Rare-earth elements (REEs) underpin a host of technologies central to modern society. As novel REE processing technologies are developed, appropriate handling of wastewaters will be important. One option is discharge of wastewaters to municipal water resource recovery facilities (WRRFs), yet little is currently understood about the potential for REEs to affect the performance of biological treatment processes. To evaluate the potential impacts, bench-scale sequencing batch bioreactors containing activated sludge and synthetic wastewater were exposed to increasing concentrations of gadolinium or yttrium salts. Nitrification inhibition with 50 ppm Gd or Y treatment and negative impacts to organic oxidation with 50 ppm Y treatment were observed. Microbiome analyses indicated changes to microbial communities as a function of REE exposure, including decreases in relative abundance of putative nitrifying bacteria. In the reactors, >95% of the added REE was insoluble, and inhibition was observed only when the soluble REE concentrations approached 1 μM. A subsequent experiment demonstrated the recovery of nitrification after cessation of Gd or Y addition. These findings suggest that due to the low solubility of Gd and Y in typical WRRF treatment conditions, high concentrations would be required to produce an inhibitory effect and that inhibition may be transient, with potential recovery after REE exposure.
Slug tests are a widely used method for characterizing aquifers because they can be performed quickly, with minimal equipment and labor, and without the need to dispose of produced water. They involve rapid displacement of the water level in a well, followed by continuous measurement of the water level as it reequilibrates. Several mathematical models available in the hydrogeology literature are summarized here. The tests are classified according to: (i) the type of perturbation (solid versus pneumatic slug); (ii) the type of response (overdamped versus oscillatory); and (iii) the type of aquifer (confined versus unconfined). Examples are included of the graphical approach using slug‐test data to estimate aquifer properties. Computational tools and software available for more complex models are mentioned briefly. Emerging works on slug tests focus on the use of slug tests to characterize heterogeneity through their integration into hydraulic tomography. They are used in multilevel single‐well tests and in cross‐hole tests with packers and with direct‐push technology to generate vertical logs of hydraulic parameters including hydraulic conductivity and storage.
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