Читать книгу Automation of Water Resource Recovery Facilities - Water Environment Federation - Страница 252

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.