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Section 9.3 water treatment

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M
Martin Brown

This document summarizes water treatment and sewage treatment processes. It describes how surface water is treated through steps like sedimentation, flocculation, filtration, chlorination and fluoridation to remove contaminants. Sewage treatment involves primary, secondary and sometimes tertiary levels to remove solids, break down organic waste biologically, and remove nutrients like nitrates and phosphates. The document also discusses how excess nutrients can cause eutrophication of waterways and pollution from heavy metals and legislation limits their levels.

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Water Treatment
Surface Water Quality Contaminants: – Suspended solids, soil (turbidity) – Pathogens (coliform indicator) – Color (decaying vegetation, algae) – Taste & odor – Other SDWA contaminants
Water Treatment Water Sedimentation Flocculation Filtration Chlorination Fluoridation pH Adjustment
Sedimentation The water is pumped into the bottom of the sedimentation tanks, so as not to disturb the clearer water at the top. The suspended particles settle to the bottom.
Filtration  Remaining suspended particles are removed by filtration. The water is allowed to pass through beds of graded sand and gravel. The sand in the filter bed acts as a filter and removes the tiny particles from the water
Chlorination This is the addition of chlorine or chlorine compounds to kill microorganisms in the water and to prevent reinfection. Both chlorine and sodium hypochlorite are added to the water to form an active disinfecting agent in the water
Fluoridation This is the addition of fluoride compounds to drinking water to prevent tooth decay. Sodium fluorosilicate is the usually fluoridating agent.
pH adjustment It may be necessary to adjust the pH of the water before it leaves the treatment plant. Tap water should have a pH in the range of 6 – 8. If the water is too acidic, lime is added to raise the pH and if the pH is too high, sulfuric acid is added.
Sewage Treatment Primary  Physical Treatment Process Secondary  Biological Tertiary Treatment Process Treatment  Chemical Process
Primary Treatment Solids and large floating debris are screened from the waste water Remaining solids are removed by allowing the waste to settle in sedimentation tanks
Secondary Treatment  Activated Sludge Method  The activated sludge method operates aerobically. The sewage is fed continuously into aerated tanks that is kept oxygenated by mechanical agitators. Aerobic Microorganisms break down organic waste in the sewage by oxidation in air to carbon dioxide and water
Tertiary Treatment Removal of nitrates and phosphates Phosphates are removed by precipitation with Aluminium Phosphate Nitrates are removed by biological nitrification. Tertiary treatment is a costly process
Eutrophication of Water  Eutrophication is caused by the overenrichment of water by nutrients such as phosphates and nitrates.
Eutrophication of Water  In effect, the nutrients behave as fertilisers that increase the growth of plants such as algae in lakes and rivers  The algae are short lived. As they decay, much of the dissolved oxygen in the water is used up, leading to the death of many forms of animal life.
Nitrate Fertilisers Waterways can also be polluted by the run-off of excess fertiliser from farmland. Excess nitrate in the fertiliser may be washed into rivers or lakes by rainwater. Eutrophication will result.
Pollution of Heavy Metals Metals with high relative atomic masses such as mercury, cadmium and lead are known as heavy metals. When recycling is inadequate, quantities of these elements are dumped e.g. Car batteries containing lead or dry batteries containing cadmium.
Pollution of Heavy Metals  Dipositive ions of these metals  e.g. Hg2+ , Cd2+ , and Pb2+ sometimes get into waterways from industrial effluent and consequently into drinking water.  These elements are cumulative poisons in that frequent exposure causes build up in the body, resulting in serious health damage. Lead ions can be removed from the effluent by precipitation
EU Legislation on Water Quality There are limits to the quantities of Hg2+ , Cd2+ , and Pb2+ ions that can be tolerated in waterways because of their toxic effects. Limits on phosphates and nitrates help to reduce the occurrence of eutrophication in waterways. Limits are also set for chemical species dissolve in drinking water.

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Section 9.3 water treatment

  • 2. Surface Water Quality Contaminants: – Suspended solids, soil (turbidity) – Pathogens (coliform indicator) – Color (decaying vegetation, algae) – Taste & odor – Other SDWA contaminants
  • 4. Sedimentation The water is pumped into the bottom of the sedimentation tanks, so as not to disturb the clearer water at the top. The suspended particles settle to the bottom.
  • 5. Filtration  Remaining suspended particles are removed by filtration. The water is allowed to pass through beds of graded sand and gravel. The sand in the filter bed acts as a filter and removes the tiny particles from the water
  • 6. Chlorination This is the addition of chlorine or chlorine compounds to kill microorganisms in the water and to prevent reinfection. Both chlorine and sodium hypochlorite are added to the water to form an active disinfecting agent in the water
  • 7. Fluoridation This is the addition of fluoride compounds to drinking water to prevent tooth decay. Sodium fluorosilicate is the usually fluoridating agent.
  • 8. pH adjustment It may be necessary to adjust the pH of the water before it leaves the treatment plant. Tap water should have a pH in the range of 6 – 8. If the water is too acidic, lime is added to raise the pH and if the pH is too high, sulfuric acid is added.
  • 9. Sewage Treatment Primary  Physical Treatment Process Secondary  Biological Tertiary Treatment Process Treatment  Chemical Process
  • 10. Primary Treatment Solids and large floating debris are screened from the waste water Remaining solids are removed by allowing the waste to settle in sedimentation tanks
  • 11. Secondary Treatment  Activated Sludge Method  The activated sludge method operates aerobically. The sewage is fed continuously into aerated tanks that is kept oxygenated by mechanical agitators. Aerobic Microorganisms break down organic waste in the sewage by oxidation in air to carbon dioxide and water
  • 12. Tertiary Treatment Removal of nitrates and phosphates Phosphates are removed by precipitation with Aluminium Phosphate Nitrates are removed by biological nitrification. Tertiary treatment is a costly process
  • 13. Eutrophication of Water  Eutrophication is caused by the overenrichment of water by nutrients such as phosphates and nitrates.
  • 14. Eutrophication of Water  In effect, the nutrients behave as fertilisers that increase the growth of plants such as algae in lakes and rivers  The algae are short lived. As they decay, much of the dissolved oxygen in the water is used up, leading to the death of many forms of animal life.
  • 15. Nitrate Fertilisers Waterways can also be polluted by the run-off of excess fertiliser from farmland. Excess nitrate in the fertiliser may be washed into rivers or lakes by rainwater. Eutrophication will result.
  • 16. Pollution of Heavy Metals Metals with high relative atomic masses such as mercury, cadmium and lead are known as heavy metals. When recycling is inadequate, quantities of these elements are dumped e.g. Car batteries containing lead or dry batteries containing cadmium.
  • 17. Pollution of Heavy Metals  Dipositive ions of these metals  e.g. Hg2+ , Cd2+ , and Pb2+ sometimes get into waterways from industrial effluent and consequently into drinking water.  These elements are cumulative poisons in that frequent exposure causes build up in the body, resulting in serious health damage. Lead ions can be removed from the effluent by precipitation
  • 18. EU Legislation on Water Quality There are limits to the quantities of Hg2+ , Cd2+ , and Pb2+ ions that can be tolerated in waterways because of their toxic effects. Limits on phosphates and nitrates help to reduce the occurrence of eutrophication in waterways. Limits are also set for chemical species dissolve in drinking water.