*3.1. Construction Phase*

The proposed location for the RED plant is on the coastline between La Carbonera lagoon and the coastal dune, outside the main SRZ1 polygon, and under the jurisdiction of the Federal Maritime Terrestrial Zone (ZOFEMAT) (A, Figures 1 and 4). At this location, several elements need to be constructed (roads and other infrastructure related to basic services). In the literature, as an example, the design of a PRO plant with a production capacity of 50 kW net output covers approximately 7000 m2 (the approximate size of a football field) [49], while the area of the Afsluitdijk RED plant covers approximately 2750 m<sup>2</sup> (measured from Google maps). The total area proposed at La Carbonera is 5250 m2, the RED plant elements will be distributed in 3000 m2 (Figures 4 and 5), which includes three tanks for the concentrated (hypersaline), diluted water (marine or fresh), and the resulting effluent (brackish marine). There will also be space for the membrane modules and for a test laboratory. Finally, spaces for storage, waste storage, sanitary facilities, and an electrical station will be added (Figure 5).

**Figure 4.** Potential location of the RED pilot power plant; the dotted lines indicate the pipelines.

**Figure 5.** Distribution of space for the proposed RED plant and ecotourism centre at La Carbonera site, 5250 m2.

La Carbonera, in spite of being a natural protected area, is becoming increasingly popular with tourists. This is encouraged since tourism is more profitable than fishing and agriculture. In line with regulations that allow minor infrastructure in the area, it is proposed that alongside the SGE plant a small ecotourism centre is built (with a total area of 2000 m2), with cabins, viewpoints, rest areas, and a section for supplies, such as kayaks and boats. (Figures 4 and 5). This centre could include a turtle camp, as the area receives three protected species (*E. imbricata*, *C. mydas* and *C. caretta*, Table 1), arriving to lay their eggs. The incubation of the eggs, laid on the beach, would improve hatching success. The huts and nesting pens could be made using local palms (*Sabal yapa*, Table 1), widely used for this type of construction [50].

The ecotourism centre would benefit from the electricity generated at the RED plant. This energy could be used to charge mobile phones, cameras, or torches, in addition to the electricity needed for the turtle camp (mainly the nurseries), and in the future, could also supply energy for electric boats. The energy generated would be sufficient to make the plant self-sufficient in electricity. An ecotourism centre would provide well-paid jobs and also limit the pressures associated with the present mode of tourism; visitors coming into the area on day trips from other towns and villages which has brought various environmental problems to this area. The biological richness of the region is accessible in only a few places, and many features of the tourism model are misguided. Excessive growth in some of these areas has led to the infilling of swamp areas to build homes, damage to the coastal dunes due to road construction, and poor management of solid waste and residual water, in particular [42] (Figure 6). Using this site solely for ecotourism, scientific development, and species conservation could help to curb such chaotic, harmful growth around La Carbonera. In addition, the mangroves, swamps, and dune vegetation would be preserved because the ecotourism centre would not be built in these ecosystems. It would also promote environmental awareness among the local population.

**Figure 6.** Aerial view of the villages around La Carbonera lagoon (the jetty is shown as a reference).

The stressors examined (Figure 3) include excavation, site preparation, placing pipelines, and the construction of platforms and modules. The impacts are similar to those of any other engineering project: removal of vegetation, habitat loss, erosion, unwanted sedimentation, soil compaction, temporary increase in turbidity, avoidance behaviour of birds and fish (due to construction noise), damage or removal of benthos, change of land use, temporary air pollution, and landscape disruption. Nevertheless, all of these will depend on the construction techniques and on the precautionary measures taken. The plant would be located on the seashore (Figure 2, section B), and as it would be affected by the tides, it is proposed that the RED plant be built on pillar-supported infrastructure (palafitte) that will allow water to flow beneath. These low-impact structures do not require extensive excavation [51] and would minimise some impacts on geomorphology and vegetation. They would also permit the free passage of wildlife, particularly useful in storm surge events. In terms of landscape disruption and land use change, as it is a small project, the impacts are expected to be minimal. Given that the site proposed for the plant will not modify the hydrodynamics of the lagoon, natural sediment transport changes are not expected. Additionally, any increase in turbidity, damage or elimination of benthos, erosion or soil compaction, and air pollution will only be temporary. In the case of vegetation and habitat loss, this location was chosen (Figure 4) precisely because there are no large patches of vegetation, except for some coastal dune species in the vicinity (Table 1).

During construction, in order to lay pipes, excavation work is required, which can lead to temporary increases in turbidity (due to sediment movement), loss of vegetation cover (terrestrial and aquatic) and associated habitat loss, removal of benthic organisms, and the release of nutrients and pollutants from the bottom of the lagoon (Figure 3).

The pumping system should be low cost and easy to maintain (to avoid algal blooms, oxygenation, and suspended particles). Three schemes are possible for the water intake in the lagoon: Marine Zone and Freshwater Zone (MW/FW); Marine Zone and Hypersaline Zone (MW/HW); or Freshwater Zone and Hypersaline Zone (FW/HW) (Figure 1). Water intakes should be close to the plant, both to minimise the impact of pumping losses on net power production and to avoid environmental impacts associated with some stressors (excavation) [52]. One option to avoid impacts from the excavation is to lay pipes on the surface and thus avoid the impacts associated with this stress factor. In the MW/FW and FW/HW schemes, the first problem is that the area where the fresh water emerges is within polygon SZAE1, and although ecotourism activities and the exploitation of flora and fauna can occur in this area with a permit, laying pipelines may not be compatible with the conservation laws of this polygon [31]. On the coasts of Yucatan, groundwater is used to supply fresh water to nearby settlements. Thus, the MW/FW and FW/HW schemes could be a possibility using wells or pumping to extract fresh or brackish water [53] in order to avoid negative effects on other processes and activities.

In the MW/HW scheme, which the plant is designed to use (Figure 4), hypersaline water would be taken from the hypersaline zone to the plant, and the pipeline would pass through parts of the SRZ1 polygon; however, low impact sustainable infrastructure would be used (Figure 4). Seawater would be brought to the plant directly from the sea, under the jurisdiction of the ZOFEMAT (Figure 4). Although another possibility for this region, where there is high radiation and evaporation, generating hypersaline conditions, would be to use seawater evaporation ponds. The volume of hypersaline water needed would therefore not be an impediment.

For the energy production expected, an inflow of 200 m3/h of both solutions is required [52]. These inflows will pass through a hydraulic network, which includes the set of pipes, pumps, and other accessories (elbows, valves, etc.) that will allow the entry and exit of the solutions. For a flow of this volume, 10-inch diameter pipes are required; hence, the impacts associated with excavation will be less. The hypersaline water must be piped to the plant; options to avoid the mangrove patches in its path exist, and special attention must be paid to the protected mangrove species (*C. erectus*, *A. germinans*, *R. mangle* and *L. racemosa*) [36]. For the discharge pipe, the same factors apply. These species are important because they provide shelter and protection areas for birds and various species of fish, and they deliver ecosystem services such as filtering water discharges from the mainland to the sea [54] and protection from wind and waves that prevents coastal erosion in an area that is also affected by hurricanes [55].

In the construction phase of the proposed design, no large-scale pipes would be installed within the lagoon; therefore, the loss of seagrasses, the release of pollutants and nutrients from the bottom of the lagoon, and the removal of organisms from the benthos would be practically nil, and the increase in turbidity due to the movement of sediments would be only temporary (weeks). Regarding the seagrass ecosystem, in areas with high salinity, seagrasses are not found; therefore, in the water intake in the hypersaline zone, there will be no damage to seagrasses [56,57]. In the hypersaline zone, the depths are 50 cm or less (Figure 2), and therefore, the hydraulic network here should be able to pump hypersaline water from different points to prevent it from drying out.

On the other hand, regarding the avoidance behaviour of birds due to construction noise (Table 1), emblematic species such as the flamingo (*Phoenicopterus ruber*, Table 1) would probably modify their distribution in the area only temporarily (weeks). In addition, with RED technology, no turbines will be used in the operation phase; hence, noise pollution would only occur during construction.

### *3.2. Operation Phase*

In this phase, the impacts associated with water pumping and water pretreatment (Figure 3) are the first that should be considered. Pretreatment of the water intake is crucial for the operation of a RED system [58]. Such a system must ensure low-cost performance and effective sediment filtration [52]. In the RED plant of Afsluitdijk, the filtration system is of the drum and gravity type, and even with small intakes of very good quality water cartridges, filters can be used [52]. The impacts associated with water pumping and pretreatment are related to the amount of fresh water and hypersaline water that will be taken from the system and pass through it. These impacts include changes to natural watercourses, as well as changes in the nutrients and salinity of the water. Possibly, and depending on the intensity, these alterations may affect native species and natural ecosystems. For instance, it is known that changes in salinity alter the reproductive and feeding behaviour of flamingos and horseshoe crabs (*Limulus polyphemus*) [59].

Another impact is the possible decrease in phytoplankton biomass retained in the filtration system. The latter is a concern at the Afsluitdijk pilot plant because large quantities of plankton, fish, and larvae must be filtered, which has ecological implications and may also have economic implications [60]. Since the biomass of microorganisms at the base of food chains is affected by this, it can lead to imbalances in the food chain and local fisheries [23,48].

Detailed hydrological studies are therefore needed to determine the amount of water that can be extracted from the lagoon and how much it can be altered without generating the above-mentioned impacts. In the case of the Afsluitdijk pilot plant, the intakes in the sea and in the lagoon area of 200 m3/h, assuming a technical potential of 1 MJ/m3 of seawater and freshwater, can produce up to 50 kW net power output [52]. In La Carbonera, taking the concentrated solution from the lagoon (hypersaline water) counteracts the effects on the biomass of microorganisms to a certain extent, since the biomass is reduced due to the hypersalinity of the area [59]. The diluted solution could be taken from wells on the coast, in which case there would be no phytoplankton, due to the lack of light. On the other hand, it could be taken directly from the sea, and this would have fewer microorganisms than that of the estuarine and marine zones of the lagoon [61].

Another stress factor is the disposal of the final by-product of the RED process—the brackish water. Even if the intended scheme is MW/HW, the water mix would have a similar or higher salinity than seawater. The change in salinity of the effluent must be calculated in the laboratory. The effluent must be discharged in an appropriate area, at the appropriate time, and the dispersion of this effluent by the hydrodynamic actions of the system should not alter the natural salinity patterns in that ecosystem [32,33].

Depending on the volume of the water, pump diffusers may be needed (alternating or slanted) in the hydraulic network, to distribute the flows in different directions within the lagoon, or into the sea [62]. The discharge of water used may not induce negative impacts at sites where the hydrodynamic performance and salinity concentrations are known prior to the design for the effluent flow. This is so at sites where there has been salinity deterioration as a result of previous anthropogenic activities.

The change in salinity is only one of the environmental conditions responsible for the variety and abundance of fish reported for this lagoon (Table 1) [39]. In addition, salinity indirectly affects the distribution of species through its role in water density and the resulting hydrodynamics [63].

The spatial/temporal variation of water masses and their salinity is important for the distribution of organisms, especially of fish, which only live under certain salinity ranges, according to their tolerance to this parameter [64–66]. This is important for the distribution of various marine species of commercial interest (Table 1). Discharging a saline effluent into the lagoon, of marine salinity or slightly higher, in the hypersaline zone, will lower the salinity in this zone and thus limit the amount of hypersaline water available for power generation. In consequence, the impact on different species may be significant.

It is important to mention that there are many characteristics that make this an environment that harbours great diversity in fishes, but high salinities have been associated with a lower richness and diversity of fish [67]. Thus, for species distributed in marine/estuarine environments, such as *S. testudineus*, *S. notata, H. clupeola*, *T. falcatus*, *L. griseus*, *L. synagris*, *F. polyommus*, *A. probatocephalus*, *E. gula*, *E. argenteus*, *M. curema, M. trichodon*, *H. unifasciatus*, *C. atherinoides* and *A. narinari*, (Table 1) [39], a potential decrease in salinity in the hypersaline zone would alter the extent of their distribution areas.

However, to avoid changes in salinity and resulting limitations in resources, it is better to discharge this effluent into the sea since, being saline and of a small volume, it will not have the same effects as if it were brine [68]. On the other hand, stressors such as the accidental release of cleaning and maintenance chemicals and electrolyte solutions must be regulated against any facility handling hazardous chemicals.

Both in the pretreatment of solutions and in the cleaning of membranes and facilities, products such as chlorine are used, which can be toxic to the environment [23,48]. Usually, chlorine is used to avoid degradation of the membranes caused by biological growth in them. There is evidence that even small amounts of chlorine (e.g., 0.1 ppm) can have ecological impacts which induce a significant reduction in the productivity of dragged phytoplankton and species diversity [23]. Similarly, electrolyte solutions should be handled with caution [69]. The electrolyte solutions are stored in the electrode compartment which also contains the electrodes and is sealed with membranes that generally have

special properties to ensure the confinement of the electrolyte, recirculating it in a closed circuit [70]. There are reports of the toxicity of this type of element [23], but there is not much information on the toxic gases or compounds that are generated in the redox process within such a compartment (depending on the redox couple and the electrolyte used). Furthermore, in these compartments, some instability in pH control sometimes occurs since anion exchange membranes have a non-negligible proton transport number. This may allow an increase in the pH of the electrode solution, accompanied by a decrease in the pH of the effluent, and this may not be environmentally acceptable if it is too high [70]. Although the pH of the effluent would only change in the event of an accidental release, it is important to note that this parameter is an indicator of water quality. It affects the toxicity of certain compounds, such as ammonia, by controlling their ionisation, as well as the bioavailability of certain pollutants, such as heavy metals. For example, water with a pH range of 6.5 to 8.5 is suitable for many biological systems. Values of over 9.0 and lower than 5.8 limit the development and physiology of aquatic organisms [71].

Finally, it is worth noting the positive impacts that an SGE plant could have on La Carbonera. In the operational phase, the pipes will provide new spaces for colonization (bioincrustation) by sessile species [1]. These structures offer heterogeneity to the habitat and appropriate surfaces for algae and sessile organisms to colonise, especially on the muddy bottom. Fish and other invertebrates will be attracted by the hard surface, the shade, the changes in turbulence, the small spaces, and eventually, by the availability of food sources [48,72]. However, this type of infrastructure can also encourage the establishment of non-native species, invasive species, and blooms of harmful algae; therefore, the extent and composition of the colonisation are difficult to predict [73,74]. In the case of La Carbonera, pipes in the hypersaline zones would be easily colonised in the short term by barnacles, which live in the carbonated structures which stick to the surfaces and may cause deterioration. For this reason, their colonisation should be treated cautiously since they also damage RED membranes, encouraging the proliferation of microorganisms which impede the free passage of water or ions and thereby reduce the functionality of the system [75]. Although the production of 50 kW of electric energy from renewable sources is by no means a technological challenge, this work aims to provide electric power using the best technologies available, producing the smallest possible footprint, in harmony with the land use of this area (i.e., [76]).

With the present technological maturity of SGE techniques, these objectives do not yet yield low costs. Firstly, because the cost of the energy depends on the establishment of the market and industry; if the technological development is successful, the manufacturing of the parts will become cheaper. In the meantime, support from public funding is to be expected [77]. On the other hand, when environmental and social aspects are prioritised, a cost–benefit analysis based only on economic variables is not sufficient [78,79]. If the community face expensive-energy versus no-energy, their decision will be controversial.

For this specific case, the goal is not to urbanise the area or provide services that will promote urbanisation. The goal is to offer services that may assist environmental protection and conservation activities held in La Carbonera Lagoon and its surroundings where only low-density ecotourism is allowed. In this sense, a pilot plant using emergent technology is both suitable and affordable. Other technologies may not be feasible at this site. For example, although an established solar energy industry exists, the installation of solar panels may increase the air temperature around them, which is undesirable in environmentally sensitive areas. Additionally, a strategy for energy storage would be needed, thus increasing the final cost. There are similar disadvantages for wind energy, with additional construction difficulties [21]. Thus, an SGE plant, with the proposed additional facilities, seems to be a good strategy for promoting education, environmental conservation, and technology development.
