Discussion by Richard D. Pomeroy, USA
Sanitary engineering textbooks frequently mention the substances responsible for sewer odors, but
aside from the known presence of hydrogen sulfide and of volatile "organic acids, the statements have
mostly been the products of speculation or are repetitions of speculative statements by others. It is
gratifying that Messrs. Thistlewayte and Goleb have taken skillful advantage of the analytical methods
now available to explore this important subject and to have published information that helps to clarify
the picture.
There is increasing evidence that mercaptans are most important in many naturally occurring foul
odors, or even odors that are only musty or mildly offensive, provided that the concentrations are very
low, and they are certainly important in many industrial activities. A well water that developed foul
odors was found to contain isopropyl and isobutyl mercaptans, traced eventually to pollution of the
ground with a commercial product. The analysis was made in this case by bubbling nitrogen through
the water and into chilled CS2, then using gas chromatography to analyze the CS2 solution. The result
was only qualitative.
Stack gases from an operation in which cattle manure was being dried in a rotary kiln was
chromatographed directly, revealing the presence of mercaptans from Ci to C4, with two C4 isomers,
in concentrations from 0.008 to 0.12 micrograms per liter of gas.
An activated sludge plant of the Los Angeles County Sanitation Districts developed odors believed
to be due to waste products from the petroleum industry. Air from the plant site was passed through a
dry-ice trap. CS2 was added, and the trap was warmed until the ice that had condensed from the water
was melted. Gas chromatography of the CS2 solution showed several mercaptans up to C5.
Thistlethwayte and Goleb have shown that the principal organic sulfur substances in sewer
atmospheres are the one- and two-carbon compounds. The higher members are probably indicative of
industrial sources.
Except where the concentrations of sulfur compounds are high enough to permit direct
chromatography, the technique developed by the authors for capturing them with mercury appears
better than attempting to absorb them in CS 2 .
Mercaptans in sewage appear to be eliminable by chlorination or oxidation. Ethyl mercaptan was
added in the amount of one mg/1 to sewage that had developed a sulfide-oxidizing culture. Dissolved
oxygen was maintained by mild aeration. Within twenty minutes the odor of mercaptan was gone.
Experiments are planned for exploring the correlation between sulfide oxidation and mercaptan
oxidation.
CD. Parker, Australia
How do the concentrations quoted compare to odour threshold values?
Reply
In response to Mr. Parker, the question is "to what extent are the various odour experiences
additive?" H 2 S alone can be detected at concentrations below 0.1 ppm by volume when first
experienced, but the intensity tends to fade. The values given in most of the literature for mercaptans
and disulphides however, are open to question on grounds of additive effects. If only pure vapours or
gases are considered the quoted concentrations are mostly close to quoted threshold values, but we
believe their odours are in fact detectable, either because of additive effects or because published
threshold values are not reliable.
289
Sanitary engineering textbooks frequently mention the substances responsible for sewer odors, but
aside from the known presence of hydrogen sulfide and of volatile "organic acids, the statements have
mostly been the products of speculation or are repetitions of speculative statements by others. It is
gratifying that Messrs. Thistlewayte and Goleb have taken skillful advantage of the analytical methods
now available to explore this important subject and to have published information that helps to clarify
the picture.
There is increasing evidence that mercaptans are most important in many naturally occurring foul
odors, or even odors that are only musty or mildly offensive, provided that the concentrations are very
low, and they are certainly important in many industrial activities. A well water that developed foul
odors was found to contain isopropyl and isobutyl mercaptans, traced eventually to pollution of the
ground with a commercial product. The analysis was made in this case by bubbling nitrogen through
the water and into chilled CS2, then using gas chromatography to analyze the CS2 solution. The result
was only qualitative.
Stack gases from an operation in which cattle manure was being dried in a rotary kiln was
chromatographed directly, revealing the presence of mercaptans from Ci to C4, with two C4 isomers,
in concentrations from 0.008 to 0.12 micrograms per liter of gas.
An activated sludge plant of the Los Angeles County Sanitation Districts developed odors believed
to be due to waste products from the petroleum industry. Air from the plant site was passed through a
dry-ice trap. CS2 was added, and the trap was warmed until the ice that had condensed from the water
was melted. Gas chromatography of the CS2 solution showed several mercaptans up to C5.
Thistlethwayte and Goleb have shown that the principal organic sulfur substances in sewer
atmospheres are the one- and two-carbon compounds. The higher members are probably indicative of
industrial sources.
Except where the concentrations of sulfur compounds are high enough to permit direct
chromatography, the technique developed by the authors for capturing them with mercury appears
better than attempting to absorb them in CS 2 .
Mercaptans in sewage appear to be eliminable by chlorination or oxidation. Ethyl mercaptan was
added in the amount of one mg/1 to sewage that had developed a sulfide-oxidizing culture. Dissolved
oxygen was maintained by mild aeration. Within twenty minutes the odor of mercaptan was gone.
Experiments are planned for exploring the correlation between sulfide oxidation and mercaptan
oxidation.
CD. Parker, Australia
How do the concentrations quoted compare to odour threshold values?
Reply
In response to Mr. Parker, the question is "to what extent are the various odour experiences
additive?" H 2 S alone can be detected at concentrations below 0.1 ppm by volume when first
experienced, but the intensity tends to fade. The values given in most of the literature for mercaptans
and disulphides however, are open to question on grounds of additive effects. If only pure vapours or
gases are considered the quoted concentrations are mostly close to quoted threshold values, but we
believe their odours are in fact detectable, either because of additive effects or because published
threshold values are not reliable.
289