5.1 Preliminary Treatment: Bar Screens, Shredders, Grit Chambers & Flow Equalization

Preliminary Treatment: Bar Screens, Shredders, Grit Chambers & Flow Equalization

Preliminary treatment represents the critical first line of defense in a municipal wastewater treatment facility. Positioned immediately at the plant headworks, these physical unit operations do not substantially reduce dissolved biochemical oxygen demand (BOD) or fine colloidal suspensions. Instead, preliminary treatment is engineered to remove coarse debris, rocks, rags, floatables, plastics, sand, and heavy abrasive mineral grit. Operating a high-performance facility requires the operator in responsible charge (ORC) to master headworks hydraulics, mechanical screening devices, grit classification dynamics, and diurnal flow equalization principles.

---

1. Purpose and Objectives of Headworks Facilities

Raw municipal wastewater carries a complex array of non-biodegradable and abrasive solids transported via the collection system. If these materials enter downstream biological and clarification processes, they cause severe operational failures:

1. Mechanical Protection: Preventing large objects (timbers, rocks, structural debris) and stringy synthetic materials (wipes, rags, plastics) from binding, ragging, or destroying raw sewage pump impellers, progressive cavity positive displacement pumps, and check valves.
2. Abrasive Wear Mitigation: Removing mineral sand, gravel, and road grit that scour pump volutes, erode piping elbows, abrade mechanical seals, and damage centrifuge bowls.
3. Process Protection: Preventing the accumulation of inert grit and heavy mineral solids inside primary clarifiers, aeration basins, and anaerobic digesters (where grit deposition permanently robs 15% to 30% of active digester volume and requires dangerous, labor-intensive manual cleaning).
4. Aeration Protection: Preventing floating plastic strips, synthetic fibers, and hair from wrapping around and blinding fine-bubble membrane diffusers or fouling dissolved oxygen (DO) sensors.

---

2. Bar Screens and Screening Mechanics

Screening is the first unit operation encountered by incoming raw wastewater. Bar screens consist of an array of parallel, inclined metal bars anchored across the influent channel.

+-----------------------------------------------------------------------------------+
|                         BAR SCREEN CLASSIFICATION MATRIX                          |
+-----------------------------------------------------------------------------------+
| Screen Category   | Clear Opening Size       | Primary Operational Function       |
+-------------------+--------------------------+------------------------------------+
| Trash Racks       | 2.0 to 6.0 in (50-150 mm)| Heavy debris, branches, logs       |
| Coarse Bar Screens| 1.5 to 3.0 in (38-75 mm) | Cans, large rags, bottles          |
| Medium Bar Screens| 0.5 to 1.5 in (13-38 mm) | Standard municipal rags, wipes     |
| Fine Bar Screens  | 0.25 to 0.5 in (6-13 mm) | High-efficiency solids capture     |
| Micro-Screens     | < 0.25 in (< 6 mm)       | Membrane bioreactor (MBR) pre-treat|
+-----------------------------------------------------------------------------------+

Screen Cleaning Mechanisms

• Manually Cleaned Screens: Set at an inclination of 30° to 45° from the horizontal to facilitate hand raking onto a perforated drainage plate. Used primarily in small facilities (< 1.0 MGD) or as emergency bypass channels. They require frequent manual attention; if neglected, screenings blinding causes upstream channel surcharging and sewer backups.
• Mechanically Cleaned Screens: Set at steeper angles (60° to 85° from the horizontal) or vertically. Mechanical rakes travel along the bar face to lift captured screenings to an overhead discharge point. Variants include front-cleaned/front-return rakes, catenary multi-rake systems, continuous reciprocating rakes, rotating drum screens, and step screens.

   Mechanically Cleaned Screen Channel Hydraulics:
   
   Influent Flow ---> [ Approach Channel ] ===> || Bar Screen || ===> [ Downstream Flow ]
                         v = 1.5 - 2.0 ft/s     ||   v <= 3.0 ||        Drop in HGL (h_L)
                                                ||    ft/s    ||
                                                ^ Measured Differential Head (2-6 inches)

Channel Velocity and Hydraulic Control

The velocity of wastewater approaching and passing through a bar screen is a vital operational parameter:

• Approach Velocity: Must be maintained between 1.5 and 2.0 ft/s (0.45 to 0.60 m/s) under all flow conditions. If the velocity drops below 1.25 to 1.5 ft/s, heavy organic and mineral solids settle out in the upstream channel bottom, decomposing anaerobically and creating hydrogen sulfide ($H_2S$) odors.
• Through-Screen Velocity: The velocity through the open bar slots must not exceed 3.0 ft/s (0.90 m/s) at peak hourly design flow. Velocities exceeding 3.0 ft/s create excessive kinetic pressure that extrudes pliable rags, plastics, and stringy debris through the bars, defeating the screen's purpose.

Differential Head and Automated Control

Automated mechanical screens are controlled primarily by differential level (differential headloss) across the screen rack, monitored using ultrasonic level sensors or differential pressure transducers:

• When debris blinds the screen face, water backs up upstream, creating a differential water surface elevation ($\Delta h$).
• A typical cleaning cycle initiates when the differential head reaches 2 to 6 inches (50 to 150 mm) of water column.
• A backup programmable timer (e.g., initiating a rake cycle every 15 to 30 minutes regardless of headloss) is always installed in parallel to prevent heavy solids compaction during nocturnal low-flow periods when differential head develops slowly.

Screenings Handling and Disposal

Raw screenings contain up to 80% to 90% water and are heavily contaminated with putrescible organic matter. Modern headworks route captured screenings through screenings washer-compactors (auger presses). High-pressure wash water flushes fecal solids and soluble organics back into the wastewater stream, while an internal screw auger dewaters and compresses the remaining plastics, paper, and rags into a dense plug (reducing volume by 50% to 70% and dry solids to 40% to 50%). The compacted screenings discharge into sealed roll-off containers for haulage to a permitted solid waste landfill.

---

3. Comminutors, Macerators, and In-Line Grinders

In some legacy facilities and pump lift stations, comminutors or in-line twin-shaft grinders are installed directly in the raw sewage flow channel or suction piping.

• Operating Principle: High-torque, counter-rotating alloy steel cutter teeth shear, shred, and macerate large solid objects into uniform particles ranging from 0.25 to 0.5 inches (6 to 13 mm), allowing them to remain in the liquid stream.
• Operational Trade-offs: While comminutors eliminate the immediate need to handle and landfill wet screenings, they do not remove plastics, condoms, hygiene products, or synthetic wipe fibers. In downstream units, these shredded synthetic fibers recombine through hydrodynamic vortices into rope-like braids (rag balls and rag ropes). These fibers wrap around aerator impellers, clog secondary clarifier sludge withdrawal pipes, and blind progressive cavity digester transfer pumps. Consequently, modern design heavily favors fine screening and total removal over comminution.

---

4. Grit Removal Principles and Technologies

Grit is defined as heavy inorganic particulate matter consisting of sand, gravel, cinders, silt, eggshells, bone fragments, seeds, coffee grounds, and heavy metal chips. Grit particles are characterized by a specific gravity ($S_G$) of 2.65 (compared to organic wastewater solids with $S_G \approx 1.05\text{ to }1.20$) and nominal diameters exceeding 0.20 mm (65-mesh sieve).

+-----------------------------------------------------------------------------------------+
|                        COMPARISON OF GRIT REMOVAL TECHNOLOGIES                          |
+-----------------------------------------------------------------------------------------+
| System Type           | Flow / Velocity Control   | Typical HRT       | Key Feature     |
+-----------------------+---------------------------+-------------------+-----------------+
| Velocity-Controlled   | Proportional / Sutro Weir | 45 to 90 seconds  | Constant 1.0    |
| Channel               | or Parshall Flume         |                   | ft/s velocity   |
| Aerated Grit Chamber  | Side air diffusers create | 3 to 5 minutes at | Helical spiral  |
|                       | helical roll motion       | peak design flow  | roll pattern    |
| Vortex Grit Chamber   | Induced tangential vortex | 20 to 30 seconds  | Fluid shear &   |
| (Pista / Hydro-Grit)  | with rotating paddles     |                   | boundary layer  |
+-----------------------------------------------------------------------------------------+

1. Velocity-Controlled Horizontal Grit Channels

Horizontal flow grit channels rely on differential gravity sedimentation governed by Stokes' Law. The hydraulic cross-section is engineered to maintain a strict, constant horizontal flow velocity of 1.0 ft/s (0.30 m/s) throughout all diurnal flow ranges:

• At 1.0 ft/s, heavy inorganic grit ($S_G = 2.65$) settles to the channel floor.
• Lighter organic solids ($S_G = 1.05–1.20$) remain in suspension and are carried out of the chamber.
• If velocity increases to $> 1.25\text{ ft/s}$, fine grit is swept out of the basin and enters downstream clarifiers.
• If velocity falls below $< 0.75\text{ ft/s}$, putrescible organic matter settles with the grit, creating severe odors and degrading grit purity.
• Constant velocity is maintained across changing depths using specialized effluent flow-control weirs: Proportional Weirs (Sutro weirs) or downstream Parshall flumes.

2. Aerated Grit Chambers

In an aerated grit chamber, air diffusers are positioned along one side of a rectangular tank, creating a continuous helical (spiral) roll pattern perpendicular to the longitudinal flow of wastewater:

• The liquid velocity of the roll pattern is controlled by adjusting air supply volume via coarse-bubble diffusers.
• As wastewater spirals through the basin with a detention time of 3 to 5 minutes, the heavier grit particles are thrown outward by centrifugal forces and settle into a collection hopper along the bottom.
• Lighter organic solids are maintained in suspension by the rising bubble curtain and roll currents.
• Operational Control: If excess putrescible organics settle with the grit, the operator must increase air flow to increase the bottom roll velocity. If fine grit carries out in the effluent, the operator must decrease air flow to reduce excessive turbulence.

3. Vortex Grit Chambers

Vortex units (such as Pista® or Hydro-Grit® basins) introduce wastewater tangentially into a cylindrical basin with a flat or sloped bottom containing a center hopper:

• An adjustable-pitch rotating paddle impeller maintains a constant circular fluid velocity.
• Centrifugal forces combined with the secondary boundary-layer floor current sweep dense grit particles toward the center floor opening, where they drop into the storage hopper.
• Fluid shear at the vortex center lifts lighter organic particles upward, returning them to the main effluent stream.
• Vortex chambers feature low hydraulic retention times (20 to 30 seconds), compact physical footprints, and high capture efficiency (> 95% of 50-mesh / 0.3 mm grit).

---

5. Grit Washing, Dewatering, and Classification

Grit harvested from collection hoppers is pumped as a thin slurry (1% to 5% total solids) using recessed-impeller vortex pumps or air-lift pumps.

$\text{Raw Grit Slurry (1-5\% Solids)} \longrightarrow \text{Hydrocyclone Degritter} \longrightarrow \text{Screw Classifier} \longrightarrow \text{Dewatered Grit (70-80\% Solids)}$

1. Hydrocyclone Degritters: The slurry enters a conical cyclone tangentially under pressure (15 to 25 psi). Centrifugal acceleration drives dense grit to the outer conical wall, discharging as a concentrated underflow slurry through the bottom apex orifice. Lighter organic water exits out the top overflow vortex finder and returns to the plant influent.
2. Grit Classifiers / Washers: The concentrated underflow drops into an inclined screw classifier or reciprocating rake classifier. A low-velocity, inclined auger slowly draws the grit up an inclined ramp while wash water sprays continuously rinse residual organic films. Free water drains back down the ramp, leaving washed, dewatered grit (60% to 80% dry solids, with $< 15%$ volatile organic content) that is clean, non-putrescible, and suitable for sanitary landfilling.

---

6. Flow Equalization Basins

Municipal wastewater generation exhibits substantial diurnal variations: peak influent flows occur during late morning and early evening (peaking factors of 2.0 to 3.0+ times average daily flow), while minimum nocturnal flows drop to 30% to 50% of average. Flow equalization basins serve as hydraulic shock absorbers.

   In-Line Flow Equalization:        Raw Influent ===> [ Equalization Basin ] ===> Constant Flow Q_avg ===> Plant
   
   Side-Stream Flow Equalization:    Raw Influent ===+===> [ Base Flow Q_avg ] ===> Plant
                                                     |             ^
                                          If Q > Q_avg             | Return Flow
                                                     v             |
                                                  [ EQ Basin Storage ]

In-Line vs. Side-Stream Equalization

• In-Line Equalization: All influent wastewater passes directly through the equalization basin. Pumping downstream from the basin delivers a constant, uniform flow rate ($Q_{\text{avg}}$) and dampened organic load to all primary and secondary processes throughout the 24-hour day.
• Side-Stream (Off-Line) Equalization: Wastewater flows directly to the treatment train during normal conditions. When peak hourly flows exceed a preset plant capacity threshold, the excess flow spills over a diversion weir into the off-line storage basin. During nocturnal low-flow periods, stored wastewater is pumped back into the main headworks.

Operational and Mixing Requirements

Equalization basins must incorporate dedicated aeration or high-energy mechanical mixing systems to prevent severe operational failures:

• Minimum Aeration Rate: 1.25 to 2.0 standard cubic feet per minute (scfm) per 1,000 gallons of storage capacity (or mechanical mixing energy of 0.02 to 0.04 hp per 1,000 gallons).
• Septicity Prevention: Without adequate dissolved oxygen, raw wastewater in equalization basins rapidly consumes dissolved nitrates, turning septic and producing toxic, corrosive hydrogen sulfide ($H_2S$) and volatile organic acids.
• Solids Deposition: Continuous mixing keeps raw suspended solids ($150–300\text{ mg/L}$) in suspension, preventing heavy organic sludge banks from forming across the basin floor.