By Jay Landers
For many small islands, seawater is abundant, but drinkable water is not. As a result, securing adequate supplies of potable water is a challenge, and conserving and extending available supplies is critical. Such is the situation on the Canary Islands, a Spanish archipelago with a population of about 2.2 million that is off the coast of northwestern Africa in the region known as Macaronesia.
In a bid to make the most of the existing water resources on the Canaries, a local research consortium is pilot-testing the use of a forward-osmosis membrane system as a means of reusing brine from a local seawater desalination facility. By using treated effluent from a municipal wastewater treatment facility to dilute the brine, the pilot plant will combine two waste streams to create reused water for irrigation purposes.
Reusing waste streams
The pilot facility is on Gran Canaria, the second-most populated of the islands that constitute the Canary Islands. On Gran Canaria, “desalinated water and water reuse accounts for more than half of the total water demand,” says Ángel Rivero Falcón, a senior engineer in the water department of the Canary Islands Institute of Technology, or ITC, a public research and development center.
The ITC and the University of Las Palmas de Gran Canaria, through its Group for the Research on Renewable Energy Systems, are conducting the pilot project under the auspices of the DESAL+ LIVING LAB platform, a public-private research effort focused on improving desalination and increasing its cost-effectiveness on the Canary Islands.
The water technology company Aquaporin A/S is providing forward-osmosis membranes for the pilot project, which is owned by the Mancomunidad del Sureste de Gran Canaria, a public regional entity that comprises three municipalities in southeast Gran Canaria.
“With this project, we are trying to make use of two current waste streams (and) producing water for agriculture,” Rivero says. The potential for “implementing a circular economy” involving the waste streams made the decision to pilot forward-osmosis membranes “very attractive,” he notes.
Less power needed
At a basic level, a forward-osmosis system consists of a feed solution and a draw solution that are separated by a semipermeable membrane. Because the draw solution has a higher concentration of total dissolved solids than the feed solution, osmosis results, with the feed solution seeking to equalize the TDS concentration in the draw solution.
As the feed solution passes through the membrane and into the draw solution, the membrane retains most of the TDS and other constituents. In this way, the feed solution is concentrated, and the draw solution is diluted, meaning that it becomes cleaner.
A draw recovery system then removes the newly cleaned water for reuse and restores TDS concentrations as necessary in the draw solution to enable the forward-osmosis process to continue.
Because it relies on osmotic pressure to generate a clean water stream, forward osmosis does not require the types of energy-intensive pumping systems that are needed to force water through membranes as part of the much more well-known reverse-osmosis process, says Jörg Vogel, the vice president of open innovation at Aquaporin.
“We are working with nature, not against it,” Vogel says. As a result, forward osmosis requires much less power to operate as compared with reverse osmosis, he notes.
The forward-osmosis system’s reduced operating pressures also mean that the membranes themselves can be thinner than those required for use in a reverse-osmosis system. “For an FO membrane, you don't need the mechanical stability that an RO membrane requires,” Vogel says. As a result, Aquaporin’s forward-osmosis membranes have thicknesses of 35 µm, as opposed to reverse-osmosis membranes that may require thicknesses of 150 µm to more than 200 µm, he notes.
Using open, very thin support membranes helps minimize the occurrence of internal concentration polarization, a condition that reduces the actual usable osmotic pressure difference between the feed and draw solutions and “thus has a direct influence on water flux and efficiency,” Vogel says.
Aquaporin’s membranes include what the company calls Aquaporin Inside, a biomimetic polyamide coating that incorporates aquaporin proteins. Aquaporin proteins are naturally occurring substances that form pores in the membranes of biological cells to facilitate the movement of water between cells. “We enhance the function (of membranes) by integrating this biomimetic component,” Vogel says.
The pilot facility comprises six hollow-fiber forward-osmosis membrane systems situated in three parallel lines having two membranes in series. Test flows consist of a 400 L/hour feed solution of treated secondary effluent and a 200 L/hour draw solution of brine from a seawater reverse-osmosis desalination facility. A 50 µm particulate filter is included before the forward-osmosis membranes.
Commissioned in April 2021, the pilot facility has tested only simulated reverse-osmosis brine to date as a draw solution.
“Until now, we have tried with a synthetic brine that simulates the salt concentration of a seawater reverse-osmosis brine of about 60 to 70 g/L,” says Noemi Melián Martel, Ph.D., a part-time associate professor at the University of Las Palmas de Gran Canaria.
“However, the pilot plant is already prepared to work with brines from two actual seawater reverse-osmosis desalination plants, and soon we will start testing them,” Melián says. “The wastewater used in the pilot plant as feed solution comes from the secondary effluent of an actual municipal wastewater treatment plant.”
“The seawater reverse-osmosis brine is presented as a potential draw solution for the forward-osmosis system because the brine has a high salinity, increasing the osmotic potential and hence the water flow,” Melián adds. In this way, the highly saline brine will enable the pilot system to achieve a “higher water recovery,” she notes.
The forward-osmosis approach to be tested at the pilot facility offers a number of potential benefits, beyond simply as a newly available water source for irrigation, Vogel says. For example, use of reverse-osmosis brine from seawater desalination facilities would decrease the concentration of any such brine needing to be discharged to the environment, potentially improving ecological conditions within receiving waters.
Meanwhile, the brine from a desalination facility represents essentially a free source of draw solution for the forward-osmosis process, Vogel says.
“As long as the desal plant runs, you have a draw solution,” Vogel notes. The ready availability of a clean draw solution having a high osmotic potential helps make the forward-osmosis process more cost-effective, Vogel says. Significantly, it obviates the need to conduct a draw recovery step, in which TDS levels must be restored to the draw solution to maintain the necessary osmotic pressure. Typically, this draw recovery process complicates efforts “to make a good economic case” for forward osmosis, he says.
Work is ongoing to determine how the waste material captured in the filters will be managed, but it could potentially be returned to the wastewater treatment plant or used as a fertilizer source, according to Rivero.
The pilot test represents a foray into an arena that has seen relatively little previous investigation, Rivero says. “So far, to our knowledge, few research groups have investigated the potential of forward-osmosis membranes in the wastewater treatment field in a pilot scale using actual seawater reverse-osmosis brine as a draw solution,” he says. “In fact, this is the first experimental forward-osmosis plant in the Macaronesian area and the first time that the circular economy concept is integrated within the wastewater-desalination nexus.”
“Depending on the results obtained, we would like to propose a scale-up system in a location where both wastewater and desalination industries are located nearby,” Rivero adds. “The main goal is to increase the water resources in the Canary Islands via the use of emerging technologies applying a circular economy.”
Funded in part by Interreg Europe, a development program of the European Union, the pilot project is part of a broader effort known as “Research and innovation towards excellence in technological efficiency, use of renewable energy, emerging technologies and circular economy in desalination,” or E5DES. E5DES focuses on desalination research and development in Macaronesia.