Environmental & Industrial Chemistry

Water Treatment

5th Year · 6th Year (Leaving Cert)

✓By the end of this lesson students will be able to define and distinguish between temporary and permanent hardness in water, and describe methods for their removal.
✓By the end of this lesson students will be able to outline the main stages involved in the purification of water for public supply, explaining the purpose of each stage.
✓By the end of this lesson students will be able to explain the phenomenon of eutrophication, its causes, and its environmental consequences.
✓By the end of this lesson students will be able to discuss the advantages and disadvantages of hard water and the process of fluoridation.

Key concepts

Hard Water

Water that does not easily form a lather with soap due to the presence of dissolved mineral ions, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions.

Temporary Hardness

Caused by the presence of dissolved calcium hydrogencarbonate (Ca(HCO₃)₂) and magnesium hydrogencarbonate (Mg(HCO₃)₂). It can be removed by boiling, which precipitates the metal carbonates.

Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

Permanent Hardness

Caused by the presence of dissolved calcium sulfate (CaSO₄), magnesium sulfate (MgSO₄), calcium chloride (CaCl₂), and magnesium chloride (MgCl₂). It cannot be removed by boiling but can be removed by adding washing soda or using an ion-exchange resin.

CaSO₄(aq) + Na₂CO₃(aq) → CaCO₃(s) + Na₂SO₄(aq)

Water Purification

The process of removing undesirable chemicals, biological contaminants, suspended solids, and gases from water to make it suitable for a specific purpose, typically drinking.

Flocculation/Coagulation

The process of adding a chemical (e.g., aluminium sulfate, Al₂(SO₄)₃) to water to cause small suspended particles to clump together into larger, heavier particles called flocs, which can then settle out.

Chlorination

The process of adding chlorine (Cl₂) or chlorine compounds to water to disinfect it by killing bacteria, viruses, and other microorganisms, making it safe for consumption.

Fluoridation

The controlled addition of fluoride compounds (e.g., sodium hexafluorosilicate, Na₂SiF₆, or hexafluorosilicic acid, H₂SiF₆) to a public water supply to reduce tooth decay.

Eutrophication

The excessive enrichment of a water body with nutrients, primarily nitrates and phosphates, leading to an overgrowth of aquatic plants and algae (an algal bloom).

Biochemical Oxygen Demand (BOD)

A measure of the amount of dissolved oxygen required by aerobic biological organisms to break down organic material present in a given water sample at a certain temperature over a specific time period (usually 5 days). A high BOD indicates significant organic pollution.

Key facts to remember

1Hardness in water is primarily caused by dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
2Temporary hardness is due to hydrogencarbonates and can be removed by boiling.
3Permanent hardness is due to sulfates and chlorides and cannot be removed by boiling.
4Washing soda (Na₂CO₃) can remove both temporary and permanent hardness by precipitating Ca²⁺ and Mg²⁺ ions as insoluble carbonates.
5The main stages of water purification are screening, flocculation, sedimentation, filtration, chlorination, and fluoridation.
6Aluminium sulfate (Al₂(SO₄)₃) is commonly used as a coagulant in flocculation.
7Chlorine is added to water for disinfection, killing harmful bacteria and microorganisms.
8Eutrophication is caused by excessive nitrates and phosphates, leading to algal blooms and subsequent oxygen depletion in water bodies.

Worked examples

Example 1

A 1.0 L sample of hard water contains 0.005 mol of calcium sulfate (CaSO₄). Calculate the mass of anhydrous washing soda (Na₂CO₃) required to remove this permanent hardness.

I1. Write the balanced chemical equation for the reaction:

II CaSO₄(aq) + Na₂CO₃(aq) → CaCO₃(s) + Na₂SO₄(aq)

III2. Determine the mole ratio between CaSO₄ and Na₂CO₃:

IV From the equation, 1 mole of CaSO₄ reacts with 1 mole of Na₂CO₃.

V3. Calculate the moles of Na₂CO₃ required:

VI Moles of CaSO₄ = 0.005 mol

VII Therefore, moles of Na₂CO₃ required = 0.005 mol

VIII4. Calculate the molar mass of Na₂CO₃:

9 Na = 23.0, C = 12.0, O = 16.0

10 Molar mass of Na₂CO₃ = (2 × 23.0) + (1 × 12.0) + (3 × 16.0) = 46.0 + 12.0 + 48.0 = 106.0 g/mol

115. Calculate the mass of Na₂CO₃ required:

12 Mass = Moles × Molar mass

13 Mass = 0.005 mol × 106.0 g/mol = 0.53 g

Answer

0.53 g of anhydrous washing soda is required.

Ensure you use the correct molar mass for anhydrous washing soda (Na₂CO₃), not hydrated forms.

Example 2

Describe the process of eutrophication, outlining its causes and consequences for an aquatic ecosystem.

I1. Causes: Eutrophication is primarily caused by an excessive input of nutrients, mainly nitrates (NO₃⁻) and phosphates (PO₄³⁻), into a water body. These nutrients typically originate from agricultural run-off (fertilisers), untreated sewage, and industrial effluents.

II2. Algal Bloom: The increased nutrient concentration leads to rapid and excessive growth of aquatic plants and algae on the water surface, known as an algal bloom. This bloom blocks sunlight from reaching submerged plants.

III3. Death and Decomposition: As the algal bloom becomes too dense, or when nutrients are depleted, the algae and other aquatic plants begin to die. These dead organic materials sink to the bottom of the water body.

IV4. Oxygen Depletion: Aerobic bacteria decompose the dead organic matter. This decomposition process consumes large amounts of dissolved oxygen from the water. The Biochemical Oxygen Demand (BOD) of the water increases significantly.

V5. Consequences: The severe depletion of dissolved oxygen makes the water anoxic, leading to the death of fish and other aquatic organisms that require oxygen to survive. This results in a loss of biodiversity, unpleasant odours (due to anaerobic decomposition), and a general degradation of water quality, making the water unsuitable for recreation or drinking.

Answer

Eutrophication is the nutrient enrichment of water, leading to algal blooms, subsequent oxygen depletion by decomposers, and ultimately the death of aquatic life.

Remember to link the algal bloom to the death of algae, then to bacterial decomposition, and finally to oxygen depletion.

Example 3

Identify the water purification stage described below and explain its purpose: 'Water is passed through beds of sand and gravel to remove any remaining suspended solids and some microorganisms.'

I1. Identify the stage: The description 'passed through beds of sand and gravel' for removing suspended solids and microorganisms corresponds to the Filtration stage.

II2. Explain its purpose: The purpose of filtration is to remove fine suspended particles, such as silt, clay, and some larger microorganisms, that were not removed during sedimentation. This process physically strains these impurities from the water, improving its clarity and reducing turbidity before disinfection.

Answer

The stage is Filtration. Its purpose is to remove fine suspended solids and some microorganisms that remain after sedimentation, thereby improving the clarity and quality of the water.

Each stage of water purification has a distinct purpose; ensure you can differentiate between them.

Common mistakes

✗Confusing the causes of temporary and permanent hardness, or incorrectly stating that boiling removes permanent hardness.
✗Mixing up the order or purpose of the different stages in water purification (e.g., thinking chlorination comes before filtration).
✗Failing to explain the full chain of events in eutrophication, particularly the link between dead algae, bacterial decomposition, and oxygen depletion.
✗Incorrectly identifying the chemicals used in water treatment, such as using 'bleach' instead of 'chlorine' for disinfection or 'lime' instead of 'aluminium sulfate' for flocculation.
✗Not being able to provide both advantages and disadvantages of hard water or fluoridation when asked.

Exam tips

★Memorise the chemical equations for the removal of temporary hardness by boiling and permanent hardness by washing soda.
★Be able to list and briefly explain the purpose of each of the six main stages of water purification in the correct sequence.
★For eutrophication, practice explaining the process step-by-step, from nutrient input to the death of aquatic life, including the role of BOD.
★Prepare to discuss the pros and cons of both hard water and fluoridation, as these are common discussion questions in the Leaving Cert exam.

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