A carbon footprint: the full water cycle in the Balearic Islands

The Balearic Islands archipelago is made up of the islands of Mallorca (923,608 inhabitants in 2019), Menorca (96,620 inhabitants), Ibiza (50,000 inhabitants), Formentera (12,200 inhabitants), and a series of smaller, practically uninhabited islands and enjoys a Mediterranean climate (see Fig. 1). Archipelagos such as the Balearic Islands face severe water scarcity problems. A review of the regional distribution of desalination capacities worldwide shows that the installed capacity for the desalination of seawater is increasing rapidly. Spain is the largest producer in the region and represents 7% of the worldwide capacity, with 70% of the Spanish plants located on the Mediterranean coast and the Balearic Islands (Lattemann & Höpner 2008). The pressures on water resources suffered by this archipelago are mainly due to the following aspects: high tourist activity, overexploitation of aquifers and marine intrusion into them (Candela et al. 2009), urban pollution, diffuse pollution from agricultural and livestock activity, and periods of drought, amongst others (García et al. 2017). This situation may worsen or intensify the pressures in a scenario of climate change, in which we currently find ourselves (García & Rodríguez-Lozano 2020). The Balearic Islands are also supplied by a combination of groundwater, which accounts for 90% of the water resources in the Balearic archipelago (Gómez et al. 2004), surface water, and desalinated seawater. Because of the high consumption of electrical energy to obtain freshwater, the production of drinking water is one of the most important sectors in the ecological transition of islands (Papapostolou et al. 2020).

The facilities that constitute the integral water cycle in Spain are varied, and the technology utilised in each one depends on the geographical area studied. This is due to the water model and the availability of resources, which varies between the islands and the Iberian Peninsula (Custodio et al. 2019). In this regard, the Balearic Islands base their own water model on the use of groundwater resources, which are the most important in quantitative and qualitative terms. In the Iberian Peninsula, the water production model relies more on surface water resources (García & Rodríguez-Lozano 2020). Therefore, the main facilities on the islands for treating drinking and wastewater are the following: groundwater wells, desalination plants, treatment plants and artificial recharge stations for the aquifers.

The methodology followed in this article is that proposed by the internationally recognised Green House Gas Protocol or GHG Protocol (Guallasamin Constante & Simón-Baile 2018). The GHG Protocol initiative arose from the union of various companies, nongovernmental organisations and other agents under the coordination of the World Resources Institute (WRI) and the World Business Council for Sustainable Development (WBCSD). The aim of this standard is to avoid heterogeneity in the methods and principles used to calculate the greenhouse gas emissions of internationally accepted companies and organisations (Guallasamin Constante & Simón-Baile 2018). Importantly, the GHG Protocol is applicable to any type of organisation and company and allows all three scopes of the carbon footprint to be calculated, making it a good fit for any carbon footprint calculation desired.

This standard makes it possible to account for the emissions of an activity using the three scopes of the carbon footprint. Scope 1 refers to direct emissions made by the company, mainly related to the burning of fossil fuels, and Scope 2 refers to indirect emissions caused by the company’s electricity consumption (Azarkamand et al. 2020). The case of Scope 3 is more particular, as it accounts for indirect emissions of the company that are related to it (suppliers, business trips, purchase of materials) (Hertwich & Wood 2018). This last scope is the most complex to address due to its open nature, but in this case, it is limited to suppliers, workers and waste management. Thus, the journeys made by the main actors related to the facility have been considered, such as the workers’ journeys to the workplace, those of suppliers and those due to waste management. To this end, information has been collected on the days worked by workers, how they travel to the treatment plant, the type of vehicle and the kilometres travelled to be able to make a realistic estimate of these emissions.

In this regard, it is necessary to bear in mind that in a water treatment or distribution facility, two types of emissions can be distinguished depending on the point or area of emission. Emissions from concessionary or authorised companies are emissions produced by activities carried out at the facility by concessionary or authorised companies. Emissions from the installation are those produced by the activities carried out by the operator itself. Therefore, the scope of the study of the installations includes maintenance work, the electrical transformer station, fuel station, clean point, drinking water supply installation and electrical network.

To obtain the data, a specific form was designed and sent to the managers of the selected facilities to obtain information on diesel consumption in litres and electricity consumption in kWh, as well as the number of suppliers and workers, their attendance at the company and the average distance travelled by their vehicles. After obtaining the data, the emission factors for the studied years of 2019 and 2020 were used to transform the data into tonnes of CO₂ equivalent, which is the unit of the carbon footprint.

Regarding the data on electricity consumption and diesel fuel consumption, the facilities had these data monitored for each year, meaning they were very accurate. However, there is less accuracy in terms of workers’ and suppliers’ vehicles, as it is very difficult to establish the exact kilometres driven by each worker to their job, as well as the possibility that they may change the type of vehicle during the years studied. This is why in Scope 3 more uncertainty arises than in Scope 1 and 2, which are more accurate as they are annual company consumption.

To calculate methane emissions from a wastewater treatment plant, the methodology proposed by the IPCC (Doorn et al. 2006) was followed, where the total organic load of the water (related to the equivalent population served by the treatment plant), the methane emission factor and the uncertainties were considered.

The facilities for which data were obtained, distributed by island, are as follows (see Table 1):

The three wastewater treatment plants (WWTP) of Menorca share the company vehicle; therefore, as they do not have diesel consumption for fixed installations, these three treatment plants have the lowest scope, Scope 1. With respect to Scope 2 of these three treatment plants, it should be noted that, as of September 2020, the diesel company vehicle was replaced by an electric vehicle; therefore, the carbon footprint of 2021 will not have values associated with Scope 1 in these treatment plants but with Scope 2, as the new vehicle is dependent on electric energy and not on fossil fuels. The charging point for this vehicle is located at the Menorca 3 wastewater treatment plant, which is where it remains at rest.

In the Menorca 3 WWTP, photovoltaic panels are installed in the plant, with a mode of self-consumption with surpluses, and the production of the panels is on the order of approximately 60,000 kWh per year. At the Menorca 4 WWTP, the company vehicle was also replaced by an electric vehicle in September 2020. The WWTP Menorca 5 responds to a larger population; therefore, it has the largest carbon footprint, as it has the highest electricity consumption of all. It also has the largest number of associated vehicles and the largest number of workers.

On the islands of Ibiza and Mallorca, Scope 2 is the highest in all cases. However, Scope 3 in Mallorca is higher than Scope 1, i.e. the use of fossil fuels by the facility, whilst in Ibiza, Scope 1 is higher than Scope 3 in the facilities studied.

With respect to Scope 3, all the treatment plants have suppliers and authorised waste management, mainly related to the treatment of sludge generated at the facility. The purified sludge undergoes digestion treatment and, once digested, is thickened and dewatered by means of centrifugal decanters (use of polyelectrolyte for thickening). The treated effluent goes, in all cases, to a submarine outfall.

Population and BOD₅ also influence the methane production of each facility, the range of which in this case is between 1 and 15 tons of methane per year, depending on the type of each treatment plant. In the digestion of sludge, whether it is done at the WWTP or elsewhere, large quantities of methane are emitted, which have not been calculated in this study. In general, they account for between 0 and 40% of the total emissions generated at wastewater treatment plants and are associated with the carbon footprint (Baeten et al. 2021). In general, the sludge produced is collected by a specialised company and treated as solid waste in a facility designed for this purpose. The final destination of the treated effluent varies in each case, although most commonly it is sent to a submarine outfall and, in some cases, it is used for irrigation in agriculture, golf courses and street washing.

Our findings also demonstrate that the facilities that treat a higher flow rate are more energy efficient (see Table 3) since wastewater treatment plants have a carbon footprint that is a function of the volume treated by the facility. However, it should be borne in mind that the value that will mark the suitability of the wastewater treatment plant is its efficiency in removing pollutants from the water, and this is only achieved when operating below the design flow rate (Collivignarelli et al. 2021).

With respect to the carbon footprint results obtained in the WWTPs, we observe that their values are similar to a study carried out in China on nine WWTPs (Gu et al. 2016), where these also showed a carbon footprint range between 0.11 and 0.45 kg/m³. In the case of the Balearic Islands, we find that this range is between 0.10 and 0.79 kg/m³ (see Table 3). There is a logarithmic relationship between (see Fig. 3) the number of tons of CO₂ emissions per Hm³ treated that are generated in each treatment plant. Therefore, in this case, the treatment plants are much more efficient the more flow they treat.

The results of the carbon footprint are also divided by scopes (see Table 2). As expected, Scope 2 for desalination plants is quite high compared to the rest of the facilities studied in this report. This is because desalination plants are large consumers of electricity for their operation, and given that Scope 2 considers emissions related to electricity consumption by the company, it has yielded high results in all the desalination plants studied. In other similar studies (Ghani et al. 2021), this same trend is observed, where electricity consumption is the main contributor to emissions associated with desalination plants. In a study developed in the Canary Islands (Spain), it is shown that the tons of CO₂ per MWh associated with the desalination sector are between 0.5 and 0.84 (Leon et al. 2021).

One of the existing alternatives that can compensate for the carbon footprint in Scope 2 is to select supply companies with an electric mix equal to zero. The electricity mix is calculated for each electricity company, considering the sources of energy generation. If the company uses entirely renewable energy sources for electricity production, the electricity mix is zero. As it incorporates non-renewable sources, the electricity mix increases due to the emissions associated with burning fossil fuels from those sources (Gopi et al. 2019).

Scope 1 is composed of the diesel consumption of the facility, as well as the fuel consumed by the vehicles associated with the facility (staff travel and activities related to the desalination plant). Finally, in Scope 3, we have high emissions mainly due to the large number of workers at the desalination plant, the number of suppliers and waste management. The trips made by the vehicles of suppliers, workers and waste management significantly increase the carbon footprint of the facilities. This is common in all those companies that provide a service, since they generally have a large number of workers and require numerous suppliers to assist them to guarantee the service (spare parts, office material, occasional maintenance by external companies, revisions, breakdowns, etc.).

The carbon footprint per volume of water withdrawn was generally lower in 2020 than in 2019 for most of the desalination plants (see Table 4). This is mainly because, in most cases, the flow captured by the desalination plant was lower in 2020 (possibly due to the decrease in tourist activity on the island, amongst others), as well as the decrease in the emission factor in the two companies that supply electricity.

It is observed that a lower volume of water captured in the desalination plant does not necessarily imply a lower carbon footprint (see Fig. 4). This is due to the following influences on the different scopes:

Figure 4 shows how the trend appears to be reversing at lower flows, as less water was desalinated overall in 2020, but the carbon footprint is larger. However, for high volumes in 2020, the footprint was reduced at similar flows. This finding can also be observed in a similar study conducted in the Canary Islands, the other Spanish archipelago, which, in turn, is an outermost region of Europe (Cruz-Pérez et al. 2022).

Finally, Figs. 5 and 6 are added where the following information can be consulted: Fig. 5 shows the relationship between the volume of water treated by the facilities and the normalised carbon footprint, per cubic hectometre; Fig. 6 shows the carbon footprint for each of the four blocks studied (desalination, wells, wastewater treatment and water distribution) for each of the four islands of the Balearic archipelago (in the red-dotted line, you can see the difference between the years 2019 and 2020).