Potable water and water treatments

• Potable water is water that has been treated or is naturally safe for humans to drink
• Chemists wouldn't call potable water pure as it contains a lot of dissolved substances
• The important thing with potable water is that the levels of dissolved salts aren't too high, its pH is between 6.5 and 8.5 and there is not any bacteria or microbes in it
• How potable water is produced depends on where you are
• Rainwater is a type of fresh water that doesn't have much dissolved in it, it can either be collected as surface water or groundwater
• In the UK, the source of freshwater used depends on the location. Surface water tends to dry up first so in warm areas, most of the domestic supply comes from groundwater.

  Even though it has low levels of dissolved substances, it still needs to be treated to make it safe before it can be used
• Treating fresh water:

  1. Filtration- a wire mesh screens out large twigs etc and then gravel and sand beds filter out any solid bits
  2. Sterilisation- the water is sterilised to kill any harmful bacteria or microbes. This can be done by bubbling chlorine gas through it or by using ozone or UV light
• In some very dry countries, there is not enough surface of groundwater and instead seawater must be treated by desalination to provide potable water- use distillation
• Sea water can also be treated by processes that use membranes- like reverse osmosis. The salty water is passed through a membrane that only allows water molecules to pass through, ions and larger molecules are trapped by the membrane so separated from the water
• Both distillation and reverse osmosis need loads of energy, so they're really expensive and not practical for producing large quantities of water
• we use water for a lot of things at home and when we flush this water down the drain it goes into the sewers and towards sewage treatment plants
• agricultural systems also produce a lot of waste water including nutrient run off from fields and slurry from animal farms
• sewage from domestic or agricultural sources has to be treated to remove any organic matter and harmful microbes before it can be put back into fresh water sources like rivers or lakes, otherwise it would make them very polluted and would pose health risks
• Industiral process produce a lot of waste that has to be collected and treated
• Industrial waste water can also contain harmful chemicals, so it has to undergo additional stages of treatment before it is safe to release into the environment
• Sewage treatment happens in several stages:

  1. screening
  2. sedimentation
  3. aerobic digestion
  4. anaerobic digestion
  5. gas and digested waste produced
• sewage treatment step 1

  Before being treated, the sewage is screened- involves removing any large bits of material as well as any grit
• sewage treatment step 2

  Then it is allowed to stand in a settlement tank and undergoes sedimentation- the heavier suspended solids sink to the bottom to produce sludge while the lighter effluent floats on top
• sewage treatment step 3

  The effluent in the settlement tank is removed and treated by biological aerobic digestion. This is when air is pumped through the water to encourage aerobic bacteria to break down any organic matter- including other microbes in the water
• Sewage treatment step 4

  the sludge from the bottom of the settlement tank is also removed and transferred into large tanks. Here it gets broken down by bacteria in a process called anaerobic digestion
• Sewage treatment step 5

  anaerobic digestion breaks down the organic matter in the sludge, releasing methane gas in the process. The methane gas can be used as an energy source and the remaining digested waste can be used as a fertiliser
• sewage treatment step 6

  For waste containing toxic substances, additional stages of treatment may involve adding chemicals, UV radiation or using membranes
• sewage treatment requires more processes than treating fresh water but uses less energy than the desalination of salt water, si could be used as an alternative in areas where there's not much fresh water