This article is continued from Part One, which was published in the October 2016 edition of revolve magazine.

Water, Water Everywhere

Assuming the C:N ratio is dialed-in correctly, the correct moisture content advances the rate of decomposition.  If the biofilm around the particles dries out, microbial activity will go dormant and composting will stop.  As piles dry out, the concentration of potential odourants in the biofilm increases, with a risk of odour emissions.  Conversely, if moisture is allowed to climb above 60 percent or so, the free air space channels between the particles clog with water.  This thicker biofilm reduces the amount of oxygen available to the microorganisms on the surface of the particle as the rate of oxygen transfer in water is much slower than the rate of transfer in air. Material with an optimum moisture content of around 50 to 55 percent has the consistency of a wrung-out sponge that is wet but not freely dripping water.  One of the challenges in composting food scraps with large amounts of vegetable and fruit material is that the plant cell walls break open under the heat of initial decomposition, flooding the pile with water.  Without adequate structural porosity to allow that flush to drain, the pore spaces in the pile will fill with water and risk formation of anaerobic conditions.

The transfer of oxygen across the biofilm requires a steady flow of air through the pile.  Whether by natural or passive means, or forced through a pile by a blower, aeration serves several critical functions in process management, including replenishment of oxygen, removal of carbon dioxide (and volatile odourants), and removal of heat.  Compost piles and windrows have both macro-aeration and micro-aeration characteristics.  Macro-aeration refers to the overall uniformity of the structural porosity of a pile.  A compost pile of wet dairy manure mixed with sawdust has low macro-aeration characteristics.  A compost pile of chipped tree waste has good macro-aeration characteristics. Good macro-aeration characteristics are necessary where passive aeration is the primary means of oxygen transfer, like in windrows.  Micro-aeration characteristics refer to how well air moves inside the pile.  Fine particles, such as those produced by processing woody wastes with a hammermill, can impede aeration rates in various areas, creating air-starved sections in a pile. 

In addition to good site design and good process management, there are a number of proactive, positive strategies compost site managers and staff can implement that will reduce the risk of off-site odour episodes.  It is helpful to break down the various compost processing activities that cause odours into a series of discrete elements, and then analyze each activity within that specific element to identify ways odours are being formed, volatilized, and dispersed off-site. By investigating each processing step composters may be able to identify specific management practices that could help minimize these odours. 

Do We Really Want This?

An important first step is to understand the odour-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.  A sample of a potential feedstock can be put into a sealed bag for several days, in order to mimic the anaerobic decomposition process.  Place 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 odours are noticed, then the compost facility operator will know to have plenty of coarse bulking agent on-hand when this feedstock arrives, in order to ensure that aerobic conditions prevail during composting.

Prompt handling of feedstocks is another important odour-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, then the pile of feedstock should be covered with a 7.5 – 10 cm layer of unscreened compost or woody grindings.  In any case, mixing and emplacing of 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 up in the morning.

As noted above, getting the mix right and keeping piles aerobic is the most important aspect of process management in odour control, but there are operational considerations that will help.  Watch the 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 7.5 – 10 cm cap of compost over it to act as a biofilter for fugitive emissions, and if windrow composting, don’t turn that windrow for the first 7-10 days (see Figure 1).  This allows primary decomposition of highly degradable organics to occur with some degree of control (note, piles must have good structural porosity for this to work).  For those windrows with adequate free air space, and assuming there are no regulatory obstacles, consider reducing turning frequencies for the first two weeks, turning only to distribute moisture from a rainstorm or for improving water distribution when irrigating.

Placing a compost cap (Photo: CalRecycle)

Placing a compost cap
(Photo: CalRecycle)

Keeping an eye on the weather, the calendar and the clock will help also.  Activities that generate odours, 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, keep odour-causing activities to a minimum.  Keeping odour-producing activities 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 neighbours are likely to be out in their yards or at public places) can also minimize odour episodes.

Keep It Clean

Two important site management practices to reduce odours 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 odour 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 through the facility workers, and soon, there are many potential fugitive odour emissions sources.

Rainwater puddles and storm water ponds are a potential source of odours.  Compost fines wash into every puddle and pond on a site and they exert strong biological and chemical oxygen demand.  This demand will quickly deplete the dissolved oxygen (DO) in the water, faster than the oxygen can be replenished across the water-air interface.   This creates anaerobic conditions in the water.  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, so, 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 odours.

Managing odours in composting is never easy and requires a constant vigilance.  Failure to do that, however, is usually a prescription for headaches, negative public relations and regulatory problems that can quickly lead to fines and shut down orders.  As the 19th Century American diplomat, Benjamin Franklin, once said, “An ounce of prevention is worth a pound of cure.”

Craig Coker is a Kiwi living in the United States and is Principal of Coker Composting & Consulting ( He can be reached at