Sensor Technology For Control Of Activated Sludge Aeration

By Robert Smith, P.E., BCEE, Ph.D., YSI

Design routines and operation strategies for activated sludge aeration systems have traditionally been based on manual control. However, rising energy costs and sustainability goals are providing incentive to implement instrumentation, control, and automation (ICA) to reduce energy consumption for aeration. Controlling dissolved oxygen (DO), for example, is reported to result in a savings of 15 to 20% of electrical costs. Once considered the weakest link of ICA, instrumentation is no longer a major barrier to automation of wastewater aeration. Technological innovations have dramatically increased the accuracy and reliability of online instrumentation and reduced the system cost. Recent innovations in online sensors include development of an optical dissolved oxygen sensor and an ion selective electrode (ISE) for ammonium.

Dissolved oxygen measurement based on traditional amperometric sensors is sufficiently accurate for control but calibration and routine maintenance of online sensors is time-consuming and costly. Innovation was needed to create a simpler, more reliable measurement. The solution was optical DO sensors based on luminescence. The principle of luminescence-based measurement is simple: the fluorescence of a luminophore is quenched in the presence of dissolved oxygen. Oxygen molecules pass through an outer sensing layer to an inner layer which is an immobilized fluorescent luminophore. The dissolved oxygen concentration is proportional to length of time it takes for the excitation flash of an LED light source to be recorded as fluorescence at a detector. A reference channel compensates for minor drift in the optics. As a result, luminescence-based sensors are extraordinarily stable and do not require calibration for the lifetime of the sensing element, a minimum of 2 years. Furthermore, there are no membranes to replace, electrodes to clean, or electrolytes to replenish.