10.1 Drinking Water Laboratory Analysis: Turbidity, pH, Alkalinity & Coliform Testing

Drinking Water Laboratory Analysis: Turbidity, pH, Alkalinity & Coliform Testing

Core Principle: Laboratory testing at public drinking water facilities provides the scientific foundation for protecting public health. Water treatment operators perform physical, chemical, and microbiological analyses daily to ensure multiple barrier pathogen removal, verify chemical dosing, and maintain compliance with Safe Drinking Water Act (SDWA) standards and the Alabama Department of Environmental Management (ADEM) Water Supply Program rules (Division 335-7).

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1. Turbidity Measurement & Regulatory Compliance

Turbidity is the principal physical indicator of drinking water quality and filtration performance. While turbidity itself is not a specific disease-causing agent, suspended particulate matter shields pathogenic microorganisms—including Cryptosporidium oocysts, Giardia cysts, bacteria, and enteroviruses—from chemical disinfectants like chlorine. Furthermore, suspended particulates can exert a significant disinfectant demand, leading to rapid residual decay and the elevated formation of regulated disinfection byproducts (DBPs).

The Nephelometric Principle

Modern drinking water compliance turbidity is determined exclusively using nephelometry (EPA Method 180.1 / Standard Methods 2130B):

• Optical Geometry: A light source (tungsten filament lamp operating at a color temperature between 2,200K and 3,000K, or an LED light source emitting at 860 nm for ISO 7027 compliance) directs an incident light beam through a clean glass sample cuvette.
• 90-Degree Scattering Detection: Suspended colloidal particles in the water scatter light in all directions. A high-sensitivity photodetector is positioned at precisely 90 degrees ($\pm 30^\circ$) relative to the incident light path to capture scattered light rather than transmitted light.
• Measurement Units: Scattered light intensity is directly proportional to particulate concentration and is reported in Nephelometric Turbidity Units (NTU).
• Difference from Spectrophotometry: Direct transmittance measurements (attenuation at 180 degrees) lack analytical sensitivity in low-turbidity finished waters ($< 1.0\text{ NTU}$). Nephelometric detection at 90 degrees isolates scattered photons against a dark background, providing high sensitivity down to $0.01\text{ NTU}$.

Instrument Calibration & Standards

Accurate nephelometric measurement requires rigorous calibration and strict adherence to standard protocols:

1. Primary Standards (Formazin & AMCO Clear):
   • Formazin: Synthesized by combining hydrazine sulfate ($(NH_2)_2 \cdot H_2SO_4$) and hexamethylenetetramine ($(CH_2)_6N_4$) in reagent-grade water. The resulting suspension yields a standardized stock of 4,000 NTU.
   • Formazin polymer chains have reproducible particle size distributions, making Formazin the universal true primary calibration standard recognized by EPA and Standard Methods.
   • AMCO Clear: Composed of calibrated styrene divinylbenzene polymer beads. AMCO Clear is commercially certified as a primary standard and does not require complex preparation or toxic hydrazine handling.
   • Primary calibration must be conducted quarterly, after optical bench maintenance, or whenever secondary standard checks fall outside acceptable tolerances.
2. Secondary Standards:
   • Sealed glass cylinders containing stabilized liquid, silicone oil, or solid optical gel with pre-assigned target values.
   • Mandatory Rule: Secondary standards are used solely for daily verification of instrument stability and drift detection. They cannot be used to recalibrate the nephelometer calibration curve. If a secondary standard check drifts outside $\pm 5-10\%$ of its baseline value, the operator must re-calibrate the instrument using fresh primary standards.
3. Sample Handling Hygiene:
   • Turbidimeter cuvettes (sample cells) must be scratch-free, thoroughly cleaned with acid detergent, rinsed repeatedly with deionized water, and dried with lint-free optical paper.
   • A light coating of high-grade silicone oil must be applied to the outer glass surface and smoothed with an optical cloth to mask micro-scratches and match the refractive index of the glass.
   • Condensation on cuvettes during warm, humid months must be eliminated using a desiccant pack or warm air purge.

Regulatory Standards under the Surface Water Treatment Rules

Under the Interim Enhanced Surface Water Treatment Rule (IESWTR) and the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), all conventional and direct filtration water systems must achieve stringent turbidity limits to ensure physical removal of Cryptosporidium:

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2. pH and Alkalinity Testing

pH and alkalinity govern the chemical equilibrium of drinking water treatment. They dictate the charge destabilization kinetics of metal coagulants, the speciation and disinfection efficacy of free chlorine, and the corrosivity or scale-forming tendencies of finished water in distribution piping networks.

Electrometric pH Analysis

pH represents the negative logarithm (base 10) of hydrogen ion activity: $\text{pH} = -\log[H^+]$. Pure water at 25°C has a neutral pH of 7.00. Because pH is an exponential logarithmic scale, a drop from pH 7.0 to pH 6.0 represents a tenfold increase in hydrogen ion concentration, while a drop to pH 5.0 represents a hundredfold increase.

• Electrode Mechanics: Measurements must be made using an electrometric combination pH electrode (Standard Methods 4500-$H^+$ B). The assembly houses a thin, pH-sensitive glass bulb membrane and an internal reference electrode (typically silver/silver chloride, $Ag/AgCl$) immersed in a concentrated potassium chloride ($KCl$) filling solution.
• The Nernst Equation: The potential difference ($E$, in millivolts) generated across the glass membrane varies linearly with hydrogen ion activity according to the Nernst relationship: $E = E_0 - \left(\frac{2.303 RT}{F}\right) \text{pH}$ Where $R$ is the universal gas constant, $T$ is absolute temperature (Kelvin), $F$ is Faraday's constant, and $E_0$ is the standard cell potential. At 25°C ($298.15\text{ K}$), the theoretical Nernstian slope is $-59.16\text{ mV}$ per pH unit.
• Calibration Protocol:
  1. The pH meter must undergo a two-point or three-point calibration at the start of each operational shift using certified NIST-traceable buffer solutions (pH 4.00, pH 7.00, and pH 10.00).
  2. Buffer 7.00 is always measured first to establish the zero-potential intercept (isopotential point where $E = 0\text{ mV}$). Next, Buffer 4.00 (for acidic to neutral waters) or Buffer 10.00 (for neutral to alkaline waters) is measured to establish the slope.
  3. Slope Verification: The meter calculates the percentage slope: $\text{Slope \%} = (\text{Actual mV/pH unit} / 59.16) \times 100$. A properly functioning electrode must demonstrate a slope between 95% and 105% (56.2 to 62.1 mV/unit). If the slope falls below 95%, the electrode must be cleaned with mild acid or enzyme cleaner, rehydrated in $KCl$ storage solution, or replaced.
• Automatic Temperature Compensation (ATC): Because the Nernst slope varies directly with absolute temperature ($T$), an integrated temperature sensor (ATC probe) must be immersed alongside the glass electrode to correct millivolt readings to standard values dynamically.

Alkalinity Titration & Chemical Speciation

Alkalinity is the measure of water's capacity to neutralize strong acids. It functions as a chemical buffer, absorbing hydrogen ions ($H^+$) released during coagulant hydrolysis and chlorine disinfection without suffering severe drops in pH. Alkalinity is reported in $\text{mg/L as } CaCO_3$.

Natural alkalinity in surface and groundwaters originates primarily from the dissolution of limestone, dolomite, and atmospheric carbon dioxide, existing in three primary forms: hydroxide ($OH^-$), carbonate ($CO_3^{2-}$), and bicarbonate ($HCO_3^-$).

Laboratory Titration Procedure (Standard Methods 2320B)

1. Measure a $100.0\text{ mL}$ representative water sample into a clean Erlenmeyer flask.
2. Add 2 to 3 drops of phenolphthalein indicator solution.
   • If the water remains clear, the pH is below 8.3, and Phenolphthalein Alkalinity ($P$) = 0.
   • If the water turns pink/magenta, the pH is above 8.3. Titrate with standardized $0.0200\text{ N } H_2SO_4$ until the pink color completely discharges (endpoint at pH 8.3). Record the titrant volume ($A$ mL).
3. Add 3 to 4 drops of bromocresol green-methyl red indicator (or methyl orange) to the same sample flask. The solution turns blue-green.
4. Continue titrating with $0.0200\text{ N } H_2SO_4$ until the indicator shifts from blue-green through a grey intermediate to a distinct light pink/orange endpoint at pH 4.5. Record the total cumulative volume of titrant used from the beginning of the titration ($B$ mL).

Mathematical Formulas

$\text{Phenolphthalein Alkalinity } (P) \text{ (mg/L as } CaCO_3) = \frac{A \times N \times 50,000}{\text{Sample Volume (mL)}}$

$\text{Total Alkalinity } (T) \text{ (mg/L as } CaCO_3) = \frac{B \times N \times 50,000}{\text{Sample Volume (mL)}}$

Where $N = 0.0200\text{ N } H_2SO_4$, and $50,000$ represents the equivalent weight conversion factor for $CaCO_3$ ($50\text{ mg/meq} \times 1,000\text{ mL/L}$). When analyzing a standard $100.0\text{ mL}$ sample with $0.0200\text{ N}$ acid, the equation simplifies directly to:

$\text{Alkalinity (mg/L as } CaCO_3) = \text{mL of } 0.0200\text{ N } H_2SO_4 \times 10.0$

Alkalinity Relationships and Speciation Table

By comparing the Phenolphthalein Alkalinity ($P$) to the Total Alkalinity ($T$), the exact distribution of hydroxide, carbonate, and bicarbonate can be calculated:

Role in Chemical Coagulation

When aluminum sulfate (alum, $Al_2(SO_4)_3 \cdot 14H_2O$) or ferric sulfate ($Fe_2(SO_4)_3$) is added to raw water, the metal ions hydrolyze to form insoluble metal hydroxide precipitates ($Al(OH)_3$ or $Fe(OH)_3$). This chemical reaction consumes natural bicarbonate alkalinity:

$Al_2(SO_4)_3 \cdot 14H_2O + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3 \downarrow + 3CaSO_4 + 6CO_2 \uparrow + 14H_2O$

• Consumption Ratio: For every $1.0\text{ mg/L}$ of commercial alum added, approximately $0.45\text{ to } 0.50\text{ mg/L}$ of natural alkalinity (as $CaCO_3$) is consumed.
• Operational Hazard: If raw water alkalinity is low ($< 20-30\text{ mg/L}$), the coagulant will strip the remaining buffer, driving finished water pH down into the acidic range ($< 5.5$). At low pH, aluminum remains soluble instead of precipitating into floc, resulting in severe floc shearing, high finished water turbidity, dissolved aluminum carryover into distribution mains, and rapid corrosion of customer plumbing. Operators must supplement alkalinity by dosing hydrated lime ($Ca(OH)_2$), caustic soda ($NaOH$), or soda ash ($Na_2CO_3$) to maintain a minimum finished water alkalinity of $30\text{ to } 50\text{ mg/L as } CaCO_3$.

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3. Microbiological Analysis: Coliform Bacteria & E. coli

Testing water for every known waterborne pathogen (Salmonella, Shigella, Vibrio cholerae, Hepatitis A, Cryptosporidium) is technically difficult, cost-prohibitive, and requires days or weeks of specialized incubation. Water utilities instead monitor for indicator organisms.

The Indicator Concept & Coliform Definitions

A valid indicator organism must be present in high numbers whenever pathogens are present, share a similar survival profile to bacterial pathogens, be incapable of proliferating in clean water mains, and be easily, rapidly, and safely detectable in the laboratory.

• Total Coliforms: A broad group of aerobic and facultatively anaerobic, Gram-negative, non-spore-forming, rod-shaped bacteria capable of fermenting lactose with gas and acid production within 48 hours at $35.0 \pm 0.5^\circ\text{C}$. Total coliforms include environmental genera (Citrobacter, Enterobacter, Klebsiella) that inhabit soil and vegetation, as well as enteric organisms.
• Fecal Coliforms (Thermotolerant Coliforms): A subset of total coliforms capable of fermenting lactose with acid and gas production at an elevated temperature of $44.5 \pm 0.2^\circ\text{C}$ within 24 hours.
• Escherichia coli (E. coli): The definitive bacterial indicator. E. coli is a thermotolerant coliform that resides exclusively in the gastrointestinal tract of humans and warm-blooded animals. Its presence in potable water indicates direct fecal contamination and the potential presence of dangerous enteric pathogens.

Analytical Methodologies

Public water systems utilize two primary EPA-approved analytical methodologies for distribution compliance monitoring:

1. Enzyme-Substrate Methods (Colilert / Colisure / Defined Substrate Technology)

The Defined Substrate Technology (DST) method simultaneously detects Total Coliforms and E. coli in a single $100\text{ mL}$ sample without requiring sub-culturing or confirmation steps:

• Sample Volume & Collection: Exactly $100\text{ mL}$ of sample is collected in a sterile polypropylene bottle containing sodium thiosulfate ($Na_2S_2O_3$) to neutralize free and combined chlorine residuals ($100\text{ mg/L } Na_2S_2O_3$ neutralizes up to $15\text{ mg/L}$ residual chlorine). Fill to the $100\text{ mL}$ line, leaving headspace for mixing.
• Specific Enzyme Substrates:
  • ONPG (ortho-nitrophenyl-$\beta$-D-galactopyranoside): Total coliform bacteria produce the intracellular enzyme $\beta$-galactosidase, which cleaves the colorless ONPG molecule to release free ortho-nitrophenol, turning the sample container distinct yellow.
  • MUG (4-methylumbelliferyl-$\beta$-D-glucuronide): E. coli produces the specific enzyme $\beta$-glucuronidase, which cleaves MUG to release 4-methylumbelliferone. When irradiated with a 6-watt, 365 nm long-wave ultraviolet (UV) light, the container produces bright blue fluorescence.
• Incubation: Incubate at $35.0 \pm 0.5^\circ\text{C}$ for 24 hours (or 18 hours for Colilert-18). A sterile comparator standard is used to distinguish weak positive yellow color from background water tint.

2. Membrane Filtration (MF) Method (Standard Methods 9222B)

• Filtration: A $100\text{ mL}$ sample is drawn through a sterile, gridded cellulose ester membrane filter ($47\text{ mm}$ diameter, $0.45\ \mu\text{m}$ pore size) using vacuum filtration. Bacterial cells are retained on the filter surface.
• Culture Medium: The filter is placed grid-side up onto an absorbent cellulose pad saturated with $1.8-2.0\text{ mL}$ of m-Endo broth (or m-Endo agar) in a tight-fitting plastic petri dish.
• Incubation: Inverted dishes are incubated at $35.0 \pm 0.5^\circ\text{C}$ for $24 \pm 2\text{ hours}$.
• Colony Identification: Total coliform bacteria ferment lactose in the medium, generating acetaldehyde. Acetaldehyde reacts with sodium sulfite and basic fuchsin to produce characteristic golden-green metallic sheen colonies. Colonies without a metallic sheen (pink, red, colorless) are non-coliforms.

Revised Total Coliform Rule (RTCR) Compliance Framework

The EPA and ADEM enforce the Revised Total Coliform Rule (RTCR) under a "find-and-fix" approach designed to detect pathways of contamination:

1. Routine Sampling: Facilities collect a designated number of distribution samples monthly based on population served, following an approved Coliform Sample Siting Plan.
2. Mandatory Repeat Sampling Actions:
   • If ANY routine distribution sample tests positive for Total Coliform, the utility must collect a mandatory set of 3 repeat samples within 24 hours of lab notification:
     • Sample 1: At the original tap where the positive sample was taken.
     • Sample 2: Within 5 active service connections upstream of the original tap.
     • Sample 3: Within 5 active service connections downstream of the original tap.
   • Ground Water Rule (GWR) Triggered Source Water Monitoring: Groundwater systems must also collect a raw water sample from every active source well and test directly for E. coli.
3. Assessments and Violation Triggers:
   • Level 1 Assessment: Triggered if a system collecting $\ge 40$ samples/month has $> 5.0\%$ positive total coliform samples, or if a small system collecting $< 40$ samples has $\ge 2$ positive samples, or if the utility fails to collect all required repeat samples. The utility conducts an internal operational investigation within 30 days to identify sanitary defects.
   • Level 2 Assessment: Triggered by an acute public health threat: an E. coli Maximum Contaminant Level (MCL) violation (e.g., an E. coli positive routine sample followed by a total coliform positive repeat, or a total coliform positive routine followed by an E. coli positive repeat, or failure to collect repeat samples following an E. coli positive routine). Also triggered if a second Level 1 assessment occurs within a rolling 12-month window. An E. coli MCL violation triggers mandatory Tier 1 Public Notification (Boil Water Advisory within 24 hours) and a Level 2 assessment by ADEM or an ADEM-approved party.

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4. Jar Testing Protocol for Coagulation Optimization

Full-scale coagulation and flocculation cannot be controlled solely by theoretical chemical calculations. Source water characteristics—turbidity, temperature, dissolved organic carbon (DOC), color, and alkalinity—fluctuate continuously with weather, rainfall runoff, and seasonal overturn. The jar test is the essential bench-scale laboratory procedure used by operators to simulate plant-scale rapid mix, tapered flocculation, and sedimentation.

Jar Testing Equipment & Reagent Preparation

• Gang Stirrer Apparatus: A multi-paddle mechanical stirrer (typically 6 stainless steel paddles) equipped with variable speed digital control ($0-300\text{ rpm}$) and an illuminated base.
• Square Jars (B-Ker Jars): Testing is conducted in 2-liter square acrylic or glass jars. Square geometry disrupts rotational fluid vortexing and creates hydraulic turbulence patterns that closely mirror full-scale baffled treatment basins.
• Stock Chemical Solutions: Primary coagulants (liquid alum or ferric sulfate) must be prepared freshly as 1.0% ($10,000\text{ mg/L}$) or 0.1% ($1,000\text{ mg/L}$) working stock solutions using distilled water. For example, adding $1.0\text{ mL}$ of a $1,000\text{ mg/L}$ stock solution into a $1.0\text{ L}$ water sample yields an exact coagulant dosage of $1.0\text{ mg/L}$ ($2.0\text{ mL}$ into a $2.0\text{ L}$ jar equals $1.0\text{ mg/L}$).

Step-by-Step Jar Testing Procedure

[Step 1: Raw Water Characterization]
Measure initial turbidity, pH, alkalinity, and temperature of fresh raw source water.
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[Step 2: Dosing & Rapid Mix Phase]
Inject incremental coagulant doses into 2-liter B-Ker jars.
Flash mix at 100-150 rpm for 60 seconds (G ≈ 700-1,000 s⁻¹).
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[Step 3: Tapered Flocculation Phase]
Reduce speed to promote interparticle collisions without shearing fragile flocs:
  • 5 minutes at 40 rpm (G ≈ 50 s⁻¹)
  • 5 minutes at 30 rpm (G ≈ 30 s⁻¹)
  • 5 minutes at 20 rpm (G ≈ 15 s⁻¹)
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[Step 4: Quiescent Sedimentation Phase]
Stop paddles and allow quiet gravity settling for 20 to 30 minutes.
Observe floc size, settling rate, and supernatant clarity.
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[Step 5: Supernatant Sampling & Analysis]
Withdraw sample from sampling port 2 inches below water surface.
Analyze settled turbidity, final pH, residual alkalinity, and pin floc carryover.

1. Raw Water Baseline: Collect 15 to 20 liters of fresh, representative raw source water. Measure and record initial water temperature, turbidity, pH, and alkalinity.
2. Jar Setup: Fill each of the 6 B-Ker jars to the 2.0-liter mark. Jar 1 is frequently left as an untreated control or loaded with the current baseline plant dosage, while Jars 2 through 6 receive incremental dosages spanning above and below current plant operations (e.g., 10, 15, 20, 25, 30, and 35 mg/L alum).
3. Rapid Mix (Flash Mix): Start paddles at 100 to 150 rpm. Rapidly inject the calculated coagulant volumes simultaneously into the center of each jar at the liquid surface. Mix for 60 seconds. This high-energy mixing simulates the plant rapid mix basin ($G \approx 700-1,000\text{ s}^{-1}$), ensuring micro-scale dispersion of coagulant species and neutralizing negative surface charges on colloidal particles within milliseconds.
4. Tapered Flocculation: Reduce paddle speeds systematically to prevent shearing of newly formed floc aggregates:
   • Stage 1: Run paddles at 40 rpm for 5 minutes ($G \approx 50\text{ s}^{-1}$).
   • Stage 2: Reduce to 30 rpm for 5 minutes ($G \approx 30\text{ s}^{-1}$).
   • Stage 3: Reduce to 20 rpm for 5 minutes ($G \approx 15\text{ s}^{-1}$).
   • Operator Observations: Note the elapsed time until visible pin floc appears. Rate floc growth on a standardized index (pin floc, grain size, pea size). Observe inter-floc water clarity.
5. Quiescent Settling: Stop paddles, lift the drive assembly clear of the water, and allow undisturbed gravity settling for 20 to 30 minutes (simulating sedimentation basin detention time). Observe settling velocities: fast ($> 1\text{ ft/min}$), moderate, or slow, and note the compaction of the settled sludge blanket at the jar bottom.
6. Supernatant Testing: Open the sampling ports located precisely 2 inches ($5\text{ cm}$) below the water surface. Discard the first 10 to 20 mL from the sampling line to flush the tube, then collect samples into clean beakers. Measure and record settled turbidity, pH, and residual alkalinity.
7. Data Evaluation & Optimum Selection:
   • Plot settled turbidity versus coagulant dosage.
   • The optimum coagulant dosage is the lowest chemical feed rate that produces a dense, rapidly settling floc resulting in a settled supernatant turbidity of $\le 1.0\text{ to } 2.0\text{ NTU}$, while preserving adequate residual alkalinity ($\ge 30\text{ mg/L}$) and avoiding excessive chemical costs or excess sludge generation.