The presence of calcium in ...

The presence of calcium in water means what you have passing through the pipe is hard water. So, you have to softening the water (soft water) before entering into the gravity fed pipes. Since your source are rivers and springs and your end point is homes for domestic water use, I recommend a system that goes this way: Source -> water reservoir -> water treatment plant (to improve water quality and also soften the water) -> water reservoir for domestic use. This system is capital intensive but the benefit is immense.