The rise and rapid adoption of the circular economy—a systems solution framework that seeks to keep materials, products, and services in circulation for as long as possible—is triggering an interesting shift in how wastewater is being viewed. One view posits that wastewater should no longer be seen as waste, owing to the possibility that industry may recover several valuable resources from it. These include water, energy, fertilizers, biofuels, biopolymers, and critical minerals.
This type of resource recovery has thrived, primarily in higher-income countries, as part of established efforts relating to general wastewater treatment. However, their uptake in the power industry has suffered setbacks, given complex water quality. Still, several research and development efforts in recent years suggest promise.
Optimizing Coal Plant Wastewater Desalination
A May 2022 study published in the journal Membranes by researchers from Dianzi University in Hangzhou, China, evaluated benefits associated with the simulation and operational optimization of coal-fired power plant wastewater treatment. The study, motivated by wastewater environmental concerns, along with water scarcity concerns, was focused on a plant system in Inner Mongolia as an example. China’s coal plants account for a large share of the nation’s energy production, and the country is exploring measures to treat and reuse power plant wastewater, especially in central and western China, which have recently been stricken with water shortages.
Coal plants, however, have a “complex water quality,” owing to high pH and chemical oxygen demand, the researchers noted. Some plant wastewater is organic, produced from gasification processes, coal-water slurry, and black water from dry coal pulverized entrained flow gasification. It mainly contains phenol, naphthalene, anthracene, and thiophene, which are refractory organics with poor biodegradability, and require biochemical treatment.
The majority of coal plant wastewater, however, is saline water, which comes from recycling systems, such as circulating water systems and chemical water station drainage. Saline water mainly contains inorganic salts, such as Cl −, SO4 2−, Na +, and Ca 2+. Desalination methods for saline wastewater treatment at coal plants include membrane technology and thermal technology. Reverse osmosis (RO) technology is also extensively employed, though these systems’ operational efficiency is greatly affected by “the feeding conditions of wastewater,” the researchers said.
The study studied the membrane treatment process. Researchers first developed a mechanistic model equation of the RO process based on the seawater desalination process and then a system model suitable for coal-power wastewater treatment. Simulation analysis and optimization analysis were also carried out. The study’s results suggest that optimization promises increases in water recovery rates of up to 20.7% depending on energy consumption, which optimization could also decrease by up to 42.6%. They said that that could amount to big savings for the nation’s coal plants.
Rare Earth Element Recovery
As the value of rare earth elements (REEs) ramps up, given their extensive use in electronics and other applications, so have efforts to recover them from unconventional streams such as coal and coal waste. In September 2021, Midwest Energy Emissions Corp. (ME2C), an environmental technologies company, announced it had completed an initial round of testing to evaluate its REE technology capture capacity and regeneration. ME2C is most prominently known for its Sorbent Enhancement Additive (SEA) system, widely used in the coal power industry for mercury emissions capture. Its REE technology, which is still under development, comprises a sorbent technology based on chemisorption and focuses on acid leaching as a capture method.
“Our technology under development, if successful, will effectively separate the REEs from the acid solution while maintaining high purity of the REEs and requiring significantly less caustic acid. Limiting the acid during the leaching/extraction process would provide a much more environmental approach for processing REEs that would allow this to be performed domestically rather than outsourced to China and third-world countries,” the company explained to POWER in September. The new technology “will also capture contaminants and other minerals from coal ash ponds and wastewater,” it said.
The company added that its technology remains “in a final phase of development as we are introducing real-world materials, acid mine drainage (AMD sludge), as well as fly ash.” So far, Penn State’s College of Earth and Mineral Sciences has evaluated the technology, and ME2C completed in-field pilot testing in March 2022. “Currently, the in-field testing is focused on the introduction of real-world materials and conducted in a lab,” it said. “The technology performed well in a lab environment with testing performed at Penn State. We continue to test at Penn State and have added a commercial lab based in Florida to validate and expedite the testing protocols.”
Plans are now underway to carry out full-scale testing with a commercial vendor. The company said it has discussed testing fly ash with a utility customer and has begun characterizing and testing fly ash from actual power plants. “With confirmation of our technology’s ability to capture certain rare earth elements, we will continue to evaluate the commercial viability, including the ability to effectively reuse this sorbent,” said Richard MacPherson, president and CEO of ME2C. The company noted multiple utilities have expressed interest in finding a potential revenue source for their coal ash ponds. “If successful, our technology will allow REEs to be easily processed from coal ash and AMD sludge and would help utilities offset the cost of their coal waste cleanup,” it said.
REE extraction from coal fly ash and power plant wastewater is also under investigation at the Department of Energy’s (DOE’s) Los Alamos National Laboratory (LANL). The lab has explored a hydrothermal process using coal ash and wastewater sludge collected from a power plant near Detroit, Michigan. A separate project DOE spearheaded by Wyonics LLC and the University of Wyoming has meanwhile successfully demonstrated the extraction of REEs from coal and fly ash into specifically designed ionic liquids (ILs) “in a simple and energy-efficient manner.” The lab said REE extraction and recovery from coal was performed “via direct treatment of coal with intelligently designed task-specific ILs. The IL chosen for these processes possesses an ideal combination of excellent solvation ability, nonvolatility, low toxicity, proven recyclability, and availability at the multi-ton scale.” Preliminary studies suggest the process can be applied to other sources of coal and ash from different U.S. mines.
Wastewater Heat Recovery
In another notable project, researchers from the Institute for Sustainable Technologies (AEE INTEC) in Gleisdorf, Austria, set out to evaluate whether it was economical for a wastewater treatment plant (WWTP) in Austria’s region of Styria to deploy a biogas combined heat and power (CHP) plant along with effluent water heat pumps to provide thermal energy for facility-wide heat demands, as well as send surplus heat to a district heating network. The facility, like most large WWTPs in Austria, produces biogas via an anaerobic digestion process. However, it currently flares 11% of its biogas.
While there is no natural gas grid connection at the facility, a biogas CHP is envisioned to replace the existing biogas boiler to provide the facility with electrical and thermal self-sufficiency. The study published in the journal Frontiers in Sustainable Cities showed that the integration of heat pumps using effluent water (which ranges between 60C and 75C) using a serial water concept could allow the facility to take advantage of lower flow temperatures while also delivering heat to the district heating network.
Algae for Animal Feed, Biofuels
Some notable projects are meanwhile exploring sector coupling. Springfield, Illinois, public power utility City Water, Light and Power has embarked on a project to utilize carbon dioxide from its 200-MW Dallman 4 pulverized coal-fired power plant and nutrients from a local WWTP to cultivate algae for animal feeds. The $2.5 million engineering-scale project announced in December 2021 will span three years.
Algae is fast-growing compared with traditional terrestrial feed crops. It also makes an attractive alternative for use in taking up CO 2 from power plants because it requires less land, according to Illinois Sustainable Technology Center (ISTC) principal investigator Lance Schideman. The project will use the algae species Spirulina because it is already Food and Drug Administration approved for use as a food ingredient and has a high protein content, “which commands higher prices.”
In the current commercial technology, managers buy liquid CO2 and various commercial fertilizers for the nutrient supply. “Using wastewater is a cost-saving in the production process and it helps to solve problems that wastewater treatment plants are experiencing in trying to minimize nutrient discharges in the environment,” Schideman added. “In Illinois, the treatment plants are under increasing scrutiny, and regulations that are now voluntary are expected to become more stringent and potentially mandatory within the next decade.”
While the Dallman 4 example doesn’t specifically utilize power plant wastewater, it illustrates a potential pathway for power plants. Although it remains challenging to deploy, algae cultivation in power plant wastewater has been much explored for its potential benefits, including as a novel source of biomass and carbon dioxide utilization.
In 2018, researchers from Shiraz University in Iran showed microalgae can be used to treat wastewater for sulfate reduction and production of microalgae biodiesel. The much-cited study’s results suggest algae species Oocystis sp. had a higher sulfate uptake rate, but that Chlorella produced more biomass.
Lithium Extraction from Geothermal Brine
If there is a prominent buzz around power plant wastewater as an unexploited resource today, it is largely centered on the recovery of freshwater, minerals, and energy from geothermal brine. Considerable interest is growing in extracting lithium—a principal component of high-energy-density batteries–from lithium-rich geothermal brines. Several companies have centered their operations on lithium mining from brine to leverage what they say is a burgeoning opportunity. These include Controlled Thermal Resources Cornish Lithium/Cornish Metals, and Vulcan Energy Resources.
Australian firm Controlled Thermal Resources (CTR) is developing a project in California’s Imperial County near the Salton Sea. The project seeks to co-produce geothermal power and lithium via direct extraction, and potentially other minerals including rare earth elements, potassium, zinc, manganese, and rubidium. The project is staged to deliver 50 MW and 25,000 tonnes of lithium in 2024. This image shows thermal mud pots at the project site. Courtesy: CTR
According to a comprehensive overview of direct lithium extraction technologies that Lawrence Berkeley National Laboratory published in late 2021, the most technologically advanced approach involves adsorption of lithium using inorganic sorbents. Other separation processes include extraction using solvents, sorption on organic resin and polymer materials, chemical precipitation, and membrane-dependent processes. The lab notes that initial studies have demonstrated that lithium extraction and recovery from geothermal brines are technically feasible. However, “challenges still remain in developing an economically and environmentally sustainable process at scale,” it says.