The practice of composting has proved to be an important alternative to landfills for recycling waste organic matter.  At least 22 states in the U.S. now have restrictions or bans on the disposal of yard trimmings in landfills.

The Environmental Protection Agency (EPA) drafted the findings of an evaluation of pathogen reduction in composting sewage sludge into a regulation known as the EPA 503 rule.  The hygiene premise in the framework of the EPA 503 rule is that the level of fecal coliform bacteria can be an indicator of the presence of Salmonella, which is very common in sludge, but is more expensive to analyze.

As more types of recycled organic matter are reaching agricultural and consumer markets, the presence Salmonella and other pathogenic bacteria–like E. coli–is of concern.

In a study published in the Journal of Food Protection, 94 market-ready, non-sludge recycled organic matter composts produced in Washington, Oregon, and California were sampled.  The researchers selected 39, 25, and 30 sampling locations from Washington, Oregon, and California, respectively.

In Washington state, the researchers sampled bagged and bulk composts; much of the bagged material was widely available at nursery centers.

According to the study, composters in Oregon employed large to very large compost pile methods, and therefore all samples were obtained from bulk processing facilities from 11 counties around the state.

In California, the researchers sampled bulk green-waste facilities in areas ranging from Northern California to Southern California.  All samples from each state were collected within a 3-week time frame.

Analysis of recycled organic matter composts produced a wide range of fecal coliform results for all regions. In Washington, 23 percent of samples exceeded the EPA 503 limit. Only one sample was positive for Salmonella.  Although the difference was not significant, composts listing manure were generally lower in fecal coliform count than those not listing manure.  Fifty-five percent of samples of bulk compost products exceeded the EPA limit.  

Researchers found that bagging operations are normally run indoors with more well-aged and stabilized material to avoid market issues, including odor and reheating.  That could be one explanation for excessive fecal coliform counts.

For Oregon, 44 percent of samples exceeded the EPA 503 fecal coliform limit; no samples contained measurable Salmonella.

The range of fecal coliforms in California was similar to those in Washington and Oregon, with only 20 percent of samples exceeding the EPA 503 limit. One facility in a noted vegetable-production area produced compost with a very high fecal coliform level.  No Salmonella was detected in any sample.

The researchers detected measurable E. coli O157:H7 in samples from three facilities. These facilities were in the large facility group and were situated within important vegetable-growing regions.  The sample with the highest E. coli O157:H7 count was retained for 3 weeks at five degrees Celsius, and was retested.  It was still positive for E. coli O157:H7.

“I can say that I am distressed to have found E. coli O157:H7 in market-ready compost batches from different facilities.,” said Will Brinton, one of the researchers who conducted the study.  “This finding is incongruous with the image of compost as a soil-building, crop-enhancing agent.”

A large number of compost samples examined in this study possessed fecal bacteria levels that exceeded the EPA 503 rule.

According to Brinton, “It would be guesswork to discern the cause or estimate the risk when E. coli O157 is present in composts. Our study sample period coincided unfortunately with the 2006 Spinach E. coli incidents in California. We have no evidence there is an association but the possibility exists.”

“I feel that there is a likely association of incidence of E. coli O157:H7 in compost and the very rapid growth of the organic matter recycling industry coupled with less than adequate development of quality standards. I see evidence of relaxation of compost-recycling standards in order to facilitate growth of that industry.  Perhaps it is time for readjustment.”

With the growth of the composting industry, the researchers suggest there should be additional research to more closely examine “critical processing factors that influence pathogen levels in finished compost. As modes of compost usage spread, there is increased concern that pathogens from compost could enter the food chain.”
“In my opinion, one way to significantly increase safeguards would be to introduce Identify Protocol (IP) tracking of inputs to compost facilities, a means to create traceability in event of problems. The next is to focus more attention on the end-product of composting rather than simply on the process of recycling organic matter. This would necessarily entail shifting quality standards to third-party agencies with credibility to address agricultural food-chain utilization separate from landfill-offsets and recycling. It is a great challenge since not all compost goes to food chain purposes, but with continued good-will on all sides, it can be accomplished,” said Brinton.