Roger Tim Haug
Editor’s Note: We’ve received several requests for BioCycle’s 2004 publication, Odor Management At Composting Facilities. It is no longer available, but the requests prompted us to pull the book off the shelf and refresh our memories. This article in Section One by long-time colleague and friend Tim Haug is a primer everyone should have. This is a slightly edited version.
Composting of biosolids, yard trimmings, manures, food wastes, municipal solid waste and other substrates has become a very popular management option. The process makes use of nature’s own microbes and produces a useful end product. In short, it accomplishes most of the things that environmentally minded folks like to see. Composting’s one drawback, however, one that is a thorn in many sides, is its potential for odor generation.
Somewhere in the history of composting, we got the idea that, if left alone, Mother Nature would be odor free. I don’t know how this idea arose, but I assure you that it’s not true, particularly with sludge materials. Presented here are 12 Theorems that support 5 Elements of odor management.
Theorem 1: Sludge Smells.
No argument here, I hope. But the corollary to #1 is that other substrates smell too. All of the substrates that enter a composting process must be viewed as potential sources of odorous molecules. This includes sawdust, yard trimmings, wood chips, MSW, food waste, and all the other substrates we may throw into the starting mix.
Theorem 2: Mother Nature Never Claimed To Be Odor Free.
On many occasions, I have heard speakers state with great pomp, “if composting is conducted properly, there will be no odors.” These speakers are never plant operators! Such statements have been implicated as a leading cause of anxiety complex among operators. Let me reassure the operators that I have never seen data to support such a claim. The starting substrates contain odorous compounds and more are formed as intermediates during the breakdown of complex substrates. Yes, this includes aerobic metabolism.
Theorem 3: Thermophilic Composting Acts Like A Heat Distillation Process.
If you want proof for Theorem 3, I offer the observation that hot sludge smells more than cold sludge. Vapor phase concentration of a compound is proportional to its vapor pressure, and the latter increases with temperature. Many odorous compounds have significant vapor pressures. Therefore, you can expect to see them in the process offgas.
Theorem 4: Some Release Of Odorous Compounds Is Inevitable During Composting.
None of what I said so far depends on the type of composting system. The only caveat is that anaerobic systems are not included. While there may be some differences between aerobic systems, their odor characteristics are all governed by the same laws of nature. Therefore ….
Theorem 5: Mother Nature Doesn’t Much Care What Compost Systems You Have. And
Theorem 6: It’s Not Nice To Fool Mother Nature With A Bad Design Or Bad Operation.
Some odors may be produced even with good design and proper operation. However, a bad design or bad operation guarantees them.
A major development in recent years [reminder that this was originally published in 1991] has been the evolution of quantitative tools to predict odor incidents. While the subject is still elusive, we have much better tools for quantifying and predicting odor emissions, their treatment, and dispersion. To support these tools it is necessary to first quantify the odor strength. This is usually accomplished by using a panel of subjects, each exposed to odor samples that have been diluted with odor-free air.
The number of dilutions to achieve a 50% positive response is termed the threshold odor concentration (TOC). In an attempt to really confuse us, the field has adopted a number of other terms such as odor unit (OU), dilutions to threshold (D/T), and effective dilutions (ED).
The mass emission from a point source is equal to the flow rate times the OU concentration. OUs are dimensionless (i.e., volume of diluted sample/volume of sample) and the mass emission has units of volume/time. Area sources are characterized by the surface odor emission rate (SOER) with units of volume/time-area.
Armed with a few theorems and surrounded by butanol equivalents, it is time to present the first elements of odor management …
Element 1: Plan For Odor Management, Before, During And After Design.
This may sound obvious, but there are numerous facilities that have “sinned” against this first commandment.
Element 2: Operate The Best You Can To Reduce Odor Emission Rates.
Good operation is vital to odor management. Good operators can make a bad design work okay, and a good design work great. Skilled operators, effective process control, and good housekeeping are powerful forces.
Biofilter to treat process air
Element 3: Contain, Collect And Treat As Much As Possible Or Go Directly To Dispersion.
If your composting system allows collection of the process gases, then you should treat them to the extent possible. If you cannot contain the process gases, then you must rely entirely on atmospheric dispersion to dilute any emissions.
Treatment methods that have had some measure of successful application to composting include chemical scrubbing; adsorption; thermal oxidation; fume furnace; catalytic; regenerative; recuperative; biofiltration; masking/neutralization.
The science and art of wet chemical scrubbing has advanced significantly. …. Wet scrubbing is a 3-stage scrubbing system designed to remove ammonia, water soluble organics and inorganics, and even water not-so-soluble compounds.
Thermal oxidation is very effective in destroying odors. … The problem with thermal oxidation is the large fuel bill at the end of the month. Alternatives to reduce the fuel bill include catalytic, recuperative, and regenerative furnaces.
A number of biofilters are in use at facilities in the U.S. Biofilters can work well in some circumstances if properly designed and properly operated. Problems with existing installations usually result from excessively high loading rates, poor flow distribution, too little moisture in the biofilter matrix, too high an inlet temperature, or inadequate dispersion of the treated gases.
Having discussed deodorization, it is important to recognize the next theorem …
Theorem 7: Odor Treatment Is Never 100%.
The only possible exception to Theorem #7 is thermal oxidation, which is capable of near complete odor destruction.
Theorem 8: Past Designs Usually Didn’t Recognize Theorem #7.
There has been a subtle but persistent tendency for the design community to ignore Theorem #7. Scrubbers are often designed with no attention to dispersion of the treated gases. It is common to see scrubbers with short stub stacks, low outlet velocities, scrubbers located near large buildings and their downwash, rain caps on top of discharge stacks, and other examples of poor dispersion design.
Theorems 7 and 8 lead to one of the most important and often ignored elements of odor management ….
Element 4: Dilution Is Part Of The Solution.
Dilution is rarely the complete solution to a problem, particularly odors. However, it is equally true that dilution must be part of the solution.
Theorem 9: The Worst Odor Never Occurs When You’re There.
Theorem 9 was developed from my experiences with the atmosphere. Another way of stating this Theorem is to say that odors usually result during worst case meteorologic (met) conditions. The likelihood of your being at the site or in the surrounding community during the worst met conditions is small. With ground level sources, the worst met conditions usually occur in the evening and early morning hours. On some days, dispersion is very limited during these periods because of strong “micro inversions,” which occur as a result of ground cooling.
Ground level dispersion is usually highest during the day, after the sun has warmed the ground surface. Because most of us work the day shift, we would likely miss a worst case incident. Managers who didn’t smell anything during the day may want to return in the evening when the met conditions may be worse.
Theorem 10: Mother Nature Always Disperses Odor, But Sometimes She Can Use A Little Help.
I think this has something to do with entropy maximum randomness, and other dimly remembered concepts. Whatever, I don’t think we have to worry about nature reconcentrating odor on us. At the same time, we needn’t just accept the dispersion that nature alone supplies. We can design systems to improve dispersion, thereby giving nature a helping hand. The term “enhanced dispersion” has been used to describe such efforts.
For point sources, such as an exhaust stack from scrubbers, dispersion can be enhanced by: 1) increasing stack height; 2) increasing stack velocity; and 3) providing forced dilution with ambient air. The latter will also increase the stack diameter, which in turn will increase the effective plume rise.
Things to avoid with a point source discharge include: 1) locating the plume within the zone of the building or stack downwash; 2) low velocity discharges from the sides or roofs of buildings, such as ridge ventilators; 3) using rain caps on roof ventilators; 4) low stack velocity; and 5) bad topography such as valleys. Avoiding bad topography is like avoiding the flu — easy to say but hard to do. The topography is always greener in the next political jurisdiction.
For ground level sources, such as open windrows and static piles, dispersion can be enhanced by: 1) providing adequate buffer; 2) using wind machines to maintain minimum flow over the area sources; and 3) using barrier walls to induce turbulence. (Providing buffer really isn’t an example of enhanced dispersion. It’s more like giving nature enough room to solve the problem herself.)
If the above measures are not adequate, the ground level source can be enclosed and converted to an elevated source. By comparison with elevated sources, ground level sources are subject to the worst met conditions and lowest dispersion. In addition, the nearest receptor will be the most affected. Therefore, ground level sources, and their surrounding topography, must be carefully considered in any odor management plan.
Every management plan needs a goal or target. It’s hard to achieve a goal if you don’t know what the goal is. This leads to the last Element in the odor plan ….
Element 5: Some Form Of Odor Criteria Must Be Established.
The industry has had a tough time with this one. Politicians want to say that “there will never be any odor.” You can tell that politicians never took statistical thermo in school or they would know that “never” is very hard to achieve. Plant managers often resort to “there will be no discernable odor beyond the plant’s boundary.” Sounds good but no one really knows what it means. Another favorite is “there must be no odor complaints.” If we only knew what ED50 caused a complaint, then we’d have something to work with. Agencies have also had problems with terms like acceptable, permissible, allowable, or tolerable odor criteria. They apply a sense of acceptance of an undesirable situation.
This leads to a somewhat controversial theorem ….
Theorem 11: You Can Stop All Of The Odor Some Of The Time, But You Can’t Stop All Of The Odor All Of The Time.
Theorem #11 is a recognition that, after all the planning and siting studies, after all the attempts to reduce emission rates, after all the collection, treatment, and dispersion, nature will periodically impose such severe met conditions that odor may occur. If the risk of odor cannot be reduced to zero, then we must establish an “acceptable odor risk.”
A number of facilities have recognized the statistical nature of odor criteria. For example, one facility established criteria of 2 OU at the property line less than 3.3 days/year and 5 OU less than 0.5 days/year, with 5 OU the estimated complaint level. Another opted for a criteria of one ED5 under all met conditions. The point is not that criteria can vary, but that they were established in the first place. The criteria provide a guide for assessing alternative designs and solutions.
The elements necessary for an odor management plan are: plan, reduce, prevent, contain, collect, treat, dilute, disperse, buffer, criteria. They’re good words by which to formulate your odor management plan.
Finally, my last theorem …
Theorem #12: Don’t Despair. Odors Can Be Managed.
Despite odor problems at some facilities, the future for composting is optimistic. The industry generally recognizes that odor compounds are likely to be released, which is a milestone of major significance.
At the time Roger Tim Haug wrote this article, he was with the City of Los Angeles’ Bureau of Engineering. Dr. Haug retired from the City as Deputy City Engineer and currently is emeritus professor of environmental engineering at Loyola Marymount University. His most recent publication (2020, CRC Press) is Lessons In Environmental Microbiology. He was recognized by BioCycle as a Trailblazer during our 60th Anniversary celebration in 2019.
