Advancements in biofilm carriers and gas-permeable membranes: assessment of zeolite technologies for shortcut nitrogen removal applications in wastewater

The partial nitrification-anammox (PNA) process and other shortcut nitrogen removal processes have been widely studied because of their potential to offer cost savings during wastewater treatment; nevertheless, sustainable examples of full-scale mainstream shortcut nitrogen removal are lacking. The recent development of novel biofilm supports, specifically, zeolite-coated hollow fiber membranes and zeolite-coated biofilm carriers, that locally concentrate ammonium are promising for enhancing mainstream PNA. The ideal application of these technologies is yet to be determined, however. In this study, zeolite-coated carriers were tested in flow-through reactors under both anaerobic and aerobic conditions and zeolite-coated hollow fiber membranes were tested in a membrane-aerated flow-through configuration with varying operating times, lumen oxygen concentrations, and with the presence and absence of amended nitrite. Under anaerobic conditions, reactors containing zeolite-coated carriers had significantly greater ammonium and total nitrogen (TN) removal (84.0 ± 16.2% and 89.4 ± 17.1%, respectively) compared to reactors containing control carriers (P = 0.005). Anaerobic ammonia oxidizing (anammox) bacteria-specific 16S rRNA (Amx) genes and two genes associated with denitrifiers (nirS and nosZ) were preferentially retained in the bulk liquid and in the carrier biofilms in zeolite-coated carrier reactors at a statistically significant level. Genes specific to aerobic ammonium oxidizers (amoA genes) were preferentially retained in the bulk liquid of the zeolite-coated carrier reactors. The aerated zeolite-coated carrier reactors also had higher ammonium removal rates (83.8 ± 10.9%) and higher TN removal rates (69.1 ± 16.1%) compared to the aerated control reactors (30.8 ± 23.4%, P = 0.002 and 37.4 ± 27.4%, P = 0.05 for ammonia and TN, respectively). Again, despite aeration, amoA genes were only preferentially retained in the liquid of the reactors containing zeolite-coated carriers. In experiments with zeolite-coated membranes, Amx genes were preferentially retained at significantly higher quantities under only two of the experimental conditions: two-week operation with 100% oxygen delivered in the membrane lumen and two-week operation with nitrite supplemented in the influent. Overall, the zeolite-coated carriers present promising potential for deployment in both anaerobic and aerated environments to enhance nitrogen removal and, in particular, the retention of anammox bacteria. The zeolite-coated membranes require more study before their optimal deployment strategy is clear.