How often do we stop to think about where our water has come from, or what processes it has been through?
Most of the water that comes out of our taps filters through a municipal water treatment plant. But what does this mean, and what is the science behind this practice?
There are two types of water treatment plants: one for drinking water and one for wastewater. Drinking water pumps into distribution pipes. Wastewater is usually pumped back into the river system
Water Treatment Plants for Drinking Water
The water originates either from a lake, a river, or even sometimes a small stream. The process from origin to your tap involves three steps.
This link shows what some plants may look like.
Sedimentation
The first step in this process the removal of dissolved solid matter suspended in the water. Chemicals call coagulants to the water, causing the sediments to stick together. Once this happens the water moves into a flocculation basin to form floc particles. These settle down into a sediment basin. Cleaner water then flows over the top.
The Sand Filter
The second step is to pass this water through a sand filter. It is possible to remove all solids with a sand trap. But this would mean it would need cleaning more often. This would reduce the efficiency of the water treatment plant.
Disinfection
Once passed through the sand trap, the water now has clarity but it still contains bacteria. The third step is to disinfect the water. There are two ways to do this – chlorine or UV rays.
Chlorine kills the bacteria but can react with any remaining organic matter. This can cause carcinogenic properties in the water.
UV rays scramble the bacteria’s DNA. This means it cannot reproduce and so become harmless if ingested. Any entering the water after UV treatment will still be harmful.
Waste Water Treatment Plants
Wastewater is sewerage – treated, separated, and returned to the environment. The solids and pollutants get removed. The organic matter is broken down and the oxygen is restored to the treated water. This takes four stages.
Preliminary
In the preliminary stage, the pant removes rubbish from the water. Large plant and non-organic matter get raked out. Some water treatment plants skim off the grease and oil at this point, others wait until the next step.
Primary
Removal of the smaller particles and solid matter is at this point in the water treatment plant. If grease and oil were not removed, this happens now using a surface skimmer.
Secondary
The agitation and aeration process. Microorganisms added here help break down organic matter into sludge.
Sludge
Gravity separates the heavier waste from the sludge. The waste moves on to landfills, the sludge moves on through a thickener. Sludge feeds into digesting tanks with anaerobic bacteria. This helps it separate stabilized sludge from the water. Disinfection of the water is the same as drinking water is and returned to the water supply.
The End of the Line
This is the science behind water treatment plants. Once these steps are complete, the water is ready. It is now pumped into the distribution system or back into the water system.
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