8.2 Preliminary Treatment: Screening, Grinding & Grit Removal

8.2 Preliminary Treatment: Screening, Grinding & Grit Removal

Preliminary treatment is the very first physical stage in a wastewater treatment facility. Unlike secondary biological treatment, preliminary treatment does not remove dissolved organic matter or nutrients; rather, its primary objective is the physical interception and removal of coarse debris, rags, plastics, floating floatables, and heavy abrasive mineral grit. Removing these coarse materials immediately upon entry into the headworks protects downstream raw sewage pumps, mechanical equipment, and piping from severe physical damage, abrasive wear, and chronic clogging.

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Core Objectives of Preliminary Treatment

1. Equipment Protection: Shield raw sludge pumps, impellers, clarifier scraper flight chains, and centrifuge dewatering units from jamming, scouring, and premature failure caused by rocks, sticks, metal items, and abrasive quartz sand.
2. Pipeline & Channel Clogging Prevention: Prevent heavy mineral sand from settling in downstream channels, aeration basins, anaerobic digesters, and sludge transport piping.
3. Volumetric Capacity Preservation: Prevent inert, non-biodegradable sand and gravel from accumulating inside anaerobic and aerobic digesters, which otherwise robs active digestion volume and requires expensive manual cleaning.
4. Downstream Process Protection: Protect fine-pore aeration diffusers from rag fouling ("ragging") and prevent plastic debris from passing through to receiving streams.

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Coarse Screening & Bar Racks

Coarse screens (or bar racks) consist of a parallel array of vertical or inclined metal bars placed across the influent headworks channel.

+-----------------------------------------------------------------------------------------+
|                        COARSE SCREEN DESIGN & OPERATIONAL CRITERIA                      |
+-----------------------------------------------------------------------------------------+
|  Bar Spacing (Clear Opening):                                                           |
|  - Standard coarse bar racks: 0.5 to 2.0 inches (12 to 50 mm).                          |
|  - Manually cleaned screens: typically 1.0 to 2.0 inches (25 to 50 mm).                |
|  - Mechanically cleaned screens: typically 0.5 to 1.0 inch (12 to 25 mm).              |
+-----------------------------------------------------------------------------------------+
|  Channel Approach Velocity Parameters:                                                  |
|  - Minimum Velocity: 1.25 ft/s (0.4 m/s) -> Prevents heavy grit deposition in channel.  |
|  - Maximum Velocity: 3.0 ft/s (0.9 m/s) -> Prevents hydraulic extrusion of rags.        |
|  - Ideal Operating Velocity: 1.5 to 2.5 ft/s (0.45 to 0.75 m/s).                        |
+-----------------------------------------------------------------------------------------+

Manually Cleaned vs. Mechanically Cleaned Screens

• Manually Cleaned Bar Screens:
  • Installed at an angle of 30° - 45° from the horizontal to facilitate hand raking with a long-tined rake onto a perforated drainage plate.
  • Used primarily in small wastewater package plants (capacity < 0.5 MGD) or installed in parallel bypass channels used only during mechanical screen maintenance.
  • Operational Disadvantage: Labor-intensive; subject to rapid blinding during morning peak flows, causing upstream channel backing and surge flow when manually cleaned.
• Mechanically Cleaned Bar Screens:
  • Installed at a steeper inclination (60° - 85° from horizontal) or vertically.
  • Categorized into front-cleaned/front-return, back-cleaned, catenary trash racks, and reciprocating rake-arm screens.
  • Automated cleaning is controlled by two primary mechanisms:
    1. Differential Water Level (Headloss): Ultrasonic level sensors or pressure transducers detect water depth differential across the screen. When blinding causes headloss to exceed a preset setpoint (2 - 6 inches of water column), the rake drive automatically initiates.
    2. Interval Timer Override: A secondary programmable timer runs the rake mechanism for a brief cycle every 15–60 minutes regardless of headloss, preventing heavy, compacted debris buildup during low-flow nighttime periods.

  Mechanically Cleaned Bar Screen Operation & Headloss Differential Trigger:

   Influent Flow ────►                                 Effluent Flow ────►
                           ┌───┐ (Motorized Drive Rake)
                           │ M │
                           └─┬─┘
   Upstream Head             │   Debris Hopper / Washer
   Water Level               ▼       ┌───┐
  ═══════════════╗        │\         │   │ (To Screw Compactor)
                 ║        │ \        └───┘
                 ║  ▲     │  \ Screen Bars (0.5" - 1.5" Spacing)
                 ║  │     │   \   (60° - 80° Angle)
                 ║ Head-  │    \
                 ║ loss   │     \
                 ║ (Δh)   │      \           Downstream
                 ║  │     │       \          Water Level
                 ║  ▼     │        \      ═══════════════╗
                 ╚════════╪═════════\════════════════════╝
                          │          \
   Approach Velocity:     │           \
   1.25 - 3.0 ft/s ───────►            \
   ─────────────────────────────────────────────────────── Channel Invert Floor

Hydraulics & Approach Velocity Control

Maintaining proper channel approach velocity is an essential operational duty:

• If Approach Velocity Drops Below 1.25 ft/s: Mineral sand, grit, and dense organic solids drop out of suspension and accumulate upstream in the channel approach invert, forming septic sludge banks.
• If Approach Velocity Exceeds 3.0 ft/s: High hydraulic dynamic pressure pushes and extrudes flexible plastics, sanitary wipes, rags, and fibrous debris directly through the screen bars, defeating the screen's purpose.

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Fine Screens & In-Stream Grinders

Fine Screens

Fine screens feature openings ranging from 0.06 to 0.25 inches (1.5 to 6 mm). They capture small solids, personal hygiene items, hair, seeds, and cellulose fibers that pass through coarse bar racks. Types include rotary drum screens, continuous step screens, and center-flow perforated band screens. Fine screens are increasingly utilized upstream of membrane bioreactors (MBRs) and advanced biological nutrient removal systems.

Comminutors & In-Stream Macerators

Comminutors and channel grinders (e.g., Muffin Monsters) consist of rotating cutting drums or dual-shafted interlocking gear-toothed cutter stacks installed directly in the sewage flow. They shred coarse solids, rags, and plastics into small fragments (typically 0.25 to 0.375 inches) without removing them from the liquid stream.

• Major Operational Pitfall: Shredded synthetic rags, baby wipes, and plastic wrappers do not biodegrade. As these shredded fibers travel downstream into aeration basins and anaerobic digesters, they encounter turbulent mixing and re-weave into rope-like rag masses ("roping"). These roped masses wrap around raw sludge pump impellers, bind floating aerators, and clog heat exchanger loops. Consequently, modern design standards widely favor fine screening with complete solids extraction over in-stream comminution.

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Grit Characteristics & Principles of Separation

What Constitutes Wastewater Grit?

Grit is defined as heavy, dense inorganic particulate matter found in raw sewage, comprising quartz sand, gravel, cinders, coffee grounds, eggshells, bone chips, metallic fragments, and road grit washed from paved surfaces during wet weather inflow events.

• Specific Gravity Comparison:
  • Inorganic Grit Specific Gravity: ~ 2.65
  • Putrescible Organic Solids Specific Gravity: ~ 1.05 - 1.20
  • Water Specific Gravity: 1.00

Separation Mechanism (Stokes' Law Kinetics)

Because inorganic grit is significantly denser (SG = 2.65) than organic sewage solids (SG ~ 1.1), grit settles through water at a substantially higher terminal velocity. Standard grit chambers are engineered to selectively settle particles with an equivalent diameter of 0.20 mm (65 mesh) or greater, while maintaining sufficient fluid velocity to keep lighter, putrescible organic matter in suspension so it flows into downstream treatment units.

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Grit Removal Technologies

1. Velocity-Controlled Horizontal-Flow Channels

In horizontal-flow grit channels, raw wastewater flows through long, narrow troughs. The design controls velocity near the selected settling target (commonly about 1.0 ft/s or 0.3 m/s) over the expected flow range:

• If Velocity > 1.0 ft/s: Grit carries through into primary clarifiers.
• If Velocity < 1.0 ft/s: Putrescible organic solids settle out with the grit, causing offensive anaerobic odors and vector attraction.
• Velocity Control Devices: To maintain a constant 1.0 ft/s velocity as water depth changes, the channel outlet is equipped with a specially contoured Sutro (proportional) weir or a downstream Parshall flume whose throat matches the channel geometry.

  Velocity-Controlled Horizontal Grit Channel with Sutro Weir:

  Influent Flow ────►                                             Effluent Flow ────►
  ┌─────────────────────────────────────────────────────────────┬──┐
  │                                                             │ S│
  │     Horizontal Velocity Maintained at EXACTLY 1.0 ft/s      │ U│
  │ ──────────────────────────────────────────────────────────► │ T│
  │                                                             │ R│ (Sutro Proportional
  │        Heavy Grit (SG 2.65) Settles to Floor:               │ O│  Weir Controls Depth)
  │        ───────►  ───────►  ───────►  ───────►               │  │
  │          \         \         \         \                    │ W│
  │           ▼         ▼         ▼         ▼                   │ E│
  │      ┌───────────────────────────────────────────────┐      │ I│
  │      │░░░░░░░░░░░░░ GRIT COLLECTION HOPPER ░░░░░░░░░░│      │ R│
  └──────┴───────────────────────────────────────────────┴──────┴──┘

2. Aerated Grit Chambers

An aerated grit chamber consists of a rectangular tank with coarse-bubble air diffusers positioned along one longitudinal sidewall, approximately 1.5 - 2.0 feet above the floor:

• Spiral Roll Mechanism: Rising air bubbles create a continuous spiral roll (helical circulation) perpendicular to the forward wastewater flow. The air injection rate is adjusted so the fluid velocity across the tank floor is 1.0 to 1.5 ft/s.
• Grit vs. Organic Partitioning: Dense grit particles have sufficient mass and momentum to overcome the rolling fluid drag and drop into a longitudinal collection hopper below the diffusers. Lighter organic particles are swept upwards by the roll and remain suspended in the flow.
• Additional Process Benefits: Aeration provides pre-aeration to freshen stale sewage, strips out dissolved H₂S gas, and floats free emulsified grease to the surface for skimming.

3. Vortex Grit Separators & Hydrocyclones

• Vortex Separators (e.g., Pista Grit): Wastewater enters a cylindrical chamber tangentially. A slowly rotating mechanical paddle maintains a constant rotational fluid vortex. Centrifugal forces roll heavy grit particles toward the center floor opening, where they drop into a collection pot, while organics exit over the top weir.
• Hydrocyclone Separators: Grit slurry pumped from a grit well enters a conical cyclone tangentially at high velocity. Centrifugal force drives dense grit to the outer cone wall, discharging out the bottom apex (underflow) into a classifier, while water and organics discharge out the top center (overflow) and return to the treatment flow.

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Grit Handling, Washing & Sanitary Disposal

Raw grit can retain water and putrescible organics, creating odor and vector problems during storage and transport. A classifier drains the slurry; a washer may scour organics from mineral particles and return the wash water to treatment. PUB2754 does not impose a universal “less than 15% organics” criterion: it directs the designer to determine whether washing is needed from the handling and final-disposal method. Operators must contain leakage and odors, characterize the material when required, and use the disposal route authorized for that material and facility.