pH control with soda ash


Thread Starter


The waste generated from our plant is acidic in nature. We neutralize the acidic waste using Soda ash. The environmental discharge pH limits of effluent leaving the plant has to be between 6 and 9, and we control at around 8.

The current theory is that if we control the effluent pH at 7 (instead of 8), we would be in theory cutting down on the amount of soda ash added and generating $$ savings.

We do record the total volume of the effluent leaving the plant, the pH values of the waste material going into the neutralizers and the pH value of the soda ash solution. The question is how do we calculate the savings value? Or to rephrase it could somebody shed light on how to calculate the soda ash saved using the incoming and outgoing pH?

Consider the alkalinity that you need to add to neutralize the acidity. Check in any basic water treatment text.

Bob Peterson

Since you can dump it at 6, why run it all the way to 8. That would seem to be a lot of extra chemical, but its sometimes difficult to get good control of pH waste, at least online, so people settle for adequate.

You might find that the cost to make this work reliably is enough that its not worth the effort.

A totration curve and a high school chemistry text book will tell you how much extra chemical you are using above the bare minimum needed. You might well find out it is not all that much.

Bob Peterson

George \(Jim\) Hebbard

ACS does not indicate >how< acidic his plant waste is before
neutralization. I'm familiar with bringing 2 -3 pH waste up to
discharge level. Since the pH is a logarithmic scale, the amount required
to neutralize a waste from 6 to 7 pH is 1/10 that from 5 -> 6, and 1/100
that from 4 -> 5. The relatively tiny amount required from 7 ("neutral")
to 8 compared to from 3 -> 4 is usually not an issue.

Keep in mind that some solids are amphoteric, and it is not unusual for
some liquids to go back down in pH if the solids are not immediately
removed or underlie the product water in a clarifier or lagoon. If 6 - 7
is the safe zone, I would use 7+ as the target with an upper limit of 8. I
wouldn't shoot for 8 but in most cases it's hard to measure the
difference. Soda ash also has a strong buffering effect in the acid range,
and sometimes, depending on the chemistry, above 7. This may make control
appear very non-linear (or non-logarithmic ^;)

George (Jim) Hebbard PE