Blower
Blowers are typically used to provide aeration in activated sludge plants and to promote aerobic digestion. They keep solids suspended in channels and in aeriated grit chambers, which is absolutely necessary for a proper treatment ecosystem. Aeration blowers receive a lot of attention from design engineers, suppliers, and end users. That is understandable since blowers account for more than 50 percent of the energy used in a typical wastewater treatment plant (WWTP). They represent “low hanging fruit” for energy conservation measures in wastewater treatment!
In some industries blower applications are specified with a single operating point consisting of the design flow and discharge pressure. Municipal wastewater treatment applications, however, usually require blower systems that provide a spectrum of flows and discharge pressures. Although this can be frustrating for suppliers, it reflects variability in the treatment process. Understanding the variable process demands on the system is critical to optimizing the performance of the blowers.
Function:
- The air supplied by the blowers to the aeration basin has several functions. The first is to supply oxygen needed for metabolizing organic compounds in the wastewater. The organic compounds are referred to as “BOD5” (biochemical oxygen demand), named after the 5-day test used to measure the concentration of these compounds. The oxygen must be dissolved in the wastewater in order to be used by the microorganisms. The diffusers use tiny bubbles of air to efficiently dissolve oxygen into the wastewater.
- Additional oxygen is required when microorganisms convert ammonia (NH3) into nitrate (NO3), a process known as nitrification. Nitrification often represents half of the total process oxygen demand.
- The combination of sludge and wastewater in the aeration basin is called mixed liquor. The air supplied by the blowers creates turbulence in the mixed liquor to maintain the sludge in suspension. Mixing also keeps the contents of the aeration basin homogeneous. In many plants mixing limitations, rather than oxygen demand, dictate the minimum air flow rate. A typical value for mixing air flow is 0.12 SCFM per square foot of aeration basin plan area.
- Most diffusers have an upper limit on air flow rate to prevent physical damage. The maximum flow varies with diffuser design.