What is the impact of city cleanliness on the environment?

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Today in Europe, 75% of the population lives in cities. 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.

The level of cleanliness of the streets determines the amount of waste that will end up in the natural environment. A clean urban space will limit this source of pollution. However, cleaning operations consume resources (energy, water) and also impact the environment (noise, dust, greenhouse gas emissions).

Users are sensitive to the cleanliness of public places and the inconvenience caused by cleaning — as evidenced by the complaints received by the road departments. Intuitively, at the scale of all cities, we can sense that the methods used to maintain a certain level of cleanliness are exerting strong pressure on the environment. However, the impact of cleaning up cities is not deemed urgent because it is not measured — making it difficult to realise the true extent of the issues at stake.

A necessary awareness

Cities hold an important contribution in the prospects of sustainability of the planet. To what degree do urban cleanliness and roadwork activities contribute to a city's environmental footprint?

Surprisingly, we have found few studies linking the cleanliness of urban centres to their impact on the environment. Recent research on the characterisation of plastic waste flows to the oceans confirms that cities are a major source of pollution. Regarding the ecological footprint of street cleaning, data is lacking.

Our own assessments with partner cities, however, lead to a harsh conclusion: cleaning up a city takes a heavy toll on the environment. Fortunately, some cities have been able to integrate the environmental dimension into the management of their cleanliness, with convincing results.

In our opinion, a need for transparency is clearly felt in our communities, in order to measure the impact of urban cleanliness on the environment. This will make it possible to identify the priorities and objectives to be achieved, to mobilise the players and to share good experiences.

Evaluating the environmental impact of urban cleanliness: some milestones

To improve, you have to be able to measure. Comprehensive ecological assessments are long and complex processes. We would like to offer a pragmatic and easy-to-use tool that will allow you to draw up a report, set objectives and steer an improvement process. This will give cities the opportunity to take virtuous steps, to communicate their progress transparently and to share good practices.

To meet these specifications, we identified the factors that weigh on the cleanliness of cities, quantified their relative importance, then made choices so as not to unnecessarily complicate the measurement. These assessments were made mainly with data from the FOEN, the study partner cities (Geneva, Basel, Zurich) and the AVPU. The ambition is to provide a working basis to converge with the players concerned towards an accepted reference method.

The model proposed here assesses the three main sources of impact selected:

• CO2 emissions from the fleet of road vehicles (sweepers, washers, utilities, etc.)
• Water consumption
• The distribution of waste into the natural environment.

We then applied this model to our partner cities. The evaluations obtained highlight the overall performance of the city and specific points where improvements could be sought.

The impact of mechanised urban cleaning

The vast majority of road vehicles still run on diesel. The emission standards of in-service vehicles are comparable. According to data from the FOEN, the combustion of one litre of diesel emits 2.61 kg of CO2 (or 3.06 kg if fuel flow is taken into account). So a sweeper that consumes 6 litres per hour and operates 700 hours per year will emit 11 tonnes of CO2 into the atmosphere. We propose to calculate the emissions of the entire fleet of utility vehicles of the road department from the diesel consumption readings, then to determine the emission classes for each city by relating these quantities to the number of inhabitants.

The number of inhabitants is readily available data, commonly used for comparing cities with one another, as it generally represents the pressure on urban space. It should be noted that this calculation disregards the grey energy of the machines (a more efficient organisation requiring fewer vehicles should in principle be favoured) and that the integration of electric vehicles would require taking into account additional data, in particular on the origin of the electricity consumed.

Table 1 — Environmental label for carbon emissions.

The use of water in the cleaning practices of each city

The consumption of water for urban cleaning purposes varies greatly from one city to another. Indeed, some cities resort to washing extensively, while others simply sweep. A sweeper consumes an average of 400 litres of water per day, part of which is collected by vacuuming. For washing techniques, high pressure lances connected to the water network or fire hydrants have average flow rates of 12 litres per minute. The washers used for washing pavements with water are equipped with tanks with a capacity of 1,000 to 7,000 litres, filled several times a day. Cities that use washing use considerable quantities of water.

The quality of the water (drinking or not) used by road cleaning depends on the cities and sometimes on the circumstances. Clean water is a precious (and expensive) resource and should not be used to wash the streets. The water, loaded with heavy metals, leaching from constructions, micro-plastics from the abrasion of tyres, then returns to the watercourses, also carrying urban waste. According to our research, the pollution of wash water does not seem to have been measured.

To measure the environmental impact of water use, we propose to compare the total annual consumption, all techniques included, with the number of inhabitants. This simple measurement does not take into account either the quality of the water (drinking water or not), its degree of pollution after washing, or the type of treatment when the water is recovered.

Table 2 — Environmental label for water consumption.

The impact of residual waste discharged into the environment

In cities, clear water (or rainwater) joins runoff water, which goes directly into the natural environment, sometimes by being filtered, without going through a treatment plant. Therefore, the drains lining the roadsides are gateways to the natural environment.

We assume that urban waste likely to pollute waterways only passes through these "gateways" (which excludes the large-scale waste frequently encountered on the streets such as PET bottles and cans). We considered that the nuisance of these wastes for the environment depends on their degradation time (without taking into account their potential toxicity). Therefore, we have ruled out organic waste and paper, which have a short lifespan. In the end, plastics and cigarette stubs constitute the urban waste with the most significant impact on the environment. Of course, this waste is only part of the ocean's pollution. Other waste, such as single-use plastics, cotton swabs, PET bottles, go through watercourses that the road department does not control.

The amount of urban waste that may pollute the environment depends on the level of cleanliness of the city and cleaning efficiency. It is measured by the cleanliness index. The longer the period of exposure of this waste, the more it will be washed away in runoff by traffic, wind and rain. As already noted, washing with water increases the rate of waste passing through the drains, and therefore the contamination of watercourses. To characterise these diffusion factors, we measured the efficiency of sweepers (rate of waste remaining after their passage). We have also fitted the drains with filters to count the waste that passes through this circuit. These results will be presented in a subsequent Clean City Lab article.

On the basis of these analyses, we propose to measure the impact of urban waste on the environment by: a cumulative duration of degradation, obtained by the quantity of plastic waste and cigarette stubs multiplied by their respective lifespan (200 years for plastics, 10 years for stubs). These durations are put into perspective according to the number of inhabitants, in order to be able to compare the cities with each other. To establish the classes, we took into account a diffusion factor between the amount of waste present on the ground and that recovered in the drains.

Table 3 — Environmental label for waste released into the environment.

Urban cleanliness, a key player in the sustainability of cities

The resources committed to cleaning up our cities generate a significant environmental footprint. European cities use an average of 35 diesel vehicles per 100,000 inhabitants, including 16 for sweeping and washing. On a European scale, for the consumption calculated previously, this represents more than two million tonnes of CO2 emitted by road departments each year.

For example, the city of Geneva, which has 200,000 inhabitants, operates 57 sweepers and washers, which consume 235,000 litres of diesel and emit 620 tonnes of CO2 per year. The city of Zurich, which has been optimising cleaning based on cleanliness indicators on a regular basis for about twenty years, emits significantly less CO2 per inhabitant. The city of Basel is committed to a zero carbon policy by 2025.

As for water consumption, according to the data available to us, a city that uses washers potentially uses 40,000 m3 per year per 100,000 inhabitants. Cities that have chosen sweeping alone consume up to 30 times less, proving that it is possible to conserve water while ensuring a high level of cleanliness.

Part of the waste from urban areas is washed into watercourses and contributes to the pollution of aquatic environments. With its directive on reducing the impact on the environment of certain single-use plastic products, the European Union is committed to a drastic reduction of this impact. Cities are mobilising today to raise awareness among users or improve the efficiency of street furniture and therefore reduce the number of waste thrown into the streets. A quantitative measure would make it possible to reinforce the most effective practices.

Proposal for a practical assessment tool for cities

This article proposes a method to measure the environmental impact of cleanliness, which no published approach currently allows. Far from being anecdotal, this impact concerns precious resources such as water, urban air and the climate (emission of tonnes of CO2).

Cities are implementing many initiatives to improve the cleanliness of their spaces and limit their impact on the environment. Transparent measurement of this impact would have the advantage of raising awareness among stakeholders, pooling their efforts, evaluating progress and sharing experiences.

In order to compare cities with each other, we offer a simple tool based on labels. The three major impacts identified — carbon emissions, water consumption and waste discharge into the environment — are measured using labels ranging from A to G. Class A-A-A means that cleanliness is of the highest standards, in terms of ecology and efficiency. Class B-F-C for example would indicate that progress could be made on washing techniques.

These environmental impact labels for the cleanliness of cities could constitute a particularly interesting incentive approach, allowing cities to evaluate themselves, identify priority areas for improvement, mobilise stakeholders and communicate progress.

The approach proposed here is a first step. It may evolve based on the experience feedback from cities and a larger basis of comparison. Organisations and cities interested in improving and deploying this tool are welcome. We invite them to join the Smart Clean Cities community.

Acknowledgements

We would like to thank the Federal Office for the Environment (FOEN) which supported the Clean City Management project, as well as the AVPU and the cities of Geneva, Basel and Zurich for their contributions.