Chemistry of Water and Water Pollution Notes

Chemistry of Water and Water Pollution

Water Pollution

  • Water pollution is any change in physical, chemical, or biological properties of water that harms living things.

  • Only a small fraction (1%) of Earth's water is usable freshwater on the surface.

Hydrologic Cycle

  • Involves processes like evaporation, sublimation, transpiration, condensation, and precipitation.

Water Pollution Parameters

  • Physical: Color, odor, turbidity, taste, temperature, electrical conductivity.

  • Chemical: pH, conductivity, salinity, hardness, BOD, carbonates, sulfates, chlorides, fluorides, nitrates, metal ions.

  • Biological: Counts of specific organisms.

Sources of Water Pollution

  • Point Source: Direct discharge of pollutants (e.g., factories, mines).

  • Non-Point Source: Diffuse sources of pollutants (e.g., agricultural runoff, construction sites).

  • Natural: Leaching of rocks, decaying organic matter, soil erosion.

  • Anthropogenic: Deforestation, industrial waste, domestic sewage, radioactive waste, synthetic chemicals, fertilizers, pesticides.

Classification of Water Pollutants

  • Organic pollutants

  • Inorganic pollutants

  • Heat/thermal pollutants

  • Suspended solids and sediments

  • Radioactive materials

Organic Pollutants

  • Oxygen demanding wastes: sewage, agricultural runoff; leads to C+O<em>2→CO</em>2C + O<em>2 \rightarrow CO</em>2.

  • Disease-causing wastes: pathogens from sewage.

  • Synthetic organic compounds: pesticides, detergents, industrial chemicals.

  • Refractory organics: pharmaceuticals, industrial chemicals, plastics.

Eutrophication

  • Excessive nutrients cause overgrowth of aquatic plants, leading to oxygen depletion and dead zones.

  • Process: fertilizers/sewage introduce nitrates/phosphates $\rightarrow$ algal bloom $\rightarrow$ competition for light $\rightarrow$ dead plants/algae $\rightarrow$ increased decomposition & oxygen use $\rightarrow$ anaerobic bacteria thrive, releasing NH<em>4NH<em>4, CH</em>4CH</em>4, H2SH_2S.

Inorganic Pollutants

  • Mineral acids, inorganic salts, metals, trace elements.

  • Sources: mining, industrial waste.

  • Heavy metals (Hg, Cd, Pb) are toxic and attack -SH bonds in enzymes.

Suspended Solids and Sediments

  • Sources: soil erosion, agriculture, mining, construction.

  • Effects: block sunlight, reduce photosynthesis, fill reservoirs.

Radioactive Materials

  • Sources: mining, research, nuclear plants, nuclear weapons.

Heat (Thermal Pollution)

  • Sources: power plants (cooling water).

  • Effects: depletes dissolved oxygen, harms aquatic life.

Water Quality Parameters

  • Standards for water quality characterization.

  • Examples include color, odor, taste, pH, specific conductance, dissolved oxygen (DO), chemical oxygen demand (COD), biochemical oxygen demand (BOD), suspended solids, total dissolved solids, metals, and anions.

pH

  • Measure of acidity or alkalinity: pH=−log[H+]pH = -log[H^+].

  • Affects chemical and biological processes.

  • Optimal range for aquatic life: 6.5-8.0.

  • Changes caused by acid rain, CO2 levels, industrial pollutants, mining.

  • High pH can lead to ammonia poisoning in aquatic organisms.

  • Low pH can cause ocean acidification and increase heavy metal toxicity.

Ocean Acidification

  • Increased atmospheric CO2CO_2 dissolves in oceans, forming carbonic acid and lowering pH.

  • Threatens shell-building organisms by reducing carbonate availability: Ca2++2HCO<em>3−⟶CaCO</em>3+CO<em>2+H</em>2OCa^{2+} + 2HCO<em>3^- \longrightarrow CaCO</em>3 + CO<em>2 + H</em>2O.

Conductivity/Specific Conductance

  • Measure of water's ability to conduct electricity.

  • Affected by dissolved solids (ions).

  • Measured in micromhos per centimeter (μ\mumhos/cm) or microsiemens per centimeter (μ\muS/cm).

Dissolved Oxygen (DO)

  • Free, non-compound oxygen in water.

  • Essential for aquatic life.

  • Sources: atmosphere, photosynthesis.

  • Low DO can lead to fish kills.

Biochemical Oxygen Demand (BOD)

  • Amount of oxygen consumed by bacteria decomposing organic material.

  • High BOD indicates high levels of organic pollution.

  • Microbial-mediated consuption of O2O_2 during 5 day period.

Chemical Oxygen Demand (COD)

  • Amount of oxygen needed to oxidize organic and inorganic materials.

  • Uses chemical oxidant under specific conditions.

Wastewater Treatment

  • Treatment methods depend on waste characteristics.

  • Preliminary Treatment: Removal of gross solids (e.g., rubbish, oil, grease).

  • Primary Treatment: Sedimentation to remove suspended solids; may include mechanical flocculation or chemical coagulation.

  • Secondary Treatment: Biological processes (aerobic or anaerobic) to remove dissolved and colloidal organic matter.

  • Tertiary Treatment: Removal of fine suspended solids, bacteria, and dissolved inorganics.

  • Sludge Treatment and Disposal: Digestion, dewatering, and disposal of sludge.