Degradable ≠ unconditional degradation
Degradable plastics refer to substances that can be degraded into environmentally harmless substances under specific natural environment conditions after use, and are a general term for a large class of degradable materials. Degradable plastics first appeared in the 1960s, and have experienced the development stage of mixing with traditional plastics, the stage of degradation through photothermal conditions, and the stage of degradation through microbial enzymatic reactions. Combining degradable plastics with traditional plastics (such as polyethylene, polypropylene) can only make the traditional plastics more easily disintegrated into small fragments, resulting in more microplastics polluting the environment, so it cannot be considered degradable. Photothermal degradable plastics have poor degradability and are constrained by natural conditions such as temperature and light intensity. For example, mulch film buried in the ground will have little effect or cannot be decomposed at all due to lack of light. Therefore, only plastics that are degradable under biological conditions, also known as biodegradable plastics, are truly degradable plastics and have the value of preventing and controlling plastic pollution.
People often have a misunderstanding: degradable plastics can degrade in any natural environment. In fact, degradable plastics have degradable properties only when they enter a specific environment, and are not degradable under any natural conditions.
Different degradable plastics can achieve plastic degradation under conditions such as compost, soil burial, and fresh water. However, compared with the soil environment, the marine environment has the characteristics of high salinity and low microbial density. Those plastic products that can be degraded by microbial enzymatic reactions on land often degrade and fail in the marine environment.
Mishandling also increases carbon emissions?
To estimate the applicable scenarios of degradable plastics and their expected production capacity, it should be evaluated from the perspective of the full life cycle of the use, consumption and disposal of degradable plastic products.
In China, bio-based polylactic acid and petroleum-based polybutylene terephthalate-adipate (PBAT) together account for 81% of the total domestic biodegradable plastic production; degradable plastic bags (38.34%) and lunch boxes Furniture (38.82%) accounted for 77% of the total consumption of degradable plastic products. Where do so many degradable plastic products go? According to the on-site investigation and empirical data calculation of the School of Environment, Tsinghua University, nearly 97% of the degradable plastics are discarded and sent to incineration and sanitary landfill by the sanitation system, about 3% directly enter the environment, and less than 0.01% enter the industry Fermentative degradation stages such as composting and anaerobic digestion.
Studies have shown that currently degradable plastics and other wastes are mixed into waste incineration and landfill facilities. In fact, compared with traditional plastics, they not only have no environmental advantages, but also degradable plastics are expensive, and the overall cost-effectiveness is extremely low.
In the case of incineration, although degradable plastics have a biodegradable chemical structure, they are not very different from traditional plastics in terms of elemental composition. The carbon emission effect after incineration and the comprehensive environmental impact are relatively close. In the case of landfill, the biodegradable characteristics of degradable plastics make the carbon emissions after entering landfill facilities higher than those of traditional plastics, and the overall environmental impact will be significantly higher than that of traditional plastics. For example, due to factors such as landfill gas combustion and discharge, and direct emission of methane from landfills, the CO2 emission equivalent of 1 kg of polylactic acid in the landfill facility is 3.1 kg, which exceeds the CO2 emission equivalent of traditional plastic incineration by about 35%.
The use of degradable plastics (bags) in the field of daily consumption and disposal into kitchen waste treatment facilities is actively promoted by many companies and experts as a feasible method to solve the problem of plastic pollution. The basis is that food waste treatment facilities use biological treatment technology, which can effectively use degradable plastics and convert them into fertilizer or gas. Moreover, the comprehensive environmental impact of degradable plastics treated by food waste treatment facilities is about 30% of incineration facilities and 15% of landfill facilities.
However, the reality is the opposite. The degradable plastic sent to the kitchen waste treatment facility will not enter the fermentation process of the facility at all. In the pretreatment of most facilities, impurities in the shape of plastic products will be removed through multiple sorting processes, and the remaining degradable plastic products that finally enter the biomass fermentation system are actually less than 5%.
The on-site investigation of Tsinghua University shows that the vast majority of degradable plastics entering biomass processing facilities such as industrial composting and anaerobic fermentation, like traditional plastics, will also be sorted out by the equipment as impurities and cannot enter the final biomass degradation. In the fermentation process, the environmental advantages of its degradation characteristics cannot be achieved.
From the point of view of recycling and regeneration, since countries in the world have not yet built facilities for recycling degradable plastics separately, and degradable plastics and traditional plastics have very little difference in appearance and color, it is difficult to pass color sorters, flotation machines, mechanical Sorting machines, etc. to achieve effective sorting. Due to the inability to separate and recycle degradable plastics and traditional plastics, the recycled value of plastic recycled products mixed with degradable plastics will be reduced by more than 20%. From this point of view, the large-scale use of degradable plastics to mix them into the existing plastic recycling system is harmful and reduces the recycling efficiency of plastic products.
