Killing Potential Pathogens in Compost – Cold and Calculated

There is hope that smaller communities and institutions who want to compost their own organic waste can do so safely and produce a pathogen free compost. Its been traditionally understood that if we can’t achieve temperatures of > 55 C during composting, we have no hope of killing the potential pathogens and the resulting compost is not safe.

Based on the information reviewed in the last few blogs, as well as our own experience with small scale composting, we are able to produce a pathogen free compost on the small scale. During the review in the last few blogs, we learned that:

  1. pathogen kill during composting is as much about the microbial community as it is about temperature
  2. 40% of compost in small scale composters met the Class A compost requirements – while 60% of large scale composters met requirements for Class A compost

Our own experience with small scale composting confirms that Class A compost can be achieved at lower temperatures. During our compost operator training in November 2016, we created a 100 kg compost recipe that included at least 25% catering waste, 10% poultry litter and yard waste. It was composted for two weeks in an insulated composter, where it achieved 55 C for 2-3 days. We knew that the material on the edges did not meet the requirements for pathogen kill. Following the two weeks of composting, we screened the material to < 1″, placed the material in a garbage bucket outside with a lid, and promptly forgot about it.

As we were preparing for the April compost operator course, we tested the material in the bucket. On November 18, 2016, the compost contained > 2080 MPN/g of both E.coli and fecal coliform. In a second measurement on March 27, 2017, the compost contained < 6.1 MPN/g of both E. coli and fecal coliform.

Was this an anomaly or could this be repeated? We made two batches of compost that included 30-50% food catering waste and the balance yard waste. Again, it was composted for two weeks, screened to < 1″, and placed in covered garbage buckets. This time we thought we would follow it along a bit more closely – and what we found was fascinating! E.coli and fecal coliform counts started high, but actually went higher before dropping and being no longer measureable!

E. coli counts in two small scale batches of compost stored in garbage buckets outside at about 20 C.

E. coli counts in two small scale batches of compost stored in garbage buckets outside at about 20 C.

The data confirm that pathogen free compost can be achieved at cooler temperatures by paying attention to the microbial community.

From our observations so far, it appears that potential pathogen kill was faster at moisture contents of 60% compared to moisture contents of 50%. We are currently testing this theory with another batch of compost that is currently sitting right beside my desk in our office!

We will report on that later as well as on practical implications of VNBC (viable but not culturable) organisms in finished compost.

In the next few blogs, we will focus on the fascinating topic of odour during the composting process.


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