Water Treatment

1. Water Treatment

2. Water Treatment • Average household uses 300 litres per day Criteria • Colourless • Odourless • Safe to drink (no active bacteria) • Fluorinated To ensure water is of an adequate standard for drinking, it must be treated

3. 7 Stages Water from Lakes and Rivers Screening Flocculation Settlement Filtration Chlorination Fluoridation pH Adjustment Water to homes and industry

5. Screening * Wire mesh 1st stage * Removes large solids and floating debris like twigs, plastics etc Flocculation * flocculating agent ( or 2nd stage flocculant) usually aluminium sulphate (alum) is added. * Makes smaller suspended solids coagulate or stick together in large clumps, so they are easier to remove at the next stage

6. Settlement * Large tanks 3rd stage * Water goes in at the bottom and rises slowly to the top, at < 2m/hr * Particles settle to the bottom * Over 90% of suspended solids removed at this stage Filtration * Large beds of sand 4th stage * Removes remaining susp solids * Sand supported on a layer of gravel * Sand cleaned regularly * Water now clear but may contain harmful bacteria

7. Chlorination * Cl2 gas is added 5th stage * Sterilises water * Very small quantities 0.2 – 0.5 ppm * Controlled carefully * Monitored by bacteriological exam of the water Fluoridation * NaF or H2SiF6 added 6th stage (hexafluorosilicic acid) * Added by law as helps to reduce dental decay by strengthening the enamel * Small quantities ~ 1ppm

8. pH Adjustment7th stage • Optimum level is between 7 - 9 • Too Acidic * may cause damage to pipes * may be corrected by addition of Ca(OH)2 (lime)to raise the pH * If very hard water, might be softened by addition of NaCO3 which is a base • Too Basic * may be corrected by addition of dilute H2SO4 to lower the pH Water is now ready for consumption and use

9. Surface Water Quality • Contaminants: • Suspended solids, soil (turbidity) • Pathogens (coliform indicator) • Color (decaying vegetation, algae) • Taste & odor • Other SDWA contaminants

10. Water Treatment Water Sedimentation Flocculation Filtration Chlorination Fluoridation pH Adjustment

11. 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.

12. 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 suspended particles from the water

13. 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

14. Fluoridation This is the addition of fluoride compounds to drinking water to prevent tooth decay. Sodium fluorosilicate is usually the fluoridating agent.

15. 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.

16. Sewage Treatment • Primary Treatment • Physical Process • Secondary Treatment • Biological Process • Tertiary Treatment • Chemical Process

17. 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

18. 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

19. 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

20. Eutrophication of Water • Eutrophication is caused by the overenrichment of water by nutrients such as phosphates and nitrates.

21. 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.

22. 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.

23. 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.

24. 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

25. 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.