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A step-by-step guide for investigating and responding to odor complaints.
Dirk Apgar, Jay Witherspoon, Richard Finger
As population centers continue to grow and expand, filling in much of the open space around wastewater
utilities, odor nuisance complaints quickly are becoming an issue that nearly all plants are having to grapple
with. No matter what your utility’s role – be it collection, conveyance, and treatment or disposal of
wastewater byproducts – growing numbers of neighbors, their proximity, and their sensitivity to odors
increase the probability that you will receive complaints, even if the odors are coming from somewhere
other than your plant or conveyance system.
So what do you do about it?
Systematically responding to odor complaints will minimize the amount of effort you expend dealing with
the issue, as well as the potential for negative outcomes such as litigation. Understanding what prompts
people to register complaints and developing an organized method of receiving, recording, investigating,
and resolving such complaints will benefit the public welfare and also may help to protect your utility from
possible legal and financial liabilities.
Understanding Nuisance Odors
To generate a nuisance, the presence of an odorous compound must be of sufficient intensity, duration,
frequency, and character to be offensive. Because these factors either singly or in combination must meet a
certain threshold before somebody is offended, not all air emissions emanating from wastewater sources
will result in nuisance odor complaints. For example, most people generally tolerate short, infrequent, and
low-level odors. Once offended, however, the threshold they consider a nuisance may be forever lowered,
and even a subsequent slight odor can illicit strong emotional responses.
General dissatisfaction with the perceived odor source can compound the problem. For example, noisy
operations, sludge trucks driving on public streets, bright and annoying lighting, and a decrepit facility
appearance all can serve to heighten the general perception of the nuisance.
Complaint Registry Made Easy
When somebody is moved to the point of complaining about an odor, any delay or inconvenience in filing a
complaint can further annoy the offended party, making it more difficult to appease them. Consequently,
utilities should have a simple process set up for receiving complaint information, and, depending on a
utility’s size, location, and staffing, there are several ways to do this.
A clearly listed telephone number offers the best means through which an individual can find out how and
where to register a complaint. If a dedicated odor complaint hotline is not appropriate for your facility, a
second listing of the general plant phone line as the number for complaints is a good idea. Of course, in
today’s high technology environment, a Web site page for registering specific odor complaints and
concerns also offers an excellent way to receive input. Personal visits by neighbors should be encouraged,
and complaints received by mail always should be acknowledged.
The least desirable means of receiving a complaint is through an elected official or governing agency. If
someone has resorted to this complaint mode, it probably means that other methods such as those noted
previously were not available or easy to work through. No matter what method a utility chooses to use for
receiving odor complaints, the system established should provide prompt feedback to those registering
An odor complaint investigation begins as soon as the complaint is received. Gathering information from
the complainant is crucial in determining the offending odor’s source. Someone who can understand and
act on the information received – typically a lead operator or plant manager – immediately should take over
the investigation. If these employees are not available, the person receiving the complaint should be trained
and equipped to obtain the appropriate information. Maintaining a professional and compassionate
demeanor is important at all times, so that the person registering the complaint knows that the issue is being
taken seriously and that an investigation will be initiated quickly. Investigators should not take offense to
the complaint or be surprised if the complainant is upset because odors can elicit strong emotional
responses. The professionalism exhibited by the person taking the call can go a long way toward calming
someone upset by nuisance odors.
Investigators should record information from the complainant systematically. A pre-prepared form for
logging this information should be available and used so that important details are requested and recorded.
Such details include the time of the perceived odor event, defining characteristics, and intensity and
frequency of the odor impacts. Investigators also should record the utility’s operating parameters, as well as
wind direction and speed, at the time and preceding the odor occurrence. Particularly important are any
process anomalies, upsets, or maintenance activities that may have lead to the release of odors from your
Investigators should not accept responsibility for the offending odors at the beginning of an investigation,
however, because the odor’s source has not yet been determined. They also should not promise to rectify a
condition that may not be controllable, only that they will investigate the odor.
After receiving the basic information, an investigator should get to the scene of the complaint as soon as
possible with the tools necessary for conducting a full investigation. The most important tools, of course,
are the investigator’s own nose, eyes, and ears, and investigators should record their own perceptions and
observations of the odor’s intensity and character.
Likewise, an investigator immediately should begin monitoring in an attempt to quantify the odor impact.
Monitoring odors in the field can be a difficult task, however, because odors detected by the complainant
may have significantly or completely abated by the time the investigator arrives on the scene. Of the most
odorous gases associated with wastewater, only hydrogen sulfide (H2S) can be measured directly in the
field at the low concentrations detectable by the human nose without expensive analytical equipment.
Humans commonly detect H2S at a concentration of 0.47 parts per billion by volume (ppbv), and the
generally accepted detection concentration ranges from 0.8 to 8 ppbv. H2S can be detected at concentrations
as low as 1 ppbv with a gold film sensor-based instrument, but often, the complaint will be due to odorous
gases other than H2S. Consequently, you will need to back up and elaborate any instrument readings with
physical observations recorded in the investigator’s notes.
Any information regarding activities in the vicinity of the odor complaint can help with qualitatively
assessing the level of the odor event. If people are enjoying picnics or sporting events in the area, for
example, then the problem probably is not severe. Conversely, if people are running from the scene, you
may have major trouble on your hands.
A camera is an invaluable tool for recording the scene and conditions of possible odor sources, as well as
any ongoing activities in the vicinity. Photos can document the site where odors were detected in relation to
the assumed odor source and any other odor sources that may have contributed to the problem such as
rotting vegetation or garbage cans. These photos can be used to help focus on probable odor sources during
the investigation, as well as later to recall specific details about the investigation.
The investigator also should record specific weather conditions to help in identifying the likely location of
the odor source and eliminate downwind sources from consideration. Wind speed and direction can be
measured with simple and inexpensive anemometers. Notes regarding precipitation and cloud cover can
help in understanding the atmospheric stability and odor dispersion, particularly if atmospheric dispersion
modeling is used to diagnose odor transport and impact.
If meeting with the complainant is a possibility, investigators should gather as much additional information
as they can, discussing details regarding the location where the odor was perceived, over how broad an
area, and whether the odor was detected indoors or outdoors. Such information will help in determining
whether the odor is coming directly from your system and impacting the neighborhood, or rather through a
dry p-trap [what is this? A p-trap is a configuration of pipe & elbows used to trap water and provide
a barrier between the sewer and the plumbing fixture (sink, toilet, etc.) The water barrier prevents
gases from the sewer from coming into the home. Look under any sink and you’ll see what I mean.
The term is used commonly in the wastewater and plumbing industries.] or plumbing roof vent and
impacting only one household. Also, the complainant should be asked whether the source is related strictly
to wastewater or if other sources possibly could be causing or contributing to the odor impact. Often,
merely raising the possibility of other odor sources can lead the complainant to recall non-wastewater odor
In addition to the complainant, the investigator also should attempt to interview other people found in the
vicinity of the odor impact, asking them about the intensity, character, duration, and frequency of any odors
they might have detected. Service people working in the area such as police officers and letter carriers can
be excellent sources of information because they cover a lot of territory surrounding the complaint site and
can provide opinions unbiased by emotional or financial attachment to the area. Contrary to local residents,
they are not as likely to fear the loss of property value or have concerns about the odor’s impact on the
quality of their home life.
Looking at System Sources
Once interviews are completed and other probable odor sources examined, you will want to take a good
look at your own possible sources of odors. At the treatment plant, you should evaluate your unit processes
and decide if any of them individually or as a group could be emitting the offending odors. Usually, the
odor emission levels will not be a surprise, but occasionally, a system upset that has gone unnoticed could
be causing higher than normal odor emissions. You also should check your odor control equipment to
verify that it is operating properly and odorous exhaust streams are not being shared with any neighbors.
Similarly, turbulent flow in the conveyance system can cause significant off gassing of odorous
compounds. Consequently, you also should check force main and siphon discharge structures, as well as
other places where turbulent conditions exist, for odor emissions. Air blockage in places like a siphon inlet
structure also can be a source of odor emissions. Here, air that has been dragged along by wastewater will
find its path of least resistance through your customers’ dry p-traps, roof vents, or a manhole. Check pump
and regulator stations to make sure ventilation of those structures is not resulting in odor emissions. As at
the treatment plant, make sure that here too any odor control equipment is operating properly.
After the investigation is completed, you hopefully will have identified the odor impact’s cause. If the odor
is in fact coming from your system, take immediate action to rectify the situation and halt the odorous
emissions. If odor emissions cannot be stopped immediately, take steps to control the odor as soon as
practical. Often, odor problems result from the plant’s design and the proximity of new neighbors. A plant
that has operated in the same mode for years can quickly become an odor source as neighbors move in
nearby. Some odor control solutions require significant planning, engineering, capital investment, and time
to implement. When this is the case, you may have to take some interim measures to decrease the odor
impacts until a final solution is in place.
Regardless of the investigation’s outcome, you should notify the complainant of your findings. Even if you
could not assign a definitive cause to the odor event, as is frequently the case, the complainant should be
made aware of all the possibilities that were investigated. Often, the complainant will be satisfied once
assured a thorough investigation has been conducted and reasonable conclusions have been drawn. You
also should encourage complainants to report any future observations of unacceptable odors, but emphasize
that they notify you as soon as possible to better enable the plant staff to investigate and, when the odors
are associated with the plant or collection system, resolve the problems.
Be sure to document the conclusions of your investigations, any recommendations for corrective actions,
and actual actions taken. If you determine that no action, or a delay in action, is required, then the odor
investigation report should clearly explain why you made this decision. The documentation should be clear
enough that anybody, especially attorneys, reviewing the records can determine that a clear, well-reasoned,
and diligent effort to resolve the complaint was undertaken.
Utility, Public Welfare at Stake
Why is investigating odor complaints so important? First of all, protecting the public welfare by protecting
all parts of the environment must be the goal of any wastewater treatment utility. Preventing air pollution
impacts by controlling odors is as fundamental a goal as preventing water pollution.
Moreover, preventing and diligently investigating and resolving nuisance odor emissions also protects the
utility. If odors become a significant problem, air pollution regulators may use their authority to require
changes that utility management has little to say about. If found legally liable for odor impacts, utilities can
incur significant costs to compensate those who have suffered. Such an outcome can occur if proof is
shown that the utility, through some act or failure to act, caused the odor emissions, which, in turn,
inhibited neighbors’ ability to use or enjoy their property. Proof also must be shown that damages, such as
decreased property values, resulted from the odor. Additionally, an existing record of unresolved odor
complaints can result in significantly increased public opposition to future wastewater projects in the
Being a good neighbor, on the other hand, offers wastewater utilities a level of community trust, making
public support for other facility programs easier to win. The key to a successful odor response program is
this public trust and a utility’s willingness to listen and respond to odor complaints and their perceived
Dirk Apgar, P.E., is the H2S corrosion and odor control program manager for the wastewater treatment
division at the King County DNRP [Department of Natural Resources and Parks please spell out this
acronym] (Seattle, Wash.; E-mail: email@example.com).Jay Witherspoon, [please provide short bio],
CH2M Hill (location? Bellevue, Washington). Richard Finger, is the west section manager for King
County DNRP’s wastewater treatment division (E-mail: firstname.lastname@example.org).
Sidebar: Investigation Checklist
[Authors: I thought this info might be more useful to the reader as a stand-alone box imbedded in the
text. Does this sound okay?]
Some useful tools or strategies to consider in investigating and documenting odor-related findings include:
- developing an odor response/complaint form to be filled out by the investigator;
- contacting local elected officials, plant staff living in impacted areas, or local commercial owners
to find out how the plant is perceived in terms of odors and their impacts;
- installing an on-site meteorological station;
- purchasing hand-held odor, ammonia, and hydrogen sulfide (H2S) monitoring equipment such as
sorbent tubes and an H2S detector;
- maintaining a dedicated file of known or confirmed odor complaints and any subsequent
resolutions, along with follow-up contacts and dates of the occurrences(?);
- offering facility tours and odor-related fact sheets on plant efforts to control odors; and/or
- developing formal intra-agency odor response processes so that any staff who handles odor
complaints knows his or her role, required actions, and responsibilities.
Your facility already may have incorporated some or all of these practices, but an audit is always beneficial
to see where needs or gaps exist in an odor response program. And, if this checklist is not enough, some
additional possibilities you might want to consider involve:
- setting up a formal community advisory panel;
- linking an odor dispersion model and odor emissions estimating model to a meteorological station
for estimating real-time odor impacts; and
- conducting direct odor source testing using various advanced odor sampling and analysis tools
both at the plant and in the community.