WTW- Dosing Two Alkaline Chemicals at Same Point in the Process

[tipp79]

Currently lime water is dosed for final water pH correction at a client's WTW.

This lime water system is unable to achieve the additional pH correction required to counteract a proposed fluoride dose and so sodium hydroxide dosing is required (and is struggling with achieving required pH at the moment at full works' flow). My preference would be to replace the lime water system for a new sodium hydroxide dosing system to achieve the full final water pH correction. However, the client needs to justify doing this over maintaining the lime water and then a smaller sodium hydroxide dosing system. While the client accepts dosing two different chemicals for the same purpose is unconventional, they have requested more justification. There is potential control issues and so the caustic would need to be dosed a certain distance downstream of the lime water.

Does anyone have experience of a WTW where two such chemicals are dosed for the same purpose and are there pitfalls I should be aware of? Thanks

 

[ashtree]

This is not an uncommon practice, with many plants dosing both lime and soda ash, or sodium hydroxide and lime.

But there are some bigger issues to consider here.
What fluoride chemical are you using that impact on the pH. I have never heard of one doing so. Even if you are using hexafurosilicic acid the dose is usually small enough that it has minimal impact on pH.

Consider why you are using lime for pH correction. Lime is often used because its cheap and readily available. But it might also be to add calcium which will reduce pipe corrosion. Calcium is sometimes required to be added for health purposes in very pure water. Lime is good at adjusting the pH but is less suitable than some other chemicals for adding alakalinity. The same can be said for sodium hydroxide. But if using liquid sodium hydroxide, this is far easier to handle and deal with than lime water, particularly if you are mixing it yourself.

Both of these chemicals should be dosed before the fluoride with fluoride being the last chemical dosed.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[peterj1]

This doesn't sound right, especially the "unable to achieve the additional pH correction". Lime should easily be able to achieve a pH of 12, maybe greater, far above that permissible for potable water. The only possible limitation is if the lime feed system is maxed out. Also, NaOH and lime react the same in water to form hydroxide ions, so in this application there is no difference between the 2 chemicals and no reason to add both. Should be just a matter of fine tuning the lime feed system. Also, unless the water is really poorly buffered, adding fluoride should have very minimal impact on pH.