7.1 Wastewater Collection & Preliminary Operations

4.1 Wastewater Collection & Preliminary Operations

The primary function of a municipal wastewater treatment plant (WWTP) is to protect public health and receiving water bodies by removing physical, chemical, and biological pollutants from raw influent. Before wastewater can undergo advanced biological or chemical treatment, it must pass through preliminary treatment processes. Preliminary operations are designed to measure flow, remove coarse solids, shred large debris, and eliminate inorganic grit. Protecting downstream equipment—such as centrifugal pumps, aeration diffusers, mechanical clarifiers, and anaerobic digesters—from severe abrasion, clogging, and mechanical damage is the principal goal of preliminary unit operations.

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1. Influent Wastewater Characteristics & Physical Parameters

Raw municipal wastewater is a complex mixture of domestic sanitary discharge, commercial waste, industrial effluents, and storm water or groundwater infiltration. Understanding influent physical characteristics enables operators to anticipate treatment challenges and optimize preliminary unit operations.

Key Physical Parameters:

• Temperature: Municipal wastewater temperatures typically range between 55°F and 75°F (13°C to 24°C), depending on season and geographic location. Temperature directly impacts biological reaction rates, gas solubility, and fluid viscosity, which affects particle settling velocity in grit chambers and clarifiers.
• Color: Fresh domestic wastewater exhibits a light grey or brownish color. As wastewater travels through lengthy collection networks under anaerobic conditions, sulfate-reducing bacteria generate sulfides, turning the wastewater dark grey or black. Black wastewater indicates septicity.
• Odor: Fresh wastewater possesses a distinct, musty odor. Anaerobic decomposition in septic sewage releases volatile compounds, predominantly hydrogen sulfide ($H_2S$)—characterized by its toxic, "rotten egg" smell—along with mercaptans, indoles, and skatoles. $H_2S$ poses severe atmospheric hazards, causes crown corrosion in concrete sewer pipes, and demands odor control scrubber systems at plant headworks.

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2. Wastewater Total Solids (TS) Breakdown

Solids in wastewater are classified based on their physical state (suspended vs. dissolved) and chemical composition (organic/volatile vs. inorganic/fixed). Accurate solids analytical testing is vital for process control and regulatory compliance under South Carolina Department of Environmental Services (DES) standards.

Laboratory Calculation Worked Example:

An operator conducts a solids test on a 100 mL raw influent wastewater sample:

• Clean glass fiber filter tare weight = $1.2500\text{ g}$
• Mass of filter + sample dried at 105°C = $1.2800\text{ g}$
• Mass of filter + residue ignited at 550°C = $1.2560\text{ g}$

$\text{TSS (mg/L)} = \frac{(1.2800\text{ g} - 1.2500\text{ g}) \times 1,000,000}{100\text{ mL}} = \frac{0.0300 \times 1,000,000}{100} = 300\text{ mg/L}$

$\text{FSS (mg/L)} = \frac{(1.2560\text{ g} - 1.2500\text{ g}) \times 1,000,000}{100\text{ mL}} = \frac{0.0060 \times 1,000,000}{100} = 60\text{ mg/L}$

$\text{VSS (mg/L)} = \text{TSS} - \text{FSS} = 300\text{ mg/L} - 60\text{ mg/L} = 240\text{ mg/L}$

$\text{VSS Percentage (\%)} = \left( \frac{240\text{ mg/L}}{300\text{ mg/L}} \right) \times 100\% = 80\%$

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3. Coarse Screening, Fine Screening & Comminution

Screening is the first unit operation at headworks, removing large floating and suspended debris (rags, wood, plastic containers, wipes) that cause pump impellers to bind.

Coarse Bar Screens:

• Bar Spacing: Coarse screens feature parallel steel bars spaced 0.5 to 2.0 inches (12.7 to 50.8 mm) apart.
• Manual vs. Mechanical:
  • Manually Raked Screens: Set at an angle of 30° to 45° to horizontal to facilitate hand raking. Common in smaller facilities or backup channels.
  • Mechanically Raked Screens: Set at 60° to 90° to horizontal. Driven by automatic timer cycles or differential level sensors. Headloss across the screen triggers automatic rake cycles when differential water depth exceeds 2 to 6 inches.
• Headloss Limits: Excessive headloss causes upstream channel backing, leading to septic solids deposition and potential channel overflow. Max allowable headloss before raking is typically 6 inches for manual and 12 inches for mechanical units.

Fine Screens & Comminutors:

• Fine Screens: Utilize wire mesh or perforated plates with openings of 0.06 to 0.25 inches (1.5 to 6 mm). Fine screens achieve higher TSS removal (5% to 15%) but require continuous mechanical cleaning and high-pressure water spray systems to prevent blinding.
• Comminutors & In-Channel Grinders: Rotating cutter drums placed directly in the influent channel that chop rags and large solids into small, uniform pieces ($\frac{1}{4}$ to $\frac{3}{8}$ inch) without removing them from the flow stream. Shredded debris flows to primary clarifiers. Exam Tip: Upstream grit removal is essential prior to comminution to prevent rapid dulling and destruction of rotating cutter teeth.

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4. Grit Removal Systems & Velocity Control

Grit consists of heavy, inorganic mineral particles such as sand, gravel, cinders, coffee grounds, eggshells, and fruit seeds. Grit particles possess a specific gravity of approximately 2.65 (compared to organic solids at 1.01 to 1.05) and nominal particle size $\ge 0.2\text{ mm}$ (65 mesh). If allowed into downstream basins, grit causes severe pump impeller abrasion, fills aeration basins and anaerobic digesters, and clogs sludge piping.

Grit Chamber Technologies:

1. Velocity-Controlled Horizontal Channel Grit Chambers:

   • Rectangular gravity channels designed to maintain a strict flow velocity of 1.0 ft/sec (0.3 m/s) (acceptable operational range: 0.7 to 1.2 ft/sec).
   • At 1.0 ft/sec, heavy inorganic grit settles to the channel floor while lighter organic matter remains in suspension and passes through.
   • Velocity control is maintained across varying flow rates using specialized channel geometry (trapezoidal channels) or outlet hydraulic structures such as Proportional Weirs or Parshall Flumes.
   • Velocity Calculation: $V = \frac{Q}{A}$, where $V = \text{velocity (ft/s)}$, $Q = \text{flow (cfs)}$, $A = \text{cross-sectional area (sq ft)}$.

2. Aerated Grit Chambers:

   • Rectangular tanks where diffused air is injected along one side near the bottom, creating a spiral or roll flow pattern perpendicular to the forward flow.
   • Air flow rate controls roll velocity. Heavy grit particles are driven outward by centrifugal force and settle into a bottom hopper, while lighter organic solids remain suspended in the spiral roll.
   • Advantage: Aeration strips volatile gases ($H_2S$), freshens septic wastewater, and allows washing of organic material from settled grit.

3. Vortex Grit Separators & Hydrocyclones:

   • Mechanically induced vortex chambers (e.g., Pista grit systems) or liquid hydrocyclones. Tangential influent entry creates a vortex; centrifugal force separates heavy grit to the outer wall and floor, while fluid lifts organic solids out through a top central weir. Compact footprint with minimal headloss.

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5. Screenings & Grit Quantities, Dewatering & Disposal

Proper handling and disposal of preliminary residuals are regulated strictly under EPA Title 40 CFR Part 503 and South Carolina DES solid waste management rules.

• Quantities Produced:
  • Coarse Screenings: 0.5 to 5.0 cubic feet per Million Gallons (cu ft/MG) of raw wastewater.
  • Grit Generation: 1.0 to 10.0 cu ft/MG (higher in combined sewer systems during storm events).
• Residual Handling: Screenings contain 80%–90% water content and high concentrations of pathogenic bacteria. Screenings are compacted using hydraulic or screw presses to reduce moisture and volume. Grit collected from hoppers is pumped as a slurry to grit washers/classifiers (screw or rake classifiers) to separate residual putrescible organic matter and dewater the grit.
• Disposal Mandates: Unwashed screenings and raw grit cannot be land-applied or incinerated in open pits. Under SC DES rules, dewatered screenings and washed grit must be transported in covered containers to a permitted municipal solid waste sanitary landfill. The material must pass the EPA Paint Filter Liquids Test (Method 9095B) to verify zero free liquids prior to landfill acceptance.