How To Control Activated Sludge With Online Sensors

By Dr. Robert Smith, Ph.D., P.E., BCEE, YSI, a Xylem brand

The goal for domestic wastewater treatment in the 21st century should be to have a minimal carbon footprint and to be 100% self-sustainable with regards to energy, carbon, and nutrients – while achieving a discharge or reuse quality that preserves the quality of receiving waters (Water Environment Research Foundation (WERF), 2009). 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 (Water Environment Federation (WEF), 2009). 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% of electrical costs (Water Environment Federation (WEF), 2006).

The focus of this white paper 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 controlsystems, 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.