting devices installed directly within wastewater channels or on pump suction piping.

• Operating Principle: High-torque, counter-rotating twin hexagonal shafts equipped with hardened cutting teeth shred solids into fine, uniform particles (1/4 to 3/8 inch or 6 to 10 mm).
• Operational Trade-offs:
  • Advantage: Eliminates the need for screenings handling, washing, odor control, and landfill transport at unmanned lift stations.
  • Disadvantage: Grinding does not remove non-biodegradable plastics and synthetic fibers. In aeration basins and digesters, shredded rags re-agglomerate into dense fibrous "rag ropes" and "mops" that wrap around turbine mixers and clog clarifier underflow pumps. Modern California WWTP designs favor complete extraction via screens over comminution.

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Grit Removal: Principles & Unit Processes

Grit consists of heavy, abrasive inorganic matter with a specific gravity ($SG$) significantly higher than organic sewage solids:

• Grit Specific Gravity ($SG$): $\mathbf{\approx 2.65}$ (silica sand, gravel, eggshells, bone fragments, coffee grounds, seeds, metal fragments, mineral silt).
• Organic Solids Specific Gravity ($SG$): $\mathbf{\approx 1.05\text{ to }1.20}$ (fecal solids, grease flocs, food waste).
• Target Separation: Standard grit chambers target particles with nominal diameter $\ge 0.2\text{ mm}$ (65-mesh) or $\ge 0.15\text{ mm}$ (100-mesh).

1. Velocity-Controlled Horizontal Grit Channels

Horizontal-flow grit channels are long, narrow rectangular channels designed to maintain a strictly controlled horizontal flow velocity of exactly 1.0 ft/s (0.30 m/s) under all flow conditions.

$\text{Target Horizontal Velocity} = 1.0\text{ ft/s} \quad (0.3\text{ m/s})$

• Velocity Dynamics:
  • If velocity $> 1.0\text{ ft/s}$ ($>1.2\text{ ft/s}$): Grit is swept out of the channel and carries over into primary clarifiers.
  • If velocity $< 1.0\text{ ft/s}$ ($<0.7\text{–}0.8\text{ ft/s}$): Light organic matter ($SG \approx 1.1$) settles along with the grit, creating septic odors and decomposing in the channel.
• Flow Control Devices: Because channel flow ($Q$) varies diurnally, water depth fluctuates. To maintain constant $1.0\text{ ft/s}$ velocity, a Proportional-Flow Weir or Sutro Weir is installed at the discharge end. The Sutro weir's curved geometry creates a linear discharge-to-head relationship ($Q \propto h$), keeping horizontal cross-sectional velocity constant across varying water depths.
• Design Retention Time: Hydraulic Retention Time (HRT) is 45 to 60 seconds; channel length typically ranges from 30 to 60 feet.

2. Aerated Grit Chambers

Aerated grit chambers are rectangular tanks where coarse bubble diffusers placed along one side near the bottom introduce air, generating a continuous spiral helical roll pattern perpendicular to longitudinal flow.

      [ Air Diffusers ] ──► Creates Spiral Roll Flow
             │
             ▼
    Transverse Velocity: 1.0 – 2.0 ft/s
             │
    ┌────────┴────────────────────────────────────────┐
    │ Heavy Grit (SG 2.65)   ──► Thrown to Bottom Trough │
    │ Organics (SG 1.05-1.2) ──► Lifted in Suspension    │
    └─────────────────────────────────────────────────┘

• Air Supply Rate: 3 to 8 standard cubic feet per minute per linear foot of channel length (scfm/ft) ($0.3\text{ to }0.8\text{ m}^3/\text{min}\cdot\text{m}$).
• Hydraulic Retention Time (HRT): 2 to 5 minutes at peak design flow.
• Process Adjustments:
  • If settled grit contains high organic content (gray color, septic odor), increase air flow to increase spiral roll velocity and scour organics.
  • If fine grit carries over in the effluent, decrease air flow to reduce spiral roll velocity and allow fine grit to settle.
• Additional Benefits: Freshens septic sewage, prevents early $H_2S$ release, and enhances surface grease separation.

3. Vortex Hydrodynamic Grit Separators (e.g., Pista, Tea-Cup)

Vortex separators utilize a cylindrical chamber with a tangential inlet that directs raw wastewater in a circular vortex path.

• Mechanics: A motor-driven rotating central impeller (paddle) maintains constant fluid rotational velocity regardless of plant flow variations. Centrifugal force and boundary-layer fluid dynamics drive heavy grit to the chamber floor, directing it down a sloped cone into a center collection hopper. Lighter organics are lifted upward into the top effluent stream.
• Design Retention Time: Extremely compact 0.5 to 1.0 minute (30 to 60 seconds) HRT.
• Removal Efficiency: Typically $>95%$ removal of 65-mesh (0.2 mm) sand with minimal organic co-settling.

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Grit Handling, Washing & Classification

[ Grit Chamber Hopper ] ──► (Air-Lift / Torque-Flow Pump) ──► [ Hydrocyclone Separator ]
                                                                      │ (Centrifugal Spin)
                                    ┌─────────────────────────────────┴─────────────────┐
                                    ▼                                                   ▼
                          [ Top Overflow: Slurry ]                            [ Bottom Apex: Grit ]
                          (Back to Headworks Stream)                                    │
                                                                                        ▼
                                                                             [ Screw Classifier ]
                                                                                        │ (Washed Dewatering)
                                                                                        ▼
                                                                             [ Dumpster Disposal ]
                                                                             (>80% DS, <15% VS)

1. Grit Slurry Pumping: Settled grit slurry (1% to 3% solids) is extracted from bottom hoppers using air-lift pumps (compressed air injection) or recessed-impeller vortex torque-flow pumps designed to handle abrasive slurries without impeller wear.
2. Hydrocyclone (Grit Cyclone): The pumped slurry enters a stationary cone tangentially at 15 to 25 psi. Centrifugal force flings dense grit outward against the conical wall, where it discharges out the bottom apex orifice. Lighter organics and water reverse flow up a central core and exit via the top overflow vortex finder back to the plant headworks.
3. Grit Classifier (Screw or Rake): The concentrated grit discharge drops into an inclined dewatering screw or reciprocating rake classifier. As the screw rotates slowly upward along an inclined trough, grit is washed with clean plant water, draining moisture back into the basin.
4. Final Disposal Quality: Cleaned, dewatered grit discharged to the disposal container should contain $>80%\text{ to }85%$ Total Solids and $<15%\text{ to }20%$ Volatile Solids.