As others have noted, the ...

As others have noted, the permeate from a membrane desalination plant will tend to be low in calcium and magnesium and their associated alkalinity.  It is important to stabilise the water so that it is not corrosive to all the downstream pipes, valves etc.  This is accomplished by adding a choice of lime, soda ash or caustic to the final water to achieve a saturated pH (pHs). To determine how much of the chosen chemical to add is quite complicated, but you may use a freely available computer programme called STASOFT downloadable from the South African Water Research Commission's website.