Water Treatment Water treatment plants receive water from

Water Treatment

• Water treatment plants receive water from different sources – rivers, lakes & reservoirs • 7 step process – screening, flocculation, settling, filtration, chlorination, fluoridation, p. H adjustment

Treatment Plant

1) Screening: • Water passes through large screens of wire mesh • This removes large suspended solids like branches, twigs or litter

Screening

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

Flocculation

3) Settling/Sedimentation: • Flocculated water pumped into bottom of large tanks • Water slowly rises to the top • Large suspended particles sink to bottom

Settling

4) Filtration: • Water passes from tanks through sand gravel beds • This removes any remaining suspended particles • Filter beds regularly cleaned

Filtration

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

Chlorination

6) Fluoridation: • Fluorine added to help prevent tooth decay • Added in form of sodium fluorosilicate • Only added in some countries (incl. Ireland)

Fluoridation

7) p. H adjustment: • Tap water should be in range of p. H 6 – 8 • If p. H too low (too acidic) lime is added • If p. H is too high (too basic) sulfuric acid is added • If the p. H is too low it can cause pipes to corrode

p. H adjustment

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

• 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

• 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

Algae Bloom

• 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)

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

• 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

• But before being dumped in water they can be removed • Removed by precipitation • There are different treatment processes for each metal

Water quality • Controlled by EU regulations • Limits have to be set because of toxic effects • Examples: Chemical: phosphates nitrates lead mercury Limit: 2. 2 p. p. m 50. 0 p. p. m 0. 01 p. p. m 0. 1 p. p. m

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

Sewage Plant

Primary Treatment • Physical process • Screening and settling • Effluent passes through large wire mesh to remove large particles and break up sewage

• 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

• 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

Secondary Treatment • Biological process • Bacterial breakdown of nutrients in effluent • Effluent oxidised aerobically (in air) by a trickling filter OR an activated sludge unit

• Trickling filter – effluent passes through bed of stones containing micro-organisms that digest the nutrients

Trickling Filter

Trickling Filter

• Activated sludge unit – effluent passes into tank containing micro-organisms that digest the nutrients. Tank is kept oxygenated by a mechanical rotar

Activated Sludge Unit

• 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

Tertiary Treatment • Removal of nitrates and phosphates • Very expensive process so not all plants carry out this stage • Removed by precipitation

• 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