We think of our drinking water as pure.

It isn't.

It has substances dissolved in it.

In some areas there is so much dissolved in it that it scales up kettles.

Sometimes we even add chemicals to our drinking water to make it safer or more healthy.

Only distilled water has nothing dissolved in it and people find its taste a bit odd.

Most of our drinking water in Britain was recently rain.

The rain ran over the surface (and sometimes below the surface) of the earth until it collected somewhere that a water company could use as a water-source.

Sometimes this is a river, occasionally it is a natural lake, more often its a man-made lake (reservoir), very occasionally its an underground feature called an Aquifer.

All these would be thought of as fresh water because they are not salty like sea-water.

But this fresh water isn't fit to drink immediately because it will be dirty and there will be microbes living in it that may make us ill.

Removing dirt is cheap an inexpensive.

A simple screen will remove most of the floating wood, rubbish, dead cats and shopping trollies etc.

Leaving the cleaner water to settle allows insoluble particles to sink - sedimentation.

This water still contains microbes because they don't sink!

It is passed through a simple sand filter.

So long as we use very fine sand (small grains) the spaces between the grains are smaller than the microbes, so the water escaping the bottom will be fit to drink.

But to be sure that there are no microbes and to kill any that get in later, we sterilise the water by adding Chlorine in concentrations low enough to kill the microbes but too low to harm us.

Other ways of sterilising water are:

1. Adding Ozone - a poisonous version of Oxygen with the formula O₃

2. Ultraviolet light

The advantage of Chlorine is it stays in the water and kills microbes that get into the system later - UV wouldn't do this.

No.

You can filter and chlorinate water polluted by soluble pesticides and the pesticides will still be dissolved in the water.

These may be harmful to humans.

So, farmers' use of chemicals is strictly controlled.

And we could filter and sterilise sea-water but it still wouldn't be potable because the level of salt is dangerous.

There is a desalination plant in London for removing salt from seawater when the city is short of a better source.

But it isn't used often because of the huge energy costs.

Most desalination plants distill the seawater, heat the water until it evaporates and then cool the water vapour until it condenses.

Some plants use Reverse Osmosis

Huge pressure is used to push salty water through a membrane that will not allow the salt through.

This is still extremely expensive but new materials like Graphene may allow the process to become much cheaper as much thinner filters that do the same job allow lower pressures to be used.

Sewage

A water treatment plant will remove sewage sludge in the ways outlined above.

The UK makes 15,000,000 kg of dried sludge a year!

The water from this will still contain dissolved substances but is safe to discharge back into rivers.

Some of this water will be removed again and cleaned up for drinking water!

The sludge is collected, sterilised and mostly used as fertiliser to grow food.

Anaerobic Digestion means that bacteria are used to break down the sludge in the absence of Oxygen.

As a by-product, Methane is made that can be used as fuel to maintain the 35^oC needed for the process to be efficient.

Aerobic treatment also uses bacteria but in the presence of Oxygen.

This time Carbon Dioxide is made instead of Methane.