MEV 013: Unit 10 - Parameters of Water Pollution

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UNIT 10: PARAMETERS OF WATER POLLUTION

10.0 Introduction

Water pollution is a critical environmental concern affecting ecosystems and human health. To assess water quality, various chemical, biological, and physical parameters must be studied. This unit focuses on key indicators such as dissolved oxygen (DO), biochemical oxygen demand (BOD), chemical oxygen demand (COD), acidity, alkalinity, and colloidal properties. It also addresses the carbonate system, ion exchange, and organic contaminants in sediments.

10.1 Objectives

By the end of this unit, you will be able to:

  • Understand the aquatic system and its key water quality parameters.
  • Explain the concepts of DO, BOD, COD, acidity, and alkalinity.
  • Describe acid-base reactions and the carbonate system in natural water.
  • Discuss the roles and behaviors of colloids in aquatic systems.
  • Explore the interaction between sediments and pollutants.

10.2 Aquatic System

The aquatic system consists of lakes, rivers, streams, and oceans, governed by complex interactions among biological, chemical, and physical factors. Parameters like temperature, pH, salinity, and oxygen levels regulate the health of these systems.

10.3 Dissolved Oxygen (DO)

DO refers to the amount of oxygen available in water for aquatic organisms. It is a primary indicator of water quality.

  • Sources: Atmospheric diffusion, photosynthesis
  • Depletion: Due to respiration, decomposition of organic matter
  • Optimal DO: >5 mg/L is ideal for most aquatic life

10.3.1 Oxygen Sag Curve

The oxygen sag curve represents the change in DO downstream from a point of pollution discharge.

  • Zones:
    1. Clean zone
    2. Decomposition zone
    3. Septic zone
    4. Recovery zone

This curve helps assess organic pollution impact over distance.

10.4 Biochemical Oxygen Demand (BOD)

BOD measures the amount of oxygen required by microorganisms to decompose organic matter in water over 5 days at 20°C.

  • High BOD indicates heavy organic pollution.
  • Standard BOD test: BOD₅ (5-day BOD)
  • Typical Values:
    • Clean water: <2 mg/L
    • Moderately polluted: 3–5 mg/L
    • Heavily polluted: >5 mg/L

10.5 Chemical Oxygen Demand (COD)

COD is the amount of oxygen required to chemically oxidize organic and inorganic compounds in water.

  • Faster than BOD test
  • Measures both biodegradable and non-biodegradable organics
  • COD > BOD indicates presence of non-biodegradable matter

10.6 Acidity

Acidity in water refers to its capacity to neutralize bases. It is mainly due to free CO₂, mineral acids, and organic acids.

  • Expressed in mg/L as CaCO₃
  • Measured by titration using standard base

10.7 Alkalinity

Alkalinity is the water’s capacity to neutralize acids. It is primarily due to the presence of bicarbonates, carbonates, and hydroxides.

10.7.1 Alkalinity and pH

  • High alkalinity buffers pH changes
  • Essential in maintaining aquatic stability
  • Expressed as mg/L CaCO₃

10.8 Acid-Base Chemistry in Natural Water: The Carbonate System

The carbonate buffering system maintains pH in natural waters through equilibrium among:

  • CO₂ (aq) + H₂O ↔ H₂CO₃ ↔ H⁺ + HCO₃⁻ ↔ 2H⁺ + CO₃²⁻

This equilibrium is influenced by:

  • pH
  • CO₂ concentration
  • Biological activity

This system resists drastic pH shifts, supporting aquatic life.

10.9 Complexation and Chelation

Chelation and complexation involve binding of metal ions with ligands (e.g., organic matter), affecting mobility and bioavailability of metals.

10.9.1 Natural and Waste Water

Natural organic matter and industrial effluents introduce chelating agents (e.g., EDTA, humic acids), which bind to metals.

10.9.2 Occurrence and Importance of Chelating Agents in Aqueous Systems

  • Influence metal transport, toxicity, and removal
  • Common in both natural water and industrial wastewater

10.9.3 Colloidal Particles in Water

Colloids are fine particles (1–1000 nm) that remain suspended due to surface charges.

  • Sources: Clays, organic matter, microbial cells
  • Not removed by simple sedimentation

10.9.4 Formation of Colloidal Particles

  • Weathering of rocks
  • Precipitation reactions
  • Organic decomposition

10.9.5 Properties of Colloidal Substances

  • Large surface area
  • Electrical double layers
  • Brownian motion

10.9.6 Classification of Colloidal Particles

  • Hydrophilic: Interact with water (e.g., proteins)
  • Hydrophobic: Repel water (e.g., metal oxides)

10.9.7 Stability of Colloidal Particles

Colloid stability depends on zeta potential, ionic strength, and pH.

  • High zeta potential = stable
  • Destabilization leads to aggregation

10.9.8 Colloidal Properties of Clays

  • High surface area
  • Cation exchange capacity (CEC)
  • Adsorb pollutants

10.9.9 Aggregation of Colloidal Particles

  • Triggered by pH changes, temperature, or salts
  • Leads to sedimentation and pollutant trapping

10.9.10 Flocculation of Colloids by Polyelectrolytes

  • Synthetic or natural polymers added to bind and settle colloids
  • Used in water and wastewater treatment

10.9.11 Flocculation of Bacteria by Polymeric Materials

  • Bacteria can form bioflocs in presence of polymers
  • Enhances treatment efficiency in bioreactors

10.9.12 Environmental Significance of Colloids

  • Aid in transport of metals and organics
  • Influence pollutant fate in aquatic systems
  • Affect water clarity and quality

10.10 Ion Exchange with Bottom Sediments

Sediments act as reservoirs and sources of ions through ion exchange processes.

10.10.1 Metals in Suspended Matter and Sediments

  • Metals bind to clay minerals and organic matter
  • Bioavailability depends on binding strength and redox conditions

10.10.2 Method for Determination of CEC (Cation Exchange Capacity)

CEC indicates the soil/sediment's capacity to hold and exchange cations.

  • Determined using ammonium acetate method

10.10.3 Method for Determination of ECS (Exchangeable Cation Strength)

ECS reflects the total strength of exchangeable cations, assessed via:

  • Ion-selective electrodes
  • Atomic absorption spectroscopy

10.11 Organic Compounds in Sediments and Suspended Matter

Organic pollutants (e.g., pesticides, PAHs, PCBs) can accumulate in sediments, posing risks to benthic organisms and entering food chains.

  • Bioaccumulation and persistence are major concerns
  • Require specialized extraction and analysis techniques

10.12 Let Us Sum Up

This unit explained critical physicochemical and biological parameters of water pollution. Understanding DO, BOD, COD, acidity, alkalinity, and the role of colloids provides a framework for assessing water quality. Complexation, sediment interactions, and organic pollutants further influence water chemistry and ecosystem health.

10.13 Key Words

  • DO-Dissolved Oxygen available for aquatic life
  • BOD-Biochemical Oxygen Demand over 5 days
  • COD-Chemical Oxygen Demand for oxidation of pollutants
  • Alkalinity-Water's ability to neutralize acid
  • Chelation-Binding of metal ions by organic/inorganic ligands
  • Colloids-Fine suspended particles in water
  • Flocculation-Aggregation of colloids into larger particles
  • CEC-Cation Exchange Capacity of sediments
  • ECS-Exchangeable Cation Strength
  • Oxygen Sag Curve-Graphical representation of DO downstream of pollution

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