4.1 Wastewater Characteristics, Screening, Shredding & Grit Removal

Wastewater Characteristics & Preliminary Treatment Mechanics

Preliminary treatment constitutes the critical first line of defense in a wastewater treatment facility. Before wastewater can undergo physical clarification or biological stabilization, large debris, rags, heavy inorganic particles, and abrasive mineral solids must be intercepted and removed. Failure in preliminary operations inevitably results in catastrophic downstream failures, including abraded pump impellers, clogged suction headers, blinded aeration diffusers, and silted anaerobic digesters.

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1. Municipal Wastewater Composition & Matrix Characteristics

Untreated municipal wastewater is overwhelmingly liquid. By weight, municipal wastewater is composed of approximately 99.9% water and only 0.1% total solids (approximately 1,000 mg/L of total matter). Despite this small percentage, the physical fractionation and biochemical characteristics of this 0.1% determine the entire operational strategy of the facility.

Physical and Chemical Fractionation of Solids

Wastewater solids are categorized by physical separation behavior and chemical volatility:

1. Total Solids (TS): All matter remaining after evaporating a wastewater sample to dryness at 103°C to 105°C.
2. Total Dissolved Solids (TDS): Solids that pass through a standard 1.5 $\mu\text{m}$ glass-fiber filter disc. Dissolved solids consist primarily of soluble organic compounds, minerals, hardness ions (calcium, magnesium), sulfates, and chlorides.
3. Total Suspended Solids (TSS): Particulate matter retained on a standard 1.5 $\mu\text{m}$ glass-fiber filter disc dried at 103°C to 105°C. TSS includes both settleable and colloidal matter.
4. Settleable Solids: The volumetric portion of suspended solids that settles out of suspension under quiescent conditions in an Imhoff cone over a 60-minute settling period, recorded in milliliters per liter (mL/L).
5. Colloidal Solids: Non-settleable particles ranging from 0.001 to 1 $\mu\text{m}$ in diameter that carry negative surface electrostatic charges, preventing natural gravity agglomeration without chemical coagulation.
6. Volatile Solids (VS) vs. Fixed Solids (FS): When dried solids are ignited at 550°C $\pm$ 50°C in a muffle furnace, the organic carbonaceous fraction volatilizes into gas (Volatile Solids), while the inorganic mineral ash remains behind (Fixed Solids). Typically, 65% to 80% of raw wastewater TSS is volatile (organic).

Typical Domestic Raw Wastewater Baseline Parameters

In standard municipal collection systems without substantial industrial contributions, untreated wastewater exhibits predictable baseline concentrations:

Domestic vs. Industrial Influent Variations

Domestic wastewater exhibits predictable diurnal variations characterized by two distinct hydraulic and organic load peaks: an early morning peak (7:00 AM to 10:00 AM) as households awaken and prepare for the day, and a secondary evening peak (6:00 PM to 9:00 PM). Minimum flows and organic loadings occur between 2:00 AM and 5:00 AM.

Industrial discharges disrupt these diurnal patterns by introducing severe shock loads:

• Food and Beverage Processing: Introduces extreme soluble organic loads with $BOD_5$ concentrations exceeding 2,000 to 5,000 mg/L, high grease fractions, and rapid acidification that depresses influent pH below 5.5.
• Metal Finishing and Plating: Discharges heavy metals (chromium, copper, nickel, zinc) and cyanides that exhibit severe biocidal toxicity toward downstream activated sludge microorganisms.
• Commercial Laundries and Cleaning Facilities: Introduces synthetic surfactants, elevated pH spikes (>10.0), and massive pulses of non-biodegradable synthetic lint.
• Inflow and Infiltration (I&I): Wet-weather stormwater ingress through leaking manholes, cracked collection gravity mains, and illicit sump connections dramatically dilutes $BOD_5$ and TSS while multiplying hydraulic throughput by 300% to 500%.

2. Coarse Screening: Trash Racks and Bar Screens

Screening physically intercepts and removes coarse floating and suspended debris from the wastewater stream. This protects mechanical assets such as raw sewage lift pumps, valves, flow meters, and pipe elbows from physical obstruction, abrasion, and catastrophic rag binding.

Trash Racks vs. Bar Screens

• Trash Racks: Heavy-duty structural steel grates featuring wide clear openings of 2 to 6 inches (50 to 150 mm) installed upstream of raw wastewater wet wells or stormwater diversion structures. They intercept large, damaging objects such as tree limbs, lumber, tires, and automotive parts.
• Bar Screens (Bar Racks): Parallel steel bars set at an angle across the influent channel to intercept rags, plastics, feminine hygiene products, wet wipes, and paper debris.
  • Coarse Screens: Clear openings between bars range from 1.5 to 2.5 inches (38 to 64 mm).
  • Medium/Fine Screens: Clear openings between bars range from 0.25 to 1.0 inch (6 to 25 mm). Modern automated facilities standardly deploy 0.25-inch to 0.5-inch fine bar screens.

Manually Cleaned vs. Mechanically Cleaned Screens

• Manually Cleaned Bar Screens: Placed at an incline of 30° to 45° from the horizontal to facilitate manual raking by plant operators into a perforated drain pan. Because manual raking is labor-intensive and subject to erratic cleaning schedules, blinding occurs rapidly during storm events, causing channel surcharging and upstream sewer backups. Manually raked screens are restricted to small plants (<0.5 MGD) or emergency bypass channels.
• Mechanically Cleaned Bar Screens: Positioned steeply at 60° to 85° from the horizontal. Automated motorized rakes continuously or intermittently clean the screen bars. Control systems activate mechanical rakes via two primary mechanisms:
  1. Differential Water Level (Head Loss): Ultrasonic level sensors measure the liquid head before and after the screen. When debris accumulates and creates a differential head loss of 2 to 6 inches (0.05 to 0.15 m), the mechanical rake initiates a cleaning cycle.
  2. Programmable Timer Override: An automated clock timer triggers raking every 10 to 30 minutes, preventing heavy debris buildup during periods when differential level sensors fail or when blinding happens uniformly.

Channel Velocity Hydraulics

Channel velocity is the paramount design and operational control parameter for bar screens:

$\text{Target Channel Approach Velocity} = 1.25 \text{ to } 3.0 \text{ ft/s (0.38 to 0.91 m/s)}$

• Minimum Velocity Limit (1.25 fps): If wastewater velocity in the approach channel drops below 1.25 fps (or 1.0 fps under absolute minimum low flow), heavy organic matter and grit settle out onto the channel invert upstream of the screen, creating putrefying septic deposits and hydrogen sulfide ($H_2S$) odors.
• Maximum Velocity Limit (3.0 fps): If velocity exceeds 3.0 fps during peak storm flows, the high hydraulic kinetic energy forces pliable debris, rags, and plastics directly through the narrow bar openings (known as "rag extrusion"), while densely compacting screenings against the bars and causing rapid channel flooding.

Screenings Handling and Disposal

Raw screenings contain foul-smelling, putrescible organic matter, pathogenic bacteria, and high moisture content (80% to 90% water). Handling involves:

1. Screenings Wash Press: Discharged screenings drop into a wash hopper where pressurized non-potable plant water (Utility Water 2 / W2) washes fecal matter and soluble organics off the rags and flushes them back into the active wastewater channel.
2. Screw Compaction & Dewatering: An internal auger conveys washed solids through a perforated cylinder, squeezing free water out and compressing the cake. This reduces screenings total volume by 50% to 75% and total weight by up to 60%.
3. Bagging & Landfill Disposal: Compacted screenings are fed into a continuous polyethylene bagging system or enclosed dumpster to contain flies, vermin, and odors, and are ultimately transported to a permitted municipal solid waste landfill. Screenings are never sent to anaerobic digesters.

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3. Shredding & Grinding Devices: Comminutors and Macerators

Comminutors and barminutors are in-line cutting devices with high-speed rotating cutter teeth and stationary shear bars. Rather than removing coarse solids from the stream, comminutors shred rags, plastics, and solids into fine slurries (typically 0.25 to 0.375 inch in size) that remain in the wastewater.

The Operational Hazards of Comminution

While comminutors eliminate the immediate operational burden of handling and hauling wet screenings, extensive utility operational experience has revealed severe downstream drawbacks:

• Re-Weaving of Fibers: Shredded textile fibers, synthetic hair, and non-biodegradable polyester wet wipes do not degrade biologically. Downstream in aeration basins and clarifiers, these shredded fibers physically re-weave and ball together into tight ropes.
• Equipment Fouling: Re-woven rag ropes wrap around raw sludge pump impellers, choke progressive cavity stator cavities, bind mechanical surface aerators, and foul dissolved oxygen (DO) sensor membranes.
• Aeration Diffuser Blinding: Shredded plastics and fibrous fuzz coat fine-bubble membrane diffusers, drastically increasing aeration blower discharge backpressure and reducing oxygen transfer efficiency (OTE).
• Digester Blanket Formation: Light, synthetic shredded plastics float in anaerobic digesters, binding with rising gas bubbles and grease to form an impenetrable crust blanket up to several feet thick that restricts biogas release and requires costly manual tank cleaning.

Because of these liabilities, modern wastewater engineering practice strongly favors fine mechanical screening and complete physical extraction over comminution.

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4. Grit Removal Systems

Grit consists of heavy, dense, inorganic particulate matter including sand, gravel, cinders, small stones, eggshells, coffee grounds, bone fragments, and metallic scale.

Physical Properties of Grit

Unlike organic wastewater solids which exhibit a specific gravity close to water ($S_g = 1.01 \text{ to } 1.05$), grit particles have a specific gravity of approximately $S_g = 2.65$ (silica sand). Grit particles are characterized as non-putrescible particles with nominal grain diameters equal to or greater than 0.2 mm (200 $\mu\text{m}$, or 65-mesh).

Failure to remove grit results in:

1. Severe abrasive erosion of metal impellers, volutes, and mechanical seals in centrifugal sludge pumps.
2. Accumulation of immovable sand and silt deposits in the dead zones of aeration basins, reducing effective hydraulic retention time.
3. Massive silting of anaerobic digesters, where heavy grit displaces up to 30% to 50% of the active digestion volume, necessitating expensive physical cleanouts.

Grit Removal Technologies

The Critical 1.0 fps Velocity Rule in Horizontal Chambers

In horizontal-flow grit channels, maintaining a velocity of about 1.0 fps (0.30 m/s) is the golden rule of wastewater operations:

• If velocity exceeds 1.0 fps (e.g., >1.2 fps): The high forward velocity scours settled grit particles from the channel floor, carrying abrasive silica sand out of the chamber and into downstream primary clarifiers and sludge pipelines.
• If velocity drops below 1.0 fps (e.g., <0.8 fps): Lighter organic fecal solids and food particles lose suspension and settle along with the grit. This produces "dirty, organic grit" that rapidly turns anaerobic, producing putrid septic odors, attracting swarms of flies, and rendering the grit unsuitable for clean municipal landfill disposal.

To hold the velocity near 1.0 fps regardless of wide fluctuations in diurnal wastewater inflow, channels are engineered with specific hydraulic control devices at their discharge end, most notably Sutro (proportional) weirs (whose discharge is directly proportional to head) or downstream Parshall flumes matched to parabolic channel invert geometries.

Grit Classification and Dewatering

Grit collected in sumps is extracted as a thin slurry (1% to 3% solids) via air-lift pumps, recessed impeller torque-flow pumps, or progressive cavity pumps. The slurry is pumped directly to a hydrocyclone (cyclone separator). Centrifugal force drives dense grit to the cyclone walls, where it discharges from the bottom underflow nozzle into an inclined screw or reciprocating rake grit classifier.

The classifier gently conveys the sand up an inclined trough, allowing water and residual light organic solids to drain back into the process basin. The discharged dewatered grit contains less than 15% to 20% organic matter and exceeds 75% to 80% dry solids, rendering it stable, odorless, and suitable for permitted municipal landfill disposal.