Odors Of Biological Origin In Water

Odors from anaerobic surface waters, groundwater, and domestic wastewater are usually from inorganic and organic gases generated by biological activity. Anaerobic decomposition of nitrogenous or sulfurous organic matter often produces gases that contain sulfur and/or nitrogen. Such gases are frequent causes of odors in water. The most common inorganic gases in water are carbon dioxide (CO2), methane (CH4), hydrogen (H2), hydrogen sulfide (H2S), ammonia (NH3), carbon disulfide (CS2), sulfur dioxide (SO2), oxygen (O2), and nitrogen (N2). Of these inorganic gases, those with an odor always contain N or S in combination with H, C, and/or O, such as H2S, NH3, CS2, and SO2.

Hydrogen sulfide, from the anaerobic reduction of sulfate (SO42-) by bacteria, usually is the most prevalent odor in natural waters and sewage. Sulfate, formed from the aerobic biodegradation of sulfur-containing proteins, is commonly present in domestic wastewater between 30 and 100 mg/L. Sulfate can arise in natural waters from sulfate minerals and aerobic decomposition of organic material. In addition to H2S, other disagreeable odorous compounds may be formed by anaerobic decomposition of organics. The particular compounds that are formed depend on the types of bacteria and organic compounds present. Table 6.6 lists a number of common odiferous inorganic and organic compounds with their odor characteristics and odor threshold concentrations when dissolved in water. Sewage carrying industrial wastes may contain other volatile organic chemicals that can contribute additional odors.

Environmental Chemistry of Hydrogen Sulfide

Under anaerobic aqueous conditions, in the presence of organic matter or sulfate-reducing bacteria, sulfate is reduced to sulfide ion (S2-):

SO42- + organic matter/sulfate-reducing bacteria —> S2- + H2O + CO2. (6.20)

Sulfide ion is a strong base, reversibly reacting rapidly in water to form HS- and gaseous hydrogen sulfide:

S2- + 2 H2O o OH- + HS- + H2O o H2S(g) + 2 OH-. (6.21)

HS- and S2- are nonvolatile with no odor. H2S is gaseous with a strong odor of rotten eggs. The equilibrium distribution between S2-, HS-, and H2S depends mainly on the pH and somewhat on the temperature. In Figure 6.7, T = 30°C.

FIGURE 6.7 pH distribution of hydrogen sulfide species in water.

Rules of Thumb

In water, S2- reacts according to Equation 6.21:

1. Raising the pH shifts the equilibrium to the left, converting the malodorous gas H2S into nonodorous and nonvolatile HS- and S2-.

2. Lowering the pH shifts the equilibrium to the right, creating more malodorous H2S gas from the nonvolatile forms, HS- and S2-.

3. Lowering the temperature shifts the equilibrium to the right (more H2S) at any pH.

4. Well water, groundwater, or stagnant surface water that smells of H2S (rotten eggs) is usually a sign of sulfate reducing bacteria.

5. Water conditions promoting the formation of H2S are

• oxidation-reduction potential = <200 mV;

Chemical Control of Odors

Depending on the odor-causing compound, chemical control of odors may be accomplished by a combination of pH control, eliminating the causes of reducing conditions, chemical oxidation and/or aeration, sorption to activated charcoal, air-stripping of volatile species, and chemical conversion (often microbially mediated, as in nitrification).

pH control

Hydrogen sulfide

For odors from H2S, raising the pH (by adding NaOH or lime) shifts the equilibrium to the left for Equation 6.21:

S2- + 2 H2O ^ OH- + HS- + H2O ^ H2S + 2 OH-. (6.21)

This converts gaseous H2S to the nonodorous ionic forms. However, the pH must be maintained above 9 for complete odor removal. Normally, odor control by removing the sulfur compounds is more practical.


Odor Characteristics and Threshold Concentrations in Water

Odor Threshold




in Water (mg/L)

Odor Characteristics

Allyl Mercaptan



Very disagreeable, garlic-like




Sharp, pungent

Benzyl Mercaptan







Pungent, irritating





Crotyl Mercaptan




Diphenyl Sulfide




Ethyl Mercaptan



Decayed cabbage

Diethyl Sulfide (ethyl sulfide)



Nauseating, ethereal

Hydrogen Sulfide



Rotten egg

Methyl Mercaptan



Decayed cabbage

Dimethyl Sulfide (methyl sulfide)



Decayed vegetables




Disagreeable, irritating




Fecal, nauseating

Sulfur Dioxide



Pungent, irritating




Rancid, skunk-like




Putrid, nauseating

figure 6.8 Fraction of hydrogen sulfide in unionized form (H2S) as a function of temperature and pH.

Lowering the pH (by adding acid) shifts the equilibrium to the right, converting the ionic forms to gaseous H2S (see Figure 6.8). At low pH, the gas can be removed from the water by air stripping in a manner similar to the air stripping of ammonia in Example 6.7. This of course does not destroy the H2S; it moves it from the water to the air. Figure 6.8 gives the fraction of hydrogen sulfide that is in the volatile form of H2S. Note that H2S behaves the opposite of NH3 (below). Stripping efficiency is increased with decreasing pH and lower temperatures.

Ammonia pH control of odors from ammonia is opposite to that for hydrogen sulfide. To use air stripping to remove odor caused by ammonia, the pH must be raised (see Figure 6.9 and Example 6.7).

figure 6.9 Fraction of ammonia in unionized form (NH3) as a function of temperature and pH. Example 6.7 Removing Ammonia by Air-stripping A wastewater flow contains 30 g/L total ammonia nitrogen

Total NH3-N = [NH3] + [NH4+], and has a 5 g/L discharge limit. The temperature varies from 0°C to 30°C and the pH is normally about 9. At what pH must the stripper be operated?

Calculation: pH must be adjusted so that enough ammonia is in the volatile form to meet the


discharge requirement. The ratio must be at least as large as the fraction of NH3-N

to be removed. Fraction of NH3-N to be removed = = 0.83.

3 30

Answer: Stripping efficiency is increased with increasing pH and higher temperatures. Figure 6.9 shows that to get 0.83 of the total NH3-N in the form of volatile NH3, the pH must be raised to about 10.8, for the worst case of 0°C. At higher temperatures, the removal efficiency will be higher. Lime (CaO) is the least expensive way to raise the pH, but it generates CaCO3 sludge. NaOH is more expensive but it does not generate sludge. Note that NH3 behaves the opposite of H2S (above).


Add oxidizing agents such as Cl2, NaOCl, H2O2, O2, KMnO4, or ClO2. They oxidize hydrogen sulfide to odorless sulfate ion, SO42-, and ammonia to odorless nitrogen compounds, including elemental nitrogen, N2.

Rules of Thumb

1. The usual chlorine dose for odor control is 10 to 50 mg/L.

2. 8.9 mg of chlorine is required to oxidize 1 mg of hydrogen sulfide, H2S.

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