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Low nitrogen (N) removal efficiency limits the potential of microalgae technology for the treatment of high nitrogen and low carbon rare earth tailings (REEs) wastewater. In this study, waste corncob was utilized as a biocarrier immobilizing Chlamydopodium sp. microalgae to realize high-efficient treatment of the REEs wastewater. In only 2.5 d, corncob-immobilized microalgae allowed the residual concentrations of N lower than the emission standards, and ammonia nitrogen (NH 4 +-N) removal rate is 83.3 mg L−1·d−1, total inorganic nitrogen (TIN) removal rate is 86.7 mg L−1·d−1, which was 18.5 times that of the previously-reported microalgae (4.68 mg L−1·d−1). Compared with other microalgae immobilization carriers, corncob possesses the ability to release available carbon sources for microalgae. Composition analysis and sugar verification experiments showed that the main content of TOC released by corncob was monosaccharide, and in a certain range, the removal rate of N was positively correlated with the TOC concentration. The utilization of biomass wastes with dual functions as biological carriers has great potential to improve the performance of microalgae, and is conducive to the development of engineering applications. [Display omitted] • Rapid removal of NH 4 +-N and TIN by microalgae was achieved in REEs wastewater. • The removal efficiency of TIN can reach 100%, and rate can reach 86.7 mg L−1·d−1. • The removal rate of N by corncob-immobilized microalgae reached the highest. • Corncob possess dual functions of biocarrier and available carbon source. [ABSTRACT FROM AUTHOR]

[Display omitted] • The co-optimization of baffle and aeration was achieved. • The processing efficiency of the optimized reactor increased by ca. 106 %. • The novel runway pool photobioreactor reduced the Light/Dark cycle time by 89.6 %. Microalgae photobioreactor (PBR) is a kind of efficient wastewater treatment system for nitrogen removal. However, there is still an urgent need for process optimization of PBR. Especially, the synergistic effect and optimization of light and flow state poses a challenge. In this study, the computational fluid dynamics is employed for simulating the optimization of the number and length of the internal baffles, as well as the aeration rate of PBR, which in turn leads to the optimal growth of microalgae and efficient nitrogen removal. After optimization, the Light/Dark cycle of the reactor B is shortened by 51.6 %, and the biomass increases from 0.06 g/L to 3.94 g/L. In addition, the removal rate of NH 4 +-N increased by 106.0 % to 1.56 mg L−1 h−1. This work provides a feasible method for optimizing the design and operational parameters of PBR aiming the engineering application. [ABSTRACT FROM AUTHOR]

Rare earth element tailings (REEs) wastewater, which has the characteristics of high ammonia nitrogen (NH 4 +-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 NH 4 +-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 NH 4 +-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 NH 4 +-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 NH 4 +-N removal. This work expands our understanding of the enhanced role of carbon-based carriers in the denitrification of REEs wastewater. [Display omitted] • The removal rate of NH 4 +-N was 127.6 mg L−1·d−1 in loofah-supported MBS system. • Loofah provides organic carbon for denitrification and maintains a stable pH. • The continuous-flow Microalgae-Bacteria bioreactor achieved 15 m/L effluent NH 4 +-N. • Microalgal nitrogen assimilation contributed to the observed increased NH 4 +-N removal. [ABSTRACT FROM AUTHOR]

[Display omitted] • The rare earth wastewater was firstly treated using a microalgae-bacteria system. • The microalgae-bacteria ratio showed positively correlated with NH 4 +-N removal. • High NH 4 +-N removal of 97.46 % was achieved at microalgae/bacteria ratio of 1:3. • Bacteria altered the microbial diversity and promoted functional microalgae growth. Microalgae-bacteria symbiosis system (MBS) appear to be a promising way for treating the rare earth elements (REEs) wastewater due to the natural symbiotic interactions between microalgae and bacteria. Herein, we investigated the effect of different inoculation ratios of microalgae and bacteria including 3:1 (MB_1), 1:1 (MB_2) and 1:3 (MB_3) on NH 4 +-N removal from REEs wastewater and analyzed the corresponding biological mechanism. The NH 4 +-N removal rate with MB_3 reached 17.69 ± 0.45 mg NH 4 +-N/L d−1, which was 2.58 times higher than that in single microalgae system. The results were further verified in continuous feeding photobioreactors and kept stable for 100 days. Metagenomic analysis revealed that the abundance of genes related to microalgae assimilation increased by 14 %-50 % in answer to photosynthesis and NH 4 +-N absorption, while that related to nitrification apparently dropped, indicating that MBS was a sustainable method capable of enhancing NH 4 +-N removal from REEs wastewater. [ABSTRACT FROM AUTHOR]

Using microalgae for rare earth elements wastewater treatment has been proved to be effective, however, the technology is challenged by the unstable performances caused by the changeable wastewater qualities and natural conditions. In this study, a corrected response surface methodology was first used to provide an optimized operation strategy for the application of wastewater treatment using microalgae, by investigating the effects and interrelations of three main factors including temperature, illumination intensity and microalgae concentration, taking rare earth elements wastewater in Southern Jiangxi as an example. Nine models were obtained using nine types of wastewater. The models were then modified by categorizing wastewaters into two classes (error range <12%) according to their initial ammonia nitrogen concentrations. The optimal conditions for ammonia nitrogen removal in class I and class II wastewater were as follows: temperature of 21.0 and 23.6 °C, illumination intensity of 46.8, 45.3 LX %, microalgae concentration of 1.64 and 1.67 g dry weight/L. The results are confirmed by stable operation of a continuous-flow photobioreactor for more than 122 days. The amount of microalgae required to treat wastewater under natural conditions is 1.5–1.8 g dry weight/L estimated by the corrected model. It implies that nearly 16,710 kg and 46,520 kg of ammonia nitrogen could be removed from two typical wastewaters with optimal microalgae dosage in 2020. The results demonstrated that corrected response surface methodology provides a powerful approach for optimizing the operational conditions for wastewater treatment by microalgae. [Display omitted] • A corrected RSM model for microalgae towards NH 4 +-N removal was first built. • Optimal conditions for REEs wastewater treatment by microalgae were accurately obtained. • Optimized NH 4 +-N removal were predicted from actual REEs watershed wastewater. [ABSTRACT FROM AUTHOR]