what input parameters and what past time steps to use is part of model design. Some ANN packages include functionality to help determine which input parameters are significant. Insignificant inputs may be removed during the calibration procedure, which often leads to better forecasts.
4.9.3 Artificial Neural Network Models Used for Process Control
Artificial neural network models are typically used as the predictive model in model predictive control (Section 4.8). In some instances, the inverse of the ANN model can be directly used for control. However, such use is not common in the environmental utilities’ field and, therefore, is beyond the scope of this document.
4.10 Control Strategies
Several control strategies that are common in wastewater treatment are discussed here; these are referenced in later sections of this chapter.
4.10.1 Lead–Lag Control
Lead–lag control is used when there are two or more devices performing the same function. By staging these devices, they can be used for a wider range of variation of demand. As the device is first needed, the lead device is started. When the first device is not sufficient to meet demand, the lag device is started. If demand is reduced, the lag device is stopped, leaving only the lead device operating. If demand is reduced with only the lead device operating, that may also be stopped.
Lead–lag control can be used with any number of devices (Figure 7.10). For example, if there are three devices available for operation, one will be the lead device, one will be the first lag, or Lag1, and the last will be the second lag, or Lag2. If both the lead and Lag1 devices are in operation and demand is not met, then the Lag2 device will be started. Likewise, if all three devices are in operation and demand is reduced, the Lag2 device will be stopped first, leaving the lead and Lag1 devices operating. There is no limit to the number of lag devices that can be used in lead–lag control, although two or three devices operated together are the most common.
FIGURE 7.10 Lead-lag control.
Lead–lag control has traditionally been used for constant-speed devices, most notably for constant-speed pumping systems. However, it can be used for variable-speed devices and allows for an extended automatic variation of range beyond that available with one device’s available modulation. For multiple variable-speed devices, the lead device will vary in speed typically using PID. Rules-based control is used to determine when to start and stop lag devices. If the lead devices’ speed control reaches its maximum while demand is increasing, a lag device will start. Typically, all devices in operation are controlled to the same speed (provided the devices are identical) so that the overall control methodology can continue to operate regardless of the number of devices in operation. If multiple devices are in operation and are operating at the minimum speed while demand is decreasing, the lowest lag device will be stopped.
Often, design of the system allows for one of the devices to be out of service. For example, with a three-pump system, only two of the three pumps might be needed for automatic operation and, therefore, one of the three pumps will not operate automatically.
Typically, selecting which device is assigned to the lead or lag level is left to an operator. Sometimes, automatic switching of the lead device is provided to ensure that all devices receive roughly equivalent use; however, in practice, there is often a device that is less efficient or has some mechanical issues that require its use to be minimized. In that instance, allowing an operator to manually assign it a lower lag rating will ensure that its use is minimized.
While an operator may choose which device has a lead or a lag value, there is often still a need for automatic reassigning of lead–lag assignment in the event of a failure or loss of availability of any specific device. For example, if the lead pump fails, automatically moving the Lag1 pump to the lead pump position and the Lag2 pump to the Lag1 position will keep the control system operating correctly in automatic mode until an operator is available.
4.10.2 Most-Open Valve Control
Most-open valve (MOV) control is commonly used to control flow or pressure to several areas via multiple valves or gates. The fluid division between each path is determined by the pressure loss through each valve and the remainder of the path. Each valve must be at an operational position (% open) that is not too high so that it can control, by opening or closing, a finite amount. However, if all of the valves’ positions are too low, it will result in inefficient energy use as the system uses energy to overcome pressure losses across each valve.
For several valves, the position of each valve will typically be different. The critical position is MOV, which needs to be controlled. In a well-designed system with good control valves, this MOV position setpoint may be as high as 95% open. If it were any higher than this, it could not open further to obtain more flow. Some valves or gates have an operational range much smaller. In some instances, the valve may achieve full flow at about 50% open and opening further will not increase flow or decrease pressure. In that instance, the MOV position setpoint might be 45% open.
The MOV position setpoint is typically a secondary or cascade control strategy. The primary strategy is to control the flow or pressure for each path. Typically, the MOV position setpoint is controlled to maintain it within an acceptable range and no further control occurs if the position remains within this range. For example, the MOV setpoint may result in no control actions if it is maintained between 80 to 90% open. In instances where there are several valves, the MOV may not be one specific valve but may represent the average of two or more of the MOVs.
5.0 PUMPING AND FLOW CONTROL STRATEGIES
5.1 Process Description
Pumping and flow control is used throughout wastewater treatment. There are two main types of pumping systems: well pumping and flow pumping. Flow control can be achieved by flow pumping and/or combined with valve or gate flow splitting.
5.1.1 Well Pumping
Well pumping is commonly used to move material away from its source. The most common forms of well pumping in WRRFs are pump stations, either within the collection system or within the facility, that are used to move wastewater to a higher elevation. Well pumping can also occur at several locations within a WRRF to move material such as waste sludge, facility drainage, groundwater, basins, or other areas where fluid flows into a well to be pumped. The main purpose of well pumping is to remove material from the source (well). Therefore, the well level is typically the process variable used for control of these systems.
5.1.2 Flow Pumping
The main purpose of a flow pumping system is to control flow from the source to its destination. In these pumping systems, source water is independently controlled and is not dependent on the pumping system for its control. Flow pumping systems include return sludge pumping from clarifiers, mixed liquor internal recycle flows within activated sludge reactors, and polymer and other chemical additive flows.
The main purpose of flow pumping is to control the amount of material discharged from the pumps. Therefore, downstream flow, or pressure, is typically the process variable used to control these systems
5.1.3 Flow Splitting
Often, flow of
