7.1 Influent Characterization & Preliminary Screening

7.1 Influent Characterization & Preliminary Screening

[!NOTE] The First Line of Defense: Preliminary treatment represents the vital physical gatekeeper of any modern wastewater treatment facility. Operating under Pennsylvania Clean Streams Law permits and Ten States Standards (Recommended Standards for Wastewater Facilities), preliminary treatment removes coarse debris, rags, plastics, and heavy mineral solids before raw sewage reaches mechanical lift pumps, flow measurement flumes, or biological treatment processes. Flawless operation of headworks screening systems prevents catastrophic equipment failures, reduces maintenance downtime, and shields downstream secondary processes from physical blinding and abrasive wear.

Municipal wastewater treatment plants (WWTPs) receive an extremely heterogeneous and chemically complex liquid stream. Raw wastewater carries domestic sanitary sewage, commercial waste, industrial discharges, and—in combined collection systems or systems experiencing excessive Inflow and Infiltration (I&I)—urban stormwater runoff containing gravel, leaves, and street trash. Preliminary treatment isolates and extracts non-biodegradable and coarse solids without attempting to treat dissolved pollutants.

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Raw Domestic Wastewater Influent Characterization

To effectively operate and monitor preliminary treatment systems, certified wastewater operators must thoroughly understand the typical chemical and physical parameter baselines of raw domestic municipal wastewater.

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|                         Raw Domestic Municipal Wastewater Parameter Ranges                        |
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| Parameter                       | Weak Sewage     | Medium (Typical)| Strong Sewage   | Units     |
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| 5-Day Biochemical Oxygen Demand | < 150           | 150 – 250 (200) | > 300 – 400     | mg/L      |
| (BOD5)                          |                 |                 |                 |           |
| Total Suspended Solids (TSS)    | < 150           | 150 – 250 (220) | > 350 – 500     | mg/L      |
| Chemical Oxygen Demand (COD)    | < 250           | 350 – 500 (450) | > 600 – 1,000   | mg/L      |
| Total Kjeldahl Nitrogen (TKN)   | < 20            | 25 – 40 (35)    | > 50 – 85       | mg/L as N |
| Ammonia Nitrogen (NH3-N)        | < 12            | 15 – 25 (20)    | > 30 – 50       | mg/L as N |
| Total Phosphorus (TP)           | < 4             | 6 – 8 (7)       | > 10 – 15       | mg/L as P |
| pH                              | 6.5 – 6.8       | 7.0 – 7.6       | 7.8 – 8.5       | Standard  |
| Total Alkalinity                | < 100           | 150 – 250       | > 300           | mg/L CaCO3|
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1. Key Analytical Parameters

• Biochemical Oxygen Demand (BOD5): Measures the quantity of dissolved oxygen consumed by aerobic microorganisms over a 5-day incubation period at $20^\circ\text{C}$ to biochemically oxidize organic matter. Typical domestic wastewater exhibits a BOD5 between 150 and 300 mg/L (averaging approximately 200 mg/L).
• Total Suspended Solids (TSS): Quantifies non-filterable particulate matter retained on a 1.5 $\mu\text{m}$ glass fiber filter disk dried at $103^\circ\text{C}$ to $105^\circ\text{C}$. Typical domestic wastewater ranges from 150 to 350 mg/L (averaging approximately 220 mg/L).
• Chemical Oxygen Demand (COD): Quantifies the total oxygen equivalent required to chemically oxidize all organic compounds (both biodegradable and non-biodegradable) using boiling potassium dichromate ($K_2Cr_2O_7$) in strong sulfuric acid. Typical domestic COD ranges from 300 to 600 mg/L.
  • The COD:BOD Ratio: A healthy domestic wastewater exhibits a COD:BOD5 ratio between 1.5:1 and 2.5:1. A ratio substantially higher than 2.5:1 or 3.0:1 indicates the presence of toxic industrial chemicals or recalcitrant, non-biodegradable synthetic organics that resist biological degradation.
• Nitrogen Forms (TKN and NH3-N): Total Kjeldahl Nitrogen (TKN) measures total organic nitrogen plus ammonia nitrogen ($NH_3 / NH_4^+$). Domestic wastewater typically contains 20 to 50 mg/L TKN (with ammonia comprising 60% to 70% of the total). Nitrate ($NO_3^-$) and nitrite ($NO_2^-$) in raw influent are virtually zero because sanitary collection sewers are generally anaerobic.
• Total Phosphorus (TP): Originates from human metabolic waste, synthetic detergents, and food residues, averaging 4 to 10 mg/L (primarily as orthophosphate and polyphosphates).
• pH and Alkalinity: Normal domestic wastewater is buffered near neutral, with pH ranging between 6.5 and 8.5 and alkalinity between 150 and 250 mg/L as $CaCO_3$. Inflow of acidic industrial wastes ($pH < 6.0$) or alkaline caustic cleaning discharges ($pH > 9.0$) severely disrupts downstream biological treatment and requires plant neutralization.

2. Per Capita Domestic Loading Baselines

In municipal engineering and operator certification calculations, standard domestic per capita daily contribution benchmarks are used to evaluate plant loadings and identify excessive infiltration or industrial inputs: $\text{Per Capita BOD}_5 = 0.17 \text{ to } 0.22 \text{ lb/person/day (nominal } 0.20\text{ lb/cap/day)}$ $\text{Per Capita TSS} = 0.20 \text{ to } 0.25 \text{ lb/person/day (nominal } 0.22\text{ lb/cap/day)}$ $\text{Per Capita Flow Rate} = 70 \text{ to } 100 \text{ gallons/person/day (gpd/capita)}$

When calculating population equivalent (PE) from measured plant loading: $\text{Population Equivalent (PE)} = \frac{\text{Plant Influent Flow (MGD)} \times \text{BOD}_5\text{ (mg/L)} \times 8.34}{0.20\text{ lb BOD}_5\text{/person/day}}$

If measured per capita flow jumps to 200 or 300 gpd/capita while influent BOD5 and TSS drop below 80 mg/L, the collection system is experiencing severe groundwater infiltration or stormwater inflow. Conversely, if BOD5 exceeds 500 mg/L with normal per capita flow, strong industrial contributors (such as dairy processing, slaughterhouses, or breweries) are discharging into the municipal sewer.

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Screening Infrastructure: Coarse Trash Racks & Bar Screens

Screening is the preliminary unit operation that physically separates coarse, waterborne solids larger than the clear openings of a rigid metal grid or perforated plate.

1. Trash Racks

Trash racks consist of heavy steel bars or structural rail sections spaced 1.5 to 4.0 inches apart. Installed at major river crossings, raw sewage lift stations, or combined sewer interceptors, their sole purpose is intercepting logs, tree branches, boulders, automobile tires, and large metal scrap before they smash mechanical equipment.

2. Manually Cleaned Bar Screens (Bar Racks)

Manually cleaned bar screens are fabricated from flat steel bars ($0.375\text{ to }0.5\text{ inch}$ thick by $2.0\text{ to }3.0\text{ inches}$ deep) set parallel in an open rectangular channel.

• Incline Angle: Oriented at an angle of 30° to 45° from the horizontal to facilitate manual raking by plant operators using special hand-tined rakes.
• Perforated Drain Plate: Screenings raked up the bars are pulled onto a perforated metal drainage plate at the top of the channel to allow captured free water to drain back into the sewage flow before manual transfer into disposal carts.
• Emergency Bypass Channel: Every manual screen installation must feature a parallel bypass channel equipped with isolation stop logs or sluice gates. When the active screen blinds during high flows or storm events, the operator must open the bypass channel to prevent sewer backups into basements or upstream street manholes.
• Labor & Safety Hazards: Manual cleaning is physically demanding and poses severe biohazard and slip/trip/fall risks. Operators must wear puncture-resistant nitrile-coated gloves, face shields, and personal gas monitors (detecting $H_2S$, $LEL$, $O_2$, and $CO$).

3. Mechanically Cleaned Bar Screens

Modern wastewater facilities utilize mechanically raked bar screens installed at steep angles (60° to 85° from horizontal, or vertical in specialized designs) with clear bar spacings of 0.25 to 1.0 inch (6 to 25 mm).

• Front-Cleaned vs. Back-Cleaned: In front-cleaned screens, mechanical rake tines engage the bars on the upstream (wetted) face, carrying screenings up to a top discharge chute. In back-cleaned screens, the rake mechanism travels down the protected downstream side and reaches through the bars from behind, preventing submerged drive chains from tangling in incoming rags.
• Continuous Catenary & Multi-Rake Screens: Heavy-duty chains carry multiple rake tines continuously past the bar rack, ensuring continuous cleaning capacity during peak storm events.
• Automated Differential Level Control: Mechanically cleaned screens should never rely solely on fixed timer control. They are controlled by differential water level sensors (such as non-contact ultrasonic transducers, submersible pressure transducers, or bubbler tubes) measuring the water surface upstream and downstream of the rack.
  • As screenings blind the bars, downstream water level drops while upstream level rises, generating differential head loss ($\Delta H$).
  • When $\Delta H$ reaches a predetermined setpoint—typically 2 to 6 inches (50 to 150 mm)—the automated controller starts the rake drive motor.
  • Once the rack is cleaned and head loss drops below the low setpoint, the drive stops.
  • An adjustable backup timer (e.g., 15 to 30 minutes) cycles the rake periodically even under low flow conditions to prevent organic solids from packing into a dense, decaying mat.
  • High-level float switches trigger an emergency continuous-run mode and an audible supervisory alarm if head loss exceeds safe channel freeboard.

4. Head Loss Dynamics & Channel Approach Velocity

Head loss across a bar screen is governed by flow velocity, open area ratio, bar geometry, and degree of clogging:

• Approach Velocity: Flow velocity in the channel immediately upstream of the screen must be maintained between 1.25 and 2.5 ft/s (0.38 to 0.76 m/s).
  • If velocity drops below 1.25 ft/s, heavier inorganic sand and grit will settle out prematurely on the channel floor upstream of the screen, creating a maintenance nightmare.
  • If velocity exceeds 2.5 to 3.0 ft/s, coarse debris and flexible plastic sheets are forced through the bar openings by hydraulic pressure (extrusion).
• Flow Velocity Through Clean Bars: Typically designed for 2.0 to 3.0 ft/s under average design flow.
• Maximum Velocity Through Blinded Bars: Under peak hydraulic conditions with 50% screen blockage, velocity through the remaining open rack area should not exceed 3.5 to 4.0 ft/s to prevent pushing captured rags and stringy materials through the bars.

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In-Channel Solids Shredding: Comminutors & Macerating Grinders

An alternative approach to physical screenings extraction is in-channel solids reduction using comminutors or macerating channel grinders.

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|                     Screening Extraction vs. In-Channel Comminution Comparison                   |
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| Feature                     | Mechanical Bar Screens            | In-Channel Comminutors/Grinders |
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| Action                      | Extracts solids from wastewater   | Shreds solids in-stream; leaves |
|                             | stream entirely                   | solids in wastewater stream     |
| Screenings Handling         | Requires washing, compaction,     | Zero screenings handling; no    |
|                             | storage bins, and landfill hauling| dumpsters or haulage costs      |
| Downstream Pump Protection  | Excellent: Removes pump-clogging  | Good: Cuts large solids to pass |
|                             | rags, wood, and plastics          | through pump impellers          |
| Aeration Basin Impact       | Clean: Minimal ragging of fine-   | Severe: Shredded fibers reweave |
|                             | bubble membrane diffusers         | into rag balls and mat diffusers|
| Anaerobic Digester Impact   | Clean: No fibrous plastic blankets| Severe: Floating hair and fiber |
|                             | or heat exchanger clogging        | blankets require manual digout  |
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1. Comminutor Mechanics

A comminutor consists of a revolving slotted drum submerged in the sewage channel. Wastewater flows into the interior of the drum and exits through its bottom. Fixed stationary shear bars and rotating tungsten-carbide teeth shred intercepted rags, paper, and soft solids into tiny fragments ($0.25\text{ to }0.375\text{ inch}$ or smaller), allowing them to pass downstream with the liquid flow.

2. High-Torque Macerating Grinders

Modern installations utilize dual-shaft, low-speed, high-torque grinders (e.g., Muffin Monster®). Two parallel counter-rotating shafts equipped with interlocking cutter teeth rotate at unequal speeds (e.g., 40 RPM vs. 60 RPM), generating intense shearing and cutting forces capable of chewing through wood, shoes, plastic bottles, and tough fibrous textiles.

3. The Downstream Pitfall: Re-weaving and Rag Mats

While comminution eliminates screenings disposal logistics, it introduces severe downstream operational problems:

• Diffuser Fouling: In activated sludge aeration basins, shredded synthetic fibers (polyester, nylon, and non-woven wet wipes) do not settle. Instead, they catch on fine-bubble membrane diffusers, creating fibrous mats that choke air distribution and drive up blower discharge pressures.
• Impeller Rag-Balling: Downstream secondary pumps and sludge recirculation pumps experience "rag-balling," where long fibers spin around the eye of the impeller, creating massive, dense plugs that cause pump cavitation, shaft deflection, and motor overload.
• Digester Scum Blankets: In anaerobic digesters, comminuted plastic scraps, hair, and synthetic fibers float to the surface, combining with grease to form an impenetrable, rubbery scum blanket several feet thick that suppresses methane gas release and requires costly manual tank cleanouts.

Consequently, modern wastewater engineering standards strongly favor mechanical bar screens with physical solids extraction over comminutors for mainstream municipal facilities.

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Screenings Volume, Washing, Compaction & Landfill Disposal

Extracted screenings are visually repulsive, foul-smelling, and heavily contaminated with human fecal pathogens. Proper conditioning, dewatering, and disposal are critical to operator hygiene, odor control, and environmental compliance.

1. Screenings Volume Generation

The volume of screenings collected varies widely depending on collection system design, community demographics, and weather conditions:

• Separate Sanitary Sewers: Typically generate 0.5 to 3.0 cu ft per Million Gallons (cu ft/MG) of treated wastewater (averaging approximately 1.0 to 2.0 cu ft/MG).
• Combined Sewers (Sanitary + Storm): Typically produce 2.0 to 5.0 cu ft/MG during dry weather, spiking to 10.0 to 15.0+ cu ft/MG during peak wet weather first-flush storm surges.
• Fine Screens (< 6 mm): Yield significantly higher volumes, typically 5.0 to 15.0 cu ft/MG, capturing fine organic fibers, seeds, and hair.

2. Screenings Washers & Washer-Compactors

Raw screenings direct from a bar rack contain over 80% to 90% water and are heavily laden with putrescible fecal organics. Modern headworks route captured screenings immediately into a screenings washer-compactor (e.g., shaftless screw press or hydraulic ram):

• Fecal Washing Cycle: High-pressure water spray wash nozzles (utilizing non-potable plant effluent, Plant Water / 2W) vigorously agitate the screenings in a washing zone. This strips digestible organic fecal matter and soluble BOD from the rags, flushing them through a perforated drainage trough back into the raw wastewater channel.
• Compaction & Dewatering: An Archimedean conveying screw drives the washed screenings against a restricted discharge cone or spring-loaded friction plug. The intense pressure squeezes out free water, reducing screenings volume by 50% to 70% and screenings total weight by 50% to 65%.
• Final Cake Solids: Washed and compacted screenings achieve 30% to 45% dry solids (TS). The resulting dewatered plug is clean, relatively odor-free, and resembles moist, shredded textile cake.

3. Landfill Disposal & Regulatory Compliance

Under Pennsylvania DEP solid waste regulations (25 Pa. Code Chapter 273 for Municipal Waste Landfills), screenings are classified as municipal sewage-derived waste and are co-disposed in permitted sanitary landfills:

• Paint Filter Liquids Test (EPA SW-846 Method 9095B): Screenings must contain zero free liquids before a commercial landfill will accept the waste. In this test, a 100 mL (or 100 g) representative sample is placed in a standard 60-mesh conical paint filter for 5 minutes. If any liquid passes through the filter, the material fails and cannot be landfilled without further dewatering.
• Odor & Vector Attraction Control: Dewatered screenings discharge directly into covered disposal roll-off dumpsters or heavy plastic continuous bagging systems. If storage dumpsters cannot be hauled immediately, operators apply hydrated lime ($Ca(OH)_2$) across the top of the screenings to elevate pH above 12, arresting biological decay, eliminating putrid odors, and repelling flies and rodents.