smart. clean. sustainable.
smart. clean. sustainable.
Clean City Lab
Plastic pollution of the marine environment is currently one of the most growing concerns. Large amounts of floating plastic are observed in the main oceanic gyres, and cities bear a significant responsibility in this leakage.
The presence of plastic has been detected on shorelines of all continents, among which cigarette butts are the most frequent litter encountered during shore clean-up operations. As we know now, this is just the tip of the iceberg. Many macro-plastics degrade into micro-plastics and depending on their density, drift over large distances or sink.
Today in Europe, 75% of the population lives in cities and the increase in the urban population represents a global trend. Cities are turning into centres of activity and recreation; social habits change; the population takes over public spaces. Waste increases — and with it, incivilities (littering, dirt, etc.). Waste that escapes collection diffuses into the environment. In particular, cities are held responsible, as the main source of input, for the issue of cigarette butt release into the waterbodies.
Various studies have been done to estimate the quantities of plastics entering the oceans globally. A study by Boucher et al. estimates that yearly 3% of all produced plastic is leaking into the environment. More specifically, a recent study conducted by the University of the Côte d'Azur investigates the cigarette butts' leakage and suggests an average release rate of 6% for the entire world. These simulations — global models — help to size the problem and identify the root causes in order to define and implement the corrective actions.
Generally, modelling the plastic leakage consists of three main steps: (i) estimating the mass of plastic ending up as waste, (ii) quantifying the fraction of mismanaged waste, and (iii) estimating the release from the mismanaged waste fraction. The models run based on a set of default assumptions such as mismanaged waste rate, littering rate, and release rate. A great challenge for estimating these rates has been cited by all model developers. Fundamentally, because these rates are hardly measured in the field and relative data are very limited.
During a case study in the city of Geneva, Cortexia tried to characterise and measure, in the field, the litter leakage from the urban area into the environment. This would help to compare the measured results with the global models' assumptions. This project completes the previous work of the company on the environmental impact of urban cleaning and the solutions to reduce cigarette butt littering. The study aims to answer: which, and how much litter is released from cities?
Cigarette butts, small pieces of plastics, and papers are the most littered items found in cities. Cortexia assumes that street litter, which is not collected by urban cleaning, leaks into the city's water runoff system. Therefore, quantities of litter which escape the mechanical cleaning process, as well as the quantities of litter found in the water runoff system, were measured for calculation of a release rate. The result was then compared to the assumptions used in release models.
Street sweepers are widely used to clean the roads from trash, dust and to avoid the growth of weed. Does this mean that after the cleaning the road is free of litter?
The monitoring of urban cleanliness, using for instance the Clean City Index, confirms that the cleanliness level after mechanical cleaning is very high. The quality of cleanliness after the work of the machines is not disputable. Nevertheless, a more detailed observation shows that if all litter is removed, this is not the case for cigarette butts. We have got used to them and no longer see the cigarette butts remaining on the road's cracks, between paving stones, and in tree grates even after a street sweeper or manual cleaning cleans a passage.
We have characterised the "cleaning efficiency" of mechanical cleaning on our reference street, which has a standard coating and does not impose any difficulties on the cleaning process. We have counted the number of litter items on the street and pavement, before and after cleaning. As mentioned before, the only remaining litters are the cigarette butts. On average, 16% of the cigarette butts remained permanently on the ground after the passage of the sweeper in the studied street. This value confirms the assumption in L. Pourchier's model — "15% cigarette butts left on the ground without collection option in high-income countries".
The amount of littered trash in an urban area, which is exposed to be leaked, depends on the littering rate, efficiency of mechanical cleaning, and frequency of cleaning. As we have seen, right after the mechanical cleaning, the road stays in the cleanest state (16% of remaining cigarette butts in the case of our reference street). Afterward, the trash keeps accumulating due to the act of littering until the next cleaning session (state of 100% dirt). On average, between each two consecutive cleaning sessions, an amount equal to 58% of total littered trash during this duration is temporarily exposed to be leaked to the natural environment.
No matter how efficient the cleaning process is, there is always a portion of the total littered trash that will be exposed to leakage, temporarily. This fact highlights the importance of anti-littering measures and reduction of littering rate in a city.
The assumption we made in our previous Clean City Lab papers is that part of the litter, which is permanently on the street, leaks to the natural environment through the city drainage system. Therefore, to monitor the amount and composition of trash leakage via the drainage system of the city, Cortexia designed a mesh strainer to place at the entrance of the drains. These meshes act like a filter, trapping the trash before it enters the urban water collection system.
Different parameters such as meteorological data (temperature, precipitation, wind), volumetric amount of water used for cleaning, and the position of the drains' gate (in calm or crowded location) were monitored during the experiment. The experiment took place in Geneva on the same pilot street with a monitoring period of 7 months.
The "harvest" over this period shows that cigarette butts and packaging plastics are the most frequently leaked wastes from the streets, at a rate of respectively 6.2 and 1.8 items per drain per day. To give an order of magnitude, there are 90 drains in this street and 20,000 in the entire city. If the leakage rate is representative of the whole city, this means that according to the study, 61,000 (daily) and approximately 22 million (yearly) cigarette butts are leaking from Geneva's streets to the waterbodies via the city drainage system.
Additionally, the study showed a strong correlation between the leakage and the position of the drains' entrance. More cigarette butts were collected by the filters which were close to commercial areas. However, Cortexia could not draw clear correlations between release rate and other measured parameters. A more detailed study needs to be done to investigate the exact contribution of each parameter on the release rate of cigarette butts.
The commonly cited release rate in the literature addresses the fraction of mismanaged waste that reaches the ocean. Mismanaged waste, according to Jambeck et al., is "the material that is either littered or inadequately-disposed. Inadequately disposed waste is not formally managed and includes disposal in dumps or open, uncontrolled landfills, where it is not fully contained." In this study, Cortexia only investigated the release rate associated with littering. It is worth mentioning that in a developed country such as Switzerland, where no municipal waste is allowed to be dumped or landfilled, the chances for inadequately disposing of cigarette waste are so low that we consider the amount of inadequately-disposed cigarette butts to be zero.
Knowing the number of cigarette butts that permanently stay on the street (permanently littered cigarette butts), as well as the number of cigarette butts that leak into the drainage system of the city (leakage), the cigarette butts' release rate from the streets was calculated. The result of this case study in the city of Geneva suggests a 2% release rate for cigarette butts.
The first and most commonly used release rate for plastics is the generic rate of 25% within 50 km of the coast, which was published by Jambeck et al. in 2015. However, recently, Boucher et al. suggested a release rate matrix calibrating this generic rate for each country according to its distance to the shore and the catchment run-off. Considering the adjustments suggested by Boucher et al., the plastic release rate for Switzerland will be 6%. Moreover, Pourchier suggests a 5.5% release rate specifically for cigarette butts and for the entire of Europe. In comparison to the literature, Cortexia's case study in Geneva suggests a smaller release rate for cigarette butts.
The simulation models are very powerful tools to make anti-littering campaigns around the world more efficient by targeting main root causes and hotspots. A brand new platform called PLASTEAX provides a good database on plastic waste management and a great opportunity to model plastic leakage across a value chain. Among many different types of plastic, cigarette butts are the most encountered litter in nearshore areas, and L. Pourchier has recently developed a global model to simulate the input of cigarette butts into the oceans.
Within the Geneva Clean City Lab, we have performed some experiments to confirm if the cities, as predicted by the models, bear an important contribution to the plastic pollution of the marine environment. The following highlights the results of this study and the conclusions:
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