10.1 Preliminary Treatment: Screening, Grit Removal & Flow Measurement

4.1 Preliminary Treatment: Screening, Grit Removal & Flow Measurement

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Objectives of Preliminary Treatment

Raw municipal wastewater enters the treatment facility laden with a wide variety of non-biodegradable, abrasive, and bulky materials. These solids include textiles, wet wipes, wood fragments, plastics, sand, gravel, eggshells, and coffee grounds. If allowed to proceed unhindered into downstream process units, these constituents create severe operational failures:

1. Equipment Protection: Coarse solids bind centrifugal pump impellers, choke check valves, and blind heat exchanger tubes. Abrasive mineral particles rapidly erode slurry pump volutes, progressive cavity stator elastomers, and centrifuge bowls.
2. Pipeline and Channel Maintenance: Heavy grit deposits in low-velocity gravity channels, anaerobic digesters, and aeration basins, drastically reducing active liquid volume and requiring costly, confined-space manual removal.
3. Biological Process Shielding: Synthetic plastics, rags, and hygiene products float through primary clarifiers, foul secondary clarifier scum troughs, coat dissolved oxygen (DO) sensor membranes, and entangle around aeration basin fine-bubble diffusers.

+-----------------------------------------------------------------------------+
|                   PRELIMINARY TREATMENT TRAIN SCHEMATIC                     |
+-----------------------------------------------------------------------------+
|  Raw Influent  -->  Coarse / Fine  -->  Grit Removal  -->  Open Channel     |
|  Wastewater          Screening           Chamber           Flow Meter       |
|                      (Bar Rack /         (Vortex /         (Parshall /      |
|                      Step Screen)        Aerated)          Palmer-Bowlus)   |
|                           |                  |                     |        |
|                           v                  v                     v        |
|                      Screenings             Grit To            To Primary   |
|                      Washer / Press       Classifier /         Clarifiers   |
|                           |               Dewatering                |       |
|                           v                  v                      |       |
|                       Landfill            Landfill                  v       |
+-----------------------------------------------------------------------------+

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Screening Technologies & Operations

Screening devices are classified according to clear opening size into coarse screens ($> 6 \text{ mm}$ or $0.25 \text{ in}$) and fine screens ($0.5 \text{ to } 6 \text{ mm}$ or $0.02 \text{ to } 0.25 \text{ in}$).

1. Coarse Bar Racks & Screens

• Manually Cleaned Bar Screens: Composed of parallel steel bars inclined at $30^\circ \text{ to } 45^\circ$ from horizontal. Bar spacing typically ranges from $25 \text{ to } 50 \text{ mm}$ ($1.0 \text{ to } 2.0 \text{ in}$). Cleaning is performed manually with a rake into a perforated drainage deck. These are restricted to small package plants or emergency storm bypass channels because debris blinding rapidly induces upstream channel surcharging.
• Mechanically Cleaned Bar Screens: Positioned at an angle of $60^\circ \text{ to } 80^\circ$ from horizontal, with bar clear openings of $12 \text{ to } 38 \text{ mm}$ ($0.5 \text{ to } 1.5 \text{ in}$):
  • Front-Cleaned, Front-Return Rakes: Rakes engage the bars on the upstream face and return on the upstream side. They are vulnerable to submerged debris jamming the bottom sprockets.
  • Back-Cleaned Screens: The rake penetrates the bars from the downstream side, preventing large logs or boulders from wedging into moving drive chains.
  • Operational Control: Mechanically cleaned screens operate automatically via differential head loss sensors (ultrasonic level transducers upstream and downstream) or an override timer. A typical head differential setpoint of $50 \text{ to } 150 \text{ mm}$ ($2 \text{ to } 6 \text{ in}$) triggers the cleaning rake cycle before upstream sewer surcharging occurs.

2. Fine Screens & High-Capture Technologies

Modern wastewater utilities, particularly those operating membrane bioreactors (MBRs) or sequencing batch reactors (SBRs), require fine screens to capture hair, lint, and micro-debris:

• Continuous Step Screens: Stepped stainless steel plates move in an alternating, counter-oscillating vertical cycle. Screenings form a continuous debris "carpet" that filters out particles smaller than the physical slot gap ($2 \text{ to } 6 \text{ mm}$) before lifting them up the step incline.
• Rotary Drum & Wedge-Wire Screens: Internally or externally fed perforated cylinders with aperture openings of $0.5 \text{ to } 3 \text{ mm}$. Rotation continuously lifts retained solids to an internal discharge chute flushed by high-pressure spray headers ($40 \text{ to } 60 \text{ psi}$).

3. Screenings Washing, Compacting & Landfill Disposal

Captured raw screenings contain up to $80-90\%$ water and substantial quantities of biodegradable fecal organics. Direct disposal creates severe vector attraction and odor violations.

• Screenings Washer-Compactors: Screenings drop into an enclosed wash-trough where high-intensity water jets wash fecal matter and soluble BOD back into the wastewater stream. A heavy-duty shafted or shaftless screw auger conveys the washed screenings through a tapered discharge tube, exerting mechanical pressure that expresses free moisture.
• Volume & Weight Reduction: Washing and dewatering reduces wet screenings volume by $50-80\%$ and produces an output cake of $35-50\%$ total dry solids.
• Typical Generation Rates: Municipal wastewater yields approximately $0.5 \text{ to } 5.0 \text{ cu ft/MG}$ ($3.7 \text{ to } 37 \text{ L/1000 m}^3$) under dry weather flows, with peaks exceeding $15-20 \text{ cu ft/MG}$ during the first flush of storm events in combined or leaky collection systems.
• Landfill Regulatory Disposal Criteria: Screenings destined for municipal solid waste landfills must comply with EPA Method 9095B (Paint Filter Liquids Test). A $100 \text{ mL}$ representative sample is placed in a standard 60-mesh conical paint filter. If any free liquid passes through the filter within 5 minutes, the material is classified as liquid waste and is legally barred from landfill disposal until further dewatered or amended with absorbent lime or sawdust.

4. Comminutors, Macerators & Inline Grinders

Comminutors and inline channel grinders utilize rotating cutting blades intermeshing with stationary combs to shred rags, plastics, and solids into small fragments ($6 \text{ to } 10 \text{ mm}$) without removing them from the flow stream.

• Operational Advantage: Completely eliminates odor, handling, and tipping fee expenses associated with headworks screenings collection.
• Critical Disadvantage: Shredded synthetic textiles, plastic film, and flushable wipe fibers do not settle well in primary clarifiers. Instead, they pass downstream into aeration basins, where hydraulic shear and turbulence cause the fibers to re-braid into dense, rope-like cords ("rag ropes" and "rag balls"). These cords wrap around turbine aerators, foul mechanical surface aerator shafts, choke sludge suction lines, and accumulate in anaerobic digester heating coils.

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Principles of Grit Removal

Grit is defined as heavy, abrasive mineral and dense organic matter that settles rapidly through liquid. It comprises silica sand, gravel, cinders, metal fragments, bone chips, seeds, and coffee grounds.

Specific Gravity Segregation

The fundamental operating principle of all grit removal units is the sharp contrast in Specific Gravity (SG) between mineral grit and putrescible organic wastewater solids:

$\text{Specific Gravity of Silica Grit} \approx 2.65$ $\text{Specific Gravity of Putrescible Organics} \approx 1.02 - 1.06$ $\text{Specific Gravity of Water} = 1.00$

According to Stokes' Law, the terminal gravity settling velocity ($v_s$) of a spherical particle in laminar flow is directly proportional to the difference between particle density and water density:

$v_s = \frac{g (\rho_p - \rho_w) d^2}{18 \mu}$

Because silica sand has a net density difference ($2.65 - 1.00 = 1.65$) approximately 30 to 50 times greater than that of organic sewage solids ($1.04 - 1.00 = 0.04$), hydraulic velocity can be precision-controlled to settle grit particles ($> 0.2 \text{ mm}$ or 65 mesh) while maintaining lighter organic solids in suspension.

+-----------------------------------------------------------------------------+
|                        GRIT SEPARATION MECHANICS                            |
+-------------------------------------+---------------------------------------+
|      SILICA GRIT PARTICLES          |        PUTRESCIBLE ORGANICS           |
|  - Specific Gravity: 2.65           |  - Specific Gravity: 1.02 - 1.06      |
|  - High Settling Velocity           |  - Low Settling Velocity              |
|  - Settles at 1.0 ft/s Channel Vel  |  - Remains Suspended at 1.0 ft/s      |
|  - Inert, Abrasive, Mineral         |  - Biodegradable Fecal / Food Solids  |
+-------------------------------------+---------------------------------------+

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Grit Removal Technologies

1. Horizontal-Flow Velocity-Controlled Chambers

Horizontal grit channels rely on maintaining an exact forward horizontal flow velocity of $0.8 \text{ to } 1.2 \text{ ft/s}$ ($0.24 \text{ to } 0.37 \text{ m/s}$), with an optimal design target of $1.0 \text{ ft/s}$ ($0.30 \text{ m/s}$):

• If velocity drops below $0.8 \text{ ft/s}$: Heavy putrescible organic matter settles with the grit, generating intense septic odors and causing the grit to fail landfill disposal standards.
• If velocity exceeds $1.2 \text{ ft/s}$: Fine grit particles ($0.20 \text{ mm}$ and smaller) are scoured off the channel floor and carried over into downstream clarifiers.
• Velocity Control Devices: In rectangular channels, flow cross-sectional area changes linearly with depth, whereas channel flow increases non-linearly. To maintain constant $1.0 \text{ ft/s}$ velocity across all flow regimes, utilities install a proportional Sutro weir at the channel outlet. The Sutro weir features curved lateral edges engineered so that the discharge head is linearly proportional to the volumetric flow rate. Alternatively, a downstream Parshall flume is sized to throttle channel water depth identically to flow changes.

2. Aerated Grit Chambers

Aerated grit chambers use fine or coarse-bubble diffusers mounted along one sidewall, approximately $0.5 \text{ to } 0.9 \text{ m}$ ($1.5 \text{ to } 3 \text{ ft}$) above the floor. The rising air column establishes a transverse spiral or helical roll flow pattern throughout the tank:

• Aeration Rates: Air supply is regulated between $3 \text{ to } 8 \text{ cfm per linear foot}$ of chamber length ($0.3 \text{ to } 0.8 \text{ m}^3/\text{min}\cdot\text{m}$). This airflow imparts a bottom scouring velocity of approximately $1.0 \text{ ft/s}$ across the tank floor.
• Hydraulic Action: Heavy grit particles, having high settling velocity, overcome the upward fluid vector and drop into a longitudinal collection hopper beneath the air diffusers. Lighter organic flocs are swept upward by the roll pattern and held in suspension until discharge.
• Process Advantages: Aeration strips dissolved hydrogen sulfide ($\text{H}_2\text{S}$), eliminating noxious odors and freshening stale, septic wastewater before primary clarification.

3. Vortex Grit Separators (Mechanically Induced & Tangential)

Vortex units (such as Pista or Tea-Cup designs) introduce wastewater tangentially into a cylindrical basin with a sloped hopper bottom. A central motorized paddle impeller rotates in the direction of flow at $15-25 \text{ rpm}$:

• Centrifugal forces drive heavy grit outward toward the vessel wall, where boundary layer friction causes it to spiral downward into a center collection hopper.
• Clean liquor and suspended organic solids spill upward over an internal circumferential weir launder.
• Settled grit slurry is pumped via air-lift or recessed-impeller vortex pumps to an elevated cyclone degritter (hydrocyclone). Centrifugal slurry acceleration in the hydrocyclone separates fine grit from water, discharging the grit cake into an inclined screw or rake classifier that washes organics back to the plant headworks and delivers dry grit ($> 75\%$ solids) to a disposal bin.

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Open Channel Flow Measurement

Accurate influent flow measurement is legally mandated under Oregon DEQ National Pollutant Discharge Elimination System (NPDES) permits. Flow data governs chemical dosing, hydraulic detention calculations, and mass pollutant reporting.

Primary Measurement Elements

Primary elements are hydraulic structures placed directly within the open channel that produce a predictable, calibrated relationship between upstream liquid depth and volumetric flow rate ($Q$).

+-----------------------------------------------------------------------------+
|                      PARSHALL FLUME HYDRAULIC PROFILE                       |
+-----------------------------------------------------------------------------+
|                                 Throat                                      |
|        Converging Section       Section         Diverging Section           |
|    =======================\                  /=======================       |
|                            \       W        /                               |
|     Flow Direction --->     \              /                                |
|    =======================/                  \=======================       |
|                                                                             |
|             * Ha Sensor                                                     |
|            (2/3 distance                                                    |
|             from throat)                                                    |
|  ~~~~~~~~~~|~~~~~~~~~~~~~~~~                                                |
|            |                \   Hb Sensor                                   |
|                              \~~~~~~~~~~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~      |
|                                             |                               |
+-----------------------------------------------------------------------------+

1. Parshall Flumes

The Parshall flume is the most ubiquitous open-channel measuring structure in wastewater engineering. It comprises three continuous sections:

1. A converging inlet section with a level floor that accelerates oncoming flow;
2. A narrow, downward-sloping throat section of width $W$ where flow passes from subcritical to supercritical velocity;
3. A diverging outlet section with an upward-sloping floor that decelerates flow and recovers hydraulic head.

• Upstream Head ($H_a$) Location: The primary level measurement point ($H_a$) is located in the converging section at a distance equal to two-thirds of the total converging wall length upstream from the throat crest.
• Free-Flow Discharge Formula: Under free-flow conditions, discharge is governed strictly by the upstream depth $H_a$ according to the empirical relationship:

$Q = C \cdot W \cdot H_a^n$

Where $Q$ is flow in cfs, $W$ is throat width in feet, $H_a$ is upstream head in feet, and $C$ and $n$ are flume-specific empirical constants ($n \approx 1.522 \cdot W^{0.026}$).

2. Palmer-Bowlus Flumes

Palmer-Bowlus flumes are designed for rapid retrofitting directly into existing circular sewer pipes and round manholes. The flume constricts the channel laterally while raising the invert slightly with a trapezoidal throat bump. Palmer-Bowlus flumes create minimal head loss (roughly one-fourth that of a comparable weir) and feature smooth transitions that prevent the deposition of rags, grit, and stringy solids.

3. Sharp-Crested Weirs

Weirs are vertical plates placed across an open channel where water cascades over a notched crest:

• V-Notch (Triangular) Weirs ($60^\circ, 90^\circ, 120^\circ$): Highly accurate at low flows because cross-sectional discharge area expands geometrically with head ($Q \propto H^{2.5}$). Primarily applied at plant effluent outfalls and tertiary filtration discharges.
• Rectangular Weirs: Employed for broad flow ranges ($Q \propto H^{1.5}$). May be suppressed (extending across the full channel width) or contracted (notched inward from sidewalls).
• Head Measurement Point: Weir head must always be sensed at a distance of three to four times the maximum anticipated head ($3-4 \times H_{\max}$) upstream of the weir crest to avoid the hydraulic drawdown curve occurring immediately above the crest.

Secondary Measurement Elements

Secondary elements measure liquid depth and convert that head into volumetric flow units via digital transmitters:

• Ultrasonic Transducers: Mounted non-contact above the liquid surface. Transmits high-frequency sound pulses and measures transit time of the reflected echo. Must incorporate integral temperature compensation sensors because sound speed in air varies by $0.6 \text{ m/s per } ^\circ\text{C}$. Transducers have an inherent "blanking distance" ($0.3 \text{ to } 0.6 \text{ m}$ or $1 \text{ to } 2 \text{ ft}$) beneath the sensor face where echoes cannot be resolved.
• Submersible Pressure Transducers: Piezoresistive diaphragm sensors submerged at channel inverts that measure hydrostatic head. Prone to signal drift from grease accumulation, silt blankets, and rag entanglement.
• Bubbler Level Detectors: Inject a continuous, metered purge of compressed air or dry nitrogen through a submerged dip tube. The pneumatic backpressure required to discharge an air bubble corresponds precisely to the hydrostatic liquid head above the tube orifice. Bubbler systems are immune to surface foam, steam, and wind disturbances.

Free-Flow vs. Submerged Flow in Parshall Flumes

When downstream downstream hydraulic restrictions or high tide/river stages back water into the flume's diverging section, supercritical flow through the throat is drowned out. This transition is quantified by the Submergence Ratio ($S$):

$S = \frac{H_b}{H_a}$

Where $H_a$ is upstream head and $H_b$ is the head measured at the throat/diverging section transition:

• Submergence Thresholds:
  • Flumes with throat width $1 \text{ to } 3 \text{ inches}$: Submergence occurs when $S > 50\%$
  • Flumes with throat width $6 \text{ to } 9 \text{ inches}$: Submergence occurs when $S > 60\%$
  • Flumes with throat width $1 \text{ to } 8 \text{ feet}$: Submergence occurs when $S > 70\%$
  • Flumes with throat width $10 \text{ to } 50 \text{ feet}$: Submergence occurs when $S > 80\%$

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Preliminary Treatment Operations Summary Matrix