pH correction of effluent project design

[Arklow1]

Hi first post!
I work for a large confectionery company. We are installing a pH correction facility to treat c 300m3 of influent discharge/24hrs of mainly acidic influent.
We are debating whether to do batch or inline pH correction or a mixture of both for our pH correction. I would appreciate your thoughts and experiences on after the influent passes through a 400m3 balancing tank is it preferable to do (A) rough inline pH correction then fine tune batch pH correction in a second 400 m3 tank or (B) in line pH correction and out to discharge and if it does nor reach the req parameters divert to a 2nd 400m3tank and repeat?
Also any information sources on inline pH correction would be really appreciated to improve my understanding of this area.

 

[bimr]

It depends somewhat on the variability of the pH and flows.

If your pH is relatively constant you can get by with an inline system. If the pH and flow rate swings wildly, then you probably should consider a two step pH correction. The first step is roughing.

One would think that the cleaning chemicals would be in the alkaline range so that if you have a large batch tank, the alkaline and acid wastes will balance out.

Here is some online resources:

http://www.phadjustment.com/index.html

 

[Arklow1]

This ph correction project is something I have recently become involved in.
The design has firstly a large balancing tank, 400m3, and then rough ph correction in line, then to a further balancing tank of 4003m for fine ph balancing in the tank before sending off site.
I would have though the first balancing tank, 400m3, will equalise out the ph to a great extent then the in line ph correction would be best before sending the effluent off site. If it was still out of spec after the ph inline correction it could be diverted to the second tank then re fed into the line for ph correction again. I could argue against the need for the second tank at all, but the whole team is in favour and money is approved for it so why not get it but use it only as back up?
Our ph and flow rates do vary over 24hrs but after entering the balancing tank , the ph and rate is smoothed out so inline ph correction is best?

 

[bimr]

You don't mention the pH. Multiple pH correction step are sometimes required when the pH is 3-4 units away from neutral.

If you are actually smoothing out the pH with a 400 cubic meter tank, why would there be another. You should have data from the process that proves your point.

If you have not installed both of the tanks, you can experiment by installing the first roughing stage. Then you should be able to see if it works.

One would assume that you have a mixer in the tank as well.

 

[Arklow1]

These are very insightful points you make.
We took effluent samples a year ago over a two week period , we are redoing the samples this week to see what our current effluent composition is.
From this new sampling we will see what our normal ph actually is. Can one see via titration what is required to get our ph to say a target of ph 7.5 +/-1. Does titration allow you to calculate say the amount of caustic/acid of a certain percentage that will be req per cubic meter, in our case mainly caustic?
I cannot see our ph been 3-4 units away from neutral after a 400m3 tank!
What typically would a ph roughing stage achieve? Why is it called roughing, it may be the final stage of ph correction? This roughing term implies to me it is not accurate.
Yes, mixer in the design , actually the mixer was located internally and accessed via chains from the top of the 400m3 tank, this is now located externally at ground level and agitated via a pumping action.
After the roughing stage how do you know if adequate mixing/normalisation has taken place? Is this based on the inline ph sensor and control used e.g. PID? Do you need at this point a shut off valve if the Ph is not correct to avoid discharging out of spec effluent or do you recirculate it back by a 3 way valve to the balancing tank?
Really appreciate your helpful comments!

 

[bimr]

The pH scale is logarithmic and as a result, each whole pH value below 7 is ten times more acidic than the next higher value. For example, pH 4 is ten times more acidic than pH 5 and 100 times (10 times 10) more acidic than pH 6.

Because of this characteristic of the pH scale, the roughing step should be used if you are trying to make a large pH correction. For example, if you start with a pH of 3, the roughing step would target a pH of 5-7. The second step would target a precise pH setpoint.

You want adequate mixing in the tank so that it is considered a complete mix. The definition of a complete mix tank implies that the effluent concentration is the same as the concentration in the tank. Power input from mechanical mixers should be approximately 0.5 horsepower per 1,000 cubic feet of tank.

You should not have to recirculate unless you have some type of equipment failure.

Most effluent regulations only require a pH range of 6-9 units.