Abstract
The Texas Water Development Board (TWDB) is advocating for funding the research and development of desalination plants in the Texas 88th Legislature. TWDB maintains that there is a need to develop, fund, and research water projects and conservation measures in order to combat growing water usage concerns. TWDB finds that by 2070, as Texas needs to accommodate a growing population, there will be a shortage of groundwater resources such that the state losses due to economic damages caused by water shortage will exceed the cost of enacting preventive measures today. Saltwater desalination plants are preventative measures that would give Texans access to other water resources besides groundwater. To ensure desalination is an efficient and practicable measure for Texas’ needs, this article will look at data from current seawater desalination plants used in the United States, the technology that can be invested in making more efficient plants, and how it compares to conventional water treatment plants. This analysis finds that Texas should fund desalination, as it has the potential to cost less than conventional water treatment plants for Texans, has the potential to become self-reliant by implementing renewable technology, and has the potential to become a great primary source of water during the summer months and a secondary source for the rest of the year.
The Efficiency of Desalination Water Plants
The most recent State Water Plan from the Texas Water Development Board (TWDB) was adopted on July 7, 2022. The TWDB found a need for the state to direct funds into developing and researching water projects and conservation measures. More specifically, TWDB has projected the future water usage of Texas and found that Texas needs to take further measures to ensure there is enough drinking water for current and future Texans. TWDB anticipates that the Texas population will increase from 29.7 million in 2020 to 51.5 million by 2070 (figure 1). Due to this population increase, TWDB projected an increase in water demand from 17.7 million acre-feet per year in 2020 to 19.2 million acre-feet per year in 2070. Texas groundwater supplies are projected to decrease from 8.9 million acre-feet per year in 2020 to 6 million by 2070. This shortage of groundwater needs to be addressed for future Texans. In order to prepare Texas for increased water demand from population growth, TWDB recommends creating over 2,400 water projects with an estimated budget of $80 billion (in 2018 dollars) over the next 47 years (The paper is not clear what, if any, discount rate is proper for these projects). If these water management strategy projects are not implemented, a severe drought may cause $153 billion per year in economic damages by 2070. In Texas, TWDB recommends 33 brackish and 7 seawater desalination water projects that would process seawater or brackish groundwater (table 1 and figure 2). This would ensure Texans have alternative water sources to help supplement groundwater. TWDB recommended 19,000 acre-feet per year of supply from groundwater desalination strategies in 2020 and recommends 157,000 acre-feet per year by 2070 and 179,000 acre-feet per year of supply from seawater desalination strategies by 2030 and 192,000 acre-feet per year by 2070. Currently, Texas is desalinating 176,000 acre-feet of brackish water a year but does not desalinate seawater. However, implementing desalination technology does come with costs. As shown in the below figures, desalination plants consume more energy than conventional water treatment plants in the state. Due to the dependence of desalination energy usage on ambient temperature, desalination can vary from being actually more energy efficient in the hottest months of the year to seven times more energy intensive in the winter (See figure 6 for details). This may be mitigated by a strategy of using desalination plants at different capacities during different seasons. Texas may need to accommodate growing electricity demand and put more funding into desalination research to find ways of reducing costs long term. This may cost Texans more in their taxes, or water bills, and that money could potentially be better used elsewhere. This analysis will look into the efficiency of desalination water plants and determine whether or not Texas should implement, fund, or research this technology further.
Desalination Plants
Desalination water plants use a treatment process called desalination that removes solids from saltwater and brackish groundwater to produce fresh water. There are different types of desalination techniques. One of the least energy-intensive and cost-effective methods is treating brackish groundwater or seawater through a semipermeable membrane. Specifically for seawater, the process is called Seawater Reverse Osmosis (SWRO) as it defines the type of water being purified and the membrane (Reverse Osmosis) being used. The SWRO process starts by intaking the saltwater from its source and pressurizing the water. Seawater then goes through pretreatment, and a pump shoots the seawater into the reverse osmosis membrane. The membrane then separates the contaminants from the seawater where the water is supplemented through final treatment and into a supply tanker whereas the contaminants are sent back to the source (figure 3).
Desalination Studies and Technology
As desalination is a very energy-intensive process, over the years technology for desalination has been researched and developed to help offset those costs. The membrane that separates the salt from water has been looked at in depth. There are multiple different types of membranes such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and nanofiltration (NF) membranes. A study by Alyson Sagle and Benny Freeman researched and found the most effective membrane we have to date is the RO membrane as it has the lowest porosity, allowing it to filter salt and other contaminants out of saltwater the best. It is also the most cost-effective method in terms of energy consumption and water produced (table 2 and table 3). Nevertheless, in the SWRO processes, the RO process is the most energy-intensive. Conventional water treatment plants’ most energy-intensive component is the pumps that pressurize, collect, and transport water through their treatment process, which SWRO similarly has as well. The RO process creates an added cost to SWRO, making conventional water treatment plants cheaper. However, for brackish groundwater RO, the energy required is 10-30 percent less than SWRO–which significantly aids in balancing the energy costs for these water plants. In order to further reduce the amount of energy and costs for brackish groundwater and saltwater, companies like Energy Recovery Inc. have created renewable technologies to help offset estimated future costs of maintaining and running a plant. Energy Recovery Inc. has introduced a PX Pressure Exchanger that reduces the energy needed to power the high-pressure pump to create the required pressure for saltwater to go through the membrane. The University of Houston has a presentation of the PX Exchanger displaying that the high-pressure concentrated brine exiting the membrane goes into the PX Pressure Exchanger and aids in pumping salt water back into the system (figure 4).
Comparing the PX SWRO system to previous estimations, Energy Recovery Inc. projected the power needed to produce water would be at 2.98 kWh/m3 (figure 5). Another projection found was the SWRO cost trend, which projected it would cost Texans only 0.47 USD/m3. Using other renewable technologies could help SWRO plants become more self-reliant and reduce the energy costs required to run these plants even more than previously projected estimates. If we look at early data from seawater desalination plants that used a thermal process, costs decreased from less than 2.50 USD/m3 and the energy required to run a facility reduced from 22.5 kWh/m3.
Desalination in Practical Use
Ensuring this technology is viable and practicable for Texas, and estimations projected by agencies and companies are coherent with real-life applications, I looked at data recorded from current desalination plants and compared it to water treatment plants in the United States. To look at an environment similar to Texas with data, I looked at one of the largest seawater desalination plants in the United States: the Tampa Bay Seawater Desalination Plant in Florida. I used this plant for the study because it is a SWRO plant on the Gulf Coast. A study published by the Édition Diffusion Presse (EDP) Sciences looked into the sustainability assessment of the water supply systems of the Tampa Bay Seawater Desalination Plant between 2016-2019. This study looked into the energy consumption of conventional water treatment plants and calculated an average value of 0.088 kWh/m3 (table 4).
The study looked at the energy consumption of the Tampa Bay Seawater Desalination Plant and calculated an average value of 0.772 kWh/m3 (table 5).
The study found that consumed energy for a desalination plant fluctuated highly, produced less solid waste, but still required more energy compared to conventional water treatment. Nevertheless, even though desalination does require more energy, the study found desalination plants require less power in the summer months (figure 6).
This is understandable as a study called “Analysis of temperature effects on the specific energy consumption in reverse osmosis desalination processes” found intake water within a temperature range of 15-40°C has a reduced specific energy consumption. However, they stated increases in the temperature in desolation tend to negatively impact the membranes in contaminant removal.
Conclusion
Looking at the data regarding the efficiency of desalination water plants’ power consumption and economic standpoint, Texas should continue to implement, fund, or research this technology further. Desalination has the potential to have less energy costs for Texans compared to conventional water treatment plants. If similar SWRO plants with renewable energy technology were implemented in coastal locations in Texas and used as a primary source during summer months and as a secondary source for the rest of the year, Texas could save tax dollars and accommodate for future water needs. It would be beneficial for the Texas 88th Legislature to continue to fund the research and development of desalination technology.
