Antiscalant dosage cannot be determined by TDS (Total Dissolved Solids). The reason is that TDS can be made up entirely of sodium and chloride which do not have a scaling potential, or it could be made up entirely of calcium and sulfate which have a very high scaling potential.
In order to determine the correct antiscalant and dosage, the ion species in the water must be identified, and their concentrations have to be calculated based on the concentration factor. The concentration factor is variable with the type of membrane (Reverse Osmosis vs. Nanofiltration) and the % salt rejection (NF membranes can vary from a 50% to 90% divalent salt rejection.)
We can then calculate the potential of the ions in the water to form a scale, and the antiscalant will be selected based on the type of scale most likely to form. For example, if there is a potential for both calcium carbonate and calcium sulfate to form, an antiscalant that can control both these scales would be selected. On the other hand, if there is a potential for calcium phosphate to form, a more exotic antiscalant would be required.
The dosage is then calculated based on the driving force for scale formation, which is dependent on the concentration of each ionic species, pH, temperature and ionic strength.
However, there are also other parameters that have to be taken into account when calculating dosage. For example, antiscalants have a greater affinity for certain surfaces, like ferric hydroxide salts, so when iron is present in the ferric state, a higher dosage is usually required.
So in order to obtain a recommendation for antiscalant selection and dosage, it is very important to perform a complete and accurate water analysis, and to measure pH and temperature immediately upon collection of the water sample.
The parameters that are required for an accurate recommendation are:
Cations: Ca, Mg, Na, Ba, Sr, Fe (specify ferrous or ferric), Mn, Al
Anions: Alkalinity, SO4, Cl, PO4, SiO2
pH, Temperature (both measured immediately upon sample collection), TDS (estimated from conductivity) and %Recovery.