6.1 Raw Wastewater Characteristics & Preliminary Systems

6.1 Raw Wastewater Characteristics & Preliminary Systems

Core Objective: Preliminary treatment represents the first physical barrier in a wastewater treatment facility. Its exclusive engineering mandate is the physical protection of downstream pumps, piping, mechanical equipment, and biological process units from abrasive wear, plugging, and inert solids deposition. Preliminary treatment provides negligible dissolved organic reduction; its success is measured by the operational reliability and longevity of downstream treatment processes.

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1. Raw Municipal Wastewater Characteristics

Municipal wastewater is a complex mixture of domestic sanitary waste, commercial discharges, industrial process water, and stormwater inflow and groundwater infiltration (I/I). Understanding the physical, chemical, and biological baseline parameters of raw wastewater is essential for licensed operators to identify process upsets, illegal industrial discharges, and wet weather infiltration surges.

Physical Characteristics

• Temperature: Typically ranges between 50°F and 70°F (10°C to 21°C), influenced by municipal water supplies, seasonal ambient weather, and industrial contributions. Temperature dictates biological kinetics, gas solubility (dissolved oxygen solubility decreases as temperature rises), and fluid viscosity (which directly impacts particle settling velocities).
• Color: Fresh raw domestic wastewater has an identifiable light brownish-gray tint. As wastewater ages in collection interceptors and enters an anaerobic (septic) state, microbial sulfate reduction produces metal sulfides, transforming the wastewater into a characteristic dark gray to black color.
• Odor: Fresh wastewater exhibits a distinctive, musty, earthy odor. Septic wastewater emits pungent, offensive odors dominated by hydrogen sulfide ($H_2S$), mercaptans, indole, and skatole, generated by anaerobic decomposition of organic sulfur compounds.
• Turbidity & Clarity: Highly turbid and opaque, containing a broad spectrum of suspended colloidal, settleable, and dissolved materials.
• Solids Distribution: Total Solids (TS) in raw wastewater comprise all matter remaining after evaporation at 103°C to 105°C, categorized into physical and chemical fractions:

$\text{Total Solids (TS)} = \text{Total Dissolved Solids (TDS)} + \text{Total Suspended Solids (TSS)}$

$\text{Total Suspended Solids (TSS)} = \text{Volatile Suspended Solids (VSS)} + \text{Fixed Suspended Solids (FSS)}$

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Chemical Constituents

Raw domestic wastewater contains organic compounds (proteins ~40%, carbohydrates ~25–50%, and fats/oils/grease ~10%) alongside inorganic nutrients and mineral salts:

• pH: Normal raw domestic wastewater ranges strictly between 6.5 and 8.5. A sudden drop below 6.0 indicates industrial acid dumping or septage discharge, which severely inhibits downstream biological nitrification and methane-producing bacteria. A pH above 9.0 indicates caustic industrial discharges.
• Biochemical Oxygen Demand ($BOD_5$): The standard regulatory parameter measuring the mass of dissolved oxygen consumed by microorganisms in 5 days at 20°C in the dark to decompose carbonaceous organic matter. Typical raw municipal range is 150 to 300 mg/L (average ~200 mg/L, or approximately 0.17 to 0.22 lb $BOD_5$/person/day).
• Chemical Oxygen Demand (COD): Measures the total oxygen required to chemically oxidize organic compounds using potassium dichromate ($K_2Cr_2O_7$) in boiling sulfuric acid. Typical municipal range is 300 to 600 mg/L. The domestic $COD:BOD_5$ ratio typically ranges from 1.5:1 to 2.5:1. A ratio exceeding 3.0:1 indicates significant industrial inputs containing non-biodegradable or toxic synthetic organic compounds.
• Total Kjeldahl Nitrogen (TKN): The sum of organic nitrogen (amino acids, urea, proteins) and ammonia nitrogen ($NH_3$ and $NH_4^+$). Typical raw wastewater concentration is 20 to 50 mg/L, with free ammonia accounting for 12 to 30 mg/L. Nitrogen exerts significant oxygen demand during biological nitrification ($4.57 \text{ lb } O_2 / \text{lb } NH_4^+\text{-N}$ oxidized).
• Total Phosphorus (TP): Ranging from 4 to 10 mg/L, occurring as orthophosphates ($PO_4^{3-}$), polyphosphates, and organically bound phosphorus originating from domestic detergents and metabolic wastes.
• Sulfates ($SO_4^{2-}$): Typically 20 to 100 mg/L. In collection interceptors where dissolved oxygen is zero, obligate anaerobic sulfate-reducing bacteria (Desulfovibrio) reduce sulfate ions to hydrogen sulfide gas ($H_2S$), which volatilizes into sewer crown headspace, dissolves in moisture droplets, and is oxidized by Thiobacillus bacteria into sulfuric acid ($H_2SO_4$), causing severe microbially induced corrosion (MIC) of concrete pipe crowns.
• Chlorides ($Cl^-$): Typically 50 to 100 mg/L above potable water baseline. In coastal New Jersey counties (Monmouth, Ocean, Atlantic, Cape May), severe tidal infiltration through submerged sewer outfalls and coastal interceptors can drive raw wastewater chlorides over 500 to 1,500+ mg/L, altering wastewater specific gravity and causing osmotic stress to biological flocs.

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Biological Characteristics & Pathogens

Raw sewage carries vast populations of non-pathogenic saprophytic microorganisms along with virulent human pathogens excreted by infected individuals or asymptomatic carriers:

1. Enteric Bacteria: Salmonella enterica (typhoid and paratyphoid fevers, gastroenteritis), Shigella dysenteriae (bacillary dysentery), Vibrio cholerae (cholera), pathogenic Escherichia coli (enterotoxigenic and enterohemorrhagic strains, e.g., O157:H7), and Campylobacter jejuni.
2. Enteric Viruses: Small (0.02 to 0.1 µm) non-enveloped particles including Poliovirus, Coxsackievirus, Echovirus, Norovirus, Rotavirus, and Hepatitis A Virus (HAV). Viruses are unaffected by screening and grit removal.
3. Protozoan Parasites: Giardia lamblia (flagellated protozoan forming thick-walled environmental cysts) and Cryptosporidium parvum (coccidian parasite forming chlorine-resistant oocysts), causing acute diarrheal illness.
4. Helminths (Parasitic Worms): Ascaris lumbricoides (human roundworm), Trichuris trichiura (whipworm), and Ancylostoma duodenale (hookworm). Helminth ova possess high specific gravity (1.15 to 1.25) and settle effectively during sedimentation, concentrating heavily in primary sludge.

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2. Preliminary Treatment Objectives

The fundamental goals of preliminary treatment are strictly physical and protective:

1. Protect Downstream Mechanical Equipment: Eliminate large debris (timbers, rocks, rags, clothing, plastics) that cause catastrophic mechanical failure, broken pump impellers, burned-out pump drive motors, and sheared shafts.
2. Prevent Pipe Clogging & Plugging: Prevent rags, wipes, and stringy fibrous debris from binding sludge withdrawal lines, suction manifolds, heat exchangers, and flow-regulating valves.
3. Prevent Abrasive Scouring: Remove dense mineral grit (sand, gravel, cinders) that erodes pump volutes, abrades progressive cavity stators, and cuts mechanical shaft seals.
4. Prevent Inactive Tank Silting: Eliminate non-biodegradable mineral grit before it enters secondary biological reactors (aeration tanks) and anaerobic digesters, where it settles into immovable bottom deposits, reducing effective tank volume and requiring costly digester shutdowns for manual cleanout.

Exam Trap Alert: Preliminary treatment provides no significant removal of dissolved $BOD_5$, nitrogen, or phosphorus. Incidental Total Suspended Solids (TSS) removal across bar screens is minimal (typically only 5% to 10% associated with large debris extraction). Never credit preliminary screening with primary organic stabilization.

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3. Coarse Screening Systems

Coarse screens intercept large debris prior to raw wastewater pumping stations or primary sedimentation basins. Screens are categorized by cleaning method and bar spacing.

Manually Cleaned Bar Racks

• Bar Spacing: Clear openings between parallel steel bars range from 1.0 to 2.0 inches (25 to 50 mm).
• Incline Angle: Set at an angle of 30° to 45° from the horizontal, allowing an operator to easily pull a manual rake up the inclined rack into a perforated drainage tray.
• Application: Restricted to emergency bypass channels, storm overflow diversion channels, or very small remote pumping stations. They require frequent manual attention; if neglected during storm events, rapid blinding causes upstream surcharging and collection system backups.

Mechanically Cleaned Bar Screens

• Bar Spacing: Clear bar openings range from 0.25 to 1.0 inch (6 to 25 mm).
• Incline Angle: Typically installed at steep angles of 60° to 85° from the horizontal, or completely vertical (90°).
• Front-Cleaned vs. Back-Cleaned Configurations:
  • Front-Cleaned Screens: The mechanical rake carriage travels along the upstream (front) face of the bar rack. Screenings are engaged directly at the channel invert and lifted upward. The major advantage is that debris cannot be forced through the bars during raking. The operational drawback is that heavy submerged logs, rocks, or large debris resting at the channel invert can jam the lower sprocket or stall the rake.
  • Back-Cleaned Screens: The mechanical drive chains, sprockets, and rake arms travel entirely on the downstream (back) side of the bar rack, with rake tines projecting through the bar openings from behind to strip debris upward. The major operational advantage is that drive mechanisms remain on the clean downstream side, shielded from abrasive grit and impacts from large floating objects. The drawback is that long stringy rags can wrap around the bars and resist stripping.

Screen Automation & Differential Head Control

Mechanically raked screens operate under dual automated control schemes to optimize electrical efficiency and mechanical wear:

1. Differential Headloss Control: Ultrasonic level transmitters measure the liquid level immediately upstream and downstream of the bar rack. As debris accumulates on the upstream face, headloss increases. When the differential level reaches a programmed setpoint—typically 2 to 6 inches (50 to 150 mm)—the differential controller triggers the screen drive motor to complete a cleaning cycle until the differential drops below the reset threshold.
2. Backup Timed Cycle: To prevent solids consolidation and odor generation during low-flow nighttime periods (when differential headloss may not trigger for hours), an adjustable interval timer initiates a 2- to 5-minute cleaning cycle every 15 to 30 minutes regardless of water level.
3. High-High Differential Alarm: If a sudden blinding event (such as a storm flush of leaves and wipes) or mechanical jam occurs, a high-high differential float switch (set at 12 to 18 inches of headloss) trips an audible SCADA alarm and automatically opens an actuated bypass channel gate to prevent upstream manhole overflow.

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4. Screenings Processing & Regulatory Disposal

Raw screenings extracted from wastewater channels consist of wet, foul-smelling masses of rags, feminine hygiene items, wet wipes, condoms, plastics, twigs, leaves, and entrapped fecal matter. Raw screenings have a bulk density of 15 to 20 lb/ft³, a moisture content of 75% to 90%, and high putrescibility.

Screenings Washer-Compactors

Modern facilities discharge screenings directly from the screen head chute into a screenings washer-compactor (washing press):

1. Washing Cycle: Inside an agitated wash zone, high-pressure washwater nozzles and rotating impellers vigorously break up fecal matter and wash soluble organic compounds back through a perforated screen basket into the wastewater channel, reducing the organic burden in the screenings.
2. Compaction & Dewatering: A heavy-duty stainless steel screw auger conveys the washed screenings into a tapered compaction zone against an adjustable discharge restriction plate or weighted cone. The mechanical pressing action squeezes out entrained water, raising the dry solids concentration to 40% to 50% dry solids and reducing screenings volume by 50% to 75% and total weight by 40% to 60%.
3. Continuous Bagging: Compacted screenings discharge into continuous plastic bagging systems (e.g., Longopac cassettes), which automatically seal the waste inside airtight polyethylene tubing, suppressing odors, fly attraction, and worker exposure to pathogens.

Regulatory Disposal Mandates (N.J.A.C. 7:26)

Under New Jersey solid waste regulations administered by the NJDEP, wastewater screenings are classified as non-hazardous municipal solid waste (MSW). Screenings can never be land-applied, composted, or processed into agricultural biosolids due to physical contamination (plastics and sharps) and pathogens.

Screenings must be hauled to a permitted sanitary landfill or waste-to-energy incineration facility. To be accepted at a sanitary landfill, the compacted screenings must pass the EPA Method 9095 Paint Filter Liquids Test (documenting that no free liquid drains through a 60-mesh paint filter during a 5-minute test period).

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5. Fine Screens & Comminution Systems

Fine Screens

Fine screens feature opening sizes ranging from 0.06 to 0.25 inch (1.5 to 6.0 mm). Common designs include continuous perforated plate band screens, rotary drum screens, and step screens.

• Application: Fine screens are mandatory upstream of Membrane Bioreactors (MBRs). Synthetic textile fibers, hair, and lint pass through coarse bar screens and wrap tightly around hollow-fiber and flat-sheet ultrafiltration membranes, causing irreversible fouling and catastrophic bundle binding. Fine screens are also used to replace primary clarifiers in compact footprints.

Comminutors & Macerators (Inline Grinders)

Comminutors and inline macerators utilize counter-rotating, hardened alloy cutter cartridges to shred incoming rags, wood, and plastics into small slurry particles (typically 1/4 inch or smaller) directly within the flow channel, allowing the shredded solids to continue downstream.

Critical Exam & Operational Principle: While comminution eliminates screenings handling and landfill hauling costs at the headworks, it introduces severe operational hazards downstream. In biological aeration tanks and anaerobic digesters, shredded synthetic fibers and plastic strips do not degrade; instead, fluid turbulence causes them to re-weave and aggregate into dense, fibrous ropes ('rag balls' or 'mop heads'). These fibrous ropes wrap around turbine mixer impellers, foul dissolved oxygen sensor membranes, plug sludge pump check valves, and bind heat exchanger tubes. Current NJDEP engineering design guidance and industry best practice strongly favor mechanical screening, dewatering, and physical extraction over comminution.

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6. Grit Removal Systems & Mechanics

Nature and Properties of Grit

Grit comprises heavy, inorganic mineral matter: sand, gravel, cinders, silt, eggshells, bone chips, coffee grounds, seeds, and metal fragments. The distinguishing physical characteristic of grit is its specific gravity, which averages approximately 2.65 (compared to putrescible organic wastewater solids with a specific gravity of 1.01 to 1.05).

If grit is not removed at the headworks, it causes:

• Rapid abrasive destruction of raw sludge pump volutes, mechanical seals, and cutting rings.
• Severe erosive scouring of pipe bends and high-pressure sludge lines.
• Settling and compaction in anaerobic digesters and aeration basins, forming concrete-like dead beds that displace active liquid volume.

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Horizontal Flow Velocity-Controlled Grit Channels

Horizontal flow channels are long, narrow rectangular conduits designed to exploit the dramatic difference in specific gravity between mineral grit ($SG \approx 2.65$) and organic matter ($SG \approx 1.02$).

• Target Operating Velocity: The horizontal flow velocity must be maintained strictly at 1.0 ft/s (0.3 m/s) (allowable operating window: 0.75 to 1.25 ft/s).
  • If velocity drops below 0.75 ft/s, lighter organic solids settle with the grit, creating putrescible, septic grit that causes intense odors and fails landfill acceptance.
  • If velocity exceeds 1.25 ft/s, fine grit (50 to 100 mesh) is scoured from the floor and carried over into downstream clarifiers.
• Velocity Control Devices: Because wastewater flow fluctuates dramatically throughout the diurnal curve, an un-throttled rectangular channel cannot maintain 1.0 ft/s across varying depths. Velocity control is accomplished by placing a specialized hydraulic control device at the effluent end of the channel:
  • Sutro Weir (Proportional Weir): A weir plate whose sides curve inward according to the formula $x \cdot \sqrt{y} = C$. The discharge over a Sutro weir is directly proportional to head ($Q \propto H$), maintaining a strictly linear velocity of 1.0 ft/s across the entire depth range in a rectangular channel.
  • Downstream Parshall Flume: Placing a Parshall flume at the discharge of a parabolic cross-section channel provides automatic depth-velocity throttling matching the 1.0 ft/s requirement.

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Aerated Grit Chambers

Aerated grit chambers introduce diffused air along one longitudinal side of a rectangular basin, inducing a transverse spiral (helical roll) flow pattern perpendicular to the forward flow through the tank.

• Helical Roll Velocity: The roll velocity across the tank floor is governed by the rate of air delivery, regulated to maintain a bottom sweep velocity of approximately 1.0 ft/s. Dense grit grains ($SG \approx 2.65$) are thrown outward by centrifugal and gravity forces to the bottom collection trench beneath the air diffusers. The lighter, lower-density organic solids ($SG \approx 1.02$) are scoured off the grit particles and carried upward by the rolling bubble plume, remaining in suspension.
• Detention Time: Designed for 2 to 5 minutes at peak hourly design flow.
• Air Supply Rate: Typically requires 3 to 8 standard cubic feet per minute (scfm) per linear foot of tank length (or 0.6 to 1.5 scfm/1,000 gallons).
• Key Operational Advantages:
  • Strips dissolved hydrogen sulfide gas ($H_2S$) and volatile foul odors into headworks scrubber ducts.
  • Pre-aerates and freshens stale, septic influent wastewater.
  • Promotes grease pre-flotation to the surface for skimming.
  • Produces exceptionally clean grit with very low organic content (< 5% volatile solids).

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Vortex Grit Separators (Pista Grit & Induced Vortex Basins)

Vortex separators are cylindrical concrete basins with a shallow conical floor and a center collection hopper.

• Operational Mechanics: Raw wastewater enters tangentially along the upper perimeter, initiating a swirling vortex. An internal motorized rotating paddle/impeller assembly operates at constant speed to induce a forced, mechanically controlled vortex. Centrifugal acceleration, combined with the boundary-layer fluid drag along the sloping floor, drives dense grit particles inward and downward toward the central hopper opening.
• Organics Suspension: The lighter organic solids are lifted by the upward central fluid vortex and carry over the effluent weir.
• Performance Characteristics: Highly compact footprint, negligible headloss (typically < 1 inch across the unit), and constant high separation efficiency (> 95% removal of 50-mesh / 300 µm sand) across a 10:1 hydraulic turndown ratio.

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7. Grit Washing, Dewatering & Classification

Grit collected in bottom hoppers is pumped as a dilute slurry (typically 1% to 3% solids) using recessed-impeller vortex pumps, air-lift pumps, or torque-flow centrifugal pumps to a grit washing and classification system.

Hydrocyclones (Cyclone Separators)

1. Inlet Acceleration: The dilute grit slurry is pumped tangentially into the cylindrical top head of the hydrocyclone under a regulated operating pressure of 15 to 25 psi (100 to 175 kPa).
2. Centrifugal Separation: The tangential fluid entry generates an intense, high-speed centrifugal vortex. Dense mineral grit particles are accelerated radially outward against the conical polyurethane wall, spiraling downward to discharge out the bottom nozzle (apex) as a concentrated grit slurry.
3. Organic Rejection: Lighter wastewater liquids, fine organics, and greases migrate to the low-pressure central core of the vortex and are discharged out the top discharge pipe (vortex finder), returning by gravity to the plant influent channel.

Grit Classifiers (Screw / Rake Dewaterers)

The concentrated hydrocyclone underflow discharges directly into the pool of an inclined screw classifier:

• Screw Dewatering Mechanics: A slowly rotating, shafted or shaftless screw auger (running at 5 to 15 RPM) lifts settled grit up a 15° to 25° inclined stainless steel trough above the liquid water line.
• Water Drainage: Entrained water drains by gravity down the trough back into the settling pool and overflows to the plant headworks.
• Final Quality: The dewatered grit discharges into a disposal roll-off container at 70% to 85% dry solids with less than 10% volatile organic content, producing an odorless, non-putrescible material that passes landfill paint filter tests.

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8. Practical Operational Scenario & Exam Traps

Practical Operational Scenario

A Class 3 wastewater treatment plant in central New Jersey experiences a sudden summer thunderstorm delivering 2.5 inches of rain in 45 minutes. The plant influent flow surges from 4.0 MGD to 14.0 MGD within 30 minutes due to intense inflow and infiltration (I/I).

• Observations: Upstream headloss on the mechanical bar screen spikes to 9 inches within 10 minutes due to an influx of road debris, plastic bottles, and flushed wet wipes. Downstream at the aerated grit chamber, the grit slurry pump discharge pressure drops, and the hydrocyclone begins spraying a wide, watery plume rather than a steady fan.
• Immediate Operational Actions:
  1. The operator switches the mechanical bar screen from differential control to continuous manual run to prevent screen blinding and surcharging the upstream interceptor.
  2. The operator checks the aerated grit chamber air supply; at 14 MGD, the liquid velocity has increased. The operator increases blower output from 4 scfm/ft to 7 scfm/ft to maintain the required helical roll velocity and prevent grit wash-through into the primary clarifiers.
  3. The operator inspects the hydrocyclone apex orifice and finds a piece of broken glass wedged in the nozzle; after isolating and clearing the blockage, operating pressure returns to 18 psi, restoring concentrated underflow to the screw classifier.

Critical Exam Traps

• Trap 1: Confusing Grit Specific Gravity with Sludge. Exam questions frequently test solids density: grit has a specific gravity of 2.65, whereas organic primary solids have a specific gravity of 1.01 to 1.05. This vast density difference is the physical basis for grit separation at 1.0 ft/s.
• Trap 2: Grit Channel Velocity Limits. Remember the exact target velocity: 1.0 ft/s. Below 0.75 ft/s, organics settle out and foul the grit; above 1.25 ft/s, grit washes out into clarifiers.
• Trap 3: Function of Sutro Weir. A Sutro weir is NOT used for flow measurement of final effluent; it is a proportional weir specifically engineered to maintain a constant horizontal velocity of 1.0 ft/s in a rectangular grit channel across fluctuating water depths.
• Trap 4: Disposal of Screenings. Screenings can never be co-disposed with sewage sludge or applied to agricultural land. Under N.J.A.C. 7:26, screenings are strictly regulated as municipal solid waste and must be landfilled or incinerated.