Estimating Mill's climate impact

Estimating Mill’s climate impact

Abstract

Mill Background

Mill is a food recycling system designed to keep food out of landfills. Mill combines a food recycler—which dries and grinds scraps into compact, nutrient-rich “Food Grounds”—with optional recycling pathways. These shelf-stable grounds can be composted locally or sent back to Mill to be turned into a chicken feed ingredient.

2023 Scoping Life-Cycle Assessment

Mill uses life-cycle assessments (LCA) to quantify the net carbon avoidance from use of the food recycler. Mill published the original version of its scoping LCA in January 2023. This initial study was completed before any production devices were in use, and was based on early field data.

New Factors Reflected in Updated 2025 LCA

In early 2025, Mill updated its LCA to incorporate new factors, which resulted in slightly different—but more accurate and representative—outcomes. These new factors included:

Updated Findings

When assessing the single scenario modeled in the initial version of the 2023 model (which assumed that before getting Mill, 80% of food scraps were landfilled, and the other 20% were composted, among other assumptions that have since changed), the 2025 model estimates 507 kg-CO2e avoided / device / year. This is slightly less than the 521 estimated in the initial study.

When assessing the scenario reflecting the average U.S. household that becomes a Mill customer, the 2025 model estimates 477 kg-CO2e avoided / device / year for improved scraps management, and a possible 258 kg-CO2e avoided / device / year for source reduction from device engagement-driven behavior change. In total, this represents 735 kg-CO2e avoided / device / year. This is based on EPA data on typical management of residential food waste in the U.S., and recent survey data of how Mill customers manage their Food Grounds.

When assessing the scenario where a household was landfilling all of their scraps before Mill, and now sends all of their Food Grounds back to Mill to be turned into poultry feed, the 2025 model estimates 899 kg-CO2e avoided / device / year (641 kg-CO2e for improved scraps management + 258 kg-CO2e for source reduction).

According to our 2023 and 2025 models, a counterfactual world without Mill appears to incur a much greater cost to our climate than a world with Mill. In other words, the emissions avoided by Mill are greater than the energy and resources invested for Mill to operate.

Background

At Mill, we’re on a mission to keep food out of landfills.

Uneaten food is the most common material in landfills and most of it comes from households. When that food decomposes in a landfill, it produces methane, a greenhouse gas that has about 80x the global warming potential of carbon dioxide over a 20-year period.

Mill combines a food recycler—which dries and grinds scraps into compact, nutrient-rich “Food Grounds”—with optional recycling pathways. Shelf-stable Food Grounds can be composted locally, fed to backyard chickens, used directly in a garden, or sent back to Mill, where they are turned into food for chickens.

We use a life-cycle assessment (LCA) to quantify the annual climate impact of the Mill Food Recycler. Based on our preliminary study, a household can avoid about one-half of a metric ton of carbon dioxide per year by using Mill.

By publishing our estimates, we invite people to help shape how we think and how we outsmart waste. In the future, as we gather data from households with Mill, we will be able to report on impact in more detailed ways. This updated 2025 study is the first step in doing so. We anticipate our numbers may continue to change as customers reduce the food scraps they generate and as sending food to landfills becomes less common in the U.S. in the future.

Scoping LCA

Our Scoping LCA is a cradle-to-grave comparison of the Mill Food Recycler (that includes the Mill Food Recycler, Mill’s Food Grounds operations, and other Food Grounds management methods) to a counterfactual scenario before having Mill in the home (where food scraps are landfilled or composted, for example).

We describe emissions avoided, where one year of Mill Food Recycler use is the functional unit, and kilograms of CO2 equivalents (kg-CO2e) is the environmental indicator.

Cradle-to-grave describes the upstream and downstream extents of the system boundaries, specifically inclusive of extraction of raw materials (to manufacture the bin) through the final fate of the food scraps or Food Grounds-derived products.

This write-up reflects an updated version of the study, completed in February 2025 (referred to as the 2025 model here on), and replaces the original study, published in February 2023 (referred to as the 2025 model here on).

External Validation

Reviewers of the 2025 scoping LCA model

Reviewers of the 2023 scoping LCA model

Approach

We carried out a series of literature reviews to identify the most reputable references available for impact accounting methodologies, assumptions, and emission factors. We then evaluated two scenarios:

Here are the definitions for each scenario:

Considerations

We conducted our Scoping LCA using both 20-year and 100-year global warming potentials (GWP20 and GWP100). This means that we applied emission factors (EF) with 20- and 100-year time horizons to convert the mass of non-CO2 greenhouse gases (GHGs) into CO2-equivalents (CO2e). Using both GWP20 and GWP100 to calculate avoided CO2e emissions allows us to understand our climate impact on both a short-term and long-term timescale.

IPCC classifies methane emission factors as either biogenic emissions or fossil emissions. Biogenic emissions are the emissions produced from a biological process, such as methane emitted from anaerobic decomposition. It is common in LCAs to omit biogenic CO2 emissions from materials of recent biological origin, such as CO2 emissions from composting, because it is considered to be a part of the short-term carbon cycle. Fossil emissions result from the combustion or oxidation of fossil fuels, such as burning jet fuel. Because fossil CO2 is not part of the short-term carbon cycle, it is accounted for in the model.

The stark difference in methane GWPs over different time frames is due to the fact that methane is relatively short-lived in the atmosphere, sticking around for roughly a decade (as compared to over a century for CO2). During its short atmospheric life, methane traps significantly more heat than CO2, thereby increasing the risk of passing potentially catastrophic climate tipping points. Even though CO2 has a longer-lasting effect, methane sets the pace for warming in the near term.

In 2018, Balcombe and colleagues did a deep dive on the appropriate uses of the 20- and 100-year methane GWP emission factors. They found that although reporting both is the safest approach, the 20-year factor was appropriate for assessing the impact of technologies targeted at mitigating methane emissions ( Balcombe et al., 2018).

Findings

It is important to re-emphasize that this is a scoping study, and that Mill made a best-faith effort to build a holistic model with fair assumptions based on our knowledge today. The 2025 model incorporated more than a year of production device operational data. This production data included food scrap generation behavior, device electricity use, and hardware maintenance frequency, for example. That said, our inputs will continue to change in the future as the product offering evolves, and as we continue to learn more about human food scrap management behavior.

When taking the difference in the CO2e impact between the counterfactual and Mill, we can quantify the emissions avoided by the use of a Mill Food Recycler (in the unit of kg-CO2e/year). Ranges, showing the values between conservative and optimistic scenarios, frame the range of possible outcomes under a given set of conservative and optimistic assumptions. For simplicity, when a single number is reported for a use case scenario, this reflects the average of the conservative and optimistic scenarios.

According to our scoping LCA, a counterfactual world without Mill appears to incur a much greater cost to our climate than a world with Mill. In other words, the emissions avoided by Mill are greater than the energy and resources invested for Mill to operate. Landfill emission avoidance is, in the base case scenario, the largest contributor toward the net environmental benefit of a Mill Food Recycler.

Below is a summary of outcomes for the base case scenario:

Behavior Change and Source Reduction

Source reduction is a potential source of impact that can be realized by a household using Mill. Source reduction is the prevention of waste through a decrease in the amount of food purchased, or more precisely, a decrease in the demand for food supply. Because of the high carbon footprint of the food supply chain, reducing one pound of food is many times more impactful than keeping one pound of food out of the landfill.

Because the Mill Food Recycler detects all food scrap mass additions, using an internal scale with a 15-gram sensitivity, Mill has gathered unique insights into food scrap generation behavior.

One such insight is the tendency for customers to experience a reduction in the mass of food scraps added to the Mill bin (by approximately 20%) over the first few months with the Mill Food Recycler in the home. The behavior change is observed consistently on a cohort basis, fleetwide. Because it is assumed that customers do not begin consuming more food once they have a Mill bin in their home, the reduction in food scraps observed might be attributable to a slight decrease in the quantity of food purchased. This theory is supported by survey data where a large portion of respondents cite changes in food preparation behavior, and a reduction in the food wasted following the adoption of Mill. A small portion of Mill customers noted a reduction in purchases of food consumed at home.

This also assumes that the 20% reduction in mass additions to the bin was not attributable to an increase in placing those food scraps in the garbage. This assumption is supported by an increase in Mill customer Net Promoter Score, a metric used to quantify customer satisfaction with a product or service, over the same period. Further, engagement, defined by the number of lid-opens per week, remains consistent over the same period, suggesting that this decrease in mass additions is not attributable to less use of the bin.