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

Abstract

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.