The process of using chlorine to disinfect and treat drinking water is different from the ways that water in pools.
Clean, safe water is vital to the overall health of every individual. Before water can be fit for human consumption or use, it must undergo a series of physical and chemical processes. The practices of water filtration and treatment vary only slightly depending on whether the end use of the water needs to comply with Federal regulatory requirements of the Safe Drinking Water Act or the Clean Water Act.
The general process for water treatment takes place as follows:
Raw water from surface water (i.e. local rivers or reservoirs) pass through a large screen. This removes the largest debris such as leaves, rocks, trash and sediment from the water. The water then flows toward a treatment tank where it undergoes rapid mixing. This phase is part of what is called "pre-treatment." During pretreatment strong disinfectant chemicals are added to the water. These chemicals are chlorine, ammonia, lime, and aluminum sulfate.
When the chlorine reacts with ammonia, it forms chloramines. These powerful new compounds are able to kill harmful bacteria and viruses. Because chloramines are highly stable and persistent, they are able to continue protecting the water long after it has left the treatment facility. This ensures that the water flowing out of our tap at home is safe to drink and free from bacteria.
The purpose of adding lime is to adjust the pH of the water. pH is a measure of how acidic something is. Water should be of neutral pH. Addition of aluminum sulfate into the disinfected water promotes even distribution of the chemicals that have been added. It also acts as a coagulant, meaning that it causes any other particles that remain in the water to stick together forming a "floc." Because flocs are made up of several particles, they become heavier than the water molecules and sink to the bottom of the mixing tank. Once the flocs have collected, they are removed. Flocs are harmless bits of sediment and can be recycled back into the environment sort of like applying mulch to a garden.
The water then undergoes another round of filtration and another series of treatment with chloramines and lime. Some cities are required to add fluoride to their drinking water supply, and that takes place during this phase of treatment. Another chemical, called orthophosphate is added to protect the pipes that the water will travel through. It works by preventing corrosion which could adversely affect the pH of the water or cause small amounts of metals to be present.
Clean, safe drinking water is then pumped from the treatment facility to consumers.
----
As far as swimming lessons, I recommend finding someone in your neighborhood who gives lessons at their homes. There's less risk of "cooties" and urination and poop in someone's private pool, especially if they only teach 4 or 5 kids at a time. Private lessons would be even better, again, at someone's home.
The lake is more dangerous for learning to swim (for reasons other than bacterial).