Methods for reducing sulfides in sewage gas

Methods are provided for selectively reducing the levels of hydrogen sulfide and organic sulfides in sewage gas to reduce or remove the odor, toxicity and corrosivity associated therewith, comprising contacting said sewage gas with a composition comprising a trisubstituted hexahydro-s-triazine.

FIELD OF THE INVENTION 
The present invention relates to methods for controlling odor, toxicity and 
corrosion of sulfides in sewage gas. More specifically, this invention 
relates to methods for reducing the hydrogen sulfide and organic sulfide 
levels in sewage gas systems. 
BACKGROUND OF THE INVENTION 
Sewage gas contains hydrogen sulfide and other organic sulfides which cause 
it to be malodorous. Also, the majority of the chemical compounds which 
cause the odor in sewage gas also cause it to be toxic and corrosive. 
Numerous sulfur-containing substances have been identified as causing the 
odor in sewage gas. Examples of these compounds are allyl mercaptan, amyl 
mercaptan, benzyl mercaptan, crotyl mercaptan, dimethyl sulfide, ethyl 
mercaptan, hydrogen sulfide, methyl mercaptan, propyl mercaptan, sulfur 
dioxide, tert-butyl mercaptan, thiocresol and thiophenol, to name a few. 
However, hydrogen sulfide is generally one of the main components of sewage 
gas, being usually contained in relatively high concentrations therein. 
Accordingly, the degree to which hydrogen sulfide is present in a sample 
of sewage gas is used as a measure of the odor intensity and corrosiveness 
of that particular sample. 
Not only will hydrogen sulfide cause an intense odor in sewage gas, this 
compound can have numerous physiological effects and can be quite 
hazardous. For example, the odor associated with hydrogen sulfide ("rotten 
eggs") can be detected when the concentration of the hydrogen sulfide is 
as low as about 0.1 parts per million of sewage gas. As the concentration 
of the hydrogen sulfide increases, various physical effects are seen, such 
as headache, nausea, throat and eye irritation, etc. Once the hydrogen 
sulfide level reaches a concentration of about 500 parts per million of 
sewage gas or more, serious life threatening effects will result, such as 
pulmonary edema, nervous system stimulation and apnea. If the hydrogen 
sulfide level were to reach a concentration of between about 1,000 to 
2,000 parts per million of sewage gas, respiratory collapse and paralysis 
resulting in death may result. 
Traditional sanitary sewer design practice has not fully acknowledged the 
importance of eliminating corrosion and controlling the odor caused by 
sulfides. This is evidenced by the widespread occurrence of these problems 
in conventional sewage treatment systems. In conventional systems, odor 
problems are managed by ventilating sewer systems so that the sewage gas 
becomes diluted with air. Although this practice may reduce gas 
concentrations to less than toxic levels and may be useful for controlling 
corrosion, large volumes of malodorous gas are produced. 
In order to address this odor problem, such air-diluted sewage gas is often 
further chemically treated. For example, offending odors can be made less 
objectionable through the use of odor-masking and counter-active agents 
such as vanillin and juniper oil. An example of this method is disclosed 
in, e.g., EPA Design Manual, "Odor and Corrosion Control in Sanitary 
Sewage Systems and Treatment Plants", EPA/625/1-85/018, pp 71-93 (1985). 
However, this approach merely involves replacing an objectionable odor 
with a more pleasant one. Accordingly, this method is generally the least 
preferred of the available techniques for reducing sewage gas odor. 
Also, strong oxidizing agents, such as chlorine, hydrogen peroxide and 
strong alkalis, such as sodium hydroxide and lime, have been used to react 
with the offending substances present in sewage gas. An example of these 
methods is set forth in "Odor and Corrosion Control in Sanitary Sewage 
Systems and Treatment Plants", Bowker et al, Noyes Data Corporation, pp 
52-60 and 71-78 (1989). However, these approaches have generally not 
proven to be commercially and/or economically successful since, although 
removal rates may appear high, the concentration of malodorous components 
in the treated sewage gas remains above threshold levels for odor 
detection. 
Since such sulfide-containing sewage gas or sludge gas (i.e., gas resulting 
from waste water or waste water constituents which have undergone 
anaerobic decomposition) is present in nearly all conventional sewage 
treatment systems, it can be seen that there is a need for an effective 
and efficient method for reducing the levels of hydrogen sulfide and other 
organic sulfides in waste water treatment systems. Such a method is 
needed, not only to remove the offensive odor and corrosivity associated 
with the sewage gas, but to reduce the possible occurrence of the adverse 
physiological effects discussed above. 
SUMMARY OF THE INVENTION 
The above-stated objectives are obtained by the present method for 
selectively reducing the levels of hydrogen sulfide and organic sulfides 
from sewage gas to remove the odor and corrosivity associated therewith, 
which comprises contacting the sewage gas with a composition comprising a 
trisubstituted hexahydro-s-triazine. This method may be used in various 
installations, such as a lift station fume exhauster or wet scrubber 
system, 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred sulfide-reducing agents for use in the present invention are 
compounds of the following Formula I: 
##STR1## 
wherein R' is hydrogen; lower alkyl, such as CH.sub.3 --, CH.sub.3 
CH.sub.2 --, CH.sub.3 CH.sub.2 CH.sub.2 --, (CH.sub.3).sub.2 CH--; 
hydroxyalkyls of lower alkyl groups, such as HOCH.sub.2 CH.sub.2 --, 
HOCH.sub.2 --, HO(CH.sub.3)CH--; and N,N-dialkylalkylene amines of lower 
alkyl groups, such as (CH.sub.3).sub.2 NCH.sub.2 -- or (C.sub.2 
H.sub.5).sub.2 NCH.sub.2 --; and R" is selected from hydrogen or lower 
alkyl, such as CH.sub.3 --, or CH.sub.3 CH.sub.2 --. As used herein 
"lower" generally means C.sub.1 to C.sub.6, and preferably C.sub.1 to 
C.sub.3. 
In particularly preferred embodiments, R' is HOCH.sub.2 --, HOCH.sub.2 
CH.sub.2 -- or HO(CH.sub.3)CH-- and R" is H or CH.sub.3. In the presently 
most preferred embodiment, the compound of Formula I is 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine. 
The compounds of Formula I employed in the method of the present invention 
may be prepared by reacting formaldehyde and a primary amine. Preferred 
primary amines for use in preparing the compounds of Formula I are: 
##STR2## 
wherein R' and R" are defined as set forth above for Formula I. The 
formaldehyde and the primary amine may be reacted in any ratio which will 
produce the desired triazines in the desired amounts. Appropriate ratios 
for specific instances will be evident to one skilled in the art, based 
upon the present disclosure. However, preferably, the formaldehyde and 
primary amine are reacted in a ratio of 1:1. 
The reaction may be carried out by any appropriate means known in the art. 
However, in the presently preferred embodiment, gaseous or aqueous 
formaldehyde may be reacted directly with the primary amine. 
Alternatively, solid paraformaldehyde may be used instead of the gaseous 
or aqueous formaldehyde and be reacted directly with the primary amine. 
The preferred procedure comprises mixing the primary amine with gaseous 
formaldehyde or an aqueous solution of formaldehyde at a temperature of 
about 35.degree. to about 60.degree. C. for about 30 to about 45 minutes. 
Once the reaction is complete, the water formed during the course of the 
reaction may be removed from the resultant reaction mixture by appropriate 
means. For example, the water may be removed by azeotropic distillation, 
distillation in vacuo, etc. The reaction mixture containing the 
trisubstituted hexahydro-s-triazines of the present invention is then 
ready for use in the present method. However, if desired, the resulting 
reaction mixture may be used directly in the method of the present 
invention without removing the water formed during the reaction. 
Commercially available trisubstituted hexahydro-s-triazines, such as 
Grotan.RTM. by Ciba-Geigy of Summit, N.J., which is a preparation of 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine, may be used in the 
present method. Such commercially available triazines are generally 
marketed for use as biocides, but may find use in the present method. 
The composition contacted with the sewage gas in accordance with the 
present method may comprise one or more of the above triazines in an 
amount of about 20 to about 80 weight percent and preferably about 25 to 
about 40 weight percent by total weight of the composition. 
The compositions useful in the present method may further comprise water 
and a lower mono- or dihydric alcohol as a medium for the triazines. 
Preferably, the alcohol is a lower alcohol of about 1 to about 3 carbons. 
More preferably, the mono- or dihydric alcohols are selected from the 
group consisting of CH.sub.3 OH, CH.sub.3 CH.sub.2 OH, (CH.sub.3).sub.2 
CHOH and HOCH.sub.2 CH.sub.2 OH. 
The compositions may contain water in an amount of about 20 to about 80 
weight percent and preferably about 48 to about 60 weight percent of the 
total composition. The compositions may also comprise about 0 to about 60 
weight percent and preferably about 12 to about 20 weight percent of the 
mono- or dihydric alcohol per total weight of the composition. 
The present method provides a selective and nearly instantaneous reaction 
with the malodorous and corrosive sulfides present in sewage gas, 
producing no precipitates, solids or deleterious environmental effects. 
Moreover, the efficacy of the reduction of hydrogen sulfide and other 
organic sulfide compounds is not affected by the concentration of carbon 
dioxide in the sewage gas, the temperature of the gas or the pressure of 
the system. However, it has been determined that good results are achieved 
when the following conditions exist: the carbon dioxide concentration in 
the gas to be treated is about 0.03 to about 5.0% by volume; the 
temperature of the gas is maintained in the range of about 0.degree. to 
about 93.degree. C.; and the pressure of the system is maintained in the 
range of about 5 to about 500 psig. However, the present method is not 
limited to the above-noted reaction conditions. 
The above triazines selectively react with sulfides present in sewage gas 
streams regardless of the CO.sub.2 level in the sewage gas, forming water 
soluble products. The water soluble products formed appear to be 
predominantly water soluble dithiazines and non-volatile organic sulfides 
as determined by .sup.13 C NMR analysis and comparison to model systems. 
Once formed, the water soluble products may be removed from the sewage 
treatment system by any appropriate means, and the system may then be 
recharged with an appropriate amount of fresh or unreacted triazine 
composition. That is, once the composition containing the triazine 
compound has reached its saturation point (i.e., once all of the triazine 
has reacted with the sulfides present in the sewage gas), the saturated 
composition may be removed and the sewage treatment system recharged with 
an appropriate amount of a "fresh" composition comprising the triazine 
compound. It can be determined that the saturation point has been reached 
by monitoring the gas exiting the scrubber for the presence of sulfides. 
The triazines useful in the invention are extremely selective in their 
ability to react with sulfides, e.g., hydrogen sulfides, carbonyl 
sulfides, carbon disulfides, etc., in the presence of any amount of carbon 
dioxide. Such selective removal of sulfides is advantageous and 
economical, particularly in systems wherein a simultaneous reduction in 
the amount of carbon dioxide is not required or desirable. Also, this 
selective removal is much more efficient than conventional systems in 
which the carbon dioxide competes with the sulfides for reaction with the 
active compound. 
In the selective removal of hydrogen sulfide and other organic sulfides 
from sewage gas, the present invention may be used in combination with any 
known, conventional sewage gas treatment method, including absorptive 
processes (such as those using activated carbon), as well as chemical 
injection treatments (such as those injecting sodium hydroxide or sodium 
hydroxide/sodium hypochlorite mixtures into the sewage system). 
The present method may be carried out by directly injecting the reaction 
product into the sewage gas flow lines, as well as directly into the 
sewage itself. Alternatively, the sewage gas may be bubbled through a 
layer of the present composition or the present composition may be 
introduced into the system by atomizers in the ducting or inlet systems 
feeding wet scrubbers. Moreover, the present method may be carried out by 
contacting the gas with the triazine compositions in wet scrubbers placed 
at appropriate points in the system. When using the wet scrubbers, 
preferably, the compositions of the invention are allowed to contact the 
sewage gas by counter-current flow or cross-flow and most preferably, 
conventional packing and/or baffling systems are used to increase the 
contact between the compositions of the invention and the sewage gas. In 
any method of introduction into the sewage system, the present composition 
should be contacted with the sewage gas for a period of time sufficient to 
reduce the sulfide levels to the desired concentration. Appropriate 
lengths of time will be evident to the artisan based upon the present 
disclosure. 
The present method may be used at any point in a waste water treatment 
system which will provide efficient and economical results. Preferably, 
the compositions containing the present reaction product are contacted 
with the sewage gas at the points of the sewage treatment system commonly 
known as the lift station fume exhauster or digester, as these are 
generally the most odorous points of the system. 
Generally, about 6.5 to about 9.5 g of a 25% aqueous solution of the 
triazine compound will effectively remove 1.0 g of hydrogen sulfide from 
sewage gas. This corresponds to about 0.03 to about 0.10 gallons of a 25% 
solution of the compound per 1 ppm H.sub.2 S per MMscf. 
Generally, in any system in which the present method is employed, the 
reaction temperature of the composition with the hydrogen sulfide and/or 
other organic sulfides in the sewage gas should be maintained at about 
32.degree. to about 200.degree. F. and preferably 60.degree. to about 
180.degree. F. 
The sulfide level in a sewage gas system may be reduced to about 0 ppm when 
the method of the present invention is employed. The determination of the 
sulfide level in the sewage gas systems may be made by various methods 
known to those skilled in the art. For example, the sulfide level may be 
determined by passing the gas exiting the system into colorimetric tubes, 
such as "Drager Tubes" which comprise lead acetate on a solid support. A 
change in color in the lead acetate indicates the presence of a specific 
concentration of organic sulfide. Alternatively, electrochemical sensors, 
metal oxide semiconductors and/or IR spectroscopy may be used to determine 
the parts per million concentration of sulfide in the sewage gas. Each of 
these methods are applied at the point of the sewage gas treatment system 
when the gas exits the system. 
The invention will now be illustrated further with reference to the 
following specific, non-limiting examples.

PREATION EXAMPLES 
Example 1 
N,N',N"-tris(2-N,N-dimethylaminoethyl)hexahydro-s-triazine 
One mole of N,N-dimethylethylenediamine was stirred at 30.degree. C. in a 
reaction flask, while one mole of aqueous formaldehyde was added dropwise 
thereto over a 40 minute period. During the addition of the aqueous 
formaldehyde, the temperature of the reaction mixture was maintained at 
50.degree.-55.degree. C. After the addition of the aqueous formaldehyde 
was complete, stirring was continued for 30 minutes. The reaction water 
was distilled off and the resulting product was isolated, purified and 
identified as the compound indicated above. 
Example 2 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine 
One mole of 37% aqueous formaldehyde was added to one mole of 
monoethanolamine at 60.degree. C. with stirring. Following the addition of 
the formaldehyde, stirring was continued for additional 30 minutes and the 
mixture was cooled to room temperature. The water of reaction was then 
removed by vacuum distillation and the 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine was isolated by 
fractional distillation in vacuo. 
USE EXAMPLES 
Example 3 
A 25% solution of N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine in a 
medium of 58.8% water and 16.2% methanol was tested in a typical scrubber 
system in a waste water treatment plant. The plant typically utilized 
activated carbon/caustic in a 4 ft. diameter by 7 ft. height scrubber. The 
system was charged with a 25% solution of the indicated triazine compound. 
Polypropylene packing was added to the scrubber to yield improved contact 
between the composition and the malodorous gases. Prior to treatment, the 
gas was generally found to contain about 35 parts per million (ppm) of 
hydrogen sulfide. After treatment was initiated with the indicated 
solution, the hydrogen sulfide level was reduced to 0 ppm in the gas 
exiting the scrubber. The hydrogen sulfide level remained at 0 ppm 
throughout 65 days of treatment. During this period, 220 gallons of the 
triazine solution were charged into the scrubber. 
Example 4 
80 gallons of a 25% solution of 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine in 58.8% water and 16.2% 
methanol was used in a scrubbing unit in a lift station. Prior to charging 
the system with the 25% triazine solution, the hydrogen sulfide level of 
the gas effluent was found to be about 18 ppm. Five minutes after charging 
the scrubber with the triazine solution, the hydrogen sulfide level of the 
gas exiting the scrubber was 0 ppm. The hydrogen sulfide level was 
maintained at 0 ppm for the 72 days in which the scrubber was charged with 
the 25% triazine solution. 
Example 5 
220 gallons of a 50% solution of 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine in 58.8% water and 16.2% 
methanol was tested in a 8 ft. diameter by 16 ft. height scrubber in a 
waste water plant lift station. Previously, this facility employed a 
conventional sodium hypochlorite solution treatment which generally 
yielded a hydrogen sulfide level of about 55 ppm in the gas exiting the 
scrubber. After the addition of the 50% triazine solution, a rapid drop of 
the hydrogen sulfide level in the exiting gas to 0 ppm was observed. The 
level of hydrogen sulfide remained constant at 0 ppm during 28 days of 
treatment. 
Example 6 
220 gallons of a 50% solution of 
N,N',N"-tris(2-hydroxyethyl)hexahydro-s-triazine in 58.8% water and 16.2% 
methanol was charged to a 6 ft. diameter by 16 ft. height scrubber in a 
waste water treatment plant mixing tank and thickener. The previous 
chemical treatment at this plant comprised the addition of a 50% sodium 
hydroxide solution to the system, typically resulting in hydrogen sulfide 
levels of about 40 ppm. After addition of the 50% triazine solution, the 
hydrogen sulfide levels in the gas exiting the scrubber were observed to 
be 0 ppm. The hydrogen sulfide level remained constant at 0 ppm for 24 
days. 
Example 7 
The following laboratory example was designed to simulate actual use 
conditions in order to demonstrate the superior effectiveness of the 
present method. 
An experimental apparatus was devised which comprised the following 
components connected in tandem: 
a lecture bottle; 
a flow meter; 
a flask; 
a flow meter; 
an infrared spectrometer; 
an air trap; 
a sodium hydroxide trap; 
a dry ice and acetone trap; 
an air trap; and 
a water trap. 
The performance capabilities of the present method to complex and retain 
the various sulfide compounds present in waste water and municipal waste 
plant effluents was determined as follows. 
Sulfide gases were introduced at a flow rate of 46-60 ml min.sup.-1 into 
the apparatus containing 10% solutions of the triazine solutions 
identified in Tables 1 and 2 as A, B and C, with subsequent monitoring of 
the process by IR spectroscopy. The sulfide gases were also treated with 
water and a conventional 10% NaOH solution for purposes of comparison. The 
results of the testing are set forth below in Tables 1 and 2. 
TABLE 1 
______________________________________ 
A B C NaOH Water 
______________________________________ 
Grams of H.sub.2 S complexed 
0.61 0.6 0.43 
.84 -- 
per gram of treating 
compound 
Moles of H.sub.2 S complexed 
4.0 2.2 4.0 1.0 -- 
per mole of treating 
compound 
Grams of H.sub.2 S complexed 
6.3 6.2 4.5 8.6 .3 
per 100 g of a 10% 
aqueous solution of 
treating compound, using 
gas comprised of 100% H.sub.2 S 
Grams of H.sub.2 S complexed 
6.3 6.2 -- 4.0 -- 
per 100 g of a 10% 
aqueous solution of 
treating compound, using 
a gas comprised of 50% 
H.sub.2 S and 50% CO.sub.2 
Grams of H.sub.2 S complexed 
6.1 6.2 -- .7 -- 
per 100 g of a 10% 
aqueous solution of 
treating compound, using 
a gas comprised of 5% 
H.sub.2 S and 95% CO.sub.2 
______________________________________ 
A = N,N',Ntris(2-hydroxyethyl) hexahydros-triazine 
B = 1,3,5trimethylhexahydro-s-triazine 
C = N,N',N",tris(2-dimethylaminoethyl) hexahydros-triazine 
TABLE 2 
______________________________________ 
A B Water 
______________________________________ 
Grams of CH.sub.3 SH complexed 
0.6 0.6 -- 
per gram of triazine 
Moles of CH.sub.3 SH complexed 
3.0 2.0 -- 
per mole of triazine 
Grams of CH.sub.3 SH complexed 
7.79 7.49 1.49 
per 100 g of 10% aqueous 
solution of triazine 
______________________________________ 
A = N,N',Ntris(2-hydroxyethyl) hexahydros-triazine (R' = HOCH.sub.2, R" = 
H') 
B = 1,3,5trimethylhexahydro-s-triazine (R' = R" = H) 
Table 1 demonstrates the effectiveness of the compounds of Formula I in 
complexing malodorous hydrogen sulfide, while Table 2 demonstrates the 
effectiveness of the compounds of Formula I in complexing malodorous 
methanethiol. As can be seen from Tables 1 and 2, the triazines of present 
Formula I are much more effective than the conventional NaOH solution in 
removing both hydrogen sulfide and methanethiol in the presence of carbon 
dioxide. This effect increases dramatically as the percentage of carbon 
dioxide in the sulfide gas increases. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification, as indicating the scope of the invention.