If sewage smells pretty much the same everywhere else, would it be safe to say that the resulting odors are caused by the same chemical compounds? That is the general premise of this article. If true, this can greatly simplify the odor control at your agency.
All Sewage is Created Equal
This premise applies not only to raw sewage, but to treated sewage in the different plant process areas. True, some raw sewage may be more septic than others and some receive chemical treatment prior to arriving at the plant to reduce the odor impact. But the original premise may still hold true in terms of the proportion of the chemical compounds, called odorants. The concentrations may be higher, but the same priority compounds are present. The same can be said for the odorants at the different plant process areas. The types of odorants present are key to choosing an effective odor treatment technology. If all headworks, primary, activated sludge, trickling filter, dissolved air floatation, filter-press dewatering, and cake truck loading processes smell alike at all treatment plants and therefore are caused by the same compounds, then the results and conclusions of OCSD’s Odor Control Master Plan could be applicable to your own facility. If true, this may save your agency a lot of headache and resources.
What OCSD found
Many odorants cause odors at treatment facilities, but not all are important. How to tell which ones are? It is easy; dividing each odorant’s concentration (C) by its odor threshold concentration (OTC) will establish the real priority for odor control. The higher the number obtained from this operation, the highest ranking in importance for that odorant. These are OCSD’s most detectable odorants:
- Hydrogen sulfide (H2S)
- Methyl mercaptan (MM)
- Dimethyl Sulfide (DMS)
- Dimethyl Disulfide (DMDS)
- Ammonia (AMM)
- 2-Methyl Isoborneol (MIB)
- 2-Isopropyl-3-Methoxypyrizine (IPMP)
Table 1 and 2 show this prioritization process for three plant process areas. It demonstrates that apparently un-important odorants, based on concentration, actually are important when shown as C/OTC (e.g. hydrogen sulfide and IPMP for trickling filters). Others that appear important (e.g. dimethyl sulfide and ammonia) actually are not important as odorants of concern for trickling filters.
Comparing your particular odor situation with that of OCSD may be easier than you think. First, verify existing lab results or run some foul air samples of your plant processes by a lab for reduced sulfur compounds, calculate their respective C/OTC values, pick the top three odorants, and then compare your results with Table 3. One thing to keep in mind is that OCSD results are a product of several iterations at each odorous plant process. The worse-case figures were chosen for the analysis. Consequently, your results may be different, but your priority odorants should be the same.
The Methyl Mercaptan Factor
OCSD did extensive tests using both chemical analysis (GC/MS) and sensory analysis (D/T and OPM) to characterize odors at both treatment facilities. After careful review, it is safe to conclude that a simple reduced sulfur test is sufficient to determine the extent of treatment required for an effective odor control. This test determines the compounds shown in the “green” family of odorants. Determining the other odorants are not critical for the design of an odor control system. Reason? Because it was found that methyl mercaptan is in fact the limiting factor. This odorant is difficult to remove because it is not as reactive as hydrogen sulfide. As a result, the available technologies that are able to remove methyl mercaptan by 90% or better (in order to avoid public nuisance) will ensure that the other odorants in the list will also be reduced below nuisance levels.
Uncovering the Key Factor: Odor Nuisance Levels
This whole analysis is useless unless we know what our target concentration at the stack should be in order to ensure that public nuisance is avoided. To this end, OCSD conducted extensive tests of the most detectable priority odorants. Several olfactometer runs were conducted coupled with chemical analyses and OPM analyses to determine the sensory curves for each priority odorant. By establishing that any odor at an odor intensity of 3 or more becomes a nuisance, the nuisance concentration for each priority odorant was determined as follows:
- Hydrogen sulfide (H2S): 1.3 ppb
- Methyl mercaptan (MM): 0.22 ppb
- Dimethyl Sulfide (DMS): 7.9 ppb
- Dimethyl Disulfide (DMDS): 0.77 ppb
- Ammonia (AMM): 4,900 ppb
- 2-Methyl Isoborneol (MIB): 0.06 ppb
- 2-Isopropyl-3-Methoxypyrizine (IPMP): 0.035 ppb
- Skatole: 0.037 ppb
- Indole: 1.1 ppb
To determine the extent of treatment needed at your facility, some modeling work must be conducted to determine the number of dilutions that the foul air from each of the plant processes experiences before leaving your fenceline. Once that is known, target concentrations for each odorant at each plant process area can be calculated by multiplying the location dilution factor by each odorant nuisance concentration. Is that simple.
 Gas chromatography/mass spectroscopy is used for most chemical analysis of foul air
 Dilution to threshold is a sensory test where 50% of the panelist agree on the number of dilutions required to barely detect an odor for a particular sample using an Olfactometer
 Odor profile method is a sensory test where panelist statistically agree on the character (how it smells like) of an undiluted foul air sample. It is expressed in units of odor intensity (how strong that smell is)