In Karachi, Pakistan, the two rivers, Lyari River and Malir River, convey raw municipal and industrial wastewaters to the Arabian Sea. People discard plastic bags here and there, which ultimately end up in the manholes of the sewerage system. The faulty sewerage system discharges its contents in the two rivers.

It was given to understand that people should use biodegradable plastics, as they degrade in the environment over time. A recent publication by the Global Programme of Action for the Protection of the Marine Environment from Land-based Activities, Marine Ecosystems Branch, Division of Environmental Policy Implementation, UNEP, titled: Biodegradable Plastics and Marine Litter Misconceptions, Concerns and Impacts on Marine Environments, reviews the fate of biodegradable plastics in the marine environment.

While the publication, which I find interesting, available at: unep.org/gpa/documents/publications/BiodegradablePlastics.pdf gives a expanded treatment, the main points are:

Synthetic polymers can be manufactured from fossil fuels or recently-grown biomass. Many plastics will weather and fragment in response to UV radiation – a process that can be slowed down by the inclusion of specific additives.

Complete biodegradation of plastic occurs when none of the original polymer remains, a process involving microbial action; i.e. it has been broken down to carbon dioxide, methane and water. The process is temperature dependent and some plastics labelled as ‘biodegradable’ require the conditions that typically occur in industrial compositing units, with prolonged temperatures of above 50°C, to be completely broken down. Such conditions are rarely if ever met in the marine environment.

Some common non-biodegradable polymers, such as polyethylene, are manufactured with a metal-based additive that results in more rapid fragmentation (oxo-degradable). This will increase the rate of microplastic formation but there is a lack of independent scientific evidence that biodegradation will occur any more rapidly than unmodified polyethylene.

A further disadvantage of the more widespread adoption of ‘biodegradable’ plastics is the need to separate them from the non-biodegradable waste streams for plastic recycling to avoid compromising the quality of the final product. In addition, there is some albeit limited evidence to suggest that labelling a product as ‘biodegradable’ will result in a greater inclination to litter on the part of the public.

Adoption of plastic products labelled as ‘biodegradable’ will not bring about a significant decrease either in the quantity of plastic entering the ocean or the risk of physical and chemical impacts on the marine environment, on the balance of current scientific evidence.

It, therefore, seems that the impression given that the biodegradable plastic is somewhat safe for the marine environment, is erroneous.

F H Mughal