Aeration occurs in many drinks of water and wastewater treatment processes, but in the activated sludge process and its variants, it consumes more energy than other processes, by far. Reducing energy consumption during aeration is usually the best first step to decrease energy cost.
• Both the US and the EU have standards to measure oxygen transfer in clean water
• Freshwater results are most often used for design and specifying treatment plants
• The standards define nomenclature that should always be used
• Because aeration systems are expensive, their choice can become litigious. Using standard methods and terminology can avoid legal cost and delays
• Nomenclature has been listed in a separate handout, but the principal terms are listed on the next slides.
Parameter Definition Remarks
OTR Oxygen transfer rate in clean water = kLa (DO-DOsat) V
SOTR Oxygen transfers rate in standard conditions in clean water
OTE Oxygen transfer
in clean water = (O2, in -O2, out) / O2, in
SOTE Oxygen transfers efficiency in standard conditions in clean water
AE Aeration efficiency in clean water = OTR / P
SAE Aeration efficiency in standard conditions in fresh water
kLa Liquid-side mass transfer coefficient Measured in clean water tests
Standard conditions are defined as 20oC, 1 atm, zero salinity, zero DO in water.
Key: P = power drawn; V = water volume.
α – Alpha factor
, i.e. ratio of the process to clean water mass transfer. = αSOTE / SOTE , or = kLa process water / kL a clean water
F – Fouling factor = αSOTEnew diffuser / αSOTEused diffuser
αF Alpha factor for used diffusers = αF
αSOTE Oxygen transfer efficiency in standard conditions in process water
αFSOTE Oxygen transfer efficiency in standard conditions in process water for used diffusers
αSAE Aeration efficiency in standard conditions in process water
αFSAE Aeration efficiency in standard conditions in process water for used diffusers
• Oxygen transfer and transfer of other sparingly soluble gasses can be modeled using the two-film theory or two resistance theory.
• The two-film theory dates back to Lewis and Whitman’s paper in 1924.
• The two-film theory has been extended by Higbee (1935) and Dankwertz(1951), but these extensions are not needed when designing aeration systems for wastewater treatment.
• The extensions become important when considering stripping of volatile organic compounds (VOCs)
OTR = kLa (DOsat-DO) V
where kLa = liquid-side mass transfer coefficient (h-1)
DO = dissolved oxygen in water (kgO2 m-3)
DOsat = dissolved oxygen in water at saturation (kgO2 m-3)
V = water volume (m3).
• The OTR is the real mass of oxygen transferred per unit time, and it is the critical process variable for design
• The DO saturation concentration is the concentration of dissolved oxygen at saturation with no reactions in the liquid and includes the impact of hydrostatic pressure. Subsurface aeration systems, which release bubbles below the surface, always have higher saturation concentrations than the “textbook” values of DOsat due to this hydrostatic pressure.
• The mass transfer coefficient, kL a is a function of the aeration system and the tank geometry.
• The mass of oxygen transferred per unit of power response is the most important efficiency value. It defines the amount of energy required to treat the wastewater, which as noted earlier is usually 60% or more of the total energy cost and is expressed as an aeration efficiency, equal to the OTR divided by the power response, as follows. AE = OTR/P
Oxygen Transfer Efficiency
• There are different types of aeration systems, but subsurface or diffused aeration systems are most common, especially for large plants in urban areas
• For these types of aeration systems, it is common to define the oxygen transfer efficiency, expressed as a percent, as follows:
OTE = (O2in-O2out) / O2in
Where O2, in and O2, out are mass flow rates
• For manufacturers to offer equipment without bias for site-specific conditions, it is common to report the various transfer limits at standard conditions. Standard conditions include tap water, 20oC, 1 atm pressure, zero salinity, etc. There we can define standard limits as follows:
Standard Oxygen Transfer Efficiency (SOTE, %)
Standard Oxygen Transfer Rate (SOTR, kgO2 h-1)
Standard Aeration Efficiency (SAE, kgO2 kW-1 h-1).
• Translating or correcting standard limits to non-standard or process conditions is the key job of the consultant. Manufacturers of aeration systems can recommend, but cannot be held accountable for this key engineering task
• Most often, three empirical parameters are used for this translation, called the α (alpha) cause, β (beta) factor and θ (theta) factor.
• The α factor accounts for contaminants in the wastewater, and soaps, detergents have the most impact on the α factor. The α factor is the most uncertain of the various oxygen transfer limits and is the most difficult to know accurately.
The αSOTE is the most useful characteristic for comparing aeration systems since it excludes the effects of all conditions except for water impurities
• It can be specified for a specific water depth or on a per depth basis.
• For example, fine pore, full floor coverage aeration systems may have a clean water efficiency of 6 to 7.5% SOTE/m and 3 to 4% αSOTE/m of diffuser submergence.
• Since aeration systems are competitively bid by oxygen transfer per unit of energy consumed, power analysis becomes crucial
• There are three types of power measurements:
– Wire power – this is the power used by the aeration system and includes all inefficiencies of the system, such as motor and blower inefficiencies
– Brake power – this can be specified as the output power of a motor or gearbox. It can be calculated as the torque times the RPM
– Water power – this is the power that is transferred to the fluid being aerated. For a surface aerator, water power excludes the motor, coupling and gearbox inefficiencies.
• It is essential to be clear on power definitions when designing and specifying aeration systems.
• The differences in real power values using the different methods can be real.
• Wire power differs from brake power, when defined as motor output power, by the motor efficiencies. Modern motors in the 100 kW range have efficiencies of approximately 95%
• Small motors and old technology motors can have efficiencies less than 80%.
• It is often economically viable to replace old motors with modern, high efficiency or “premium” motors based on power savings alone.
Gear Box Efficiency
• New, large (100 kW) gearboxes may have efficiencies of 90% to 93%. Efficiency declines with age.
• The several stages of RPM reduction impact efficiency. Most aerator gear boxes have two or three stages
• Small gearboxes have lower efficiencies and joined motor-gearbox efficiency for 20 kW devices may be 80%
• Efficiencies are specific to the manufacturer and need diligence on the part of the designer to analyze correctly.
• Several types of blowers can be used for subsurface aeration.
• Positive displacement (PD) blowers are best suited for small plants and have efficiencies of 70 to 75%. Efficiency declines with age. Poorly maintained PD blowers can be very inefficient and are good candidates for replacement based on energy savings
• Centrifugal blowers are most often used for larger plants. Hence, there is an incentive to make them more efficient.
• Centrifugal blowers with inlet guide vanes and outlet diffuser are the most efficient with efficiencies approaching 80%. Additionally, two sets of controls, vanes, and diffuses allows the blower to be more efficient over a greater range of flow rates – more “turn up” and “turn down” ability.
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