Activated Sludge

The goal for domestic wastewater treatment in the twenty-first century should be to have a minimal carbon footprint and to be 100 percent self-sustainable with regards to energy, carbon, and nutrients—while achieving a discharge or reuse quality that preserves the quality of receiving waters. Sustainability with respect to energy requires both conservation and production. The obvious target for energy conservation is the activated sludge aeration system because it accounts for 25 to as much as 60 percent of total plant energy use. Implementation of instrumentation, control, and automation (ICA) is critical to reducing energy consumption for aeration. Controlling dissolved oxygen (DO), for example, is reported to result in a savings of 15 to 20 percent of electrical costs.
The focus of this series is the use of online instrumentation for control of wastewater treatment aeration. Once considered the weakest link of ICA, instrumentation is no longer a major barrier due to recent technological breakthroughs. The adoption of solid-state electronics and digital communication technology has dramatically increased the functionality of the measurement system. Modern online instruments are capable of measurements at a frequency, accuracy, and reliability suitable for process control at a reasonable cost. Furthermore, additional sensor options are available, including ion selective electrode (ISE) sensors for ammonium and nitrate and a new, more reliable optical DO measurement technology.
However, it is not just the measurement technology that has improved. Computing power is now practically free and no longer a limiting factor. It has enabled the development of inexpensive, commercial wastewater treatment simulators that capture the accumulated knowledge and understanding of the process and bring it right to operators’ and consultants’ desktops. It has enabled more widespread usage of distributed control systems, including programmable logic controllers (PLC). New blower technologies have improved blower efficiency and increased turn-down capability allowing air supply to more closely match air demand. For example, single-stage centrifugal blowers equipped with inlet guide vanes and variable outlet vane diffusers makes it possible to operate the blower at its highest efficiency point, not only at the design condition but also within a greater range outside of the design condition (Environmental Protection Agency (USEPA), 2010). As a result, control strategies that are based on the needs of the process have emerged.
The simplest control method is manual adjustment of blowers and valves based on manual samplings and measurements using laboratory or handheld instruments to maintain desired setpoints. This is a form of the “Sneakernet” because it requires operators to physically transfer information by walking from process to instrument to control element. It was common in the early days following passage of the Clean Water Act, especially for facilities required only to meet secondary treatment standards for TSS and BOD. Furthermore, treatment facilities were originally designed based on hefty safety factors applied to empirical standards for maximum loading conditions. In this case, the design priority was meeting discharge permit requirements under worst case conditions not providing flexibility to optimize operation for typical daily loads. Nonetheless, an attentive operator can observe the effect of manual adjustments on process performance and optimize the operating scheme for daily, weekly, and seasonal variations based on a formal or informal set of constraints. The main drawback of this strategy is that the system is never optimized. System settings are optimum for only one loading condition (at most) depending on the availability of the operator and cannot closely reflect diurnal variations in demand.
Furthermore, no adjustments can be made on days that the operator has more urgent tasks or on days the plant is not fully staffed, such as on weekends or holidays. The natural tendency is to adjust the system conservatively as a contingency to assure permit compliance while the process is not tended. The tradeoff is that treatment performance suffers and operating costs for energy and chemicals are higher than necessary.
Despite this disadvantage, manual is still the most common aeration control method used today. There are several reasons for this including the perceived unreliability of online instrumentation, the complexity of advanced control systems, and the cost to implement automatic control. One of the most important reasons is utilities’ unwillingness to adopt a higher level of automation. There is a resistance to change and a strong desire to keep things simple and not invest in training required to maintain online instrumentation. In other words, if it has worked for forty years why change now?
On the other hand, more stringent treatment requirements, the cost of energy, and the drive for sustainability are providing the incentive for more advanced control. Compliance with discharge permits now requires advanced treatment for removal of nitrogen and phosphorus requiring more stringent control of DO over a range of setpoints and a flexibility to adapt to wastewater loads that is beyond the capability of manual control. Furthermore, higher energy costs and greatly diminished availability of grant and low-interest loan funds mean that treatment goals must be achieved at lower energy consumption with minimum construction. The logical solution is increased usage of ICA.
In the next installment of this series, we’ll take a closer look at open loop control, also known as sequencing, where control elements are turned on and off based on a timer and relays, and closed loop control, in which the process response is measured by online instrumentation and implemented in SCADA using PLCs and motorized variable actuators. By the time this series concludes, we’ll also present a clear breakdown of cost benefits of greater control. ◆
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