1. Introduction
Seawater Reverse Osmosis (SWRO) is the most popular water treatment technology for desalination operations globally [
1]. The SWRO process generates high-quality freshwater, but it also results in the production of a brine that should be discharged [
2]. The environmental impact of brine discharges from SWRO plants on the marine environment is a key concern in desalination development [
3,
4].
Hypersaline effluents from SWRO plants are usually discharged into the sea, as this costs much less compared to other disposal methods [
5]. Brine discharge can double the salinity of seawater intake and result in the formation of a high-density saline plume that tends to follow the slope of the seabed. Such high-salinity flows may harm benthic communities in the brine discharge area [
6,
7,
8,
9,
10]. The dilution of brine discharge in a marine environment depends upon many factors such as: the disposal method (coastal or submarine outfall), hydrodynamic characteristics of discharge area (waves, currents and tides), and bathymetry of the seabed, among others [
5,
6,
11,
12]. Brine dilution in a marine environment can be increased by methods such as the use of diffusers (single or multiport diffusers), bypassing seawater before discharge, or mixing brine with other effluents [
8,
13,
14,
15]. However, the use of these measures can increase the energy consumption of desalination plants [
16].
The San Pedro del Pinatar SWRO plants located on the southeast of the Spanish Mediterranean coast began operation in 2006 with a maximum installed capacity of 130,000 m
3, which represented a maximum brine discharge production of 159,000 m
3. They are located near
Posidonia oceanica seagrass meadows that are protected by national and European regulations. The brine discharged from both plants is disposed into the sea through a 4790 m outfall to ensure that the discharge point is situated below the lower limit of the
Posidonia oceanica meadow [
17,
18,
19].
Initially, no further measures were considered in relation to this project for the protection of the marine environment. However, environment monitoring results showed high salinity values around the brine discharge area and negative effects on the abundance and diversity of benthic communities present there [
8,
20]. Therefore, in 2010, it was decided to install a diffuser piece at the end of the submerged outfall to maximize effluent dilution and mitigate the identified environmental impact [
14]. Diffusers are devices used to maximize the dilution process of brine discharges with the nearby seawater, in order to enhance jet exit velocity and therefore the mixing process. The outfall of the San Pedro del Pinatar SWRO plants has only one diffuser at the end of the submerged outfall which creates an inclined dense jet to achieve maximum mixing process efficiency [
14]. Subsequent monitoring indicated a drastic reduction in salinity values in the brine discharge area along with a significant recovery of the abundance and diversity of benthic organisms [
8].
Even though the installation of diffuser pieces at the end of submerged outfalls reduces the environmental impact of desalination plants, it increases energy consumption as special equipment is required to increase the discharge pressure before the effluent can be released into the sea.
The objective of this study is to assess the energy consumption and economic cost associated with the installation of a diffuser piece at the end of the outfall of SWRO plants to meet environmental protection requirements and minimize the environmental impact associated with the discharged brine. Here the San Pedro del Pinatar SWRO plants are used as a case study.
4. Discussion
Our work presents the first systematic evaluation of the energetic and economic cost of discharging brine safely into the marine environment by incorporating a diffuser piece at the end of a submerged outfall. The novelty of this study is to evaluate for the first time the extra energy and economic cost associated with the installation of a brine pumping system related with the installation of a diffuser piece on the submerged outfall. The arrangements at San Pedro del Pinatar I and II SWRO plants are used as a case study. The use of a diffuser promoted the recovery of the abundance and diversity of the benthic fauna affected by the brine discharge [
8] at a low economic and energy cost. The results indicate that there are cost-effective means to meet environmental protection requirements and minimize the environmental impact associated with the discharged brine.
The diffuser piece installed in the brine discharge of San Pedro del Pinatar I and II SWRO plants requires higher pressure and higher velocity pumping of the effluent to maximize the brine discharge dilution in the marine environment [
14]. The results showed that, from 2012 to 2019, the operation of these pumps consumed only 0.229% to 0.843% of the total energy required for the operation of the San Pedro SWRO plants. In addition, this energy consumption amounted to between 10 and 30 kWh per thousand cubic meters produced.
The costs incurred for discharging the brine ranged from 1336 € in 2013 to 89,390 € in 2019. Energy consumption and economic cost are directly related to the production of desalinated water; thus, higher output from the San Pedro del Pinatar SWRO plants implied more brine discharge and higher pumping energy [
1,
12]. Since 2015, the freshwater production has increased to approximately 20 Mm
3, representing an increase of approximately 42% in the maximum freshwater production capacity of both plants. The annual variability observed in the fresh water production has been explained with the availability of water from different sources and prioritizing the cheapest water resources available [
1,
21]. Therefore, there was a significant reduction in desalinated water production in 2013 and 2014 due to the abundance of water from other resources during these years [
21].
On the other hand, the operation of the pumps made up 0.23% and 0.84% of the total energy consumption of the plant from 2012 to 2020, with an average value of only 0.563% of the total energy cost of the San Pedro SWRO plants. This indicates that the use of diffusers is an effective measure that can be adopted by SWRO plants for complying with environmental regulations in a cost- and energy-effective manner.
The results can be compared with other methods described in the literature. The desalination plants at Alicante use a seawater bypassing system to comply with the environmental regulations and to reduce the influence area of brine. An average additional cost of 1.7% was incurred [
16]. Additionally, these plants use an irrigation program to maintain the groundwater level in the saltmarsh and compensate for the effect of the intake system on the saltmarsh, but with lower energy consumption than the dilution system [
22]. The results obtained in this study show that the use of a diffuser piece in the outfall represents an average energy consumption of 0.57%, which is considerably lower than the seawater bypassing method used in Alicante SWRO plants with similar desalinated water production capacity. Moreover in the Javea Desalination Plant with a constant dilution ratio of 4 parts of water to 1 part of brine, the seawater bypassing system also has higher energy consumption [
13].
The installation of the brine discharge pumping system in the San Pedro del Pinatar SWRO plants, in combination with the use and configuration of the diffuser piece on the submerged outfall, ensures effective dilution of the discharged brine in the surrounding environment and drastically reduces the affected area [
14,
20,
23,
24]. The installation of diffuser pieces is deemed one of the best methods globally to minimize the environmental impact of brine discharge and maximize the dilution of the brine discharge influence area [
15,
25,
26].
Finally, this study demonstrates that using a diffuser piece system to meet environmental requirements entails a very low economic cost for SWRO plants. Further, using diffusers entails a lower economic cost than using other methods, such as seawater bypassing systems [
16].