Dear all,

It will be greatly appreciated if any of you could please provide some help  with my query. I am looking into changing from soda ash to caustic for final water pH correction but am having difficulty calculating the amount of caustic required from the jar tests I have done.  

I made up a stock of 1000mg/L of soda ash solution and found that I needed to add 2.5ml per 1L of sample water for the necessary pH correction. In contrast, I found I only needed to add 0.5ml per 1L of sample water for 1000mg/L caustic soda solution.

I am unsure as to how then calculate the amount of chemical required at full scale where the average flow is 173.6 L/s. Based on the jar tests, how much caustic at 10% strength is estimated to be required? Similarly, how much soda ash at 35% strength would be needed?

Currently, the plant uses 31.8T of soda ash (35%) per year and I would like have an idea of the amount of caustic that would be needed.

Many thanks,

Alex

You have already established that the mass of NaOH required is 20% of the mass of soda ash. Other factors may influence your decision:

1. Are you buying soda ash as a 35% solution?

2. NaOH is typically sold as a 50% solution  (25% solution having the  
   lowest freezing point).

3. Consider these points and the buffering provided by the soda ash    
(although the NaOH may control the pH satisfactorily with a feedback control system).

grrun

You should purchase and install a pH probe at your outfall and tie it into a loop with your chemical feed pump and flow meter so that you will be assured of supplying sufficient voume of NaOH to meet whatever pH level you choose.

~Eric