Fixed film process for the treatment of waste water utilizing interfacial oxygen transfer

A process and apparatus for the treatment of waste water utilizing a downflow or upflow system incorporating a suitable media or biological growth wherein air is sparged into the media on an intermittent or pulsed flow basis at selected time intervals and the pulsed application of air allows the trapping of the air bubbles in the media bed based at least in part on particulate media size, shape and biofilm growth and thereby provides a more significant avenue of oxygen transfer due to a prolonged exposure of the air bubbles with the biomass and the absorption of oxygen directly from the film of the bubbles to the biofilm in addition to transfer from surrounding liquid due only to oxygen diffusion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is directed towards a fixed film, pulsed air process for the 
treatment of waste water wherein oxygen transferred to the biomass is 
enhanced by direct contact of the biomass with bubbles entrapped and 
prolonged within a media bed. 
2. Description of the Prior Art 
In the treatment and purification of waste waters a number of processes 
exist which involve the growing of bacteria used to degrade the waste 
water components either in a suspended culture or on a fixed or movable 
media. The latter method is commonly known as a fixed film biological 
process and generally appears to have certain advantages over the commonly 
used suspended culture or activated sludge processes. One such distinct 
and well recognized advantage is the ability to develop a greater biomass 
concentration per unit volume and thereby to decrease the necessary 
reactor size for such treatment. This has the obvious advantages of 
reduced land area and capital cost requirements. Another important 
advantage is directed to the positive control of the biomass inventory 
which results in a more reliable treatment performance. 
Prior to approximately 1970, most fixed film systems consisted of passing 
waste water over non-submerged rock or plastic media towers followed by a 
secondary clarification. The utilization of plastic media was looked upon 
as an improvement over the previously used rock media system since it had 
an increased surface area per unit volume and a greater void volume to 
facilitate air flow through the media and improve oxygen transfer to the 
liquid and therefore to the biomass growth on the media. 
A system resulting in similar performance and cost features incorporates 
the rotation of a biological contactor where the media is attached to a 
shaft and rotated through waste waters. The waste waters are retained in 
tanks to insure contact with the biomass on the media. Aeration to the 
system occurs due to the fact that the rotating media is exposed to 
atmosphere approximately 60% of the time. 
Prior art systems designed to incorporate submerged fixed film application 
were based on the requirement of using relatively small sized particulate 
media exhibiting high surface area characteristics in order to increase 
the fixed film biomass concentration per unit volume and thereby decrease 
the reactor size. Most of the prior art fixed film designs were upflow 
systems utilizing a variety of oxygen transfer methods. High rate 
biological oxygen demand requirement in the reactors of these systems were 
satisfied by presaturating the feed water with oxygen. Downflow systems 
are also recognized in the prior art wherein a packed bed fixed film 
system is utilized and waste water is fed to the top of the bed so that it 
flows downward through the packed bed media. The intended waste water 
treatment occurs by solid particle filtration and biological oxidation of 
soluble organic material by the biomass growing on the media. 
Specifically, a downflow fixed film biological media reactor has been 
introduced in France (biological aerated filter) wherein air is introduced 
into an intermediate location in the filter bed depth and the media depth 
below the air sparging point is utilized and is necessary for solids 
filtration. 
While submerged fixed film systems are known to have reduced reactor size 
and capital cost requirement, typical continuous air sparging processes 
generally result in a greater energy requirement then previously used rock 
or plastic media towers. Such energy requirements in continuous flow 
sparged fixed film systems are dependent on the amount of oxygen that 
could be transferred to the liquid in the reactor from the sparging gas 
passing upward through the reactor. 
The above set forth prior art processes and techniques base their 
oxygenation design on a fundamental oxygen transfer mechanism known in the 
prior art as the "two film" theory. Application of this theory, which was 
established in 1928, generally states that the oxygen is first transfered 
from a gas bubble into the bulk liquid and the biomass bacterial cells 
consumes the dissolved oxygen due to diffusion of dissolved oxygen from 
the liquid to the biofilm rather than from any direct engagement or 
contact with the gas bubble itself. 
Accordingly, there is a recognized need in the prior art for a fixed film 
waste water treatment system having the advantages of a reduced reactor 
size and capital cost requirement while at the same time having minimal or 
at least reduced energy requirements to satisfy the oxygenation needs of 
the biofilm or system bio-mass. 
SUMMARY OF THE INVENTION 
The present invention relates to a process and apparatus for performing 
such process directed to the purification treatment of waste water. 
Generally, such treatment involves the application of a pulsed 
oxygenation, fixed film process incorporating a means of oxygen transfer 
to the biological cell of the biomass which may herein be termed 
"interfacial transfer." The submerged fixed film process is designed 
either as an upflow or downflow system. 
More specifically, interfacial transfer involves oxygen transfer or 
absorption by the biomass based on a direct contact between the biofilm 
and surface of a gas or air bubble. This of course differs from the two 
film theory wherein the oxygen from the air is first dissolved into the 
surrounding liquid and then subsequently absorbed by the biomass directly 
from the liquid as versus from direct contact with the gas bubble itself. 
Therefore, interfacial transfer may be considered an additional pathway 
for oxygen transfer based on the recognition that a sparged gas bubble can 
be in contact with the fixed film biomass (biofilm) surface as well as the 
flowing liquid in the biological reactor. This additional oxygen transfer 
pathway, which is not described or encompassed by the two film theory, can 
account for a greater amount of oxygen transfer in a fixed film system 
than that described by the two film theory. Further, an important feature 
of the present invention is related to the action of sparged gas bubbles 
in a packed bed reactor. It has been found that based on the particular 
media size, media shape and biofilm growth, the sparged gas bubbles do not 
flow freely upward through the bed after exiting from the gas sparging 
device. Instead, bubbles can and do become trapped or prolonged in the 
void spaces of the media by the biological growth. Once the gas bubbles 
which are sparged into the system reach certain size, their buoyancy is 
sufficient to carry them to the top of the packed bed and out of the 
system. As the sparge rates increase, such as in a continuous flow 
sparging system, these gas bubbles are used less efficiently for direct 
oxygen transfer since their time in the packed bed reactor is decreased 
due to the more rapid formation of larger bubble sizes in the void spaces. 
The aforementioned inefficiency is thereby based on less time of the 
individual bubbles being spent in the media resulting in less time for 
interaction between the biological cells of the biomass and the film or 
surface of the bubbles themselves. Accordingly, by prolonging the time 
that the individual bubbles remain trapped within the voids of the media, 
contact is increased between the biomass and the bubbles and more oxygen 
is transferred per unit volume of sparged gas to reduce the quantity of 
sparged gas required to satisfy the oxytgen demand in a given biological 
reactor. This in turn reduces the energy required for biological treatment 
and reduces the disadvantage normally associated with continuous flow 
sparge systems. The sparge air contact and the contact time is increased 
in the process of the present invention by providing sparge gas air 
intermittently or at a selected pulsed rate to a fixed film system. The 
subject technique can also be applied to suspended growth activated sludge 
systems that use diffused air for oxygen transfer. 
In practice, a downflow or upflow reactor containing a suitable media for 
biological growth preferably having a predetermined effective size of 1-6 
mm and a depth of from 3-20 feet may be utilized. Any suitable media such 
as sand, anthracite, activated carbon, pumice, pea gravel or like material 
may be used as a suitable media. Air is provided by a distribution system 
located below or at the bottom of the fixed film media wherein the air 
percolates into the bed through a communication zone between the media and 
an underdrain. 
Contrary to certain prior art systems, the air is not applied continuously. 
Instead, the air supply is provided intermittently or at a pulsed rate at 
selected intervals. The ratio of air on time to air off time may be equal 
but could also be varied dependent upon the particular application and/or 
waste water concentration, etc. 
The media size selected and the biogrowth in the fixed film system results 
in trapping the air bubbles in the bed or more particularly in the voids 
between the media particles. If a continuous air application rate is used, 
the trapped bubbles are forced out of the bed to decrease the air bubble 
retention time. Thus, air is not used efficiently, and continuous air flow 
minimizes the contact time between the bubble and biological film. This 
reduction of contact time reduces the oxygen transfer and treatment 
efficiency. 
In the pulsed or intermittent application of the sparged air, the gas 
bubbles stay in the bed longer and a greater quantity of oxygen per unit 
of air supplied. Increased contact between the biofilm and the gas volume 
applied is achieved. Thus, oxygen transfer is controlled less by the two 
film mechanism and more by the "interfactial transfer" pathway to reduce 
total air application rates. As set forth above, this results in lower 
energy requirements compared to the continuous air flow systems prevelant 
in the prior art. 
During the interval when air is not applied to the media bed, more 
efficient filtration of solids occurs throughout the entire length of the 
media bed since agitation therein is minimized. During air application, 
solids filtration continues, but not as efficiently, due to contact 
between the biofilm and the solids. The overall result, however, is a 
filtration efficiency that is adequate for waste water treatment solids 
removal requirements. The process and attendant apparatus of the present 
invention may also be operated in a manner to accomplish nitrification and 
denitrification. However, the accomplishment of such steps will likely 
require more reactor volume than used for only BOD (biological oxygen 
demand) removal in that nitrification requires more volume.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
With regard to FIG. 1, the present invention is directed towards a pulsed 
air, fixed film purification process for the treatment of waste waters. In 
the performance of the subject process, a downflow packed bed reactor 
generally indicated as 10 is utilized. An upflow packed bed can also be 
used. The medium 12 of the packed bed preferably has an effective size of 
1-6 mm. and the media depth, in the preferred embodiment, may vary 
anywhere from 3-20 feet depending on the process design requirements. 
Further, design requirements relate to the media size and biological 
growth characteristics wherein such must be suitable to trap sparged gas 
bubbles within the void spaces of the media. Accordingly, suitable media 
material may be sand, anthracite, volcanic ash, pea gravel, activated 
carbon or other like material. 
Sparged air is provided by a distribution header or chamber 14 placed at 
the bottom of or below the packed bed media 12. Distribution of the 
processed air may occur from header or chamber 14 from the top of the 
underdrain system 16 schematically represented in FIG. 1. Alternately, the 
header 14' may be mounted essentially within the underdrain system 16 
wherein the underdrain system may comprise a clay, metal or synthetic tile 
or chamber 16' structured to include a plurality of distribution apertures 
17 formed over an upper or exposed surface thereof. The air header may 
also be located directly on top of the underdrain system just below the 
media. 
In practice, the reactor 10 may incorporate use of multiple cells as 
schematically and collectively represented in FIG. 3 as 20 and 20', 
wherein common air blowers or compressors 22 operating continuously 
supplies the sparged air to a group of cells such as 20 and 20'. The air 
flow is stopped to one or more of the cells at any given time. For 
example, if two cells are utilized, the air flow rate from blower 22 is 
pulsed intermittently at various time intervals between the two cells. 
This results in air ON/OFF time intervals being a function of the system 
design factors including the BOD loading rate, the waste water BOD 
concentration, the air sparge rate, the media size, and the media depth. 
Again with reference to FIG. 3, the ON/OFF sparge gas application to each 
cell 20 may be controlled by automatically operated valves 24 or 
alternately by a rotating mechanical device 26 that would open and close 
ports for an air manifold system generally indicated as 28 connected to 
the process air inlet line 30 (FIGS. 1 and 3) for each cell. The air 
ON/OFF period may typically vary from 10 to 30 seconds or to as much as 
30 minutes or more, again depending on the particular application for 
which the subject process is intended. It should be further noted that the 
process is capable of nitrification and denitrification application, to be 
described in greater detail hereinafter, wherein during such application 
the difference between the ON/OFF time would be longer depending on the 
packed bed reactor detention time. 
As best shown in FIG. 4 and as set forth above, the well-known "two film" 
theory is based on the fact that oxygen is first transferred from a gas 
bubble A into the surrounding liquid 30. For a fixed film system, it is 
acknowledged that the dissolved oxygen first diffuses across the stagnant 
film located between the biomass generally indicated as 34 and the flowing 
liquid 30. Once the oxygen diffuses across the film, it diffuses into the 
biomass bacterial cells comprising the biomass 34 for respiration and 
enzyme mediated oxidation-reduction reactions. Therefore, bubble A is 
shown in the flowing liquid 30 in a biological system and bubble B is in 
contact with the biofilm liquid film or biomass volume 34. Due to this 
contact, bubble B will be able to transfer oxygen to the biomass at a 
higher rate than bubble A even though both bubbles are identical in size, 
surface area, and oxygen partial pressure. Bubble A conforms to the two 
film theory resulting in an oxygen transfer rate as a function of the 
liquid dissolved oxygen concentration. Due to biological respiration, the 
dissolved oxygen concentration within the biofilm stagnant volume 34 is 
lower than in the liquid 30. Thus, the dissolved oxygen concentration 
dissolution driving force gradient for oxygen dissolution from bubble B is 
much greater than that from bubble A. Therefore, a high rate of oxygen 
transfer occurs due to this higher gradient of bubble B and also due to 
the shorter distance required for the diffusion of oxygen from bubble B to 
the biomass 34 compared to bubble A. As set forth above, this oxygen 
transfer pathway is herein termed "interfacial transfer" and can account 
for a greater amount of oxygen transfer in a fixed film system than that 
described by the two film theory. 
Further with regard to the application of air at intermittent intervals, 
when sparge air is not applied to the packed bed during the OFF part of 
the ON/OFF interval, the gas bubbles trapped in the voids of the packed 
bed continue to transfer oxygen to the biomass via the interfacial 
transfer pathway. If the biological respiration rate is extremely great, 
the partial pressure of oxygen in the gas bubble contacting the biomass 
will be depleted rapidly. In this case, the difference between the ON/OFF 
intervals has to be decreased to shorten time intervals to replenish 
oxygen in the gas bubbles in the void spaces. For a less active biological 
system, the difference between the ON/OFF intervals would be of longer 
duration, and the air flow rate would also be lower. 
Accordingly, the subject process reduces energy requirements for the 
sparged fixed film downflow packed bed reactor and results in efficient 
filtration of solids entering or produced in the bed in spite of air 
sparging throughout the bed depth. This is a result of the fact that the 
air is sparged intermittently and during the air OFF time, minimal 
agitation occurs in the packed bed. Small particles are more easily 
captured on the biofilm in the bed or adhered to the other particles 
trapped in the bed. The larger particles formed are more effectively 
trapped so that solids do not easily escape the packed bed during the air 
ON period which provides more agitation within the bed. The air ON period 
is short enough so that influent solids cannot reach the bottom of the 
packed bed via the liquid flow before the next air OFF period. Based on 
the above, a bottom filtration zone below the air header is not required. 
Therefore, the process and the accompanying structure for performing the 
subject process has the advantage of allowing the entire reactor depth to 
be made available for biological degradation of the waste water. An 
additional advantage is that a higher dissolved oxygen effluent is 
possible since oxygen is available throughout the entire media depth 12 
(FIG. 1). As set forth above, the subject process incorporating the 
downflow packed bed system may also be designed and operated to accomplish 
denitrification of the waste water and thus nitrogen removal in addition 
to nitrification. During the air ON time and part of the OFF time, 
sufficient oxygen should be available to allow nitrifying bacteria 
attached to the biofilm to oxidize ammonia to nitrite and nitrate. During 
the OFF period, oxygen will be depleted within the depth of the biofilm 
attached to the media. Nitrite or nitrate produced will diffuse into this 
depth to provide an electronic acceptor to satisfy the oxygen demand 
within this biofilm depth. The nitrite or nitrate is then reduced to 
nitrogen gas products and recirculation of the effluent may be provided to 
enhance the efficiency of nitrification-denitrification performance. 
Media modifications such as the use of dual media for a different 
embodiment of the preferred fixed film system is considered for the 
treatment of some waste waters. More specifically, a finer, more dense 
media may be used at the bottom of the media bed 12 to increase filtration 
efficiency. This could be a sand material wit anthracite above the lower 
portion. It could also be a garnet material located beneath the sand 
media. 
Yet another embodiment of the present invention comprises the utilization, 
in the subject process, of a probe means 40 used in combination with a 
micro-processor 42 (FIG. 3) wherein the probe and micro-processor or 
similar controller are specifically structured to sense dissolved oxygen 
in the effluent from the media bed or at an intermediate point in the bed 
and automatically control the air sparge rate and attendant ON/OFF time 
intervals. The probe, microprocessor 40, 42 combination will observe the 
rate of increase of the dissolved oxygen concentration during sparging 
and/or rate of decrease during the OFF time. The air sparge rate and the 
ON/OFF time intervals will automatically be adjusted by the microprocessor 
as a function of the rate of change of the dissolved oxygen 
concentrations. 
With reference to FIG. 5, another embodiment of the present invention is 
the use of pulsed air in activated sludge systems. As in the packed bed 
system of FIG. 1, the activated sludge process may include a multi-stage 
system consisting of a number of treatment cells and include zone of high 
oxygen demand. This can be accomplished by a staged activated sludge 
system. The first two stages 50 and 52 are smaller than the final stage 53 
to accomplish a high level of oxygen respiration per unit volume. This 
condition rapidly depletes the oxygen concentration in the activated 
sludge floc to greatly increase the oxygen transfer rate due to bubble 
contact with the floc. It should be noted that the pulsed air approach as 
set forth in the present invention can be used in a variety of activated 
sludge designs and configurations. It may also be used in systems where 
the mixing method is separate from the air sparge method. Diffused air 
activated sludge systems using short ON/OFF intervals could benefit from 
this approach. 
It should be further emphasized that the pulsed air concept would be 
applicable to other types of biological systems to maximize oxygen 
utilization efficiency from sparge gases and minimize energy requirements 
normally associated with continuous flow sparge systems. An upflow packed 
bed reactor 10' (FIG. 6) would be operated in a similar fashion as 
discussed with regard to downflow packed bed reactor 10 (FIG. 1). In this 
embodiment, processed air is introduced below the media bed 12 and an 
influent similarly is introduced below the media bed wherein effluent is 
taken off as indicated from the reactor 10'. An underdrain may be of the 
type as represented with regard to the embodiment of FIG. 7. 
Sparged air is provided by a distribution header for chamber 14 placed at 
the bottom of the media bed 12. Distribution of the processed air may 
occur by an alternate embodiment including delivery conduit 14" having the 
configuration represented in FIG. 7 and disposed on top of a synthetic 
tile or chamber 16' which may be structured to include a plurality of 
distribution apertures or other porous formation over an exposed surface 
thereof.