Water Pollution and Its Prevention. Chapter 17. Water Pollution defined as the presence of a substance in the environment that, because if its chemical composition or quantity, prevent the functioning of natural processes and produces undesirable environmental and health effects.”
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Remove grit and debris – water passes through a series of screens. The water is slowed to allow the grit go settle out. This is taken to a land fill. (you’d be amazed at what goes down a toilet)
it uses organisms and their metabolism to feed on the colloidal and dissolved organic material and break it down to CO2 and water. The water flows over a series of rocks on which live organisms, detritus feeders, live. Oxygen is provided for the system.
The activated sludge is also processed by detritus feeding organisms. The water is aerated (this is the activation part) as it moves through the system. As organisms feed on each other they form clumps called floc, This water goes into another settling tank where the organisms settle out. It is mostly water . 90% of the organic material is removed. The organisms that settle out are put back into the clarifier tank. Surplus amounts of the activated sludge are taken out from time to time and put in with the raw sludge. So the end product is carbon dioxide, water and mineral nutrients that remain in the water solution
After the secondary treatment the resultant mixture is very nutrient rich. Heretofore, this was not considered a problem. Now we know that extra nutrients leads to cultural eutrophication. Today, secondary activated sludge systems have been added to remove nutrients ad oxidize detritus. This is called biological nutrient removal.