8.1 Wastewater Characterization, Preliminary Screening & Primary Clarification

8.1 Wastewater Characterization, Preliminary Screening & Primary Clarification

1. Influent Wastewater Characterization & Strength Parameters

Municipal wastewater treatment plants (WWTPs), also classified as Publicly Owned Treatment Works (POTWs), receive a complex mixture of domestic sanitary discharges, commercial wastes, and permitted industrial effluents conveyed through underground collection systems. In North Carolina, understanding the baseline chemical, biological, and physical parameters of raw influent wastewater is critical for certified biological wastewater operators. It establishes the design basis for process loading, operational control, and compliance with National Pollutant Discharge Elimination System (NPDES) permits administered by the North Carolina Department of Environmental Quality (NC DEQ) Division of Water Resources (DWR).

Domestic vs. Industrial Wastewater Characteristics

• Domestic Wastewater: Generated from residential plumbing fixtures (toilets, sinks, showers, washing machines). It possesses a predictable biochemical composition with an organic carbon-to-nitrogen-to-phosphorus ratio ($BOD_5 : N : P$) near $100 : 5 : 1$, which provides an ideal balanced nutritional substrate for aerobic biological decomposition.
• Industrial Wastewater: Discharges from food processing plants, dairies, textile mills, pharmaceutical facilities, metal finishing operations, and chemical manufacturing can introduce extreme hydraulic surges, high-strength soluble organic loads, toxic heavy metals (e.g., copper, nickel, zinc, hexavalent chromium), cyanide, and extreme pH excursions (< 5.0 or > 9.0). Facilities in North Carolina receiving non-domestic waste must enforce a Pretreatment Program under 15A NCAC 02H .0900 to protect biological processes from toxic inhibition and pass-through violations.

Physical Appearance, Temperature, and pH

• Color & Turbidity: Fresh municipal wastewater is typically light brownish-gray with a soapy, cloudy appearance. As wastewater travels through lengthy collection networks with flat slopes, warm temperatures, or sluggish velocities, dissolved oxygen ($DO$) is depleted. Anaerobic, sulfate-reducing bacteria (Desulfovibrio) reduce sulfate ($SO_4^{2-}$) to hydrogen sulfide gas ($H_2S$), which reacts with iron to form ferrous sulfide ($FeS$), turning septic wastewater dark gray or pitch black.
• Odor: Fresh domestic wastewater possesses an earthy, musty, or oily odor. Septic wastewater emits a pungent, rotten-egg odor characteristic of hydrogen sulfide ($H_2S$), accompanied by volatile organic acids, mercaptans, and indoles. Hydrogen sulfide is not only an odor nuisance but also lethal at elevated concentrations (> 100 ppm paralyzes the olfactory nerve) and corrosive to concrete structures (crown corrosion via microbial oxidation to sulfuric acid, $H_2SO_4$).
• Temperature: Untreated wastewater typically ranges from 50°F to 70°F (10°C to 21°C). Seasonal variations directly dictate biological kinetic reaction rates. Bacterial metabolism roughly doubles for every 10°C (18°F) increase within mesophilic ranges (up to ~38°C / 100°F).
• pH: The normal pH range of untreated domestic wastewater is 6.5 to 8.0 standard units (SU). Deviations outside 6.0 to 8.5 signal industrial dumping, chemical spills, or excessive acid/caustic infiltration.

Diurnal Flow Variations

Collection networks experience distinct diurnal (24-hour) flow cycles influenced by human activity and residential water usage:

1. Morning Peak (7:00 AM – 10:00 AM): Characterized by a sharp increase in flow, TSS, and BOD as residents wake, bathe, and prepare food.
2. Afternoon Lull (1:00 PM – 4:00 PM): Flow stabilizes at a moderate, steady rate.
3. Evening Peak (6:00 PM – 9:00 PM): A secondary peak caused by evening cooking, dishwashing, and laundry.
4. Nocturnal Minimum (2:00 AM – 5:00 AM): Flow drops to its lowest rate. Water passing through the plant during these early morning hours represents base sanitary flow combined with groundwater Infiltration and Inflow (I/I).

Untreated Domestic Wastewater Strength Parameters

Operators must memorize the standard baseline concentrations of medium-strength domestic wastewater, as these numbers form the foundation of mass loading calculations across all NC WPCSOCC exam grades:

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2. Preliminary Treatment Unit Operations

Preliminary treatment is the first line of defense at a wastewater treatment facility. Its sole objective is to condition raw wastewater by removing coarse debris, inorganic grit, rags, plastics, and large floating objects that would otherwise clog channels, abrade pump impellers, bind mechanical aerators, and settle as unmanageable inert deposits in biological basins and anaerobic digesters.

Raw Influent Wastewater
          │
          ▼
┌──────────────────────────────────────────┐
│  Coarse Bar Screens & Trash Racks        │ ──► Large Debris (2"-6" spacing)
└──────────────────────────────────────────┘
          │
          ▼
┌──────────────────────────────────────────┐
│  Mechanically Cleaned Bar Screens        │ ──► Screenings (1/2"-1" spacing)
│  (Differential Head Loss / Timer)        │     (Washed, Compacted, Landfilled)
└──────────────────────────────────────────┘
          │
          ▼
┌──────────────────────────────────────────┐
│  Grit Removal System                     │
│  - Horizontal Channel (1.0 ft/sec)       │ ──► Inorganic Grit (Sand/Gravel/Eggshells)
│  - Aerated Chamber / Vortex Separator    │     (Dewatered, Landfilled)
└──────────────────────────────────────────┘
          │
          ▼
   To Primary Clarification

Coarse Bar Screens, Mechanically Cleaned Screens & Trash Racks

• Trash Racks: Installed at deep pump stations or major river outfalls with wide bar clear openings of 2 to 6 inches (50 to 150 mm). They intercept heavy debris, lumber, logs, and tree branches during storm events.
• Coarse Bar Screens: Formed of parallel vertical or inclined steel bars with clear openings of 1/2 inch to 1 inch (12 to 25 mm). In modern treatment facilities, mechanically cleaned bar screens operate automatically:
  • Differential Head Loss Control: Ultrasonic level transmitters or bubbler tubes measure water level upstream and downstream of the rack. When debris blinds the screen, upstream head rises. A differential head of 2 to 4 inches (50 to 100 mm) triggers a raking cycle.
  • Clock-Timer Backup: A programmable timer initiates cleaning cycles (e.g., every 15 to 30 minutes) regardless of head loss to prevent rags from binding tightly to the bars.
  • Emergency Bypass: An adjacent, manually cleaned bar rack inclined at 30° to 45° must be installed to handle overflow during mechanical breakdown or power failure.
• Fine Screens: Rotary drum or step screens with openings ranging from 0.06 to 0.25 inches (1.5 to 6 mm), frequently employed ahead of membrane bioreactors (MBRs) to capture fine hair, lint, and seeds.

Comminutors and In-Line Macerators

Comminutors and heavy-duty macerators consist of rotating cutting cylinders, cutter teeth, and stationary shear bars positioned directly in the influent channel. Rather than extracting rags, wipes, and plastics from the flow stream, comminutors shred solids into fine, uniform particles (typically 1/4 inch or smaller) to pass harmlessly through raw sewage pumps.

• Operational Trade-Offs: While comminutors eliminate the health hazards and disposal logistics of managing raw screenings, the shredded plastic, latex, and fibrous materials remain suspended in the wastewater. Downstream, these materials re-weave into rope-like rags (rag balls), blind fine bubble diffusers, and form impenetrable fibrous crusts on the surfaces of anaerobic digesters. Consequently, modern engineering practice strongly favors screen extraction and physical removal over comminution.

Screenings Management, Washing, and Disposal

Extracted screenings consist of putrescible organic matter mixed with sanitary wipes, plastics, and rags. Untreated screenings create severe odor problems and harbor pathogens and disease vectors (flies, rodents).

1. Screenings Washers/Presses: Screenings drop into a hopper equipped with wash-water sprayers that dislodge and return trapped fecal matter and soluble organics back into the wastewater flow stream.
2. Compaction: A heavy-duty screw auger dewaters and compresses the washed screenings, reducing total screenings volume by 50% to 70% and weight by 40% to 60%.
3. Bagging and Disposal: Compacted screenings are continuously fed into sealed continuous bagging cassettes or dumpsters for haul-off to an approved municipal sanitary landfill in compliance with state solid waste rules.

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3. Grit Removal Systems & Hydraulics

Nature and Hazards of Grit

Grit is defined as non-biodegradable, heavy inorganic particulate matter having a specific gravity ($SG$) of approximately 2.65 (characteristic of silica sand). Grit includes sand, gravel, cinders, silt, eggshells, bone chips, coffee grounds, and metal fragments. By contrast, organic wastewater solids have a specific gravity close to that of water ($SG \approx 1.01 \text{ to } 1.05$).

Allowing grit to enter downstream secondary processes causes severe operational damage:

• Extreme abrasive wear on raw sewage pump impellers, mechanical seals, and piping elbows.
• Accumulation in aeration basins, diminishing effective biological treatment volume.
• Settling in the dead zones of anaerobic and aerobic digesters, creating massive, compacted sandbeds that can only be removed through hazardous, expensive manual basin entry and vacuum mining.

Horizontal-Flow Velocity-Controlled Channels

Horizontal-flow grit channels exploit the difference in specific gravity between mineral grit ($SG \approx 2.65$) and organic solids ($SG \approx 1.02$). The channel is designed to maintain a horizontal flow velocity of:

$\text{Target Velocity} = 1.0 \text{ ft/sec} \quad (\text{Permissible Range: } 0.8 \text{ to } 1.2 \text{ ft/sec})$

• If velocity drops below 0.8 ft/sec: The hydraulic energy is insufficient to keep lighter organic solids in suspension. Putrescible organics settle out with the grit, creating a foul-smelling, septic sludge that fails landfill acceptance criteria.
• If velocity exceeds 1.2 ft/sec: The scouring shear force suspends fine inorganic grit (silica sand), washing it directly into downstream primary clarifiers and aeration basins.
• Hydraulic Velocity Control Devices: Because flow varies diurnally, an unconstrained rectangular channel cannot maintain a constant 1.0 ft/sec velocity at differing depths. Velocity is precisely governed by installing a Sutro (proportional) weir or downstream Parshall flume, which automatically adjusts cross-sectional discharge depth proportionally to incoming flow.

Aerated Grit Chambers

An aerated grit chamber introduces compressed air through coarse bubble diffusers mounted along one side of a rectangular basin, inducing a continuous spiral (helical) roll pattern perpendicular to the forward wastewater flow.

• The roll velocity is regulated by adjusting air flow rates (typically 3 to 8 cubic feet per minute [cfm] per linear foot of tank length).
• The circular roll velocity is tuned to approximately 1.0 to 1.5 ft/sec along the tank floor. The centrifugal force and gravitational settling allow dense grit particles to drop into a longitudinal bottom hopper, while lighter organic particles are swept upwards and carried through the chamber.
• Detention time is typically 2 to 5 minutes at peak flow.

Vortex Grit Separators

Modern facilities utilize vortex grit separators (e.g., Pista grit chambers). Wastewater enters tangentially, generating a circular vortex path. A centrally mounted, motor-driven paddle impeller rotates in the direction of flow, maintaining a stable fluid velocity regardless of influent flow variations. Centrifugal forces roll dense grit down a sloping floor into a central bottom collection sump, while buoyant organic matter rises and exits over an effluent weir. From the sump, grit is extracted by a recessed-impeller grit pump or air-lift system and sent to a hydrocyclone separator and screw classifier to wash and dewater the grit prior to landfill disposal.

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4. Primary Clarification / Sedimentation

Purpose and Unit Removal Efficiencies

Primary clarification is a physical treatment process that provides a calm, quiescent zone where settleable suspended organic solids sink to the tank floor under gravity, while floatable materials (fats, oils, grease, soaps, and plastics) rise to the water surface.

In North Carolina municipal systems, a properly designed and operated primary clarifier achieves the following baseline removal efficiencies:

$\mathbf{\text{TSS Removal: } 50\% \text{ to } 65\%}$ $\mathbf{\text{BOD}_5 \text{ Removal: } 25\% \text{ to } 35\%}$

Note: The $BOD_5$ removed in primary clarification represents insoluble, particulate matter. Soluble and colloidal $BOD_5$ cannot settle by gravity and passes directly to the secondary biological process.

Influent (100% BOD5, 100% TSS)
              │
              ▼
  ┌───────────────────────┐
  │  Primary Clarifier    │ ──► Floatable Scum (FOG, Plastics)
  │  (Gravity Settling)   │
  └───────────────────────┘
       │             │
       │             ▼
       │       Primary Sludge (3% - 6% Total Solids)
       │       - Removes 50% to 65% of TSS
       │       - Removes 25% to 35% of BOD5
       ▼
  Primary Effluent
  (70% BOD5, 40% TSS remaining)
  Sent to Secondary Aeration Basins

Basin Configurations and Mechanical Components

Circular Center-Feed Clarifiers

• Wastewater enters through an inverted central pipe into a center feed well (energy dissipating baffle), which reduces entrance velocity and distributes flow radially outward in all directions toward the perimeter.
• The tank floor slopes gently downward toward the center (slope of approximately 1 inch per foot or ~8%).
• A motor-driven rotating bridge supports scraper plows that slowly push settled primary sludge along the sloped floor into a central sludge hopper.
• Surface skimmer blades attached to the rotating bridge sweep across the surface, collecting floatable scum and depositing it into a scum box/trough.
• Clarified water exits by flowing under a scum baffle (which retains floating grease) and over adjustable V-notch (weir) plates into an effluent collection launder.

Rectangular Longitudinal Clarifiers

• Wastewater enters at one end through distribution baffles and moves horizontally along the tank length at low velocity.
• Flight-and-Chain Scrapers: Endless motor-driven conveyor chains fitted with fiberglass or wooden flights drag along the tank floor toward the influent end, pushing settled sludge into an inlet collection hopper. On their return path at the water surface, the same flights push floating scum toward a slotted pipe scum skimmer.

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5. Operational Parameters, Calculations & Troubleshooting

Hydraulic Detention Time (DT)

Hydraulic detention time represents the average time a unit volume of wastewater remains inside the settling basin. Standard design criteria specify a detention time of 1.5 to 2.5 hours under average daily flow.

$\text{Detention Time (hours)} = \frac{\text{Basin Volume (gallons)}}{\text{Influent Flow Rate (gallons/day)}} \times 24 \text{ hr/day}$

• If DT < 1.5 hours: Hydraulic velocities are too fast; settleable solids do not have sufficient time to fall to the floor, causing solids carryover over the effluent weirs.
• If DT > 2.5 to 3.0 hours: Wastewater becomes stagnant; anaerobic conditions develop, leading to septicity and floating sludge.

Surface Overflow Rate (SOR)

Surface Overflow Rate (also referred to as surface loading rate) measures the volume of wastewater applied daily per square foot of clarifier water surface area. It dictates the vertical settling velocity required for a particle to reach the tank floor before the water exits over the weirs.

$\text{SOR } (\text{gpd/ft}^2) = \frac{\text{Influent Flow Rate (gpd)}}{\text{Clarifier Surface Area } (\text{ft}^2)}$

$\text{Circular Surface Area } (\text{ft}^2) = 0.785 \times (\text{Diameter})^2 = \pi \times (\text{Radius})^2$ $\text{Rectangular Surface Area } (\text{ft}^2) = \text{Length} \times \text{Width}$

• Standard Operating Range: 600 to 1,200 gpd/ft² under average daily flow conditions (maximum allowable peak hourly rate typically 1,500 to 2,000 gpd/ft²).
• Higher overflow rates wash fine suspended solids out of the clarifier and directly into downstream aeration basins.

Weir Overflow Rate (WOR)

Weir Overflow Rate quantifies the volume of water exiting per linear foot of effluent weir per day. Excessive weir loading causes high localized approach velocities that draw settled sludge off the clarifier floor.

$\text{WOR } (\text{gpd/linear ft}) = \frac{\text{Influent Flow Rate (gpd)}}{\text{Total Active Weir Length (ft)}}$

• Standard Design Range: 10,000 to 20,000 gpd/linear foot.

Primary Sludge Withdrawal Management

Raw primary sludge is dense, dark, and contains high concentrations of settleable organic matter. Sludge total solids ($TS$) typically range from 3.0% to 6.0% solids (30,000 to 60,000 mg/L).

• Withdrawal Frequency: Primary sludge should be pumped intermittently on timed cycles (e.g., pumping for 5 to 15 minutes every 2 to 4 hours) rather than in large, infrequent batches or continuous high-volume pumping.
• Density Monitoring: Operators utilize sight glasses, in-line ultrasonic density meters, or manual grab sampling from the pump petcock to verify sludge thickness. Pumping should cease as soon as the sludge turns from thick blackish-brown to watery gray.

Operational Troubleshooting Matrix