11.2 Physical & Chemical Water Quality Analysis

9.1 Physical & Chemical Water Quality Analysis

Physical and chemical laboratory testing forms the cornerstone of water and wastewater treatment plant control, process optimization, and regulatory compliance. Certified operators in South Carolina must understand the underlying chemistry, analytical procedures, instrument calibration, and quality control requirements mandated by the South Carolina Department of Environmental Services (SC DES) under the Safe Drinking Water Act (SDWA) and Clean Water Act (CWA).

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Physical Water Quality Parameters

Physical parameters reflect the sensory and particulate characteristics of water. Though some physical attributes are classified as secondary aesthetic standards, parameters like turbidity directly impact disinfection efficacy and public health.

Turbidity & Nephelometry

Turbidity is an optical measurement of water clarity representing the light-scattering and light-absorbing properties of suspended matter, such as clay, silt, finely divided organic matter, plankton, and microscopic organisms.

• Measurement Principle: Regulatory turbidity analysis utilizes nephelometry, where a light beam is passed through a sample and a photo-detector measures the intensity of light scattered at a 90-degree angle relative to the incident light path. Results are reported in Nephelometric Turbidity Units (NTU).
• Calibration Standards: Instruments must be calibrated using Formazin primary standards or EPA-approved secondary standards (such as StablCal or Gelex secondary formulation). Formazin is a polymer suspension produced by mixing hydrazine sulfate and hexamethylenetetramine. Primary calibration is performed quarterly or following major maintenance, while secondary standards verify instrument accuracy daily.
• Regulatory Standards: Under SC DES drinking water regulations for conventional surface water treatment plants:
  • Combined filter effluent (CFE) turbidity must be (\le 0.3 \text{ NTU}) in at least 95% of monthly samples.
  • CFE turbidity must never exceed 1.0 NTU at any time.
  • Individual filter effluent (IFE) must be continuously monitored; spikes above 0.5 NTU or 1.0 NTU trigger mandatory reporting and filter performance evaluations.
• Analytical Best Practices: Sample cuvettes must be scrupulously cleaned with non-abrasive detergent, rinsed with ultra-pure water, dried with lint-free wipes, and coated with a thin layer of silicon oil to mask micro-scratches. Outgassing (air bubbles) creates false high readings and must be eliminated by gentle swirling or vacuum degassing.

Color & Odor Determination

• True vs. Apparent Color: Apparent color includes color from both dissolved substances and suspended matter in an unfiltered sample. True color is measured after removing suspended particulates by centrifuging or filtering the sample through a 0.45 (\mu\text{m}) membrane filter. Color is reported in Platinum-Cobalt (Pt-Co) units (also called Hazen units), where 1 unit corresponds to 1 mg/L platinum as chloroplatinate ion. The secondary drinking water standard for true color is 15 Pt-Co units.
• Threshold Odor Number (TON): Odor is evaluated using human panel dilutions at 60°C. The Threshold Odor Number (TON) represents the dilution ratio at which odor is just barely detectable: [ \text{TON} = \frac{A + B}{A} ] where (A) is the volume of original sample (mL) and (B) is the volume of odor-free dilution water (mL). A standard total volume of 200 mL is maintained. For example, if 50 mL of sample diluted with 150 mL of odor-free water yields the threshold odor, the (\text{TON} = \frac{50 + 150}{50} = 4). The secondary standard for drinking water is 3 TON.

Temperature

Water temperature influences chemical reaction rates, gas solubility, biological activity, and disinfectant contact time ((CT) values). Cold water reduces disinfection kinetics and coagulant reaction speed while increasing dissolved gas capacity. Temperature measurements must be recorded in situ using a calibrated NIST-traceable thermometer (precision (\pm 0.1^\circ\text{C})).

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Chemical Water Quality Parameters

pH Measurement & Electrometric Calibration

pH is the negative logarithm of hydrogen ion activity ((\text{pH} = -\log[\text{H}^+])). Measurement is performed electrometrically using a glass measuring electrode paired with a reference electrode (e.g., (\text{Ag/AgCl})) or a combination electrode.

• Nernst Equation & Temperature Compensation: The electrode potential shifts with temperature according to the Nernst equation: [ E = E_0 + \frac{2.303 R T}{n F} \log(\text{a}_{\text{H}^+}) ] At 25°C, the theoretical Nernstian slope is 59.16 mV per pH unit. Instruments feature Automatic Temperature Compensation (ATC) to adjust the slope calculation based on sample temperature.
• Calibration Protocol: SC DES requires a daily 2-point or 3-point calibration bracketing the expected sample pH range using fresh standard buffer solutions:
  • pH 7.00 (Neutral reference buffer setting zero-potential / offset)
  • pH 4.01 (Acidic range slope buffer)
  • pH 10.01 (Alkaline range slope buffer)
• Slope Verification: Acceptable electrode slope ranges between 95% and 105% (56.2 to 62.1 mV/pH unit). Electrodes below 95% slope must be cleaned, reconditioned in 0.1 M HCl, or replaced. Glass electrodes must never be stored in deionized water, which leaches ions; store in 3 M KCl storage solution.

Alkalinity Titration

Alkalinity is the quantitative capacity of water to neutralize hydrogen ions (acids). It serves as a vital chemical buffer preventing rapid pH drops during coagulation (alum addition) or nitrification. Principal species are Hydroxide ((\text{OH}^-)), Carbonate ((\text{CO}_3^{2-})), and Bicarbonate ((\text{HCO}_3^-)).

• Titration Procedure: A 100 mL sample is titrated with standard 0.02 N (0.01 M) Sulfuric Acid ((\text{H}_2\text{SO}_4)):
  • Phenolphthalein Alkalinity (P-Alk): Titrated to pH 8.3 (phenolphthalein color change from pink to clear), neutralizing all hydroxide and converting carbonate to bicarbonate.
  • Total Alkalinity (T-Alk): Titrated to pH 4.5 (bromcresol green-methyl red indicator change from blue-green to pink), neutralizing all bicarbonate to carbonic acid ((\text{H}_2\text{CO}_3)).
• Calculation Formula: [ \text{Alkalinity (mg/L as CaCO}3\text{)} = \frac{A \times N \times 50,000}{V{\text{sample}}} ] where (A) = mL of standard (\text{H}_2\text{SO}4) titrant, (N) = normality of acid (0.02 N), and (V{\text{sample}}) = sample volume in mL. (Note: (0.02 \times 50,000 = 1,000), so for a 100 mL sample, (\text{Alkalinity} = A \times 10)).

Hardness Determination

Water hardness is caused by polyvalent metallic cations, predominantly Calcium ((\text{Ca}^{2+})) and Magnesium ((\text{Mg}^{2+})).

• EDTA Titrimetric Method: Total hardness is determined by titrating with Ethylenediaminetetraacetic acid (EDTA) at pH 10.0 (\pm 0.1) (buffered using an (\text{NH}_4\text{OH-NH}_4\text{Cl}) buffer solution) using Eriochrome Black T (EBT) indicator. The EDTA chelates (\text{Ca}^{2+}) and (\text{Mg}^{2+}) ions, driving a sharp color change from wine-red to pure blue.
• Classification: Total hardness is expressed in mg/L as (\text{CaCO}_3):
  • Soft: 0 – 60 mg/L
  • Moderately Hard: 61 – 120 mg/L
  • Hard: 121 – 180 mg/L
  • Very Hard: > 180 mg/L

Total Dissolved Solids (TDS) & Specific Conductance

• Total Dissolved Solids (TDS): Gravimetric determination performed by filtering sample through a glass fiber filter (2.0 (\mu\text{m})), evaporating filtrate in a pre-weighed dish at 180°C (\pm 2^\circ\text{C}) to constant weight. Drying at 180°C releases occluded water and converts bicarbonate to carbonate.
• Electrical Conductivity (EC): Measures current-carrying capacity of dissolved ions in microsiemens per centimeter ((\mu\text{S/cm})) normalized to 25°C.
• TDS to Conductivity Ratio: For typical natural waters and treated effluents, an empirical ratio exists: [ \text{TDS (mg/L)} \approx (0.55 \text{ to } 0.70) \times \text{Conductivity }(\mu\text{S/cm}) ]

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Disinfectant Residual Testing Methods

Disinfection compliance requires continuous verification of chlorine species in distribution and treatment processes.

In DPD analysis, Free Available Chlorine consists of hypochlorous acid ((\text{HOCl})) and hypochlorite ion ((\text{OCl}^-)). When ammonia is present, chloramines (monochloramine (\text{NH}_2\text{Cl}), dichloramine (\text{NHCl}_2), and trichloramine (\text{NCl}_3)) form Combined Chlorine. DPD titrations or colorimetric kits must be analyzed within 15 minutes of sample collection to prevent chlorine volatilization or degradation.