PRINCIPLES AND PRACTICE
QUICK-TO-IMPLEMENT ODOR REDUCTION TECHNIQUES
A round-up of odor management tools, concepts and ongoing research gives operators some ready-to-use strategies.
Nora Goldstein

THE ART and science of managing odors at composting and other types of organics recycling facilities continues to evolve. In some cases, what were once experimental approaches have now become common — and effective — practices. While some mitigation and control steps require complete facility overhauls, many are a matter of better understanding and handling of feedstocks, timing steps like pile breakdown and windrow turning when the winds are favorable, and employing quick-to-implement control measures.

This article provides a round-up of principles and practices that are either being used successfully or are being researched. Articles in BioCycle throughout 2002 will explore many of these areas in more depth.

TRIED AND TRUE FUNDAMENTALS

William Toffey, biosolids recycling manager for the city of Philadelphia, has had a tremendous amount of experience with odor management – both at its aerated static pile composting facility and with its land application program. We asked Toffey for his “quick ideas” about composting odors – what he feels are some of the bottom line odor control essentials. Here is his response:

1. Get your moist feedstocks, particularly food residuals and grass, into a composting mix before they start to rot. With biosolids, use well digested biosolids and take them directly to composting after dewatering, rather than stockpiling.

2. Make sure the moisture content of the mix is optimal (55 percent to 65 percent moisture range in the case of our biosolids-wood chip mix), so that you don’t form anaerobic pockets, which can be a source of nitrogen and sulfur odors.

3. Keep the pile well aerated. Without adequate oxygen, and unless excess heat can dissipate, aerobic composting will be lost, replaced by odor generating anaerobic processes. Good aeration can be accomplished in several ways. However, a common mistake is to presume that mixing the pile incorporates oxygen, when in fact the oxygen is used up quickly. A good strategy is to use a generous proportion of coarse textured amendment so that the pile is porous, heat and moisture can readily escape, and oxygen is drawn in.

4. If at all possible, use a biofilter to scrub odorous process air. If not, a compost pile blanket seems to provide some biofiltration.

5. Use common sense management practices. Consider reducing compost handling during periods of stagnant, humid summer climate conditions, or during times of the day, such as early morning and early evening, when neighbors are most likely to be outdoors. Also, locate your operations as far as possible from neighbors who are in the direction of prevailing winds. If you are doing windrow composting, recognize that odors are strongest during the first several turns, and time your passes to avoid adverse weather and wind.

6. Finally, psychologists have learned that people’s reactions to odors can be influenced by their knowledge of the odor source. Invite your neighbors to visit your facility and learn of your operations, and give them a chance to be reassured that the odors are harmless. Also, give them a point of contact at your facility to whom they can register complaints, and then do what you can to respond to those complaints.

MANAGING ODORS BY MANAGING FEEDSTOCKS

The editors of BioCycle are working on a new compost operator training module series. The first installment covers feedstock management. The following is based on information in the first chapter:

Feedstocks have a major effect on the type and amount of odors generated at a facility. Therefore, proper selection and preparation of feedstocks should be on the front burner of odor management. Where particularly odorous feedstocks are handled, site selection and process management become more critical.

Each feedstock has a characteristic odor quality. The odor may not be apparent when the material is fresh but it is released or intensified once decomposition commences. Furthermore, the odor may be acceptable, perhaps even pleasant, at normal levels but becomes overwhelming when the material is amassed in large quantities, such as at a composting facility. In addition to odor character, feedstock degradability is a factor. Some materials simply decompose faster. They release the odors at a faster rate and require greater aeration to remain aerobic.

The potential for odors to be generated during the process is also dependent on the physical characteristics of the feedstocks. In addition to degradability, moisture content and free air space (FAS) determine how easily the process can become anaerobic. Both moisture and FAS influence the movement of air and transfer of oxygen to the decomposing materials. FAS is closely related to other physical characteristics including porosity, structure and bulk density. In fact, FAS is closely correlated with bulk density, which is an easier characteristic to measure.

Highly degradable feedstocks should be mixed with slowly degradable materials to get a mix that degrades at a moderate pace. This is especially important with composting methods that rely on passive aeration like windrows. As a gauge to how well mixes will aerate, feedstocks can be combined to achieve a bulk density between 800 and 1000 lbs/cy (this should also produce a reasonable FAS). Finally, moisture content has a direct impact on odor generation and movement of oxygen through air space to particles. To start, moisture should be in the 50 to 60 percent range, but over the entire period of composting, the range is 40 to 60 percent — depending on the porosity and structure of the compost mix — to avoid saturating conditions that increase bulk density and slow oxygen transfer.

MITIGATION MEASURES

When the Baltimore biosolids composting facility was built in the 1980s, a chemical scrubbing system was installed for odor control. Odor complaints persisted and the operator at the time, Professional Services Group, focused in on the basic compost recipe. “We get biosolids from two different sources and one is a lot more odorous than the other,” says David Hill, project manager with U.S. Filter, which now operates the plant. “It was decided to amend the recipe.”

At the time, chipped raw wood with a high moisture content was being used as a bulking agent, along with sawdust. The flat chips had a tendency to shingle and compress, and in combination with the sawdust, says Hill, resulted in a very dense compost recipe. “A switch was made to using tub ground pallets, which produces a very different kind of wood chip. The pallet wood is kiln dried and primarily hard wood. It is much drier, has a different shape than the raw wood and creates a lot more pore space in the compost mix.”

Another major change was the addition of a high carbon wood ash. “That adsorbs the odor producing volatile organic compounds that would normally blow off of the compost,” he adds. “It holds odors down until the compounds are broken down by the composting process.” The ash is purchased from a wood boiler operation. It is mixed in with the sawdust that is stored in a silo. Fresh wood chips and recycled wood chips are stored in separate silos. All three amendments are loosely combined on a feed belt, with biosolids added on top. The belt feeds into two McLanahan paddle mixers. After mixing, materials are composted in an aerated, agitated bed system.

The wood ash takes effect pretty quickly because it is distributed throughout the mix. “Basically, there have not been any odor complaints since wood ash was added to the recipe,” says Hill. “We haven’t used our scrubber for odor control for almost three years. Our permit requires us to run it four hours/week, but we only run water through the system.” He adds that the high carbon wood ash is the most expensive ingredient in the compost mix, primarily because it is getting more difficult to procure because of air quality restrictions that have led to more efficient burners at boiler plants that produce less ash. In terms of the finished compost, the ash has a positive impact. “The best effect is that it darkens the color of the compost and the perception is that the darker a soil or compost is, the richer it is,” says Hill. “So the visual aspects are very positive.” There is minimal impact on nutrient characteristics and pH.

In Marlborough, Massachusetts, a biosolids/MSW cocomposting plant also has been testing use of wood ash and getting good results. Mixed MSW and biosolids are first put in rotating drums, then are conveyed to a trommel screen to separate out contaminants. During this trial phase, high carbon wood ash is added in with the material passing through the two inch holes. “We add one bucket of wood ash to about nine buckets of the compost feedstocks,” says Bob Spencer of Waste Options, Inc., which owns and operates the plant. The company also has been experimenting with adding wood ash right before the final screen. “This step is very effective,” he adds. “There is still some odor but it doesn’t create nuisance conditions when the compost is stockpiled in soil blenders’ yards, which is how most of our compost is marketed.”

Waste Options also installed a misting system ahead of the trommel screen to apply the BAT 506 odor control product manufactured by Global Odors Control Technologies. “This application step definitely reduces odor production in the plant,” notes Spencer. It has been particularly effective in the composting hall where there are 8,000 cubic yards of material in aerated piles at any one time. “We are not sending as much odorous air out to the biofilter for treatment, and there is less opportunity for fugitive emissions.”

The Global Odor product used in Marlborough has more available carbon in it to balance out the variation in the feedstocks received at the plant that cause daily changes in the C:N ratio. “The BAT 506 has a very high compound oxygen content. It contains amino acids, sulfates, other minerals, various protein derivatives and other ingredients,” explains David Hill of Global Odor. “Essentially, it keeps degradation going inside the composting pile, controlling the generation of odorous gases.” Two ounces are added to each ton of material. It is mixed with water, which acts as the carrier. To be most effective, the piles or windrows should not be disturbed for 28 to 30 days after formation and any initial agitation. “During that time, the material stratifies within the mix in a certain way resulting in very effective deodorization,” he adds.