The system (left) is typical of a acid and caustic pump being controlled by a pH controller.  Systems often use proportional control from the pH controller to adjust the feed rate of the pumps as they near the desired pH setting.  Mixers are normally used in the batch tank.  For continuous systems, a 10 to 15 minute retention time is normally required. 

Metering pumps are often used in a self-priming or suction lift condition.  The pumps are also able to pump into a pressurized line however, a bypass valve will enhance the ability of the system to reprime.  The bottom of the suction line will normally incorporate a foot valve (combination of strainer and check valve).