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Principles of Water Quality


Water quality refers to the chemical, physical, and biological characteristics of water, determining its suitability for various uses (drinking, agriculture, recreation, and ecology). Key parameters include pH, electrical conductivity (EC), biochemical oxygen demand (BOD), and chemical oxygen demand (COD).

1. Suspended and Dissolved Solids

  • Suspended Solids (SS): These are undissolved particles (silt, clay, sand, organic matter) suspended in water.

    • Measurement: Total Suspended Solids (TSS) in milligrams per liter (mg/L).
    • Impact: Cause turbidity, reducing light penetration and harming aquatic life. Can carry pollutants.
    • Example: Construction or agricultural runoff.

  • Dissolved Solids (DS): These are substances completely dissolved in water, forming ions (salts, minerals, gases).

    • Measurement: Total Dissolved Solids (TDS) in mg/L, often estimated by conductivity.
    • Impact: Affect taste, aquatic life, irrigation, and industrial use. Can indicate pollution (high salt/metal concentrations).
    • Example: Salinity in coastal areas, mineral leaching from rocks.

2. Electrical Conductivity (EC)

  • Definition: Measures water's ability to conduct electricity, reflecting the concentration of dissolved ions. Higher ion concentration, higher EC.
  • Units: Microsiemens per centimeter (µS/cm) or millisiemens per centimeter (mS/cm).
  • Factors: Primarily influenced by dissolved salts, minerals, and metals.
  • Example: High in seawater, low in pure water.
  • Significance: Indicates potential salinity issues, industrial contamination, or high nutrient levels (leading to eutrophication).

3. pH of Water

  • Definition: Measures water acidity or alkalinity on a scale of 0-14 (7 is neutral).
  • Significance: Affects solubility and toxicity of chemicals (e.g., heavy metals, nutrients). Most aquatic life thrives in a pH range of 6.5-8.5.
  • Impact:
    • Acidic water (pH < 6): Can leach heavy metals (lead, copper) from pipes, harmful to humans and aquatic life.
    • Basic water (pH > 8.5): Affects nutrient availability, causes scale formation in pipes.
  • Example: Acid rain (pH < 5.6) from fossil fuel burning acidifies water bodies.

4. Trace Constituents

  • Definition: Elements or compounds (heavy metals, trace nutrients, organic pollutants) present in small amounts but with significant impacts.
  • Significance: Can be toxic to aquatic life, harm human health, and disrupt ecosystems.
  • Example: Mercury accumulation in the food chain.

5. Biochemical Oxygen Demand (BOD)

  • Definition: Amount of oxygen consumed by microorganisms to decompose organic matter in water (typically measured over 5 days at 20°C).
  • Units: Milligrams per liter (mg/L).
  • Significance: Indicates organic pollution level. High BOD suggests high levels of biodegradable material (sewage, food waste), depleting oxygen and harming aquatic life.
  • Example: Untreated sewage discharge in a river.

6. Chemical Oxygen Demand (COD)

  • Definition: Total oxygen required to oxidize both biodegradable and non-biodegradable organic substances in water.
  • Units: Milligrams per liter (mg/L).
  • Significance: Measures total oxygen demand, including non-biodegradable substances (COD is usually higher than BOD).
  • Example: Industrial effluents containing organic chemicals.

Comparison of BOD and COD

ParameterBOD (Biochemical Oxygen Demand)COD (Chemical Oxygen Demand)
DefinitionOxygen demand from microbial activityOxygen demand from both biological and chemical processes
PurposeMeasures biodegradable organic matterMeasures total organic matter
Typical Range1-300 mg/L for natural waters20-500 mg/L for polluted waters
UsageIndicator of organic pollution and oxygen depletionEstimates pollution load, especially when BOD is impractical
DecompositionBiologically degraded by microorganismsCan be chemically oxidized, including non-biodegradable compounds
ExampleSewage water, food wasteIndustrial effluents, chemical runoff

Summary

Water quality is assessed through various parameters. Understanding these principles is crucial for assessing water suitability, implementing effective water treatment, and promoting sustainable water management.

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