2.3 Physical, Chemical & Biological Characteristics of Raw Water

2.3 Physical, Chemical & Biological Characteristics of Raw Water

Water treatment operators must monitor and interpret a broad spectrum of physical, chemical, and biological parameters to adjust chemical dosages, optimize unit treatment processes, and ensure finished drinking water satisfies all state and federal Primary and Secondary Maximum Contaminant Levels (MCLs).

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Physical Properties of Raw Water

Physical parameters characterize the aesthetic, optical, and thermal qualities of raw water, directly impacting treatment process efficiency and consumer acceptance.

1. Turbidity

Turbidity is an optical measurement of water clarity, quantifying the degree to which light is scattered and absorbed by suspended particles (silt, clay, fine organic matter, inorganic minerals, and microscopic organisms) rather than transmitted in straight lines.

• Measurement Unit: Measured in Nephelometric Turbidity Units (NTU) using a nephelometer (turbidimeter), which directs a light beam through a sample and measures light scattered at a $90^\circ$ angle relative to the incident light path.
• Operational Significance:
  • Suspended particles shield pathogenic bacteria, viruses, and protozoan cysts from chemical disinfectants (free chlorine, chloramines, chlorine dioxide) and ultraviolet (UV) light irradiation.
  • Elevated raw turbidity increases coagulant chemical demand and accelerates sludge accumulation in sedimentation basins.
  • Under the EPA/MoDNR Surface Water Treatment Rule, combined filter effluent turbidity must remain $\le 0.3\text{ NTU}$ in at least $95%$ of measurements each month and must never exceed $1.0\text{ NTU}$.

2. Color: True Color vs. Apparent Color

• Apparent Color: The total visual color of an unfiltered raw water sample. It results from the combined optical effect of dissolved chemical compounds and suspended particulate turbidity.
• True Color: The color of a water sample after all suspended particulate turbidity has been removed by centrifugal separation or filtration through a $0.45\ \mu\text{m}$ membrane filter.
  • Causes: True color originates from dissolved natural organic matter (NOM), particularly humic and fulvic acids leached from decaying leaves, peat, and soil humus, as well as dissolved iron and manganese.
  • Measurement: Quantified against standard visual comparator solutions on the Platinum-Cobalt (Pt-Co) Scale (Standard Methods APHA color units). The Secondary MCL for color is $15\text{ Pt-Co units}$.

3. Taste and Odor: The Threshold Odor Number (TON)

Raw water taste and odor issues originate from biological metabolites, decaying vegetation, industrial discharges, or dissolved gases (such as hydrogen sulfide, $\text{H}_2\text{S}$).

• Threshold Odor Test: An operator evaluates odor intensity by progressively diluting an odorous water sample with odor-free blank water at $60^\circ\text{C}$ until no discernible odor is detected.
• Threshold Odor Number (TON) Formula: $\text{TON} = \frac{A + B}{A}$ where $A$ is the volume of the raw odorous sample (in $\text{mL}$) and $B$ is the volume of odor-free dilution water (in $\text{mL}$) required to yield a mixture with barely perceptible odor.
  • Example Calculation: If $25\text{ mL}$ of raw water ($A$) is diluted with $175\text{ mL}$ of odor-free water ($B$) to reach the detection threshold: $\text{TON} = \frac{25 + 175}{25} = \frac{200}{25} = 8.0$
• Regulatory Threshold: The EPA Secondary MCL for odor is a TON of $3$.

4. Temperature Effects on Water Treatment

Water temperature drives both physical and chemical treatment dynamics:

• Fluid Viscosity & Stokes' Law: As water temperature drops, dynamic fluid viscosity increases significantly (water at $35^\circ\text{F}$ is nearly twice as viscous as water at $80^\circ\text{F}$). According to Stokes' Law, particle settling velocity is inversely proportional to fluid viscosity: $v = \frac{g (\rho_p - \rho_w) d^2}{18 \mu}$ where $\mu$ is dynamic viscosity. Cold winter water retards floc settling in sedimentation basins, causing floc carryover onto granular media filters.
• Reaction Kinetics & Coagulation: Cold water slows chemical reaction rates, requiring increased coagulant dosages, longer rapid-mix dispersion, and expanded flocculation detention times.
• Disinfection Contact Time ($CT$): Disinfection chemical kinetics proceed more slowly in cold water. To achieve mandatory log-inactivation of Giardia and viruses, operators must provide substantially higher $CT$ values (disinfectant concentration $\times$ contact time in $\text{mg}\cdot\text{min/L}$) during winter operations.
• Gas Solubility: Colder water holds higher concentrations of dissolved gases, including dissolved oxygen (DO) and carbon dioxide ($\text{CO}_2$).

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Chemical Characteristics of Raw Water

Chemical parameters determine water stability, corrosivity, coagulant chemical requirements, and compliance with drinking water quality standards.

1. pH & Hydrogen Ion Activity

• Definition: The negative base-10 logarithm of hydrogen ion activity: $\text{pH} = -\log[\text{H}^+]$. The scale ranges from $0$ (strongly acidic) to $14$ (strongly basic), with $7.0$ representing chemical neutrality at $25^\circ\text{C}$. Because the scale is logarithmic, a drop of 1.0 pH unit represents a 10-fold increase in hydrogen ion concentration.
• Operational Importance:
  • Coagulation Windows: Aluminum sulfate (alum) operates optimally within a pH range of $5.8–7.5$, whereas ferric sulfate operates across a broader window of $4.0–9.0$.
  • Chlorine Speciation: At $\text{pH } 6.0$, over $95%$ of free chlorine exists as hypochlorous acid ($\text{HOCl}$), a potent germicide. At $\text{pH } 8.5$, over $90%$ dissociates into the hypochlorite ion ($\text{OCl}^-$), which is $80–100$ times less effective as a disinfectant.
  • Corrosivity: Secondary MCL guideline for finished water pH is $6.5–8.5$.

2. Alkalinity & Buffer Capacity

Alkalinity is the quantitative capacity of water to neutralize acids, acting as a chemical buffer against rapid pH fluctuations. It is expressed in units of $\text{mg/L as CaCO}_3$.

• Primary Chemical Species: Bicarbonate ($\text{HCO}_3^-$), Carbonate ($\text{CO}_3^{2-}$), and Hydroxide ($\text{OH}^-$).
• Coagulation Alkalinity Consumption: Metal coagulants are acidic salts that react directly with raw water alkalinity to form insoluble metal hydroxide precipitates: $\text{Al}_2(\text{SO}_4)_3 \cdot 14\text{H}_2\text{O} + 3\text{Ca(HCO}_3)_2 \longrightarrow 2\text{Al(OH)}_3\downarrow + 3\text{CaSO}_4 + 14\text{H}_2\text{O} + 6\text{CO}_2$
  • Stoichiometry: Every $1.0\text{ mg/L}$ of alum added consumes approximately $0.45–0.50\text{ mg/L}$ of natural alkalinity (as $\text{CaCO}_3$).
  • Operational Rule: If raw water alkalinity is insufficient ($<30–45\text{ mg/L}$), adding alum causes a severe pH drop ("pH crash"), halting floc formation and causing soluble aluminum to pass through filters. Operators must add supplemental alkalinity—such as hydrated lime ($\text{Ca(OH)}_2$), caustic soda ($\text{NaOH}$), or soda ash ($\text{Na}_2\text{CO}_3$).

3. Hardness

Hardness is caused by divalent and polyvalent metallic cations dissolved from sedimentary rock formations (limestone, dolomite, gypsum), expressed as $\text{mg/L as CaCO}_3$.

• Primary Cations: Calcium ($\text{Ca}^{2+}$) and Magnesium ($\text{Mg}^{2+}$).
• Hardness Classifications:
  • Carbonate Hardness (Temporary Hardness): Hardness chemically associated with bicarbonate ($\text{HCO}_3^-$) and carbonate ($\text{CO}_3^{2-}$) anions. It precipitates out of solution as calcium carbonate scale when heated.
  • Non-Carbonate Hardness (Permanent Hardness): Hardness associated with sulfate ($\text{SO}_4^{2-}$), chloride ($\text{Cl}^-$), or nitrate ($\text{NO}_3^-$) anions. It cannot be removed by boiling; removal requires chemical lime-soda softening or ion exchange.

4. Total Dissolved Solids (TDS)

Total Dissolved Solids (TDS) represents the aggregate mass of all dissolved inorganic mineral ions and organic molecules smaller than $2.0\ \mu\text{m}$. High TDS increases mineral taste, pipe corrosion, and water heater scaling. The EPA Secondary MCL for TDS is $500\text{ mg/L}$.

5. Iron ($\text{Fe}$) and Manganese ($\text{Mn}$)

Iron and manganese commonly occur in anaerobic groundwater and stagnant reservoir bottom waters in their soluble, reduced divalent oxidation states:

• Soluble Forms: Ferrous iron ($\text{Fe}^{2+}$) and Manganous manganese ($\text{Mn}^{2+}$).
• Aesthetic Problems: When exposed to oxygen or chlorine, they oxidize into insoluble precipitates—ferric hydroxide ($\text{Fe(OH)}_3$, reddish-orange staining) and manganese dioxide ($\text{MnO}_2$, dark brown/black staining).
• Secondary Standards: Iron MCL is $0.3\text{ mg/L}$; Manganese MCL is $0.05\text{ mg/L}$.

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Biological & Microbiological Characteristics

Raw surface waters and shallow groundwater sources contain diverse microorganisms, some of which present severe public health hazards.

                  Increasing Disinfection Resistance ──►

  Bacteria ────────────────► Viruses ────────────────► Protozoa
  (E. coli, Salmonella)      (Norovirus, Hep A)        (Giardia cysts, Cryptosporidium oocysts)
  [Chlorine Inactivates      [Small (20-100 nm),       [Thick outer oocyst wall,
   Rapidly in Minutes]        Need High CT/UV]          Highly Chlorine Resistant,
                                                        Requires Filtration/UV/Ozone]

Pathogen Classifications

1. Pathogenic Bacteria: Single-celled prokaryotic organisms ($0.5–5.0\ \mu\text{m}$).
   • Examples: Salmonella enterica (typhoid and salmonellosis), Shigella dysenteriae (bacillary dysentery), Vibrio cholerae (cholera), Campylobacter jejuni, and Shiga toxin-producing Escherichia coli O157:H7.
   • Control: Highly susceptible to conventional chemical oxidants (free chlorine, chloramines, chlorine dioxide).
2. Enteric Viruses: Ultramicroscopic infectious particles ($0.02–0.10\ \mu\text{m}$ / $20–100\text{ nm}$) containing DNA or RNA wrapped in a protein capsid.
   • Examples: Enteroviruses, Norovirus, Rotavirus, and Hepatitis A virus.
   • Control: Due to their sub-micron size, viruses easily pass through granular media filters unless effective preceding chemical coagulation is applied. The Surface Water Treatment Rule treatment train must achieve 4-log ($99.99%$) virus removal/inactivation credit using approved barriers.
3. Protozoan Parasites (Cysts & Oocysts): Single-celled eukaryotic organisms that produce protective survival shells:
   • Giardia lamblia: Forms oval cysts ($8–14\ \mu\text{m}$) that cause giardiasis. Cysts exhibit moderate resistance to free chlorine; requires 3-log ($99.9%$) removal/inactivation.
   • Cryptosporidium parvum: Forms spherical oocysts ($4–6\ \mu\text{m}$) with a rigid multi-layer shell. Oocysts are extremely resistant to free chlorine disinfection (a CT of several thousand $\text{mg}\cdot\text{min/L}$ is required for 1-log inactivation, which is impractical for drinking water plants).
   • Primary Barriers against Crypto: Coagulation and granular media filtration, microfiltration/ultrafiltration membranes, ultraviolet (UV) light irradiation, or ozone ($\text{O}_3$). Regulated under the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR).

Indicator Organisms & The Revised Total Coliform Rule (RTCR)

Because isolating specific pathogenic organisms is technically difficult, expensive, and time-consuming, water systems monitor indicator organisms:

• Total Coliform Group: Aerobic and facultatively anaerobic, Gram-negative, non-spore-forming, rod-shaped bacteria that ferment lactose with gas and acid production within 48 hours at $35^\circ\text{C}$. Total coliforms originate from animal intestinal tracts, soil, and vegetation, serving as a broad indicator of treatment performance and distribution system integrity.
• Escherichia coli (E. coli): A specific subgroup of fecal coliform bacteria that originates exclusively from the feces of warm-blooded animals. Detection of E. coli confirms recent fecal contamination and indicates immediate pathogen risk, triggering an MCL violation and requiring immediate public notification.

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Reservoir Limnology, Stratification & Seasonal Turnover

Deep raw water storage reservoirs and lakes undergo pronounced thermal and chemical transitions driven by solar heating, seasonal temperature swings, and wind mixing.

                    SUMMER STRATIFICATION
  ════════════════════════════════════════════════════════ Surface Water
  EPILIMNION        Warm ($68-85°F$), High DO, Sunlight, Active Algae
  ────────────────────────────────────────────────────────
  THERMOCLINE       Rapid Temp Drop (>= 1°C per meter / 1.8°F per ft)
  (Metalimnion)     Sharp Density Gradient (Barrier to Mixing)
  ────────────────────────────────────────────────────────
  HYPOLIMNION       Cold ($39-50°F$), Stagnant, Dense
                    Anoxic (DO = 0 mg/L)
                    High Fe2+, Mn2+, H2S, NH3, MIB & Geosmin
  ════════════════════════════════════════════════════════ Reservoir Bottom

The Three Thermal Strata in Summer

During late spring and summer, solar radiation heats the surface layer of a deep lake. Because warm water is less dense than cold water, the reservoir separates into three distinct non-mixing zones:

1. Epilimnion: The warm, less dense upper water layer ($68^\circ\text{F}–85^\circ\text{F}$). It remains well-mixed by surface winds, receives abundant sunlight, maintains high dissolved oxygen ($7.0–12.0\text{ mg/L}$) through atmospheric contact and photosynthetic oxygen production, and supports active algal populations.
2. Metalimnion (Thermocline): The intermediate transition zone characterized by a rapid decrease in water temperature with increasing depth—defined as a temperature drop of at least $1^\circ\text{C}$ per meter ($0.55^\circ\text{F}$ per foot) of depth. The resulting sharp density gradient acts as a physical barrier preventing water exchange between the upper and lower layers.
3. Hypolimnion: The deep, cold ($39^\circ\text{F}–50^\circ\text{F}$), dense bottom layer isolated from atmospheric oxygenation and sunlight.

Anoxic Hypolimnion Chemistry

As dead plankton and organic debris sink into the dark hypolimnion, aerobic heterotrophic bacteria decompose the organic mass, rapidly consuming all available dissolved oxygen. Once the hypolimnion becomes anoxic ($\text{DO} = 0.0\text{ mg/L}$), anaerobic bacteria switch to alternative terminal electron acceptors, driving chemically reducing reactions:

• Insoluble oxidized iron ($\text{Fe}^{3+}$) is reduced to soluble ferrous iron ($\text{Fe}^{2+}$).
• Insoluble oxidized manganese ($\text{Mn}^{4+}$) is reduced to soluble manganous manganese ($\text{Mn}^{2+}$).
• Sulfate ($\text{SO}_4^{2-}$) is reduced to hydrogen sulfide gas ($\text{H}_2\text{S}$), producing a rotten-egg odor and high chlorine demand.
• Organic nitrogen and nitrates ($\text{NO}_3^-$) reduce to ammonia ($\text{NH}_3$).

Seasonal Overturn (Turnover) Dynamics

• Fall Overturn: In autumn, ambient air temperatures drop and solar radiation declines. The epilimnion cools, increases in density, and sinks. When the surface water temperature reaches approximately $39.2^\circ\text{F}$ ($4^\circ\text{C}$)—the temperature of maximum water density—the entire water column becomes isothermal (uniform temperature and density from top to bottom). Moderate autumn winds then mix the entire reservoir from surface to bottom.
  • Impact on Water Treatment: Overturn suddenly disperses the stagnant, anoxic hypolimnetic water throughout the entire water column. Treatment plants experience abrupt spikes in raw water iron, manganese, hydrogen sulfide, natural organic matter, and taste/odor compounds. Chlorine demand surges, and coagulant dosages must be immediately re-optimized via jar testing.
• Spring Overturn: Following ice melt in northern climates, surface waters warm from $32^\circ\text{F}$ to $39.2^\circ\text{F}$, destabilizing winter stratification and initiating complete spring mixing.

Algal Blooms & Taste and Odor Metabolites

Warm water, abundant sunlight, and excessive nutrient runoff (phosphorus and nitrogen) induce rapid growth of cyanobacteria (blue-green algae), such as Microcystis, Anabaena, Aphanizomenon, and Planktothrix.

• Cyanotoxins: Algal blooms can release potent hepatotoxins (Microcystin, Cylindrospermopsin) and neurotoxins (Anatoxin-a), threatening human and animal health.
• Taste and Odor Metabolites:
  • Geosmin (trans-1,10-dimethyl-trans-9-decalol): Imparts an intense earthy, muddy taste and odor.
  • 2-Methylisoborneol (MIB): Imparts a sharp musty, damp-cellar taste and odor.
  • Human Olfactory Sensitivity: Humans can detect both geosmin and MIB at extremely trace concentrations of $5–10\text{ nanograms per liter}$ (parts per trillion).
  • Treatment Strategies: Standard chlorination and conventional coagulation do not effectively remove geosmin or MIB. Removal requires feeding Powdered Activated Carbon (PAC), filtration through Granular Activated Carbon (GAC), chemical oxidation with potassium permanganate ($\text{KMnO}_4$) or ozone ($\text{O}_3$), or advanced oxidation processes (AOP).

Multi-Level Intake Management

To manage stratification and algal blooms, raw water intake structures on deep reservoirs are equipped with multi-level intake ports at various depths. Operators select intake gates that draw water from depths situated below surface algal scums and above the stagnant, anoxic hypolimnion.