Effect of organic toxicants on the activity of denitrifying granular sludge.

Denitrification plays a key role in the biological nitrogen removal from the wastewater using granular sludge as the integral part of a high-rate denitrification technology. It is helpful to evaluate the effect of typical organic toxicants on the activity of denitrifying granular sludge for the application of denitrification technology. In this study, four typical organic toxicants, namely, penicillin, chloramphenicol, 2,4-dinitrophenol and polymyxin B sulphate were used to assess the effect of organic toxicants on the activity of denitrifying granular sludge. The results of individual toxicity indicated that penicillin, chloramphenicol and 2,4-dinitrophenol had significant inhibition, whose half-inhibitory concentrations were 0.534, 0.162 and 0.474 g/L with respective inhibitory magnitudes of 90.79%/(g/L), 282.5%/(g/L) and 138.83%/(g/L). Polymyxin B sulphate showed no significant inhibition. The results of combined toxicity indicated that the binary mixture of penicillin and chloramphenicol had an antagonistic effect, both the binary mixture of penicillin and 2,4-dinitrophenol and the binary mixture of chloramphenicol and 2,4-dinitrophenol had additive effects. The ternary mixture of penicillin, chloramphenicol and 2,4-dinitrophenol had a partial additive effect.

Feasibility and optimization of a novel upflow denitrification reactor using denitrifying granular sludge for nitric acid pickling wastewater treatment.

Stainless steel is highly valued for its superior resistance to corrosion. However, the pickling process involved in stainless steel production generates abundant NO3--N, causing health and environmental risks. To address this issue, this study proposed a novel solution utilizing an up-flow denitrification reactor and denitrifying granular sludge for treating NO3--N pickling wastewater under high NO3--N loading. It was found that, the denitrifying granular sludge exhibited stable denitrification performance with the highest denitrification rate of 2.79 gN/(gVSS·d) and average removal rates of NO3--N and TN of 99.94% and 99.31%, respectively, under optimal operating conditions of pH 6-9, temperature 35 °C, C/N ratio 3.5, hydraulic retention time (HRT) 11.1 h and ascending flow rate 2.75 m/h. This process reduced carbon source usage by 12.5-41.7% as compared to traditional denitrification methods. These findings demonstrate the efficacy of combining granular sludge and an up-flow denitrification reactor for treating nitric acid pickling wastewater.

Comprehensive evaluation of the long-term effect of Cu2+ on denitrifying granular sludge and feasibility of in situ recovery by phosphate.

With increased industrial development, vast heavy metals are inevitably discharged into wastewater. Cu2+ is one of the most hazardous heavy metals in biotreatment. However, the potential effect of Cu2+ on denitrifying granular sludge is still unknown. This work assesses the response of denitrifying granular sludge to Cu2+ stress from multiple aspects. The denitrifying granular sludge could tolerate 5 mg L-1 Cu2+, while the nitrogen removal efficiency decreased to 48.5% under 10 mg L-1 Cu2+. Enzyme activity and carbohydrate metabolism were inhibited, and the denitrifying bacteria were washed out under Cu2+ stress. The resulting deteriorated state was reversed by phosphate. The nitrogen removal efficiency recovered to 99% after 10 days, and the enzyme activity also recovered to the original level. Membrane transport, transcription and cellular processes were promoted. Overall, the results of this work provide a feasible strategy to rapidly restore the metabolic activity of denitrifying granular sludge under Cu2+ stress.

Effects of ZnO nanoparticles on high-rate denitrifying granular sludge and the role of phosphate in toxicity attenuation.

The increasing release of engineered nanoparticles (NPs) from consumer products has raised great concerns about their impacts on biological wastewater treatment. In this study, the widely-used ZnO NP was selected as a model NP to investigate its impact on high-rate denitrifying granular sludge in terms of sludge properties and community structure. A hormesis effect was observed during short-term exposure, in which the specific denitrification activity (SDA) was stimulated by 10% at 1 mg L-1 ZnO NPs, but inhibited by 23% at 5.0 mg L-1 ZnO NPs. When continuously exposed to 2.5 mg L-1 ZnO NPs, the nitrogen removal capacity of the denitrification reactor was nearly deprived within 15 days, and the relative abundance of the dominant denitrifying bacterium (Castellaniella) was decreased from 51.0 to 8.0%. Meanwhile, the dehydrogenase activity (DHA) and the content of extracellular polymeric substance (EPS) significantly decreased to 22.3 and 61.1%, respectively. Nevertheless, the presence of phosphate substantially weakened the adverse effects of ZnO NPs on the SDA, EPS, DHA and the relative abundance of functional genes even exposed to 6.25 mg L-1 ZnO NPs, which was associated with the fact that the level of Zn(II) released from ZnO NPs was significantly reduced in the presence of phosphate. Therefore, the toxicity of ZnO NPs may be mainly attributed to the release of toxic Zn(II) and could be attenuated in the presence of phosphate. Overall, this study provided further reference and meaningful insights into the impact of engineered NPs on biological wastewater treatment.

Sustainable bioreduction of toxic levels of chromate in a denitrifying granular sludge reactor.

Biological removal of chromate [Cr(VI)] in the presence or absence of nitrate by granular sludge biofilms was investigated in batch experiments and in a sequencing batch reactor (SBR). Denitrifying granular sludge cultivated from activated sludge was able to directly reduce Cr(VI) in the presence of an electron donor. Bioreduction was dependent on the initial Cr(VI) and the granular sludge concentrations. Bioreduction of Cr(VI) was followed by Cr(III) precipitation or entrapment in the granular sludge which was corroborated with decrease in total soluble Cr and increase in inorganic content of biomass. Batch experiments revealed that Cr(VI) addition has no major influence on high-strength nitrate (3000 mg L-1) denitrification, but nitrite denitrification was slowed-down. However, SBR experiment demonstrated successful denitrification as well as Cr(VI) removal due to enrichment of Cr(VI)-tolerant denitrifying bacteria. In fact, stable SBR performance in terms of complete and sustained removal of 0.05, 0.1, 0.2, 0.3, 0.5 and 0.75 mM Cr(VI) and denitrification of 3000 mg L-1 was observed during 2 months of operation. Active biomass and electron donor-dependent Cr(VI) removal, detection of Cr(III) in the biomass and recovery of ~ 92% of the Cr from the granular sludge biofilms confirms bioreduction followed by precipitation or entrapment of Cr(III) as the principal chromate removal mechanism. Metagenomic bacterial community analysis showed enrichment of Halomonas sp. in denitrifying granular sludge performing either denitrification or simultaneous reduction of nitrate and chromate.

Rapid start-up of the anammox process: Effects of five different sludge extracellular polymeric substances on the activity of anammox bacteria.

This study investigated the rapid start-up of the anaerobic ammonium oxidation (anammox) strategy by inoculating different biomass ratios of denitrifying granular sludge and anammox bacteria. The results demonstrated that two reactors (R1 and R2) were rapidly and successfully started-up on days 25 and 28, respectively, with nitrogen removal rates (NRRs) of 0.70kg/(m(3)·d) and 0.72kg/(m(3)·d) at biomass ratios of 10:1 (R1) and 50:1 (R2). The explanation for rapid start-up was found by examining the effect of five different sludge extracellular polymeric substances (EPS) on the activity of anammox bacteria in the batch experiments. Batch experiments results first demonstrated that the denitrification sludge EPS (DS-EPS) enhanced the anammox bacteria activity the most, and NO2(-)-N, NH4(+)-N removal rates were 1.88- and 1.53-fold higher than the control with optimal DS-EPS volume of 10mL. The rapid start-up strategy makes possible the application of anammox to practical engineering.

Effects of inorganic salts on denitrifying granular sludge: The acute toxicity and working mechanisms.

It is highly significant to investigate the toxicity of inorganic salts to denitrifying granular sludge (DGS) and its mechanism since the application of high-rate denitrification is seriously limited in the treatment of saline nitrogen-rich wastewaters. The batch experiments showed that the IC50 (half inhibition concentration) and LC50 (half lethal concentration) of NaCl, Na2SO4 and Na3PO4 on DGS were 11.46, 21.72, 7.46 g/L and 77.35, 100.58, 67.92 g/L respectively. Based on the analysis of specific denitrifying activity, the live cell percentage, the cell structure, and the DNA leakage, the toxicity of low salinity was ascribed to the inhibition of denitrifying activity and the toxicity of high salinity was ascribed to both the inhibition of denitrifying activity and the lethality of denitrifying cell.