Chemistry of water

Water, a critical component of the Earth's system, is essential for all known forms of life. Understanding its chemistry is vital for numerous scientific fields, including environmental science, biology, and chemistry. The unique properties and behaviors of water arise from its molecular structure and interactions.

Molecular Structure of Water

• Chemical Formula: H₂O
• Molecular Geometry: Bent or V-shaped
• Bond Angle: Approximately 104.5 degrees
• Polarity: Polar molecule with a dipole moment due to the difference in electronegativity between hydrogen (H) and oxygen (O) atoms.

Key Properties of Water

1. Polarity

   • Water is a polar molecule, meaning it has a partial positive charge on the hydrogen atoms and a partial negative charge on the oxygen atom.
   • This polarity allows water to form hydrogen bonds, which are responsible for many of its unique properties.

2. Hydrogen Bonding

   • Each water molecule can form up to four hydrogen bonds with neighboring water molecules.
   • Hydrogen bonds are relatively strong intermolecular forces, giving water a high boiling point, melting point, and surface tension compared to other molecules of similar size.

3. High Specific Heat Capacity

   • Water can absorb a lot of heat energy without a significant increase in temperature.
   • This property helps moderate the Earth's climate and is crucial for maintaining stable temperatures in organisms.

4. High Heat of Vaporization

   • A considerable amount of energy is required to convert water from a liquid to a gas.
   • This property is essential for processes like perspiration and cooling in living organisms.

5. Density and Ice Formation

   • Water is most dense at 4°C. As it freezes, it becomes less dense, which is why ice floats on liquid water.
   • The floating ice insulates the water below, protecting aquatic life in cold environments.

Important Concepts in Water Chemistry

1. Dissolved Oxygen (DO)

   • The amount of oxygen dissolved in water is crucial for the survival of aquatic organisms.
   • DO levels can be affected by temperature, salinity, and the presence of pollutants.

2. Biochemical Oxygen Demand (BOD)

   • BOD measures the amount of oxygen required by microorganisms to decompose organic matter in water.
   • High BOD indicates high levels of organic pollution, which can lead to oxygen depletion and harm aquatic life.

3. Chemical Oxygen Demand (COD)

   • COD measures the total quantity of oxygen required to oxidize both organic and inorganic substances in water.
   • It provides an estimate of the water's pollution level.

4. pH

   • pH measures the acidity or alkalinity of water on a scale of 0 to 14.
   • Natural water bodies usually have a pH between 6.5 and 8.5. Extreme pH levels can be harmful to aquatic life.

5. Redox Potential (Eh)

   • Redox potential indicates the tendency of water to gain or lose electrons.
   • It is important for understanding the oxidation-reduction reactions occurring in water, which affect the fate of pollutants and nutrients.

Concept of DO, BOD, COD

Dissolved Oxygen (DO)

Definition

Dissolved Oxygen (DO) refers to the amount of oxygen that is present in water. It is a crucial parameter for assessing water quality, as it directly impacts the health of aquatic ecosystems.

Importance

• Essential for the respiration of aquatic organisms such as fish, invertebrates, and aerobic bacteria.
• High DO levels typically indicate good water quality and a healthy ecosystem.
• Low DO levels can lead to hypoxia (oxygen deficiency), which can cause stress or death to aquatic life.

Factors Affecting DO

1. Temperature: Colder water can hold more oxygen. As temperature increases, DO decreases.
2. Salinity: Saltier water holds less oxygen. Higher salinity reduces DO.
3. Photosynthesis: Aquatic plants and algae produce oxygen during photosynthesis, increasing DO levels.
4. Respiration and Decomposition: Organisms consume oxygen for respiration. Decomposition of organic matter by bacteria also consumes oxygen, decreasing DO levels.
5. Water Movement: Flowing or turbulent water, such as in rivers or streams, usually has higher DO levels due to increased aeration compared to stagnant water.

Measurement

• DO is measured in milligrams per liter (mg/L) using a DO meter or a chemical titration method such as the Winkler method.

Biochemical Oxygen Demand (BOD)

Definition

Biochemical Oxygen Demand (BOD) is the amount of oxygen required by aerobic microorganisms to decompose organic matter in water over a specific period, usually 5 days at 20°C (BOD5).

Importance

• Indicates the level of organic pollution in water.
• High BOD values suggest high levels of biodegradable organic matter, which can lead to oxygen depletion.
• Used to gauge the effectiveness of wastewater treatment processes.

Factors Affecting BOD

1. Organic Matter: The more organic waste present in the water, the higher the BOD.
2. Microbial Population: A higher concentration of microorganisms can increase BOD due to more active decomposition.
3. Temperature: Higher temperatures can accelerate microbial activity, increasing BOD.
4. Presence of Toxic Substances: Toxic substances can inhibit microbial activity, leading to lower BOD values.

Measurement

• Water samples are incubated in the dark at 20°C for 5 days.
• The initial DO is measured, and the DO after 5 days is measured. The BOD is the difference between these two measurements.

Chemical Oxygen Demand (COD)

Definition

Chemical Oxygen Demand (COD) measures the total quantity of oxygen required to oxidize both organic and inorganic substances in water using a strong chemical oxidant.

Importance

• Provides a quick measure of the organic pollutant load in water.
• Unlike BOD, COD includes both biodegradable and non-biodegradable substances, giving a more comprehensive assessment of water quality.
• Useful for monitoring and controlling water pollution, especially in industrial effluents.

Factors Affecting COD

1. Concentration of Organic and Inorganic Matter: Higher concentrations increase the COD.
2. Type of Organic Matter: Certain types of organic compounds are more resistant to oxidation, affecting COD values.
3. Presence of Oxidizable Inorganics: Inorganic substances that can be oxidized also contribute to COD.

Measurement:

• A water sample is treated with a strong oxidizing agent, usually potassium dichromate in an acidic solution.
• The sample is heated, and the amount of oxidant consumed is measured, often using a colorimetric or titrimetric method.
• The result is expressed in milligrams of oxygen per liter (mg/L).

Comparison of DO, BOD, and COD

Sedimentation

Definition

Sedimentation is the process by which particles suspended in a fluid (usually water) settle out of the fluid by gravity. It is a fundamental process in both natural environments and engineered systems, used for water and wastewater treatment. Key components in sedimentation tanks is described as:

Inlet:

1. Flow Regulation: Raw water or wastewater enters the sedimentation tank through an inlet pipe.
2. Velocity Reduction: As the water enters the tank, its velocity decreases, allowing suspended particles to settle.

Sedimentation Zone:

1. Particle Settlement: Suspended particles in the water, including organic matter, silt, and other solids, settle to the bottom of the tank due to gravity.
2. Clarified Water: Clean, clarified water rises to the surface and moves towards the outlet.

Scum and Sludge Collection:

1. Scum Removal: Lighter, floating materials such as oils, grease, and other organic matter form a layer of scum on the surface of the water.

   • Scum removal mechanisms such as skimmers or scrapers remove the scum layer and direct it to a collection system for further treatment or disposal.

2. Sludge Collection: Heavier settled solids accumulate at the bottom of the tank as sludge.

   • Sludge is periodically removed from the bottom of the tank using mechanisms like sludge rakes or suction pumps.
   • Collected sludge is pumped to a sludge treatment facility for further processing, such as dewatering, digestion, or disposal.

Outlet:

1. Effluent Discharge: Clarified water, free from suspended solids, exits the sedimentation tank through an outlet pipe.
   • The effluent may undergo further treatment processes, such as filtration or disinfection, before being discharged into receiving water bodies or distribution systems for potable water supply.

Key Considerations

• Hydraulic Retention Time (HRT): The duration water spends in the sedimentation tank influences its efficiency in removing suspended solids.
• Baffle Arrangement: Baffles or weirs installed in the tank help control the flow of water and ensure proper settling of solids.
• Sludge Recirculation: Some sedimentation tanks employ sludge recirculation systems to enhance particle settling and improve overall treatment efficiency.
• Maintenance: Regular inspection, cleaning, and maintenance of sedimentation tanks are essential to ensure optimal performance and prevent operational issues.

Principles of Sedimentation

1. Gravity Settling: The primary driving force for sedimentation is gravity. Particles with a higher density than the fluid will naturally settle downwards.

2. Stokes' Law: This law describes the settling velocity of small spherical particles in a fluid. Stokes' Law applies to small, spherical particles in laminar flow conditions.

Types of Sedimentation

1. Discrete Particle Settling

   • Occurs when particles settle individually without interacting with each other.
   • Typical in dilute suspensions.

2. Flocculent Settling

   • Particles agglomerate to form larger flocs, which settle more rapidly.
   • Common in natural water bodies and wastewater treatment where coagulation and flocculation occur.

3. Hindered Settling

   • Occurs at higher particle concentrations where interactions between particles slow the settling rate.
   • Particles create a network that impedes their own movement.

4. Compression Settling

   • Occurs when the concentration of particles is so high that the particles at the bottom are compressed by the weight of the particles above.
   • Typical in thick sludge layers in wastewater treatment.

Applications of Sedimentation

1. Water Treatment

   • Used to remove suspended solids from raw water to produce clear potable water.
   • Typically involves coagulation, flocculation, and sedimentation steps.

2. Wastewater Treatment

   • Sedimentation tanks (clarifiers) are used to remove settleable solids from wastewater.
   • Primary sedimentation removes large particles, while secondary sedimentation follows biological treatment processes to remove biomass.

3. Mining and Mineral Processing

   • Sedimentation is used to separate valuable minerals from ore slurry based on their density differences.

4. Environmental Engineering

   • Helps in controlling soil erosion by allowing sediment to settle in designed retention basins.
   • Used in stormwater management to reduce sediment load in runoff.

Design of Sedimentation Tanks

1. Rectangular Sedimentation Tanks

   • Common in water and wastewater treatment plants.
   • Flow is horizontal, allowing particles to settle as water moves through the tank.
   • Typical dimensions: length-to-width ratio of 4:1 to 6:1.

2. Circular Sedimentation Tanks

   • Often used for secondary sedimentation in wastewater treatment.
   • Water enters at the center and flows radially outward, with solids settling to the bottom.
   • Sludge is collected at the center by a rotating scraper mechanism.

3. Inclined Plate Settlers (Lamella Clarifiers)

   • Consist of a series of inclined plates to increase the effective settling area.
   • Particles settle on the plates and slide down into a sludge collection zone.
   • More compact and efficient compared to conventional tanks.

Factors Influencing Sedimentation

1. Particle Size and Density: Larger and denser particles settle faster.
2. Fluid Viscosity: Higher viscosity slows down the settling process.
3. Temperature: Higher temperatures reduce fluid viscosity, increasing settling velocity.
4. Flow Conditions: Laminar flow conditions are ideal for sedimentation. Turbulence can keep particles suspended and reduce settling efficiency.
5. Concentration of Suspended Solids: Higher concentrations can lead to hindered or compression settling.

Challenges and Considerations

1. Floc Breakup: Shear forces can break up flocs, reducing settling efficiency.
2. Resuspension: Strong currents or disturbances can re suspend settled particles.
3. Sludge Management: Accumulated sludge must be periodically removed and properly treated or disposed of.
4. Chemical Addition: Coagulants and flocculants are often added to enhance particle aggregation and improve settling.

Coagulation

Coagulation is a vital step in the water treatment process aimed at removing suspended particles, colloids, and organic matter from raw water. It involves the addition of coagulants to destabilise and aggregate particles, facilitating their removal through subsequent sedimentation or filtration.

1. Coagulation

• Purpose: The primary objective of coagulation is to neutralize the electrostatic charges on suspended particles, promoting their aggregation into larger, settleable flocs.
• Mechanism: Coagulants, typically metallic salts or polymers, interact with charged particles in water, neutralizing their charges and forming tiny, insoluble precipitates called floc.

2. Types of Coagulants

• Inorganic Coagulants: Common examples include aluminum sulfate (alum), ferric chloride, and polyaluminum chloride (PAC). These coagulants form metal hydroxide precipitates that aid in floc formation.
• Organic Coagulants: Polymers such as polyDADMAC (poly diallyl dimethyl ammonium chloride) and polyacrylamide are used to enhance coagulation by bridging and enmeshing particles to form larger flocs.

3. Coagulation Process

1. Coagulant Addition: Coagulants are added to the raw water stream through rapid mixing or in-line injection. The dosage is carefully controlled based on water quality parameters such as turbidity, pH, and alkalinity.
2. Rapid Mixing: Immediately after coagulant addition, the water undergoes rapid mixing to ensure thorough dispersion of the coagulant and promote particle collision.
3. Flocculation: The mixed water enters a flocculation basin where gentle agitation promotes the growth and aggregation of flocs. Slow mixing allows smaller particles to collide and adhere to form larger, settleable flocs.
4. Settling or Sedimentation: The flocculated water flows into sedimentation basins or clarifiers, where gravity facilitates the settling of flocs to the bottom of the tank.
5. Filtration: After sedimentation, the clarified water undergoes filtration to remove any remaining fine particles and flocs, ensuring high-quality treated water.

4. Factors Influencing Coagulation

• pH: Optimal coagulation occurs within a specific pH range, typically between 6.5 and 7.5 for most coagulants. pH adjustment may be necessary to enhance coagulation efficiency.
• Coagulant Dosage: The amount of coagulant added depends on factors such as raw water quality, turbidity, and the desired level of particle removal.
• Water Temperature: Coagulation is more effective at higher temperatures due to increased particle collision rates and reaction kinetics.
• Mixing Intensity: Proper mixing ensures uniform dispersion of coagulant and enhances particle collision and floc formation.

5. Monitoring and Control

• Jar Testing: Laboratory jar tests are conducted to determine the optimal coagulant dosage and mixing conditions for specific water sources.
• Online Monitoring: Turbidity meters and online particle counters provide real-time data on water quality parameters, allowing operators to adjust coagulant dosages accordingly.
• Process Optimization: Continuous optimization of coagulation parameters ensures efficient particle removal and consistent water quality.

6. Challenges and Considerations

• Residual Aluminum: Inorganic coagulants may leave residual aluminum in treated water, requiring additional treatment or management.
• Chemical Handling: Proper storage, handling, and dosing of coagulants are essential to ensure worker safety and prevent environmental contamination.
• Impact on Treatment Efficiency: Coagulation effectiveness may be affected by variations in raw water quality, necessitating periodic adjustments to treatment processes.

7. Benefits of Coagulation

• Improved Water Quality: Coagulation effectively removes suspended particles, turbidity, color, and organic matter from raw water, producing clear, potable water.
• Enhanced Treatment Efficiency: Coagulation enhances the performance of downstream treatment processes such as sedimentation, filtration, and disinfection.
• Compliance with Regulations: Coagulation plays a critical role in meeting regulatory standards for drinking water quality and ensuring public health and safety.

Coagulation is a fundamental process in water treatment that promotes the removal of suspended particles and impurities from raw water. By effectively neutralizing charges and facilitating floc formation, coagulation contributes to the production of high-quality treated water essential for various domestic, industrial, and municipal applications. Proper coagulant selection, dosage optimization, and process control are key to achieving efficient and reliable water treatment outcomes.

Filtration

Filtration is a crucial unit operation in water treatment plants aimed at removing suspended solids, microorganisms, and other impurities from water to produce clean, potable water suitable for various applications. It involves passing water through a porous medium or filter bed, which traps and retains suspended particles while allowing purified water to pass through.

1. Types of Filtration

• Granular Media Filtration: Involves passing water through a bed of granular media such as sand, anthracite coal, or multimedia filters.
• Membrane Filtration: Utilizes semipermeable membranes with fine pores to physically separate particles and impurities from water. Includes microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.

2. Mechanism of Filtration

1. Particle Capture: As water flows through the filter medium, suspended particles, microorganisms, and colloids are intercepted and trapped within the pores or void spaces of the filter media.
2. Depth Filtration: Filtration beds with varying particle sizes and densities provide multiple layers of filtration, allowing for the entrapment of particles throughout the depth of the filter medium.
3. Cake Formation: In some filtration processes, a thin layer of captured particles, known as a filter cake, forms on the surface of the filter medium, enhancing particle removal efficiency.

3. Filtration Processes

• Direct Filtration: Involves the passage of untreated water through a filtration medium without pre-treatment such as coagulation or sedimentation.
• Conventional Filtration: Preceded by pre-treatment processes like coagulation, flocculation, and sedimentation to optimize filtration efficiency and particle removal.

4. Filtration Media

• Sand: Most commonly used media in granular media filtration. Provides effective removal of suspended solids and impurities through physical straining and adsorption mechanisms.
• Activated Carbon: Highly porous media with adsorptive properties, effective in removing organic contaminants, odors, and taste compounds from water.
• Anthracite Coal: Provides superior filtration performance due to its high density, angularity, and resistance to fouling and sedimentation.

5. Filtration Equipment

• Gravity Filters: Operate under gravity flow without the need for additional pumping. Includes rapid gravity filters and slow sand filters.
• Pressure Filters: Employ pressure to force water through the filter medium, allowing for higher flow rates and improved efficiency. Examples include multimedia filters and cartridge filters.

6. Process Control and Monitoring

• Filtration Rate: Controlled flow rates ensure proper contact time between water and filter media, optimizing particle removal efficiency.
• Backwashing: Periodic backwashing of filtration media helps remove accumulated debris and restore filtration capacity.
• Effluent Quality Monitoring: Regular analysis of filtered water quality parameters such as turbidity, total suspended solids (TSS), and microbiological counts ensures compliance with regulatory standards.

7. Benefits of Filtration

• Particle Removal: Filtration effectively removes suspended solids, turbidity, and impurities from water, producing clear, visually appealing water.
• Microbial Reduction: Membrane filtration processes provide effective removal of bacteria, viruses, and protozoa, enhancing microbiological safety.
• Improved Water Quality: Filtration contributes to the production of high-quality, potable water suitable for drinking, industrial processes, and recreational activities.

Filtration is a critical process in water treatment that facilitates the removal of suspended solids, microorganisms, and impurities from water to produce clean, safe, and potable water. By utilizing various filtration media and processes, water treatment plants can achieve efficient particle removal and consistently meet regulatory standards for water quality. Proper process control, monitoring, and maintenance are essential to ensure optimal filtration performance and reliable operation of water treatment systems.

Redox potential

Redox potential, also known as oxidation-reduction potential or ORP, is a fundamental parameter in environmental water chemistry that measures the tendency of a system to undergo oxidation or reduction reactions. It plays a crucial role in understanding and controlling various biogeochemical processes in natural aquatic systems, wastewater treatment, and water quality management.

1. Redox Reactions

• Oxidation: Involves the loss of electrons by a chemical species, leading to an increase in its oxidation state or a decrease in its electron density.
• Reduction: Involves the gain of electrons by a chemical species, resulting in a decrease in its oxidation state or an increase in its electron density.

2. Redox Couples

• Redox reactions involve the transfer of electrons between two chemical species, known as redox couples.
• Oxidizing Agent: The species that accepts electrons and undergoes reduction.
• Reducing Agent: The species that donates electrons and undergoes oxidation.

3. Measurement of Redox Potential

• Electrochemical Potential: Redox potential is measured using a reference electrode, typically a standard hydrogen electrode (SHE) or a saturated calomel electrode (SCE), and a working electrode immersed in the water sample.
• Unit: Redox potential is expressed in millivolts (mV) relative to a standard reference electrode.

4. Importance in Environmental Water Chemistry

• Biogeochemical Processes: Redox potential influences various biogeochemical reactions such as mineral dissolution, metal speciation, nutrient cycling, and microbial metabolism.
• Wastewater Treatment: Redox potential plays a critical role in biological wastewater treatment processes such as activated sludge, anaerobic digestion, and denitrification.
• Water Quality Management: Monitoring redox potential helps assess the redox status of natural water bodies, identify sources of contamination, and evaluate the effectiveness of water treatment processes.

5. Redox Potential and Microbial Activity

• Anaerobic Conditions: Low redox potential indicates anaerobic conditions favorable for microbial reduction reactions, such as sulfate reduction, methanogenesis, and denitrification.
• Aerobic Conditions: High redox potential indicates aerobic conditions conducive to oxidative processes, such as nitrification and organic matter oxidation.

6. Factors Influencing Redox Potential

• Oxygen Availability: Oxygen is a potent oxidizing agent, and its presence influences redox potential. Aerobic conditions typically result in higher redox potentials, while anaerobic conditions lead to lower redox potentials.
• pH: Redox potential is pH-dependent, with higher pH values favoring reduction reactions and lower pH values favoring oxidation reactions.
• Presence of Electron Donors/Acceptors: The availability of electron donors (e.g., organic matter) and electron acceptors (e.g., oxygen, nitrate, sulfate) in the environment influences redox potential.

• The redox potentials of aqueous solutions clearly depend on both the dissolved oxygen, dissolved hydrogen, and hydrogen ion concentrations (pH). More acidic solutions favor aerobic conditions and more positive redox potential, and more alkaline solutions favor anaerobic conditions.

  As the concentration of molecular oxygen increases, the redox potential increases.

  As the concentration of molecular oxygen decreases, the redox potential decreases.

  As the concentration of molecular hydrogen increases, the redox potential decreases.

  As the concentration of molecular hydrogen decreases, the redox potential increases.

  As the concentration of hydrogen ions increases (and pH decreases), the redox potential increase.

  As the concentration of hydrogen ions decreases (and pH increases), the redox potential decreases.

7. Applications in Water Quality Management

• Redox Buffering: Redox potential serves as a buffer against rapid changes in the redox state of aquatic systems, maintaining chemical equilibrium and stability.
• Water Treatment Optimization: Monitoring redox potential helps optimize water treatment processes by controlling microbial activity, nutrient removal, and contaminant degradation.
• Bioremediation: Redox potential is utilised in bioremediation strategies to enhance the degradation of organic pollutants and remediate contaminated environments.

Redox potential is a critical parameter in environmental water chemistry that reflects the balance between oxidation and reduction processes in natural aquatic systems. By measuring redox potential, scientists and water quality professionals can assess the redox status of water bodies, evaluate microbial activity, and optimize water treatment processes to ensure the protection and preservation of aquatic ecosystems and human health. Understanding the principles of redox chemistry and its applications is essential for effective water quality management and environmental stewardship.