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«Final Report of the Cold Compost Project Prepared by The Cornell Waste Management Institute Ithaca, NY Ellen Z. Harrison Director May 2004 Cold ...»

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Hygienic Implications of

Small-Scale Composting in New York State

Final Report of the

Cold Compost Project

Prepared by

The Cornell Waste Management Institute

Ithaca, NY

Ellen Z. Harrison


May 2004

Cold Compost Project – Final Report


This project was made possible by funding received from the Cornell University Agricultural

Experiment Station and Cornell Cooperative Extension. Useful comments on the fact sheet

developed to reflect this project were received from Allison Hornor (Horticulture, Cornell University), Uta Krogmann (Rutgers University), Kevin Mathers (Cornell Cooperative Extension of Broome County) and Adam Michaelides (Cornell Cooperative Extension of Tompkins County). Dan Olmstead (CWMI) was the primary author of this report. Joe Regenstein (Food Science, Cornell University) was a collaborator on the project. Will Brinton (Woods End Research Laboratory) provided helpful insights regarding pathogens in compost. While essential to the quality of this project, these persons do not necessarily endorse the findings and conclusions. CWMI is responsible for the content of the report.

Abstract Small-scale composting is an effective way to recycle organic wastes generated in the home and/or community. Little research has been done to determine potential human health risk of composts generated on a small scale. Bacteriologic testing of twenty composts from across New York State representing a wide variety of small-scale composting practices and situations was conducted. No statistical relationships were found between concentrations of total coliforms, fecal coliforms, enterococci, Escherichia coli, Salmonella spp., and Clostridium perfringens, indicating that none of these organisms could be considered a good indicator of general microbial presence. Compared with microbial standards for sewage sludge composts, these composts generally met those standards. Basic compost parameters were also analyzed. Water holding capacity ranged from 50% to 246%, organic matter 9% to 80.5%, C to N ratio 10.4 to 29, Total Kjeldahl Nitrogen 0.185% to 2.419%, density 24 lb/ft3 to 82 lb/ft3, solids 27.7% to 75.6%, moisture 24.4% to 72.3%, pH 6.54 to 8.65, and Solvita maturity from 3 to 7. No statistically significant relationships at the p=0.1 level were found between microbial concentrations and compost parameters. However, the relationship between pH and TKN was close to the statistical cut off, with higher pH and TKN associated with higher concentration of microbes. An unanticipated finding was that the two laboratories used for bacteriological testing employed different methodologies to look for the same bacteria which may account for some of the discrepancy in results between the labs. Researchers and composters alike need to ensure Cornell Waste Management Institute Cold Compost Project – Final Report methods appropriate for compost are used. The results of this research led to a recommendation to follow good hygiene practices (such as washing hands) when working with composts. Similar practices are advisable when dealing with any soil material since these too may contain bacterial pathogens.

Keywords Small-scale compost; backyard compost; on-site compost; home compost; community compost;

compost pathogens; compost hygiene Acknowledgements



Project Objectives


Materials and Methods

Project Duration

Site Selection

Sampling Protocol

Microorganisms of Interest

Indicator Organisms

Pathogenic Organisms

Microbial Testing

Statistical Analysis


Physical Parameters

Bacterial Concentrations

Comparison of Pathogen Concentrations Reported by Laboratory 1 and Laboratory 2

Relation of Compost Physical Parameters to Microbial Concentration

Results of Multiple Regression Analysis

Correlations between Physical Compost Parameters and Microbial Concentrations

Relationship among Microbes

Relationship of Compost Practices and Microbial Concentration




Figure 1. Hierarchy of Coliform Bacteria

Figure 2. Example of a Correlation Graph between C.

perfringens and E. coli

Table 1. Comparison of Laboratory Methods

Table 2. Ranges of Small Scale Compost Physical Parameters

Table 3. Ranges of Small-Scale Compost Bacterial Parameters for Laboratory 1

Table 4. Ranges of Small-Scale Compost Bacterial Parameters for Laboratory 2

Appendix A. Survey Sample and Summary of Results

Appendix B. Sampling Protocol for Compost Piles

Appendix C. Physical Data

Appendix D. Bacterial Data

Appendix E. Between Lab ANOVA Results of Microbial Concentrations

Appendix F. Microbe Regression Analysis

Appendix G. Independent Samples t-tests Microbes vs. Management Practices

Appendix H. Small Scale Guidance Fact Sheet

Cornell Waste Management InstituteCold Compost Project – Final Report

Project Objectives The overall goals of this study were to: 1) determine the prevalence of selected human pathogens in composts generated in typical small-scale composting systems in New York State; 2) develop guidance for composters operating small-scale systems for, minimizing pathogen risks; and 3) train and educate Extension educators and others about minimizing exposure to pathogens from small-scale composting systems.

Introduction A majority of solid waste generated in the United States is organic material that can be recycled through composting (USEPA 1999). On-site composting of yard trimmings and food scraps at homes, businesses, and institutions is the most environmentally sound approach to organic waste recycling since it avoids transportation impacts and the impacts of large centralized facilities. It also makes the resulting compost available for use by the generator. To be successfully used, however, the quality of the compost must be appropriate for its intended purpose. For use in gardens, hygienic quality in regard to pathogenic organisms is an important quality criteria.

This work focuses on the hygienic quality of composts produced in small-scale compost systems at homes, schools and multi-family residences. While disease causing organisms represent only a very small fraction of the microbial community in compost piles, but there are factors that need to be considered. A literature search revealed very few data on this subject.

Research has shown that compost achieving the “temperature/time” regime required for proper operation of large, permitted composting facilities is effective in pathogen destruction (although subsequent recontamination of the compost and regrowth of microorganisms can be a problem) (Bollen 1990; ODEQ 2001). Although it is commonly believed that reaching temperatures of 55°C for 3 days is sufficient to essentially eliminate bacterial pathogens (Yanko et al. 1995), recent work suggests that the control of bacterial pathogens in composting is more complex and not simply the result of thermal treatment (Droffner and Brinton 1995). Salmonella, E. coli, and other bacteria survived high temperatures for a significant time (Droffner and Brinton, 1995), but whether the high temperature resistant strains are pathogenic is unknown (Droffner and Brinton 1994). Moisture level, for example, is also important in the survival of E. coli through the

Cornell Waste Management InstituteCold Compost Project – Final Report

composting process (Droffner and Brinton, 1995). It has been suggested that microbial competition is also important in the destruction of pathogenic organisms in compost. If so, if finished composts with low levels of competing microorganisms become inoculated with pathogens, there would be an increased potential for high pathogen levels due to regrowth in the absence of competition..

A review of abstracts on manure composting and pathogens suggest the following:

• Most of the research work is done on fairly controlled compost piles, in contrast with what may actually take place on farms or at homes or schools (Skjelahugen 1992; Cooperband and Middleton 1996; Graft-Hanson et al. 1990).

• The data, even in these cases, is inconsistent. Some piles seem to rapidly lose organisms (Schleiff and Dorn 1997; Graft-Hanson et al. 1990; Forshell and Ekesbo 1993), while others take much longer (Slawon et al. 1998). In other cases, minimal change was observed (Kikuchi and Ataku 1998; Tiquia et al. 1998) or in one case the number of organisms actually increased with time (Mote et al. 1988).

• The actual organisms studied varied, but E. coli and Salmonella are a recurring theme, because they are two organisms associated with animal manures, and presumably also of food wastes, that are of concern to human health (Skjelahugen 1992; Cooperband and Middleton 1996; Schleiff and Dorn 1997). These organisms are used by US Environmental Protection Agency and many states as “indicator” organisims for products derived from sewage sludges. The term “indicator organism” is discussed in the Materials and Methods section.

• The attempt to relate critical processing and compost pile factors to the outcome of composting with respect to pathogen concentrations has received minimal attention.

• Commonly used methods for the detection of Salmonella and Listeria may fail to detect those present (Yanko et al, 1995; Droffner and Brinton 1995).

A literature review and discussions with experts turned up almost no information on the topic of safety in regard to pathogens for small-scale composting systems. One article, published by German authors, did find that small compost systems do not generate adequate heat to kill human pathogens such as Salmonella (Roth 1994).

Cornell Waste Management InstituteCold Compost Project – Final Report

Information about the time/temperature behavior of pathogens in the temperature range of interest is also limited. Data used for food service establishments where food temperatures are directed to be above 60°C or below 4.4°C are not directly relevant to these compost systems.

In large-scale composting operations, pathogen concerns may arise if either; 1) adequate temperatures are not achieved for sufficient duration to ensure pathogen destruction, or 2) recontamination occurs after the composting process is successfully achieved. The Oregon Department of Environmental Quality (2001) states that regrowth of bacterial pathogens may occur when there is available carbon, adequate moisture, and a lack of competitive organisms. In small composting systems, these conditions are frequently the norm.

Most home and small institutional and commercial compost systems do not reach 55° C, or if they do, composts may not maintain temperatures for sufficient lengths of time for pathogen reduction. The temperature as recorded (when this is done) is often hottest toward the core of the pile and cooler along the pile’s edges. Given the less systematic nature of turning in most of the smaller compost systems, it is likely that even with piles that self-heat, not all of the compost will be subjected to the higher temperatures. Thus, if pathogens are present, they may persist through the composting process.

Another concern raised by Droffner and Brinton (1995) and Yanko et al. (1995), is whether the standard techniques used for microbiological characterization of pathogens are effective with compost samples. As Droffner and Brinton’s experiments with Listeria demonstrate, the enrichment media does not enrich for those organisms that survived the high temperature regime in the compost. Comparing five methods for enumeration of Salmonella in composts and sludges, Yanko, et al. (1995), found that the EPA approved methods significantly under counted.

Thus, standard methods may not accurately measure pathogens in the compost.

Cornell Waste Management InstituteCold Compost Project – Final Report

Materials and Methods Project Duration This project took place over a 3-year period and included two separate sampling events. The first began in April of 2001 and was completed in January of 2002. This period of time is referred to as the “early” sampling period. The second sampling event began in September of 2002 and was completed by the end of October of the same year. The second sampling period is referred to as the “late” sampling period.

Site Selection Twenty sites across New York State were selected to participate in this study. Of these, 6 participated only in the early round of sampling, while the remaining 14 participated in the full study with samples analyzed both in 2001 and 2002. Data regarding compost management was also obtained for each site through a questionnaire. (See Appendix A for a copy of the questionnaire and a summary of the results). Sites were identified by Cornell Cooperative Extension educators in New York City, Tompkins County, and Schuyler County, New York who work with home, school and multi-family residential composters.

Sites for which data were collected include 10 homes, 6 communal compost piles (at community gardens, multi-family residences, or the workplace), 2 schools and one dormitory residence.

Sampling Protocol Each sample consisted of 16 representative grab samples gathered from the compost pile, using standard collection techniques to prevent contamination and obtain a representative sample (See Appendix B). Each composite grab sample was placed in a 5-gallon bucket lined with a clean garbage bag. Using clean vinyl gloves, the contents were mixed thoroughly to provide as uniform a composite sample as possible. Two testing laboratories were used. For each laboratory, 2 heavy-duty 1-quart Zip-locTM bags were filled using a portion of the composite sample, and clearly labeled.

The sealed bags were packed in insulated styrofoam containers with ice to minimize both microbial growth and death. Samples going to laboratory #1 for analysis were dropped off in Cornell Waste Management Institute Cold Compost Project – Final Report person after sampling was finished for the day. Samples going to laboratory #2 were shipped overnight.

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