In the last wlog we discussed what wastewater is and why it is important to be aware of what happens to it. In this wlog, we will see how exactly the sewage is converted to clean drinking water. I hope that you will admire the brilliance of engineering and science, just as much as I do, by the end of this wlog. Once we flush our toilets, everything goes down the mysterious hole and never is talked about. However, what magic happens, that converts the flushed sewage to clean and safe drinking water?
After being flushed from the toilet, all the sewage goes to a wastewater treatment plant. Once the sewage influents enter the plant, it undergoes “pretreatment”, which removes bigger solids like baby wipes, cloths, diapers, coins, toys and pieces of wood and rock that may have entered the sewerage. It is important to remove all these, otherwise, they might clog and damage the parts of the sewage treatment infrastructure. After the pretreatment, the sewage mixture is homogenous and proceeds to primary treatment.
Primary treatment is basically churning the sewage to separate the inorganic contaminants from the water. The sewage is stored in huge tanks and is churned with the help of huge rods at a “settling velocity”. These inorganic contaminants mostly are made of chemicals and detergents used for cleaning purposes; They include artificial fragrances, preservatives, optical brighteners, etc. Mostly, the recovered sludge from the primary treatment is incinerated. To combat the pollution that is produced through this, we can make a shift to organic and eco friendly cleaning agents. There are many good brands available in the market, however, it is important to beware of “greenwashed” products. These products are inorganic, or chemical based, but claim to include natural substances. It is necessary to make sure that the products we use are free of borax, caustics, metasilicates and phosphates. Natural substances like soap nuts combined with lemon or vinegar could be used as a substitute to laundry cleaning agents. Yet, please make sure what suits your clothes, before making the switch.
After primary treatment the sewage effluents move to secondary treatment. In this part of the process the sewage is treated with bacteria that digest the organic waste suspended in the water. Then the sewage effluents are sent to a clarifier where the biosolids are churned out. The separated biosolids can be used as compost after they clear testing. After this, the sewage undergoes tertiary treatment, in which the nutrients like nitrogen and phosphorous are removed. Depending on the concentration of the contaminants in the influent sewage, the process till now can be repeated or the water is sent to the next stop. At this point, the water is safe to be released in the environment. However, it must complete one last step before it can come through our taps as drinking water.
The last step is disinfection. There still might be some pathogens and toxic chemicals that may have escaped despite of all the previous treatment. Hence, to make the water completely safe, disinfection is carried out in three different ways. First is chlorination or chemical disinfection. It kills all the microbes and combines with harmful chemicals and can be filtered out. Once it is removed from the water, the water becomes safe to drink. The other way is Ultraviolet radiation treatment. The UV rays destroy the genetic material of the pathogens so that they don’t multiply. The last method is ozone treatment. An electric current is sent through the water; This converts the dioxygen molecules to ozone. Ozone destroys the cell wall of bacteria and causes rapid damage of the cell. After this the water, is safe to drink and can be released in rivers, lakes and streams for agricultural and other purposes.
Despite all the infrastructure being in place, many wastewater plants don’t function. Bill Gates has funded a lot of research to create toilets that can treat sewage and convert it into energy and clean drinking water. This might be the future of our zero waste lives.
Till then, it is important that we, as responsible citizens, make sure our local authorities are running the wastewater treatment plants and taking care of our environment.