How is sewage treated? There are several different methods, but sewage generally goes through four steps before the water is safe enough to drink.
Sewage isn’t only the wastewater from our toilets. It can be other wastewater from our houses, containing soaps and household products. It can also be industrial wastewater, containing chemicals and other byproducts of industrial manufacturing processes. Whatever the source of the wastewater, the goal of the sewage treatment plant is the same. The goal is to remove the water and make it safe for reuse, and to remove the contaminants and dispose or degrade them safely.
We are very lucky to have a reliable sewage system. Different countries have different systems, but America has approximately 16,000 sewage treatment facilities for a population of about 350 million, which means one plant for every 25,000 people. Even today, some countries don’t have sewage treatment systems that they can rely on and the 6th goal of the UN’s sustainable development goals (SDGs) is to ensure clean water and sanitation for all. 2 billion people in the world don’t have access to safe drinking water. In the countries that do have a good sewage system, they haven’t existed for that long. Throughout most of history, people have used rivers for sewage disposal. As long as populations stay small, this can work. However, once people started to live in cities, that system breaks down. The idea of sewers seems to have been invented by the Romans, but still most people just disposed of sewage in the streets and the sewers only carried the wastewater to the rivers. It took the Industrial Revolution and the cholera outbreaks that kept occurring for an English doctor, John Snow, to work out the connection between sanitation and health. The Industrial Revolution had brought large numbers of people to the cities and there was still no effective sewage system. Sewage was still pumped into the River Thames. However, Hamburg was the first city to have a real sewage system in 1842, and it wasn’t until 1914 that Edward Arden and William T. Lockett worked out how to treat the sewage with a system that is still in use today.
So, how is sewage treated? The first step is to remove the large bits. The water is carried from toilets, baths, sinks, showers, drains, and industrial facilities into the sewers. As it enters the sewage treatment plant, it is filtered through a screen made of bars. This removes things that have been washed down drains and things that might break the equipment, such as cans, sticks, pieces of plastic, and so on. The incoming water is also filtered for grit. Small stones can be carried in the water and may block the treatment plant pipes, so the rate of water is slowed down so the grit sinks to the bottom.
The second stage is to remove the suspended solids and organic matter. The water is poured into tanks where it is allowed to settle. Once the water has slowed down, the large solids and organic matter sink to the bottom of the tank, where they can be removed. At the same time, oil, grease, and lighter solids float to the top of the tank where they can be skimmed off.
The third stage removes as much of the organic matter as possible. This stage will usually get out about 85% of the organic matter and is done with bacteria. There are two main ways of doing this. The first way is to pump the wastewater in through a filter bed, which is made up of chips of limestone and coal. These chips have a large flat surface area where bacteria can grow. As the wastewater filters through the chips, the bacteria attach to the organic matter and decompose it. The second method uses a large rotating shaft with lots of parallel discs mounted on it. These discs also have a large surface area for the bacteria to grow and the bacteria break down the organic matter. The second system is more commonly used these days because it mixes the wastewater and allows more bacteria to come into contact with the organic matter. In both systems the water has to be aerated to help the bacteria work.
Once the process is finished, the water is pumped into another tank where the bacteria sink to the bottom, forming a sludge. This sludge is pumped back into the last tank to be used again. Then the water is disinfected, usually with chlorine. Finally, once the water is cleaned and disinfected, it is fed back into rivers and the water system. A sewage treatment plant can cope with about 13 million liters of sewage every day. And this is what I learned today.
Sources
https://www3.epa.gov/npdes/pubs/bastre.pdf
https://en.wikipedia.org/wiki/Sewage_treatment
https://www.thameswater.co.uk/about-us/responsibility/education/the-sewage-treatment-process
https://www.wearewater.org/en/sewage-the-trace-of-our-history_281141
https://www.un.org/sustainabledevelopment/water-and-sanitation/