INTRODUCTION: The quantity of plastic waste in the sea is rising rapidly. To many, the news that up to 12 million tonnes of plastic is entering the World’s oceans every year is a source of both dismay and confusion. How does it get there? There is an assumption that this must surely be the work of some other irresponsible (or downright criminal) human beings. However while the vague notion of the faceless malefactor upending skip-loads of garbage into the swell provides some tangibility (however implausible) to the concept, the reality is that all plastic-users, no matter how conscientious, are contributing to ocean plastic. According to Greenpeace the 4 major sources are:
1) Accidental loss at sea of commercial stock and fishing equipment, plus illegal dumping.
2) Land litter blowing into storm drains, rivers and waterways, then flowing unhindered into the sea.
3) Microbeads from cosmetic products and microfibres from plastic-based clothing (like Polyester) that are deliberately (if unknowingly) washed away from sinks and washing machines into grey water and thereafter (because of their small size) bypassing filtering systems, ending up in the sea.
4) Plastics that are ‘responsibly’ put into recycling or landfill systems, that are then lost into the environment at point of collection or transferral, or blown away because of the open nature of the treatment sites.
Put into context, points 2-4 make up around 80% of the plastic waste that finds its way into the sea.
To compassionate humans, the considerable harm caused when this detritus is consumed by sea-going mammals, fish, birds and smaller organisms is rightfully of great concern. Many responsible traders are now offering single-use items – packaging, take-away containers and the like – made of a variety of alternatives to plastic such as paper, cardboard or biocompostable plastics (plastic-like compounds derived from plant materials which will compost rapidly when certain conditions are met). What happens, however, when these alternative materials end up as ocean litter, as they are bound to do unless we can stem entirely the flow of waste into the seas? Does bioplastic decompose in salt water any faster than a petrochemical plastic? This experiment seeks to make a comparison between various materials commonly found in single-use packaging or take-away containers (and hence which would make up the kind of litter that finds its way to the sea) and the rate at which they decay in salt water. The focus is particularly on the behaviour of biocompostable plastics.
HYPOTHESIS: Biocompostable plastics will show some evidence of decomposition in salt water i.e. clouding, swelling and eroding, but are expected to have retained their original form (cup, fork etc.) and will not have completely broken down within the 2-year duration of the experiment. Items made from paper and cardboard are expected to decompose rapidly, swelling and then shredding apart, loosing their shape within 6 months. Wooden items are expected to show deterioration and swelling, but to be still largely recognisable at 2 years. Aluminium is expected to remain unchanged at 2 years, except for some de-colouration of the exterior printing. Steel is expected to show some rusting early on, but to have retained its original form at 2 years. Petrochemical plastic controls are expected to remain unchanged at 2 years. Since rate of decomposition is to be partially measured by weighing the items, it is noted that an initial weight gain is expected for porous compounds as they absorb water and accumulate salt deposits, before they gradually lose mass due to decomposition.
MATERIALS: 11 subjects are to be included in this experiment, as follows:
Biocompostable Plastics: Vegware forks (white); Natcorn ‘biobased’ forks (white); Vegware drinking cups (clear #7 PLA)
Petrochemical Plastics (as controls): Plastico ‘Sunlite’ forks (white #6 Polystyrene); Solo® drinking cups (clear #1 PETE); drinking straws (black, plastic type unknown)
Other: steel bottle caps (NB these have a plastic coating and seal which is expected to remain unchanged during the experiment and may act to preserve the steel); compressed card burger boxes; aluminium drinks cans (NB these have a plastic coating which is expected to remain unchanged during the experiment and may act to preserve the aluminium); paper drinking straws; wooden forks
EQUIPMENT: Second-hand plastic grocer’s crate; nylon filter mesh; second-hand nylon rope; marine-grade stainless steel zip ties, stamped with identifying letters; thermometer; weighing scales.
METHOD: All items are to have their individual masses recorded before entry into the water, and one of each item kept back from the experiment for visual comparison. Materials likely to deteriorate rapidly are to be tagged for identification with marine-grade stainless steel zip ties. All materials will then be contained inside a large bag made from double-walled fine filter mesh (nylon), which is secured to a plastic crate to open it out and allow free movement of the items. This crate is to be weighted down with pebbles and secured to a railing with the nylon rope. The bag is to be submersed in tidal seawater inside the wet dock of Boathouse 4 at the Portsmouth Historic Dockyard for 2 years, with 1 of each item being retrieved monthly for 10 months, then again at 12 months, 18 months and the final items removed at 2 years. After removal from the water, subjects are to be dried and weighed, and their condition visually recorded. Sea temperature is also to be recorded at monthly intervals for the first year.
CONSIDERATIONS: While the experiment seeks to replicate as closely as possible the conditions to which litter in the oceans would be exposed there are some notable exclusions. The first is the reduced levels of UV light due to the wet dock being inside a (naturally lit) industrial building. UV light has a degrading action upon many materials (including petrochemical plastics) and so it can be surmised that the subjects of this experiment will not degrade as quickly as if they were exposed to direct sunlight. Secondly, even though they will be exposed to tidal currents inside the wet dock, they will be protected from mechanical erosions that would occur on a piece of litter being churned in waves. Lastly, the fine mesh of the nylon bag, vital to prevent any small pieces escaping from the experiment into the sea, will also protect the subjects from the actions of all but the smallest organisms. It would be irresponsible to allow animals to come into contact with the subjects of the experiment, but it is noted that in the marine environment the actions of invertebrates and fish would have a bearing on the speed of decomposition. In summary, whatever speed the subjects of this experiment degrade it is assumed that it will be at a decelerated rate compared to litter moving freely in the open ocean.
DISCLAIMER: This is not a controlled scientific experiment, it is an amateur study using domestic equipment and materials. The degree to which degradation of the subjects has progressed will be largely subjective.
Scientific it may be not, however the myriad conditions found in our oceans make a real-world experiment such as this just as valid as that conducted in a lab with all variables measured and controlled. A sun-baked Caribbean bay offers differing conditions to an sub-ice Arctic swell, and plastic has found its way to every corner of the planet, we now know.
Anyone with experience of the sea will tell you it cannot be predicted, that it is ever-changing. Some of that change will have been affected by our human influence, and some by other forces. One can only hope, with this increasing awareness and concern for the watery ecosystems that make up, after all, 99% of the living space of our planet, that future changes will be for the better.
Endnote: The subjects were submerged in the wetdock on 23/06/18.