The quality and marketability of a compost product is directly related to the prevention of contaminants in the feedstock material. In addition to excessive contamination being a possible enforcement issue, even a small percentage of contamination can give the appearance to the general public that a composter is processing or storing “garbage” rather than a desirable commodity.

Title 14 Section 17852(a)(32) defines "physical contamination" or "contaminants" as human-made inert products contained within feedstock, including, but not limited to, glass, metal, and plastic. These types of contaminants can be minimized either at the point of generation/collection or through an on-site load check program.

Contaminants such as glass, plastic, and metal affect the aesthetic marketability of the final product and in some cases may be hazardous. Other types of contamination such as clopyralid or infestations may also effect marketability.

Contaminants such as pathogens and heavy metals affect the usability and safety of the finished product. This type of contamination is managed though careful control of feedstock and testing of the final product, which is addressed in 14 CCR Sections 17868.2 and 17868.3.

Great improvements have been made in public awareness and education to prevent contamination at the point of generation. However, contamination (along with odor) remains one of the most difficult obstacles faced by operators.

Green Material:

Recent studies of curbside green material collection programs indicates that green material may contain excessive levels of contaminants. On-going education and outreach programs need to be comprehensive and sufficient to reach all sectors of the population.

Here are a few factors to consider about green material contamination:

  • No amount of food material is allowed in green material. Green material containing food waste is municipal solid waste (MSW).
  • Curbside green material is not source separated if the amount and types of contaminants cause the waste to be classified as MSW.
  • Green material is source separated (separated at the point of generation) plant material and contains no greater than 1.0 percent physical contaminants by weight and has met the load checking requirements in Section 17868.5 which specify that a minimum of one percent of daily incoming feedstock volume or at least one truck per day must be inspected visually by trained personnel.
  • Green material does not include food material, biosolids, or mixed solid waste. Green material becomes mixed solid waste when it contains 1.0% or more physical contaminants by weight.
  • Physical contaminants are human-made inert products contained within feedstocks, including, but not limited to, glass, metal, and plastic. Paper is not considered a physical contaminant when evaluating the 1% contaminant level.

Determining Compliance:

There are three basic methods to determine compliance with the 1% physical contaminant amount allowed in green material. Attachment 2 in Advisory 58 contains a comparison chart that can be used as a visual tool to estimate the percentage composition of contaminants in a material. The chart contains diagrams representing a range of percentage compositions. By comparing the diagrams to a representative sample of the host material, the user can visually determine which diagram best matches the sample. The chart is based on percent by volume, so to use it to determine percent by weight, the user will need some idea of the relative densities of the contaminants and the host material.

  1. Look at the load being received after it is dumped out in one spot. An estimation of the amount and types of physical contaminants by weight in the load would be a rough guess based on the visual observation of the load. This is a very low level of analysis and not very accurate.
  2. Have a portion of the load dumped in a spot and have the truck move up, have more of the load dumped and have the truck move up, etc. This method will help reveal more of the contaminants in the load. The heavier components will have dropped to the bottom of the load and the lighter contaminants such as the plastics and polystyrenes will have moved towards the top of the load. A more accurate estimation may be made based on the amounts and types of contaminants observed in the spread load.
  3. Have the load spread out and obtain a large representative sample. Remove the physical contaminants and weigh the amount as compared to the sample weight. If the representative sample has greater than 1% physical contaminants by weight or any food waste, the green material is considered MSW. This is the most accurate method for determining compliance.

Questions and answers regarding contamination of green material and land application may be found in the FAQs. We are currently analyzing the Three part Test and Green Material Contamination. It is anticipated that regulatory changes may occur as a result of the investigation.

Food Waste:

Food waste represents approximately 15.5 percent of the material landfilled in California. There are important CalRecycle initiatives exploring diverting food waste from landfills.

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