Dear Nasir,

I do not know why the pH goes up further till app. 10. The reaction of metabisulfite and Chlorine at this elevated pH is little documented, so the "cause" for pH going up further remains unknown to me. Or, maybe something else is going on?
I have however one remark: at pH of 9, the hypochlorite you dose has lost most of its disinfection power. The pH should be around 7.5 or less.
I can imagine that your RO installation is suffering from scaling at these elevated pH values.

In order to restore the pH of the RO-effluent at the desired value, you should do a lab test: make some titration curves with the acid you intend to use. Then make a mass balance (based on equivalents to dose) in order to choose the right pump rate range. Full scale: install a good pH sensor which automatically controls (PI-control) pump dosing rate of acid. Sensor is downstream dosing point and not too far away from it (minimise dead-time), but ensure excellent mixing before reaching the pH-sensor.
controlling pH at 7.00 is not that simple, so good parameterisation of controller is required.

best regards,

DS