The Science Behind Turning Wastewater into Clean Water
The subject of the future of the world’s water supply is once again on the agenda following a recent UN report that forecasts “a rapid increase in the use of treated wastewater for farming and other purposes worldwide”.
While no specific volume increase was mentioned, the joint study between Tottori University and U.N. University’s Institute for Water, Environment and Health (UNU-INWEH) cited rising costs and decreasing supply as the primary drivers behind the projected increase.
So if we’re to rely increasingly on wastewater for our future needs, it’s probably a good time to get familiar with the science behind turning toxic by-product into clean, usable water.
We all remember osmosis from high school science class. Well, one of the more prominent wastewater management techniques is reverse osmosis (or RO for short), which is exactly what it sounds like – osmosis in reverse. Desalination plants use RO to remove salt from seawater and it can also be used to remove toxic impurities from industrial wastewater. While significant amounts of energy are required for RO, the process itself is straightforward – pressurized tanks push waste water through a membrane, which filters out impurities (like salt or iron), leaving behind safe water that can be reused.
As you’ll no doubt recall from various cooking exploits, when you add any kind of oil to water, the two don’t mix together – instead, they separate into layers with the oil floating on the water’s surface due to its thicker density.
Machines called oil skimmers (or oil water separators) rely on this law of physics to remove oil from water. And while there are a plethora of different oils skimming machine types that work in slightly different ways, they all use a ‘skimming’ action to scoop the oil from the water’s surface – think of it as the industrial equivalent of using a spoon to scoop olive oil from the surface of the water you’re boiling your pasta in. Oil skimming is often used as a first step in waste water treatment before other processes remove further impurities.
Fresh water naturally has a pH of level of seven, which means it is neither acid nor alkali. When water is used in disciplines like manufacturing and mining, this often changes, with the water being too acidic or alkaline to use again or dispose of safely. pH control systems create reusable water by returning those levels to normal; if the pH is five or below (i.e. too acidic), alkalis like lime are added; if it’s six or above (too alkaline), acids (like sulfuric acid) are put in. It’s exactly the same process as you may have used to treat your garden’s soil or your fish tank’s water.
If any one element could be called the ‘poster boy’ of global warming, it’s carbon. The irony is that although carbon is often followed by words like ‘emissions’ and ‘footprints’, it’s extremely useful in purifying industrial wastewater.
You may not have realised, but if you have a tap water filter jug at home, the filter section is made from ‘activated carbon bed’, which in layman’s terms is just plain old charcoal. The reason charcoal is used is because carbon readily bonds (i.e. ‘sticks together’) with many other types of atoms. The porous, dimpled surface of charcoal accelerates this process, so it’s an ideal, ready-made natural water purifier.
The exact same technique is used to clean impurities from wastewater, just on a much larger scale; wastewater is pumped into tanks with activated carbon beds, and the chemicals in the water simply ‘stick’ to the carbon. This means the same water can be used again and again for the same manufacturing processes without having to taint the world’s diminishing drinking supply.