3.1 Preliminary Treatment & Primary Clarification

3.1 Preliminary Treatment & Primary Clarification

Raw Wastewater Characteristics & Influent Dynamics

Municipal wastewater represents a complex, dynamic mixture of water (greater than 99.9% by weight) and suspended, colloidal, and dissolved solids (less than 0.1%). In Virginia municipal systems, typical raw domestic wastewater characteristics exhibit average concentrations of:

• Biochemical Oxygen Demand ($\text{BOD}_5$): 150–250 mg/L
• Total Suspended Solids (TSS): 150–300 mg/L
• Total Kjeldahl Nitrogen (TKN): 25–45 mg/L (as N)
• Total Phosphorus (TP): 4–8 mg/L (as P)
• Fats, Oil, and Grease (FOG): 50–100 mg/L
• pH: 6.5–8.0

Diurnal Flow Fluctuations

Wastewater treatment facilities experience predictable diurnal flow patterns. Peak influent flows and organic loadings typically occur twice daily: a primary morning surge between 07:00 and 10:00 as residential domestic water use peaks, and a secondary evening peak between 18:00 and 21:00. Minimum flows occur during overnight hours (02:00 to 05:00). In combined sewer systems or separate sanitary sewers subject to Inflow and Infiltration (I&I), precipitation events can cause sustained peak flows that exceed dry-weather design averages by 200% to 500%. Preliminary and primary treatment units must absorb these hydraulic surges without passing debris or losing settleable solids to downstream biological stages.

Industrial Pretreatment & Prohibitions

Under EPA General Pretreatment Regulations (40 CFR Part 403) and Virginia Department of Environmental Quality (DEQ) VPDES pretreatment mandates, industrial discharges into Publicly Owned Treatment Works (POTWs) are strictly regulated. Specific prohibited discharges include:

1. Flammable or Explosive Substances: Closed-cup flashpoint below 140°F (60°C), such as gasoline, kerosene, or industrial solvents, which pose severe fire and explosion risks in sewer lines and headworks channels.
2. Corrosive Wastes: Any discharge with a pH below 5.0 (causing structural corrosion of concrete channels and steel mechanisms) or above 12.5 (unless the facility design accommodates extreme alkaline buffering).
3. Solid or Viscous Obstructions: Heavy greases, waxes, slaughterhouse offal, or uncomminuted food wastes that accumulate and cause line blockages.
4. High-Temperature Discharges: Discharges exceeding 104°F (40°C) at the plant headworks, which accelerate biological decomposition, generate toxic hydrogen sulfide ($\text{H}_2\text{S}$), and inhibit nitrification microorganisms.
5. Toxic Gases & Vapors: Atmospheric releases of hydrogen sulfide, sulfur dioxide, or cyanide capable of causing acute worker health hazards.

---

Preliminary Treatment Operations

Preliminary treatment is the first line of defense in a wastewater facility. Its primary objective is to remove or reduce coarse debris, rags, plastics, sand, gravel, and heavy inorganic matter before raw wastewater reaches pumps, pipes, aeration diffusers, and mechanical digesters.

Raw Wastewater Influent 
       │
       ▼
┌────────────────────────────────────────┐
│ Coarse Bar Screening (1.5–3.0 ft/s)    │ ──► Screenings to Washer/Compactor & Landfill
└────────────────────────────────────────┘
       │
       ▼
┌────────────────────────────────────────┐
│ Grit Removal Chambers (1.0 ft/s)       │ ──► Grit to Hydrocyclone/Classifier & Disposal
└────────────────────────────────────────┘
       │
       ▼
┌────────────────────────────────────────┐
│ Comminution / Fine Screening (Optional)│
└────────────────────────────────────────┘
       │
       ▼
 Clarified Wastewater to Primary Clarifiers

Screening Systems & Hydraulics

Screening devices remove large objects (wood, branches, rags, plastic containers, hygiene products) that could clog suction lines, bind centrifugal impellers, or foul sensor probes.

Approach Velocity Principles

The velocity of wastewater entering and flowing through a bar screen channel must be strictly maintained within an operating envelope of 1.5 to 3.0 ft/s (0.45 to 0.91 m/s):

• Lower Boundary (<1.5 ft/s): Velocity becomes insufficient to keep heavy organic and mineral solids in suspension. Sand, grit, and fecal solids settle in the approach channel upstream of the screen, creating septic anaerobic conditions and releasing foul odorous $\text{H}_2\text{S}$ gas.
• Upper Boundary (>3.0 ft/s): Excessive hydraulic velocity pushes flexible plastics, wipes, and rags through the bar openings by hydrodynamic deformation. High velocities also wedge debris tightly into the bar matrix, jamming mechanical rakes and increasing channel headloss.

Operators monitor the differential headloss ($\Delta h$) across the screen rack. A clean bar screen exhibits a headloss of 2 to 6 inches of water column. When headloss reaches 6 to 12 inches (or on an automated timer cycle), the mechanical rake system initiates to clean the bars and transport screenings to a washing compactor, where organic matter is washed back into the waste stream while compacted solids (at 40–50% dryness) are discharged to a dumpster for landfill disposal.

---

Grit Removal Systems

Grit consists of heavy, non-putrescible inorganic particles including sand, gravel, cinders, eggshells, bone chips, coffee grounds, and heavy seeds. Grit particles typically exhibit a specific gravity ($S_g$) of approximately 2.65, whereas organic wastewater solids have a specific gravity of 1.02 to 1.05. This substantial density differential allows selective hydraulic separation.

If not removed at the headworks, grit causes severe operational problems:

• Abrasive wear on pump impellers, mechanical seals, and piping elbows.
• Accumulation in aeration basins, digesters, and clarifier hoppers, reducing effective tank volume and requiring costly manual cleanouts.
• Clogging of sludge transfer lines and check valves.

Grit Chamber Technologies

                    Horizontal Velocity = 1.0 ft/s (0.3 m/s)
                    ──────────────────────────────────────►
 Wastewater Inflow  ┌──────────────────────────────────────────────┐ Wastewater Outflow
 ═════════════════► │   Organic Solids Stay Suspended (Sg = 1.02)  │ ═════════════════►
                    │                                              │ (To Clarification)
                    │     ▼    ▼    ▼    ▼    ▼    ▼    ▼          │
                    │   Grit Settles to Floor (Sg = 2.65)          │
                    └───────┬──────────────────────────────┬───────┘
                            │                              │
                            ▼                              ▼
                       Grit Hopper                    Grit Hopper

1. Velocity-Controlled Horizontal Flow Grit Channels

These rectangular channels utilize gravity sedimentation controlled at an exact horizontal velocity of 1.0 ft/s (0.30 m/s). At this velocity, particles with $S_g \ge 2.65$ and diameters $\ge 0.2\text{ mm}$ (65-mesh sand) settle to the floor within a detention time of 45 to 90 seconds. Organic solids remain in buoyant suspension. Flow velocity is maintained across variable diurnal flow rates using specialized control flumes, such as Parshall flumes or Sutro (proportional) weirs installed at the channel outlet.

2. Aerated Grit Chambers

Aerated grit chambers introduce compressed air through coarse bubble diffusers located along one sidewall of a rectangular tank. The rising air bubbles establish a continuous spiral roll (helical) hydraulic flow pattern. The rotational velocity of the roll (typically 1.5 to 2.0 ft/s at the periphery) allows heavy grit to disengage from the streamlines and settle into a collection trough beneath the diffusers, while lighter organic solids remain suspended in the core roll. Hydraulic retention time ranges from 2 to 5 minutes at peak flow.

3. Vortex Grit Separators (Pista / Hydrocyclone Type)

Vortex units introduce wastewater tangentially into a cylindrical basin with an inverted cone bottom. A mechanically driven flat-bladed impeller rotates in the center, establishing a controlled toroidal vortex. Centrifugal force and boundary layer fluid dynamics drive dense grit particles toward the center floor opening, where they drop into a lower grit storage sump, while clarified liquid spirals upward and overflows into the effluent channel.

Grit Washing, Dewatering & Disposal

Grit extracted from collection sumps contains 15% to 50% putrescible organic matter. To prevent severe odors, fly breeding, and landfill rejection, grit slurries are pumped to hydrocyclones and grit classifiers (inclined screw or reciprocating rake washers). The hydrocyclone uses centrifugal acceleration to separate grit from organics and water; the inclined screw then conveys the washed grit out of the water pool, draining moisture to produce a clean grit cake containing less than 15% volatile solids and greater than 75% dry solids.

Primary Sedimentation & Clarification

Primary clarifiers represent the second stage of physical treatment. By providing quiescent conditions with minimal hydraulic turbulence, primary sedimentation tanks allow settleable organic solids to sink to the tank floor under gravity, forming raw primary sludge, while oils, greases, and floatables rise to the surface to form scum.

                                  Scum Skimmer Arm
Influent Feed                        │
(Center Well)                        │        Effluent V-Notch Weirs & Baffles
    │                                ▼               │
    ▼                       ┌────────────────┐       ▼
┌───────┐                   │   Scum Layer   │    ┌─────┐ Clarified Primary Effluent
│       │                   └────────────────┘    │     │ ═════════════════════════►
│       │ ────►  Quiescent Settling Zone (HRT 1.5–2.5 hr)│ (To Secondary Aeration)
│       │                                         └─────┘
└───────┘                   ┌────────────────┐
                            │ Settled Sludge │
                            └────────────────┘
                                     │
                                     ▼
                             Sludge Scraper Flight
                                     │
                                     ▼
                           Bottom Sludge Hopper ──► To Primary Sludge Pump (4–7% Solids)

Clarifier Configurations & Mechanical Components

Primary clarifiers are designed in two principal geometries:

1. Circular Center-Feed Clarifiers: Wastewater enters through a central vertical riser pipe into a circular feed well (influent baffle) designed to dissipate kinetic energy and distribute flow radially outward. A rotating bridge mechanism drives bottom scraper plows that push settled sludge toward a central collection hopper, while a surface skimmer arm sweeps floating scum into a scum beach box for separate pumping.
2. Rectangular Chain-and-Flight Clarifiers: Wastewater enters through an influent distribution baffle across the tank width and travels longitudinally toward effluent launders. Motorized endless chains with wooden or fiberglass flights travel slowly along the floor (0.5 to 3.0 ft/min) dragging settled sludge into an influent sludge hopper. On the return pass along the liquid surface, the flights skim scum toward a transverse slotted scum pipe.

Core Design & Operational Parameters

Operators and regulatory engineers evaluate primary clarifier performance using three fundamental hydraulic and loading metrics:

$\text{Hydraulic Retention Time (HRT)} = \frac{\text{Tank Volume (gal)}}{\text{Influent Flow Rate (gal/day)}} \times 24\text{ hr/day}$

$\text{Surface Overflow Rate (SOR)} = \frac{\text{Influent Flow (gpd)}}{\text{Clarifier Surface Area (sq ft)}}$

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

Removal Efficiencies & Performance Dynamics

A properly designed and operated primary clarifier achieves:

• Total Suspended Solids (TSS) Removal: 50% to 70%
• Biochemical Oxygen Demand ($\text{BOD}_5$) Removal: 25% to 35%
• Fats, Oil, and Grease (FOG) Removal: 50% to 60%
• Settleable Solids Removal: 90% to 95%

Because soluble BOD cannot settle by gravity, primary sedimentation removes only the particulate carbonaceous fraction. The remaining dissolved and colloidal BOD passes forward to biological treatment.

Primary Sludge Pumping & Operational Control

Raw primary sludge settles to the floor as a dense, fibrous mass with a solids concentration of 4.0% to 7.0% total dry solids (TS). Because primary solids decompose rapidly in warm temperatures, sludge must be pumped to stabilization units (such as anaerobic digesters or gravity thickeners) on frequent, automated time cycles.

Sludge Pumping Pitfalls

• Rat-Holing (Coning): Occurs when primary sludge pumps are operated for too long or at excessively high flow rates. The pump pulls a narrow core of overlying water through the center of the sludge hopper, leaving dense sludge banked along the hopper slopes. The pumped sludge becomes thin and watery (<2% solids), overloading digesters with unnecessary water volume. Operators eliminate rat-holing by setting timers to pump frequently for short durations (e.g., 3 minutes every 30 minutes rather than 30 minutes every 5 hours).
• Septic Sludge Blankets & Gas Lifting: If sludge pumping frequency is too low, anaerobic decomposition begins on the clarifier floor. Methanogens and acid formers generate methane ($\text{CH}_4$) and carbon dioxide ($\text{CO}_2$) bubbles that attach to sludge flocs, floating large clumps of black, foul-smelling sludge to the surface ("clumping" or "boiling"), causing permit violations for effluent TSS.