Theoretical oxygen demand

Theoretical oxygen demand is like when you have some yummy snacks and you want to eat them all, but you know it might take a long time to finish them. For example, imagine you have a plate of cookies, a glass of milk, and a bag of chips. You know that if you eat all of them, you will feel full and maybe a little bit sleepy, but you also know that it will take your body some time to digest all that food.

Similarly, when we have polluted water, we want to clean it up so that it's safe for people and animals to use. But just like you need time to digest your snacks, the water needs time to clean itself up too. This process is called biological oxygen demand (BOD).

BOD measures how much oxygen is needed by microorganisms in the water to break down the pollutants. Just like you need energy to digest your snacks, these microorganisms need oxygen to break down pollutants. The theoretical oxygen demand (TOD) is the amount of oxygen that would be needed in a perfect world, where all the pollutants are broken down completely.

So, if we go back to our snack example, the TOD would be the total amount of energy needed to digest all the snacks completely. In the same way, TOD tells us how much oxygen is needed to clean up polluted water completely. However, in reality, pollutants are not always fully cleaned up, just like you might not finish all your snacks. But by measuring TOD and BOD, we can understand how much work needs to be done to make the water clean and safe again.