How to Control Compost Odors

The first and last concern everyone has about composting is the smell. Compost odors that offend (malodors) can make life very difficult, and coexistence with neighbors sometimes impossible. In other words; all your investment is at risk when you generate malodors. 

The good news is that with care in design and diligence in operation odors can be controlled to levels that are acceptable to the neighboring airshed. The best defense against malodors is a great facility design. At Green Mountain Technologies, we design facilities that produce great compost with minimal malodors. With over 400 systems around the globe, none of our clients’ facilities have ever been shut down due to malodors. We’ve been able to accomplish this through our standard design practices. 

Managing Your Compost Pile’s Oxygen Levels 

When designing a facility, it’s vital that you manage air flow. Oxygen levels within the pile should be above 13% at all times. The composting process is very hungry for oxygen in the first 1 to 2 weeks. The aeration system must be designed to meet this need, or malodors are formed. We target designs that provide up to 5 cubic feet per minute (CFM) per cubic yard of biologically fresh and degradable feedstocks like grass and food waste. This provides enough airflow to cool a pile 7 feet deep or less. Since most loader operators over stack the initial piles to save time and effort, the piles may overheat. But they do not run out of oxygen, simply because it takes about 9 times more airflow to cool a pile than to provide adequate oxygen. 

While more odors will be driven off by higher heat (think heat of vaporization of different compounds) the ability to manage pile temperatures allows the operator to improve odors by lowering pile heights back to the design level.  GMT uses our temperature probes and reversing air flow dampers to keep a pile’s temperature in control. 

Adding Bulking Materials

However, it is also more than just the interstitial spaces between globs of goo that need oxygen, the insides of the globs do too. This is why bulking material can be so important to the composting process, and management of odor. Composting is a surface edge biological function. It is very important to add lightweight fine granular material like sawdust or ground coconut husk (coir) to both absorb excess water and to be a surface to coat the goo onto so that the aerobic bacteria can eat and breathe in comfort.  Otherwise, their evil cousins, anaerobic bacteria, start going crazy inside the goo generating reduced sulfur compounds that smell quite foul. We find that a rule of thumb to start with is 1 part goo to material to 3 parts bulking material can keep you out of trouble, and you can experiment on either side of that amount to find. We strongly suggest mixers for gooey feedstocks. 

Using Biofilters and Biocovers

Once you have your oxygen levels under control and bulking materials added, you have optimized the composting process. All that is left is polishing the emissions from your piles. This will make odors acceptable to even the most discerning noses in your air shed. 

In the first week to 10 days, you can use biofilters and biocovers to contain and treat the odor compounds biologically. Old composted woody material doesn’t contain much food for bacteria and it has a significantly large surface area that can hold a thin film of water. The perfect home for hungry bacteria. Did you know that odor compounds are food for bacteria? They are easy to absorb and provide energy for the bacteria to thrive on. Did you also know that most odor compounds are hydrophilic, meaning they love to attach themselves to water molecules? The rest is done by nature to reduce odors by over 80% before they leave the biocover or biofilter.

The trick is to keep the odor controlling media moist by watering overhead twice a day during dry warm weather, and just allowing condensation to form in cool moist weather. Designing biofilters is more complex and requires its own post, but biocovers are pretty simple; cover a freshly built pile with 6 inches to 12 inches of screened composted overs, and water overhead. When you turn the pile (more on this later) the cover materials continue to help adsorb odors within the pile as well.

Having Odor Issues? Give Us a call!

At Green Mountain Technologies, we’re proud of the fact that no facility we’ve designed has ever been shut down due to odor. With smart design, proper facility management, and state of the art composting technologies we are able to stop malodors. We also work with existing composting facilities to help get their odors under control. 

Have any questions about odors at your compost site? Contact us! We’ll be more than happy to find a custom odor solution for you compost facility.