Buffered loofah supported Microalgae-Bacteria symbiotic (MBS) system for enhanced nitrogen removal from rare earth...

Rare earth element tailings (REEs) wastewater, which has the characteristics of high ammonia nitrogen (NH₄⁺-N) and low COD. It can cause eutrophication and biotoxicity in water which is produced in high volumes, requiring treatment before final disposal. Microalgae-Bacteria symbiotic (MBS) system can be applied in REEs wastewater, but its low extent of nitrogen removal and instability limit its application. By adding biodegradable carrier as both carbon source and carrier, the system can be stabilized and the efficiency can be improved. In this work, the extent of NH₄⁺-N removal reached 100% within 24 h in a MBS system after adding loofah under optimal conditions, and the removal rate reached 127.6 mg NH₄⁺-N·L^-1·d^-1. In addition, the carbon release from loofah in 3 d reached 408.7 mg/L, which could be used as a carbon source to support denitrification. During 90 d of operation of the MBS system loaded with loofah, the effluent NH₄⁺-N was less than 15 mg/L. At phylum level, Proteobacteria were dominant which accounted for 78.2%. Functional gene analysis showed that enhancement of microalgae assimilation was the main factor affecting NH₄⁺-N removal. This work expands our understanding of the enhanced role of carbon-based carriers in the denitrification of REEs wastewater.