We may only use disposable plastic products for a few minutes, but a regular plastic bag takes 20 years to decompose in the natural environment. Disposable coffee cups lined with plastic film take 30 years, plastic straws take 200 years, plastic bottles take 450 years, and some baby diapers take 500 years... Plastic pollutants cause the death of 100000 marine mammals every year, and out of approximately 130 marine mammals, 81 have accidentally ingested or been entangled in plastic.
The harm of plastic products to the environment has become notorious, so more and more "biodegradable plastics" and "compostable plastics" have entered people's sight. These plastics are indeed more environmentally friendly choices than traditional plastics. Let's first take a look at what these plastics are.
The degradation methods of biodegradable plastics mainly include biodegradation, photodegradation, and chemical degradation. Although they may sound environmentally friendly, a large part of them are still made from petroleum and other products as the main raw materials, and the decomposed products may still be harmful to the environment. The characteristic of compostable plastic is that it does not produce any harmful by-products after degradation, so it can be used as fertilizer. The raw materials for compostable plastics are corn, sugarcane, etc. After chemical processing, they can obtain properties similar to petroleum based plastics (i.e. plastics made from petroleum and other materials). For example, compostable plastic bowls like cardboard are made of bamboo fibers or other plant fibers.
In theory, these compostable plastics can gradually degrade into carbon dioxide, water, and other biomass over time, but if they are simply thrown into the environment, the degradation process may take much longer than we imagine.
A study found that after burying a certain compostable plastic in soil for 27 months, it still maintains its original shape, but its load-bearing capacity decreases.
Scholars also placed bags made of polylactic acid (PLA) in seawater to test their degradation ability and found that they still did not begin to degrade after 428 days. This experiment also tested a so-called biodegradable plastic, which is a petroleum based plastic mixed with plant cellulose (PP and PET plastics were used in the experiment, which belong to traditional plastics), and found that even with the addition of cellulose, there was not much improvement in the degradation ability of traditional plastics. In contrast, bags made from natural and regenerated cellulose can be completely biodegraded in about 35 days.
So even though they are called 'compostable' or 'biodegradable', these plastics themselves are not as environmentally friendly as their names suggest. Even if it ends up in a landfill, it doesn't necessarily mean finding the best home. According to scientists' calculations, some plastic products (such as PLA) in landfills may take over 100 years to be completely degraded at 20 ℃. The garbage in the landfill is stacked layer by layer, forming an anaerobic environment. Under these conditions, some compostable plastics release methane during the degradation process. Methane and carbon dioxide are both greenhouse gases, but methane has about 25 times the ability to retain heat in the atmosphere compared to carbon dioxide.
For compostable plastics, their best destination is professional composting equipment. The temperature here is maintained above 40 ℃, and plastic waste will encounter leachate rich in microorganisms. One of the functions of leachate is to keep the reaction environment moist. In addition, microorganisms can be added regularly to continuously decompose the waste. The generated gas can provide power to the entire equipment through a generator. In this type of equipment, compostable plastic can be completely decomposed in approximately 28 days.
