SALT Feasibility Study

PGS has developed a plastic collection concept using seismic vessels to combat the problem of marine plastic pollution. This desktop review evaluates the potential and feasibility of PGS’ proposed novel concept for collecting pelagic plastic waste in the North Atlantic.

The review found that knowledge on the spatial and temporal distribution of macroplastics in the surface ocean is scarce, particularly for the North Atlantic.

Existing studies suggest that pelagic microplastic densities are too low for surface plastic collection to be efficient, but densities may be higher in certain areas and/ or seasons, or during flush-out events. Floating plastic densities are likely to be higher close to the coast, particularly in rivers and river mouths, but this is also where impact on marine life of the clean-up technology may be the greatest. Organisms may be affected directly through physical interaction with the cleanup technology, or indirectly for example if floating habitats are removed with the plastics.

To evaluate the efficiency of the plastic collection concept, there is a need for field data on the spatial and temporal distribution of macro-plastic. The type of plastics should also to be recorded as it can identify the sources of the litter and the potential for recycling of recovered litter. Knowledge on the spatial and temporal distribution of marine life is also needed in order to reduce undesirable interactions. It is recommended that these knowledge gaps are filled before investing in further development of technology to collect plastic litter at sea as this knowledge is fundamental to how and where to implement clean-up technologies.

Summary and recommendations

PGS has developed a concept for collection of plastic waste at and near the sea surface by combining a bubble curtain with a boom arrangement and collection unit. This desktop study assesses key aspects of implementing such a technology in the North Atlantic. This includes an evaluation of knowledge on the spatial and temporal distribution of plastic in the ocean to determine the availability of plastic in the ocean for the clean-up technology, the feasibility of the bubble plume lifting marine plastic debris to the surface, the likely collection efficiency of the technology and the potential negative impacts of the cleaning technology on marine life. Legal considerations and the opportunities for marine plastics to enter a circular economy through recycling are also discussed.

A review of synthesis and modeling studies identified the Gulf of Mexico and the Caribbean Sea as areas where plastic could be available seasonally in high densities due to likely high flows of input from runoff and rivers during wet seasons. Other potential river hotspots were off the Amazon basin outlet in Brazil, in Guatemala/Honduras, as well as Costa Rica and off the coast of Nigeria. Clean-ups are likely to be the most efficient in rivers and possibly in- or close to river mouths. A lack of field studies on the spatial and temporal distribution of pelagic plastic waste makes it impossible to determine the availability of plastics in the ocean and thereby the efficiency of the proposed cleanup technology. Larger plastic particles are likely to be the target of the technology, and while there are no field studies on the horizontal distribution of macroplastics, it is likely that majority of the larger plastic fragments will be found at or near the sea surface. The few studies documenting pelagic macroplastics in the North Atlantic, suggests that collection efficiency will be low.

While it is more likely that high concentrations of larger plastic particles will be found close to the coast, this is also where the potential negative impact on marine organisms of the clean-up technology is likely to be high. Furthermore, since the operations will be within national economic zones, permissions from local authorities to operate will be needed.

Studies on the spatial and temporal distribution of floating marine plastics in the identified focus areas, as well as retention time, should be conducted to evaluate the potential availability of plastic litter. The amount and type of plastics available need to be established to investigate recycling opportunities. While air bubbles have been developed in zooplankton and pelagic fisheries, there are no studies on the use of this technology to lift plastic items. Laboratory and field studies on the ability of the bubble curtain to lift relevant macroplastic items to the surface should also be conducted.

Ecological impacts were evaluated using the Gulf of Mexico as a case study. The clean-up technology may have a negative impact on marine life and floating habitat through capture or encounter with marine life. Of particular concern are commercial and vulnerable species, that may be affected at different life stages, as well as Sargassum mats (floating brown macroalgae) that are considered essential fish habitat. The impact of the air curtain on plankton should be evaluated and bycatch of all species, including organic debris, should be monitored. It may be possible to limit negative environmental impacts by accounting for seasonal variations and dial migrations of marine organisms in the area, as well as scouting for biological activity at the surface.

Data on marine litter in general, and surface macro litter specifically, is scarce. Vessels can contribute to filling these knowledge gaps by monitoring marine litter during operations.

Key recommendations for pilot studies

All information collected should be open-access as this is a field with large knowledge gaps and it is only through sharing knowledge a better understanding of the state of marine plastic pollution, its impacts and solutions can be achieved.

Filling the knowledge gaps on the spatial and temporal distribution of ocean plastics, as well as marine life the clean-up technology may interact with, is vital to evaluating the efficiency of collecting pelagic ocean plastics and prevent negative impacts on the ecosystem through undesirable interactions. It is recommended that these knowledge gaps are filled before investing in further development of technology to collect plastic litter at sea as this knowledge is fundamental to how and where to implement clean-up technologies.

The following section lists the type of studies that should be conducted to evaluate the feasibility of implementing the clean-up technology proposed by PGS.

Lab experiments and small-scale experiments

Before initiating a pilot testing, some laboratory experiments, or possibly highly controlled field-testing, are recommended to improve the understanding of the behavior of plastics encountering the air curtain.

  1. While there are some differences between stationary bubble plumes in current and towed plumes in quiescent water, a flow chamber to simulate tow is still likely the best place to start as this allows trials to run continuously for some time. Following this, further targeted trials can be conducted where the plume is towed across long tanks in short bursts.

  2. Trials needs done with plastic items of different types and sizes to determine how size and buoyancy influences entrainment and thus the effectiveness of the air curtain in lifting plastics.

  3. Trials also needs done to determine how long raised items remain at the surface. This is critical to determining the maximum distance behind the air curtain the collection boom can be towed.

  4. Conduct trials with the collection boom to determine the angle of the skirts at different tow/current speeds.

During pilot testing, but before embarking on the sailing route (i.e., when considerable modifications to the setup are still readily feasible), we recommend conducting small-scale tests near port to improve understanding of the physical characteristics of the air curtain.

  1. Tow the air curtain (without the collection booms) for a short distance at the target tow speed and film, using both underwater and aerial drones, to determine (1) the continuity of the air curtain, (2) the vertical angle of the plume, (3) the horizontal shape of the air curtain while towed, and (4) the distance behind the air hose which the plume surfaces.

  2. Repeat the above trial, this time to measure upwelling flow generated by the air curtain. Use dye as in Grimaldo et al. (2011).

  3. Trials should also be done to test the practical aspects of sampling and monitoring during operations. For example, sampling nets could be suspended from the vessel both in front of and behind the clean-up device as illustrated in Fig. 1. Vertical sampling nets in advance of the air  curtain, regularly spaced between the air curtain and the collection boom, and behind the boom would allow field testing of: (1) the effectiveness of the air curtain in concentrating particles (and potential bycatch) at the surface, (2) the location of plastics and organisms concentrated in front of- or behind where the air curtain surfaces, (3) the duration for which plastics and organisms remain concentrated at the surface, and (4) the fate of plastics and organisms after hitting the collection booms. If successfully arranged, such a setup could be used at intervals or throughout operations during the pilot study sailing route.

  4. Trials should be done to determine the rapidity by which the booms and collection net can be reliably reeled in or otherwise disabled; same for the air supply hoses and bubble curtain. This knowledge is critical to being able to minimize catastrophic bycatch if e.g., essential habitat such as Sargassum mats or aggregations of whales are encountered as it indicates the distance ahead of the vessel which must be monitored for these. 

Field experiments along the sailing route

Apart from using visual surveys, field experiments will require permission from the respective countries where the studies will take place. 

During the pilot study itself, sampling to monitor bycatch and testing of best practices to avoid it must be conducted.

  1. Regular analyses of organic content in the collection net must be conducted to identify both quantity and taxa of bycatch. Note that this will require a small laboratory or work space on board with microscopes, as well as staff with the ability to key organisms.
  2. Regular analyses of the contents of vertical samplings nets in different positions.
  3. Regular monitoring with cameras to observe behavior and fate of organisms (and plastics) encountered.
  4. Skills training and development of best practises with bridge staff to identify “hazards” (e.g., Sargassum mats, aggregations of whales or other large animals).
  5. Based on all of the above, guidelines should be developed for when operations should be paused. These guidelines should be developed by a marine biologist.

The efficiency of the clean-up technology and the possibility to recycle the litter depends on the amount and type of plastic litter available for clean-up. Since the clean-up technology is targeting meso- and macro-plastic, the pilot study should focus on quantifying these size categories.

  1. Sampling should be done in a gradient from rivers to river outlets, to coastal and offshore locations.
  2. Visual surveys can be used to identify and quantify surface plastics. Methods for ship-based observation of floating litter are described in Appendix 1.
  3. Net sampling without using the bubble plume is needed in order to estimate how much plastic is floating in the surface ocean and therefore potentially available for the clean-up technology. The mesh size of the net should be the same as the one PGS will use in the collector unit of the clean-up technology. To capture mesoplastic particles the mesh size should not exceed about 5 mm.
  4. Knowledge of the vertical distribution of litter is important to determine how deep the bubble curtain should be deployed and the degree to which collection below surface will generate more litter. The  depth of the sampling should be determined based on the technical feasibility of how deep the bubble curtain of the clean-up technology can be deployed. Multi-level net tows, such as those conducted by Reisser et al (2015), should be used to determine the vertical distribution of the litter. However, the bubble plume should not be used during these tests as this could affect the vertical distribution of items in the water column. If a sampling technology such as that illustrated in Figure 1 is developed, then the sampling nets in front of the clean-up technology could capture the sample needed for analysis. Note that a net at the surface is also needed to cover the surface water items.

To provide information relevant for up-stream solutions to prevent plastic pollution, the recovered litter should be source identified. Furthermore, knowledge of the weight and quality of the plastics will give information on the amount of plastic available for clean-ups and the possibility for recycling of the plastics.

  1. For each plastic item recovered the source, weight, type of plastic, degree of degradation and cleanliness should be recorded. The OSPAR monitoring protocol (OSPAR 2010), which focuses on the source of plastics, can be extended and modified to document the relevant information. The indices on degree of degradation and cleanliness should be developed in dialogue with relevant companies that recycle marine plastics.

At what time plastic pollution may be in particularly high concentrations and how long time it remains at these concentrations is relevant to determine if clean-ups could be more efficient at specific times of the year and how quickly the clean-up technology has to be deployed to catch the litter before it is flushed out to sea.

  1. Year-round monitoring and close monitoring of large plastic pollution events should be conducted. Monitoring during wet-season should be given priority.
  2. If a larger plastic pollution event takes place during the pilot survey period, these should be monitored to document the evolution of the event from first detection to it is almost impossible to detect.

Key terminology

Marine litter«any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment. Marine litter consists of items that have been made or used by people and deliberately discarded into the sea or rivers or on beaches; brought indirectly to the sea with rivers, sewage, storm water or winds; accidentally lost, including material lost at sea in bad weather (fishing gear, cargo); or deliberately left by people on beaches and shores» (UNEP 2005)

Marine debris: has been used as a synonym to marine litter, but could also include detached natural fragments as well as pieces of litter.

Marine plastic debris/marine plastic litter/marine plastic pollution/marine plastics: refers to the plastic fraction of marine litter.

Waste«any substance or object which the holder discards or intends or is required to discard» (EU 2008).

 

There are many definitions describing the size fractions of plastic. Generally, particles < 5 mm are defined as microplastics and macroplastics are > 5 mm. The definitions used in this report are generally as follows:

  • Nanoplastic: < 100 μm (Koelmans, Besseling, and Shim 2015)
  • Small microplastics: 0.33-1.00 mm (Eriksen et al. 2014)
  • Large microplastics: 1.01-4.75 mm (Eriksen et al. 2014)
  • Mesoplastic: 4.76-200 mm (Eriksen et al. 2014)
  • Macroplastic: > 200 mm (Eriksen et al. 2014)