June 15, 2012 | General

Going On Offense Against Odors

Proactive management techniques, such as mixing incoming putrescent feedstocks within an hour of receipt at the facility, are effective tools to prevent odors. Part III

Craig Coker

BioCycle June 2012, Vol. 53, No. 6, p. 25

This article series examines the intricacies of odor management: how and where odors are generated, measured, and perceived; how they are managed through good process control; proactive operations management; how they are controlled with technology; and how to manage the public outreach related to organics recycling odors.
There are a number of proactive, positive strategies composting site managers and staff can implement to greatly reduce the risk of off-site odor episodes. It is helpful to break down the various compost processing activities that cause odors into a series of discrete elements, and then analyze each activity within that specific element to determine how odors are being formed, volatilized and dispersed off-site. By investigating each processing step, specific management practices can be identified that could help minimize these odors.
The discrete elements reviewed in Part III of this odor management series are: Feedstock Receipt, Processing and Mixing; Active Composting; and Site and Facility Management. Tips are provided by composting facility operators, researchers and project consultants.

Feedstock Receipt, Processing And Mixing

Liquefaction of food waste during transport can lead to delivery of odorous materials. Mixing that material upon delivery with amendment or covering it with unscreened compost or wood grindings will help mitigate odor generation. Photo courtesy of Brooks Contractor, Goldston, NC

An important first step is to understand the odor-causing potential of each feedstock and either reject the feedstock or be sure the facility can effectively handle the material. A feedstock acceptance protocol is a good tool to understand the nature of incoming materials before deciding to accept them. With a protocol in place, a generator would supply a sample of the feedstock, a portion of which can be sent to a lab for analysis of its compostability parameters that were discussed in Part II of this series (see “Odor Defense Strategy,” May 2012).
Another portion of the sample can be put into a sealed bag in order to mimic the anaerobic decomposition process. Keep the sealed bag in a warm place for two to three days (a car dashboard works well) and then have someone whose sense of smell has not be compromised by working at the composting facility open the bag and give an indication of the intensity and unpleasantness of the smell. If objectionable odors are noticed, then the composting facility operator will know to have plenty of coarse bulking agent on-hand when this feedstock arrives, to ensure that aerobic conditions prevail during composting.
Having stockpiles of certain odor-abating materials will also help. “I would add that a good stock of brown material (wood) should always be kept ready for adding to any wet odorous green material such as grass clippings to allow good air flow and drainage through the pile,” says Gavin Bartlett of Shorts Composting in Berkshire, United Kingdom. “By monitoring the oxygen, moisture and carbon dioxide levels you should be able to turn compost when it’s fully aerobic and produce minimal odors.” Sharon Barnes of Barnes Nursery in Huron, Ohio echoes Bartlett’s comment: “Always maintain a sufficient stockpile of processed yard waste to cover any unexpected odor event. Always place the more putrescent material on an improved surface so that should an issue occur the operator can get to the pile, regardless of the weather.”
Putrescent feedstocks should always be a concern to operators. Commercial food scraps collection trucks have potential to deliver odorous materials, due in part to food scraps sitting in hot trucks for more than two to three days, and in part from liquefaction of the contents during transport. “Always prepare your site to receive sloppy food wastes with something like leaves, grindings, or similar amendment that will absorb material containing lots of liquid,” says Heidi Ringhofer of Western Lake Superior Sanitation District in Duluth, Minnesota.
Prompt handling of feedstocks is another important odor-minimizing strategy. If possible, the operator should get incoming feedstock processed and mixed with amendment within one hour of receipt. If that is not possible, cover the material with a 3- to 4-inch layer of unscreened compost or woody grindings. In any case, mixing and placing those feedstocks in a windrow or in an aerated pile should be attempted by the end of the day. If a load comes in late, it might be necessary to cover it with compost or grindings and then mix it in first thing the following morning.
CalRecycle (formerly the California Integrated Waste Management Board (CIWMB)) published a series of odor mitigation strategies composters can consider (CIWMB, 2007). Table 1 is an excerpt from this detailed list of recommended activities related to feedstock receiving and mixing.

Active Composting

As noted in Part II, getting the mix right and keeping piles aerobic is the most important aspect of process management in odor control, but there are operational considerations that will help. Observe loader operators to be sure they are not driving up on a pile or windrow to place materials, which will compact under the weight of the loader and compress out the free air space. Once a pile or windrow is built, put a 3- to-4-inch cap of compost over it to act as an in situ biofilter for fugitive emissions. If windrow composting, don’t turn that windrow for the first 7 to 10 days. This allows primary decomposition of highly degradable organics to occur with some degree of control (note that good structural porosity is a must for this to work). For those windrows with adequate free air space, and assuming there are no regulatory restrictions, consider reducing turning frequencies for the first two weeks, turning only to distribute moisture from a rainstorm or for improving water distribution when irrigating.
Most of the odorous chemicals that vaporize from a compost pile or windrow are highly soluble in water. This phenomenon is the main advantage of covering piles with a micropore fabric, which forms a layer of odor-absorbing water on the underside of the cover (the moisture layer traps the odorants and keeps them from volatilizing). Alternatively, one can apply water to a pile by misting or spraying to knock down odorants. One of the reasons the air smells clean after a rainstorm is the “scrubbing” effect of rain on pollutants in the air. Buyuksonmez (2011) studied the effect of watering windrows prior to turning in a study for the San Joaquin Valley Air Pollution Control District (SJVAPCD), and found that volatile organic compounds (VOCs) were reduced by 19 percent by watering for 20 minutes. The term “VOCs” is used to describe a large category of compounds, many of which are odorous at composting temperatures, thus if VOCs are reduced, many odors also will be reduced. The SJVAPCD is developing a new requirement that composters in its district must water windrows before turning to reduce VOCs.
Keeping an eye on the weather, the calendar and the clock also helps. Activities that generate odors, like mixing, turning windrows and moving fresh piles, should be minimized at certain times, provided that operations can tolerate the disruptions. For example, when the air is heavy and still — defined as a wind speed below 4 miles/hour and a less than 10°F difference between the ambient and dew point temperatures — keep odor-causing activities to a minimum (Das, 2000). Restricting odor-producing activities to between 10 am and 3 pm, when the sun has heated the atmosphere to promote good vertical mixing, and refraining from those activities late in the afternoons on Fridays and the days before holidays (when neighbors are likely to be out in their yards or at public places) can also minimize odor episodes.

Site And Facility Management

The two most important site management practices to reduce odors are rigorous housekeeping and water management. Housekeeping is always important at a composting facility, as every bit of stray organic matter not incorporated into a pile is a potential odor source. It requires dedication to focus an hour per day on housekeeping patrol, where stray bits of mashed food scraps or clumps of grass clippings are picked up and put into a pile. Managerial complacency about housekeeping can quickly spread to the facility workers, and soon, there are so many potential fugitive odor emissions that it becomes almost impossible to get the site cleaned up and back into shape.
Rainwater puddles and storm water ponds are a potentially onerous source of odors. Compost fines wash into every puddle and pond on a site and they exert strong biological and chemical oxygen demand. This demand quickly depletes the dissolved oxygen (DO) in the water, faster than the oxygen can be replenished across the water-air interface. Anaerobic conditions in the water are created with the resultant formation of hydrogen sulfide. Larger storm water ponds, if not mixed and aerated, will stratify during the summer into different levels of temperature and DO. Compost fines washed into those ponds will accelerate consumption of DO; in the fall season, when stratification ends with cooler temperatures, the layers in the pond will mix together, bringing anaerobic waters to the surface with a release of odors. This is what caused the green waste composting facility at the Southeastern Public Service Authority in Virginia Beach, Virginia to shut down after Tropical Storm Gaston washed fines into the facility’s pond in September 2008.
Part II of this series noted the importance of understanding the local weather at a composting facility site. “One strategy that I’ve had success with at sites already experiencing issues is to install an on-site weather station — or in some cases a wind sock or a flag — and start tracking weather data (wind speed and direction primarily) and then use that data to correlate between specific operations such as grinding, turning and screening and complaints,” says Matt Cotton of Integrated Waste Management Consulting LLC in Nevada City, California. “In some cases this can help pinpoint a specific odor-causing event and perhaps suggest a need to reschedule those events until conditions improve. You can get a decent recording weather station for around $1,000 and it can really help you better understand your site — ideally before the complaints start.”
As Tim Haug, author of Practical Handbook of Compost Engineering, famously observed, ‘the prevailing winds rarely prevail,’ so having a detailed record of your own site-specific conditions can be a very useful tool. Doesn’t work in every case, but you’d be surprised how well it can work if you can identify times/conditions that you can operate without impact versus times/conditions that are causing issues. In some circumstances a simple (non-recording) wind sock or a flag can provide useful clues.”
Ensuring good operational flexibility is another key part of the battle against odors. “I always recommend to composters that they size equipment bigger than necessary to reduce specific processing times for grinding, mixing, turning and screening,” said Jeff Gage of Compost Design Services at a recent U.S. Composting Council presentation (Gage, 2012). “It’s important to keep all your equipment operating properly to minimize odors, so you should keep critical spares in stock, especially for long lead time items, and have rental backup sources for common equipment like loaders.”
Along with good process design and control, positive and proactive operational management measures are another tool in the tool chest composters can use to prevent off-site odor episodes. Part IV of this series will examine odor control technologies in use at composting facilities.
Craig Coker is a Contributing Editor to BioCycle and a Principal in the firm Coker Composting & Consulting (, near Roanoke, VA. He can be reached at

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