Methane = biodegradable plastics

Globules of PHA made from methane in cells of bacteria extracted to make biodegradable plastics
Polyhydroxyalkanoate (PHA) globules in cells of bacteria

As we have seen, both carbon dioxide and methane tend to get a ‘bad press’ as they are primarily regarded as greenhouse gases.  However, both gases can also be used as feedstock for a number of processes that convert them into valuable products. Gases that are currently seen as a problem should really be regarded as the basis of new industries.

Fortunately, both gases are available in concentrated forms that enable these processes to be carried out economically:

  • Carbon dioxide – can be taken from flue gases at coal-fired power stations and from anaerobic digestion plants to grow microalgae that produce high-value protein, pharmaceuticals and nutraceuticals.
  • Methane –  can be taken from from landfills and anaerobic digestion plants and converted by bacteria into biodegradable plastics.

Anaerobic digestion facilities and carbon dioxide/methane

You will note that anaerobic digestion plants can provide both carbon dioxide and methane. I will return to the idea of turning anaerobic digestion plants into biorefineries in a later blog.

In this blog I will look at converting methane into biodegradable plastics. My next blog will cover the use of carbon dioxide.

Problems with non-biodegradable plastics

Most plastics, such as polyethylene and polypropylene, are not biodegradable. This means that they accumulate in the environment resulting in the pollution of land and the oceans. This accumulation is increasing seen as a major environmental problem. 

Plastic waste prevention system analysis & applications, 2018

Making biodegradable plastics from methane using bacteria

Polyhydroxyalkanoates (PHA) are a group of biodegradable biopolymers produced naturally as minute globules within some types of bacteria under nutritional stress such as excess carbon and reduced nitrogen.

The globules can represent up to 90% of the bacteria’s dry weight. The mechanical properties of PHAs are useful, in that they can be modified, they are biodegradable, and their production does not depend on petroleum-based feedstocks. The main type of PHA produced by bacteria is polyhydroxybutyrate (PHB). Over 300 strains of bacteria have been identified as being able to produce PHB.

Mango Materials, 2018

Typically, the bacteria producing PHAs are grown with sugars as the carbon source. This can represent up to 40% of the production costs. To reduce these costs, several waste streams have been trialled as a carbon source, including whey waste, sugar industry waste, agricultural crop waste and glycerol. Such systems normally require an expensive sterilisation stage. 

Methane, from landfills, natural gas, and as a component of biogas produced by anaerobic digestion facilities, has been demonstrated to be an energy-rich feedstock and a much cheaper alternative to the above feedstocks.


Sustainable bio-plastic production through landfill methane recycling. 2018

A commercial example of converting methane into biodegradable plastics – Mango Materials

I first came across this company through a genuinely exciting presentation by Dr. Anne Schauer-Giminez, Vice President of Customer Engagement at Mango Materials, (Using biogas to produce high value biodegradable plastic) at the Biocycle Conference in Baltimore, October 2014.

Anne explained how the company – based in the San Francisco Bay Area – had developed a patent-protected process to use bacteria to produce PHB (polyhydroxybutyrate) a type of PHA (polyhydroxyalkanoate). This can be made from the methane component of the biogas produced at an anaerobic digestion (AD) plant or from the methane produced in landfill sites. The PHB is used to produce biodegradable plastic. If co-located at an AD plant the process has the potential to significantly improve the economics of the facility.

Formula for polyhydroxybutyrate (PHB)
Formula for polyhydroxybutyrate

The process has the advantage of using cheap methane gas to make PHB rather than the more expensive feedstocks used by some other processes. As the plastics produced are biodegradable, the Mango Materials process is closed-loop.

Mango Materials closed-loop system converting methane into PHB

The Mango Materials concept has caught the imagination of many.  Molly Morse, CEO of Mango Materials, was recently interviewed by former astronaut Cody Coleman, along with Cyrus Wadia VP of Nike, on the role that both companies can play in the circular economy. See my other website for some videos of Molly and Anne.

NASA and Mango Materials

NASA have agreed to fund Mango Materials – partnering with the Colorado School of Mines –  to develop a membrane bioreactor system that can be used in ‘outer space’ environments, such as the International Space Station, to produce a biopolymer on-demand from methane gas. This is a really exciting use of the technology.

In this modified design for NASA, the bioreactor will allow bacterial growth and biopolymer production to take place in microgravity environments by providing the methane gas through membranes. The biopolymers could possibly be used for 3D printing applications. The project will also explore ways in which wastes from the process can be recycled back to minimize the required inputs. A feasibility analysis will be carried out to evaluate the use of the process on long-term space missions.

I think this technology not only finds an excellent use for methane by helping to replace non-biodegradable plastics, but can also make us rethink the role of AD facilities.