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Mercaptan Frequenlty Asked Questions


Mercaptans are colorless liquids, which are insoluble in water and possess a characteristic offensive smell. FreshAWL® breaks the bonds that tie bacteria to sulfur which causes the creation of hydrogen sulfide, mercaptans and other foul odors.
Why are foul odors detectable around wastewater treatment plants?

Odors are a natural part of the substances that are delivered to and treated at any wastewater treatment plant. Odors are typically contained within the wastewater treatment plant site; but occasionally odors drift from the plant site depending on weather conditions and wind direction. Routine processing operations are designed to reduce the amount of odors present; however, certain weather conditions and equipment maintenance may lessen the effectiveness of these routine odor control measures.

What causes these foul rotten-cabbage odors?

Mercaptans smell like rotting cabbage and FreshAWL WOW-Air will eliminate them.
Mercaptans smell like rotting cabbage and FreshAWL® WOW-Air™ will eliminate them.

Most of the odors detected in and around wastewater treatment plants are signals that the natural organic treatment process is working. Organic matter is decomposing with the assistance of good bacteria and pollutants are being removed from the wastewater. In normal aerobic (oxygen rich) condition, these good bacteria utilize oxygen in their natural process of respiration.

There are long pipe runs and enclosed anaerobic (absence of oxygen) digesters where the bacteria substitute sulfur in place of oxygen for their respiration. The by-products of sulfate respiration are sulfides and reduced sulfur compounds such as mercaptans. Sulfide is the leading cause of the odor and corrosion problems associated with biosolids, which occurs when the sulfides are mixed with air and form hydrogen sulfide. As the following table Odorous Compounds In Wastewater shows, three major odorous compounds naturally occurring in the treatment process, hydrogen sulfide, amines and mercaptans(see Mercaptan definition from Brittanica in the following column), are detectable by the human nose at extremely low concentrations:

Odorous Compounds In Wastewater

Compound NameRecognition Threshold parts per millionOdor Description

Allyl mercaptan


Disagreeable, garlic



Pungent, irritating

Amyl mercaptan


Unpleasant, putrid

Diisopropyl amine



Dimethyl amine


Putrid, fishy

Ethyl amine


Ammonia like

Ethyl mercaptan


Decayed cabbage

Hydrogen sulfide


Rotten eggs



Fecal, nauseating

Methyl amine


Putrid, fishy

Methyl mercaptan


Rotten cabbage

From Table 2.1, Odor Control in Wastewater Treatment Plants, 1995, WEF & American Society of Civil Engineers

Definition of MERCAPTANS (Thio-alcohols), organic chemical compounds of the type R.SH (R = an alkyl group).

The name is derived from mercurium captans, in reference to the reaction these compounds readily undergo mercuric oxide to form crystalline mercury derivatives. The mercaptans may be prepared by the action of the alkyl halides on an alcoholic solution of potassium hydrosulphide; by the reduction of the sulpho-chlorides, e.g. C2H5SO2C1 (chlorides of sulfonic acids), by heating the salts of esters of sulfuric acid with potassium hydrosulphide, and by heating the alcohols with phosphorus pentasulfide.

They are colorless liquids, which are insoluble in water and possess a characteristic offensive smell.

On oxidation by nitric acid they yield sulfonic acids.

They combine with aldehydes and ketones, with elimination of water and formation of mercaptals and mercaptols. Methanethiol (also known as methyl mercaptan) is a colorless gas with a smell like rotten cabbage. It is a natural substance found in the blood and brain of humans and other animal as well as plant tissues. It is disposed of through animal feces. It occurs naturally in certain foods, such as some nuts and cheese. It is also one of the main chemicals responsible for bad breath and the smell of flatus. The chemical formula for methanethiol is CH3SH; it is classified as a thiol. It is sometimes abbreviated as MeSH.

Ethanethiol (also known as ethyl mercaptan) is an organic compound with the formula CH3CH2SH. It consists of an ethyl group, CH3CH2, attached to a thiol group, SH. Its structure parallels that of ethanol, but with S instead of O. The presence of S leads to many different properties, most notably the infamous odor of EtSH.

Ethanethiol has a strongly disagreeable odor that humans can detect in minute concentrations. The threshold for human detection is as low as one part in 2.8 billion parts of air. Its odor resembles that of leeks or onions. Ethanethiol is intentionally added to butane and propane to impart an easily noticed smell to these odorless fuels, that otherwise pose the threat of fire, explosion and/or asphyxiation.

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