Water Quality Parameters Quiz

Chemical Examination of Water

• Biochemical oxygen demand (BOD) measures the amount of dissolved oxygen in milligrams used by aerobic bacteria growing on organic matter in a water sample at 20°C over 5 days.
• BOD units are mg/L at 20°C over 5 days.
• Increasing BOD indicates increasing organic matter and water pollution, and increasing microbial content, including pathogens.
• BOD analysis indicates water pollution and treatment efficacy.

BOD Estimation and Water Quality

• Very good water quality: BOD = 1-2mg/L (low organic matter)
• Fair water quality: BOD = 3-5mg/L (moderate organic matter)
• Poor water quality: BOD = 6-9mg/L (high organic matter and microbial content)
• Very poor water quality: BOD = 10- more than 10mg/L (very polluted water)
• Extremely toxic water: BOD = 100 more than 100mg/L (sewage water)

BOD Estimation Methods

• Dilution method: requires dilution, seeding, and a blank sample
• Manometric method: simple, no dilution, no seeding, and no blank sample, with direct BOD value reading

Chemical Oxygen Demand (COD)

• COD measures the oxygen amount used by reactions in a measured solution
• COD unit is mg/L
• COD examination is less specific than BOD, measuring all chemically oxidized substances

Dissolved Oxygen (DO)

• DO is the atmospheric oxygen that dissolves in water and is used by fish and aquatic animals to breathe
• DO concentrations of healthy water should be above 6.5-8 mg/L
• DO levels vary depending on factors such as:
  • Running water: high DO levels
  • Cold temperature: increased DO levels
  • Presence of dead plants: decreased DO levels due to decomposer use
  • Water depth: affects DO levels

Total Hardness (TH)

• TH is the sum of the molar concentrations of calcium and magnesium in water (ppm, mg/L, or mmol/L units)
• Drinking water should have:
  • Ca 2+: 40-80ppm
  • Mg 2+: 20-30ppm
  • Total water hardness: 2-4 mmol/L
• Iron presence in drinking water causes a brownish rust-like color
• Hard water is characterized by:
  • White lime scale formation in water heaters and kettles
  • Lack of foam formation when using soap
  • High hardness gives a bitter taste of water

pH and Toxicity

• pH is the hydrogen ion concentration in water, with a range of 0-14
• Drinking water pH should be between 6.5-8.5
• pH estimation alone does not provide a full picture of water properties
• Toxic substances include heavy metals, pesticides, and PFAS, indicating industrial pollution