Optimization of Energy Efficiency, Operation Costs, Carbon Footprint and Ecological Footprint with Reverse Osmosis Membranes in Seawater Desalination Plants

This article shows the optimization of the reverse osmosis process in seawater desalination plants, taking the example of the Canary Islands, where there are more than 320 units of different sizes, both private and public. The objective is to improve the energy efficiency of the system in order to save on operation costs as well as reduce the carbon and ecological footprints. Reverse osmosis membranes with higher surface area have lower energy consumption, as well as energy recovery systems to recover the brine pressure and introduce it in the system. Accounting for the operation, maintenance and handling of the membranes is also important in energy savings, in order to improve the energy efficiency. The energy consumption depends on the permeate water quality required and the model of the reverse osmosis membrane installed in the seawater desalination plant, as it is shown in this study.


Introduction
Seawater desalination in water treatment plants has evolved considerably in the last five decades, in which the desalination process and its technology have changed and become more and more profitable and efficient. Initially, the water desalination process was a thermal process, but it has been changing with scientific technological advances towards a process of reverse osmosis, which dominates the current market [1][2][3][4][5].
Following the state of the art in water desalination and the evolution of this process not only at the regional Canary level but also at national and international levels, there are now different desalination processes, such as Vapor Compression (VC), Multi-Effect Distillation (MSF), Multi-Stage Distillation (MED) and reverse osmosis, which currently account for 65% of all the processes used around the world [4][5][6][7].
The main objective is to study the improvements in seawater desalination, based on the reduction of energy consumption in the production of fresh water. Consequently, reverse osmosis is the most suitable process due to its lower energy consumption per cubic meter of water produced, and therefore it occupies a privileged position in the sector. So far, in the 21st century, research efforts in water desalination have focused on advances in reverse osmosis membranes, with higher surface area and lower energy consumption, as well as energy recovery systems to recover the brine pressure and introduce it in the system, reducing the energy consumption of the desalination process [8][9][10].
The operation, maintenance and handling of the membranes have been studied in detail, due to their importance in energy savings, detailing how to optimize all the processes in which they are involved to improve energy efficiency [7].
In the same way, we analyze data from the different seawater desalination plants we visited, obtaining data on thousands of hours of operation in many cases. We have developed techniques to improve the energy efficiency of seawater desalination membranes in strict compliance with the water quality parameters established by national and international regulations, or even by organizations such as the World Health Organization [11][12][13][14][15][16][17][18][19].
To carry out a general cost analysis of the components or elements of the plant and their operation, it is necessary to determine the direct costs, indirect costs and other considerable expenses for this purpose [20][21][22][23][24].
Among the direct costs, we can highlight the acquisition cost of the elements, both initial and replacement, and among the most significant expenses are those related to the initial capital investment, operation and maintenance [25][26][27].

Materials and Methods
As stated earlier, energy consumption depends on the permeate water quality required and the reverse osmosis membrane model installed in the desalination plant. Therefore, we developed a methodology, in the following equations, to calculate the permeate qualitycost ratio [15][16][17][18][19].

General Analysis of Element and Operation Costs
In this sense, and as a guide, according to data from a construction compan salination plants in Gran Canaria with more than 100 references in the market, i be noted that the cost of the membranes in a seawater desalination plant repres proximately 13% of the total investment in the facility's equipment. The rest of t ponents (high-pressure pump, booster pump, pressure pipes, pre-treatment, etc sent 87% of the total amount, not including industrial profit and before taxes [1,2 Table 1 and Figure 2 show all the significant variables that affect operating cubic meter of water produced [3,[4][5][6]7].

General Analysis of Element and Operation Costs
In this sense, and as a guide, according to data from a construction company of desalination plants in Gran Canaria with more than 100 references in the market, it should be noted that the cost of the membranes in a seawater desalination plant represents approximately 13% of the total investment in the facility's equipment. The rest of the components (high-pressure pump, booster pump, pressure pipes, pre-treatment, etc.) represent 87% of the total amount, not including industrial profit and before taxes [1,2,28]. Table 1 and Figure 2 show all the significant variables that affect operating costs per cubic meter of water produced [3][4][5][6][7].

General Analysis of Element and Operation Costs
In this sense, and as a guide, according to data from a construction company of desalination plants in Gran Canaria with more than 100 references in the market, it should be noted that the cost of the membranes in a seawater desalination plant represents approximately 13% of the total investment in the facility's equipment. The rest of the components (high-pressure pump, booster pump, pressure pipes, pre-treatment, etc.) represent 87% of the total amount, not including industrial profit and before taxes [1,2,28]. Table 1 and Figure 2 show all the significant variables that affect operating costs per cubic meter of water produced [3,[4][5][6]7].  In this sense, it is demonstrated that the cost of energy consumption in the pumps and mainly in the high-pressure pump is by far the most significant of a seawater desalination plant, and we can reduce it considerably with the introduction of last-generation reverse osmosis membranes, which were confirmed to be suitable through the same through-plant pilots [29,30].
If the membranes are not replaced, an action that has the lowest cost of those studied, this will have a negative impact with a considerable increase in the energy consumption of the high-pressure pump, which very significantly affects the cost per m3 of water produced, as discussed below [31][32][33][34].
In Figures 3 and 4, the most important issues of this model are represented, which are the costs, energy consumption, water quality and environment. In this sense, it is demonstrated that the cost of energy consumption in the pumps and mainly in the high-pressure pump is by far the most significant of a seawater desalination plant, and we can reduce it considerably with the introduction of last-generation reverse osmosis membranes, which were confirmed to be suitable through the same through-plant pilots [29,30].
If the membranes are not replaced, an action that has the lowest cost of those studied, this will have a negative impact with a considerable increase in the energy consumption of the high-pressure pump, which very significantly affects the cost per m3 of water produced, as discussed below [31][32][33][34].
In Figures 3 and 4, the most important issues of this model are represented, which are the costs, energy consumption, water quality and environment. A reduction in energy consumption will have a direct impact on environmental improvement and we study this through the carbon footprint produced by these desalination plants and their ecological footprint, with the latter as a future line of action. The corresponding diagram according to Figure 4 is shown below. To produce a quantity of water from a reverse osmosis plant, a quantity of electrical energy must be consumed, and to generate this energy in a conventional electrical network, emissions in the form of greenhouse gases are emitted.
The magnitude of these emissions depends on the set of technologies that make up the energy generation system of the electrical network to which the water production In this sense, it is demonstrated that the cost of energy consumption in the pumps and mainly in the high-pressure pump is by far the most significant of a seawater desalination plant, and we can reduce it considerably with the introduction of last-generation reverse osmosis membranes, which were confirmed to be suitable through the same through-plant pilots [29,30].
If the membranes are not replaced, an action that has the lowest cost of those studied, this will have a negative impact with a considerable increase in the energy consumption of the high-pressure pump, which very significantly affects the cost per m3 of water produced, as discussed below [31][32][33][34].
In Figures 3 and 4, the most important issues of this model are represented, which are the costs, energy consumption, water quality and environment. A reduction in energy consumption will have a direct impact on environmental improvement and we study this through the carbon footprint produced by these desalination plants and their ecological footprint, with the latter as a future line of action. The corresponding diagram according to Figure 4 is shown below. To produce a quantity of water from a reverse osmosis plant, a quantity of electrical energy must be consumed, and to generate this energy in a conventional electrical network, emissions in the form of greenhouse gases are emitted.
The magnitude of these emissions depends on the set of technologies that make up the energy generation system of the electrical network to which the water production A reduction in energy consumption will have a direct impact on environmental improvement and we study this through the carbon footprint produced by these desalination plants and their ecological footprint, with the latter as a future line of action. The corresponding diagram according to Figure 4 is shown below.
To produce a quantity of water from a reverse osmosis plant, a quantity of electrical energy must be consumed, and to generate this energy in a conventional electrical network, emissions in the form of greenhouse gases are emitted.
The magnitude of these emissions depends on the set of technologies that make up the energy generation system of the electrical network to which the water production plant is connected. The energy produced by this set is often referred to as the energy mix, which tends to depend largely on the territory and energy policy [3,4].
In relation to territorial dependence, electricity networks generally have energy mixes that cause higher greenhouse gas emissions, as they generally have systems based on lower performance technologies. These electrical energy production technologies can mainly be classified as two types: Conventional and renewable [3].
Within the conventional technologies, which have a direct impact on the carbon footprint of the installations, several can be considered: Diesel engines, gas turbines, combined cycles and steam turbines, which generally have different performances and quantities of emissions. On the other hand, there are technologies based on renewable energies, such as solar photovoltaic, wind, waves, etc. [4,5].
Therefore, in order to reduce greenhouse gas emissions, it is possible to propose the generation of electrical energy necessary for water production in the same facility through hybrid energy systems. These hybrid energy systems can be composed of several types of technologies, in which the largest amount of energy from renewable sources tends to be integrated with the support of an energy storage system or conventional technology such as a diesel engine [3].
Therefore, a methodology can be proposed to achieve the stable operation of a highefficiency diesel engine with a small integrated autonomous diesel engine and a photovoltaic solar energy generating system to power a reverse osmosis plant, thus reducing the greenhouse gas emissions associated with water production. This application would be very useful in hotel complexes, private facilities, industries, isolated areas, etc. [3].
For the specific case of seawater desalination plants in the Canary Islands, with regard to the production of seawater desalination plants, the following permeate flows can be confirmed: Gran Canaria (220,870 m 3 /d), Tenerife (106,034 m 3 /d), Fuerteventura (90,755 m 3 /d) and Lanzarote (87,480 m 3 /d). These produce a significant carbon footprint with respect to the overall footprint of each island, especially on Fuerteventura and Lanzarote. In this sense, renewable energies can make a great contribution, mainly through wind and solar photovoltaics. For example, Fuerteventura and Lanzarote are windy islands with high solar radiation all year round, which also have large areas of flat land suitable for these installations. These installations could be for the energy consumption of public desalination plants, or for those that are private, which are normally smaller and can also be self-supplied with renewable energies and a diesel engine for the security of the electricity supply at all times without resorting to the island network, as may be the case of hotels or isolated areas where the electricity network does not reach. In Gran Canaria and Tenerife, it is also possible to implement this, although the orography is more complicated throughout the year in the coastal areas where the seawater desalination plants are located, as the solar radiation and the winds are quite significant, especially in the months between June and September with sunnier days and trade winds. Therefore, the possibility of introducing renewable energies for the supply of electricity to seawater desalination plants in the Canary Islands is studied in order to reduce the carbon footprint and the ecological footprint of the sector, due to the considerable influence of the whole archipelago.

Results
Taking into account these parameters, the typical production of a seawater plant of 100,000 m 3 /d, Equation (7) explained above and the reverse osmosis membrane software, we obtain the common results presented in Tables 3-5. In Table 3, there is a pressure difference essentially every year, due to the age of the membranes. At start up, in year 0, the elements are new so they need less feed pressure than in years 1 to 5. This is because fouling and scaling could damage the membranes little by little, and consequently, the feed pressure increases every year. This shows that the pressure measured in year 1 grows more in the first year, and from year 2, it is constant at 1.2 bar.
Consequently, one can observe from Figure 5 that the pressure varies over 5 years without replacing the membranes, whereas the energy consumption of the pump increases accordingly.
In Table 4, feed temperature is low (17 • C), and due to this, the feed pressure is higher than in Table 5 where the feed temperature is high (27 • C). At start up, the feed pressure is 6-7 bars higher at 17 • C than at 27 • C. After 5 years, without replacement, the pressure difference is even higher between the minimum and maximum temperature, at around 9-10 bars.  In Table 4, feed temperature is low (17 °C), and due to this, the feed pressure is higher than in Table 5 where the feed temperature is high (27 °C). At start up, the feed pressure is 6-7 bars higher at 17 °C than at 27 °C. After 5 years, without replacement, the pressure difference is even higher between the minimum and maximum temperature, at around 9-10 bars.  In Table 6, we show the existing seawater desalination plants in the Canary Islands, including consumption, and the introduction of renewable energies.      Table 7 shows the existing seawater desalination plants in the Canary Islands including the carbon and ecological footprints.     Figure 6 shows the most significant plants in the Canary Islands, in terms of size, that produce the largest share of the ecological footprint mentioned above. Moreover, the positions of the RO desalination plants are shown on the map, including the permeate flow of each one in the picture. Considering the type of specific environmental impact indicators [10], the results are classified according to the non-renewable technology and island in Table 8 (2019).  Considering the type of specific environmental impact indicators [10], the results are classified according to the non-renewable technology and island in Table 8 (2019).  Table 9 presents the above values per MW of installed power on each island. Similarly, we can calculate the CO 2 footprint per MWh taking into account the thermal consumption by technology and island in Tables 10 and 11.

Conclusions
The most important conclusions obtained from this study are the following: -By reducing the operation costs outlined in this article, it is possible to improve the energy efficiency of the system. - To reduce the carbon footprint and ecological footprint, the energy consumption needs to be decreased.
-There are different results regarding the optimization of energy efficiency and environmental footprints. -These conclusions of the study may serve as a tool for the decision-making processes related to improving energy efficiency in seawater reverse osmosis plants. -The main objective was to study the improvements in seawater desalination based on the reduction of energy consumption in the production of fresh water. -Reverse osmosis is the most suitable process due to its lower energy consumption per cubic meter of water produced. -Reverse osmosis membranes with higher surface area have lower energy consumption, as well as energy recovery systems to recover the brine pressure and introduce it in the system, reducing the energy consumption of the desalination process. -Considering the operation, maintenance and handling of the membranes is also important in energy savings, in order to improve energy efficiency. -Energy consumption depends on the permeate water quality required and the model of the reverse osmosis membrane installed in the desalination plant.