Understanding how and where biodegradation occurs is essential for designing sustainable, compliant and market-ready products.
Definition of Biodegradation
You may come across many different definitions for biodegradation on the web; but put simply, biodegradation is the breakdown or decomposition of materials by microorganisms.
Specifically, biodegradation is the natural process where microorganisms break down complex materials into forms that can be used by living systems. Bacteria and fungi are typical examples of microorganisms that are involved in the biodegradation process.
Importantly, biodegradation is driven by biological activity and differs from general degradation, which can occur through physical or chemical means.
Biodegradation vs. Biodegradability
Biodegradation refers to the actual process of biological breakdown by microorganisms. It’s the action of microbes transforming materials into simpler substances.
Biodegradability, on the other hand, is the potential, or capacity, of a substance to undergo biodegradation under specific conditions. Biodegradability is often assessed through standardized testing to determine whether, and how quickly, a material will break down in a particular environment (e.g. soil, freshwater, compost).
Understanding the distinction is key: not all biodegradable materials behave the same in different environments, and claiming “biodegradability” requires proven data to back it up.
How Biodegradation Works
Biodegradation occurs when microorganisms break down organic substances into simpler substances like carbon dioxide, water, and biomass. The microorganisms, such as bacteria and fungi, consume the organic matter and produce the byproducts as part of their metabolic process. The end products after complete biodegradation are safely reintegrated into ecosystems.
How Long Does Biodegradation Take?
The speed of the biodegradation process depends heavily on environmental conditions and the structure of the material. Depending on the material that is being broken down and the environmental conditions, the degradation time can vary drastically from days to decades.
Key factors influencing biodegradation include:
- Chemical structure and molecular weight: Simpler, low-molecular weight substances have the potential to degrade more easily.
- Environmental conditions: Oxygen availability, moisture, temperature and pH all impact microbial activity.
- Microbial presence: The diversity and concentration of microbes present determine how fast and completely materials can break down.
- Bioavailability: The solubility, sorption, physical state, and structure complexity of a material can all impact how “available” the material is to microorganisms, influencing the extent and rate of biodegradation.
Some materials can degrade in days under ideal conditions, while others can persist for decades in less favorable environments.
Types of Biodegradation
Biodegradation can generally be broken down into two main types: anaerobic and aerobic.
Anaerobic Biodegradation
Anaerobic biodegradation is when the biodegradation process occurs in the absence of oxygen, typically in landfills or anaerobic digesters. The result of anaerobic biodegradation is biogas, which consists largely of methane along with smaller amounts of carbon dioxide and hydrogen. Since biogas contains methane, which is harmful to our atmosphere, it is often collected and used for eco-friendly power generation. It has been noted that anaerobic conditions can produce slower biodegradation rates compared to aerobic conditions.
Aerobic Biodegradation
Aerobic biodegradation takes place in the presence of oxygen. Unlike anaerobic biodegradation which can release potentially harmful methane into the atmosphere, the result of aerobic biodegradation is mainly carbon dioxide. Since the infrastructure needed to manage aerobic biodegradation is much simpler, this method is usually preferred over anaerobic biodegradation.
Types of Environments Where Biodegradation Occurs
There are many different environments where your products may eventually end up. Each different location has a unique set of environmental conditions that can affect how a material degrades. Understanding the specific conditions of each end-of-life environment is critical for accurate biodegradability claims and product design.
With this being said, there are a few overarching conditions that are commonly examined: Aerobic Soil, Freshwater, Marine, Industrial Compost, Home Compost and various Anaerobic environments.
Soil Biodegradation
Biodegradation in soil occurs in an open environment, such as the ground outside your home with naturally occurring microbes. Conditions vary greatly with weather, season and region. The temperature and humidity will be relatively the same as the atmospheric conditions around the soil. This process typically occurs slower relative to home composting and industrial composting. Some of the most common standards for soil biodegradability testing are ISO 17556 and ASTM D5988.
Freshwater Biodegradation
Freshwater biodegradation typically occurs in wastewater treatment plants. The core function of most biological wastewater treatment is to remove organic pollutants, suspended solids, and nutrients from municipal and industrial wastewater before it’s discharged back into the environment. This removal is overwhelmingly achieved through microbial biodegradation. While most freshwater biodegradation takes place in wastewater treatment plants, it can also occur in rivers, lakes and streams which have cooler temperatures and less optimized conditions for biodegradation to occur. OECD 301 and OECD 302 are some of the most common standards for freshwater biodegradability testing.
Marine Biodegradation
Marine biodegradation occurs in oceans and seas, where oxygen levels and microbial concentrations can vary greatly. Biodegradation in these environments, typically occurs at atmospheric temperatures. Conditions including salinity, temperature and low microbial concentrations can slow the biodegradation process. This results in marine biodegradation generally being slower than freshwater biodegradation. ASTM D6691 and OECD 306 are some of the most common methods for marine biodegradability testing.
Industrial Composting Biodegradation
While biodegradation is an important aspect of making an industrially compostable product, not all biodegradable materials are compostable. There are several parts to a product being compostable, but this piece will focus on biodegradation alone. Industrial composting is an established process for transforming biodegradable waste of biological origin into stable, sanitized products. The compost conditions usually range from 50-70 degrees Celsius and moisture levels are normally kept within 40%-60% for a minimum of 21 active days. The degradation rate of industrial composting is generally faster than home composting due to the increased temperature and higher microbial concentration.
European Bioplastics gives an overview of industrial compost conditions. Raw materials, intermediates and final articles are typically required to meet passing thresholds in specifications, such as ASTM D6400 or EN 13432, to be labeled industrially compostable.
Home Composting Biodegradation
Much like industrially composting, biodegradation also plays a key role in making a home compostable product, but there are other aspects that need examined before declaring something as home compostable. Biodegradation in home compost generally relies on atmospheric temperatures and can occur in one of two ways: Backyard composting with a pile, or Vermicomposting (worm composting). If organized extremely well, the compost temperature conditions can still reach up to 70 degrees Celsius. The degradation rate of home composting is still relatively slower than industrial composting.
While there are no regulations around home composting, the EPA does provide guidelines and suggestions.
Summary
Overall, biodegradation is an important step in a circular economy. As its core, it involves microorganisms, such as bacteria or fungi, breaking down materials into usable, less complex compounds. This can happen either anaerobically (in the absence of oxygen) or aerobically (in the presence of oxygen). When it comes to aerobic biodegradation, this can take place across a wide range of environmental conditions, each of which has a unique effect on the biodegradation process.