1. Introduction to Water Quality Testing & Oxygen Parameters

Water quality testing relies on physical, chemical, and biological parameters to evaluate the ecological health of aquatic ecosystems and quantify organic pollution loads. Among these, Dissolved Oxygen (DO), Biochemical Oxygen Demand (BOD), and Chemical Oxygen Demand (COD) are the three fundamental oxygen-based indicators used globally by environmental scientists and regulatory agencies (such as the CPCB) to assess water contamination, monitor sewage discharges, and manage wastewater treatment processes.

WATER QUALITY OXYGEN METRICS

1. Dissolved Oxygen (DO)
   - Definition: Amount of free, uncombined O₂ gas dissolved in water.
   - Indicator of: Immediate aquatic life sustainability and health.

2. Biochemical Oxygen Demand (BOD)
   - Definition: O₂ consumed by aerobic bacteria to decompose BIODEGRADABLE organic waste.
   - Indicator of: Microbial organic pollution load (5-day incubation test).

3. Chemical Oxygen Demand (COD)
   - Definition: O₂ required to chemically oxidize TOTAL organic matter (biodegradable + non-biodegradable).
   - Indicator of: Total chemical and industrial pollution load (COD > BOD always).

2. Dissolved Oxygen (DO)

A. Definition & Ecological Significance

• Definition: Dissolved Oxygen (DO) is the measure of free, non-compound oxygen gas (O₂) dissolved in an aquatic volume, expressed in milligrams per liter (mg/L) or parts per million (ppm).
• Vital Role: DO is essential for the cellular respiration of fish, aquatic invertebrates, amphibians, and aerobic decomposers. It governs the self-purification capacity of natural streams and rivers.

B. Sources of Dissolved Oxygen in Water

1. Air-Water Interface Transfer: Atmospheric oxygen dissolves directly into surface water through diffusion and physical aeration driven by wind turbulence, waves, and river currents. Running streams (lotic systems) maintain higher DO levels than stagnant ponds (lentic systems) due to a larger surface area exposed to mechanical mixing.
2. Photosynthetic Generation: Aquatic autotrophs—including phytoplankton (diatoms, dinoflagellates, cyanobacteria) and submerged macrophytes—absorb dissolved CO₂ and release O₂ gas as a metabolic byproduct during daylight hours.

C. Factors Influencing DO Solubility

• Water Temperature (Inverse Relationship): Gas solubility decreases as water temperature increases. Warm water holds significantly less dissolved oxygen than cold water. Thermal pollution from industrial cooling discharges lowers DO levels, causing thermal shock and respiratory stress in fish.
• Water Salinity (Inverse Relationship): As salt concentration rises, the solubility of oxygen in water decreases. Marine and brackish waters hold less DO than freshwater at the same temperature and pressure.
• Organic Waste & Microbial Activity: The addition of organic sewage or agricultural waste stimulates rapid bacterial multiplication. Aerobic bacteria consume DO rapidly to digest organic matter, driving DO down.
• Sunlight & Turbidity: Suspended silt or heavy algal blooms reduce light penetration, halting photosynthesis in deeper layers and lowering DO production.

D. Critical DO Thresholds for Water Quality

DISSOLVED OXYGEN (DO) HEALTH SCALE

≥ 8.0 mg/L ───────────────► Clean Water (Thriving Aquatic Life)
    │
 < 8.0 mg/L ───────────────► Contaminated Water (Early Pollution Signal)
    │
 < 4.0 mg/L ───────────────► Heavily Polluted Water (Hypoxia & Fish Kills)
    │
 < 1.5 mg/L ───────────────► Anoxic State (Anaerobic Decay: H₂S, NH₃, CH₄)

3. Biochemical Oxygen Demand (BOD)

A. Definition & Biological Concept

• Definition: Biochemical Oxygen Demand (BOD) is the total amount of dissolved oxygen required by aerobic microorganisms to decompose the biodegradable organic matter present in a given sample of water over a specified time and temperature.
• Direct Relationship: Higher organic pollution -> Increased microbial activity -> Higher BOD -> Lower Dissolved Oxygen (DO). Thus, BOD is directly proportional to organic waste concentration and inversely proportional to DO:

\[\text{BOD} \propto \text{Organic Waste Load} \quad \text{and} \quad \text{BOD} \propto \frac{1}{\text{DO}}\]

B. Standard Testing Protocol (BOD₅ at 20°C)

• Incubation Parameters: BOD is measured by incubating a sealed water sample at 20°C for 5 days in total darkness. Darkness is mandatory to prevent photosynthetic algae from producing oxygen, which would distort the test results.

• Calculation Formula:

  \[\text{BOD}5 \text{ (mg/L)} = \frac{\text{DO}{\text{initial}} - \text{DO}_{\text{final}}}{\text{Dilution Factor}}\]

• CPCB Bathing Standard: The Central Pollution Control Board (CPCB) mandates that water used for outdoor bathing must maintain a BOD < 3.0 mg/L.

C. Water Quality Categorization Based on BOD Values

D. Limitations of the BOD Parameter

1. Measures Only Biodegradable Waste: BOD accounts solely for biologically digestible organic material; it completely ignores non-biodegradable synthetic chemicals, toxic plastics, heavy metals, and persistent organic pollutants.
2. Long Test Duration: Standard BOD tests require 5 full days, making them unsuitable for real-time emergency pollution monitoring.
3. Toxicity Interference: If the wastewater contains toxic heavy metals (e.g., Hg, Pb, Cd) or cyanides, the test bacteria may be killed, producing a falsely low BOD reading despite high chemical pollution.

4. Chemical Oxygen Demand (COD)

A. Definition & Chemical Principle

• Definition: Chemical Oxygen Demand (COD) is the amount of oxygen (or equivalent chemical oxidant) required to chemically oxidize all organic compounds (both biodegradable AND non-biodegradable) as well as oxidizable inorganic chemicals in a water sample.
• Scope: Provides a comprehensive measurement of the total pollution burden in industrial effluents and municipal sewage.

B. Standard Testing Protocol

• Chemical Oxidant: Uses a powerful chemical oxidizer—Potassium Dichromate (K₂Cr₂O₇)—in a concentrated solution of Sulfuric Acid (H₂SO₄) with a Silver Sulfate (Ag₂SO₄) catalyst.
• Digestion: The water sample is boiled under reflux with K₂Cr₂O₇ for 2 hours. The remaining unreacted dichromate is titrated to determine total oxygen equivalent consumed.
• Advantage of Speed: Delivers results in 2 to 3 hours, compared to 5 days for a BOD₅ test.

C. Comparison & The BOD / COD Ratio

• Universal Rule: The COD value is ALWAYS higher than the BOD value for any given water sample (COD > BOD). This is because chemical dichromate oxidation breaks down complex, biologically resistant materials (such as lignin, cellulose, synthetic dyes, pesticides, and petroleum hydrocarbons) that bacteria cannot digest.

TOTAL ORGANIC & OXIDIZABLE MATTER IN WATER
  │
  ├── Biodegradable Organic Waste ───────────► Measured by BOD & COD
  │
  ├── Non-Biodegradable Organic Waste ───────► Measured ONLY by COD
  │
  └── Oxidizable Inorganic Compounds ────────► Measured ONLY by COD

Result: COD Value is ALWAYS Greater than BOD Value (COD > BOD)

D. The BOD / COD Ratio as a Biodegradability Index

The numerical ratio of BOD to COD (\(\text{BOD} / \text{COD}\)) serves as an operational index to determine the biodegradability of wastewater and select the appropriate treatment process:

• Ratio > 0.6 (Highly Biodegradable): Indicates municipal domestic sewage or food-processing waste. Easily treated using biological sewage treatment plants (STPs) such as activated sludge, trickling filters, or oxidation ponds.
• Ratio 0.2 – 0.5 (Partially Biodegradable): Indicates mixed municipal-industrial wastewater containing recalcitrant chemicals. Requires extended aeration or combined physical-biological treatment.
• Ratio < 0.2 (Non-Biodegradable / Recalcitrant): Indicates toxic industrial effluents (e.g., tanneries, chemical plants, textile dyeing). Biological treatment will fail; requires chemical oxidation, advanced Fenton oxidation, activated carbon adsorption, or membrane filtration.

5. The Oxygen Sag Curve (Streeter-Phelps Model)

When a point-source discharge of untreated organic sewage enters a flowing river stream, a distinct spatial and temporal pattern of oxygen depletion and recovery develops downstream, known as the Oxygen Sag Curve.

           THE OXYGEN SAG CURVE IN A FLOWING RIVER

  DO (mg/L)
   8.0 ──┐  Zone 1   │  Zone 2   │   Zone 3   │  Zone 4
         │ Degradation│ Active    │  Recovery  │ Clear Water
   6.0 ──┤            │ Decay     │            │
         │            │           │            │
   4.0 ──┤            \           │  /─────────┼──────── Standard DO (≥ 8 mg/L)
         │             \ Critical │ /          │
   2.0 ──┤              \  Dip   /             │
         │               \______/              │
   0.0 ──┴───────────────┴──────┴──────────────┴────────────►
        Outfall         Lowest DO             Clean
        Point            Point                River

A. Two Simultaneous Competing Processes

The shape of the curve is governed by two simultaneous, opposing processes defined by the Streeter-Phelps equation:

1. Deoxygenation: Aerobic microorganisms consume dissolved oxygen to decompose incoming organic waste, creating an oxygen deficit.
2. Reaeration: Atmospheric oxygen dissolves across the air-water interface to replenish lost oxygen and restore baseline DO.

B. The Four Distinct River Pollution Zones

1. Zone of Degradation (Immediately Downstream of Outfall)

• Location: Extends a short distance downstream from the sewage discharge point.
• Characteristics: Water becomes dark and turbid; coarse suspended solids settle out forming sludge beds.
• Chemical Profile: BOD spikes rapidly; DO begins to drop sharply as aerobic bacteria initiate organic breakdown.
• Biota: Sensitive fish species migrate away; floating algae decline due to reduced light penetration.

2. Zone of Active Decomposition (Maximum Pollution Zone)

• Location: Follows the degradation zone; represents the most severely polluted stretch of the river.
• Characteristics: DO falls to its absolute lowest point—the Critical DO Dip—dropping below 1.5 to 4.0 mg/L. Water turns black and emits foul odors due to anaerobic gas release (H₂S, NH₃, CH₄).
• Biota: Fish disappear completely. Highly tolerant pollution indicator species thrive in the mud, including Sludge worms (Tubifex tubifex), Bloodworms (Chironomid larvae), and Sewage fungus.

3. Zone of Recovery

• Location: Extends downstream as the organic waste load is progressively digested and depleted.
• Characteristics: The rate of atmospheric reaeration exceeds the rate of microbial deoxygenation. Water turbidity decreases, BOD drops steadily, and DO levels rise back toward normal.
• Biota: Anaerobic bacteria disappear; green algae, diatoms, rooted macrophytes, and freshwater crustaceans re-establish.

4. Zone of Clear Water

• Location: The final downstream stretch where natural self-purification is complete.
• Characteristics: DO returns to its unpolluted baseline (≥ 8.0 mg/L) and BOD drops to clean background levels (< 2.0 mg/L).
• Biota: Sensitive aquatic life, clear-water game fish (e.g., Trout, Mahseer), and stonefly larvae return.

6. Bio-Indicators of Water Quality

Certain organisms have specific tolerance limits for oxygen deficit and organic contamination, acting as biological indicators of aquatic health:

7. Comprehensive Comparative Matrix: DO vs. BOD vs. COD

Quick Revision

• Dissolved Oxygen (DO): Measure of free O₂ in water (mg/L or ppm). Source: Air-water interface aeration and plant photosynthesis.
• DO Thresholds: Clean water ≥ 8.0 mg/L; Contaminated < 8.0 mg/L; Heavily polluted < 4.0 mg/L; Anoxic < 1.5 mg/L.
• Solubility Drivers: Temperature up -> DO down (thermal pollution); Salinity up -> DO down; Waste up -> DO down.
• Biochemical Oxygen Demand (BOD): O₂ consumed by aerobic bacteria to break down biodegradable organic waste.
• BOD Test Protocol: Measured over 5 days at 20°C in the dark (BOD₅) to block photosynthetic O₂ production.
• BOD Scale: 1–2 mg/L (Very Good); 3–5 mg/L (Fair); 6–9 mg/L (Poor); ≥ 10 mg/L (Very Poor/Severe). CPCB bathing limit is < 3 mg/L.
• Chemical Oxygen Demand (COD): O₂ required to chemically oxidize both biodegradable AND non-biodegradable organics using K₂Cr₂O₇ + H₂SO₄.
• Universal Rule: COD is ALWAYS greater than BOD (COD > BOD). Test takes 2–3 hours.
• BOD / COD Ratio: > 0.6 = Highly biodegradable (use biological STPs); < 0.2 = Non-biodegradable/toxic (requires chemical/tertiary treatment).
• Oxygen Sag Curve: Spatial curve of DO drop and recovery downstream of sewage outfall (4 Zones: Degradation -> Active Decomposition -> Recovery -> Clear Water).
• Critical DO Dip: Occurs in Zone of Active Decomposition; DO drops to lowest point (< 1.5–4.0 mg/L), fish die, Tubifex worms thrive.
• Bio-Indicators: Daphnia (BOD indicator); Stonefly larvae & Trout (Clean water); Tubifex worms (Severe organic pollution); E. coli (Fecal sewage).

Memory Map

1. Water Quality Testing & Oxygen Metrics --> Dissolved Oxygen (DO): Available O₂ gas for aquatic respiration. --> Biochemical Oxygen Demand (BOD): Bacterial O₂ demand for biodegradable organic waste (5-day test). --> Chemical Oxygen Demand (COD): Chemical O₂ demand for total organic + inorganic waste (2-hour K₂Cr₂O₇ test).

2. Dissolved Oxygen Dynamics --> Inputs: Atmospheric aeration across air-water interface + Photosynthesis by Phytoplankton. --> Temperature & Salinity Rise -> Reduces O₂ solubility -> Drives DO down. --> DO Benchmarks: ≥ 8.0 mg/L (Clean) | < 4.0 mg/L (Heavily Polluted) | < 1.5 mg/L (Anoxic State -> Anaerobic H₂S, NH₃, CH₄).

3. BOD vs. COD Relationships --> Organic Waste Input -> Increases microbial decay -> Spikes BOD/COD -> Depletes DO. --> Value Hierarchy: COD > BOD (Always, because chemical dichromate oxidizes recalcitrant organics). --> Biodegradability Index (BOD/COD): > 0.6 -> Biological STP | < 0.2 -> Chemical/Tertiary Treatment.

4. Oxygen Sag Curve (Streeter-Phelps Stream Response) --> Outfall -> Zone 1: Degradation (BOD spikes, DO drops). --> Zone 2: Active Decomposition (Critical DO Dip < 1.5 mg/L, Fish die, Tubifex worms thrive, Anaerobic odor). --> Zone 3: Recovery (Reaeration > Deoxygenation, BOD falls, DO rises). --> Zone 4: Clear Water (DO recovers ≥ 8 mg/L, Clean biota returns).

5. Bio-Indicator Application --> Daphnia -> BOD & Toxicity sensitivity. --> Stonefly Larvae & Trout -> High DO / Unpolluted streams. --> Tubifex & Bloodworms -> Anoxic mud / Organic sewage load. --> E. coli -> Fecal coliform contamination.

Must Remember

• DO Bathing Benchmark: Clean natural water has DO ≥ 8.0 mg/L (10 ppm average in fresh water).
• Anoxic Threshold: When DO drops below 1.5 mg/L, aerobic oxidation halts and anaerobic bacteria produce foul H₂S, NH₃, and CH₄.
• BOD₅ Test Conditions: Incubated for 5 days at 20°C in complete darkness to prevent photosynthetic O₂ interference.
• CPCB Bathing Limit: CPCB mandates BOD < 3.0 mg/L for outdoor bathing water quality.
• COD > BOD Rule: COD is always higher than BOD because Potassium Dichromate oxidizes non-biodegradable compounds (lignin, cellulose, plastics) that bacteria cannot digest.
• BOD / COD Ratio Guide: BOD/COD > 0.6 indicates highly biodegradable sewage suitable for biological STPs; BOD/COD < 0.2 indicates toxic recalcitrant waste.
• Critical DO Dip: The point of maximum DO deficiency on the Oxygen Sag Curve occurring in the Zone of Active Decomposition.
• Daphnia Sensitivity: Water fleas (Daphnia) are primary biological indicator organisms used to assay BOD and water toxicity.
• Indicator Species: Tubifex worms and Chironomus bloodworms indicate severe organic sewage pollution and low DO.

Key Terms

• Dissolved Oxygen (DO): Concentration of free dissolved oxygen gas available in water for aquatic organisms, measured in mg/L or ppm.
• Biochemical Oxygen Demand (BOD): The amount of dissolved oxygen needed by aerobic bacteria to decompose biodegradable organic waste in a water sample over 5 days at 20°C.
• Chemical Oxygen Demand (COD): The oxygen equivalent required to chemically oxidize total organic (biodegradable and non-biodegradable) and oxidizable inorganic matter using dichromate.
• Oxygen Sag Curve: The characteristic spatial profile of dissolved oxygen depletion and recovery along a flowing river downstream of an organic waste discharge.
• Deoxygenation: The consumption of dissolved oxygen by aerobic microorganisms during the decomposition of organic waste.
• Reaeration: The replenishment of dissolved oxygen in a water body via atmospheric gas absorption across the air-water interface.
• Critical DO Dip: The minimum point of dissolved oxygen concentration along the Oxygen Sag Curve where deoxygenation rate equals reaeration rate.
• Anoxic Zone: A water layer with DO < 1.5 mg/L where aerobic life disappears and anaerobic bacteria produce foul gases.
• Biodegradability Index (BOD/COD): The ratio indicating the proportion of biologically digestible organic waste in a wastewater sample.
• Biological Indicator: An organism whose presence, absence, or population health indicates the specific environmental quality or pollution status of a water body.

1-Line Bits / Short Points

• Unpolluted natural fresh water contains a Dissolved Oxygen (DO) concentration of approximately 8.0 mg/L or higher.
• Water with a DO concentration below 4.0 mg/L is classified as heavily polluted.
• When DO falls below 1.5 mg/L, anaerobic bacteria take over, generating toxic H₂S, NH₃, and CH₄.
• Dissolved oxygen in water originates from air-water interface diffusion and autotrophic photosynthesis.
• An increase in water temperature reduces the solubility of dissolved oxygen.
• Thermal pollution from power plants reduces oxygen solubility and accelerates fish metabolism.
• Running streams (lotic systems) have higher DO levels than stagnant ponds (lentic systems) due to greater surface aeration.
• Biochemical Oxygen Demand (BOD) measures oxygen needed by aerobic microbes to digest biodegradable organic waste.
• Higher organic sewage input into a river increases BOD and decreases DO.
• Standard BOD tests are conducted over 5 days at 20°C in complete darkness (BOD₅).
• BOD samples are incubated in darkness to prevent algae from producing oxygen via photosynthesis.
• CPCB mandates that outdoor bathing water must maintain a BOD level below 3.0 mg/L.
• Water with a BOD value of 1–2 mg/L is categorized as very clean.
• Water with a BOD value of 10 mg/L or higher is categorized as severely polluted.
• BOD measures only biodegradable organic matter and fails to detect non-biodegradable toxic chemicals.
• Chemical Oxygen Demand (COD) measures oxygen required to chemically oxidize all organic and oxidizable inorganic matter.
• COD testing uses Potassium Dichromate (K₂Cr₂O₇) in concentrated Sulfuric Acid (H₂SO₄).
• COD test results are obtained in 2 to 3 hours, whereas BOD tests take 5 days.
• The COD value of a water sample is always higher than its BOD value (COD > BOD).
• Chemical oxidation breaks down resistant compounds like lignin, cellulose, and synthetic polymers that bacteria cannot digest.
• A BOD/COD ratio greater than 0.6 indicates that wastewater is highly biodegradable and suitable for biological STPs.
• A BOD/COD ratio below 0.2 indicates toxic or recalcitrant industrial waste requiring chemical treatment.
• The Oxygen Sag Curve plots the spatial profile of DO depletion and recovery downstream of a sewage outfall.
• The Streeter-Phelps model balances two opposing forces: microbial deoxygenation and atmospheric reaeration.
• The Oxygen Sag Curve consists of four zones: Degradation, Active Decomposition, Recovery, and Clear Water.
• The Critical DO Dip occurs in the Zone of Active Decomposition where DO drops to its lowest level.
• Sludge worms (Tubifex tubifex) and bloodworms thrive in the anoxic mud of the Zone of Active Decomposition.
• Water fleas (Daphnia) serve as primary biological indicator organisms for measuring BOD and water toxicity.
• Stonefly larvae and trout require high DO (≥ 8 mg/L) and serve as indicators of clean, unpolluted water.
• High coliform bacteria (E. coli) counts indicate sewage pollution and gastrointestinal pathogen risks.