Methods for removing odors from process airstreams

After aerating sewage sludge in a composting process, a stream of process air is treated to remove odors therefrom by injecting into the airstream an atomized mixture of dilute sulfuric acid and dilute surfactant to remove ammonia and odorous organic compounds therefrom. The airstream is then oxidized with bleach to remove sulfides and treated with a hydrogen peroxide solution to remove chlorine introduced by the bleach while maintaining the oxidation reaction. A dilution fan is used to further dilute the scrubbed airstream as the airstream is exhausted to the atmosphere.

BACKGROUND OF THE INVENTION 
This invention relates generally to methods of and apparatus for removing 
odors from airstreams. More particularly, this invention relates to 
methods of and apparatus for neutralizing odors in a relatively low 
temperature airstream by removing malodorous constituents from the 
airstream with finely divided liquid droplets of reagents with which the 
constituents react. 
Offensive odors are characteristic of many chemical process industries and 
are often difficult and costly to eliminate or control. Of particular 
interest with respect to the instant invention is the technology of 
controlling odors emitted by waste water sludge composting facilities, 
wherein the sludge is biologically degraded into a stable end product. The 
end product is a sanitary, odor-free humus-like material which is used as 
a soil conditioner for yards, gardens, and athletic fields. By composting 
waste water sludge, a material which until recently was considered to be a 
waste, has become a valuable soil conditioning product which is generally 
less costly than the top soil or peat moss it replaces yet produces better 
results. Wide utilization of this resource has been curtailed because 
composting facilities typically produce undesirable odors. The problem is 
compounded because composting sites are most conveniently near waste water 
treatment plants, which themselves are located in or near urban or 
suburban areas. It is, therefore, necessary to minimize the odor emitted 
by composting facilities. This invention is directed to that end. 
While the technology disclosed in the instant application is of particular 
significance with respect to waste water sludge compost facilities, it 
also is applicable to industries such as, but not restricted to, petroleum 
refineries, rendering plants, pet food producers, flavor and fragrance 
producers, and fermentation processes. While there are presently no 
practical federal regulations for odor control, local laws and local 
political pressure is often sufficient to discourage introduction of such 
process industries; and, if industries are already established, local laws 
are sufficiently stringent to enable authorities to act on complaints and 
level stiff fines and, in some cases, even close plants. 
In view of these considerations, a number of approaches have been tried to 
control odors. The simplest is merely to dilute odorous gas with large 
quantities of air. Another approach has been to mask offensive odors with 
more pleasant ones, but this is not particularly successful in that, to 
some people, the new odor is as unpleasant as the original odor; and, 
downstream of the plant, the mixture is often not sustained. In any event, 
any odor emanating from a plant, whether pleasant or unpleasant, is 
suspect. Incineration has been attempted, but this is a very costly 
approach in that an entire odorous gas stream must be subjected to 
combustion temperatures In a situation where the process has no combustion 
stage or the airstream is saturated with moisture, such as with process 
air from a composting system, combustion for odor control is not at all 
practical. 
Another common approach to odor control is by chemical neutralization of 
odorous compounds. The odorous gas is conventionally washed with an 
aqueous solution of a reactive chemical in spray towers, packed beds, and 
the like. Chemicals commonly used for this purpose include permanganates, 
dichromates, acids, hypochlorite solutions, hydrogen peroxide, and other 
common oxidizing agents. Exemplary processes are described in British 
Patent No. 1,152,705 and U.S. Pat. No. 3,923,955. Another approach for 
chemical neutralization of odors is described in U.S. Pat. No. 4,125,589, 
in which odoriferous constituents are removed from essentially saturated 
gas streams utilizing relatively small volumes of liquid in very finely 
divided droplet form with relatively long gas-liquid contact times. This 
process was improved upon by the process described in U.S. Pat. No. 
4,238,461, wherein the droplet size was further controlled. Further 
exemplary of the prior art are U.S. Pat. Nos. 4,225,566 and 4,416,861. 
U.S. Pat. No. 4,225,566 utilizes two or more liquid reagents, with one 
being dispensed above another in a reaction vessel through which waste gas 
is passed, while U.S. Pat. No. 4,416,861 utilizes a two-stage odor control 
system, wherein exhaust air from fat-rendering plants is fogged with 
sulfuric acid. 
Utilization of the foregoing technology has resulted in removal of a 
substantial portion of the odor in process air from a composting facility 
with which the instant inventors have worked. However, the remaining odor 
is still sufficiently pervasive to precipitate a substantial number of 
complaints from citizens. While this remaining odor does not present a 
nuisance which is sufficient to shut down the plant or result in fines, it 
can affect the plant's reputation and can hinder the erection of more 
composting facilities. In that it is to everyone's benefit to erect and 
utilize composting facilities for management of sewage sludge, 
substantially controlling all odor is of utmost importance if this concept 
is to gain wide acceptance. 
In controlling the remaining odor-causing constituents, the inventors 
fogged the process air with a surfactant solution. This is a unique 
concept, not taught in the prior art for applications such as a composting 
facility or other large processing operation. However, the prior art does 
disclose utilizing detergent to wash odors and grease from cooking stove 
hoods, as is exemplified in U.S. Pat. Nos. 3,841,062; 4,351,652; and 
4,753,218. There is no suggestion in these patents that fogging with 
surfactants in combination with other scrubbing reagents would result in 
improved removal of the remaining odor-causing constituents from a stream 
of relatively low temperature air which has odor-causing constituents 
generated by a sludge composting system or similar facility. 
In view of these and other considerations, there was a need for methods of 
and apparatus for removing odor-causing constituents from airstreams, 
which constituents remain as residuals when prior processes are utilized. 
SUMMARY OF THE INVENTION 
In view of the aforementioned considerations, it is an object of the 
instant invention to deodorize process gas streams such as airstreams 
emitted by industrial facilities or the like as completely as possible by 
removing substantially all odor-causing constituents therefrom. 
In view of this and other objects, the instant invention contemplates 
methods of and apparatus for scrubbing a stream of air which includes 
undesirable active organic compounds by treating the stream of air with 
atomized liquid surfactant-containing droplets to remove the undesirable 
organic compounds therefrom and removing the surfactant-containing 
droplets retaining the undesirable organic compounds from the stream of 
air. 
The method and apparatus further include continuously recirculating the 
surfactant solution through the stream of air while performing the steps 
of treating the stream with the surfactant-containing droplets and then 
removing the surfactant solution from the airstream. The above-described 
methods and apparatus are particularly useful in removing odor-causing 
compounds such as ketones, alcohols, aldehydes, terpenes, and alkyl 
benzenes from airstreams such as the exhaust airstreams of air utilized to 
aerate piles of composting wastewater sludge. 
In an even more particular application of the instant invention, the stream 
of exhaust gas, such as air which includes undesirable odor-causing 
compounds, is scrubbed by treating the airstream with an atomized 
surfactant-containing acid solution. The airstream is then treated with a 
bleach solution to oxidize sulfides therein and finally treated with a 
hydrogen peroxide solution to remove chlorine therefrom. By treating the 
airstream with an atomized surfactant solution, a number of odor-causing 
constituents are removed from the airstream, which constituents would 
remain as residuals if the airstream had only been treated with acid, 
bleach, and hydrogen peroxide. 
Again, the concept is of special significance with respect to deodorizing 
process air used to aerate piles of wastewater sludge in a composting 
operation. However, any airstream having noxious hydrocarbons therein may 
benefit from the teachings of this invention. 
Upon further study of the specification and appended claims, further 
objects of this invention will become apparent to those skilled in the art 
.

DETAILED DESCRIPTION 
(a) First Embodiment of the Invention 
The following description is directed to utilizing the instant invention 
with a composting facility; however, the principles of the instant 
invention may be applied to odor control of other chemical process 
operations and industries such as petroleum cracking and refining plants, 
rendering plants, pet food plants, flavor and fragrance production, and 
fermentation processes and plants. 
Referring now specifically to FIG. 1, showing a composting facility 
designated generally by the numeral 10, sewage sludge is trucked to the 
facility in trucks 11 and mixed at a mixing station 12 with wood chips 
from a wood chip storage station 13 to form a sludge/wood chip mixture 14, 
of which a composting pile 15 is comprised. The composting pile 15 is 
surrounded by a blanket of compost 16, which serves as an insulating layer 
and to help control odor emitted by the wood chip/sludge mixture 14. The 
compost pile 15 rests on a deck 17 which has a manifold, designated 
generally by the numeral 18, comprised of a number of air pipes 19 which 
are perforated. The manifold 18 is connected to a suction air blower 20, 
which draws air through the composting pile 15 into the perforated pipes 
19 and exhausts the air through a header pipe 21. 
The air in the header pipe 21 contains a number of malodorous compounds 
which are removed by the scrubbing system illustrated in FIGS. 2, 3, and 
7. These compounds may include ammonia and organic sulfides as primary 
offenders and a number of other odor-causing compounds, such as ketones, 
alcohols, aldehydes, terpenes, alkyl benzenes, and the like, as secondary 
odor-causing constituents. While the secondary odor-causing constituents 
are not generally as offensive as the primary constituents, they still 
cause odor which is sufficiently unpleasant to raise complaints from 
citizens living proximate and downwind of the composting facility 10. 
Generally, the composing pile 15 is aerated for 21 days and then screened 
at a screening station 25 to remove the wood chips from the mixture. The 
resultant compost 26 is then stored and cured for approximately one month 
before being sold as a conditioner for lawns, gardens, athletic fields, 
and the like. The compost 26 is a useful, highly valuable product, the 
production of which could be curtailed if odor problems associated with 
the production are not solved. The instant invention solves the odor 
problem and thus promotes the production of commercially valuable compost 
26. 
Referring now to FIG. 2, the composting pile or piles 15 of FIG. 1 are 
preferably contained within a large industrial building 30, which totally 
encloses the compost piles in order to prevent ground level discharge of 
offensive odors. Fresh air is introduced into the building through an 
inlet 31, which serves as the process air which passes through the compost 
pile 15. The building 30 is also provided with exhaust fans 32, which 
disperse building air which is not passed through the compost piles at a 
height of approximately 40 feet from ground level. The exhaust fans 32 
have a capacity to exhaust air at one million cubic feet per minute. 
The process air in exhaust line 21 is heated by the decomposing compost 
piles to approximately 135.degree. F. The process air is passed through an 
odor scrubber, designated generally by the numeral 40, which, in 
accordance with the principles of the instant invention, removes almost 
all odor-causing constituents from the process air. The process air is 
discharged at a height of approximately 110 feet from ground level through 
a half-million cubic feet per minute dilution fan 41, which mixes fresh 
air with the treated process air. 
Referring now to FIG. 3, where the odor scrubber 40 is shown in detail, it 
is seen that the process airstream indicated by the arrow A enters the 
odor scrubber 40 through a lower inlet 45, rises in a first tower, 
designated generally by the numeral 46, falls through a second tower, 
designated generally by the numeral 47, and rises through a third tower, 
designated generally by the numeral 48, before exhausting through an 
outlet 49 where the treated outlet air is mixed with fresh air by the 
500,000 cubic feet per minute dilution fan 41. 
Considering now with more particularity the scrubbing process which takes 
place within the scrubber 40 of FIG. 3, it is seen that each tower 
performs a different process, the processes themselves being interrelated. 
In the first tower 46, a surfactant-containing solution of dilute sulfuric 
acid removes ammonia and organic compounds from the airstream. The second 
tower 47 removes organic sulfides, primarily dimethyl disulfide, from the 
airstream with bleach, while the third tower 48 keeps the oxidation 
reaction of the tower 47 going while removing chlorine produced in the 
second tower 47. 
Considering the first tower 46, it is seen that the airstream passes first 
by an atomizing nozzle 50, which introduces an atomized spray comprised of 
a surfactant-containing solution of dilute sulfuric acid. The diameter of 
the droplets of the atomized spray average approximately 8 microns so that 
some of the mixture of sulfuric acid and surfactant is carried along with 
the airstream to initially react with the ammonia and active organic 
compounds entrained therein. The active organic compounds may include 
malodorous compounds, such as ketones, alcohols, aldehydes, terpenes, and 
alkyl benzenes, and comprise a relatively small, yet readily detectable 
component of the exhaust air which, in the absence of ammonia and 
sulfides, is unmasked and can generate citizen complaints. 
As the airstream ascends the tower 46, it is treated with a recycled 
surfactant-containing sulfuric acid solution collected from the port 51 at 
the bottom of the first tower, which is pumped by a pump 52 through a fine 
spray nozzle 53 disposed downstream of the atomizing nozzle 50 and a 
coarse spray nozzle 54 disposed downstream of the fine spray nozzle 53. 
The nozzles 53 and 54 are connected to the pump 52 by a recirculation line 
56. Nozzles 53 and 54 serve several purposes. By recirculating the 
solution collected at port 51, any unreacted chemicals can be utilized, 
which increases the chemical efficiency of the system. By introducing an 
additional 50 gpm through two nozzles, the amount of water available for 
solubilization is greatly increased. The compounds are mostly rendered 
soluble by the acid/surfactant solution in atomizing nozzle 50 and 
additionally rendered soluble by the recirculated solution in nozzles 53 
and 54. The droplets of the coarse and fine spray nozzles 53 and 54 are of 
sufficient size to fall through the rising airstream A. By introducing a 
fine spray from fine spray nozzle 53 above the atomized spray from the 
nozzle 50 and subsequently introducing a coarse spray from the coarse 
spray nozzle 54, most of the droplets injected into the tower 46 are 
agglomerated on coarser droplets and on one another so as to fall through 
the airstream for collection at a sump to which drainage port 51 is 
connected. Thereafter, the liquid collected at the sump is recirculated 
through fine and coarse spray nozzles 53 and 54 under pressure by the pump 
52. 
The airstream A is neutralized in the tower 46 by injection of the 
surfactant-containing sulfuric acid solution. In order to ensure that the 
airstream A is continuously neutralized, the recirculated sulfuric 
acid-surfactant mixture is constantly monitored by a pH monitor 60 
connected to the recirculation line 56. If the pH drops below the set 
point, due to addition of too strong an acid solution, the acid feed rate 
is reduced; if the pH rises above the set point, the amount of acid 
flowing through nozzle 50 is increased. This is accomplished in accordance 
with standard technology by simply adjusting the speed of the acid feed 
pump. Water from a water line 64 is constantly introduced into the feed 
line 56 via an inlet 66 keep the recirculating sulfuric acid-surfactant 
mixture dilute and to waste the undesirable constituents removed from the 
airstream A. 
The neutralized airstream A then passes down through the second tower 47, 
where a bleach solution of sodium hypochlorite is introduced for oxidizing 
organic sulfides such as dimethyl disulfide and other similar malodorous 
constituents. The sodium hypochlorite is introduced through a staggered 
pair of atomizing nozzles 71 and 72, which treat the neutralized airstream 
with a dilute solution of sodium hypochlorite. The atomized sodium 
hypochlorite droplets have an average diameter of about 8 microns and are 
entrained with and carried along with the airstream A down to the bottom 
of the second tower 47. As the airstream drops in the tower 47, the 
oxidation reaction takes place to remove sulfides from the airstream. 
During the sodium hypochlorite treatment, the further addition of 
surfactant, perferably mixed and dispensed with the dilute sodium 
hypochlorite solution, may further enhance removal of residual organic 
compounds. 
In order to maintain the oxidation reaction as the airstream rises in the 
third tower 48 before exiting through exhaust 49, the airstream is then 
treated with a relatively weak solution of hydrogen peroxide dispensed 
from a reservoir 76. The hydrogen peroxide also removes chlorine from the 
airstream so that odor of chlorine is not present in the airstream. Even 
in small concentrations, chlorine can be irritating; and, in any event, it 
is preferable to have the exhaust from the tower 48 as odor free as 
possible. This process also further solubilizes and removes organic 
compounds. In the illustrated embodiment, hydrogen peroxide is introduced 
in the rising airstream A through an atomizing nozzle 77, which atomize 
the hydrogen peroxide to an average diameter of approximately 8 microns. 
The dilute solution of hydrogen peroxide is piped from the reservoir 76 to 
the nozzle 77 through a line 78 and is continuously recirculated from a 
supply which pools in a sump 80 at the bottom of the third tower 48. 
During hydrogen peroxide treatment, the addition of further surfactant, 
preferably mixed and dispensed with the dilute hydrogen peroxide solution, 
may further enhance removal of residual organic compounds. 
Hydrogen peroxide pooling in the sump 80 of third tower 48 is recirculated 
in a line 81 by pump 82 and dispensed in a relatively fine spray by nozzle 
83 positioned above the atomizing nozzle 77. The relatively small atomized 
droplets from nozzle 77 agglomerate on the relatively large droplets from 
the spray nozzle 83 and precipitate out of the airstream A to pool at the 
sump 80 of the tower 48 so as to be recirculated through line 81 by the 
pump 82. The airstream A flowing from the exhaust 49 has now been scrubbed 
to remove odor-causing constituents and, as illustrated in FIG. 2, is 
mixed with a very large volume of fresh air by the air dilution fan 41. 
In order to properly dilute each of the scrubbing liquids, i.e., the 
acid/surfactant mixture, the sodium hypochlorite bleach and the hydrogen 
peroxide wash, water is constantly introduced through a line 90, which has 
feed lines 91-95 connected to each liquid line. The line 91 is connected 
by a chemical feed pump 100 to the sodium hypochlorite line 73 so as to 
mix with sodium hypochlorite dispensed from reservoir 74. Line 92 
replenishes water through the recirculation line 56 for the 
acid/surfactant mixture. Line 93 provides dilution water for the acid 
input. Line 94 provides dilution water for the hydrogen peroxide in line 
78, and line 95 maintains the water content for recirculating hydrogen 
peroxide in line 81. 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiment is, therefore, 
to be construed as merely illustrative and not limitative of the remainder 
of the disclosure in any way whatsoever. 
Example of the First Embodiment 
In a specific installation utilized to treat process air as the air is 
exhausted from a waste-water sludge composting facility such as that shown 
in FIGS. 1 and 2, the stream of process air A moving at the rate of about 
25-30,000 cubic feet per minute is processed by cylindrical towers 46, 47, 
and 48, each having a height of about 60 feet and supplied by Quad 
Environmental Technologies Corporation of Highland Park, Ill. The first 
tower 46 has a diameter of about 41/2 feet, with the nozzles 50, 53, and 
54 positioned at heights of about 10 feet, 18 feet, and 27 feet, 
respectively, from the base on the tower 46. The nozzle 50 is an atomizing 
nozzle, available from Quad Environmental Technologies Corporation. It 
dispenses a dilute solution of sulfuric acid (H.sub.2 SO.sub.4) in the 
range of about 2.0 to 3.0 percent and is mixed with a surfactant to a 
solution in the range of 0.05 percent to 0.50 percent by volume, and 
preferably about 0.15 percent. The nozzle 50 dispenses the mixture as 
droplets in the range of 5 to 20 microns in diameter, in the preferable 
range of 6 to 10 microns in diameter, and preferably about 8 microns in 
diameter, at a rate in the range of 1 to 4 gallons/ minute, and preferably 
about 3.0 gallons/minute. The current surfactant utilized is a detergent 
in the form of retail TIDE detergent manufactured by Procter & Gamble 
under U.S. Pat. Nos. 4,318,818; 4,702,857; 4,304,679; 4,597,898; 
4,561,998; 4,507,219; and 4,515,705, each of which patents is hereby 
incorporated by reference. The fine and coarse spray nozzles 53 and 54 are 
nozzles available from Bete Fog Nozzle Corp., Model Nos. WL1530 and 
NCM1012K-30, respectively, and dispense the recirculated mixture at a 
total rate of about 60 gallons per minute. 
From the first tower 46, the airstream expands in second tower 47, which 
has a diameter of about 12 feet and dispenses a dilute solution of sodium 
hypochlorite in the range of 0.05 to 1.5 percent, and preferably about 
0.80 percent, from nozzles 71 and 72, which are atomizing nozzles 
available from Quad Environmental Technologies Corporation and dispense 
the sodium hypochlorite solution at a rate of about 1.5 gallons per minute 
each for a preferred total of about 3 gallons per minute with an 
acceptable total range of 1.0 to 4.0 gallons per minute. The dilute sodium 
hypochlorite solution is atomized as droplets ranging from 1 to 20 microns 
in diameter and preferably about 8 microns in diameter. 
The airstream A is then slightly compressed as it flows into tower 48, 
which has a diameter of 10 feet. The atomizing nozzle 77 is positioned 
about 7 feet from the floor of the 60-foot tower and dispenses a dilute 
solution of hydrogen peroxide (H.sub.2 O.sub.2) a rate in the range of 1 
to 6 gallons/minute, and preferably about 3 gallons/minute, the atomizing 
nozzle again dispensing the solution in droplet sizes ranging from 1 to 20 
microns, and preferably about 8 microns. The spray nozzle 83 is a full 
60.degree. cone Model No. TF32NN, available from Bete Fog Nozzle Corp., 
and is positioned 15 feet above the atomizing nozzle 75. The recirculated 
hydrogen peroxide solution is pumped through the nozzle 83 at a rate in 
the range of 30 to 35 gallons/min. 
As is seen in FIG. 4, the surfactant feed rate is a little over 3 gallons 
per day to remove more than 90% of organic compounds remaining after 
treatment with sulfuric acid and sodium hypochlorite, which removes about 
90% of the odor-causing components of the stream, i.e., ammonia and 
dimethyl disulfide. 
As is seen by the chromatographs of FIG. 5A-5C, the organic compounds, as 
exemplified by the tall spikes, remain in the airstream A from inlet to 
outlet if the airstream is not treated with a surfactant. As is seen in 
FIGS. 6A-6C, the detectable amounts of active organic compounds initially 
in the airstream A, as indicated by the tall spikes in FIG. 6A, have 
disappeared almost completely at the outlet, as is seen in the absence of 
tall spikes in FIG. 6C, the organic compounds having been initially 
diminished at the end of the oxidation stage, as is seen in FIG. 6B. 
(b) Second Embodiment of the Invention 
Referring now to FIG. 7, there is shown a second embodiment of the 
invention, wherein airstream B is treated to remove organic compounds such 
as hydrocarbons which might be produced from chemical plants such as 
petrol chemical plants and other chemical plants having malodorous 
emissions. In the apparatus of FIG. 4, a gas stream, such as an airstream 
B, enters the inlet of scrubbing tower 201 and is exposed to surfactant 
dispensed from an atomizing nozzle 202, similar to the atomizing nozzle 50 
of FIG. 3. The atomizing nozzle 202 is supplied from a reservoir 203, 
which is filled by recirculating solution containing surfactant. 
Disposed above atomizing nozzle 202 is a fine spray nozzle 206 and a coarse 
spray nozzle 207, which dispenses relatively large droplets that fall 
through the airstream B as the airstream rises in the tower 201, causing 
the droplets dispensed by the fogging nozzle 202 to agglomerate thereon 
and pool in a sump 208 at the bottom of the tower 201. The dilute 
surfactant pooled at the bottom of tower 201 drains through a line 209 and 
is recirculated by a pump 210 through a line 211 to the spray nozzles 206 
and 207. 
Malodorous organic compounds are removed from the airstream B by the 
surfactant sprays from nozzles 206, 207, and 202 utilizing relatively 
dilute amounts of surfactant solution. For example, an airstream of 25,000 
to 30,000 cubic feet per minute is treated with a surfactant solution of 
approximately 0.10 percent to remove odor-causing organic compounds such 
as ketones, alcohols, aldehydes, terpenes, and alkyl benzenes. Since the 
detergent solution is recirculated, it is necessary for such a volume of 
air to add only 3 to 6 gallons of surfactant to the solution each day. 
(c) Third Embodiment of the Invention 
Referring again to FIG. 3, in situations where the airstream A contains 
little or no ammonia, such as certain refineries and smelters, painting 
facilities, and the like, it is not necessary to treat the airstream with 
sulfuric acid in order to reduce ammonia. Accordingly, the treatment 
utilizes a weak surfactant solution in the tower 46, followed by the 
bleach treatment and hydrogen peroxide treatment previous described in the 
full description of FIG. 3. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. The 
addition of surfactants to the liquids dispensed in the aforedescribed 
processes enhances solubility of the organic compounds being removed from 
airstreams while apparently increasing the efficiency of the atomizing 
process by decreasing droplet size. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention and, without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.