Activated carbon treatment of oxygenated wastewater

An integrated system for treating wastewater containing biodegradeable organic contaminants by oxygenation thereof in the presence of activated sludge in an enclosed oxygenation zone with at least 50% oxygen feed gas and removal of the residual organic contaminants by adsorption in an activated carbon adsorption zone. At least part of the oxygen-depleted vent gas discharged from the oxygenation zone is concurrently flowed upwardly through the adsorption zone with the oxygenated effluent to maintain aerobic biological conditions in the adsorption zone for physical adsorption and biochemical oxidization of the residual organic contaminants therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a system for treating wastewater 
containing biodegradeable organic contaminants by oxygenation in contact 
with active biomass followed by contacting with an activated carbon zone 
for removal of residual organic contaminants. 
2. Description of the Prior Art 
A common method for treating wastewater such as municipal sewage or 
industrial effluents to remove biodegradeable organic contaminants is by 
the activated sludge process. According to this process, the sewage with 
or without primary clarification is thoroughly mixed with 
oxygen-containing gas in the presence of aerobic microorganisms in the 
activated sludge. The organic matter contained in the water is thereby 
absorbed and biochemically oxidized by the activated sludge 
microorganisms. Subsequently the activated sludge is separated, e.g., by 
gravity settling, and the purified effluent is discharged into a receiving 
stream or body of water. 
While the activated sludge process is one of the most effective and 
economic wastewater treatment processes available today, it does not 
achieve complete purification. The effluent, as for example from a 
municipal activated sludge treatment plant, will contain some oxidizable 
material including biodegradeable organic matter representing residual 
biochemical oxygen demand (BOD). 
Until comparatively recently, atmospheric air has been employed as the sole 
source of oxygen in activated sludge plants. In recent years however, this 
system has been vastly improved by the use of high purity oxygen gas as 
the oxidant in the manner taught by U.S. Pat. Nos. 3,547,812 to 3,547,815, 
to J. R. McWhirter et al. In the practice of oxygenation of wastewater as 
taught by the McWhirter et al. patents, at least one enclosed covered 
oxygenation chamber is employed wherein the liquid undergoing treatment is 
intimately contacted in the presence of activated sludge with 
oxygen-enriched gas from an over-lying gas space to dissolve the oxygen 
necessary for aerobic biological activity. Such oxygenation systems 
provide substantial advantages over prior art treatment systems wherein 
atmospheric air is used as the oxidant in open aeration chambers. For 
example, the closed chamber oxygenation system is able to operate at 
biological suspended solids levels several times greater and aeration 
detention periods several times less than those of air aeration systems 
while maintaining comparable or better overall levels of treatment. Such 
advantages are a consequence of the higher mass transfer driving force for 
oxygen-enriched gas relative to air, which permits higher dissolved oxygen 
levels to be achieved with economic levels of volumetric oxygen transfer 
rate per unit of power input. In spite of these advantages however, closed 
chamber oxygenation systems still produce effluents which contain some 
small quantity of residual biodegradeable contaminants. 
It is known in the art to "polish" or post-treat the effluent from the 
activated sludge secondary treatment system by contacting the effluent 
with activated carbon to provide for removal of the residual organic 
contaminants in the wastewater. Such tertiary treatment has in fact proven 
effective in providing high overall adsorptive removals of total organic 
carbon (TOC) and the consistuent biochemical oxygen demand (BOD) from the 
wastewater due to the morphology of activated carbon which provides an 
extremely large surface area for physical absorption, e.g., 1200 to 1400 
meters.sup.2 per gram of activated carbon. 
Recent studies have shown that the absorptive capability of activated 
carbon for organic contaminants of wastewater can be enhanced by the 
promotion of aerobic conditions in expanded beds of activated carbon in 
which biological growth is allowed to develop on the activated carbon 
surfaces. In a paper by Weber, W. J., Jr., Friedman, L. D., and Blum, R., 
Jr., entitled "Biologically Extended Physical Chemical Treatment", 
presented at the Sixth International Conference on Water Pollution 
Research, Jerusaleum, in 1972, it was reported that an expanded bed 
activated carbon adsorption system operated under aerobic conditions and 
treating clarified primary effluent wastewater comprised of approximately 
75% domestic waste and 25% industrial waste with a total organic carbon 
(TOC) concentration in the range of 10-40 milligrams per liter had 
demonstrated better performance (approximately 15% higher TOC removal) 
than a corresponding anaerobic activated carbon system operated under the 
same conditions and had demonstrated a removal capability of nearly 70% by 
weight adsorption of organic material during nine months of continuous 
treatment. This performance was markedly superior to that predicted by 
saturation data obtained from measurements of absorption isotherms and 
suggested that the observed enchancement of the effective capacity could 
be attributed to bacteriological activity on the surfaces of the carbon 
substrate. Subsequent work in the field has borne out the existence of 
this mechanism as providing an in situ reactivation of the activated 
carbon adsorbent by biological assimilation of surface absorbed 
biodegradeable contaminants thereby providing a longer operating life for 
the adsorbent before saturation occurs and regeneration as for example by 
thermal reactivation is necessary. 
It is an object of the present invention to provide an improved system for 
treating wastewater containing biodegradeable organic contaminants by 
oxygenation followed by contacting with an activated carbon zone utilizing 
the above-described mechanism. 
Another object is to provide a system of the above type which is 
characterized by high oxygen utilization and low power consumption. 
Other objects and advantages of this invention will be apparent from the 
ensuing disclosure and appended claims. 
SUMMARY OF THE INVENTION 
This invention relates generally to a system for treating wastewater 
containing biodegradeable organic contaminants by oxygenation followed by 
contacting with activated carbon for removal of residual organic 
contaminants. 
More specifically, the method aspect of the invention relates to treatment 
of wastewater containing biodegradeable organic contaminants by 
oxygenation in contact with the active biomass including the steps of 
mixing the wastewater with activated sludge and feed gas containing at 
least 50% oxygen by volume in an enclosed oxygenation zone for sufficient 
duration to biochemically oxidize carbon food in the wastewater and form 
oxygenated liquor of reduced BOD content. Oxygen-depleted vent gas 
containing 20-70% oxygen by volume and the oxygenated liquor are 
discharged from the oxygenation zone. The oxygenated liquor is separated 
into effluent containing residual biodegradeable organic contaminants and 
sludge, and at least part of the sludge is recycled to the oxygenation 
zone as the aforementioned activated sludge therefor. 
The improvement in the method aspect of the invention resides in removing 
the residual organic contaminants from the effluent by cocurrently flowing 
the effluent and at least part of the vent gas from the oxygenation zone 
upwardly through an activated carbon adsorption zone. The vent gas is 
introduced to the adsorption zone at a rate sufficient to maintain aerobic 
biological conditions in the adsorption bed and maintain a dissolved 
oxygen concentration of at least 2 parts per million (p.p.m.) in the 
effluent being flowed therethrough for physical adsorption and biochemical 
oxidization of the residual organic contaminants in the adsorption zone. 
Organic contaminant-depleted effluent water and oxygen-depleted waste gas 
are discharged from the absorption zone. 
The apparatus aspect of the invention relates to a system for treating 
wastewater containing biodegradeable organic contaminants by oxygenation 
in contact with active biomass including an enclosed oxygenation vessel 
with means for introducing wastewater and activated sludge to the 
oxygenation vessel and conduit means for introducing at least 50% oxygen 
by volume feed gas at superatmospheric pressure to the vessel. Gas-liquid 
contacting means are positioned within the vessel for mixing of the 
wastewater, activated sludge and oxygen-containing gas therein to form 
oxygenated liquor. Gas vent means are provided for discharging 
oxygen-depleted vent gas from the vessel. Passage means are included for 
transferring oxygenated liquor from the oxygenation vessel to means for 
separating the oxygenated liquor into effluent containing residual 
biodegradeable organic contaminants and activated sludge. The apparatus 
also includes means for recycling the separated sludge from the separating 
means to the means for introducing activated sludge to the oxygenation 
vessel, and means for discharging effluent containing residual 
biodegradeable organic contaminants from the separating means. 
The improvement in the apparatus aspect of the invention relates to means 
for removing the residual organic components from the effluent. Such means 
comprise an adsorbent vessel with enclosing side and bottom walls 
containing a bed of particulate activated carbon adsorbent bearing against 
the side walls thereof. The bed is supported within the adsorbent vessel 
at its lower end and unconfined at its upper end. Means joined to the gas 
vent means are provided for introducing the oxygen-depleted vent gas 
discharged from the oxygenation vessel into the adsorbent vessel at its 
lower end, with means joined to the effluent discharge means for 
introducing the effluent containing residual biodegradeable organic 
contaminants into the adsorbent vessel at its lower end. Further means are 
provided for discharging oxygen-depleted waste gas and final effluent 
water from the adsorbent vessel at its upper end, to provide cocurrent 
upward flow of the introduced oxygen-depleted vent gas and effluent 
through the adsorbent bed and maintain aerobic biological conditions 
therein for physical adsorption and biochemical oxidization of the 
residual organic contaminants in the effluent flowed therethrough. 
In a particularly preferred aspect, the method and apparatus of the 
invention are suitably employed to treat wastewater containing 
biodegradeable organic contaminants by activated sludge in oxygenation 
wastewater treatment systems of the type as described hereinearlier and 
disclosed and claimed in McWhirter et al. U.S. Pat. Nos. 3,547,812 to 
3,547,815, incorporated herein to the extent pertinent. As used herein the 
term "aerobic conditions" means that viable aerobic microorganisms -- 
i.e., living aerobic species capable of biologically assimilating organic 
contaminants and dissolved oxygen to yield energy for respiration and 
cellular growth -- are present on the surfaces of the activated carbon 
adsorbent in the absorption zone. Such conditions are readily determinable 
as for example by oxygen uptake and respiration measurements performed on 
the adsorption zone, as described hereinafter. 
Operation in accordance with the present invention has been found to 
provide substantial enhancement of the removal capability of the 
wastewater treatment system by virtue of the fact of in situ reactivation 
of the activated carbon adsorbent, by biological renewal of the adsorbent 
active surfaces. The invention permits increased adsorption bed life to be 
achieved with increased residual organic contaminant removal as compared 
to straight adsorption and accomplishes these improvements more 
economically than would be the case if pure oxygen or air were employed to 
maintain aerobic conditions in the adsorption zone. In long term 
adsorption zone operation the oxygenation zone vent gas has been found to 
be superior to air and, unexpectedly, generally equivalent to pure oxygen 
in achieving extended adsorbent bed life -- i.e., the period during which 
the adsorbent bed is able to actively remove the residual organic 
contaminants from the activated sludge-treated effluent. Particular 
advantage has been found in the application of the invention to removal of 
acidic organic contaminants from wastewater, due to the carbon dioxide 
content of the oxygenation zone vent gas and the pH characteristics of the 
oxygenated effluent introduced to the adsorption zone. 
In summary the oxygenation zone vent gas has been found to be generally 
competitive with pure oxygen as an aeration medium for the adsorption zone 
from a performance standpoint (both are superior to air) and to be 
superior to pure oxygen from an economy standpoint. The present invention 
allows the further benefit, not available from separate use of air or pure 
oxygen, of operating the oxygenation activated sludge system at an 
increased oxygen feed rate thereby permitting a reduction in the oxygen 
dissolution power requirements for the oxygenation zone.

DESRCIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings, FIG. 1 shows a generalized schematic block 
diagram of a wastewater treatment system according to the invention. 
Influent BOD-containing wastewater, as for example sewage or industrial 
wastewater, enters the activated sludge secondary treatment system 3 in 
line 1. This influent wastewater may be subjected to primary treatment, 
upstream of the activated sludge secondary treatment, by chemical 
treatment, primary sedimentation or other well-known primary treatment 
steps. Feed gas containing at least 50% oxygen by volume is introduced to 
the activated sludge treatment zone in conduit 2. 
In the secondary treatment zone, the introduced feed gas and wastewater are 
mixed with activated sludge in an enclosed oxygenation zone. The enclosed 
oxygenation zone may be any of various commonly employed types, such as a 
contacting pipe through which wastewater, oxygen gas and activated sludge 
are passed under turbulent flow conditions to achieve the requisite 
mixing. Alternatively, the oxygenation zone may be of a type as disclosed 
in the aforementioned McWhirter et al. U.S. Pat. Nos. 3,547,812 to 
3,547,815, incorporated herein to the extent pertinent. Furthermore, the 
specific activated sludge process employed may be of various types 
including for example contact stabilization and extended aeration. 
Regardless of the specific activated sludge process employed, however, the 
wastewater, activated sludge and oxygen-containing gas are intimately 
mixed in the enclosed aeration zone for sufficient duration to 
biochemcially oxidize carbon food in the wastewater and form oxygenated 
liquor of reduced BOD content. 
After the requisite period of BOD conversion is completed, the resultant 
oxygenated liquor is discharged from the oxygenation zone and separated 
into activated sludge and purified effluent containing some residual 
biodegradeable organic contaminants. At least part of the separated 
activated sludge is recirculated to the oxygenation zone to maintain 
effective biological action on the influent wastewater. The separated 
purified effluent is discharged from the activated sludge secondary 
treatment system in line 4. 
During the oxygenation mixing treatment, oxygen from the feed gas is 
dissolved in the liquid undergoing treatment and utilized by the biomass 
-- i.e., sludge microorganisms -- for biological assimilation of the 
carbonaceous food in the wastewater. Carbon dioxide is formed as a product 
of the oxidation process and evolves into the gas phase, together with 
nitrogen, argon and other gases which are dissolved in the influent 
wastewater and stripped from the liquid phase during oxygenation. As a 
result the oxygen purity of the introduced oxygen feed gas declines during 
the oxygenation process and an oxygen-depleted gas, containing for example 
50% oxygen, 30% carbon dioxide and 20% nitrogen by volume is formed. This 
oxygen-depleted gas is vented from the activated sludge system in line 5, 
either at the end of the oxygenation step in a batch type process, or 
continuously or intermittently in a continuous process wherein the oxygen 
feed gas is introduced continuously or intermittently. 
In accordance with the present invention, the effluent in line 4 separated 
from the oxygenated mixed liquor in the activated sludge treatment and at 
least part of the vent gas flowing in line 5 are introduced to an 
activated carbon adsorption zone. In the adsorption zone, at least one bed 
of particulate activated carbon adsorbent is provided. The bed is 
supported in a containing vessel at its lower end and unconfined at its 
upper end to accomodate upflow expansion. The effluent introduced to the 
adsorption zone in line 4 and the vent gas introduced in line 5 are 
cocurrently flowed upwardly through the bed of activated carbon to expand 
the same, with the adsorbent-contacted effluent water being discharged 
from the adsorption zone in line 7 and oxygen-depleted waste gas being 
discharged in line 8. 
As discussed above, at least part of the vent gas from the oxygenation zone 
is passed to the adsorption zone. The exact portion of the vent gas so 
used will depend on the initial oxygen feed gas concentration, the oxygen 
utilization in the activated sludge treatment, the vent gas composition 
and the contaminant characteristics of the wastewater being treated, 
particularly the contaminants in the effluent passed from the activated 
sludge zone to the activated carbon contacting step, together with the 
hydraulic behavior characteristics of the activated carbon bed. With 
respect to the latter, the volumetric flow rate of the gas entering the 
adsorbent bed must be sufficient to ensure adequate vapor-liquid 
distribution in the bed. On the other hand the flow rate of vent gas to 
the adsorbent bed should not be so large relative to the liquid flow that 
channeling is induced by the formation of excessively large gas bubbles. 
Thus any excess vent gas not needed in the activated carbon treatment step 
is discharged from the treatment system through lines 6 upstream of the 
adsorbent zone. 
In the adsorption zone, the oxygen-depleted vent gas, containing 20-70% 
oxygen by volume, and preferably 40-60% oxygen by volume is introduced to 
the bed of activated carbon at a rate which is sufficient to maintain 
aerobic biological conditions in the adsorbent bed and maintain a 
dissolved oxygen concentration of at least 2 p.p.m. in the effluent being 
flowed therethrough. Under aerobic biological conditions, growth of viable 
aerobic microorganisms develop on the surfaces of the particulate 
activated carbon adsorbent. This growth permits the biodegardeable 
adsorbed contaminant species to be biologically assimilated by the biomass 
on the adsorbent particle, thereby removing the contaminant from the 
active sites on the activated carbon on which the contaminant was 
originally adsorbed. Such biological removal thus renews the active 
surface of the adsorbent for continued adsorption of sorbable 
contaminants. In this manner the capacity of the activiated carbon is 
appreciably enhanced relative to that obtained with straight adsorption so 
that the bed of activated carbon is able to remain on-stream in service 
for a considerably longer time than would be possible with straight 
adsorption before regeneration, as for example by backwashing or thermal 
regeneration, is necessary. 
It will be observed that the above-described biological surface renewal of 
the adsorbent will increase removal of both the residual biodegradeable 
organic contaminants in the activated sludge secondary treatment effluent 
and also the non-biodegradeable adsorbable contaminants therein, since the 
removal of the biodegradeable contaminants by biological assimilation will 
free those active adsorbent sites on which the biodegradeable species 
would otherwise subsist in the absence of biological activity and so makes 
those sites available to both biodegradeable and non-biodegradeable 
species. In this fashion the adsorbent particles provide a synergistic 
effect by supporting and retaining the biological growth in the bed and by 
retaining the adsorbed biodegradeable species in contact with the biomass 
for sufficient duration to permit biological assimilation of those 
species. 
In order to provide biological growth in the activated carbon bed, the bed 
may in practice be "seeded" with viable aerobic microorganisms upon 
start-up of the treatment system in the same manner as the secondary 
activated sludge treatment step is initiated, by introduction of cultures 
of the desired microorganisms into the treatment system under suitable 
nutrient and process conditions to ensure high growth rate and rapid 
stabilization of the microbial population. Alternatively, the tertiary 
activated carbon unit may be brought on stream after such start-up of the 
activated sludge segment of the system, with a small portion of the 
activated sludge being flowed with the secondary treatment effluent to the 
adsorbent bed, so that the activated sludge microorganisms adsorb on the 
activated carbon particles in the adsorbent bed and thus become available 
to effect biological removal of adsorbed biodegradeable contaminants 
therein. 
The presence of the requisite aerobic biological condition in the activated 
carbon adsorbent bed in operation may readily be determined by oxygen 
component mass balance calculations performed around the adsorption zone 
on the inlet and outlet streams associated therewith, with a concurrent 
determination of the oxygen uptake rate (OUR) taking place in the 
adsorbent bed. Alternatively, the aerobic biological condition may be 
determined by unsteady-state biological respiration measurements and 
calculation of the OUR for the adsorbent bed by calculational procedures 
analogous to those used in the steady-state case. 
To insure that adequate levels of biological activity take place in the 
adsorption zone, it is necessary in the practice of the present invention 
to maintain a dissolved oxygen concentration of at least 2 p.p.m. in the 
effluent flowed through the adsorption zone. Under such conditions the 
effluent contains the minimal level of oxygen necessary to sustain the 
microorganisms in the adsorbent bed in an active state. Although the 
effluent as discharged from the activated sludge treatment may contain 
significant levels of dissolved oxygen, as for example 4-6 p.p.m., such 
dissolved oxygen residuum would quickly be consumed in the inlet region 
adsorption zone and render the downstream portion of the zone anaerobic if 
the vent gas from the oxygenation zone were not flowed through the zone to 
maintain the necessary dissolved oxygen conditions. To secure high levels 
of aerobic biological removal per unit weight of adsorbent and to minimize 
the amount of adsorbent required in the adsorption zone, it is preferred 
in the broad practice of the present invention to maintain a dissolved 
oxygen concentration of at least 5 p.p.m. in the effluent being flowed 
through the adsorption zone. 
Although the foregoing description has been directed to the utilization of 
a single unitary adsorbent bed in the adsorption zone, it will be apparent 
that more than one adsorbent bed may be advantageously employed. Thus, for 
example, it may be desirable in some instances to employ multiple 
adsorbent beds in series with the effluent being passed sequentially 
through the serial beds either together with the oxygen-containing waste 
gas from a preceding bed or with separate introduction of portions of the 
oxygenation zone vent gas to each of the serial beds. The latter gas 
introduction arrangement has the advantage that oxygen-containing gas may 
be introduced to the respective beds in amounts proportioned with respect 
to the biodegradeability characteristics of the contaminants in the 
effluent being treated therein. In such manner, the oxygen inputs to the 
serial adsorbent beds in the adsorption zone may optimally varied to 
achieve the highest contaminant removals and utilization of oxygen in the 
vent gas for the particular wastewater being treated. In addition, the use 
of multiple adsorbent beds in the adsorption zone permits one or more beds 
to be taken off-stream for regeneration while maintaining continuous 
active adsorption treatment in other, on-stream beds. 
As discussed hereinearlier, in the Summary section, the present invention 
is based on the discovery that the waste gas discharged from an 
oxygenation activated sludge system can be utilized in an activated carbon 
adsorbent bed to sustain aerobic biological conditions and greatly prolong 
bed life before the sorptive capacity of the bed is exhausted and 
regeneration is necessary, to an extent far greater than that achieved by 
atmospheric air and roughly equivalent to that achieved by the use of pure 
oxygen gas as the aerating medium in the adsorption zone, and that in some 
instances the waste gas from the oxygenation zone is actually superior to 
pure oxygen in providing improved removal of total organic carbon (TOC) 
and biochemical oxygen demand (BOD) in the adsorption step as well as 
providing extended bed life. For example, it has been found in some cases 
involving the treatment of wastewater containing moderate concentrations 
of acidic organic components, the use of oxygenation zone vent gas in the 
adsorption zone has been observed to have enhanced the removal of these 
components by adsorption/bioxidation in the adsorption zone relative to 
the use of pure oxygen. 
It has also been found that under this invention sufficient utilization of 
oxygen may be achieved from the oxygenation zone vent gas in the 
adsorption zone to permit the feed rate of oxygen gas introduced to the 
oxygenation zone to be increased to levels which would otherwise be 
uneconomical if the oxygenation and adsorption zones were not integrated 
in the manner of this invention. Such increase in oxygen gas feed rate 
provides a significant performance advantage in reducing the energy 
requirements for oxygen dissolution in the oxygenation zone of the 
process. This advantage derives from the higher oxygen partial pressures 
in the oxygenation zone which are associated with a greater flow rate for 
the oxygen feed gas and which permit less power to be used in achieving 
the same level of oxygenation treatment relative to oxygenation systems 
operating with conventional oxygen feed gas flow rates. 
In the conventional enclosed chamber oxygenation systems operated according 
to the prior art, oxygen-depleted aeration gas is vented from the 
oxygenation chamber either intermittently or continuously to maintain a 
suitably high partial pressure in the gas phase therein, as for example at 
least 300 mm. Hg. Such partial pressure levels are necessary to achieve 
both a high percentage absorption from the oxygen feed gas and to maintain 
a high energy utilization efficiency for oxygen dissolution and mixing of 
the activated sludge biomass suspension, in order to obtain economic 
operation of the wastewater treatment system. In accordance with the 
present invention, economical operation can be obtained with significantly 
higher oxygen partial pressures in the oxygenation zone than 
conventionally employed, while simultaneously using substantially less 
power in the oxygenation step, as for example 13% less power than a 
corresponding oxygenation chamber not integrated in the manner of this 
invention, while achieving the same degree of wastewater BOD removal 
therein. 
The present invention also affords the advantage of providing a treated 
effluent having a moderately high dissolved oxygen concentration (D.O.) 
together with low BOD and chemical oxygen demand (COD). In addition to 
removing a major portion of the BOD/COD in the wastewater treatment 
system, it is usually desirable to fortify it with dissolved oxygen (DO). 
This is done so as to provide a quantity of dissolved oxygen along with 
the effluent which approaches or attains a value equivalent by any BOD or 
COD still remaining in the effluent. The object is to prevent such BOD and 
COD from contributing toward an oxygen-deficient condition in the 
receiving stream. In some instances, an additional amount of dissolved 
oxygen is also desirable over and above that which is required to satisfy 
the residual BOD or COD of the effluent. This additional DO is desired so 
as to actually improve the quality of the receiving water above the level 
it would possess absent the discharge of effluent. This is in recognition 
of the fact that natural waters receive pollution from sources other than 
a "closed" wastewater treatment system, e.g., run-off of rainwater, 
effluent upstream and downstream of the municipal boundaries, and 
unauthorized "dumping" or drainage of pollution within the municipality. 
The present invention achieves a dissolved oxygen fortification of the 
effluent which serves partially to overcome such difficulties and 
accordingly permits improvement of the quality of the receiving waters for 
the treated wastewater effluent, in addition to providing improvement in 
overall levels of wastewater treatment. 
FIG. 2 illustrates a specific apparatus configuration which may be used to 
practice one embodiment of the present invention. Influent wastewater 
containing biodegradeable organic contaminants is introduced to the 
enclosed oxygenation vessel 10 through line 11. Recirculated activated 
sludge is introduced to the oxygenation vessel in line 13, although it may 
be desirable in some instances to commingle the recycle activated sludge 
directly with the influent waste-water in line 11 by joinder of conduit 13 
thereto so as to effect introduction of both wastewater and activated 
sludge to the oxygenation vessel through line 11. 
As shown, the enclosed oxygenation vessel 10 is in the form of a liquid 
storage enclosure with vertical partition walls 45a-b and 45b-c spaced to 
provide three chambers 10a, 10b and 10c as oxygenation stages. The 
partition walls extend substantially to the floor of the storage enclosure 
and are joined thereto in fluid-tight relation. Flow of oxygenated liquid 
(liquor) through the stages of the oxygenation zone is provided by 
restricted flow opening 46 in the lower portion of the first-second 
chamber common partition wall 45a-b, and restricted opening 47 in the 
lower portion of the second-third common partition wall 45b-c. Unconsumed 
oxygen-containing gas flows through the oxygenation zone from chamber to 
chamber through restricted openings 48 near the top of the partition 
walls. The liquid storage enclosure is enclosed by a common cover 49 
leak-tightly joined to the upper ends of the chamber partition walls to 
form gas spaces in each of the chambers. Accordingly, back mixing of 
oxygen-containing gas within the liquid storage enclosure from a 
succeeding chamber to a preceding chamber is avoided as long as a slight 
pressure differential is maintained. 
In the illustrated system the wastewater and activated sludge introduction 
lines 11 and 13 are disposed to introduce the wastewater and activated 
sludge to the first oxygenation chamber 10a. Sludge is recycled to the 
first oxygenation chamber 10a at a rate sufficient to maintain the desired 
total solids concentration (MLSS), as for example 6000 milligrams/liter 
(mg/l), and volatile suspended solids concentration (MLVSS), as for 
example 4500 mg/l. Broad suitable ranges for these parameters are 
2500-10,000 mg/l., MLSS and 2000-8000 mg/l MLVSS. The food-to-biomass 
ratio may be in the range of 0.2 - 1.55 gm BOD/ (day x gm MLVSS), for 
example about 0.68. The recycled sludge concentration is in the range of 
10,000 - 50,000 mg/l. 
Feed gas containing at least 50% oxygen by volume and preferably at least 
80% oxygen by volume is introduced in sufficient quantity to maintain 
dissolved oxygen concentration in the liquid in the oxygenation vessel at 
2-10 mg/l and for example 6 mg/l. Oxygen flow control valve 12a may be 
automatically adjusted in response to sensed D.O. level or to gas pressure 
in the overhead gas spaced as monitored by suitable sensor and 
transmitting means (not shown). 
Gas-liquid contacting means are positioned in each of the oxygenation 
chambers for mixing the oxygen-containing gas and liquor and 
simultaneously continuously circulating the oxygen-containing gas against 
the wastewater-activated sludge liquor therein. These gas-liquid 
contacting means for each chamber include blades 15 submerged in the 
liquor and joined by a rotatable shaft 50 to suitable drive means such as 
gearbox and motor 51. The fluid circulation means comprise withdrawal 
conduit 17 joined to the cover 49, compressor 16, return conduit 52 in 
flow communication with the inlet side of hollow shaft 50 driven by motor 
51, and sparger 16a positioned at the lower end of the shaft 50 beneath 
blades 15. Small oxygen-containing gas bubbles are discharged from the 
sparger 16a by the pressure of the pump 16 and are distributed through 
each chamber in intimate contact with the wastewater-activated sludge 
liquor and rise through the liquor to the surface where the unconsumed 
portion disengages into the gas space along with the oxidation reaction 
product gases. Gas-liquid contacting aeration devices are commonly rated 
by the so-called "air standard transfer efficiency" which identifies the 
capability of the device to dissolve oxygen from air into zero D.O. tap 
water at one atmospheric pressure and 20.degree. C. Suitable devices are 
those which have an air standard transfer efficiency of at least 1.5 lb. 
O.sub.2 per HP - hr. For these purposes the power used in rating the 
device is the total power consumed both for agitating the liquor and for 
gas-liquid contacting. 
In the first oxygenation chamber 10a, the influent wastewater, recycle 
activated sludge and introduced oxygen feed gas are mixed and the 
oxygen-containing gas is simultaneously continuously recirculated against 
the wastewater-activated sludge suspension for sufficient duration, as for 
example 20 - 30 minutes, to form a first oxygenated liquor and first 
unconsumed oxygen-containing gas. The resultant first oxygenated liquor is 
transferred from the first oxygenation chamber to the second oxygenation 
chamber through the restricted flow opening 46 in the first - second 
chamber partition wall 45a-b. Simultaneously, the first unconsumed 
oxygen-containing gas from the first oxygenation chamber is passed to the 
second oxygenation chamber through restricted passageway opening 48 in 
partition wall 45a-b. In the second oxygenation chamber 10b, the 
so-transferred first oxygenated liquor and first unconsumed 
oxygen-containing gas are mixed and the oxygen-containing gas is 
simultaneously continuously recirculated against the liquor for sufficient 
duration, which may again be on the order of 20-30 minutes, to form second 
oxygenated liquor and second unconsumed oxygen-containing gas of lower 
oxygen purity than the first unconsumed oxygen-containing gas. From the 
second oxygenation chamber the second unconsumed oxygen-containing gas and 
second oxygenated liquor are respectively passed through partition wall 
45b-c openings 48 and 47 into the third oxygenation chamber 10c for final 
mixing and recirculation therein by the third gas-liquid contacting means 
to form finally oxygenated liquor and oxygen-depleted vent gas. The 
oxygen-depleted vent gas, containing 20-70% oxygen by volume and for 
example 50% oxygen is discharged from the final oxygenation chamber 10c 
through gas vent line 22. 
In the above-described oxygenation zone, the oxygenated liquor from each 
stage is discharged and preferably introduced to the next succeeding stage 
in concurrent flow relation with the unconsumed oxygen-containing gas for 
mixing and recirculation of the fluids in the subsequent stage. Cocurrent 
gas-liquor flow through a multiplicity of oxygenation stages is preferred 
to satisfy the inherent variation in BOD or the water to be treated and of 
the succeeding partially oxygenated liquor. The feed gas representing the 
highest purity oxygen is contacted with the feed wastewater in the first 
stage. Accordingly, the first or feed gas stage has the highest oxygen 
partial pressure and thus the highest oxygen transfer potential. Therefore 
the high oxygen demand in this stage can be supplied without excessive 
power consumption. 
The oxygenated mixed liquor is discharged from the third and final 
oxygenation chamber 10c through discharge flow conduit 19 and is 
introduced to clarifier 18 for separation into supernatent purified liquor 
and activated sludge. The mixed liquor enters the clarifier via stilling 
well 56 which serves to distribute the mixed liquor uniformly in the 
clarifier and to damp any excessive velocities of the introduced liquor 
flow which may otherwise detrimentally affect the separation efficiency of 
the clarifier. Clarifier constructions are well known to those skilled in 
the wastewater treatment art and may for example include rotatable scraper 
53 at the lower end to prevent coning. The activated sludge is withdrawn 
through bottom conduit 21 and at least part, e.g. 85 percent by weight 
thereof, is recycled through pump 54 in conduit 13 to the first 
oxygenation chamber 10a for mixing with BOD-containing feed water and 
oxygen-containing feed gas. Any excess sludge produced in the oxygenation 
treatment may be delivered to waste or further treatment steps, e.g. 
aerobic digestion, by waste conduit 55. The purified effluent liquid is 
removed in clarifier 18 by overflow into weir trough 57 and discharged 
from the clarifier in line 58. 
The purified effluent liquid discharged from the clarifier of the activated 
sludge zone in line 58 will contain some residual biodegradeable organic 
contaminants which have not been biologically assimilated by the activated 
sludge biomass in the oxygenation zone. For example, with a moderately 
high strength industrial/municipal wastewater entering the treatment 
system, e.g. on the order of 600 mg/l BOD, the BOD of the effluent from 
the activated sludge segment of the system may be as high as 80-100 mg/l. 
This effluent is pressurized in liquid pressurizing pump 59 in line 58 to 
a pressure, e.g. 10 to 20 psig, sufficient to overcome the head loss 
across the downstream adsorbent bed 75. As indicated earlier, this liquid 
stream may contain a significant D.O. concentration residuum from the 
oxygenation step, as for example 5 mg/l and is saturated in highly soluble 
CO.sub.2 with respect to the oxygen-containing gas in the final 
oxygenation stage gas space. 
The oxygen-depleted vent gas discharged from the final oxygenation chamber 
10c, containing for example 50% oxygen by volume, may have a pressure of 
only a couple of inches of water at the point of discharge into the vent 
line 22. Any excess vent gas not required for the further adsorption step 
is wasted from the system through line 23. The remaining vent gas in line 
22 is joined with recycle gas from line 25 and the resultant combined 
stream is pressurized in blower 24 to a pressure which may be on the order 
of that of the liquid in line 58 which has been pressurized by pump 59, 
e.g. 10-20 psig. 
The pressurized liquid in line 58 is introduced to the lower end of 
adsorbent bed 75 and the pressurized gas in line 22 is introduced thereto 
via sparger device 68 disposed in the plenum space 64. The adsorption zone 
75 includes an adsorbent vessel 60 with enclosing side and bottom walls 
containing a bed 61 of particulate activated carbon adsorbent bearing 
against the side walls of the adsorbent vessel. The activated carbon may 
be in granular form with an average mesh size of about 8 mesh. The 
adsorbent bed 61 is supported within the adsorbent vessel at its lower 
end, resting upon a small volume of particulate refractory material which 
in turn is supported by a support screen or grid 63 fixedly joined to the 
side walls of the vessel. The purpose of the refractory materials, which 
may be in the form of small diameter balls, is to trap any particulate 
solids which have not been separated in the clarifier 18 and which might 
tend to clog the bed of finely sized activated carbon granules, and also 
to prevent attrition of activated carbon by abrasion of the adsorbent 
against the support screen 63. If the suspended solids in the effluent 
from clarifier 18 are relatively high, it may be desirable to employ 
suitable filter means in line 58 upstream of the adsorption zone for 
removal of a major part of the suspended solids. It will be appreciated 
that in some instances it may not be necessary to utilize such support 
media layer 62, as where the adsorbent utilized is highly 
abrasion-resistant and where the level of solids in the effluent from the 
clarifier 18 is suitably low. 
The adsorbent bed is unconfined at its upper end 65 in the adsorbent vessel 
to allow for free rise and expansion, i.e. partial fluidization by the 
upflowing effluent liquid and oxygen-containing gas dispersion. 
Oxygen-depleted waste gas disengaging from the contacted liquid at the 
gas-liquid interface 69 is collected in the upper plenum space of the 
adsorbent vessel and discharged in waste gas discharge line 71. A portion 
of this waste gas, if necessary for efficient hydraulic behavior of the 
adsorbent bed, or desirable for high utilization of the oxygen in the vent 
gas passed to the adsorbent vessel, may be recycled through line 25 
joining the waste gas discharge line 71 with the vent gas line 22 to 
augment the gas being introduced to the adsorbent bed. Final effluent 
water, depleted in residual biodegradeable organic contaminants is 
discharged from the adsorbent vessel above the interfacial level 65 of the 
expanded adsorbent bed, through line 70. 
By means of the foregoing arrangement cocurrent upward flow of the 
introduced oxygen-depleted vent gas and effluent through the adsorbent bed 
is provided and aerobic biological conditions are maintained in the 
adsorbent bed for physical adsorption and biochemical oxidization of the 
residual organic contaminants in the effluent flowed through the adsorbent 
bed. The oxygen-depleted vent gas and effluent are intimately commingled 
in the expanded adsorbent bed and the extended surface area of the 
particulate adsorbent functions as contacting area to enhance the 
gas-liquid contact. In this fashion oxygen in the introduced vent gas is 
dissolved in the cocurrently flowing effluent and made available to the 
biological growths on the active surfaces of the adsorbent particles. 
Expanded bed operation permits the biological growths on the particles to 
reach appreciable size before backwashing is necessary, without clogging 
of the interstital flow areas between the adjacent adsorbent particles in 
the bed by extended growth. Expanded bed operation also renders the 
adsorbent bed highly resistent to plugging by residual suspended solids in 
the effluent passed to the adsorption zone from the clarifier. As 
discussed hereinabove, the volumetric flow rate of gas introduced to the 
adsorbent bed must be adequate to ensure proper liquid-gas distribution. 
For a bed as shown and described in connection with FIG. 2 the volumetric 
vent gas flow rate to the bed may for example need to be at least 10% of 
the volumetric flow rate of effluent entering the bed to ensure sufficient 
liquid-gas distribution. Under such conditions, the final effluent water 
discharged from the adsorption zone in line 70 to final receiving waters 
may have a D.O. level of about 4 mg/l together with a negligible level of 
contaminants therein. The provision of such high quality effluent may be 
achieved with 90-95% utilization of the oxygen supplied in the feed gas to 
the system, and at a significantly lower expenditure of power in the 
oxygenation step than could be achieved without the present invention. 
In the activated sludge process, the recycle sludge for the oxygenation 
step consists essentially of flocculent agglomerates of aerobic organisms 
which have the ability, in the presence of sufficient dissolved oxygen, to 
first absorb, then assimilate and oxidize the organic material of the feed 
water. This adsorption-assimilation sequence occurs in the oxygenation 
zone of the present process, and in the FIG. 2 embodiment the sequence is 
substantially completed upstream of the separation zone 18. The 
wastewater-activated sludge cuontact time in oxygenation zone 10 for 
organic food adsorption-assimilation may for example be between about 10 
minutes and 24 hours. This time varies depending upon the strength (BOD 
content) of the wastewater, the type of pollutant, solids level in 
aeration and temperatures. A maximum retention period of 24 hours will 
usually provide adequate time to remove BOD from effluent, to activate the 
sludge, and perform a reasonable degree of autooxidation (endogeneous 
respiration) if desired. When several oxygenation stages are employed, the 
retention period in the oxygenation zone refers to the total time the 
biomass solids (the total bacteria present) together with the BOD of the 
feed water are held in all oxygenation stages. In the FIG. 2 embodiment 
where the oxygenation zone 10 is entirely upstream of the clarifier 18, 
the contact time is calculated as the total liquid volume of stages 10a, 
10b and 10c divided by the volumetric flow rate of combined BOD-containing 
feed water and activated sludge recycle. In the FIG. 3 embodiment wherein 
the oxygenation zone 130 is partly upstream and partly downstream 
clarifier 129 (as discussed hereinafter in detail), the contact time for a 
given quantity of BOD plus biomass is calculated as the sum of the contact 
times upstream and downstream of the intermediate clarifier. The upstream 
contact time is calculated by dividing the upstream liquid volume of 
stages 130a and 130b by the volumetric flow rate of combined 
BOD-containing feed water and activated sludge recycle. The downstream 
contact time is calculated by dividing the downstream liquid volume of 
stage 130c by the sludge discharge rate from the clarifier flowing to 
stage 130c. 
It is desired to extend oxygen treatment on the biomass beyond the period 
required to assimilate and oxidize the wastewater's BOD, then the fraction 
of the organisms of the sludge which themselves are destroyed and consumed 
by biological oxidixation can become significant. Carrying (endogeneous 
respiration) to an extreme in the oxygenation zone should be avoided 
because it reduces the activity of the biomass to be recycled in the 
return sludge and impairs its settlability. Moreover, retaining the 
treated biomass under long term oxygenation is expensive because liquid 
enclosure tankages become prohibitively large and power consumption is 
greatly increased. 
If the sole objective is to assimilate and oxidize the BOD of the influent 
BOD-containing water, than a relatively short contact time will suffice. 
Contact times not exceeding 180 minutes are usually satisfactory in 
multiple staged-cocurrent flow oxygenation systems treating relatively low 
strength municipal waste liquids, e.g. having up to about 300 mg/l. BOD. 
For higher strength wastes as for example those discharged from 
petrochemical plants, longer contact times on the order of 5-12 hours are 
necessary to yield an effluent of comparable purity. 
In the practice of this invention, it is preferred to maintain high 
suspended solids concentration in the oxygenation zone. The BOD-containing 
water-activated sludge volatile suspended solids content is preferably at 
least 3,000 mg/l. One reason for this preference is that the solids 
concentration in the oxygenation zone affects the rates of the biochemical 
reactions occurring in the system. In the treatment of municipal sewage, 
the suspended solids comprise: (1) biologically oxidizable organic 
material, (2) non-biologically oxidizable organic material, and (3) 
non-oxidizable non-organic material. The non-organic material such as sand 
and grit, and the non-biologically oxidizable material such as 
polyethylene particles or cellulose are undesirable but unavoidable 
components of the BOD-containing water, e.g., sewage, entering the 
aeration zone. Normally relatively large particles, e.g. wood chips, are 
usually removed in a pretreatment step. 
The major fraction of the total solids in the mixed liquor, e.g., 70 
percent thereof, consists of bacterial floc (biomass) in the activated 
sludge recirculated from the clarifier to the oxygenation zone. The higher 
the concentration of bacteria, the more rapid will be the adsorption and 
assimilation of BOD, assuming other requirements are also met such as 
dissolved oxygen supply. 
FIG. 3 is a schematic view of another apparatus embodiment of the invention 
adapted for contact stabilization treatment of wastewater. This apparatus 
differs from the previously described multi-stage system in that clarifier 
129 is positioned intermediate between second oxygenation chambers 130b 
and third oxygenation chamber 130c. The oxygenation zone is thus divided 
into two sections, the first comprising stages 130a and 130b and the 
second section comprising stage 130c. The advantage of this arrangement is 
that only the reduced volume stream of concentrated activated sludge is 
processed in the third oxygenation chamber downstream from clarifier 129. 
More specifically, BOD-containing wastewater, and the recycle activated 
sludge joined therewith from recycle line 115, are introduced in line 111 
to the first oxygenation chamber 130a in liquid storage enclosure 180 
enclosed by cover 149. Feed gas containing at least 50% oxygen by volume 
is introduced in conduit 112, having feed gas flow control valve 113 
therein, to the first oxygenation chamber. In the first oxygenation 
chamber, the wastewater, activated sludge and oxygen feed gas are mixed by 
gas-liquid contacting means comprising a rotatable surface impeller 122a 
and drive means 117a therefor. The rotatable surface impeller throws 
massive sheets of liquor into the enclosed gas space above the liquor 
level and thereby performs the fluid recirculation function. The mixing 
and recirculation are performed in the first oxygenation chamber 130a for 
sufficient duration, e.g., 10 to 20 minutes, for the biomass present to 
partially adsorb the BOD and to form first oxygenated liquor and first 
unconsumed oxygen-containing gas. 
The first oxygenated liquor and first unconsumed oxygen-containing gas are 
cocurrently flowed from the first oxygenation chamber 130a to the second 
oxygenation chamber 130b through restricted gas and liquor flow openings 
in the partition wall therebetween extending from the floor of liquid 
storage enclosure 180 to cover 149. In the second oxygenation chamber the 
transferred gas and liquor are further mixed and gas is recirculated 
against the liquor by second gas-liquor contacting means comprising 
rotatable impeller 122b and drive means 117b. Such mixing and fluid 
recirculation is carried out for sufficient time, which may again be on 
the order of 10-20 minutes to complete the BOD adsorption by the biomass 
to form second oxygenated liquor and second unconsumed oxygen containing 
gas. The oxygenated liquor overflows internal weir 114 in the second 
oxygenation chamber and is discharged from the oxygenation chamber in line 
127. This oxygenated liquor is passed to the clarifier separating means 
129. In the clarifier, the terminal outlet end of conduit 127 is disposed 
within concentric baffle 128. The baffle serves to form a stilling well 
for the oxygenated liquor introduced to the clarifier by the conduit and 
preferably extends from above the liquid level in the clarifier to a point 
intermediate this level and the clarifier's conical bottom. In the 
clarifier the oxygenated liquor is separated by the settling out of 
activated sludge solids. Motor 130 drives a slowly rotating rake across 
the clarifier bottom to prevent "coning" of the dense settled sludge. In 
the clarifier, a solids-depleted purified supernatent liquid is formed 
which rises in the clarifier to the liquid-air interface and overflows 
weir 126 into the associated trough for discharge through conduit 134. The 
settled BOD-enriched sludge is withdrawn through conduit 150 and at least 
a portion thereof is pressurized by pump 151 for flow thereof to third 
oxygenation chamber 130c. The third oxygenation chamber is provided in 
liquid storage enclosure 181 having cover 146 gas-tightly joined to the 
upper ends of the side walls of the liquid storage enclosure. Third 
gas-liquid contacting means are provided in the third oxygenation chamber 
comprising rotatable bladed impeller 122c and motor drive means 117c. 
Unconsumed oxygen-containing gas is flowed from the oxygenation zone first 
section (stages 130a and 130b) to the oxygenation zone second comprising 
oxygenation chamber 130c in transfer line 152 having flow control valve 
means 153 disposed therein. 
In the third oxygenation zone 130c the second unconsumed oxygen-containing 
gas and BOD-enriched sludge are mixed and sludge is recirculated agaist 
the overlying gas for sufficient time for the sludge to assimilate the BOD 
and form activated sludge and third unconsumed oxygen-containing gas. The 
activated sludge is withdrawn from the third oxygenation zone in line 137 
and is at least partly recycled through conduits 135 and 115 to the first 
oxygenation zone 130a as the activated sludge therefor. The conduits 135 
and 115 are joined by recycle sludge pump 136. The volume ratio of 
recycling active sludge/BOD-containing water may be maintained in the 
range of 0.1 to 0.5. This ratio may be maintained by varying the speed of 
pump 136. Any sludge not needed for recirculation may be removed from the 
system through sludge waste line 139 having control valve 138 therein. 
The third unconsumed oxygen-containing gas is discharged from the third 
oxygenation chamber 130c as vent gas containing 20-70% oxygen by volume in 
line 123. At least part of this vent gas is diverted into line 140 for 
passage to the activated carbon tertiary treatment step. Any excess vent 
gas not needed in the tertiary treatment step is discharged from the 
system through valve 141. 
In the tertiary treatment, the effluent from the clarifier 129 in line 134 
containing residual biodegradeable organic contaminants is pressurized in 
liquid pressurizing pump 132 to a level sufficient to overcome the 
pressure drop across the adsorption zone. Likewise the oxygen-dpeleted 
vent gas diverted into line 140 is pressurized to suitable pressure level 
by blower 131. The pressurized effluent liquid in line 134 is then joined 
with a first part of the pressurized vent gas in line 140 diverted by 
branch conduit 158. The resultant gas-liquid mixture is introduced into 
the lower end of first adsorbent vessel 160, into the lower plenum space 
168 through sparger device 171 which may for example comprise a porous 
ceramic plate sparging surface. 
As in the previously described system, the adsorbent vessel 160 has 
enclosing side and bottom walls and contains a bed of particulate bearing 
against the side walls thereof. The bed is supported in the vessel at its 
lower end upon support grid 166 and is unconfined at its upper end to 
allow expansion of the bed during operation, as for example by 5 to 15 
volumes %. Oxygen-depleted waste gas disengages from the contacted liquid 
at the liquid-gas interface 175 in the upper plenum space in the adsorbent 
vessel and is discharged from the system in line 177. The effluent flows 
upwardly through the bed cocurrently with the simultaneously introduced 
gas, from the lower plenum space 168 upwardly to above the upper surface 
173 of the expanded adsorbent bed and is discharged in line 170 from the 
adsorbent vessel. In this arrangement, aerobic biological conditions are 
maintained in the adsorbent bed for physical adsorption and biochemical 
oxidization of the residual organic contaminants in the effluent flowed 
therethrough. 
Intervessel conduit 170a is joined to the first effluent water discharge 
line 170 for transferring the first effluent water from the first 
adsorbent vessel 160 into the second adsorbent vessel 161 at its lower 
end. Prior to such introduction a second part of the diverted vent gas in 
line 159 is joined with the first effluent water in conduit 170a to form a 
gas-liquid mixture which is passed into the lower plenum space 169 in 
vessel 161 via sparger 172. The construction of the second adsorbent 
vessel 161 is similar to that of the first vessel 160. The former contains 
a bed of particulate activated carbon adsorbent supported on grid 167 in 
flow communication with the upper plenum gas space in the vessel for 
discharging second oxygen-depleted waste gas from the vessel, and line 179 
for discharging second effluent from the second adsorbent vessel at its 
upper end. Thus, the introduced oxygen-depleted vent gas and first 
effluent water cocurrently flow upwardly through the second adsorbent bed 
and maintain aerobic bacteriological conditions therein for physical 
adsorption and biochemical oxidization of the residual organic 
contaminants in the effluent flowed through the adsorbent bed. In the 
adsorbent vessel the bed is expanded to a height corresponding to the 
interface 174 and contacted vent gas disengages from the contacted liquid 
at the gas-liquid interface 176. 
The treatment system of FIG. 3 may suitably employ a control circuit for 
regulating the proportion of oxygen-depleted vent gas which is passed to 
the adsorption zone, in order to maintain a predetermined D.O. 
concentration in the final effluent water discharged from the adsorption 
zone and thereby insure optimal maintenance of aerobic bacteriological 
conditions in the constituent adsorbent beds. The control system for the 
FIG. 3 system comprises a dissolved gas probe 143 for sensing the disolved 
oxygen concentration of the effluent water discharged from the system in 
line 179, with D.O. sensing transmitting means 144 coupling the probe 143 
and controller 146, which in turn is linked by control signal transmitting 
means 145 to the actuator 142 on vent gas valve 141. In practice probe 143 
is preferably of a type which generates a difference in electrical 
potential between an electrode immersed in the effluent water and a 
reference electrode isolated from the liquid. The potential difference 
signal generated by probe 143 may then be transmitted by wire 144 to 
controller 146 for generation of a valve actuator control signal in 
response to the sensed dissolved oxygen concentration which is then 
transmitted to valve actuator 142 to cause opening or closing of valve 141 
to the desired extent so as to maintain a predetermined dissolved oxygen 
concentration in the effluent water discharged in line 179. 
Thus, if the D.O. in the effluent water drops below the set point level due 
to insufficient introduction of vent gas to the adsorption zone the 
controller-activator elements operate to further close the valve 141 so 
that correspondingly more vent gas is diverted to the adsorption zone in 
line 140. In this manner the predetermined D.O. level in the effluent 
water in line 179 is maintained despite variation in liquid loading and 
biodegradeability characteristics of the contaminants in the wastewater 
being treated. In accordance with the invention, the effluent water in 
line 179 must contain at least 2 p.p.m. D.O. and preferably at least 4 
p.p.m. D.O. to insure that efficient aerobic biological conditions are 
maintained in the adsorption zone. Below 2 p.p.m. D.O., the biological 
growth in the adsorbent beds is limited to ineffective levels since there 
is insufficient oxygen concentration driving force to penetrate thickness 
of the adherent biomass layers associated with attached growth. As a 
result, comparatively low levels of biodegradeable organic contaminant 
removal are achieved in the activated carbon treatment. In addition, below 
2 p.p.m. D.O., the activated carbon adsorption zone tends to run at least 
partially anaerobic with formation of noxious hydrogen sulfide gas in the 
bed and low biodegradeable organic contaminant removal levels. With the 
introduction of vent gas from the activated oxygenation zone to the 
adsorption zone it is possible to overcome such problems with high 
dissolved oxygen levels, which may in practice be as high as 8-10 mg/l. 
without excessive compression power expenditure in pressurizing the liquid 
and vent gas streams entering the zone. 
Although the foregoing description has been directed to the use of 
partially expanded adsorbent beds, it may be desirable in some 
applications to employ fluidized beds of particulate activated carbon 
adsorbent in the adsorption zone. Alternatively, pulsed beds of activated 
carbon could be employed wherein increments of the spent (loaded) carbon 
are removed in a direction countercurrent to the wastewater flow for 
thermal regeneration. The regeneration of the activated carbon adsorbent 
in the specifically described embodiments of the invention may be carried 
out in any suitable manner, by conventional methods as are well known in 
the art. 
One of ordinary skill might conclude that discarding the vent gas from the 
oxygenation zone and feeding pure oxygen separately to the adsorption zone 
would achieve performance which is superior to that obtained when gas from 
the oxygenation zone is reused in the adsorption zone. For example, if 60 
percent oxygen utilization were obtained in each of the two zones on 
independent oxygen feed streams, the overall oxygen utilization is also 
about 60 percent. Moreover, feeding pure oxygen to the adsorption zone 
would appear to provide a higher oxygen partial pressure in the adsorption 
zone as compared to the integrated oxygen system of this invention wherein 
CO.sub.2 and nitrogen evolved in the oxygenation zone are carried forward 
into the adsorption zone. This presumption is strengthened when CO.sub.2 
equilibrium effects are considered. The large volume of the mixed liquor 
flowing through the oxygenation zone provides large holding capacity for 
dissolved CO.sub.2. As a result, the major portion of the CO.sub.2 
resulting from oxidation of BOD remains in solution and does not 
contaminate the oxygenation gas. Nonetheless, the effluent from the 
activated sludge step is flowed at moderate flow velocity over the 
adsorbent particles in intimate admixture with the oxygen-containing gas, 
so that conditions are created which would appear to be conducive to 
release of CO.sub.2 in large amounts in the adsorption zone. Futhermore, 
the use of separate feed oxygen gas in the adsorption zone would appear to 
be superior to the reuse of oxygenation zone vent gas for the reason that 
the vent gas, even though it may have a relatively high oxygen 
concentration, e.g. 50-60% by volume, in some instances, will have a flow 
rate which is only a small fraction of the oxygen feed gas flow rate. 
Thus, the actual physical volume of oxygen in the vent gas which is 
available for further utilization is small and this in fact has been a key 
to the economic success of the recently developed oxygenation systems, 
which `lose` through venting only a very small amount of the oxygen fed to 
the system. Accordingly, the vent gas, containing only a small volume of 
oxygen which additionally is heavily diluted by CO.sub.2, nitrogen and 
other gas constituents, would appear to be substantially inferior to the 
use of a separate stream of oxygen feed gas to the adsorption zone. 
Despite the foregoing considerations, it has been unexpectedly found that 
(1) there is a sufficient amount of oxygen in the gas vented from the 
oxygenation zone to meet the oxygen requirements of the adsorption zone, 
(2) the vent gas is generally equivalent to pure oxygen in extending the 
operating life of the activated carbon adsorbent beyond the life achieved 
with straight adsorption and with the same effluent water requirements in 
both cases, and (3) higher oxygen utilization at the same oxygenation 
power expenditure or, alternatively, lower power requirement for the same 
oxygen utilization level can be achieved with the vent gas as compared to 
a separate stream of oxygen feed gas. In line with these differences, the 
use of oxygenation zone vent gas has been found to be significantly 
superior to pure oxygen from an economy standpoint. 
The remarkable oxygen economy of the system is achieved by a fortuitous and 
unobvious matching of the oxygen requirements for biologically assisted 
adsorptive removal of residual biodegradeable organic contaminants and for 
DO-enrichment on one hand, and the oxygen requirements for purification by 
biochemical oxygenation in the activated sludge step (on the other hand). 
This matching is in part due to the very high oxygen utilization 
efficiency and remarkably complete removal of BOD which are achievable 
when oxygen is effectively used in the oxygenation-activated sludge step. 
The reasons for the closely equivalent performance of vent gas relative to 
the use of pure oxygen in the adsorption zone is not fully understood, but 
may be due in part to the fact that under the mixing conditions required 
for oxygenation of the influent wastewater, the oxygen-containing gas 
vented from the oxygenation zone tends to be close to equilibrium with 
respect to the oxygenated liquor discharged from the zone. Thus, when the 
vent gas and the effluent separated from the mixed liquor are again 
brought into contact in the adsorption zone, the dissolved gases in the 
effluent are in approximate equilibrium with the corresponding components 
in the vent gas and hence the former do not evolve from the liquid phase 
and pass into the gas phase in such appreciable amounts as to further 
impair the mass transfer driving force (gas phase concentration) for the 
oxygen component. On the other hand, such dissolved gas components -- 
i.e., CO.sub.2, nitrogen and the like -- may be appreciably stripped from 
the liquid in the adsorption zone consisting of separate oxygen feed gas 
and effluent and contribute to a lowered mass transfer performance and 
consequent lower utilization of oxygen for the separate feed arrangement. 
The following examples illustrate the specific advantages of the present 
invention, as compared to prior art systems lacking the improvement 
features of the process and apparatus of this invention. 
EXAMPLE I 
In the first test five parallel activated carbon adsorption units were 
concurrently operated for treatment of activated sludge secondary effluent 
in order to evaluate the present invention. The effluent was obtained from 
the East clarifier at the South Charleston Waste Treatment Works, where an 
air activated sludge system is employed to treat a combined wastewater 
comprising 70% industrial petrochemical waste and 30% municipal sewage. 
The experimental apparatus consisted of five glass columns each 3.5 cm. in 
diameter and 122 cm. in height and containing a 61 cm. deep bed of 
granular activated carbon (Westvaco "Nuchar" WV-G, 12 .times. 40 mesh, 
manufactured by Westvaco Corporation, New York, N.Y. 10017). Each column 
was simultaneously fed wastewater effluent in an upflow mode, with 
introduction of the effluent at the bottom of the bed and removal of 
contacted effluent water at the top thereof, at a nominal rate of 485 
ml/hr. (1 bed volume/hr.) by an individual micro-bellows pump from a 
common 55-gallon feed storage drum. Gas feed to each column was introduced 
through diffusion stones at the bottom of the column at a rate of 10 
standard milliliters/minute. The gas feed streams were varied for the five 
columns as follows: Column 1 = pure oxygen; Column 2 = 50% O.sub.2, 30% 
CO.sub.2 and 20% N.sub.2 by volume (representative of typical oxygenation 
system vent gas); Column 3 = air; Column 4 = pure nitrogen; column 5 = 
straight adsorption -- no gas. The purpose of the column 4 and 5 operation 
was to identify purely mechanical or hydraulic effects of gas flow through 
the bed. 
The above-described system was operated for an extended period of 
approximately 1200 hours. The BOD in the wastewater effluent fed to the 
adsorbers varied during the test from about 7 to 160 mg/l and the TOC 
showed a variation of from about 80 to 160 mg/l. All five adsorbent 
columns were backwashed at the same frequency, once every 13 days, to 
prevent plugging of the columns by biomass or effluent suspended solids. 
At various times, approximately once every 24 hours, determinations were 
made of biochemical oxygen demand (BOD) and total organic carbon (TOC) 
remaining in the final effluent water withdrawn from the columns. All 
measurements during these tests were conducted in accordance with the 
standard practices of the waste treatment industry as for example outlined 
in "Standard Methods for the Examination of Water and Wastewater," 
published by the American Public Health Association, Inc., 13th ed. 
(1971). 
In the early stages of the test, all of the first three columns (1, 2, and 
3) utilizing an oxygen-containing gas flow therethrough were about equally 
effective in enhancing BOD and TOC removals, compared to straight 
adsorption. TOC removal was not complete in any of the adsorbers due to 
the presence of residual organic contaminants, which were neither readily 
biodegradeable nor easily adsorbed. The ungassed and nitrogen-fed 
adsorbers (columns 4 and 5) were exhausted after a cumulative throughput 
of 320 liters of effluent (640 bed volumes) based on BOD removal. 
Exhaustion was not as rapid on a TOC removal basis in these beds because 
of their continued removal of non-biodegradeable organic contaminants. 
The results of BOD and TOC analyses toward the end of the run, at 440 
liters of effluent cumulative throughput (880 bed volumes) and 401 liters 
of effluent cumulative throughput (802 bed volumes) respectively, are set 
forth below in Table I. 
TABLE I 
__________________________________________________________________________ 
BOD Removal at 440 1. Throughput 
TOC Removal at 401 1. Throughput 
Influent 
Effluent Influent 
Effluent 
Adsorber 
Applied Concentration 
Concentration Concentration 
Concentration 
Column 
Gas (mg/l) (mg/l) % Removal 
(mg/l) (mg/l) % Removal 
__________________________________________________________________________ 
1 pure 55 14 74.8 95 33 65.3 
oxygen 
2 50%O.sub.2, 30%CO.sub.2 
20%N.sub.2 (by 
55 12 78.2 95 30 68.4 
volume) 
3 Air 55 28 49.2 95 37 61.1 
4 pure 
nitrogen 
55 60 nil 95 43 54.8 
5 Straight 
adsorption- 
55 60 nil 95 48 49.5 
no gas 
__________________________________________________________________________ 
In the latter stages of the run, the data show that the columns through 
which pure oxygen and vent gas mixture were passed (columns 1 and 2) were 
significantly more effective in maintaining high removals of BOD relative 
to the column through which air was flowed (column 3), while the anaerobic 
columns (columns 4 and 5) were completely exhausted, i.g., had completely 
lost their adsorptive capacity for BOD and thus required regeneration. As 
shown, the pure oxygen and vent gas mixture were about equally effective, 
with the latter providing somewhat higher % BOD removal than the former. 
The fact that the data for the anaerobic columns (4 and 5) showed higher 
BOD effluent concentrations than the influent BOD level may be 
attributable to anaerobic decomposition of inorganic and aerobically 
non-biodegradeable organic contaminants in the liquid fed to the 
adsorption zone into additional aerobically biodegradeable species and/or 
decomposition of easily biodegradeable species into less biodegradeable 
components. 
The same general performance advantages as described above for BOD removal 
were obtained in the pure oxygen and vent gas mixture gassed columns for 
TOC removal, although the differences in % removal between the various 
columns was not as great in the case of BOD removal. As mentioned, the 
anaerobic columns 4 and 5 still retained capacity for removal of TOC even 
though their BOD removal capacity was exhausted. Such behavior indicates 
that the activated carbon adsorbent had a lower adsorptive capacity for 
the biodegradeable portion of the feed, in the absence of in situ 
biological regeneration of the absorbent active surfaces, than for some of 
the non-biodegradeable organic constituents whose removal appears as TOC 
reduction. 
The foregoing results show the advantages of the present invention relative 
to prior art activated carbon adsorption systems, as including a greatly 
increased adsorption adsorbent bed life before exhaustion on a BOD removal 
basis and improved removal of BOD and TOC in the adsorption zone. Although 
their benefits were also attained by the use of air and pure oxygen, the 
vent gas mixture was slightly superior in performance, particularly in the 
later stages of the adsorption run. 
EXAMPLE II 
In this test, the above-described columns 1, 2 and 3 were operated to test 
their capacity for removal of acidic organic contaminants. Such 
contaminants are frequently encountered in the treatment of industrial 
wastewaters, such as chemical plant or refinery wastes. 
For this test, the operation of columns 1, 2 and 3 as described in Example 
I was continued through an additional study phase with the secondary 
effluent feedstock "spiked" with approximately 500 mg/l of acetic acid. 
Columns 4 and 5 were shut down during this phase of the study since they 
were already exhaused on a BOD removal basis. In this test the operation 
continued for an extended period of time corresponding to approximately 
600 liters of secondary effluent cumulative throughput (1240 bed volumes), 
under the same flow conditions as described in Example I. Daily analyses 
were conducted for feed and effluent pH and acetic acid concentrations 
were monitored by flame ionization gas chromatography conducted at 
140.degree. C based on 3 ml. samples of the wastewater. 
The results of the test are set forth in Table II below, showing feed and 
effluent pH and acetic acid (HAc) concentration values at various 
cumulative throughput levels of secondary effluent (feed). These results 
show that column 2, through which the oxygen-containing vent gas mixture 
was flowed, performed somewhat better than column 1 in which pure oxygen 
was passed through the adsorbent bed at cumulative throughput levels of up 
to 524 liters of secondary effluent (0.6 mg HAc/l for column 2 versus 1.3 
mg HAc/l for column 3). Pure oxygen was superior to the vent gas mixture 
at higher cumulative throughput valves (above 524 liters), but, as 
discussed more fully hereinafter, the use of the vent gas mixture permits 
significant improvement in oxygen utilization and oxygenation horsepower 
requirements to be achieved relative to the use of pure oxygen as the gas 
medium in the adsorption zone. The vent gas mixture (column 2) was clearly 
superior to air (column 3) at the higher throughput levels, at 565 
cumulative liters of feed and above. 
TABLE II 
__________________________________________________________________________ 
Acetic Acid Removals 
Cumulative Feed, 
liters 512 524 536 565 585 
HAc HAc HAc HAc HAc 
Sample pH mg/l 
pH mg/l 
pH mg/l pH mg/l 
pH mg/l 
__________________________________________________________________________ 
Feed 7.1 22 6.8 79 6.7 127 6.7 382 6.7 440 
Col. 1 Effluent water 
7.7 1.0 7.5 1.3 7.5 0.5 7.7 110 7.5 153 
(Oxygen) 
Col. 2 Effluent water 
7.0 0.7 6.5 0.6 6.3 2.0 6.7 174 6.6 211 
(vent gas mixture) 
Col. 3 Effluent water 
7.4 4.0 7.0 0.9 6.8 nil 7.0 348 6.8 330 
(air) 
__________________________________________________________________________ 
The data show that for the column through which the oxygen-containing vent 
gas mixture was passed (column 2), the pH of the effluent cocurrently 
flowed therethrough generally dropped slightly by up to 0.4 pH units 
across the adsorption zone. Such drop in attributable to the CO.sub.2 
content of the vent gas mixture. The moderate pH lowering effect of the 
vent gas mixture is believed to contribute significantly to the observed 
enhancement of acetic acid removal by adsorption and biooxidation, as 
compared to the adsorbent bed through which air is passed (column 3). In 
the general practice of the invention, the pH of the effluent containing 
residual biodegradeable organic contaminants which is passed to the 
adsorption zone should be in the range of from about 5.5 to 7.5, so as not 
to adversely effect the aerobic bacterial growth present in the adsorption 
zone. Where acidic organic contaminants are present in the effluent from 
the activated sludge zone, the pH of the effluent is desirably in the 
range of about 6 to 7, in order to provide a favorable pH environment for 
adsorption and biological assimilation of the acidic contaminants. 
EXAMPLE III 
A comparative performance evaluation was undertaken to assess the 
performance of an illustrative embodiment of the present invention against 
that of an oxygenation activated sludge and activated carbon adsorption 
treatment system wherein oxygen feed gas streams are separately introduced 
to the oxygenation and adsorption zones. 
The illustrative system representing the present invention was of a general 
type as shown in FIG. 2 featuring multiple oxygenation chambers in the 
liquid storage enclosure, but with two identical liquid storage enclosures 
disposed in side-by-side relationship, each processing one-half of the 
influent wastewater flow to the system and discharging oxygenated liquor 
to a single clarifier. This system employed a single carbon column 
adsorbent vessel, downstream from the activated sludge secondary treatment 
unit, as the adsorption zone for the system. The separate oxygen gas feed, 
unintegrated system also had the same apparatus configuration, with the 
exception that the vent gas from the third oxygenation chamber was 
discharged to the atmosphere and a separate oxygen feed gas stream, of the 
same composition as that introduced to the first oxygenation zone in the 
activated sludge unit, was used to supply oxygen-containing gas to the 
adsorbent bed. 
The above-described systems were designed with the constraint that the 
volumetric flow rate of the oxygen-containing gas entering the adsorber 
column must be at least 10% of the volumetric flow rate of effluent 
entering the column to ensure adequate vapor-liquid distribution. The 
adsorber column used Westvaco "Nuchar" 8 .times. 40 mesh as the adsorbent 
and was sized to process 1 bed volume of wastewater per hour at the 
wastewater flow rate conditions considered. In each system the effluent 
and the oxygen-containing gas enter the adsorber column at a temperature 
of 35.degree. C and pressure of 15 psig. This effluent enters the column 
with a dissolved oxygen concentration of 5 mg/l and is saturated in 
CO.sub.2 with respect to the vent gas discharged from the third and final 
stage of the oxygenation zone. In each system the purified liquid effluent 
water is withdrawn at the upper end of the adsorber column at a 50% 
saturation of oxygen with respect to the waste gas discharged from the 
column. These conditions are based on system operation such that the 
oxygen transferred from the gas phase to the liquid phase in the 
adsorption zone equals the oxygen consumed by aerobic biological growth 
therein. 
In this evaluation, two distinct sets of process conditions were considered 
for each of the two comparison systems, as denoted hereinafter by "Case I" 
and "Case II". These process conditions, together with the system 
apparatus parameters for each case, are set forth in Table III below. 
TABLE III 
______________________________________ 
Example III - Process Conditions and 
System Apparatus Parameters 
Oxygenation System Design Basis 
Case 1 Case 2 
______________________________________ 
Feed wastewater flow, million 
gallons per day 1.5 8.5 
Feed Wastewater BOD.sub.5, mg/l 
4500 550 
Mixed liquor volatile suspended 
solids mg/l 4500 4500 
Feed Gas Composition (by volume) 
98% O.sub.2, 
98% O.sub.2, 
2% Ar 2% Ar 
Feed Gas Pressure, inches H.sub.2 O 
2-5 2-5 
(gauge) 
Feed Gas Flow Rate, lbs. O.sub.2 /hr 
3490 2240 
Vent Gas Composition (by volume) 
38% O.sub.2, 
40% O.sub.2, 
54% CO.sub.2, 
45% CO.sub.2, 
2% Ar, 5% Ar, 
6% H.sub.2 O, 
5% H.sub.2 O, 
0% N.sub.2 
5% N.sub.2 
Vent Gas Flow Rate, lb. moles/hr 
89.2 24.0 
Vent Gas Pressure, psia 
14.7 14.7 
Mixed Liquor D.O., mg/liter 
5 5 
Number of Stages in Oxygenation 
4 3 
Zone* 
Liquid Volume of each Stage in the 
Oxygenation Zone, gallons 
253,900 590,280 
Clarifier Cross-Sectional Area, 
2500 14,166 
Feet.sup.2 
STE of Gas-Liquid Contacting Devices 
in each Stage of Oxygenation 
Zone, lbs. O.sub.2 /Bhp-hr 
5.3 3.3 
Sludge Recycle Rate, % 
39 39 
Oxygenation zone retention time, hrs 
32.5 10.0 
Food to biomass ratio, lb BOD.sub.5 /lb 
MLVSS-day 0.75 0.30 
Oxygen requiement, Lbs O.sub.2 /lb BOD.sub.5 
removed 1.05 1.45 
Secondary effluent BOD.sub.5, mg/l 
100 100 
Carbon Adsorption Zone Design Basis 
Organic contaminant-depleted 
effluent water BOD.sub.5, mg/l 
50 50 
Oxygen requirement, Lbs O.sub.2 /lb BOD.sub.5 
removed 1.0 1.0 
Effluent water D.O., mg/l 
4 4 
Configuration Vertical, co- 
upflow current 
Percent Oxygen in Waste Gas 
22 29 
(by volume) 
Waste Gas Flow Rate, lb. moles/hr 
4.6 17 
Waste Gas Pressures psia 
14.7 14.7 
Waste Gas Recycle Volumetric 
Flow Rate ft.sup.3 hr. (at 95.degree. F, 15 psig) 
0 616 
Suspended Solids in Effluent 
Flowed to Adsorption Zone 
&lt;100 ppm &lt;100 ppm 
______________________________________ 
*In both cases, there were two trains in the oxygenation zone, each with 
the tabulated number of stages therein 
In Case I, the two systems were evaluated for a high strength wastewater 
having a BOD.sub.5 content of 4500 mg/l, such as is representative of 
typical chemical plant or refinery wastes. In Case II, the integrated 
(this invention) and unintegrated treatment systems were compared on the 
basis of a feed wastewater with 550 mg/l BOD.sub.5, representative of a 
combined municipal/industrial wastewater. 
The results of the evaluation are shown in Table IV below for both Case 1 
and Case 2. 
TABLE IV 
__________________________________________________________________________ 
Case 1 Case 2 
System Integrated 
Unintegrated 
Integrated 
Unintegrated 
Parameter Carbon Adsorber 
Carbon Absorber 
Carbon Adsorber 
Carbon Adsorber 
__________________________________________________________________________ 
Oxygenation System O.sub.2 
Utilization (%) 
69.0 69.0 86.0 80.5 86.0 
Total System O.sub.2 
Utilization (%) 
69.7 69.2 92.6 86.7 86.7 
Oxygenation Zone Mixing 
and Fluid Recirculation 
Power Consumption, HP 
480 480 450 390 450 
Power Required to 
Compress Gas Flowing 
to Carbon Column, HP 
2.99 2.93 16.6 16.6 16.6 
__________________________________________________________________________ 
As shown by the data, the oxygen, utilization in Case 1 in the oxygenation 
zone was designed to be the same in both the integrated system according 
to the present invention and in the unintegrated system. Under such 
conditions, the power required for mixing and fluid recirculation in the 
oxygenation zone were thus also the same. The overall oxygen utilizaton 
for the total system (activated sludge secondary treatment and activated 
carbon tertiary treatment) was however marginally higher for the 
integrated system of this invention relative to the unintegrated system. 
The importance of the data in Case 1 lies in the fact that even at the 
higher BOD levels of this Case, the vent gas discharged from the 
oxygenation zone contains an adequate amount of oxygen to carry out the 
required BOD removal in the adsorption zone. The power required to 
compress the gas flowed to the carbon column in the integrated systems is 
about 2% higher than in the unintegrated case owing to the smaller volume 
of gas in the latter case. 
In Case 2, two sets of calculations were made for the integrated system to 
separately compare the integrated and unintegrated systems at equal 
oxygenation zone power consumption and again at equal overall oxygen 
utilization levels. The power required to compress the oxygen-containing 
gas flowed to the carbon column in all comparison systems was the same, 
16.6 horsepower (HP). Comparing first the integrated and unintegrated 
systems at equal oxygen utilization and power consumption in the 
oxygenation zone, the data show the total system oxygen utilization for 
the integrated system to be approximately 7% higher (92.6% v. 86.7%) than 
for the unintegrated system. When the two Case 2 systems are operated to 
obtain the same total system oxygen utilization level (86.7%), the 
integrated system achieves an oxygenation system oxygen utilization in the 
activated sludge step of 86.0% of the oxygen introduced to the oxygenation 
zone in the feed gas. Such oxygen utilization value is 6.8% higher than 
the value of 80.5% for the integrated system, but to obtain such 
utilization value, as necessary to achieve the same overall O.sub.2 
utilization of 86.7%, the unintegrated system requires a power expenditure 
of 450 horsepower for mixing and fluid recirculation in the oxygenation 
zone whereas only 390 horsepower are required in the oxygenation zone 
integrated with the adsorption zone in the manner of this invention. Thus 
the integrated system of this invention is characterized by a power 
consumption in the oxygenation zone which is 15.2% less than that required 
in the unintegrated system. Such striking reduction in power requirement 
under the present invention is a consequence of the fact that the 
integrated system is able to operate at significantly higher oxygen 
partial pressure in the oxygenation zone. With such higher oxygen partial 
pressure, the mass transfer driving force for oxygen dissolution is 
consequently increased so that correspondingly less mixing power is 
required in the oxygenation zone to effect such dissolution, all while 
achieving the same overall oxygen utilization level as in the unintegrated 
system. 
the significence of the Case 1 and Case 2 results when taken together, is 
based on the fact that in most wastewater treatment systems the feed 
wastewater flows are associated with frequent and often substantial 
variation in strength (BOD content) of the liquid to be treated and hence 
the oxygen requirements for BOD removal in the system. Thus, in a system 
treating a wastewater with a moderate BOD level which is however subject 
to peak or "shock" loadings of high BOD waste, the integrated arrangement 
of this invention is able to provide efficient tertiary removal of 
residual contaminants in the activated carbon adsorption zone without loss 
of the high levels of oxygen utilization necessary for economic operation 
of the wastewater treatment system. Furthermore, it is common practice to 
overdesign a treatment plant in deference to anticipated long-range BOD 
load increases. For example, a plant may go onstream at half its design 
capacity, with the full (design) load not expected until a number of years 
later. Under such conditions the integrated arrangement of the present 
invention is able to provide efficient tertiary treatment residual BOD 
removal over the operating life of the treatment plant without major 
alteration of the treatment apparatus. 
The maximal benefits under the present invention are realized when the 
residual organics in the oxygen activated sludge effluent (adsorber feed) 
are predominantly biodegradable. Removal of bio-refractory organics is 
also enhanced in the absorber to the extent that the biological activity 
continuously opens up adsorptive sites by destroying the adsorbed 
biodegradable organics. The removal of marginally degradable organics can 
be enhanced by providing sufficient effluent retention time for microbical 
adaption to occur during the period of exposure of the biological growths 
to the residual organic material. 
The present invention allows operation of the oxygen activated sludge unit 
at a higher wastewater flow rate, with consequent capital savings from the 
smaller volume requirement for the oxygenation zone. The utilization of 
the off-gas in a downstream adsorber alternatively permits the operation 
of the upstream activated sludge unit with increased oxygen feed, thereby 
reducing the dissolution horsepower requirements, as shown in the above 
Example III. 
The economic benefits occurring in the adsorption zone operated in 
accordance with the present invention relative to anaerobic and air-gassed 
zones include increased treatment efficiency in a given sized adsorbent 
bed and longer on-stream adsorbent bed life, resulting in reduced 
regeneration frequency, which in turn is associated with reduced 
regeneration furnace size (and investment) and reduced regeneration 
operating costs. Frequent regeneration is also undesirable for the reason 
that it entails greater carbon losses by attrition and burning (in the 
case of thermal regeneration) and higher costs for make-up carbon. 
Although preferred embodiments have been described in detail, it will be 
further appreciated that other embodiments are contemplated only with 
modification of the disclosed features, as being within the scope of the 
invention.