What enables the continuous adjustment of chemical feed in a treatment plant?

Feed-forward control is the correct answer because it refers to a proactive approach in which adjustments are made to the chemical feed based on anticipated changes in water quality or flow before these changes actually occur. This allows for immediate responses to fluctuations in the incoming water characteristics, ensuring that the treatment processes maintain optimal performance and stability.

In a treatment plant setting, feed-forward control monitors influent parameters and uses this information to predict and adjust chemical dosing accordingly. For instance, if an increase in turbidity is anticipated due to a predicted storm event, the system can increase coagulant dosage in advance, thereby enhancing the treatment process and improving overall water quality.

Other options represent different control strategies but do not enable the same level of anticipation and responsiveness as feed-forward control. Feed-back control, for example, reacts only after a change has occurred, which may not be sufficient in situations where quick adjustments are necessary. Proportional control adjusts feed based on the current state of the system but also relies on feedback mechanisms, while manual control does not provide the continuous or automatic adjustments needed for optimal operation.