Biodegradability: the principles behind the standards

According to the OECD, “readily biodegradable” chemicals will rapidly and completely biodegrade in natural environments under aerobic conditions. The test requirements are: 60% mineralization to be reached in a 10-day window within a 28-day period of the test. This stringent pass level is considered to show the complete ultimate degradation of the test substance, as the remaining 40% is assumed to be assimilated by the biomass or to be present as products of biosynthesis. According to “REACH” (European Regulation 1907/2006) whenever a substance is classified as “readily biodegradable”, it can be assumed it will undergo rapid and ultimate biodegradation in any biologically-active environment and therefore there is no need for further investigation. The test requirements of European standard EN 17033 (Plastics - Biodegradable mulch films for use in agriculture and horticulture - Requirements and test methods) are: 90% biodegradation to be reached in a 2-year period. This is a much slower rate than that of readily biodegradable chemicals and it could cause concerns about interactions between the mulch film and the environment. However, we must recognise that biodegradation only occurs at the surface of solid plastics and therefore measured biodegradation is an “apparent rate”, being the average between the fast rate occurring at the surface and the rate in the inner part of the plastic particles, which is zero. Studies have shown the biodegradation rate at the surface of biodegradable polymers is very fast, comparable to readily biodegradable chemicals, the limiting factor being the available surface area. From a chemical viewpoint, the biodegradable polymers used in mulch films are effectively suitable for fast biodegradation. From a physical viewpoint, the permanence of any specific product will be affected by thickness (i.e., the effective surface area) and environmental conditions (temperature, water activity, nutrients, microbial population, etc.).