Removal of tetracycline in nitrification membrane bioreactors with different ammonia loading rates: Performance, metabolic pathway, and key contributors.

Membrane bioreactors (MBRs) have been widely applied for the treatment of wastewater that contains high concentrations of both ammonium and antibiotics. Nonetheless, information about tetracycline (TC) removal in nitrification MBRs with high ammonium loading rates (ALRs) is still very limited. Herein, the fate of TC at four different concentrations of 1, 5, 20, and 50 mg/L in three parallel lab-scale nitrification MBRs with different ALRs (named AN50, AN500, and AN1000) were investigated in this study. Excellent nitrification performance and high TC removal efficiency (90.46%) were achieved in AN1000 at influent TC concentration of 50 mg/L. Higher ALRs promoted the removal of TC at lower influent TC concentration (≤5 mg/L), while no significant difference was observed in TC removal efficiencies among different ALRs MBRs at higher influent TC concentration (≥20 mg/L), implying that the heterotrophic degradation could be strengthened after long-term exposure to high concentration of TC. Batch tests demonstrated that adsorption and biodegradation were the primary TC removal routes by nitrification sludge, of which both autotrophic ammonia oxidizers and heterotrophic microorganisms played an important role in the biodegradation of TC. FT-IR spectroscopy confirmed that amide groups on the sludge biomass contributed to the adsorption of TC. Mass balance analyses indicated that biodegradation (63.4-88.6% for AN50, 74.5-88.4% for AN500 and 74.4-91.4% for AN1000) was the major mechanism responsible for the removal of TC in nitrification MBRs, and its contribution increased with influent TC concentration, while only 1.1%-15.0% of TC removal was due to biosorption. TC was progressively degraded to small molecules and the presence of TC had no notable effect on membrane permeability. These jointly confirmed TC could be effectively removed via initial adsorption and subsequent biodegradation, while biodegradation was the primary mechanism in this study.

Nitrification performance and bacterial community dynamics in a membrane bioreactor with elevated ammonia concentration: The combined inhibition effect of salinity, free ammonia and free nitrous acid on nitrification at high ammonia loading rates.

The responses of the operational performance and bacterial community structure of a nitrification membrane bioreactor (MBR) to elevated ammonia loading rate (ALR) were investigated. Effective nitrification performance was achieved at high ALR up to 3.43 kg NH4+-N/m3·d, corresponding to influent NH4+-N concentration of 2000 mg/L. Further increasing influent NH4+-N concentration to 3000 mg/L, the MBR system finally became completely inefficient due to the combined inhibition effect of salinity, free ammonia and free nitrous acid on nitrification. Ammonia-oxidizing bacteria (AOB) Nitrosomonas were enriched with the increase of ALR. The relative abundance of Nitrosomonas in the sludge with ALR of 2.57 kg NH4+-N/m3·d was up to 14.82%, which were 9-fold and 53-fold higher than that in seed sludge and the sludge with ALR of 0.10 kg NH4+-N/m3·d, respectively. The phylogenetic analysis of AOB amoA genes showed that Nitrosomonas europaea/mobilis lineage are chiefly responsible for catalyzing ammonia oxidation at high ALRs.

Effect of Increasing C/N Ratio on Performance and Microbial Community Structure in a Membrane Bioreactor with a High Ammonia Load.

Herein, the responses of the operational performance of a membrane bioreactor (MBR) with a high ammonium-nitrogen (NH4+-N) load and microbial community structure to increasing carbon to nitrogen (C/N) ratios were studied. Variation in the influent C/N ratio did not affect the removal efficiencies of chemical oxygen demand (COD) and NH4+-N but gradually abated the ammonia oxidization activity of sludge. The concentration of the sludge in the reactor at the end of the process increased four-fold compared with that of the seed sludge, ensuring the stable removal of NH4+-N. The increasing influent COD concentration resulted in an elevated production of humic acids in soluble microbial product (SMP) and accelerated the rate of membrane fouling. High-throughput sequencing analysis showed that the C/N ratio had selective effects on the microbial community structure. In the genus level, Methyloversatilis, Subsaxibacter, and Pseudomonas were enriched during the operation. However, the relative abundance of ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) involved in nitrification declined gradually and were decreased by 86.54 and 90.17%, respectively, with influent COD increasing from 0 to 2000 mg/L. The present study offers a more in-depth insight into the control strategy of the C/N ratio in the operation of an MBR with a high NH4+-N load.

Characterization of cadmium biosorption by inactive biomass of two cadmium-tolerant endophytic bacteria Microbacterium sp. D2-2 and Bacillus sp. C9-3.

In this study, two cadmium-tolerant endophytic bacteria (Microbacterium sp. D2-2 and Bacillus sp. C9-3) were employed as biosorbents to remove Cd(II) from aqueous solutions. The influence of initial pH, initial Cd(II) concentration, adsorbent biomass, temperature and contact time on Cd(II) removal were investigated. Results showed that the Langmuir isotherms were found to best fit the equilibrium data, and the maximum biosorption capacities were found to be 222.22 and 163.93 mg/g at a solution pH of 5.0 for Microbacterium sp. D2-2 and Bacillus sp. C9-3, respectively. The biosorption kinetics followed well pseudo-second-order kinetics. Fourier transform infrared spectroscopic analysis suggested that the hydroxyl, carboxyl, carbonyl and amino groups on Microbacterium sp. D2-2 and Bacillus sp. C9-3 biomass were the main binding sites for Cd(II). The results presented in this study showed that Microbacterium sp. D2-2 and Bacillus sp. C9-3 are potential and promising adsorbents for the effective removal of Cd(II) from aqueous solutions.