Question: How do you make water clean

Craig Chalmers answered on 20 May 2020:

The most commonly used method in the UK is probably a DAF & RGF plant, technology which hasn’t really changed much for 100 years. This would typically consist of 3 main processes:

In the first process, a ‘dissolved air flotation’ (DAF) tank, a chemical is added to the water which will attach itself to any solid particles (for example, soil) within the water, and then air is blown into the bottom of the tank. Air bubbles will float up through the tank, and will catch the chemical added and any solids attached to it up to the surface. This mix of chemicals and solids (called ‘sludge’) can then be skimmed off the top of the tank, whilst cleaner water below the surface passes onto the next stage.

Second, the water will pass through a ‘Rapid Gravity Filter’ (RGF). This is simply a big tank, with a layer of sand (and sometimes a layer of gravel underneath it) at the bottom of it. The water flows into the top of the tank, through this layer of sand and out the bottom of the tank, whilst particles within the water will get stuck in the sand.

Finally, the water will pass to a ‘disinfection’ stage – where the water will usually be dosed with chlorine in some form before being retained in a tank for a period of time, to allow the chlorine to kill any bacteria within the water.

However, the above is just an example of a typical large water works in the UK – there are many different types of process that can be used, depending on the quality of the water to begin with, the amount of water the works needs to treat, and the standards that need to be met. Famously, the water in NYC isn’t ‘treated’ (though they do dose with chlorine to kill germs) as they have put so much effort into ensuring it isn’t contaminated in the first place, whilst in other locations additional steps may be added to deal with particular issues in that area.