12.1 Water & Wastewater Laboratory Procedures & QA/QC

12.1 Water & Wastewater Laboratory Procedures & QA/QC

Water and wastewater treatment facilities rely entirely on accurate, defensible laboratory data to verify treatment efficacy, protect public health, maintain environmental compliance, and control physical, chemical, and biological unit operations. In Missouri, drinking water facilities must adhere to analytical methodologies approved under the Safe Drinking Water Act (SDWA), while wastewater utilities operate under National Pollutant Discharge Elimination System (NPDES) operating permits administered by the Missouri Department of Natural Resources (MoDNR) pursuant to Clean Water Act (CWA) guidelines codified in 40 CFR Part 136 and Standard Methods for the Examination of Water and Wastewater (published jointly by APHA, AWWA, and WEF).

An operator must master both the theoretical chemical principles and practical laboratory bench techniques for routine process parameters and legal compliance testing.

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Biochemical Oxygen Demand (BOD5 & CBOD5)

Theoretical Principle & Test Conditions

The 5-Day Biochemical Oxygen Demand (BOD5) test quantifies the amount of dissolved molecular oxygen consumed by heterotrophic microorganisms while decomposing putrescible organic matter in a water sample under standardized laboratory conditions:

• Incubation Duration & Temperature: Exactly 5 days (120 hours ± 3 hours) at 20.0°C ± 1.0°C.
• Complete Darkness: Incubation bottles must be stored in total darkness to prevent photosynthetic algae from generating oxygen, which would artificially mask true bacterial oxygen consumption.
• Aqueous Matrix: Standard 300 mL glass BOD bottles equipped with flared necks and ground-glass tapered stoppers. Water is maintained in the flared lip to form an airtight water seal, preventing atmospheric re-aeration during incubation.

Dilution Water Preparation & Nutrient Buffers

Because raw wastewater exerts high oxygen demand that would quickly exhaust all available dissolved oxygen (≈ 8.5 - 9.0 mg/L at saturation), samples must be diluted with nutrient-fortified deionized water containing:

1. Phosphate Buffer Solution (maintains pH 7.2 for optimal bacterial kinetics);
2. Magnesium Sulfate (MgSO4), Calcium Chloride (CaCl2), and Ferric Chloride (FeCl3) solutions (supplying essential trace micronutrients).

• Dilution Water Blank Criterion: To ensure dilution water purity, an unseeded dilution water blank must exhibit a dissolved oxygen depletion of no more than 0.20 mg/L over the 5-day incubation period.

Standard Methods Validation Criteria

For a BOD5 test result to be legally valid for NPDES compliance reporting, the sample dilution must satisfy two strict criteria:

1. Minimum DO Depletion: The sample mixture must deplete at least 2.0 mg/L of DO over the 5 days (DOi - DOf ≥ 2.0 mg/L).
2. Minimum Residual DO: The sample mixture must maintain a residual DO of at least 1.0 mg/L at the end of the 5 days (DOf ≥ 1.0 mg/L).

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|                         BOD5 TEST VALIDITY BOUNDARIES (300 mL BOTTLE)                   |
+-----------------------------------------------------------------------------------------+
|  Initial DO (DOi): typically ~8.5 - 9.0 mg/L at saturation                              |
|  Final DO (DOf): MUST BE >= 1.0 mg/L (prevents anaerobic conditions)                    |
|  Depletion (DOi - DOf): MUST BE >= 2.0 mg/L (ensures measurable statistical precision)  |
|  Dilution Water Blank Depletion: MUST BE <= 0.20 mg/L (verifies high-purity water)       |
|  GGA Check Standard Recovery: 198 ± 30.5 mg/L (acceptance range: 167.5 to 228.5 mg/L)   |
+-----------------------------------------------------------------------------------------+

Unseeded vs. Seeded BOD Mathematical Formulations

1. Unseeded BOD Formula (used when samples contain an active, native microbiological population, such as raw municipal influent or settled primary effluent): BOD5 (mg/L) = (DOi - DOf) / P where:

• DOi = initial dissolved oxygen concentration of the diluted sample (mg/L)
• DOf = final dissolved oxygen concentration of the diluted sample after 5 days (mg/L)
• P = decimal dilution fraction = Sample Volume (mL) / Total Bottle Volume (300 mL)

1. Seeded BOD Formula (mandatory for disinfected effluents, industrial wastes, high-temperature discharges, or acidic/alkaline samples where viable biological seed must be added): BOD5 (mg/L) = [(DOi - DOf) - (Bi - Bf) × f] / P where:

• Bi = initial DO of the seed control blank bottle (mg/L)
• Bf = final DO of the seed control blank bottle after 5 days (mg/L)
• f = ratio of seed volume in sample bottle to seed volume in seed blank bottle = (% seed in sample) / (% seed in blank)

Carbonaceous BOD (CBOD5) vs. Nitrogenous Demand

Autotrophic nitrifying bacteria (Nitrosomonas and Nitrobacter) oxidize ammonia into nitrite and nitrate, exerting Nitrogenous Biochemical Oxygen Demand (NBOD) (4.57 mg O2 per mg NH3-N). In secondary effluents, nitrification can falsely inflate organic carbon loading values. To isolate Carbonaceous BOD (CBOD), a chemical nitrification inhibitor such as TCMP (2-chloro-6-(trichloromethyl)pyridine) is added to the bottle prior to incubation to suppress nitrifiers.

Glucose-Glutamic Acid (GGA) Standard

To verify seed viability and analytical technique, a Glucose-Glutamic Acid (GGA) primary standard (150 mg/L reagent-grade glucose + 150 mg/L reagent-grade glutamic acid) is analyzed with every batch. Standard Methods mandates that a 2% dilution (6.0 mL of GGA in 300 mL) must yield a mean BOD5 of 198 ± 30.5 mg/L (valid acceptance range: 167.5 to 228.5 mg/L).

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Solids Analysis: Gravimetric Determination (TSS & VSS)

Total Suspended Solids (TSS)

Total Suspended Solids (TSS) represents the non-filterable particulate fraction of a water sample retained on a standardized filter disk:

1. Filter Media: Standard binderless 1.5 µm glass fiber filter disk (Whatman 934-AH, Gelman A/E, or Millipore AP40) seated in a Gooch crucible or vacuum filtration funnel.
2. Filtration Protocol: A well-mixed sample aliquot (yielding a dry residue between 2.5 and 200 mg) is pulled through the filter using vacuum. The filter is washed with three successive 10 mL rinses of reagent-grade deionized water to dissolve and wash out entrapped soluble mineral salts.
3. Drying & Desiccation: The filter is transferred to a drying oven maintained at 103°C - 105°C for at least 1.0 hour, removed with tongs, cooled to room temperature inside a desiccator charged with active color-indicating desiccant (silica gel or Drierite) to prevent atmospheric moisture absorption, and weighed on an analytical balance sensitive to 0.1 mg (0.0001 g). The drying cycle is repeated until constant weight is achieved (weight change < 0.5 mg).

TSS (mg/L) = [(Weight of filter + dry residue in g - Weight of tare filter in g) × 1,000,000] / Sample Volume (mL)

Volatile Suspended Solids (VSS) & Fixed Suspended Solids (FSS)

To determine the biological/organic portion of suspended solids:

• Combustion in Muffle Furnace: The dried TSS filter is placed inside a porcelain crucible and ignited in a muffle furnace at 550°C ± 50°C for 15 to 20 minutes.
• Significance: Organic carbonaceous matter volatilizes into CO2 and H2O gas, leaving behind incombustible inorganic mineral ash (Fixed Suspended Solids, FSS):

VSS (mg/L) = [(Weight of filter + dry residue in g - Weight of filter + ash residue in g) × 1,000,000] / Sample Volume (mL) FSS (mg/L) = TSS (mg/L) - VSS (mg/L)

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Turbidity Measurement (Nephelometric Method)

Turbidity is an optical property expressing the clarity of water, caused by suspended and colloidal matter such as clay, silt, finely divided organic matter, and microorganisms scattering and absorbing light.

• Nephelometric Principle: Light is passed through a sample cell, and a photodetector measures the intensity of light scattered at a 90° angle relative to the incident light beam. The measurement is reported in Nephelometric Turbidity Units (NTU).
• Calibration Standards: The primary calibration standard for turbidity instruments is Formazin (a polymer suspension synthesized from hydrazine sulfate and hexamethylenetetramine; stock = 4,000 NTU). Secondary standards (sealed styrene divinylbenzene copolymer beads, AMCO-Clear) are utilized for daily verification.
• Cell handling: Use clean, unscratched matched cells, orient them consistently, remove bubbles and fingerprints, and follow the approved method and instrument instructions. Apply index-matching oil only when that procedure calls for it; oil is not a universal requirement for every turbidimeter and cell.

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Potentiometric pH Measurement & Buffer Calibration

Principle of Measurement

pH represents the negative logarithm of hydrogen ion activity. Measurement is performed potentiometrically using a combination pH electrode containing a glass sensing membrane and an internal reference half-cell (silver/silver chloride, Ag/AgCl, in saturated KCl electrolyte).

Calibration Protocol

• Two-Point or Three-Point Bracket Calibration: The pH meter must be calibrated daily before use using at least two standard, NIST-traceable buffer solutions that bracket the expected sample range:
  • For neutral-to-basic waters (7.0 - 8.5): Calibrate with pH 7.00 and pH 10.00 buffers.
  • For acidic waters (4.5 - 7.0): Calibrate with pH 7.00 and pH 4.00 buffers.
• Automatic Temperature Compensation (ATC): Electrode voltage output changes with temperature according to the Nernst equation (59.16 mV/pH unit at 25°C). An ATC temperature probe automatically adjusts the meter's response slope.
• Electrode Storage: Combination electrodes must be stored in saturated KCl storage solution or pH 4.0 buffer—never in deionized or distilled water, which leaches electrolyte ions from the reference junction.

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Alkalinity & Total Hardness Titrations

Alkalinity Titration

Alkalinity measures the capacity of water to neutralize acids (buffering capacity), primarily contributed by bicarbonate (HCO3-), carbonate (CO3^2-), and hydroxide (OH-) ions.

• Phenolphthalein Alkalinity (P-Alkalinity): Titrated with standardized 0.0200 N H2SO4 to an endpoint of pH 8.3 (phenolphthalein indicator turns from pink to colorless). Measures all hydroxide and half of carbonate.
• Total Alkalinity (T-Alkalinity or M-Alkalinity): Titrated with 0.0200 N H2SO4 to an endpoint of pH 4.5 (bromocresol green-methyl red indicator transitions from green-blue to light pink). Measures total buffering capacity.

Alkalinity (mg/L as CaCO3) = [Titrant Volume (mL of 0.0200 N H2SO4) × 1,000] / Sample Volume (mL) (When a 100 mL sample is titrated with 0.0200 N H2SO4, each 1.0 mL of titrant equals 10.0 mg/L of alkalinity as CaCO3.)

Total Hardness Titration

Hardness is caused by divalent metallic cations, primarily calcium (Ca2+) and magnesium (Mg2+).

• EDTA Titrimetric Method: Sample is buffered to pH 10.0 ± 0.1 using an ammonium chloride-ammonium hydroxide buffer. Eriochrome Black T (EBT) indicator is added, binding with calcium/magnesium ions to produce a wine-red color.
• Endpoint: Standardized 0.0100 M EDTA (ethylenediaminetetraacetic acid) chelation solution is titrated until all divalent cations are sequestered, releasing free dye and producing a sharp color change to pure blue.

Total Hardness (mg/L as CaCO3) = [Titrant (mL of 0.0100 M EDTA) × 1,000] / Sample Volume (mL)

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Chlorine Residual Analysis (DPD Colorimetric Method)

Disinfection efficacy is verified by measuring residual chlorine species using the DPD (N,N-diethyl-p-phenylenediamine) colorimetric method (Standard Methods 4500-Cl G) on a spectrophotometer or filter photometer at 515 nm:

1. Free Available Chlorine (HOCl + OCl-): Sample is mixed with DPD buffer and indicator. Free chlorine reacts instantly to oxidize DPD into a magenta/pink compound (Wurster dye).
2. Total Chlorine: Potassium iodide (KI) is introduced into the vial. Combined chlorine species (monochloramine NH2Cl, dichloramine NHCl2, and trichloramine NCl3) catalytically oxidize iodide to iodine, which then oxidizes DPD to intensify the magenta color.
3. Combined Chlorine: Calculated by subtraction: Combined Chlorine (mg/L) = Total Chlorine (mg/L) - Free Chlorine (mg/L)

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Microbiological Examination: Coliform Bacteria & E. coli

Total coliforms and E. coli are regulatory indicator groups. A positive indicator result signals a treatment or distribution-system integrity concern and triggers the applicable rule response; it does not directly enumerate every possible waterborne pathogen.

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|                       MICROBIOLOGICAL COMPLIANCE TESTING METHODS                        |
+-----------------------------------------------------------------------------------------+
|  1. Membrane Filtration (MF):                                                           |
|     - 100 mL sample filtered through 0.45 µm sterile cellulose ester membrane.           |
|     - Total Coliform: m-Endo agar at 35.0°C ± 0.5°C for 24 ± 2 hr -> Golden-green       |
|       metallic sheen colonies.                                                          |
|     - Fecal Coliform: m-FC broth at 44.5°C ± 0.2°C (water bath) for 24 ± 2 hr -> Blue   |
|       colonies.                                                                         |
+-----------------------------------------------------------------------------------------+
|  2. Multiple Tube Fermentation (MTF / MPN):                                             |
|     - Presumptive Phase: Lauryl Tryptose Broth (LTB) at 35°C (gas in Durham tube).      |
|     - Confirmed Phase: Brilliant Green Bile Broth (BGBB) at 35°C (Total Coliform) or    |
|       EC Broth in 44.5°C water bath (Fecal Coliform). Result reported as MPN/100 mL.    |
+-----------------------------------------------------------------------------------------+
|  3. Enzyme Substrate Method (Colilert / Quanti-Tray):                                   |
|     - ONPG Substrate: Total coliforms produce β-galactosidase -> Yellow color.          |
|     - MUG Substrate: E. coli produces β-glucuronidase -> Bright blue-green fluorescence |
|       under 365 nm UV light.                                                            |
+-----------------------------------------------------------------------------------------+

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Quality Assurance & Quality Control (QA/QC) Protocols

Environmental data submitted to MoDNR must be legally defensible and scientifically sound under formal QA/QC programs:

• Trip Blanks: Sealed containers of analyte-free deionized water prepared in the laboratory that accompany sample containers to the field and back without being opened. Evaluates container contamination and shipping cross-contamination.
• Field Blanks: Analyte-free water transferred into a clean sample container at the field sampling site. Detects contamination introduced by ambient environmental air, dust, or sampling technique.
• Laboratory Duplicates (Replicates): Split aliquots of a single field sample analyzed independently to quantify analytical precision, calculated as Relative Percent Difference (RPD): RPD (%) = [|Result1 - Result2| / ((Result1 + Result2) / 2)] × 100
• Matrix Spikes (MS) & Matrix Spike Duplicates (MSD): A known concentration of target analyte added to a client sample before processing to measure analytical accuracy and matrix interferences, reported as Percent Recovery (%R) (standard target: 80% - 120%): % Recovery = [(Spiked Sample Conc. - Unspiked Sample Conc.) / Known Spike Added] × 100
• Chain of Custody (COC): A legally binding tracking document documenting every individual who took possession of a sample from field collection, transport, laboratory receipt, analytical extraction, to final disposal.