hydrosphere - water treatment
sedimentation
water is allowed to remain static to let suspended solids such as silt settle
screens
metal grids or meshes are used to remove vegetation such as litter and plastic
aeration
bubbles of air of water sprays are used to aerate the water and ensure a high dissolved oxygen content.
water sources that are anaerobic may contain hydrogen sulfide from the decay of dead organic matter, making it smell bad
some dissolved metals that are toxic, or give water a bad taste, are removed by aeration as they become insoluble
flocculation
clay particles do not settle put in the sedimentation lagoons because electrostatic charges on their surfaces cause them to repel each other
these can be neutralised by adding flocculants such as aluminium sulfate or polyelectrolytes. These are mixed quickly with the water then passed into the clarifier tank where the particles are allowed to settle
filtration
filters are used to remove any remaining suspended solids or bacteria - these often involve slow flow through layers of sand and gravel
activated carbon filters
particles of activated carbon are used to remove organic chemicals such as pesticides, which absorb onto the carbon particles
sterilsation
addition of chlorine, ozone, or exposure to uv light, is used to sterilise the water and kill pathogens
pH control
sterilisation with chlorine can make the eater to acidic, pH adjusted by adding lime
flouridation
flourides added to improve dental health of drinking water
ion exchange
toxic ions such as those of lead, mercury and arsenic can be removed from water using ion exchange resins. the toxic ions absorb onto polymer beads or particles of xeolite
reverse osmosis
removing salt from saltwater by desalination is expensive and is only carried out when no water is available
saline water = filtered = high pressure through partially permeable membranes of very small polyamide tubes = fresh water collected = half amount of original = very salty water returned to sea
distillation
water is boiled by heating and/or reducing the pressure, Steam that is produced is condensed and collected