Summary of "ES200_ES250_WQ1"

Overview — main ideas and lessons

Earth is ~71% water, but only ~2.5% of that is fresh water; most fresh water is unavailable for direct use (large fractions locked in glaciers and deep groundwater).
Water is renewed by the hydrological cycle (evaporation, condensation, precipitation, transpiration) driven by solar energy, but practically it is a finite resource that requires careful management.
Natural water composition is shaped by movement through compartments (ocean, atmosphere, rivers, groundwater) and by physical/chemical processes (dissolution, precipitation, biological activity).

Water availability and renewability

Global breakdown:
- ~96.5% of the planet’s total water is saline (oceans); the remainder includes atmospheric and other reservoirs.
- Of the ~2.5% fresh water, about 69% is in glaciers, ~30.1% is groundwater, and ~1% is surface water (rivers, lakes).
The hydrological cycle renews water but does not imply unlimited availability; distribution and quality vary regionally.

Hydrological cycle — key processes

Main processes: evaporation, condensation, precipitation (rain/snow), transpiration, convection, freezing/melting, runoff, and groundwater flow.
Movement among compartments alters dissolved and particulate constituents, producing characteristic water chemistries in each compartment.

Composition of natural waters — characteristic patterns

Rainwater
- Often resembles strongly diluted seawater because marine aerosols act as cloud condensation nuclei.
- Dominant ions: Na+ and Cl–.
- Typical natural rain pH ≈ 5.7 (acidified by dissolved CO2). Acid rain has pH < 5.7 due to NOx and SO2 (forming nitric and sulfuric acids).
River water
- Composition governed by three principal controls:
  - Precipitation dominance (rain-fed salts),
  - Rock dominance (mineral dissolution — e.g., Ca2+, Mg2+ from carbonates),
  - Evaporation concentration effects.
- The Gibbs diagram (plot of TDS vs. Na/(Na+Ca)) is commonly used to infer the dominant control.
Seawater
- High total dissolved salts with Na+ and Cl– dominant.
- Relative enrichment of Na and Mg, depletion of Ca and carbonate due to CaCO3 precipitation during concentration.
- Typical pH ≈ 8.1 (photosynthesis can remove CO2 and raise pH).
Groundwater
- Tends to be richer in divalent cations (Ca2+, Mg2+), bicarbonate, and silica because of rock weathering.
- Usually has lower dissolved oxygen and often requires aeration before use.

Why water-quality parameters are needed

To assess fitness for specific uses (drinking, agriculture, industry, aquaculture) and to design treatment processes.
Pollution (definition):

Presence or change in physical, chemical, or biological properties at concentrations/levels that interfere with beneficial uses or ecosystem functioning.
Most pollution is anthropogenic, though some hazards (e.g., certain algal toxins) can be naturally occurring.

Classification of water-quality parameters

Chemical: organic and inorganic contaminants (nutrients, heavy metals, synthetic organics).
Physical: temperature, color, turbidity, froth, total suspended solids (TSS), total dissolved solids (TDS).
Biological: pathogens (bacteria, viruses, protozoa), algae, weeds.
Physiological (sensory): taste and odor compounds.

Key chemical and biological parameters — meanings and importance

Chemical Oxygen Demand (COD)
- Surrogate measure of the total oxidizable organic matter.
- Defined as the oxygen equivalent required to oxidize organics to CO2 and H2O.
- Measured by using a strong chemical oxidant (potassium dichromate); consumption is reported in mg O2/L.
Biochemical Oxygen Demand (BOD)
- Measures the biodegradable fraction of organic matter (what microbes will consume).
- Typical test: incubate a seeded/diluted sample at 20°C and measure dissolved oxygen (DO) decrease over 5 days (BOD5).
- BOD is often modeled as first-order decay (initial biodegradable load L0 and decay coefficient k).
- Example regulatory targets: BOD discharge limit for inland surface water ≈ 30 mg/L; drinking water target ≈ 2 mg/L.
Dissolved Oxygen (DO)
- Critical for aquatic life; a healthy range is typically 7–9 mg/L; many fish fail to thrive below ~5 mg/L.
- Solubility governed by Henry’s law and affected by temperature (solubility decreases with rising temperature), salinity/ionic strength (higher ionic strength lowers gas solubility), and biological oxygen demand (microbial consumption).
- Organic discharges create a downstream DO “sag,” commonly conceptualized with the Streeter–Phelps model that balances BOD decay, re-aeration, and advection.
Inorganics and nutrients
- Ammonia, nitrate, phosphate: excess causes eutrophication and algal blooms, which can lead to anoxia.
- pH and alkalinity:
  - Alkalinity is the acid-neutralizing capacity (mainly HCO3–, CO3^2–, OH–).
  - Often expressed as mg/L as CaCO3 (meq/L × 50 mg/meq).
  - Alkalinity buffers pH changes.
- Hardness:
  - Mainly due to Ca2+ and Mg2+.
  - Temporary (carbonate) hardness removed by boiling (decarbonation and precipitation); permanent (non-carbonate) hardness requires chemical treatment or ion exchange.
Total Dissolved Solids (TDS) and salinity
- TDS = total mass of dissolved constituents per unit volume; used to classify water as freshwater, brackish, or seawater.
- Conventional freshwater threshold: TDS < ~1,000 mg/L. Ocean salinity ≈ 34–36 ppt.
- Practical advisory: household reverse-osmosis (RO) units are not recommended where incoming TDS < 500 mg/L because some mineral content is desirable.
Physical parameters
- Color: measured in Hazen or Platinum–Cobalt units; indicates dissolved organics, dyes, or metal complexes.
- Turbidity: light scattering by suspended colloids; measured in NTU with a nephelometer (90° scattering). Desirable turbidity ≤ ~5 NTU.
- Total Suspended Solids (TSS): gravimetric mass of suspended matter per volume.
- Temperature: affects DO solubility; thermal effluent limits typically restrict the allowable temperature rise (e.g., ≤ ~5°C above the receiving water).
- Froth: suggests presence of surfactants, detergents, or saponins.
Physiological (taste & odor)
- Caused by compounds such as phenols, chlorophenols, hydrogen sulfide, amines, indole/scatole; these affect acceptability even if other chemical standards are met.
Biological parameters
- Pathogens: E. coli, Salmonella, Clostridium, viruses, protozoa; algal toxins and aquatic weeds.
- Major cause of waterborne disease, especially in developing regions; disinfection is typically required if pathogens are present.

Regulatory examples and practical limits (lecture examples)

COD discharge limit into receiving waters (example cited for India): ≤ 250 mg/L (industry discharge example).
BOD:
- Inland surface water discharge: ≤ 30 mg/L.
- Drinking water supply target: ≤ 2 mg/L.
DO: desirable range 7–9 mg/L; harmful to fish below ~5 mg/L.
Turbidity: desirable ≤ 5 NTU.
Color: desirable ≤ 5 Hazen units.
Effluent thermal limit: temperature rise not to exceed ~5°C above the receiving water.
TDS: freshwater defined as < ~1,000 mg/L; advisory against household RO use where incoming TDS < 500 mg/L (government advisory referenced).

Methodologies and stepwise procedures

COD (dichromate titrimetric method) — outline
1. Prepare 0.25 N potassium dichromate (K2Cr2O7) oxidant solution.
2. Mix the water sample with acidic dichromate solution containing H2SO4, Ag2SO4 (catalyst), and HgSO4 (to complex chloride and prevent its oxidation artifact).
3. Digest (reflux/sealed vessel) at ~150°C for 2 hours to oxidize organics.
4. Cool the digest; excess dichromate remains.
5. Titrate remaining dichromate with ferrous ammonium sulfate (FAS, Fe2+) using ferroin indicator:
  - Dichromate oxidizes Fe2+ → Fe3+ until dichromate is consumed.
  - Endpoint indicated by ferroin color change.
6. Calculate dichromate consumed (initial − remaining) and convert to mg O2/L (using the equivalent factor; O2 equivalent weight = 8 mg/equivalent).
BOD (BOD5) — basic procedure
1. Seed and dilute sample as required.
2. Measure initial DO (DO0).
3. Incubate sealed samples at 20°C in the dark for 5 days.
4. Measure DO after 5 days (DO5).
5. Compute BOD5 = DO0 − DO5 (with adjustments for dilution and seed oxygen uptake).
6. For kinetics: measure DO decline over time and fit a first-order decay model to estimate L0 and k.
DO and Streeter–Phelps (oxygen sag) modeling — concept
- Mass balance on a river reach: change in DO = re-aeration from atmosphere − oxygen consumption by BOD decay ± advection.
- Re-aeration term is proportional to the DO deficit (DOsat − DO) with a reaeration coefficient that depends on flow and geometry.
- BOD decay is modeled as first-order (k · BOD).
- Solving the differential equations predicts DO sag downstream of an organic discharge.
Alkalinity titration — outline
1. Titrate sample with a strong acid to specified endpoints (using appropriate indicators or pH endpoints).
2. Quantify contributions from OH–, CO3^2–, and HCO3–.
3. Express results in meq/L and/or mg/L as CaCO3 (multiply meq/L by 50 mg/meq).
TDS measurement (gravimetric)
1. Filter sample (commonly 0.45 µm for the dissolved fraction) and collect filtrate in a weighed dish.
2. Evaporate and dry to constant weight.
3. Weigh residue and calculate mg/L TDS based on sample volume.
TSS measurement (gravimetric)
1. Filter a known volume through a pre-weighed filter (appropriate pore size for suspended solids).
2. Dry filter and retained solids to constant weight.
3. Calculate TSS (mg/L) = (final filter weight − initial filter weight) / sample volume.
Turbidity measurement
- Use a nephelometer (90° scattered light) and read in NTU.
Hardness — distinction and treatment
- Temporary (carbonate) hardness: due to Ca/Mg bicarbonates; removed by boiling (CO2 loss → carbonate precipitation).
- Permanent (non-carbonate) hardness: due to Ca/Mg sulfates and chlorides; requires chemical softening (lime-soda) or ion exchange.

Key causes, effects, and management implications

Major pollutants: organic loading (COD/BOD), nutrients (N, P), pathogens, salts, heavy metals, surfactants, thermal loads.
Effects:
- Oxygen depletion and DO sag,
- Eutrophication and algal blooms leading to anoxia,
- Toxicity to aquatic life and humans,
- Taste and odor issues,
- Scaling and fouling from hardness,
- Thermal stress to ecosystems.
Management approach:
- Characterize water using chemical, physical, biological, and physiological parameters.
- Treat according to intended use (drinking, industrial, agricultural).
- Disinfect to control biological risks.
- Control point-source discharges and upstream inputs to prevent DO sag and eutrophication.

Speakers, models, and referenced sources

Lecturer: instructor for ES200 (primary speaker in the lecture).
Scientific methods/models cited:
- Gibbs (1970) — Gibbs diagram for river water composition (TDS vs. Na/(Na+Ca)).
- Streeter–Phelps model — DO sag modeling (re-aeration and BOD decay balance).
- Henry’s law — governs gas solubility (CO2, O2) in water.
Standards/guidance referenced:
- Indian regulatory examples for COD, BOD, turbidity, color, and other limits were cited.
- A government advisory regarding household RO use when incoming TDS < 500 mg/L was mentioned.

Note: some numeric values and procedural details in the original transcript may have been garbled. The above focuses on intended concepts, typical procedures, and relative trends rather than verbatim numeric transcription.