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Automation and Process Control

Manual

Manual operation is the traditional method for wastewater process control. Manual adjustments of control elements are made to maintain parameters within desirable ranges, such as dissolved oxygen. Early implementation of manual control utilized physical samples and standard laboratory measurements for biochemical oxygen demand (BOD) and total suspended solids (TSS) as the most common sources of data to calculate aggregate parameters including food/microorganism (F/M) ratio and sludge volume index (SVI). Furthermore, wastewater treatment facilities were designed based on hefty safety factors applied to empirical standards for maximum loading conditions. The design priority was meeting discharge permit requirements under any circumstance, not providing flexibility to optimize operation for typical daily loads.

Open Loop control

The simplest form of automatic process control is open-loop control using a timer. Typical implementations of this strategy include the intermittent operation of blowers, mixers, and pumps. An attentive operator can observe the effect of manual adjustments on process performance with the aid of manual samplings and measurements and optimize the operating scheme for daily, weekly, and seasonal variations based on a formal or informal set of constraints.

Feedback control

The next level of control is automatic control implemented in SCADA using online measurements. Changing wastewater characteristics cause a disturbance to the process. The disturbance causes a response by the process, a higher or lower dissolved oxygen (DO) concentration, for example, that can be measured. The measurement is reported to a controller, which reacts to eliminate the difference between the measurement and the desired setpoint by sending a control signal to an actuator.

The most common and inexpensive form of feedback control is on/off control. The process is controlled using simple relays that start and stop equipment in response to a disturbance. Another benefit is that on/off control can be retrofitted into existing facilities with minimal modifications. The typical application is in batch or semi-batch processes such as blower control for cyclic activated sludge-based on online measurements of DO. The main drawbacks are process instability and actuator wear as a result of oscillation above and below the setpoint.

Feedforward control

Feedforward control consists of measuring the disturbance and calculating the required response. A common implementation of this strategy is an extension of cascade control for feedback control based on DO. An ammonium controller sits on top of the DO controller and receives input from online instruments measuring flow and ammonium, calculates the load, and determines the DO setpoint. The benefit is that suppression of DO and bleed-through of ammonia into the effluent can be eliminated because the controller anticipates the aeration demand instead of reacting. The drawback is that it requires additional sensors to measure ammonium and a model to calculate the response based on the disturbance. A feedback loop may also be needed to adjust the model-predicted response.

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