Compost odor control

 

At composting facilities, the decomposition of organic materials produces odorous compounds and chemical emissions. These odors can affect the environment and nearby communities, potentially leading to compliance issues and involvement from regulatory authorities.

Why compost produce odours at different stages of composting?

 

Compost produces odours at various stages of the composting process due to microbial decomposition.  Microbes break down the feedstock and release gases such as carbon dioxide, sulfur compounds, and moisture.

The intensity of smell depends on the type of composting process used: aerobic (with air) or anaerobic (without air). Aerobic composting produces milder odours, while anaerobic composting can generate stronger, more unpleasant smells.

 

Primary sources of compost odors at commercial facilities

 

• Raw organic materials that have begun to decompose

• Final product stockpiles that are not properly managed

• Improper mixing of feedstocks and the production of leachate without adequate drainage

• Strong odours can also arise during the mixing and preparation stages, as this process breaks up areas where materials have started to rot without oxygen (anaerobic decay). Additionally, adding air to the organic materials either through aeration systems or turning often generates the strongest smells at the facility.

Because of these impacts, many countries enforce strict regulations on windrow composting operations. 

Aerobic vs. anaerobic composting: Which method helps control compost odors?

 

In aerated pile composting, air is evenly distributed and microbes convert organic matter into carbon dioxide, moisture and heat.

A study of biosolids composting found that forced aeration reduced ammonia, formic acid and acetic acid concentrations by 72%, 57%, and 11% respectively versus windrow piles without the aeration treatment (Rosenfeld P, Grey M, Sellew P., 2004). Thus, aerobic process tends to emit fewer odor intensive compounds.

Anaerobic composting (without enough oxygen) is much more likely to produce strong, unpleasant odors:

Conrnell composting found, when oxygen is limited for instance due to high moisture, compaction, or large piles that restrict air flow, anaerobic microbial pathways dominate. These lead to the production of volatile sulfur compounds (VSCs) like hydrogen sulphide (H₂S), dimethyl sulfide (DMS), dimethyl disulfide (DMDS), mercaptans, and volatile fatty acids (VFAs)  many of which smell like rotten eggs, cabbage, or sewage.

 

What compost odors reveal about the composting process?

 

If ammonia smell is strong, it means the compost pile has a feed mix problem and nitrogen presence is excessive.

Solution: Add more carbon such as leaves and wood shavings.

If there is a musty smell, it means the mix has too much moisture that can lead to produce sulphurous odor if not corrected on time.

Solution: Add a bulking agent as soon as you start to smell mustiness.

 In order to control odors, the best solution in open windrow composting is to turn the compost pile.

 

Why odours from commercial composting facilities are serious health hazards? 

 

Commercial composting facilities emit bioaerosols and microbial volatile organic compounds (VOCs) into the air during aerobic decomposition, especially when the compost is disturbed. 

Bioaerosols include: 

 

Microorganisms: bacteria (e.g., actinomycetes), fungi (e.g., molds), viruses, algae, and pollen. 
Biomolecules: endotoxins (from gram-negative bacteria), beta-glucans (from fungi). Emission levels vary by facility size, technology, wind, and microbial content. These emissions return to background levels between 100 to 1400 meters away from the facility downwind.

 

Microbial VOCs include:

 

Emitted from decomposing plant material and microbes. Not strongly correlated with bioaerosol concentrations. 
Common VOCs: acetic acid, acetone, limonene, toluene, benzene derivatives, xylene, alcohols, and terpenes (e.g., pinene, camphene).  Some VOCs were detected up to 800 meters downwind at notable concentrations. 

Emission levels are affected by composting methods: 
Aerated static piles produce lower emissions of certain compounds (e.g., 72% less ammonia) compared to windrows. 

In essence, both bioaerosols and VOCs are key air contaminants from composting, with their spread and concentration influenced by composting technique, feedstock, and environmental conditions.

References

• Rosenfeld P, Grey M, Sellew P. Measurement of biosolids compost odor emissions from a windrow, static pile, and biofilter. Water Environ Res. 2004 Jul-Aug;76(4):310-5. doi: 10.2175/106143004×141898. PMID: 15508421.
• https://compost.css.cornell.edu/odors/odor.html