8.1 Wastewater Characteristics, Flow Variations & Sampling
Key Takeaways
- Untreated domestic wastewater typically contains 200–300 mg/L BOD5, 200–300 mg/L TSS, 20–40 mg/L TKN (15–25 mg/L NH3-N), 4–10 mg/L Total Phosphorus, and 100–250 mg/L total alkalinity as CaCO3 at a neutral pH of 6.8–7.5.
- Total solids comprise Total Suspended Solids (TSS, retained on a 1.5 µm glass fiber filter) and Total Dissolved Solids (TDS), with volatile fractions (VSS/VDS ignited at 550°C) representing organic biological matter and fixed solids representing inorganic mineral ash.
- Wastewater hydrographs exhibit a diurnal curve with dual daytime peaks (morning 7–9 AM, evening 6–8 PM) and minimum nighttime flow (2–5 AM); wet weather Inflow and Infiltration (I&I) severely distorts baseflows and hydraulic loading.
- Regulatory compliance requires grab sampling for volatile or rapid-decay parameters (pH, temperature, DO, residual chlorine, fecal coliform/E. coli, oil & grease) and 24-hour flow-proportional composite sampling for composite pollutant loads (BOD5, COD, TSS, nutrients).
- EPA 40 CFR Part 136 mandates thermal preservation at ≤6°C (on ice, without freezing), chemical preservation (H2SO4 to pH <2 for nitrogen/phosphorus species), and adherence to strict holding times (e.g., 6 hours for coliform, 48 hours for unpreserved BOD5, 28 days for preserved nutrients).
8.1 Wastewater Characteristics, Flow Variations & Sampling
Operating a wastewater treatment facility requires a comprehensive understanding of the physical, chemical, and biological properties of incoming raw sewage. Municipal wastewater is approximately 99.9% water and 0.1% dissolved and suspended solids. Despite this seemingly small fraction of solids, that 0.1% contains high concentrations of putrescible organic matter, nutrients, pathogenic microorganisms, toxic compounds, and abrasive debris. A certified operator must accurately monitor, sample, and quantify these constituents to ensure process stability, protect downstream biological systems, and maintain strict compliance with National Pollutant Discharge Elimination System (NPDES) permits administered by the Missouri Department of Natural Resources (MoDNR).
Domestic vs. Industrial Wastewater Characteristics
Raw wastewater entering a collection and treatment facility is broadly classified into domestic and industrial origins:
Domestic Wastewater
Domestic wastewater (sanitary sewage) originates from residential dwellings, commercial facilities, and institutional restrooms. It consists of human physiological wastes, wash water, food preparation residues, and household cleaning chemicals. Typical domestic sewage is remarkably consistent in its per-capita loading metrics under dry weather conditions:
- Per-Capita Biochemical Oxygen Demand (BOD₅): 0.17 - 0.22 lb/person/day (0.08 - 0.10 kg/person/day)
- Per-Capita Total Suspended Solids (TSS): 0.20 - 0.25 lb/person/day (0.09 - 0.11 kg/person/day)
- Per-Capita Hydraulic Discharge: 70 - 100 gallons/person/day (265 - 380 L/person/day)
Industrial Wastewater & High-Strength Discharges
Industrial wastewater originates from manufacturing, agricultural processing, metal plating, chemical refining, and food/beverage industries (such as dairy operations, slaughterhouses, and breweries). Industrial discharges can drastically alter the hydraulic and pollutant loading of a treatment plant by introducing:
- High-Strength Organic Loads: Breweries, dairies, and food processors can introduce BOD₅ concentrations exceeding 2,000 - 10,000 mg/L, causing severe oxygen depletion in secondary aeration basins.
- Extreme pH Fluctuations: Acidic industrial washes (e.g., metal pickling at pH < 4.0) or highly alkaline cleaning solutions (e.g., bottle-washing caustic baths at pH > 11.0) can inhibit or kill biological nitrifiers and heterotrophic biomass.
- Toxic & Inhibitory Compounds: Heavy metals (copper, zinc, chromium, nickel), cyanide, phenols, and synthetic surfactants can disrupt biological flocs, inhibit enzymatic activity, and pass through untreated into receiving waters.
- High Temperature & Fats, Oils, and Grease (FOG): Commercial food processors discharge large volumes of emulsified animal fats and vegetable oils that solidify in collection sewers, foul preliminary screens, and blind clarifier weirs.
Municipalities enforce Industrial Pretreatment Programs under the Clean Water Act (CWA) and MoDNR rules (10 CSR 20-6) to require significant industrial users (SIUs) to pretreat corrosive, flammable, or toxic discharges before discharging into public sewer networks.
Physical Parameters & The Total Solids Matrix
Physical Characteristics of Raw Sewage
- Temperature: Typically ranges between 50°F - 70°F (10°C - 21°C). Warm temperatures accelerate biological metabolic rates, increase oxygen consumption, and decrease gas solubility (DO saturation decreases as temperature rises). Cold winter wastewater slows biological kinetics, particularly autotrophic nitrification.
- Color & Odor:
- Fresh, Aerobic Sewage: Light brownish-gray color with a mild, earthy, or soapy odor.
- Septic, Anaerobic Sewage: Dark gray to jet black color with a pungent "rotten egg" odor caused by hydrogen sulfide (H₂S) gas generated by sulfate-reducing bacteria under anaerobic conditions.
- Turbidity: High cloudiness caused by finely divided colloidal and suspended solids.
The Complete Wastewater Solids Classification Matrix
Understanding solids partitioning is critical for sizing clarifiers, calculating solids retention time (SRT), and managing sludge digestion units. Total solids are categorized based on filtration and thermal combustion:
+-----------------------------------------------------------------------------------------+
| TOTAL SOLIDS (TS) |
| (Residue remaining after drying liquid sample at 103°C - 105°C) |
+-------------------------------------------+---------------------------------------------+
|
┌──────────────────────────────┴──────────────────────────────┐
▼ ▼
+-------------------------+ +-------------------------+
| TOTAL SUSPENDED SOLIDS | | TOTAL DISSOLVED SOLIDS |
| (TSS) | | (TDS) |
| (Retained on a 1.5 µm | | (Passes through 1.5 µm |
| glass fiber filter) | | filter into filtrate) |
+------------+------------+ +------------+------------+
| |
┌──────┴──────┐ ┌──────┴──────┐
▼ ▼ ▼ ▼
+-----------+ +-----------+ +-----------+ +-----------+
| VOLATILE | | FIXED | | VOLATILE | | FIXED |
| SUSPENDED | | SUSPENDED | | DISSOLVED | | DISSOLVED |
| SOLIDS | | SOLIDS | | SOLIDS | | SOLIDS |
| (VSS) | | (FSS) | | (VDS) | | (FDS) |
| (Combusts | | (Remains | | (Combusts | | (Remains |
| at 550°C; | | as ash; | | at 550°C; | | as ash; |
| organic) | | mineral) | | organic) | | mineral) |
+-----------+ +-----------+ +-----------+ +-----------+
- Total Solids (TS): All matter remaining after evaporating a well-mixed sample in a drying oven at 103°C - 105°C until constant weight is achieved.
- Total Suspended Solids (TSS): The non-filterable particulate fraction retained on a standardized 1.5 µm glass fiber filter disk (such as Whatman 934-AH), dried at 103°C - 105°C. Typical domestic influent range: 200 - 300 mg/L.
- Total Dissolved Solids (TDS): The filterable fraction passing through the 1.5 µm glass fiber filter into the filtrate flask, dried at 180°C. Consists of dissolved mineral salts (calcium, sodium, chloride, sulfates) and soluble organic molecules. Typical domestic influent range: 300 - 600 mg/L.
- Volatile Suspended Solids (VSS): The fraction of TSS that combusts and volatilizes when heated in a muffle furnace at 550°C ± 50°C for 15–20 minutes. VSS represents the organic biological fraction (microorganisms, food scraps, fecal matter) and typically accounts for 70% - 85% of raw wastewater TSS.
- Fixed Suspended Solids (FSS): The inorganic mineral residue (sand, grit, clay, silt) remaining in the crucible after ignition at 550°C. Calculated as: FSS = TSS - VSS.
- Settleable Solids: The volume of suspended particles that settle by gravity to the bottom of a 1.0-liter conical Imhoff cone within a 60-minute settling period, reported in units of mL/L/hr. Typical raw domestic wastewater contains 5 - 20 mL/L/hr of settleable solids.
Chemical Water Quality Parameters
| Parameter | Standard Domestic Influent Range | Analytical Significance & Process Impact |
|---|---|---|
| BOD₅ (5-Day Biochemical Oxygen Demand) | 200 - 300 mg/L | Measures dissolved oxygen consumed by heterotrophic microorganisms oxidizing organic matter over 5 days at 20°C in the dark. Standard permit compliance parameter. |
| COD (Chemical Oxygen Demand) | 400 - 600 mg/L | Measures total oxygen equivalent required to chemically oxidize all organic compounds using boiling potassium dichromate (K₂Cr₂O₇) in sulfuric acid with a silver catalyst (2-hour test; COD ≈ 1.5 - 2.5 × BOD₅). |
| TOC (Total Organic Carbon) | 80 - 200 mg/L | Direct instrumental combustion analysis oxidizing carbon to CO₂; rapid method taking minutes. |
| Total Kjeldahl Nitrogen (TKN) | 20 - 40 mg/L as N | Total concentration of Organic Nitrogen plus Ammonia Nitrogen (TKN = Organic N + NH₃-N). Excludes oxidized nitrate (NO₃⁻) and nitrite (NO₂⁻). |
| Ammonia Nitrogen (NH₃-N) | 15 - 25 mg/L as N | Soluble inorganic nitrogen derived from urea hydrolysis. Exerts heavy biological oxygen demand during nitrification (4.57 mg O₂ per mg NH₃-N oxidized). |
| Total Phosphorus (TP) | 4 - 10 mg/L as P | Sum of orthophosphates (PO₄³⁻), polyphosphates (detergents), and organically bound phosphorus. Crucial nutrient driving surface water eutrophication. |
| pH | 6.8 - 7.5 Standard Units | Optimal biological window (6.5 - 8.5). Values outside this range inhibit activated sludge bacteria and nitrifiers. |
| Total Alkalinity | 100 - 250 mg/L as CaCO₃ | Chemical buffering capacity against acid. Nitrification consumes 7.14 mg of alkalinity as CaCO₃ for every 1.0 mg of NH₃-N oxidized to nitrate. |
Carbonaceous vs. Nitrogenous Oxygen Demand
Total oxygen demand in wastewater comprises two distinct phases:
- Carbonaceous Biochemical Oxygen Demand (CBOD): Exerted by heterotrophic bacteria metabolizing organic carbonaceous compounds. This reaction dominates the first 5 to 8 days of incubation.
- Nitrogenous Biochemical Oxygen Demand (NBOD): Exerted by specialized autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) oxidizing ammonia to nitrite and nitrate: NH₄⁺ + 1.5 O₂ → NO₂⁻ + H₂O + 2 H⁺ NO₂⁻ + 0.5 O₂ → NO₃⁻
Total Theoretical Oxygen Demand for Nitrification = 4.57 mg O₂ per mg NH₃-N oxidized
To prevent nitrification interference during standard CBOD₅ testing, an inhibitory chemical compound (such as 2-chloro-6-(trichloromethyl) pyridine, TCMP) is added to suppress nitrifying organisms in the test bottle.
Diurnal Flow Variations & Infiltration and Inflow (I&I)
The Municipal Diurnal Hydrograph
Municipal wastewater flow rates follow a predictable diurnal curve that directly mirrors community water usage habits:
- Morning Peak (7:00 AM - 9:00 AM): Flow and pollutant concentration spike as residents awake, shower, prepare breakfast, and flush toilets. A corresponding mass loading peak arrives at the plant between 8:00 AM and 11:00 AM (dependent on collection system transit time).
- Afternoon Trough (1:00 PM - 3:00 PM): Flow stabilizes at a moderate baseline.
- Evening Peak (6:00 PM - 8:00 PM): Secondary flow and organic load surge corresponding to evening cooking, dishwashing, laundry, and bathing.
- Minimum Night Flow (2:00 AM - 5:00 AM): Flow drops to the daily minimum (25% - 50% of average daily flow) as residential water consumption ceases.
Wastewater Diurnal Flow Hydrograph (Dry Weather vs. Wet Weather I&I Surge):
Flow
▲
│ Wet Weather Storm Peak (Inflow Surge)
│ . - - - - - - - .
│ . .
│ . . - - - - - - - - - - (High Infiltration)
│ /
│ / Morning Peak
│ / (7-9 AM) Evening Peak
│ / ┌───┐ (6-8 PM)
│ / │ │ ┌───┐
│ / │ │ Midday │ │
│ / │ │ Trough │ │
│ ───┘ │ │ ┌───┐ │ │ Dry Weather
│ │ └───┘ └───-┘ │ Baseflow
│ │ └───┐
│ ───────────────┘ └───┐ Minimum Night Flow (2-5 AM)
│ └───────────
└─────────────────────────────────────────────────────────────► Time of Day (24 Hr)
12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM 12 AM
Infiltration & Inflow (I&I) Dynamics
During storm events, clean extraneous water enters sanitary collection sewers, causing severe hydraulic overloading:
- Infiltration: Groundwater that enters sewer pipes, deteriorated pipe joints, cracked lateral connections, and leaking manhole walls located below the water table. Infiltration is a slow, steady response that creates a prolonged, elevated baseline flow that persists for days or weeks after rainfall events.
- Inflow: Direct stormwater runoff that enters collection systems through surface pathways, including unsealed or submerged manhole pick holes, roof downspouts, yard drains, sump pump discharges, and illicit cross-connections with municipal storm sewers. Inflow is an immediate, acute surge that spikes simultaneously with rainfall intensity.
Severe I&I dilutes incoming wastewater (lowering BOD₅ and TSS concentrations while multiplying total hydraulic volume), reduces clarifier detention times, washes solids out of primary clarifiers, hydraulically overloads secondary biological processes, and can cause sanitary sewer overflows (SSOs).
Sampling Methodologies: Grab vs. Composite
Obtaining representative samples is the legal foundation of environmental compliance under MoDNR operating permits. An unrepresentative sample invalidates all subsequent laboratory analyses.
Grab Samples
A grab sample is an individual, discrete aliquot of wastewater collected at a single specific location over a brief interval (typically less than 15 seconds).
- When Mandatory: Grab samples are mandatory for parameters that are volatile, degrade rapidly, adhere to sample collection tubing, or change chemical state during composite storage.
- Parameters Requiring Grab Sampling:
- pH (must be analyzed on-site within 15 minutes of collection)
- Temperature (analyzed immediately in the field)
- Dissolved Oxygen (DO) (analyzed immediately in the field)
- Residual Chlorine (analyzed immediately within 15 minutes)
- Fecal Coliform & Escherichia coli (bacteriological decay occurs rapidly)
- Oil and Grease (FOG) (immiscible; adheres to container walls and automatic sampler intake lines)
- Volatile Organic Compounds (VOCs) (purgeable organics volatilize into headspace)
- Cyanide and Sulfides (rapid oxidation and chemical conversion)
Composite Samples
A composite sample consists of multiple individual sample aliquots collected over an extended timeframe (standardly a 24-hour period) and combined into a single master container.
- Time-Proportional Composite: Collects equal sample aliquot volumes at uniform, fixed time intervals (e.g., 100 mL every 60 minutes for 24 hours), regardless of fluctuating flow rates. This method is acceptable only when flow is completely constant.
- Flow-Proportional Composite (Flow-Weighted): Samples are collected in direct proportion to the wastewater flow rate. This can be achieved by:
- Constant Time / Variable Volume: Sampling at uniform time intervals (e.g., every 30 minutes), but varying the collected aliquot volume proportionally to the instantaneous flow rate.
- Constant Volume / Variable Time: Sampling a fixed aliquot volume (e.g., 200 mL) after a preset volume of sewage passes the primary flow meter (e.g., one aliquot every 50,000 gallons). This is the EPA/MoDNR standard of preference.
- Parameters Requiring 24-Hour Flow-Proportional Compositing: BOD₅, CBOD₅, COD, TSS, TKN, Ammonia Nitrogen (NH₃-N), Total Phosphorus (TP), and Total Dissolved Solids (TDS).
Sample Preservation Protocols & Clean Water Act Holding Times
Under EPA standard methods (40 CFR Part 136) and MoDNR rules, samples must be properly preserved immediately upon collection to halt biological respiration, chemical precipitation, and volatile stripping.
| Parameter | Container Material | Required Preservation Protocol | Maximum Permissible Holding Time |
|---|---|---|---|
| pH, Temperature, DO, Residual Cl₂ | Glass or Plastic (HDPE) | None (Analyze immediately on-site) | 15 minutes |
| Fecal Coliform / E. coli | Sterile Glass or Polypropylene | Cool to ≤6°C; add Sodium Thiosulfate (Na₂S₂O₃) if chlorine is present | 6 hours (for NPDES compliance) |
| BOD₅ / CBOD₅ | Plastic (HDPE) or Glass | Cool to ≤6°C on wet ice (do not freeze) | 48 hours |
| Total Suspended Solids (TSS) | Plastic (HDPE) or Glass | Cool to ≤6°C on wet ice (do not freeze) | 7 days |
| Ammonia (NH₃-N), TKN, Total Phosphorus | Plastic (HDPE) or Glass | Acidify with Sulfuric Acid (H₂SO₄) to pH < 2.0; Cool to ≤6°C | 28 days |
| Chemical Oxygen Demand (COD) | Plastic (HDPE) or Glass | Acidify with Sulfuric Acid (H₂SO₄) to pH < 2.0; Cool to ≤6°C | 28 days |
| Oil and Grease (FOG) | Wide-mouth Glass only (with PTFE liner) | Acidify with Hydrochloric (HCl) or Sulfuric (H₂SO₄) to pH < 2.0; Cool to ≤6°C | 28 days |
| Metals (Total - Cu, Zn, Pb, etc.) | Polyethylene (acid-washed) | Acidify with Nitric Acid (HNO₃) to pH < 2.0 | 6 months (Mercury: 28 days) |
In wastewater laboratory testing, which of the following accurately describes the relationship between Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS)?
According to EPA 40 CFR Part 136 and MoDNR sampling regulations, which sample type and preservation protocol is required for compliance reporting?
An operator analyzing collection system hydraulics observes that during a heavy rainstorm, wastewater flow at the plant headworks spikes instantly within 30 minutes of rainfall initiation, whereas baseflow remains elevated for two weeks following the storm. How are these two hydraulic phenomena properly classified?
Why is monitoring raw wastewater alkalinity critical for wastewater utilities operating biological nitrification processes?