How is water polluted?
How is water polluted in the cities?
The city is a place where large number of people live in the small area. Each of them uses water and everyone pollutes it by doing laundry, draining the pasta, taking a shower, washing the car etc. - that's how sewage is created. A lot of people usually produces a lot of wastewater.
The wastewater produced in the households are treated as pollutants because they contain leftovers, faeces, detergents (detergents, soaps, shampoos), bacteria and parasites. Fortunately, this type of wastewater is easily to be dealt with in the wastewater treatment plants.
Waste water is formed not only in our households and flats, but also in schools, offices, factories, hospitals and restaurants. But the worst wastewater is produced by the factories - this kind of pollutants are called industrial effluents. They can be very toxic and are very difficult to purify.
Wastewater in the city
There is also a type of wastewater that is typical for the city - it is so called precipitation wastewater. This is rain water, most of which does not seep into the ground but flows on sidewalks and streets and rinses streets off dirt, dust, oils and grease from cars, the salt used during winter to remove snow etc.
There was a practice some 200 years ago in the cities to pour the dirty water right into the street. The impurities flowed down the street by the street gutter - a special water canal for the impurities. Imagine the smell of the city in the summer.
POWER OF PLANTS!
The power plants in the city produce hot water which is the result of cooling of the machinery that is located in the plant. Hot water is the pollutant itself. it must be cooled off before it is poured into the river because, as you remember, gases like oxygen are poorly soluble in warm water. When there is no oxygen in the river the fish suffocate.
Ask your mother to boil two eggs. When they are boiled, put one of them immediately into a bowl of cold water, leave the second one on the saucer. After a few minutes, touch both eggs with caution. Which of them has cooled off? And what temperature is water in the bowl? Is it still as cold as in the beginning?