Few Houstonians wake each morning concerned that the clean water that pours from their faucets could run dry, but this is a reality that could affect future generations. In fact, the United States Bureau of Reclamation, USBR, is proactively researching more efficient, cost effective ways to desalinate brackish groundwater and surface water sources.
“You have no choice but to move to worse and worse source waters,” said Shankar Chellam, professor of civil and environmental engineering at the UH Cullen College of Engineering. “So maybe we don’t think very much about our water in Houston, but you go to other areas of Texas and Central Oklahoma, and it’s already a crisis.”
Chellam received $150,000 from the USBR to research ways to remove contaminants and salt from brackish surface water in Foss Reservoir in Foss, Okla., located several hours north of Houston. The City of Houston, Foss Reservoir Master Conservancy District and the University of Houston supported the project with in-kind contributions.
Growing population pressures and symptoms of drought, which include overutilization of groundwater, increased concentrations of existing pollutants and addition of new pollutants, have taken their tolls on water supplies around the world. Consequently, the relatively clean surface water and groundwater supplies available 50 years ago are essentially nonexistent today, Chellam said.
“So the qualities of the water supplies are decreasing as we are forced to expand production,” Chellam said. “Old technologies are incapable of dealing with these issues.”
The extended drought has caused high evaporative losses in Foss Reservoir, and the remaining water has higher-than-usual salt content without the diluting effects of rain. The water district has relied on electrodialysis reversal, an expensive desalination method, in the face of the drought, which is expected to continue for several more years. It is a scenario that has become more common among water districts in arid areas across the country.
The most common method for salt filtration in the U.S. is reverse osmosis, which is also the gold standard for removing high levels of salt from seawater and ocean water. However, the salt content is so high in ocean water that the energy required for desalination is prohibitively expensive for public water utilities in all but a few areas of the world where limited freshwater supplies make it necessary, Chellam said.
“We want to implement less expensive processes so the induction into an actual technology can happen faster,” Chellam said. “We don’t always want to use the most expensive method even though we know it does a very good job.”
Chellam’s earlier work with nanofiltration membranes focused on improved methods for eliminating contaminants such as bacteria, viruses, organic matter and inorganic chemicals from surface waters that serve as municipal water supplies. This project aims also to remove salt from the mix.
Nanofiltration membranes, which are less expensive than reverse osmosis membranes, could potentially fill a niche for waters that fall somewhere on the salt spectrum between relatively unsalted lakes, such as Lake Conroe, and briny oceans.
Polymer nanofiltration membranes have irregular-shaped, nano-sized pores that filter salt and contaminates by size exclusion and charge repulsion. The filters mimic most natural systems with negative charges that repel negatively charged contaminants.
The challenge is to understand the mechanisms by which contaminants pass through filters to determine ways to improve their removal, to optimize flow of water without irreversibly clogging the filters and to determine ways to regenerate the filters when they clog. Chellam plans to evaluate electrocoagulation and electroflotation, novel pretreatment methods, to prevent clogging.
“Nanofilters are useless if they clog rapidly,” Chellam said. “They need to last five or 10 years, or they become unreasonably expensive.”
Water conservation is another benefit of nanofiltration. Desalination with reverse osmosis membranes typically recovers 75 to 80 percent of the feed water, which means 20 to 25 percent of the water is lost, said Kevin McCalla, special counsel for the Texas Commission on Environmental Quality’s Office of Water, in an email.
“It basically improves treatment technologies so they’re not wasting as much water,” Chellam said. “It strengthens water supplies, especially in times of drought.”
In Texas, 13 water treatment plants desalinate surface water, and three more, including one in Galveston County, are approved for construction, McCalla said. All the facilities use reverse osmosis membranes with the exception of one, which uses electrodialysis reversal.
The total combined maximum amount of water desalinated at the 13 plants ranges from 15 to 22 million gallons per day, which means three to six million gallons are lost each day in Texas from these plants, based on 2010 data.
In the next couple of years, Chellam, his graduate student, Mutiara Ayu Sari, and his collaborators from Austin’s USBR office, Collins Balcombe and Anna Hoag, expect to lay the theoretical and experimental framework for meaningful implementation of this technology.
“Every environmental engineer is working to protect the environment we all interact with, whether it’s clean air, clean soil or clean water, and they’re all important for our quality of life,” Chellam said. “Safe, clean drinking water is an important environmental issue that affects every single human.”