It is simple. Nothing is ...
Published by Joseph Cotruvo, President at Joseph Cotruvo & Associates, Water , Environment and Public Health
It is simple. Nothing is perfect but the closest you can get to that is ozone, or UV but they several have other problems i.e., substantial pretreatment requirements, no disinfectant residual, and they are expensive and difficult to operate, so they are not accessible for many water systems.. Realistically, more than 100 years of experience demonstrate that, chlorine is probably the best all around disinfectant, except for cryptosporidium. However, that is managed by filtration. And chlorine is acccessible, cost effective, and easy to use for just about every water system.
Where are you getting these questions? Are you writing a book or teaching a class?