Design and Operation of Hybrid Aeration System

37 Pages·2006·0.66 MB·English

Design and Operation of
Hybrid Aeration System
SAWEA
Marc Rhodes – Envirex Products
2005
It is generally agreed that fine bubble aeration has the highest
oxygen transfer efficiency of any aeration device in clean water.
Some typical empirically measured value:
(cid:132) Fine Bubble Aerator Clean Water Efficiency (SAE) = 4.4
kg/hr/kW
(cid:132) Mechanical Aerator Clean Water Efficiency (SAE) = 1.9 kg/hr/kW
However, efficiencies are different in wastewater
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In wastewater, contaminants affect aeration efficiencies by altering the process
variables
(cid:132) Variables affecting oxygen transfer rate during aeration include:
(cid:132) alpha = KLa Waste water
KLa Clean water
= mass transfer coefficient in waste water
mass transfer coefficient in clean water
Alpha is the most variable factor includes affects of loading, suspended solids,
mixing, etc.
(cid:132) field process dissolved oxygen (D.O.) desired to be maintained.
(cid:132) Beta (saturation factor - corrects for dissolved solids in wastewater)
(cid:132) Theta (corrects for temperature)
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(cid:132) Barometric pressure
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(cid:132)Many variables are different in wastewater than in clean
water therefore,
Aeration equipment performs differently in clean water than
wastewater
(cid:132)Some variables change throughout the wastewater
process
So, aeration equipment performs differently at
different locations in the wastewater process
(cid:132) Each type of aeration equipment is uniquely
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affected by the process variables
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Alpha
Most important process variable affecting
(cid:132)
aeration efficiency is Alpha:
(cid:132)Variations in Alpha are caused primarily by
surfactants in wastewater
(cid:139) Surfactant concentrations differ throughout the
wastewater treatment process due to breakdown by
biological activity
(cid:139) Alpha values of 0.2 to 1.2 are reported
(cid:139)Alpha above 1.0 are for mechanical aeration in the
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Effect of Surfactants on
Diffused aeration
(cid:132) Surfactant molecules form a rigid surface on bubbles
(cid:132) Aeration difficulty decreases with smaller bubbles
liquid
(cid:139) Lower Alpha for fine bubble than for coarse bubble air
Mechanical aeration
(cid:132) Surfacants reduce surface tension resulting in formation of smaller
liquid droplets
(cid:139) Increases available surface area for transfer
(cid:139) Alpha factors can be greater than 1.0 liquid
air
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Effect of Surfactants on:
(cid:132)Fine Bubble Aeration at Front of Process
(cid:139)Relatively high concentration of
surfactants
(cid:139)Overall effect is decreased oxygen
transfer efficiency
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Effect of Surfactants on:
(cid:132)Fine Bubble Aeration Further Into Process
(cid:139)Reduced concentration of surfactants
(cid:139)Overall affect is increased oxygen
transfer efficiency
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Effect of Surfactants on:
(cid:132)Mechanical Aeration at Front of Process
(cid:139)Relatively high concentration of surfactants
(cid:139)Decreased water droplet size
(cid:139)Increased air transfer efficiency from air to
droplet
(cid:139)Alpha factors can be greater than 1.0
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Effect of Surfactants on:
(cid:132)Mechanical Aeration Further into Process
(cid:139)Reduced concentration of surfactants
(cid:139)Increased water droplet size
(cid:139)Decreased air transfer efficiency from air to
droplet
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