Physical Contaminant Testing Of Composts

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Courtesy of BioCycle Magazine

Changes are in progress in multiple countries on weight-based limits for physical contaminants in composts. The net effect may be higher quality compost products.

Maintaining and improving compost product quality is vitally important for ensuring continued confidence in organic waste recycling industries around the world. One quality criterion of particular interest from the general public’s perspective is physical contamination (also known as foreign matter or inerts content) from such materials as plastic, glass and metal. This contamination makes “poor quality” compost immediately apparent compared to other quality criteria that require some form of laboratory analysis for their detection.

The quality of compost products is highly dependent on the quality of the feedstock materials. Therefore, considerable effort has been made in recent years to educate the public and stakeholder groups on feedstocks destined, either knowingly or unknowingly, for composting. As such measures start to have a positive effect, they hopefully will result in more consistent or even higher quality compost than is already produced. This in turn can lead to tighter limits for physical contaminants in compost.

In the United Kingdom (UK), for example, the Quality Meats Scotland (QMS) agricultural assurance scheme has successfully driven the physical contaminants limit for compost quality down by 50 percent on land used for producing Scotch beef and lamb branded meat (Table 1). Representing 90 percent of Scotland’s breeding cattle population and 80 percent of the breeding sheep flock, QMS’ influence is clear on the quality of compost going to the Scottish agricultural land bank (land suitable for compost application). The result appears to be the most stringent weight-based target for physical contaminants in compost products in the world. The limits set by QMS are being considered for wider implementation this year by the Scottish Environmental Protection Agency (SEPA) for regulating the handling and use of compost outputs in Scotland (SEPA, 2016).

Changes are also in progress elsewhere on limits for physical contaminants in composts. In the U.S., as of January 1, 2018, the limit for physical contaminants in compost will be lowered from 1.0 to 0.5 percent on a dry weight basis in the state of California, which mirrors the limit set by the Washington State Department of Transportation (WSDOT). A limit of 0.1 percent by weight will also be applied for film plastics in California, as they tend to have a significantly lower weight to surface area ratio compared to most other physical contaminants. (The California rule states: “no more than 20% by dry weight of this 0.5% shall be film plastic greater than 4 mm,” thus 0.5% x 20% = 0.1%.) A specific limit for film plastics should help to standardize the quality of different compost products. Similarly, a 0.1 percent by weight limit for film plastics was introduced in Germany at the beginning of 2016.

Weight Based Laboratory Testing

With fairly similar weight limits, there appears to be consistency between different countries in terms of the testing method used for physical contaminants. On a basic level this is true in that physical contaminant testing in the three countries mentioned involves screening, manual isolation of physical contaminant fragments, and quantification on a weight basis. However, when comparing individual country test methods a number of differences become apparent (Table 2). For example, in Germany and the UK, physical contaminant testing is carried out on dried or partially dried material, whereas in the U.S., fresh material is used for a wet-based sieving process.

Differences both in the original design, and interpretation, of the written methods can clearly lead to more or less stringent physical contaminant testing. An example of a design difference between the country methods is fragment size, where the limit in Germany and the UK is 2 mm (about 0.08 inches) whereas 4 mm (about 0.16 inches) is used in the U.S. The test in the U.S. therefore has a greater tolerance in terms of physical contaminants as small contaminant fragments (2-4 mm) found in compost products are ignored. The exclusion of the 2-4 mm fraction is perhaps justified though in terms of correct and consistent fragment analysis carried out by laboratories, as this fraction presents the greatest challenge for analysts in judging “suspect” fragments isolated from compost as physical contaminants or not. This becomes a potentially greater issue where small fragments must be placed in specific categories, as recently demonstrated in a research project using the UK method to assess sources of variability between different commercial laboratories (Echavarri-Bravo et al., 2016).

Read the full article in BioCycle Magazine

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