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Water Treatment
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Water treatment plants receive water from different sources – rivers, lakes & reservoirs
7 step process – screening, flocculation, settling, filtration, chlorination, fluoridation, pH adjustment
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Treatment Plant
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1) Screening: Water passes through large screens of wire mesh This removes large suspended solids like branches, twigs or litter
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Screening
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2) Flocculation: A flocculating agent called aluminium sulfate added to water Acts as a coagulant – makes small suspended particles stick together and form large ones
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Flocculation
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3) Settling/Sedimentation:
Flocculated water pumped into bottom of large tanks Water slowly rises to the top Large suspended particles sink to bottom
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Settling
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4) Filtration: Water passes from tanks through sand and gravel beds This removes any remaining suspended particles Filter beds regularly cleaned
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Filtration
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5) Chlorination: Chlorine added to sterilise water – killing any micro-organisms Added in very small forms – just enough to keep water sterile Added in form of chlorine gas or sodium hypochlorite
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Chlorination
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6) Fluoridation: Fluorine added to help prevent tooth decay Added in form of sodium fluorosilicate Only added in some countries (incl. Ireland)
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Fluoridation
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7) pH adjustment: Tap water should be in range of pH 6 – 8 If pH too low (too acidic) lime is added If pH is too high (too basic) sulfuric acid is added If the pH is too low it can cause pipes to corrode
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pH adjustment
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Eutrophication Eutrophication is the over-enrichment of water by nutrients like phosphates and nitrates This is reason for tertiary stage of sewage treatment as it removes phosphates & nitrates
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Occurs when agricultural waste like fertiliser and animal slurry are dumped in water
Fertiliser and animal slurry are full of these nutrients Use of fertiliser and control of dumping are vital to prevent this pollution
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The nutrients in the water act like fertilisers to plants and increase growth of plants and algae in lakes and rivers Can see the algae bloom formed
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Algae Bloom
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Algae do not live long As they decay they consume the oxygen dissolved in the water By using up this dissolved oxygen they cause the death of animal life (animals living in the water and those that rely on that water supply)
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Heavy metal pollution Heavy metals = metals with high relative atomic masses Examples = mercury (Hg), cadmium (Cd) and lead (Pb) Sources = car batteries or dry batteries
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Have started recycling batteries which will improve level of pollution
But if dumped in water can pass into drinking water All these metals are poisonous and if build up in body can cause serious illness
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But before being dumped in water they can be removed
Removed by precipitation There are different treatment processes for each metal
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Water quality Controlled by EU regulations
Limits have to be set because of toxic effects Examples: Chemical: Limit: phosphates p.p.m nitrates p.p.m lead p.p.m mercury p.p.m
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Sewage Treatment Domestic sewage dumped untreated in waterways for years Huge source of pollution to water supply Some areas now have sewage plants to treat the effluent 3 stage process – primary, secondary and tertiary treatment
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Sewage Plant
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Primary Treatment Physical process Screening and settling
Effluent passes through large wire mesh to remove large particles and break up sewage
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Passes through long grit channels on way to settling tanks to remove more particles
Grit channels regularly cleaned Enters settling tank at bottom and slowly rises to top
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Sludge settles at bottom and is removed regularly
Machine skims surface of water to remove any floating particles Effluent flows from top of tank on to secondary stage
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Secondary Treatment Biological process
Bacterial breakdown of nutrients in effluent Effluent oxidised aerobically (in air) by a trickling filter OR an activated sludge unit
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Trickling filter – effluent passes through bed of stones containing micro-organisms that digest the nutrients
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Trickling Filter
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Trickling Filter
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Activated sludge unit – effluent passes into tank containing micro-organisms that digest the nutrients. Tank is kept oxygenated by a mechanical rotar
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Activated Sludge Unit
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Aerobically digested effluent then passes into settlement tanks
It is now safe to pass into waterways where it will be diluted and less harmful to environment But still contains nitrates and phosphates so moves on to tertiary stage
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Tertiary Treatment Removal of nitrates and phosphates
Very expensive process so not all plants carry out this stage Removed by precipitation
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Phosphates react with aluminium sulfate (flocculating agent) and are removed by filtration
Nitrates removed by biological denitrification – effluent left in tanks with no air so uses the oxygen bonded to nitrogen in nitrates to digest effluent
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