9.1 Wastewater Characteristics & Preliminary Treatment

9.1 Wastewater Characteristics & Preliminary Treatment

[!NOTE] Arizona Regulatory & Operational Context: In Arizona, wastewater utilities operate under stringent discharge and aquifer protection standards administered by the Arizona Department of Environmental Quality (ADEQ). Under Arizona Administrative Code (A.A.C.) Title 18, Chapter 9 (Aquifer Protection Permits) and Chapter 11 (Arizona Pollutant Discharge Elimination System - AZPDES), reclaimed water facilities often target Class A+ or Class A standards to permit direct aquifer recharge, agricultural reuse, and urban turf irrigation. Meeting these standards requires thorough preliminary removal of coarse debris and inorganic grit before wastewater enters biological processes.

Municipal wastewater treatment represents a critical barrier for public health and environmental protection. Untreated wastewater carries thousands of physical, chemical, and biological constituents originating from domestic residences, commercial businesses, and industrial facilities. Managing this complex matrix requires a rigorous understanding of wastewater characterization, diurnal hydraulic and organic fluctuations, and the mechanics of preliminary treatment.

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Physical, Chemical & Biological Constituents of Wastewater

Municipal wastewater is approximately 99.9% liquid water and only 0.1% (1,000 mg/L) total solids. However, that minute 0.1% fraction contains organic matter, suspended debris, nutrients, toxic minerals, and virulent human pathogens.

                                  Total Solids (TS)
                           (100% of solids in wastewater)
                                         │
                  ┌──────────────────────┴──────────────────────┐
                  ▼                                             ▼
      Total Suspended Solids (TSS)                  Total Dissolved Solids (TDS)
      (Non-filterable, >1.5 µm filter)               (Filterable, <1.5 µm filter)
      Typically 150 - 300 mg/L                      Typically 400 - 1,000+ mg/L
                  │                                             │
         ┌────────┴────────┐                           ┌────────┴────────┐
         ▼                 ▼                           ▼                 ▼
   Volatile (VSS)     Fixed (FSS)                Volatile (VDS)     Fixed (FDS)
   (Organics, 550°C)  (Inorganics/Ash)           (Soluble organics) (Salts/Minerals)
      70% - 80%         20% - 30%                   ~40%               ~60%

1. Solids Classifications

• Total Solids (TS): All matter remaining as residue after evaporation of the sample at 103°C to 105°C in a drying oven.
• Total Suspended Solids (TSS): The non-filterable particulate fraction retained on a standard glass-fiber filter disc (nominal pore size ~1.5 µm) after drying at 103°C–105°C. Raw municipal wastewater typically contains 150 to 300 mg/L of TSS.
• Total Dissolved Solids (TDS): The filterable fraction that passes through the 1.5 µm filter and remains after oven drying. In Arizona, raw influent TDS frequently ranges from 500 to 1,200+ mg/L, heavily influenced by high mineral hardness and salinity in imported drinking water supplies (e.g., Central Arizona Project water).
• Settleable Solids: Suspended solids that settle out of quiescent suspension under gravity within a calibrated Imhoff cone over a 60-minute settling period, reported in milliliters per liter (mL/L). Typical domestic wastewater contains 5 to 20 mL/L of settleable solids.
• Volatile vs. Fixed Solids: When dried solids are ignited in a muffle furnace at 550°C ± 50°C, organic matter combusts into gases ($CO_2$, $H_2O$, $N_2$), leaving behind inert mineral ash:
  • Volatile Suspended Solids (VSS): Represents the combustible, organic biological fraction (typically 70% to 80% of TSS in raw sewage).
  • Fixed Suspended Solids (FSS): The inorganic, non-combustible ash fraction (grit, sand, silt, clays), representing 20% to 30% of raw TSS.

2. Organic Strength Indicators: BOD5, COD, and TOC

• Biochemical Oxygen Demand ($BOD_5$): Measures the quantity of dissolved oxygen (expressed in mg/L) consumed by aerobic microorganisms to biochemically oxidize carbonaceous organic matter over a standardized 5-day incubation period at 20°C in the dark. Raw domestic municipal sewage exhibits a typical $BOD_5$ concentration of 150 to 300 mg/L (averaging ~200 mg/L).
• Chemical Oxygen Demand (COD): Measures the total equivalent quantity of oxygen required to chemically oxidize all organic compounds (both biodegradable and non-biodegradable refractory compounds) into $CO_2$ and $H_2O$. The test utilizes boiling potassium dichromate ($K_2Cr_2O_7$) in concentrated sulfuric acid ($H_2SO_4$) with silver sulfate catalyst over a 2-hour reflux. Because COD captures refractory organics that bacteria cannot metabolize, COD is always higher than $BOD_5$:

$\text{Municipal COD:BOD}_5\text{ Ratio} = 1.5:1 \text{ to } 2.5:1$ $\text{Raw Municipal COD} = 300 \text{ to } 600\text{ mg/L}$

• Total Organic Carbon (TOC): Measures organic carbon content directly by high-temperature catalytic combustion (or UV-persulfate oxidation), converting organic carbon atoms into carbon dioxide ($CO_2$), which is quantified by an infrared analyzer. Raw municipal TOC typically ranges from 80 to 200 mg/L.

3. Nitrogen Forms & Phosphorus

• Total Kjeldahl Nitrogen (TKN): The analytical sum of Organic Nitrogen (proteins, urea, amino acids) and Ammonia Nitrogen ($NH_3$ and $NH_4^+$). Raw municipal wastewater contains 20 to 40 mg/L as N of TKN (approximately 60% ammonia-N and 40% organic-N).
• Nitrite ($NO_2^-$) and Nitrate ($NO_3^-$): Nearly absent in fresh raw municipal wastewater (<0.5 mg/L as N) because raw sewage is devoid of dissolved oxygen, preventing autotrophic nitrification.
• Total Phosphorus (TP): Present as soluble orthophosphates ($PO_4^{3-}$), polyphosphates (detergent builders), and organically bound phosphorus. Raw sewage contains 4 to 10 mg/L as P.

4. Arizona Climate & High-Temperature Wastewater Dynamics

In Arizona's desert basins (Phoenix, Tucson, Yuma, Lake Havasu City), raw wastewater exhibits unique seasonal dynamics:

• Influent Temperature: Summer influent sewage temperatures frequently reach 28°C to 32°C (82°F to 90°F). According to biological reaction kinetics (Arrhenius relationships), microbial metabolic rates approximately double for every 10°C rise in temperature.
• Rapid Sewer Septicity: High temperatures cause rapid dissolved oxygen depletion in gravity collection mains. Obligate anaerobic sulfate-reducing bacteria (Desulfovibrio) reduce sulfate ($SO_4^{2-}$) to dissolved hydrogen sulfide ($H_2S$ and $HS^-$). When turbulence occurs at manholes and headworks, $H_2S$ gas strips into the air space.
• Biogenic Crown Corrosion: Chemolithoautotrophic Acidithiobacillus bacteria inhabiting the moist, aerobic pipe crown oxidize $H_2S$ gas into concentrated sulfuric acid ($H_2SO_4$, pH < 1.0), destroying concrete pipe crowns, headworks structures, and metal sluice gates.

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Diurnal Flow Variations & Loading Hydrographs

Municipal wastewater flows follow predictable diurnal (24-hour) cycles driven by domestic water use patterns:

Flow (MGD)
  ▲
  │                    Morning Domestic Peak
  │                       (7 AM - 10 AM)
  │                             ┌───┐                 Evening Domestic Peak
  │                            ┌┘   └┐                   (6 PM - 9 PM)
  │                           ┌┘     └┐                      ┌───┐
  │                          ┌┘       └┐                    ┌┘   └┐
  │        Average Flow ─────┼─────────┼────────────────────┼─────┼─────
  │                         ┌┘         └┐                  ┌┘     └┐
  │                        ┌┘           └┐                ┌┘       └┐
  │   Nighttime Minimum   ┌┘             └┐              ┌┘         └┐
  │    (2 AM - 5 AM)     ┌┘               └──────────────┘           └───
  │       ┌─────┐       ┌┘
  │───────┘     └───────┘
  └──────────────────────────────────────────────────────────────────────► Time
   12 AM   3 AM   6 AM   9 AM   12 PM   3 PM   6 PM   9 PM   12 AM

• Nighttime Minimum (2:00 AM – 5:00 AM): Flows drop to 20%–40% of average daily flow as domestic water consumption ceases. Wastewater detention time in collection pipes increases dramatically, promoting septicity and odor generation.
• Morning Peak (7:00 AM – 10:00 AM): Flows surge to 150%–200% of average daily flow as residents wake, shower, flush toilets, and prepare food. Organic concentrations ($BOD_5$, TSS) peak simultaneously, creating a mass loading spike.
• Afternoon Dip (1:00 PM – 4:00 PM): Intermediate stabilization as residential activity subsides.
• Evening Peak (6:00 PM – 9:00 PM): A secondary surge driven by cooking, dishwashing, and laundry.
• Arizona Seasonal 'Snowbird' Influx: In southern and central Arizona communities (such as Yuma, Mesa, Apache Junction, Green Valley, and Casa Grande), the winter seasonal population increases local wastewater flows and organic loadings by 20% to 40% between November and April.

Preliminary Treatment Objectives

Preliminary treatment is the physical entryway of a wastewater reclamation plant. Its core objectives are purely protective:

1. Protect Mechanical Equipment: Prevent large rocks, timber, rags, wet wipes, plastics, and debris from damaging raw sewage lift pumps, binding valves, and puncturing sludge transfer lines.
2. Prevent Pump & Pipe Abrasion: Remove sharp, dense inorganic sand, gravel, and eggshells that prematurely erode pump impellers, wear volute casings, and scour piping.
3. Eliminate Downstream Accumulation: Prevent inert, non-biodegradable grit from settling and compacting inside primary clarifiers, biological aeration basins, and anaerobic digesters (where grit deposition can steal 20%–40% of active digester volume).

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Screening Technologies: Bar Racks vs. Mechanically Cleaned Fine Screens

Screening physically intercepts and removes rags, sanitary products, plastics, and stringy materials before they enter the treatment stream.

Coarse Bar Racks (Trash Racks)

• Clear Openings: 1.0 to 2.0 inches (25 to 50 mm).
• Placement: Located at the deep influent pump station inlet or plant headworks channel.
• Function: Catches catastrophic debris (dimensional lumber, concrete rubble, large automotive parts, tree branches).
• Cleaning: Manually raked in small rural package plants; mechanically driven in medium-to-large utilities.

Mechanically Cleaned Fine Bar Screens

• Clear Openings: 0.25 to 0.5 inches (6 to 12 mm). Advanced facilities (such as Membrane Bioreactor [MBR] plants) employ ultra-fine screens with 1.0 to 3.0 mm perforated plates or wedge-wire drums.
• Configuration Types:
  • Front-Cleaned / Front-Return: The cleaning rake approaches the bar rack from the upstream side, travels upward carrying screenings, and discharges at the top. This configuration prevents debris from carrying over to the downstream side.
  • Back-Cleaned: The rake operates behind the screen bars, where mechanical chains and sprockets are shielded from heavy impact. However, long rags can wrap around the bars and bypass the rake mechanism.
• Automation & Differential Headloss: Fine screens operate automatically via differential headloss level transmitters (ultrasonic or pressure transducers measuring the water level differential upstream and downstream of the screen). When debris blinds the screen, upstream water rises:

$\Delta H = H_{\text{upstream}} - H_{\text{downstream}}$

When $\Delta H$ exceeds the pre-set setpoint (typically 2 to 6 inches of headloss), the drive motor activates to clean the rack. A backup programmable interval timer (e.g., initiating a cycle every 15 to 30 minutes) ensures periodic raking during low-flow periods to prevent organic stagnation.

Screening Quantities, Washing, Compaction & Disposal

• Screenings Production: Municipal wastewater generates 0.5 to 5.0 cubic feet of screenings per Million Gallons (cu ft/MG) treated (averaging ~2.0 to 3.0 cu ft/MG).
• Screenings Washer-Compactor: Raw screenings contain up to 80% putrescible fecal organics and water. The screenings drop from the bar screen into a washer-compactor, where high-pressure wash water washes soluble fecal matter and organic carbon back into the wastewater flow channel. A rotating stainless-steel screw auger then forces the washed screenings through a tapered discharge cone, squeezing out moisture to achieve >40% to 50% dry solids content and reducing screenings volume by 50% to 75%.
• Disposal: Dewatered screenings are discharged into covered, leak-proof roll-off bins and transported to a permitted municipal solid waste landfill under Arizona solid waste rules.

Comminutors and Macerating Grinders

• Mechanism: A comminutor or macerating grinder (such as a dual-shafted Muffin Monster) incorporates counter-rotating hardened steel cutter teeth that shred large solids, rags, and plastics into small fragments (typically 1/4 to 3/8 inch) directly in the liquid stream without removing them.
• Operational Trade-offs: While comminutors eliminate screenings handling, odor bins, and hauling fees, they create severe downstream operational headaches. Shredded rags and wet wipes re-weave in biological aeration basins and anaerobic digesters, forming dense "rag balls" and "rag ropes" that bind primary sludge pump impellers, wrap around surface aerators, and foul level sensors. Modern Arizona wastewater engineering design heavily favors screening and total removal over in-stream comminution.

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Grit Removal Systems: Hydraulics, Mechanics & Classification

Wastewater grit consists of heavy, abrasive inorganic particles with a specific gravity (SG) ranging from 1.9 to 2.65 (primarily silica sand, fine gravel, silt, cinders, eggshells, coffee grounds, bone fragments, and seeds). Organic sewage solids, by comparison, have a specific gravity of 1.02 to 1.2 (nearly buoyant in water).

1. Horizontal Velocity-Controlled Grit Chambers

• Operating Principle: Designed to maintain a constant horizontal flow velocity of strictly 1.0 ft/sec (0.3 m/s) regardless of fluctuating diurnal flow rates.
• Hydraulic Control Devices:
  • Proportional (Sutro) Weir: A specially contoured vertical weir plate positioned at the channel outlet where the discharge flow is directly proportional to head ($Q \propto H$), maintaining constant velocity in a rectangular channel.
  • Parshall Flume: A calibrated downstream Venturi flume positioned after a parabolic or trapezoidal grit channel that forces depth to adjust in exact proportion to flow.
• The 1.0 ft/sec Hydraulic Critical Balance:
  • If Velocity > 1.0 ft/sec: Excessive turbulence prevents grit from settling. Abrasive grit carries over into primary clarifiers, damaging pumps and filling anaerobic digesters.
  • If Velocity < 1.0 ft/sec: Organic fecal matter and grease settle out alongside the grit. The settled grit becomes septic and putrid, generating dangerous hydrogen sulfide odors and complicating landfill disposal.

2. Aerated Grit Chambers

• Mechanism: A rectangular basin where coarse bubble air diffusers placed along one longitudinal wall introduce air, inducing a continuous spiral roll (helical) flow pattern perpendicular to the longitudinal flow path.
• Process Control: The air injection rate is tuned so that the circulating liquid velocity along the chamber floor is approximately 1.0 ft/sec. Heavier inorganic particles with higher settling velocities escape the roll and drop into a longitudinal grit collection hopper beneath the diffusers. Lighter organic solids are held in suspension by the rolling current and carried out with the effluent.
• Operational Advantages: Hydraulic detention time is 2 to 5 minutes at peak flow. The injected air provides pre-aeration, freshening septic incoming sewage, stripping noxious volatile compounds, and preventing anaerobic odors.

3. Vortex (Induced Swirl) Grit Chambers

• Mechanism: A circular chamber with a tangential influent entry and a slow-speed, motor-driven rotating impeller paddle located in the center (e.g., Pista Grit or Grit King).
• Physics: The rotating paddles induce a controlled, three-dimensional vortex flow. Centrifugal force, boundary layer fluid dynamics, and toroidal flow drive dense grit toward the center floor opening into a lower collection hopper. Lighter organic solids are washed off the grit particles and lifted upward into the peripheral effluent weir.
• Performance: Extremely compact footprint, minimal headloss, and highly efficient capture (>95% removal of grit larger than 50-mesh / 0.30 mm).

Grit Washing, Classification & Disposal

• Grit Extraction: Settled grit is extracted from hoppers using air-lift pumps, recessed-impeller vortex pumps, or inclined screw conveyors.
• Grit Classifier / Hydrocyclone: The raw grit slurry (containing 70%–90% water and residual organic matter) is pumped into a hydrocyclone separator, which utilizes centrifugal separation to concentrate dense solids. The concentrated slurry discharges onto an inclined screw or reciprocating rake classifier that slowly conveys grit up an incline above the water line, allowing water and organic matter to drain back into the plant flow.
• Clean Grit Properties: A well-operated classifier yields clean, dewatered grit containing >80% mineral solids and <10%–15% volatile organic material, suitable for sanitary landfill cover or disposal.