There’s no way to stop water from multiple sources getting into the water supply. Potable water could be contaminated by water utilized in chemical plants, cooling systems, stagnant fire suppression systems, and main line ruptures out of the sewer.
To put backflow protection systems in key places, utilities, and public will be supported by companies for back flow prevention. It is likely that there are hundreds of them in your town, and that’s just the beginning. Though many of them have low risk, it’s wiser to be cautious than not to be.
Back pressure and back syphoning are two types of back flow to be avoided. Back pressure is a sign that the water supply to the facilities is not sufficiently pressurized. Water flows backwards to the drinking supply in order to keep it from becoming so.
Back syphoning happens when necessary measures aren’t followed in order to fill up the contaminated areas as pressure released elsewhere causes the water to be syphoned into drinking water sources. The following video gives more information.