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PAKISTAN COUNCIL OF SCIENTIFIC & INDUSTRIAL
RESEARCH LABORATORIES COMPLEX, KARACHI
OFF: UNIVERSITY ROAD, KARACHI-75280.
AERATION
UNITS...

 Unit process in which air and water are brought
into intimate contact.
 The contact time and ratio of air to water must
be sufficient for exchange sufficient oxygen.
Advantages
 Providing O2 for purification and improving
overall quality.
 CO2 reduction-reduces the corrosion.
 Raising the pH.
 VOC removal
 Effective method for bacterial control

Methods of aeration
 Diffused aeration
 Spray aeration
 Turbine aeration
 Surface aeration

Diffused aeration
 Providing maximum water surface per unit volume of
air.
 Air bubbles brought with water in a mixing or contact
chamber.
 A common way to aerate water is via diffused air.
 Air is pumped through some sort of diffuser to
generate small bubbles.

 Usually gas is injected into the bottom of the
aeration tank and is allowed to rise to the surface
in an open tank.
 The rising bubbles transfer oxygen to the water,
as well as transport bottom water to the surface.
 The bubbles raising through water create
turbulence.
 Untreated water is allowed to enter the tank from
top and exit from bottom.

Efficiency of diffused aeration can be improved:
 Fine bubbles (0.2 cm dia) as compared to coarse bubble
(2.5 cm dia)
 By increasing water depth (9-15 ft)
 By improving the basin geometry (width to depth ratio
not exceed 2)
 By increasing the retention time (10-30 min)

Typical diffused aeration system looks like:

There are a large variety of diffuser types. For example
ceramic plates

These plates are arranged on manifolds at the
bottom of aeration tanks as shown here.

Other types of diffusers include coarse aerators

To determine the oxygen transfer rate in these
diffused aeration systems, first define the
pressure difference from top to bottom of the
tank.
At the surface:
14.7(1 0.032 AlPsurfac t)e  
Alt = altitude in thousands feet above sea level
Psurface has units of psi

H = depth of tank (depth of discharge point) in feet.
62.4 H
P P (psi)bottom surface 144

 

Mechanical Aeration
Basically there are two types of mechanical aeration.
Turbine Aeration:
 In this system coarse bubbles are injected into the
bottom of the tank and then a turbine shears the
bubbles for better oxygen transfer.
 Efficiency of turbine aerators is generally higher than
diffused aeration.

Surface Aeration:
 In this case a mixing device is used to agitate the surface
so that there is increased interfacial area between liquid
and air.
 There are many different proprietary types of surface
aerators .

Common surface aerators

Design consideration for mechanical aerators is
usually based on Eckenfelder and Ford equation.
 T 20C Cw lN N (1.02)0 9.17
 
    
 
Notice that there is no depth consideration for
mechanical aeration.

Where as:
 N = actual transfer rate (lb-O2/hr)
 N0 = manufacturer specified transfer rate ( lb/hr)
for clean water, 20oC, zero DO.
 Cw = saturation value for oxygen for wastewater
under operating conditions.
 9.17 = saturation DO for clean water, 20oC.
 Cl = the design oxygen concentration in the aeration
basin.
 T = Temp.
 α = oxygen transfer correction factor for waste
water

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