CWC Technology Brief

A PROTOCOL FOR ASSESSING COMPOST STABILITY IN THE FIELD: Development, Evaluation, and Feasibility of Implementation

The objective of this project was to develop a simple method for assessing compost stability in the field. Such a method would help public agencies gain confidence in using compost products. Compost producers would also benefit from a simple, inexpensive stability test. A simple stability test could be especially significant for increasing the use of compost in public works projects.

The use of compost products in public works projects presents a large potential market for this recycled commodity. A key task in using compost for public works projects, is the development of specifications for compost by public agencies. Despite the implementation of compost procurements specifications, it is not economically feasible to analyze every load of compost for the parameters specified. Consequently, a significant potential exists that a compost product delivered on site may not meet the procurement specifications. If this did occur, there is no procedure for testing the compost on site to insure that it meets the specification.

Compost stability or how well decomposed the product is, can be seen as the single most important parameter for assessing compost quality. Negative impacts on plant growth resulting from the use of a compost product are commonly attributed to the product not being sufficiently mature.

The field method was developed by collecting a total of 90 samples from five yard debris composting facilities over a five week period. A series of samples comprising raw yard debris to finished product were collected during each site visit. The samples were split and analyzed for quantitative parameters and assessed for qualitative characteristics.

The quantitative stability analyses included: carbon dioxide respiration rate, the cress seed germination test and a growth trial. Each of these quantitative procedures are briefly described as follows: The carbon dioxide respiration rate procedure entails the incubation of a compost sample at 30øC and subsequent daily determination of the weight of CO2 -carbon evolved (>5 mg/g compost carbon - day) indicate the compost is unstable.

The cress seed germination procedure entails the germination of water cress seeds in a five percent (dry weight basis) compost : water extract. A germination index is produced by taking the product of percent germination and root elongation and dividing by 100. An index below 60 is an indication the compost is not completely stable and when used may inhibit plant growth.

In this project, the growth trial entailed the growth of radishes in a compost : peat based potting mix (1:2 volume basis) under grow lights. After 12 days, the above ground portion of the radish plants was dried and weighed. The effect of each sample on the growth of radishes was presented as a percent of a non-amended control. A compost resulting in less than 80 percent growth compared to the control would be considered unstable.

The qualitative assessments were conducted by the following four different groups:

· King County Soils and Materials Testing Laboratory
· Department of Transportation
· Cedar Grove Composting, Inc.
· E & A Environmental Consultants, Inc. (E&A)

The qualitative assessments included: odor, color, and particle characterization. A fourth qualitative variable, amount of sample that floats, was only determined by E&A.

The resulting data was analyzed by multivariate linear regression. Equations for predicting the quantitative variables based on the significant qualitative or field variables were developed.

The statistical analyses revealed R2 values between 0.5 and 0.6. This indicated predictive formulas based on qualitative field parameters can explain approximately 50 to 60 percent of the variability in the quantitative stability parameters - respiration rate and cress seed germination. Odor and color were noted to be the most viable field parameters for predicting the quantitative stability parameters. Consequently, a field method for assessing compost stability based on color and odor was developed.

The method entails the use of a color template and odor descriptions, for determining the color and odor values of a compost sample. The resulting values are assigned to a color/odor matrix that defines stable and unstable compost.

The statistical analyses indicate that at this time the test should not be used for inspecting and rejecting loads of compost delivered to a site. However, the method does have promise as a tool for assessing compost in the field while halting further deliveries until quality can be verified for questionable loads. The field procedure may be especially useful for people with very little experience using compost.

The field method for assessing compost maturity may be useful for the following activities.

· Inspection tool for requiring additional testing
· Warning contractors product is substandard
· Component of specifications

As an inspection tool, field inspectors and landscape architects could spot check loads of compost to help insure they are receiving a stable product. If the product was found to fail the test procedure and there is sufficient time, the inspector may have the product analyzed to insure it meets the specifications. Alternatively, if there is insufficient time to test the product, the inspector could warn the processor or contractor that the product is substandard. Having the test procedure as a component of specifications would aid significantly in its use as a field inspection tool. As a component of compost specification the field procedure would provide a more clear definition to the processor and contractor what a stable compost product is.

The test procedure could be readily used as a component of compost specifications. Several compost specifications have been noted to use subjective phrasing such as "The compost product must be well decomposed and have a soil like odor". This subjective wording could be replaced with the following statement. "Compost shall be well decomposed as demonstrated by passing the color and odor components of the field stability procedure".

The project was conducted with the assistance of Cedar Grove Composting, Inc., a compost processor located in Maple Valley, Washington

This technology brief was prepared by the Clean Washington Center. The Clean Washington Center is the Managing Partner of the Recycling Technology Assistance Partnership (ReTAP). ReTAP's mission is to advance industry's use of recycled materials through technology extension services. ReTAP is an affiliate of the national Manufacturing Extension Partnership (MEP), a program of the U.S. Commerce Department's National Institute of Standards and Technology. ReTAP is also funded by the U.S. Environmental Protection Agency and the American Plastics Council.

Report Dated: October 1994

Fact Sheet Update: October 1994