Hi kedardahivale !!
1.You need to consider the flow rate for proper Dosing.Depending on the flow rate you are suppose to control the speed of the dosing pump.Tune a PID in PLC which works in a way like "If the flow is more,proportionally the dosing should be more, hence increase the speed of the dosing pump".And then in this case provide a Correction signal to your PID from the turbidity Analyzer in the form of a Closed Loop.
If you don't have a Variable Frequency Drive to control the speed of the pumps you can:-
2.Using Turbidity as an analog input to the PLC
You can set Limits of Turbidity.
Example: When the Turbidity is MORE than X Value, Trigger START command to the pump.
And when turbidity is LESS than X Value, STOP the pumps.
Else if you still can't arrange a Variable Frequency Drive to control the speed, let me know, I have a solution for it.
Regards.