Environmental Issues (Water Pollution)

Water Pollution (from Environmental Chemistry)

Water pollution is the contamination of water bodies, such as lakes, rivers, oceans, and groundwater, by substances that render it unusable for drinking, cooking, swimming, and other purposes. It affects aquatic life and human health.

Causes Of Water Pollution:

Water pollution can be caused by various sources:

1. Domestic Sewage:

• Composition: Contains biodegradable organic matter, human excreta, detergents, pathogens (bacteria, viruses).
• Effects:
  • Oxygen Depletion: Biodegradation of organic matter by aerobic bacteria consumes dissolved oxygen ($O_2$) in water, leading to a decrease in Biochemical Oxygen Demand (BOD). If BOD is high, aquatic life can suffocate.
  • Eutrophication: Sewage often contains nitrates and phosphates, which act as nutrients for algae. Excessive algal growth (algal bloom) depletes oxygen when they decompose, leading to the death of fish.
  • Spread of Diseases: Pathogens in sewage can cause waterborne diseases like cholera, typhoid, dysentery.

2. Industrial Effluents:

• Sources: Discharge from industries like mining, manufacturing, power plants, chemical industries.
• Pollutants:
  • Heavy Metals: Mercury ($Hg$), Lead (Pb), Cadmium (Cd), Chromium (Cr) are toxic and can bioaccumulate in food chains.
  • Organic Chemicals: Solvents, pesticides, synthetic detergents, phenols.
  • Inorganic Chemicals: Acids, alkalis, salts, phosphates, nitrates.
  • Thermal Pollution: Discharge of hot water from power plants and industries raises the temperature of water bodies, decreasing dissolved oxygen and harming aquatic life.
• Effects: Toxic to aquatic life, contaminate drinking water, cause cancer, affect reproductive systems.

3. Agricultural Runoff:

• Sources: Fertilizers and pesticides used in agriculture are washed into water bodies by rainwater.
• Pollutants:
  • Nitrates and Phosphates: From fertilizers, cause eutrophication.
  • Pesticides and Herbicides: Toxic to aquatic organisms and can enter the food chain.

4. Other Sources:

• Oil Spills: Large-scale pollution from tanker accidents or offshore drilling.
• Radioactive Waste: Discharge from nuclear power plants.
• Plastic Waste: Accumulation of non-biodegradable plastic debris in oceans harms marine life.
• Thermal Pollution: Discharge of hot water.

International Standards For Drinking Water:

Drinking water quality standards are set by national and international organizations to ensure public health. The World Health Organization (WHO) provides guidelines for drinking water quality.

Key parameters and acceptable limits (WHO Guidelines):

• Coliform Bacteria: Must be absent in 100 mL of sample. Indicates faecal contamination.
• Total Dissolved Solids (TDS): < 1000 mg/L (for good taste), < 2000 mg/L (acceptable).
• pH: 6.5 - 8.5.
• Hardness (as $CaCO_3$): < 60 mg/L (excellent), 60-120 mg/L (good), 120-180 mg/L (fair), > 180 mg/L (poor).
• Fluoride ($F^-$): < 1.5 mg/L (to prevent dental fluorosis).
• Lead (Pb): < 0.01 mg/L.
• Arsenic (As): < 0.01 mg/L.
• Nitrate ($NO_3^-$): < 50 mg/L (to prevent methemoglobinemia).
• Sulphate ($SO_4^{2-}$): < 400 mg/L.
• Chloride ($Cl^-$): < 250 mg/L.
• Pesticides: Levels must be below detection limits or specific guideline values.

Note: Indian standards (BIS - Bureau of Indian Standards) are also relevant within India.

Water Pollution And Its Control (from Environmental Issues)

Domestic Sewage and Industrial Effluents:

Domestic Sewage:

• Pollution Load: High BOD due to biodegradable organic matter. High phosphate and nitrate content from detergents and human waste. Presence of pathogenic microorganisms.
• Impact: Oxygen depletion, eutrophication, spread of diseases.
• Treatment Methods: Sewage is treated in sewage treatment plants (STPs) through several stages:
  • Primary Treatment: Physical processes like screening (removes large solids) and sedimentation (settling of suspended solids to form sludge).
  • Secondary Treatment: Biological processes where microorganisms degrade organic matter. Activated sludge process and trickling filters are common methods. This stage significantly reduces BOD.
  • Tertiary Treatment: Advanced treatment processes to remove specific pollutants like nutrients (N, P), residual chemicals, or pathogens. Includes disinfection (e.g., chlorination, ozonation, UV irradiation).

Industrial Effluents:

• Pollution Load: Highly variable depending on the industry. Can contain heavy metals, toxic organic compounds, acids, alkalis, thermal pollution.
• Impact: Toxic to aquatic life, contaminate drinking water, bioaccumulation, eutrophication.
• Treatment Methods: Specific treatment methods are tailored to the type of effluent.
  • Neutralization: For acidic or alkaline effluents.
  • Precipitation: For heavy metals (e.g., adding lime to precipitate metal hydroxides).
  • Adsorption: Using activated charcoal to remove organic pollutants.
  • Oxidation/Reduction: To convert toxic compounds into less harmful ones.
  • Filtration: To remove suspended solids.
  • Boiling/Distillation: For thermal pollution.
  • Reverse Osmosis: For removing dissolved salts and specific contaminants.

A Case Study Of Integrated Waste Water Treatment:

Concept: Integrated Waste Water Treatment combines conventional sewage treatment methods with ecological approaches, often involving wetlands or constructed aquatic ecosystems. The goal is to treat wastewater efficiently while also creating a habitat and improving the aesthetic value.

Example: Constructed Wetlands:

• Description: Artificial wetland systems designed to utilize natural processes involving aquatic plants, soil, and microorganisms to remove pollutants from wastewater.
• Process: Wastewater flows slowly through a bed of gravel or sand containing specific aquatic plants (e.g., reeds, cattails).
  • Physical Filtration: The substrate traps larger suspended solids.
  • Adsorption: Pollutants adhere to the surfaces of soil particles and plant roots.
  • Biodegradation: Microorganisms attached to the plant roots and substrate degrade organic pollutants and nutrients.
  • Phytoremediation: Plants absorb some pollutants, including heavy metals and excess nutrients.
• Advantages: Low energy consumption, cost-effective, aesthetically pleasing, provides habitat for wildlife, can be used for secondary and tertiary treatment.
• Application: Can be used for treating domestic sewage, agricultural runoff, and some industrial effluents.

Integrated approach in action: A facility might first employ primary sedimentation, followed by treatment in a series of constructed wetlands (e.g., a subsurface flow wetland followed by a surface flow wetland) for secondary and tertiary treatment, before releasing the treated water or reusing it.