PYTR Class

Learning Objectives

1. Explain the concept of water quality, including its chemical, physical, and biological characteristics
2. Define biochemical oxygen demand (BOD) and interpret its significance in assessing water pollution.
3. [AHL] Outline and analyse how water quality is measured using a Water Quality Index (WQI).
4. [AHL] Summarise the drinking water quality guidelines set by the WHO and compare them to local standards.

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Part 1: Water Quality Assessment

Water quality monitoring in freshwater systems involves assessing:

• Dissolved oxygen (O₂)
• pH levels
• Temperature
• Turbidity (clarity)
• Concentrations of nitrates, phosphates, specific metals, and total suspended solids

Purpose of data collection:

• To inform and guide management strategies for reducing pollution in aquatic environments

Standard water quality tests can be conducted on drinking water, rivers, and other sites using portable equipment that detects:

• Nitrate and nitrite ions
• Free chlorine
• Chloride and fluoride ions
• Water hardness
• Heavy metals (e.g., lead)

Common water quality indicators for rivers include:

• Biochemical Oxygen Demand (BOD)
• Turbidity
• Ammonia levels
• Dissolved oxygen levels

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Two main methods of measuring water quality:

• Direct measures:
  • Involve chemical testing of water samples
  • Identify concentrations of pollutants or harmful substances
  • Examples: nitrates, phosphates, heavy metals
  • Known as chemical indicators of water quality
• Indirect measures:
  • Involve studying the aquatic organisms (fish, insects, invertebrates)
  • High biodiversity indicates good water quality
  • Lack of aquatic life suggests poor water quality
  • Known as biological indicators of water quality

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Summary of measures:

Indicator	Method	What the Results Show
Dissolved Oxygen	– Meter and probe or specialized chemical testing kit – Measured as percentage saturation – Bacteria use oxygen to break down organic pollutants	– 75% saturation = healthy, clean water – 10–50% = polluted water – <10% = raw sewage – Low oxygen = potential pollution (e.g., raw sewage) – Impacts aquatic life and food supply
pH	– pH probe, pH meter, or indicator paper/test strips – Compare to a color chart – Record the pH value (e.g., 8)	– pH 1–6 = acidic – pH 7 = neutral – pH 8–14 = alkaline – Changes may occur over long periods and may not link to a single source
Phosphate	– Test kit – Follow instructions – Measured in mg/dm³	– < 5 mg/dm³ = clean water – 15–20 mg/dm³ = polluted water
Nitrate	– Test strips or test kit – Follow instructions – Measured in mg/dm³	– 4–5 mg/dm³ = clean water – 6–15 mg/dm³ = polluted water – Over 50 ppm unsafe for drinking – May originate from fertilizers or manure – Contributes to eutrophication
Salt (Chloride)	– Test kit, meter, or sensor – Follow instructions – Measured in mg/dm³	– 20,000 mg/dm³ = seawater – 100–20,000 mg/dm³ = tidal/brackish water
Ammonia	– Test strips or test kit – Follow instructions – Measured in mg/dm³	– 0.05–1.0 mg/dm³ = clean water – >1–10 mg/dm³ = polluted – 40 mg/dm³ = sewage – Often linked with fertilizers or manure
Turbidity	– Secchi disc or turbidity tube – Record depth where disc markings disappear	– High turbidity = high suspended sediment – May indicate organic pollutants – Low visibility depth = dirtier water
Temperature	– Measured using a waterproof thermometer	– Lower temperature = more dissolved oxygen – Higher temperature = less dissolved oxygen

Part 2: Biological Oxygen Demand (BOD)

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BOD Methodology

1. Collect a known volume of water sample.
2. Measure the initial dissolved oxygen level.
3. Seal the sample in an airtight container and store in the dark at 20°C for 5 days (darkness prevents photosynthesis from adding extra oxygen).
4. Measure the dissolved oxygen level again after 5 days.
5. BOD is calculated as the difference between the initial and final oxygen levels.

BOD Analysis

• Biological Basis:
  • Aerobic organisms use oxygen during cellular respiration.
  • A higher number of organisms or faster respiration rates at a site leads to increased oxygen use, resulting in a higher BOD.
• Pollution Indicator:
  • High BOD levels can indicate organic pollution (e.g., sewage, silage).
  • Organic pollutants cause an increase in microorganism populations, which consume more oxygen.
  • This can reduce oxygen levels, possibly leading to anaerobic decomposition.
  • Anaerobic decomposition may produce harmful gases like methane (CH₄), hydrogen sulfide (H₂S), and ammonia (NH₃).

Part 3: [AHL] Water Quality Index (WQI)

• Developed in the 1960s; widely used by water scientists.
• Water quality has declined in many areas, due to both natural causes (e.g. tropical cyclones, El Niño) and human activities (e.g. intensive farming, mining, waste disposal).
• Poor water quality is a global issue affecting both high-income countries (HICs) and low-income countries (LICs).
• Factors such as population growth, improved living standards, and climate change are expected to worsen water quality.
• Common parameters: temperature, turbidity, dissolved oxygen (DO), suspended solids (SS), total dissolved solids (TDS), faecal coliforms (FC), biological oxygen demand (BOD), and nitrate nitrogen.
• Some models exclude suspended solids, microbiological contaminants, and toxic compounds due to high testing costs and limited lab access.

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Drinking Water Standards

• No universal national standards exist for drinking water.
• WHO provides guidelines; local governments set their own standards.
• Standards typically target levels of fluoride, lead, nitrates, and selenium.
• Few countries enforce legal standards—exceptions include:
  • The European Drinking Water Directive.
  • The Safe Drinking Water Act (USA).

Parameter	WHO	European Union	USA	Canada	India
Fluoride	1.5 mg/l	1.5 mg/l	4 mg/l	No standard	1.0 mg/l
Lead	No standard	10 µg/l	15 µg/l	10 µg/l	0.5 µg/l
Nitrate	50 mg/l	50 mg/l	10 mg/l	No standard	45 mg/l
Selenium	40 µg/l	10 µg/l	50 µg/l	10 µg/l	0.01 µg/l

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