Mains water in Phuket is not intended to be drunk at the tap. Depending on the season it can carry chlorine and its by-products, sediment and heavy metals, and there is a bacteriological risk wherever the network is old or intermittent.
As on every island and coastal network, two more constraints apply: salinity, and pressure loss when demand peaks. Both matter for a system connected to the mains. Pre-filtration protects the stages downstream from sand, rust and dirt, and the reverse osmosis membrane, at 0.0001 micron, removes heavy metals and dissolved contaminants.
Where the line cannot be relied on at the times the site needs water most, an Air system removes the dependency altogether.
Is tap water safe to drink? Read the guide