Automation of Water Resource Recovery Facilities. Water Environment Federation

level should be made infrequently, with a single change made each day or so because the blanket level typically does not change quickly.

7.4.2 Sludge Removal Control—Valve to Common Pump

      If more than one primary clarifier shares a dedicated pump, the control gets more complicated. In this instance, each primary sludge tank valve is controlled with an “open time”. The sludge pump (along with any grinders associated with the pump) will typically run continuously or as long as any associated tank’s sludge valve is open. The valves for each clarifier will typically cycle in order, with the clarifier valve that has been closed the longest opening next. In general, the open-time frequency for each valve should be short enough so that the clarifiers waiting to remove sludge do not cause plugging at the pipe inlet and to prevent rat holing. For example, four clarifiers sharing the same pump might each have their valve open time at 10 minutes, resulting in a closed time of 30 minutes for each clarifier valve. If sludge blanket levels are known, the relative open time for each clarifier can be trimmed slightly so that the blankets remain close across all clarifiers. As stated previously, adjustments to time because of blanket level should typically be made infrequently, with a single change made each day or so because the blanket level typically does not change quickly.

7.4.3 Scum Trough Operation Control

      Rotating scum troughs are turned to allow the scum to enter the trough. The main control is frequency and timing of the rotation. The trough needs to rotate at the time the scum rake or flight is approaching the trough to maximize the amount of scum collected. In addition, it is important to have enough water along with scum to help the scum travel along the trough and to the pit so that the scum does not accumulate in the trough. At the same time, too much water could overload the scum pit and downstream scum removal and/or treatment processes.

      Typically, a discrete position indicator for the skim arm (i.e., circular clarifier mechanisms) or flight (i.e., rectangular clarifier mechanisms) is provided to indicate when the position of the flight is optimal to lower the trough. The controller will count these events and lower the trough when the count reaches the setpoint. The count setpoint can be lower (perhaps 1) when there is a large scum accumulation and set higher when there is less accumulation. There is also a “down-time” setpoint. When the trough is at the low position for a time equal to the down-time setpoint, it is raised again.

7.4.4 Scum Pit Level Control

      Scum pit level control is similar to other well-level pumping. Typically, constant-speed pumps are used and there is a high “start-level” setpoint and a low “stop-level” setpoint. When the level reaches the start-level setpoint, the scum pump is started. When the level reaches the stop-level setpoint, the scum pump is stopped.

      Scum tends to stratify in the scum pit, with the scum rising to the top and the water falling to the lower level. In some systems, the pit is mixed to allow a more homogeneous mixture, which will be easier to pump than the concentrated scum. In these instances, the mixer is started when the level reaches the “start pump” setpoint and is run for a time equal to a setpoint. When the mixer stops, the scum pump is started.

8.0 ACTIVATED SLUDGE SYSTEM CONTROL STRATEGIES

      8.1 Dissolved Oxygen Control

      8.1.1 Process Description

      The aerobic section of an activated sludge reactor must maintain a minimum oxygen concentration to provide enough oxygen to microorganisms for them to achieve removal efficiencies. In activated sludge reactors without BNR, oxygen is needed to provide adequate carbonaceous oxygen demand. With nutrient removal, oxygen is used to both achieve nitrification via ammonia removal and to ensure an environment for phosphorus accumulating organisms to absorb excessive phosphorus.

      Oxygen is typically supplied to the aerobic reactor via air. Air contains about 21% oxygen. This can be accomplished by using mixers to entrain air into the reactor or by using compressed air that is diffused into the reactor. Use of compressed air is significantly more common. Blowers that supply this air are typically the largest source of energy use at a WRRF. There may be several blowers available to allow for variable demand of air. Most blowers are centrifugal and their output is typically varied by using guide vanes or inlet valves. Variable-frequency drives (VFDs), which are used to vary the speed of the motor, are gaining popularity for use in centrifugal blowers and are the most common method to vary the output of a positive-displacement blower.

      Oxygen is typically distributed to several reactors and, often, to specific areas within each reactor. A valve is typically used at each addition point to control the amount of air feed at that location. As the valves’ positions modulate, the pressure within the overall air header extending from the blowers is affected. Blower output is typically modulated to maintain header pressure within an acceptable range. Pressure that is too high could cause the blower to surge. In addition, the higher the pressure maintained, the more energy use of the blower.

8.1.2 Process Variables

      The following are typical process variables needed for control:

      • Dissolved oxygen concentration,

      • Air header pressure, and

      • Airflow.

8.1.3 Controlled Variables

      The following are typical controlled variables used for automatic control:

      • Blowers’ start–stop,

      • Blower speed control,

      • Blower guide vane position control, and

      • Individual air valve position control.

8.1.4 Control Strategies

      The main control strategy for aeration control is to vary the individual air valve’s position in each reactor or reactor area to maintain the dissolved oxygen level in that area at a setpoint. As dissolved oxygen drops below the setpoint, the valve is opened, and, as dissolved oxygen rises above the setpoint, the valve closes. In some instances, airflow to each reactor is monitored and used to modulate the individual air valve’s position. Dissolved oxygen, in this instance, is used in a cascade loop to modify the airflow setpoint.