There was a time when odors from places like manufacturing plants and wastewater treatment facilities were considered simply facts of life. Community residents looked upon the aroma of paper mills, for example, as the smell of money and jobs.

It’s different today, observes Robert Bowker, principal of Bowker & Associates, a consulting engineering firm in Portland, Maine, that specializes in the control of odors and corrosion in sewers and wastewater treatment plants.

People today in general are much more sensitive to odors, and they expect remedies. That’s putting pressure on wastewater treatment plant managers and operators. Bowker observes that solutions to odor problems can be elusive and, once found, challenging to fix completely. He advocates a systematic, scientific approach to diagnosing and correcting problems. He shared his experiences and ideas in an interview with Treatment Plant Operator.

TPO: What has changed in terms of public perception of odors?

Bowker: Twenty years ago, people would be more likely to say, ‘Oh, that’s the treatment plant; we notice that once in a while; we’re used to it.’ Today people are more aware of their environment, and they also understand that by putting enough pressure on the local municipality, they can sometimes cause changes to occur. The minimum standard of what odor is acceptable has changed and is becoming more stringent.

TPO: What is your role in solving odor problems at treatment plants?

Bowker: Most of the work I do is at the early stages of a problem — when people are starting to complain — and evaluating what is responsible for the complaints. Sometimes the plant personnel don’t know where to begin.

Granted, operators are very familiar with their plants, and they often have some idea what is causing the odor — what process is potentially responsible — but that is actually pretty difficult to determine. You have many sources of odor in a treatment plant — the headworks, the primary clarifiers, sludge handling operations and dewatering, and other places where odors can be emitted.

One challenge is to figure out where you’re going to get the best bang for the buck. What is the priority? If I cover the grit chamber and treat that air, what will be the result? I come in and actually collect air samples from these different processes and come up with a scientific way to rank the sources of odor. Here’s your number one source; now let’s look at what you can do to knock that down.

TPO: Identifying the source of odor seems simple. What makes it so difficult?

Bowker: You have to look at it in terms of how many pounds of odor are being released from each source. You might have a sludge holding tank that’s got a real intense odor — and there is a technique for measuring the strength. Some sources might have strong odors, yet release relatively little odorous air.

On the other hand, you might have a primary clarifier where you walk up next to it and it doesn’t smell that bad, but you have much more surface area emitting odor, and so the pounds of odor leaving that process are more than the pounds of odor leaving the sludge holding tank.

People will invite me to walk around and help them figure out which direction they should be heading. But without taking some basic measurements and collecting some data, it’s tough. One thing that can help is to do what’s called odor modeling. You take those calculations of how many pounds of odor are being released from a source and put them into a dispersion model, and it will predict how far downwind that odor may drift and where it might be a problem.

TPO: How is it possible to quantify odors?

Bowker: We can collect samples of the air and send it to a lab — usually two kinds of labs. One type of lab does what’s called an odor panel analysis of the samples. They use an olfactometer to make up known dilutions of the odor. Then they present that diluted air to a group of trained panelists. The object is for the panelists to determine at what dilution they can first start to detect the odor. Then the lab calculates the strength of the odor in terms of dilutions to threshold. That is, how many times do I have to dilute this odor with clean air before 50 percent of the odor panel can no longer detect it?

The other kind of lab actually analyzes the odor constituents using a gas chromatograph. There are a lot of sulfur compounds that contribute to the odor. The main one is hydrogen sulfide, but there’s also methyl mercaptan, dimethyl sulfide, and others. If you’re looking for a way to treat that air, it’s very helpful to know what’s in it.

If it’s primarily hydrogen sulfide, that’s one thing, but if you get into solids handling, dewatering and sludge holding, you start seeing these other compounds that are formed under anaerobic conditions — organic sulfur compounds that are more difficult to treat, chemically or biologically.

TPO: Do all odors largely originate at the treatment plant?

Bowker: No, a lot depends on the characteristics of the wastewater coming into the plant. Is it septic (anaerobic) so that it already contains hydrogen sulfide? Usually, plants that have septic sewage coming in have odor problems.

We look at the collection system to see how that’s contributing to odors. Big contributors of hydrogen sulfide are force mains. Sewage is detained in these enclosed pipes, and it becomes anaerobic pretty quickly. In gravity sewers, sewage tends to get aerated and stay fresh. Yes, it smells like sewage, but it’s only when it’s allowed to go anaerobic and generate hydrogen sulfide that the odor really gets intense. Normally, we would collect sewage samples and measure sulfide content, dissolved oxygen, pH, and oxidation reduction potential (ORP).

TPO: Can anything be done in the collection system to counteract odors?

Bowker: Yes. One option for force mains that’s quite economical is injecting pure oxygen to keep the wastewater aerobic. Bleach is sometimes used, although that can be expensive. One approach that has been quite popular in the past 10 or 15 years is injecting products containing nitrate. If you add nitrate to sewage that is going septic, it acts as a source of oxygen for the bacteria. They will use nitrate as an oxygen source rather than using sulfate and degrading it into hydrogen sulfide. Hydrogen peroxide is another product that’s used to knock out hydrogen sulfide. Iron salts like ferrous sulfate and ferrous chloride can be used to precipitate the sulfide and tie it up to keep it from being released as a gas.

TPO: Once you have all the necessary data and can quantify the odor, how do you go about finding a solution?

Bowker: Most plants will do a phased approach. Maybe we’ve done some sampling and modeling and found that the headworks and the sludge holding tank are contributing about 70 percent of the odor emissions. Then the challenge is to figure out how much control is enough — where we can get the most bang for the buck.

We come up with cost estimates for different control strategies. For example, this solution costs so many dollars per pound of odor removed. Suppose we estimate that an aerated grit chamber is spitting out 40 percent of the total odor from the plant. If you treat just that, will it solve the odor problem? Probably not, but it will help. Then you start looking at the primary clarifiers and other sources, and probably the dollars per pound of odor start going up.

A municipality usually can’t sink all the money into a project that it may want to and solve everything at once — that’s just economic reality. The objective is to get the odors in the community down to a level that people find acceptable.

TPO: How do you go about collecting the air to be treated?

Bowker: You have to decide from which processes you want to pull air for treatment. To cite one example, a primary clarifier is often a fairly significant source of odor. If you have sewage containing hydrogen sulfide sitting in a tank not being stirred up much, you may not detect a lot of odor, but once you stir it up and create turbulence, you release those odorous gases.

In a primary clarifier, most of that turbulence is at the effluent launder where the sewage cascades over the weir. About 50 to 90 percent of the odor is released at the effluent launder. So you might look at strategies like covering the effluent launders and the turbulent areas, collecting that air, and conveying it to some form of treatment.

TPO: What treatment options are available?

Bowker: Traditionally, odor treatment systems have included chemical scrubbers, where the odor comes in the bottom of a tower containing a packing material, and you circulate a chemical solution of bleach and caustic soda through that packing. That process absorbs and destroys the odorous compounds, and the exhaust is discharged out the top.

These systems are very space-efficient, but they involve the handling of hazardous chemicals. I’ve seen a trend in the last 10 to 15 years that operators justifiably don’t want to handle 50 percent caustic soda if they don’t have to. That has tended to push people toward other options.

Another traditional technology is activated carbon. It’s effective for a pretty wide range of odorous compounds, and there have been a lot of developments in improving the carbon chemically so that it does a better job of removing hydrogen sulfide. There are some carbons where you can actually regenerate its ability to remove hydrogen sulfide by soaking it in water. The big disadvantage of carbon is you don’t want to use it on a very strong odor. It will work in those conditions, but it won’t work very long because the capacity of the carbon gets exhausted.

TPO: What about biological treatment? Is that equally effective?

Bowker: Biofilters have been around for 20 or 30 years. They traditionally contain organic material, such as wood chips, a blend of compost, or bark mulch. The air is passed up through this bed. You need a sprinkler system or some way to keep that bed moist. The air is treated by bacteria as it passes up through the media.

They have a pretty large footprint, but if you have the area, they are very economical and effective. The disadvantage is that the organic media, although inexpensive, will degrade over time and will need to be replaced in about three to five years.

In the last 10 years or so we’ve seen a new technology, new to the U.S. anyway, called bioscrubbers or biotrickling filters. They look similar to a chemical scrubber. It’s a vertical vessel with a packing material inside designed to grow bacteria. The packing will be some type of plastic mesh or lava rock. The air is passed up through that media, which is kept moist with an irrigation system.

Unlike biofilters, bioscrubbers usually require nutrients to be added. They are extremely effective for hydrogen sulfide. You can get 99 percent removal, and the only thing you have to worry about is a small nutrient tank that you have to feed occasionally. The drawback is that these systems can be less effective on odors caused by more complex sulfur compounds.

TPO: What basic advice would you give to a treatment plant staff facing a persistent plant odor problem?

Bowker: One thing I always say: Whatever it is, keeping it moving. Anytime you let material hang around for a length of time, particularly sludge, it will go south very quickly. That applies to liquid sludge or sludge cake. Keep it moving, get it out. Cleanliness can help — flushing the weirs, keeping sludge holding tanks clean. It might not solve the problem, but it can go a long way to minimize the odor released. The other thing is — don’t ignore your neighbors. If they perceive that there’s a problem, there’s a problem.

There are certainly things like chemicals that plant staff can try on their own. Operators are pretty sharp people. They try their own solutions, and a lot of times they work. Other times, on more complex problems, it helps to have some advice. An odor expert can look at things holistically, evaluate the plant and the collection system, and put them on the right road to success.