by Catherine Ortega Klett, NM WRRI Program Manager
Lei Hu, a PhD graduate student in the Department of Civil Engineering at New Mexico State University, is working on a grant project funded by the New Mexico Water Resources Research Institute (NM WRRI) titled “Recovery of Ammonium and Magnesium from Produced Water by Struvite Precipitation.” His faculty adviser is Dr. Yanyan Zhang, also of the Department of Civil Engineering at NMSU.
Oil and natural gas industries generate huge amounts of produced water containing a complex mixture of organic contaminants and very high concentrations of inorganic salts. Pollution problems related to the ammonium (ionized ammonia) discharge of produced water commonly include eutrophication and dissolved oxygen depletion in water bodies, as well as toxicity to aquatic life. Therefore, high concentrations of ammonium in the produced water is a major hurdle for the treatment and reuse of the produced water. A particularly promising chemical treatment for the removal and simultaneous recovery of ammonium and magnesium from the produced water is to precipitate it out in the form of a relatively insoluble compound of magnesium, phosphorous and oxygen (as phosphate), ammonium, and water, known as struvite. This white crystalline mineral can also be put to good use as a slow release chemical fertilizer, which therefore helps to make this approach to water treatment economically viable.
The main objective of this project is therefore to simultaneously recover ammonium and magnesium from produced water by struvite precipitation. The optimal conditions for struvite formation are being explored by considering the different effects of salinity, organic composition, solution pH, concentration ratios of the various ionic components that react to form struvite, and the presence of other competitive metal ions, such as those of calcium and potassium, on the production of struvite. For example, by the addition of excess phosphate to the produced water, calcium will also precipitate out as calcium phosphate, along with the struvite. The removal of such calcium from produced water can help to minimize the calcium-based scale formation that fouls membranes that are used to carry out the desalination of produced water.
The phosphate that is needed for these precipitation reactions can potentially be obtained partially from the wastewater itself, and from the anaerobic sludge of wastewater treatment plants. It is therefore anticipated that these precipitation processes will be found to provide an economical pathway for the removal of ammonia, magnesium, phosphate, and calcium from produced water, while at the same time providing a beneficial mineral for use as a chemical fertilizer. Considering the low cost and relatively simple technology, the struvite and associated calcium precipitation processes have considerable economic potential for large-scale applications.
Lei Hu expects to graduate with a doctoral degree in civil engineering in July 2021. He came to NMSU from China where he received bachelor and master degrees from China University of Mining & Technology in Beijing. Lei presented his project at the New Mexico Produced Water Conference in Santa Fe last November. He met with an engineer from the Mosiac Company who was very interested in the research and who also believes that struvite precipitation is a good method to recover ammonium and magnesium from produced water.
Lei also presented his project at the NM WRRI’s 63rd Annual New Mexico Water Conference in October 2018. He says, “I was so happy to meet other students conducting water-related research. My career goal is to become an expert in the wastewater treatment field and solve some water pollution problems.” Lei added that the NM WRRI student grant was very helpful in providing him with the financial support to work on developing new research about produced water.