Biodegradable Plastic Landfill Decomposition

Overview: Biodegradable Plastics in Landfills

Biodegradable plastics are often promoted as greener alternatives to conventional plastics, but their actual behavior in landfills is very different from what most people expect. Landfills are engineered to be relatively dry, compacted, and low in oxygen so that waste remains stable over long periods. These conditions are almost the opposite of what most biodegradable plastics need in order to break down efficiently.

This calculator provides an educational estimate of how long common biodegradable plastic types might take to decompose under typical landfill conditions. It focuses on three broad categories:

• PLA (polylactic acid) – a plant-based plastic used in some cups, packaging, and 3D printing filaments.
• PHA (polyhydroxyalkanoates) – bioplastics produced by microbes, used in certain packaging and specialty applications.
• Starch-based plastics – materials that include a substantial portion of starch, sometimes blended with conventional plastics.

The tool lets you adjust landfill temperature, moisture, and dominant oxygen conditions to see how these factors change the estimated time to significant decomposition.

How the Landfill Decomposition Model Works

The calculator uses a simplified model that estimates the time to substantial decomposition, expressed in years. The estimate is based on a baseline decomposition time for each plastic type and then adjusted by multipliers for temperature, moisture, and oxygen availability.

In conceptual terms, the model assumes:

• Each plastic type has a base decomposition time under standard landfill-like conditions.
• Warmer temperatures generally speed up microbial activity and chemical reactions.
• Higher moisture levels generally support microbes and increase transport of nutrients and degradation products.
• More oxygen (aerobic conditions) often allows faster biodegradation than low-oxygen (anaerobic) conditions, particularly for many bioplastics.

Mathematically, the model is written as:

Where:

• T = estimated decomposition time (years) under the chosen conditions.
• B = base decomposition time (years) for the selected plastic type, under standard conditions (25 °C, 40 % moisture, largely anaerobic).
• f_t = temperature factor.
• f_m = moisture factor.
• f_o = oxygen factor.

In the current configuration:

• Base times B are set to median literature-based estimates:
  PLA: 80 years; PHA: 40 years; Starch-based: 5 years (under the standard reference conditions).
• The temperature factor is:
  f_t = 1 + 0.05 × (T_c − 25), where T_c is temperature in °C.
• The moisture factor is:
  f_m = 1 + 0.02 × (M − 40), where M is moisture in percent by mass/volume (simplified).
• The oxygen factor f_o is:
  2 for conditions with notable aerobic pockets, and 1 for predominantly anaerobic conditions.

In plain language: the base time is adjusted downward (faster decay) when temperature, moisture, and oxygen conditions are more favorable for biodegradation. Because these factors multiply together, strong shifts in more than one variable can have a large combined effect.

Model Inputs and Assumptions

To use the calculator, you provide three main environmental inputs plus the plastic type.

Plastic type

The model treats PLA, PHA, and starch-based plastics as broad categories. Within each category, there are many specific formulations, additives, and product designs. The base times (B) reflect approximate median values from a mixture of experimental data and expert judgment.

• PLA: typically slow in landfills; requires sustained high temperatures and oxygen (such as in industrial composting) to degrade quickly.
• PHA: often more readily biodegradable in a wider range of environments, but still sensitive to moisture and oxygen levels.
• Starch-based plastics: often break down faster, though many commercial products contain non-starch polymers that may persist.

Landfill temperature (°C)

Typical municipal landfill core temperatures are often in the range of about 20–60 °C, depending on climate, landfill design, depth, and biological activity. The model assumes a roughly linear relationship between temperature and degradation rate around the reference value of 25 °C.

In simple terms: slightly warmer landfills may allow faster degradation, while cooler ones slow it down.

Moisture level (%)

Moisture refers to the approximate water content in the waste mass. Many landfills aim to limit moisture to reduce leachate generation, but some modern designs (e.g., bioreactor landfills) add liquids to enhance decomposition. The model uses 40 % moisture as a reference, with higher moisture increasing the estimated rate and very low moisture reducing it.

Put simply: drier waste tends to break down more slowly, even if it is labeled as biodegradable.

Dominant oxygen conditions

Most of the volume of a conventional landfill is anaerobic (very low oxygen). However, there can be aerobic pockets near the surface, or in designs that actively circulate air or leachate. The model simplifies all of this into just two choices:

• Aerobic pockets: conditions where oxygen is present frequently enough to significantly support aerobic microbes.
• Anaerobic: conditions dominated by low oxygen, as in the deep, compacted body of most landfills.

In the model, choosing aerobic pockets roughly doubles the effective rate compared with anaerobic conditions, all else being equal.

Interpreting the Results

The calculator output is an estimated number of years until substantial decomposition of the selected plastic under the specified conditions. This is intentionally a broad estimate, not a precise prediction for any given landfill or product.

Here is how to interpret the number you see:

• Long time spans (many decades or more): indicate that the plastic is likely to remain largely intact in typical landfill conditions, even if it is labeled as biodegradable.
• Moderate time spans (tens of years): suggest that, under favorable conditions, substantial breakdown may occur within a human lifetime, but still slowly compared with composting.
• Shorter time spans (under ~10 years): are most likely in warm, moist, more oxygenated landfill environments or for the more readily degradable plastic categories, such as starch-based materials.

Some practical ways to use the output include:

• Comparing materials: see how PLA, PHA, and starch-based plastics differ in estimated landfill persistence under similar conditions.
• Sensitivity to conditions: explore how much the estimated time changes when you adjust temperature, moisture, or oxygen conditions within realistic ranges.
• Educational insight: understand why a product marketed as biodegradable may still last for decades in a conventional landfill.

The result should not be interpreted as a guarantee that a given product will fully mineralize (convert to CO₂, water, and biomass) within the estimated time. Fragmentation into smaller pieces may happen much earlier than complete biodegradation.

Worked Example

To illustrate how the model behaves, consider a hypothetical case of PLA in a relatively warm and slightly moist landfill with some aerobic pockets:

• Plastic type: PLA
• Landfill temperature: 35 °C
• Moisture level: 50 %
• Dominant oxygen conditions: aerobic pockets

Step 1: Choose the base time B for PLA (80 years).

Step 2: Calculate the temperature factor f_t using 35 °C:

• f_t = 1 + 0.05 × (35 − 25) = 1 + 0.05 × 10 = 1 + 0.5 = 1.5

Step 3: Calculate the moisture factor f_m at 50 %:

• f_m = 1 + 0.02 × (50 − 40) = 1 + 0.02 × 10 = 1 + 0.2 = 1.2

Step 4: Set the oxygen factor f_o for aerobic pockets:

• f_o = 2

Step 5: Compute the estimated time T:

• T = B / (f_t × f_m × f_o) = 80 / (1.5 × 1.2 × 2)
• 1.5 × 1.2 = 1.8; 1.8 × 2 = 3.6
• T = 80 / 3.6 ≈ 22.2 years

Under these fairly favorable landfill conditions, the model suggests that substantial decomposition of PLA might occur in a few decades, rather than the ~80-year baseline. However, conditions in many real landfills are cooler, drier, and more anaerobic, which would push the time estimate back toward many decades or longer.

This example highlights that even a plastic designed to be compostable in industrial facilities can still persist for a long time in a landfill.

Comparison of Plastic Types

The table below summarizes how the three plastic categories compare conceptually in landfill-like environments, using the base times and general behavior patterns as a guide.

These values are not guarantees; they are educational reference points to compare materials and highlight how important environmental conditions are.

Limitations, Assumptions, and Appropriate Use

This model is intentionally simple and should be used for learning and rough comparisons, not for detailed engineering or regulatory decisions. Some key limitations and assumptions include:

• Simplified conditions: Real landfills vary widely in temperature, moisture, compaction, and gas management over time and depth. The model uses a single average temperature and moisture level.
• Median-based base times: The base times (B) are representative mid-range values, not strict upper or lower bounds. Actual degradation times may be much shorter or longer.
• Linear response to temperature and moisture: Biological and chemical reactions in real systems often follow nonlinear patterns; the linear factors here are only approximate.
• Binary oxygen treatment: Oxygen conditions are simplified to either anaerobic or aerobic pockets. In reality, oxygen levels can vary continuously and change with time.
• No explicit treatment of additives or blends: Many products include fillers, plasticizers, stabilizers, or blends with conventional plastics that significantly affect degradation.
• Fragmentation vs biodegradation: The model considers "substantial decomposition" as an integrated concept. It does not separately track fragmentation into microplastics versus full mineralization to CO₂, water, and biomass.
• No site-specific calibration: The calculations are not calibrated to any particular landfill or region. They are not suitable for compliance, permitting, or emissions forecasting.
• Educational intent: The tool is best used to explore how changes in conditions affect degradation times and to understand why landfill disposal is generally not the preferred route for many biodegradable plastics.

In other words, this calculator gives a helpful big-picture view, but it should not replace detailed analyses carried out by waste management professionals or environmental scientists.

Environmental Context and Practical Implications

From an environmental perspective, the main message is that labeling a product as biodegradable does not guarantee that it will break down quickly in a landfill. Conditions in industrial composting facilities are designed to keep materials warm, moist, and well aerated, often with active turning or forced aeration. Landfills, by contrast, are engineered primarily for containment and long-term stability.

Some practical implications include:

• Source separation: For materials like PLA that are intended for industrial composting, separating them from general waste streams is crucial for them to realize their designed environmental benefits.
• Design for end-of-life: Product designers and policymakers should consider realistic disposal routes. If most items are likely to end up in landfills, investing only in compostability may not significantly reduce long-term persistence.
• Methane and greenhouse gases: Faster degradation in anaerobic conditions can produce methane, a potent greenhouse gas, if gas capture systems are incomplete. This aspect is beyond the scope of this calculator but is important for overall environmental impact.
• Microplastic risks: For starch-based blends and other composite materials, the biodegradable portion may disappear, leaving behind smaller fragments of persistent polymers. Fragmentation without full mineralization can still pose environmental risks.

In plain language: making plastics biodegradable is only part of the solution; how we collect, sort, and treat waste is just as important.

Sources and Data Background

The base times and qualitative behavior in this model are informed by a range of sources, including:

• Laboratory and pilot-scale studies on PLA and PHA degradation under simulated landfill and composting conditions.
• Field measurements from municipal and bioreactor landfills, including temperature and moisture profiles.
• Review articles and reports from academic and government organizations on bioplastics, compostability standards, and waste management practices.

Because the published data span different experimental conditions and experimental designs, the values here should be seen as approximate mid-range figures rather than definitive benchmarks. When making policy or design decisions, consult primary literature, local waste management authorities, and environmental engineers for site-specific information.

Overall, this calculator is meant to clarify expectations around biodegradable plastics in landfills and to support more informed discussions about material choice, collection systems, and disposal strategies.