Aeration - 2...

Introduction...

In aerobic metabolism oxygen acts primarily as an electron acceptor for catabolism. As the activated sludge process is designed to be substrate - limiting, metabolism sets the rate of oxygen demand. The function of the aeration system is to transfer oxygen to the liquid at such a rate that oxygen never becomes the limiting factor in process operation ( i.e., never limits the rate of organic utilization or other metabolic functions ). It is the engineer's responsibility to ensure that aeration systems are adequate. This requires an understanding of the basic principles of gas transfer as well as some familiarity with the different types of aeration devices that are available.

Fundamentals of Gas Transfer...

All solutes tend to diffuse through solutions until the composition is homogeneous throughout. The rate at which solutes diffuse across a uniform cross - sectional area depends on the molecular size and shape and the concentration gradient of that substance. Matter moves spontaneously from a region of high concentration toward a region of lower concentration, and the more the concentration is decreasing, the more the diffusion rate increases. This can be expressed by writing the concentration gradient term as ( - aC / aY ), where C is the concentration and Y the distance. If ( aM / at ) represents the rate at which M grams of solute cross the reference plane, " Fick "s first law of diffusion states that ;

( aM / at ) = - ( D L ) ( A ) ( aC / aY )

where ; aM / at : rate of mass transfer ( mass time - 1 ), D L : diffusivity constant ( area time - 1 ), A : cross - sectional area across which the solute is diffusing ( area ) and aC / aY : concentration gradient ( i.e., the change in concentration with distance, mass volume - 1 length - 1 ).

The simplest concept of a gas transfer process is the " stationary liquid film theory ". This theory suggest that at the interface between the gas phase and the liquid phase, there exists a stationary liquid film in which gas molecules are concentrated. The gas concentration is not homogeneous throughout the liquid film but rather decreases from the saturation concentration given by " Henry "s law to some lower concentration at the film / bulk liquid boundary. Figure given below illustrates the stationary liquid film theory.



In this figure, C S represents the saturation concentration of the gas in the liquid as predicted by " Henry "s law, C the concentration of the gas in the bulk of the liquid, Y F the thickness of the film, and C L the gas concentration at the film / bulk liquid boundary. Applying " Fick "s first law of diffusion to this situation gives ;

( aM / at ) = - ( D L ) ( A ) ( aC / aY F )

In this case, A represents the interfacial area of contact between the gas and liquid phases. Since the liquid film thickness is small ( only a few molecules thick ), it is possible to approximate the differential quantity ( aC / aY F ) with linear approximation ;

( aC / aY F ) ==== ( C S - C ) / ( Y F )

The system, as described by the linear approximation, is shown in figure given below. For the linear approximation, equation reduces from a partial differential equation in time and space to an ordinary differential equation in time.

( dM / dt ) = - ( D L ) ( A ) [ ( C S - C ) / ( Y F ) ]



Dividing both sides of the equation by V ( the volume of the liquid phase ), the equation becomes ;

[ ( 1 / V ) ( dM / dt ) ] = - ( D L ) ( A / V ) [ ( C S - C ) / ( Y F ) ]

Since the term [ ( 1 / V ) ( dM / dt ) ] has the units of mass volume - 1 time - 1 or concentration per unit time, this term can be expressed by the differential equation ( dC / dt ) ;

[ ( 1 / V ) ( dM / dt ) ] = ( dC / dt )

( dC / dt ) = - ( D L ) ( A / V ) [ ( C S - C ) / ( Y F ) ]

Because the value of the film thickness is normally unknowm, it is usually combined with D L to define a new constant term.

K L = ( D L ) / ( Y F )

where ; K L represents the gas transfer coefficient and has the units of length time - 1 . K L can be incorporated into the equation to give an expression which has the form ;

( dC / dt ) = - ( K L ) ( A / V ) ( C S - C )

This equation represents the change in concentration to be expected as molecules diffuse from a region of high concentration to a region of low concentration so that the concentration is decreasing with time. When the gas concentration increases with time during the aeration process, the negative sign is dropped and equation reduces to ;

( dC / dt ) = ( K L ) ( A / V ) ( C S - C )

In most cases the interfacial area of contact, A, is difficult to determine. To circumvent this problem, a second constant, K L a, is introduced. This constant has a value equal to the product of K L and ( A / V ) ;

K L a = ( K L ) ( A / V )

K L a is defined as the overall gas transfer coefficient and has the units of time - 1 .

( dC / dt ) = ( K L a ) ( C S - C )

[ ( dC ) / ( C S - C ) ] = ( K L a ) ( dt )

- ln ( C S - C ) ==== ( K L a ) ( t ) + Constant of integration

If C = C 0 at t = 0, the constant of integration has the value - ln ( C S - C 0 ) ;

- ln ( C S - C ) = ( K L a ) ( t ) - ln ( C S - C 0 )

ln [ ( C S - C 0 ) / ( C S - C ) ] = ( K L a ) ( t )

This implies that a semilog plot of [ ( C S - C 0 ) / ( C S - C ) ] versus t will give a linear trace with a slope equal to ( K L a / 2.3 ) in log base. Some typical C S values for pure water are given in table shown below.

Temperature ( O C ) C S ( mg / L )
0 14.62
2 13.84
4 13.13
6 12.48
8 11.87
10 11.33
12 10.83
14 10.37
16 9.95
18 9.54
20 9.17
22 8.83
24 8.53
26 8.22
28 7.92
30 7.63
Atmosphere contains 21 % oxygen, chloride concentration = 0.00 mg / L and pressure = 1 atm.

Factors Affecting Oxygen Transfer...

Such factors are ; ( 1 ) oxygen saturation, ( 2 ) temperature, ( 3 ) wastewater characteristics and ( 4 ) degree of turbulance.

Oxygen Saturation...

The saturation concentration of oxygen in water depends upon salinity, temperature, and the partial pressure of the oxygen in contact with the water. " Eckenfelder " and " O'Connor " suggest that the saturation concentration may be obtained from the following equation ;

C S - 760 = [ 475 - ( 2.65 ) ( S ) ] / ( 33.5 + T )

where ; C S - 760 : saturation value of oxygen at a total atmospheric pressure of 760 mm Hg ( mg / L ), S : dissolved solids concentration in the water ( g / L ) and T = temperature ( O C ).

Many workers correct for the presence of dissolved salts by introducing a BETA factor, defined as ;

BETA = ( Saturation concentration in wastewater ) / ( Saturation concentration in tap water )

The value of oxygen saturation given by the equation may be corrected for prevailing pressure by applying the expression ;

C S = ( C S - 760 ) [ ( P - p ) / ( 760 - p ) ]

where ; P : prevailing barometric pressure ( mm Hg ) and p : saturated water vapor pressure at the temperature of the water ( See the table given below ).

Temperature ( O C ) Vapor pressure ( mm Hg )
0 4.5
5 6.5
10 9.2
15 12.8
20 17.5
25 23.8
30 31.8

Temperature...

Temperature affects the overall oxygen transfer coefficient according to the following expression ;

K L a T = ( K L a 20 C ) ( 1.020 ) T - 20

" Dobbins " have proposed another approach wherein K L a is corrected for both temperature and viscosity effects ;

[ ( K L a 1 ) / ( K L a 2 ) ] = { [ ( T 1 ) ( MU 2 ) ] / [ ( T 2 ) ( MU 1 ) ] }

where ; T : temperature ( O K ) and MU : absolute viscosity.

Wastewater Characteristics...

Surface - active agents such as short - chain fatty acids and alcohols are soluble in both water and oil solvents. The hydrocarbon part of the molecule is responsible for its solubility in oil, while the polar carboxyl or hydroxyl group has sufficient affinity to water to drag a short - length nonpolar hydrocarbon chain into aqueous solution. These molecules will concentrate at an air / water interface, where they are able to locate their hydrophilic group in the aqueous phase, which allows the hydrophobic hydrocarbon chain to extend into the vapor phase ( see figure given below ). This situation is energetically more favorable but creates a concentration of molecules or " film " that retards molecular diffusion. Hence, resistance to oxygen transfer is increased and, consequently the value of K L a is decreased. To compensate for the effects of surface - active agents on oxygen transfer, an ALPHA factor is introduced, where ;

ALPHA = ( K L a of wastewater ) / ( K L a of tap water )

Turbulence...

" Eckenfelder " and " Ford " report that the degree of turbulence in the aeration tank will influence the value of ALPHA as follows ;

( 1 ) Under near - quiescent conditions ( a lower degree of turbulence ), fluid motion has little effect on ALPHA because the resistance to diffusion in the bulk of the liquid is greater than the film resistance.

( 2 ) Increasing fluid agitation to a moderate degree decreases the resistance to diffusion in the liquid bulk so that film resistance will control the diffusion rate. At this point ALPHA is depressed to a minimum value.

( 3 ) A further increase in fluid agitation will produce a high degree of turbulence and break up the film. Under such conditions ALPHA will approach unity.

The effects of turbulence on ALPHA, as developed by " Mancy " and " Okun " are illustrated by figure given below.

Oxygen Transfer Rates...

The oxygen transfer rate of a particular aeration device quoted by a manufacturer applies only for standard conditions and a specific tank geometry. Standard conditions mean that the aerator was tested with tap water at zero dissolved oxygen concentration, 20 O C and 760 mm Hg atmospheric pressure. Thus, a manufacturer's figure for the rate of oxygen transfer must be modified for process conditions. This can be done by incorporating the factors that affect the oxygen transfer rate into the equation to give ;

( dC / dt ) ACTUAL = ( ALPHA ) ( K L a 20 C ) { [ ( P - p ) / ( 760 - p ) ] ( BETA ) ( C S ) - C }

Under standard conditions where C = 0 ;

( dC / dt ) STANDARD = ( K L a 20 C ) ( C S )

To determine the design oxygen transfer rate, equations given above must be combined as ;

( dC / dt ) ACTUAL / ( dC / dt ) STANDARD

Determination of K L a and ALPHA Values...

Either the steady - state or non - steady - state test is used to determine aeration equipment characteristics under process conditions. The basic formulation used in the steady - state test is developed by modifying the equation such that ;

( dC / dt ) OVERALL = ( K L a ) ( C S - C ) - R

where ; R : the oxygen utilization rate of the biomass and has the units mass volume - 1 time - 1 . The C S term reflected in the equation is specific for process conditions. At steady - state conditions the rate of oxygen transfer by the aeration system is equal to the rate of oxygen utilization by the biomass, implying that ( dC / dt ) OVERALL is equal to zero. Thus, equation given above can be solved for K L a to give ;

K L a = ( R ) / ( C S - C )

where ; K L a : overall oxygen transfer rate in wastewater at process conditions, time - 1 . This K L a value indicates the effects of surface active material, TDS, temperature, and partial pressure.

In the non - steady - state test, final effluent or supernatant from sttled mixed liquor is generally used. The laboratory procedure for this test is as follows ;

( 1 ) Adjust the liquid temperature to that expected in the field.

( 2 ) Deoxygenate the liquid in the test basin using sodium sulfite with a cobalt chloride catalyst. The cobalt chloride dose should be no greater than 0.05 mg / L. The reaction between sodium sulfite and oxygen is as follows ;



Theoretically, 7.9 mg / L of sodium sulfite is required for each mg / L of oxygen present. However, it is common practice to add 1.5 to 2.0 times this amount to ensure complete deoxygenation.

( 3 ) Oxygenate the liquid using the same type of aeration device to be used under process conditions.

( 4 ) Tabulate the dissolved oxygen concentration at various time intervals and sampling points until oxygen saturation is reached. Recommended sampling points for steady - state test is given in figure shown below ;



( 5 ) A plot of log [ ( C S - C 0 ) / ( C S - C ) ] versus time will give a linear trace of slope ( K L a ) / ( 2.3 ) ( See figure given below ).



( 6 ) Repeat the same procedure using tap water as the test liquid.

Some typical values of ALPHA and BETA are given in tables shown below.



Design of Aeration Systems...

The costliest item in the activated sludge process is the aeration system. Because of this, its design is critical if the treatment facility is to be cost - effective. Manufacturers of aeration devices will usually quote a figure for the oxygen transfer rate in terms of the mass of oxygen that the aerator can introduce into water per unit of time per unit of power input. This figure quoted by the manufacturer will be valid only under standard conditions and the specified tank geometry. It will, therefore, be necessary to adjust the manufacturer's figures to those which more realistically describe actual process conditions. Current methods used to transfer oxygen in aerobic biological wastewater treatment process include ;


( 1 ) Compressed air diffusion...



( 2 ) Submerged turbine aeration...



( 3 ) Low - speed surface aeration...



( 4 ) Motor - speed ( high ) surface aeration...


As a guide to system selection, diffused aeration not be used when the oxygen utilization rate exceeds 40 mg / L . h. Low speed surface aerators are acceptable as long as the oxygen utilization rate is less than 80 mg / L . h. However, when the oxygen utilization rate exceeds 80 mg / L . h, submerged turbine aeration should be the method of choice. Furthermore, in areas where freezing temperatures are experienced for long periods winter months, either diffused or submerged turbine aeration is preferred over surface aeration.

Diffused Aeration...

Fiffused air systems operate by blowing compressed air through diffusers. Compressed air is provided by compressors ( " blowers " ) operating at a pressure sufficient to overcome the head created by frictional losses in the air piping system and the static head of liquid above the diffuser.


Centrifugal " blower "...

The diffusers are positioned near the bottom and along the side wall of the aeration tank to effect oxygen transfer and mixing, or are evenly spaced across the bottom of the aeration tank. They may be attached to either fixed or retractable mountings. Such systems will generally provide adequate mixing with air flows of 20 to 30 standard cubic feet per minute ( scfm ) per 1,000 cubic feet of tank volume. Other commonly used figures for air flow in these systems are 1 cubic feet per gal wastewater treated or 1,000 cubic feet per lb BOD applied to the aeration tank. There are basically two types of diffusers. One type produces small bubbles by passing compressed air through a porous medium prior to its discharge into the liquid. The porous medium used is either a material composed of silicon dioxide or aluminum oxide grains held by a ceramic binder or plastic - wrapped ( e.g., " Sa