Andrada Dumitrascu of Balclutha Primary School asks :-

How do people clean dirty water?

Johannes Bosch, a water quality chemist with the Palmerston North City Council, responded.

When you mention cleaning dirty water it could mean quite a few different things. I will give you some idea of what could be done for drinking water.

Normally it is not necessary to clean rainwater when it is collected in household tanks but occasionally you need to add a bit of chlorine (Janola) to it to kill a few bugs. Rainwater seeps into the ground, collects in aquifers and sometimes it is pushed to the surface as artesian wells. Water from aquifers has been filtered through the soil and often have very few or no bugs that are harmful to us. Some supplies are so good that nothing needs to be done to them and the Christchurch basin is an example of a good clean artesian supply.

When a town water supply is cleaned they would use a combination of treatments to suit the water. Solids are often associated with bugs and people know instinctively not to drink water that looks murky. Particles in river and dam water can be filtered through sand, like aquifers, and similar to swimming pool filters that have sand in them. They might coagulate some unwanted particles with long chain molecules called polymers. The particles are normally allowed to settle to remove them before filtration. Groundwater with lots of iron and manganese changes colour because of oxygen in the air, and has a taste. Permanganate or chlorine added before a sand filter can remove them.

Activated charcoal like in the filter of a fish tank can remove some organics that cause tastes and smells. Some chemical reactions depend on pH to work properly and something like lime might be added to increase the pH. The last step before people get to drink the water is usually adding some chlorine to keep the water bug free.

Some water has metals and other compounds that need to removed to be safe to drink and there are complicated methods like reverse osmosis and ion exchange needed to clean it. Some countries also use evaporation to collect drinking water from seawater.

When you go tramping you can get tablets with chlorine or iodine to disinfect water. Boiling water works very well and does not leave a taste.

Here are three Internet sites with some stuff to look at http://www.groundwater.org/kc/kc.html http://www.epa.gov/safewater/kids/index.html http://en.wikipedia.org/wiki/Drinking_water