By Jeff Gunderson
In the coming decades, soaring global energy demand driven by booming population growth and expanding commercial and industrial activity is expected to create an overwhelming need for water supplies. Indeed, the International Energy Agency has reported that the amount of freshwater consumed for world energy production is on track to double within the next 25 years.
With water withdrawals associated with electrical power production -- primarily for cooling processes -- accounting for one of the largest water uses in the U.S. and worldwide, new water reduction strategies and more sustainable water treatment solutions are increasingly needed in the power industry for conserving freshwater resources.
As an industry, power producers are very aware of the challenges surrounding water availability and future requirements associated with thermo-electric generation, and especially, how to mitigate the use of water, said Rick Higginbotham, corporate account executive with GE Water & Process Technologies.
"We are seeing significant movement toward sustainable solutions, including either the study of water reuse or actual implementation," he said. "It's not just interest, but a lot of time and resources are either going into the research and development of new technologies or towards improving existing processes to minimize water use."
In terms of water needs for the power industry, Higginbotham believes greater understanding needs to be established concerning the difference between water usage and consumption as it relates to thermo-electric power production. "There is a significant amount of water that is required to generate energy, but less than 3 percent of the water that's used is actually consumed or evaporated," he said. "That's why compared to reducing water use, there is even greater emphasis on cutting consumption."
Evaporative Cooling Water
With the largest allocation of water in energy production required for cooling, increasingly more attention is being focused on improvements and innovations in cooling tower processes.
Brent Giles, a senior analyst at Boston, Mass.-based Lux Research, said the current, prevailing trend in the power industry is towards cooling towers that incorporate water recycling and closed-loop cooling. "Single-pass cooling systems have fallen out of favor, even though these units are still quite common in many existing plants," he said.
Another trend is the adoption of dry-cooling systems, Giles noted, which offer considerable water savings but need to be balanced against the tradeoffs that include lower heat transfer efficiency, and therefore, higher operating and capital costs.
One technology holding potential is an innovation developed by United Kingdom-based Modern Water which employs forward osmosis technology for producing evaporative cooling make-up water from impaired, very low-quality source water, Giles said.
According to Modern Water, the company's forward osmosis process utilizes a recirculating "osmotic agent" system that transfers pure water from a feedwater (such as seawater) to the regeneration (permeate extraction) system, enabling for the production of high-quality process water at significant cost savings.
Municipal Source Water
Another developing trend in the power industry -- largely driven by regulation and prevailing costs -- includes turning to municipal wastewater as a feedstock for power plants.
"No longer can a plant be built next to a lake or a river and be able to access free water," Higginbotham said. "The cost of water has become a driving consideration."
But with the use of municipal wastewater come new challenges, particularly in relation to high salinity levels. "When that water is reused over and over again, the salts become very concentrated," he said. "Those salts can precipitate out on the metal surfaces, resulting in a loss of heat transfer and reduced performance."
GE has recently developed a high-stress/high-heat resistant polymer -- called Stress Tolerant Polymer (STP) -- which helps greatly increase the range in which difficult-to-treat waters can be managed and cost-effectively treated.
"Prior to the development of this polymer, the industry was limited in terms of salt saturation indices and thermal load," Higginbotham said. "Our STP provides higher flexibility in the design of a treatment program for large cooling systems and greatly expands the opportunities to use gray water make-up -- a source that was never before considered due to treatment limitations or overall costs."
On the back end of processes, significant progress in wastewater treatment solutions is enabling power producers to treat wastewater streams both more effectively and efficiently. With cooling tower blowdown, improved treatment solutions are allowing for higher levels of water recovery, ultimately maximizing reuse opportunities. Advancements in treatment technologies also provide plants with key solutions for complying with new and increasingly stringent regulations.
Patrick Randall, director for the North American market with Aquatech, said strict wastewater discharge limits are driving greater adoption of zero liquid discharge (ZLD) technology, which in some cases is now required by regulation.
"Within our ZLD process, ultrafiltration is a big area of focus for us," he said. "We are actively working on improvements to ultrafiltration membranes that will lead to enhanced ZLD performance and water recovery in excess of 90 percent."
Another ZLD process innovation from Aquatech includes fractional electrodeionization, which is used for producing the final polished water that is fed into boilers.
"Electrodeionization is a common technology, but we were able to improve the performance by making it what we call fractional," Randall said. "This involves splitting the process into two different cells within the unit, enabling this technology to be more forgiving for various different feedwaters."
In addition to cooling tower blowdown, ZLD technology is also being utilized to treat flue gas desulfurization (FGD) -- or scrubber -- wastewater. In recent years, tougher air emission standards have required new and existing coal-fired power plants to install FGD systems for removing sulfur oxide emissions. These systems effectively remove these contaminants from emissions but produce difficult, high-strength waste streams that are characterized with high chloride content in addition to metals and suspended solids.
"The EPA has come out with new electric power guidelines that restrict FGD discharge, driving treatment in this area," Randall said. "Recently, in New Hampshire, the EPA determined that ZLD represented the best available technology for treating that waste stream."
A More Holistic Approach
In line with the larger shift in the power industry towards the adoption of more sustainably-focused water and wastewater solutions, more interest is also being shown in approaching challenges more holistically, paving the way for smarter decision making and more effective integration of technologies.
"In evaluating a design or a suite of options, the total water balance of the plant should be factored in -- every point in and every point out, including how these points interrelate and affect the system as a whole," said Higginbotham. "The largest use and consumption of water in a power facility is dedicated towards cooling, but if we can develop and implement solutions that are better integrated and targeted for higher performance in that process, we can effectively reduce the amount of water required for electrical generation."
Consistent with this approach is the increasing use of data and analytics with treatment systems for driving higher efficiency and greater connectivity in terms of decision making.
"A meaningful advance in the industry is to use instrumentation and the data that is available not only for monitoring but for analyzing the system in a predictive way," Higginbotham said. "Smart systems allow us to model the waters, salt concentrations and thermal cycles to a degree where we can predict our critical parameters exactly and what specifically needs to be done to a system in order to increase performance for handling difficult or harder-to-treat wastewater."
About the Author: Jeff Gunderson is a correspondent for Industrial WaterWorld. He is a professional writer with over 10 years of experience, specializing in areas connected to water, environment and building, including wastewater, stormwater, infrastructure, natural resources, and sustainable design. He holds a master's degree in environmental science and engineering from the Colorado School of Mines and a bachelor's degree in general science from the University of Oregon.