Typically people do use two sensors, one up stream and the other well down stream.
The problem is that the control tubing has to be very gentle, the response time will be long.
Injection is done with quills that place the chemical near the center of the pipe. If you have both acid and caustic injection those will be separate quills because different metallurgy is required. Don't go cheap on the quills, if they fail the acid will be injected along the wall of the pipe and eat right through it.

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P.E. Metallurgy, Plymouth Tube

The pH change reaction proceeds very quickly , but you will need to ensure that you have thorough mixing so that what your downstream probe is seeing is a completely mixed and reacted sample.

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

In-line pH control of stream is almost impossible if there are rapid variations in the inlet pH. A tank with a few minutes of residence time makes control much easier and more accurate.

I agree with the above. Controlling pH inside a 16" pipe will be extremely difficult, if not impossible, especially in case of variations of flow and/or variations of inlet pH, as mentioned by Compositepro. In addition to that, these pH electrodes foul very quickly and become completely useless. With wastewater stream, for this scenario to happen it could be just a matter of days.

Some Alkylation units in refineries employ special neutralization basins with mixing, with basin walls made of special concrete and high-metallurgy mixer devices. This helps in avoiding localized pH excursions, and in avoiding differences between pH in the measurement point and pH of the bulk fluid which essentially ruins all control efforts. Perhaps you should consider something similar and also make sure there is a proper sample off-take system for reliable pH measurement.

Dejan IVANOVIC
Process Engineer, MSChE

Hi,
The process design and process control is decided based on the range of ph change in the wastewater and the required acceptable range of ph after adjustment.
But generally speaking the in-line ph adjustment is not a good idea unless the ph swinging of the wasterwater is low and at the same time the acceptable range of target ph is wide (e.g. target ph = 6- 9).
If for whatever reason you don't want to put a static mixer, the pipe length before DGF(Dissolved gas Flotation vessel) should be minimum 150D, I mean 150*16" or 60 otherwise the coagulant (and flocculant?) may not work properly. Ask the coagulant vendor for the range of coagulator application.
Hope this help

THANK YOU ALL FOR YOUR HELPFUL ADVISE.

ROKER

As the other have mentioned, if the pH is widely variable, it will be difficult to achieve precise control in a pipe.

Because of the logarithmic nature of the pH scale, pH control is usually done in two stages. The first stage adjustment will bring the pH within the desired range. The second stage adjustment fine tunes the pH to a precise setpoint.

Quote (EdStainless)

Typically people do use two sensors, one up stream and the other well down stream.
The problem is that the control tubing has to be very gentle, the response time will be long.
Injection is done with quills that place the chemical near the center of the pipe. If you have both acid and caustic injection those will be separate quills because different metallurgy is required. Don't go cheap on the quills, if they fail the acid will be injected along the wall of the pipe and eat right through it.

Just experienced this first-hand a month ago. Turns out the quill was too short and only protruded into the water steam about 1/2". Mixing water and sulfuric acid that close to a steel pipe wall doesn't bode well for the steel.

Photo is just water spraying out but not really an easy fix since this header can't come offline without taking down a whole lot of other equipment...

THANKS AGAIN, IT SEEMS IT IS A COMPLEX BUT INTERESING ISSUE.

REGARDS,
ROKER