1.2 Physical, Chemical & Biological Source Water Parameters

Physical Water Quality Parameters

Physical parameters describe the optical, aesthetic, and thermal properties of raw water. These parameters dictate coagulant dosages, clarifier surface overflow rates, and filtration run times.

Turbidity

Turbidity is the optical measure of water clarity, quantifying the extent to which suspended colloidal matter (clay, silt, finely divided organic debris, algae, and microorganisms) scatters and absorbs light rather than transmitting it in straight lines.

• Measurement Standard: EPA Method 180.1 mandates nephelometry, utilizing a tungsten-filament light source and a photodetector positioned at a 90-degree angle to the incident beam to measure scattered light in Nephelometric Turbidity Units (NTU). Calibration standards are formulated using synthetic formazin polymer suspensions.
• Operational Importance: Turbidity particles shield bacteria and viruses from chlorine contact, rapidly consume chemical disinfectants, increase coagulant demand, and accelerate filter head loss accumulation.

Color: Apparent vs. True

Color in raw water originates from natural mineral leaches (iron, manganese) or decaying organic vegetation (tannins, lignins, humic acids, and fulvic acids).

• Apparent Color: The color of an untreated, unfiltered water sample, caused by both dissolved compounds and light-scattering suspended particulate matter.
• True Color: The color remaining after all suspended turbidity is physically removed by centrifuging or passing the sample through a 0.45-micrometer membrane filter. True color reflects dissolved colloidal organic matter.
• Scale & Standard: Measured against the Platinum-Cobalt (Pt-Co / Hazen) scale, where one unit corresponds to 1 mg/L platinum as chloroplatinate ion. The EPA Secondary Maximum Contaminant Level (SMCL) for color is 15 Pt-Co units.

Taste and Odor: Threshold Odor Number (TON)

Raw water taste and odor arise from decaying vegetation, actinomycetes, industrial spills, and biological algae metabolites. Odor intensity is quantified using the Threshold Odor Number (TON) dilution protocol per Standard Methods 2150:

$TON = \frac{A + B}{A}$

Where $A$ is the volume of odor-bearing sample (in mL) and $B$ is the volume of odor-free dilution water (in mL) required so that odor is barely perceptible to a trained panel at 60°C. The EPA aesthetic SMCL is 3 TON.

Temperature and Fluid Mechanics

Water temperature exerts profound kinetic and hydrodynamic effects across the treatment train:

1. Dynamic Viscosity & Stokes' Law: As water cools, its dynamic viscosity increases sharply. Water at 0°C (1.79 cP) has twice the viscosity of water at 25°C (0.89 cP). Under Stokes' Law, particle settling velocity ($v_s$) is inversely proportional to dynamic viscosity ($\mu$): $v_s = \frac{g (\rho_p - \rho_w) d^2}{18 \mu}$ In winter operations, cold water cuts particle settling velocities by up to 50%, causing floc carryover onto filters unless basin surface overflow rates are reduced.
2. Coagulation Kinetics: Cold water slows chemical hydrolysis and particle collision rates, demanding higher coagulant doses, polymer aids, and longer flocculation detention times.
3. Disinfection Contact Requirements: Disinfectant reaction kinetics decelerate in cold water. Achieving 3-log Giardia inactivation with free chlorine requires significantly higher residual concentrations or longer contact times ($CT$) at 5°C than at 20°C.

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Chemical Parameters: Buffer Capacity, Mineral Matrix, and Organics

Chemical profiling dictates chemical feed rates and identifies disinfection byproduct precursor liabilities.

Alum Feed: Al2(SO4)3·14H2O + 6HCO3- ---> 2Al(OH)3(s) + 6CO2 + 14H2O + 3SO4(2-)
[ Consumes ~0.5 mg/L Alkalinity as CaCO3 per 1 mg/L Alum dosed ]

pH and Buffering Alkalinity

• pH: The negative logarithm of hydrogen ion activity ($pH = -\log[H^+]$). Controls chemical speciation. During chlorination, pH dictates the equilibrium between potent hypochlorous acid (HOCl) and weaker hypochlorite ion (OCl⁻) with a $pK_a$ of 7.54 at 25°C. At pH 6.5, over 90% exists as HOCl; at pH 8.5, less than 10% exists as HOCl.
• Alkalinity: The capacity of water to neutralize strong acids without significant pH depression, expressed in mg/L as $CaCO_3$. Primarily composed of bicarbonate ($HCO_3^-$), carbonate ($CO_3^{2-}$), and hydroxide ($OH^-$) ions.
• Operational Demand: Each 1.0 mg/L of commercial alum added consumes approximately 0.45 to 0.50 mg/L of natural alkalinity as $CaCO_3$. If raw water alkalinity is low (<30–45 mg/L), alum addition drives pH below the optimum solubility window for aluminum hydroxide precipitation (pH 5.8–6.8), dissolving toxic soluble aluminum into finished water. Operators must dose supplemental alkalinity (hydrated lime, soda ash, or sodium hydroxide).

Total Dissolved Solids (TDS) and Conductivity

• Total Dissolved Solids (TDS): The total concentration of dissolved inorganic minerals and organic matter passing through a 2-micrometer filter, dried to constant mass at 180°C. EPA SMCL is 500 mg/L.
• Specific Conductance: Electrical conductivity measured in micromhos per centimeter or microsiemens per centimeter ($\mu S/cm$) normalized to 25°C. For natural waters, TDS correlates empirically to conductivity: $TDS \text{ (mg/L)} \approx (0.55 \text{ to } 0.70) \times \text{Specific Conductance } (\mu S/cm)$

Total Organic Carbon (TOC) and Specific UV Absorbance (SUVA)

Natural Organic Matter (NOM) reacts with free chlorine to form regulated, carcinogenic Disinfection Byproducts (DBPs): Total Trihalomethanes (TTHM, MCL = 0.080 mg/L) and Five Haloacetic Acids (HAA5, MCL = 0.060 mg/L).

• Total Organic Carbon (TOC): Measures all dissolved and particulate carbonaceous organic matter in mg/L.
• UV-254 Absorbance: Measures light absorption at 254 nm ($cm^{-1}$), directly proportional to aromatic, humic content.
• Specific UV Absorbance (SUVA): Calculated to characterize organic precursor amenability to coagulation: $SUVA \text{ [L/(mg}\cdot\text{m)]} = \frac{\text{UV-254 } (cm^{-1})}{\text{TOC } (mg/L)} \times 100$

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Biological Parameters and Pathogens of Concern

Surface sources harbor biological organisms ranging from nuisance bloom-forming algae to lethal microbial pathogens.

Algal Blooms, Cyanotoxins, and Metabolites

Phosphorus over-enrichment ($P > 0.02\text{ mg/L}$) combined with warm, calm water initiates cyanobacteria (blue-green algae) blooms:

1. Operational Impacts: Diatoms (Asterionella, Synedra) secrete extracellular polymeric substances that clog rapid sand filters within hours. Photosynthetic carbon dioxide consumption drives diurnal raw water pH up to 9.0–10.0.
2. Cyanotoxins:
   • Microcystins: Potent hepatotoxins causing liver hemorrhaging. EPA 10-day Health Advisory guidelines: 0.3 µg/L for bottle-fed infants and children under 6; 1.6 µg/L for school-age children and adults.
   • Cylindrospermopsin: Cytotoxin targeting liver and kidneys. EPA Health Advisory: 0.7 µg/L for infants, 3.0 µg/L for adults.
   • Operating Precaution: Never apply strong pre-chlorine oxidants to raw water containing high concentrations of intact cyanobacteria cells. Chlorine lyses cellular membranes, releasing massive concentrations of dissolved intracellular toxins into the water column. Rely on physical cell removal (coagulation-clarification) and powdered activated carbon (PAC) adsorption.
3. Taste and Odor Compounds: Cyanobacteria (Anabaena, Microcystis) and actinomycetes synthesize geosmin (earthy odor) and 2-methylisoborneol (2-MIB) (musty odor). These non-toxic tertiary alcohols possess extreme human sensory detection thresholds of 5 to 10 nanograms per liter (ng/L or parts per trillion). They are immune to standard chlorine oxidation and must be removed using PAC ($10\text{--}30\text{ mg/L}$) or ozone.

Pathogens of Public Health Significance

Microbial Size Comparison:
Viruses (0.02 - 0.08 µm) < Bacteria (0.5 - 3.0 µm) < Cryptosporidium (4 - 6 µm) < Giardia (8 - 14 µm)

• Cryptosporidium parvum/hominis: Protozoan parasite forming durable, spherical oocysts (4–6 µm) with a thick, multi-layered protein wall. Virtually impervious to standard drinking water chlorine dosages (a 2-log inactivation with free chlorine requires $CT > 7,000\text{ mg}\cdot\text{min/L}$). Protection relies on physical barrier removal (coagulation, sedimentation, and granular filtration) or low-dose ultraviolet (UV) disinfection or ozone.
• Giardia lamblia: Protozoan flagellate forming oval, thick-walled cysts (8–14 µm) causing giardiasis. Moderately resistant to chlorine; regulated under the SWTR, mandating 3-log (99.9%) removal/inactivation via filtration and validated chlorine $CT$ values.
• Enteric Viruses: Ultramicroscopic (0.02–0.08 µm) pathogens (Enteroviruses, Norovirus, Rotavirus, Hepatitis A). Readily pass through coarse media unless coagulated into chemical floc, but highly susceptible to free chlorine (4-log inactivation achieved at low $CT < 6\text{ mg}\cdot\text{min/L}$ at 10°C).
• Coliform Indicators: Under the Revised Total Coliform Rule (RTCR), Escherichia coli (E. coli) serves as the primary fecal indicator. Any confirmed E. coli violation triggers acute health warnings and Tier 1 public notification within 24 hours.