Soda ash is more expensive than lime and more soluble than lime. It probably has less impurities than lime and would not require slaking, or in the case of hydrated lime, be as difficult to disperse. Both lime and soda ash may be used sequentially to soften water depending on the composition of the raw water. If the raw water had primarily calcium hardness, the soda ash may have been used to precipitate the calcium as well as adjust the pH. I would do a search for "lime softening" for more information.

grrun        

Personally, I don't have much faith in calculating dosage rates because changes in pH around the neutal point are so sensitive with strong acids and bases. Any chemistry text will probably have a graph of the change in pH with additions of strong acids or bases. I would either monitor the pH manually or use a pH control system because of the variability in commercial sodium hydroxide would cause dosage calculations to be unreliable. Also, the dissolved minerals in water provide another variable to deal with. Anyway, I'm not much help for the theoretical calculations.

grrun        

Sewell:

You might also consider a magnesium hydroxide slurry rather than either soda ash or lime, for both reasons of cost and pH controllability.

Regards,

Orenda

Sals,

My personal experience with lime slakers has been such that I would avoid them unless I had redundant equipment and the manpower to maintain them. Metering liquid alkalis is far more reliable than slaking and metering lime slurries. Obviously, the chemical costs for lime are lower and because the costs of maintenance and reliability (which was a primary concern in my operation) are not as easily determined, they are usually overlooked by designers. Treatment plant operators are often the victims of designers whose main focus is minimizing costs that can be easily documented.

Sincerely, grrun          

Hi

one problem with buying slurries of lime is the relatively low concentrations, typically 18% solids. This means large numbers of tanker movements, and the subsequent CO2 emissions.

My company provide a 60% Magnesium hydroxide based product, which reduces tanker movements, and also gives excellent pH control. Unlike lime, the pH buffers out at about pH 10 greatly reducing the risk of overdosing, on biological plants, causing bacterial die off.

regards

Alistair