*2.7. Environmental Impact Assessment*

To ascertain the potential impacts of SGE implementation at La Carbonera, an Environmental Impact Assessment (IEA) was conducted [44–47]. The selection of the environmental impacts was based on the available scientific literature, such as [1,21–23].

The impact analysis used the [48] classification framework proposed for the impact assessment of other ocean renewable energies. The analysis was conducted for three phases: construction, operation, and decommissioning. This EIA considered stressors, receptors, and environmental responses and impacts. Stressors are those characteristics of the environment that may change due to the implementation of SGE in construction, operation, and decommissioning. Receptors are elements of the ecosystem with the potential for some form of response to the stressor, including the various biotic and abiotic components of the ecosystem with the potential to be affected. Effects, or responses, are how these receptors change, without indicating magnitude or significance. Finally, impacts address the severity, intensity, or duration of the effects and cover the direction of the effect, which can generate positive or negative outcomes.

The impacts of this technology are similar in the construction phase to those of seawater desalination plants, wastewater treatment plants, or other renewable energy plants. Since there is currently insufficient on-site experience on the impacts, the stressors considered were based on the analysis of the components of the PRO and RED technologies (Figure 2). This figure gives a general view of the main components of both technologies. In section A, the pipes and the filtering system are shown. In section B, depending on the technology used, and the expected production, the site will require several, repeated PRO or RED modules (membranes in both cases, in RED also electrolyte solutions), as well as hydraulic installations (turbines in PRO), sanitary installations and storage facilities for inputs and waste. Section C shows the electrical installations for power distribution/storage (should include cabling, towers, etc.). In both PRO and RED, a final by-product is a brackish water or seawater, depending on the scheme used (SW/FW, SW/HW, or FW/HW).

**Figure 2.** Components and processes involved in RED and PRO technologies that may produce environmental impacts in coastal systems. Abbreviations: PE (pressure exchanger), SE (electrolyte solution), AEM (anion exchange membrane), CEM (cation exchange membrane). The components by sections are shown (A, B and C).

For the determination of receptors, shown in Figure 2 (A–C), general impacts, similar to other engineering projects in coastal systems, are expected, including the following:


Finally, from the revision of biotic and abiotic features of the system shown in the case study, an analysis of Stressors, Receptors, and general Responses to the implementation of SGE in potential systems was presented.

It is hoped that this work can serve as a guide for other cases elsewhere.

### **3. Results and Discussion**

A general analysis of the environmental impacts of PRO and RED technologies in three phases (construction, operation, and decommissioning) is presented. These impacts can also be applied to other potential systems that use salinity gradients for energy production (river mouths, estuaries, coastal lagoons). The receptors highlight those characteristics of coastal environments that may be susceptible to change due to the SGE (Figure 3). The responses were analysed for each receptor, revealing the potential impacts of this technology (Figure 3).

In this overview of possible impacts on potential systems, it is important to mention that these may be minor or major impacts, depending on several factors, such as the size of the plant, the characteristics of each system (biotic and abiotic), and scheme used for energy harvesting (natural solutions or anthropic effluents). In the case of La Carbonera, the exploitation scheme proposed is in line with the regulations in force and adapted to the specific characteristics of the site.

The scheme for La Carbonera is the hypothetical implementation of net output of 50 kW RED power plant of the size and production of the RED prototype located in Afsluitdijk, in the Netherlands. This plant included three water storage tanks, two pretreatment systems (concentrated and dilute solution), and eight membrane stacks. The proposed infrastructure is therefore low impact.

For the size and energy production of this hypothetical plant, it is important to consider first that many of the environmental impacts in each phase will depend on the characteristics of the site where the plant is built. Therefore, some aspects taken into account to select a location, with consideration to possible responses, are reported in Figure 3 and include the following:


**Figure 3.** Stressors, receptors, and possible effects due to the implementation of SGE in La Carbonera lagoon in a three-stage scheme (construction, operation, and decommissioning) for the main components of the PRO and RED technologies. The letters indicate the number of stressors according to the phase of the project.

From these considerations, installing the RED plant behind a small jetty in the lagoon, A in Figure 1, would induce significantly greater environmental impacts than with option B of Figure 1. First, this result is because the excavation for the pipelines would be greater, as the hypersaline water must come further, and all the impacts associated with the various stressors (C1, C2, and C3, Figure 3) must be considered. Second, the pipeline would pass through several mangrove patches, and therefore, the vegetation here and its associated fauna would be severely affected. Thirdly, although the availability of seawater at this location (at the mouth of the lagoon) would be close to the RED plant, the interruption to the tidal flow entering the lagoon, due to the constant intake of this solution, would generate various negative impacts. One of the main impacts possible is that large areas of the hypersaline zone that depend on this sea–lake exchange could dry out. Exposed sediments would therefore be salinised, and large areas of mangroves would dry out. In addition, the interruption of this flow would change the hydrodynamic conditions that favour the distribution and abundance of the various fish species reported in Table 1. Likewise, this would affect the sedimentation processes at the lagoon mouth. On the other hand, the discharge of effluent from this location into the sea would also imply greater excavation and the laying of pipes and damage to the mangrove and associated fauna (Figure 3).

Although the proposed infrastructure is small, the correct location of a plant and especially of the collecting and discharging zones may result in huge differences in terms of impacts: in this example, locating the plant at site A may generate several negative impacts that can be avoided with the location and design of plant at B (Figure 1). The design process, including location selection and analyses to be considered, are presented next.
