Biodegradation of nitrophenol compounds and the membrane fouling trends in different submerged membrane bioreactors

Recent advances in biological removal of nitroaromatics from wastewater

Various nitroaromatic compounds (NACs) released into the environment cause potential threats to humans and animals. Biological treatment is valued for cost-effectiveness, environmental friendliness, and availability when treating wastewater containing NACs. Considering the significance and wide use of NACs, this review focuses on recent advances in biological treatment systems for NACs removal from wastewater. Meanwhile, factors affecting biodegradation and methods to enhance removal efficiency of NACs are discussed. The selection of biological treatment system needs to consider NACs loading and cost, and its performance is affected by configuration and operation strategy. Generally, sequential anaerobic-aerobic biological treatment systems perform better in mineralizing NACs and removing co-pollutants. Future research on mechanism exploration of NACs biotransformation and performance optimization will facilitate the large-scale application of biological treatment systems.

Solids retention time dependent, tunable diatom hierarchical micro/nanostructures and their effect on nutrient removal

The hierarchical three-dimensional (3D) micro/nanostructures of diatoms make them a promising biomaterial for fabricating nanomaterials, producing bioactive pharmaceuticals or nutraceuticals, and removing micropollutants. For diatom production in a continuous flow system, little is known how bioreactor operating parameters, especially solids retention time (SRT), affect the 3D structures of diatoms. This study demonstrated that tunable diatom micro/nanostructures could be produced by varying the SRT of membrane bioreactors (MBRs). A diatom strain (Stephanodiscus hantzschii) was cultivated in two identical MBRs with a fixed hydraulic retention time (HRT) of 24 h and staged SRTs from 5, to 10, and to 20 d. As SRTs increased from 5 to 20 d, important characteristics of diatom micro/nanostructures showed linear decreases: the diameters of foramina on the areola layer decreased from 170 ± 10 to 130 ± 12 nm, the numbers of nanopores per cribrum layer decreased from 20 ± 3 to 12 ± 2, and the specific surface areas of the diatoms decreased from 36.01 ± 1.27 to 12.67 ± 2.45 m²·g^-1. However, the average diatom heights increased from 2.9 ± 0.3 to 3.9 ± 0.4 µm, while diatom cell diameter (5 µm) and nanopore size (20 nm) remained unchanged. The silicon content of diatoms also linearly increased with SRT. The decrease in diatom porosity and increase in silicon content were probably due to the reduced diatom growth rates (likely resulting in less pores) at increasing SRTs, which also facilitated silica deposition as the overall diatom population stayed longer in the MBRs. As the SRTs increased from 5 to 10, and to 20 d, the nitrate (NO₃^-) removal efficiency decreased from 75% to 70%, and to 60%, respectively, whereas phosphorus (P) removal efficiency increased from 74% to 80%, and to 90%, respectively. The opposite trends in efficiencies were because NO₃^--N was removed by cellular uptake and biomass waste whereas P was mainly removed through diatom-assisted chemical precipitation.

Improvement of sludge dewaterability by energy uncoupling combined with chemical re-flocculation: Reconstruction of floc, distribution of extracellular polymeric substances, and structure change of proteins

This study innovatively combines energy uncoupling and chemical re-flocculation helped to accelerate residual sludge dewatering. Ferric chloride (FeCl₃) and 3, 3', 4', 5-tetrachlor-osalicylanilide (TCS) were employed as the flocculant and uncoupler, respectively. The results showed that the specific resistance to filtration (SRF) and the water content of sludge filtered cake fell dramatically from 11 × 10¹² m/kg and 80.2% to 1.1 × 10¹² m/kg and 77.1% respectively, when the addition of TCS ranged from 0 to 0.12 g/g VSS with flocculation conditioning. The distribution of sludge extracellular polymeric substance (EPS) was altered radically after adding TCS, leading to the collapse and fragmentation of EPS, causing the reduction and formation of fragmentized sludge flocs. Meanwhile, the stretching and deformation vibrations of CO and NH bonds suggested the strong attack between TCS and EPS proteins, while variations of the main secondary structures of protein (i.e. α-helix, β-sheet and random coil) indicated the loose structure of proteins and enhanced hydrophobicity. Consequently, the cracked and loose structure of residual sludge resulted in the release of bound water. After TCS addition combined with chemical re-flocculation, the channels of sludge water discharge were widened, guaranteeing the discharge of sludge water. Therefore, the sludge dewaterability was elevated under the energy uncoupling combined with chemical re-flocculation. As well, the application of TCS would not destroy sludge cells, in which bioenergy (sludge carbon source) could be retained and effectively utilized in the subsequent disposal process. The findings reported here not only widen our perception of the energy uncoupling technology, but also encourage researchers to explore both effective and economic methods on the basis of energy uncoupling, aiming to achieve high-efficiency of reduction and dewatering in the future.

Coupling of membrane-based bubbleless micro-aeration for 2,4-dinitrophenol degradation in a hydrolysis acidification reactor

Micro-aeration hydrolysis acidification (HA) is an effective method to enhance the removal of toxic and refractory organic matter, but the difficulty in stable dosing control of trace oxygen limits its wide application. Membrane-based bubbleless aeration has been proved as an ideal aeration method because of its higher oxygen transfer rate, more uniform mass transfer, and lower cost than HA. However, the available information on its application in HA is limited. In this study, membrane-based bubbleless micro-aeration coupled with hydrolysis acidification (MBL-MHA) was exploited to investigate the performance of 2,4-dinitrophenol (2,4-DNP) degradation via comparing it with bubble micro-aeration HA (MHA) and anaerobic HA. The results indicated that the performances in MBL-MHA and MHA were higher than those in HA during the experiment. 2,4-DNP degradation rates under redox microenvironments caused by counter-diffusion in MBL-MHA (84.43∼97.28%) were higher than those caused by co-diffusion in MHA (82.41∼94.71%) under micro-aeration of 0.5-5.0 mL air/min. The 2,4-DNP degradation pathways in MBL-MHA were nitroreduction, deamination, aromatic ring cleavage, and fermentation, while those in MHA were hydroxylation, aromatic ring cleavage, and fermentation. Reduction/oxidation-related, interspecific electron transfer-related species, and fermentative species in MBL-MHA were more abundant than that in MHA. Ultimately, more reducing/oxidizing forces formed by more redox proteins/enzymes from these rich species could enhance 2,4-DNP degradation in MBL-MHA.

Single and joint inhibitory effect of nitrophenols on activated sludge

In this study, single and joint inhibitory effects of nitrophenols on activated sludge and variations on the content of extracellular polymeric substances (EPS) were investigated. Results indicate that the nitrophenols adversely affected the organic and NH₃-N removal of activated sludge and the adverse effect of nitrophenols on autotrophic bacteria was higher than that on heterotrophic bacteria. Further, 2,4-dinitrophenol (2,4-DNP) demonstrated the highest inhibitory effect, followed by 4-nitrophenol (4-NP) and 2-nitrophenol (2-NP), and the combined effects of binary and ternary nitrophenols induced additive toxicity. At various concentrations and toxicant ratios, 2,4-DNP, as the dominant toxic nitrophenol, was the major contributor to the joint inhibition effects of the mixed nitrophenols. At lower concentrations of 2-NP (below 100 mg/L), 4-NP (below 50 mg/L), and 2,4-DNP (below 10 mg/L), large amounts of both tightly bound EPS (TB-EPS) and loosely bound EPS (LB-EPS) were secreted for the normal physiological activities of the microbiological cells. After further stimulation with higher concentrations of 2-NP (above 100 mg/L), 4-NP (above 50 mg/L), and 2,4-DNP (above 10 mg/L), the inhibitory effect of nitrophenols on bacterial metabolism evidently increased. However, the EPS production sharply reduced, particularly with respect to protein production. Parallel factor analysis for TB-EPS and LB-EPS further confirmed that the major proteins were tyrosine, tryptophan, and aromatic proteins. Thus, this study provides new insights into the inhibitory effects of mixed nitrophenols, which are frequently found in pharmaceutical and petrochemical effluents.

Biological-based control strategies for MBR membrane biofouling: a review

Membrane bioreactor (MBR) technology has been paid extensive attention for wastewater treatment because of its advantages of high effluent quality and minimized occupation space and sludge production. However, the membrane fouling is always an inevitable problem, which causes high operation and maintenance costs and prevents the wide use of MBR technology. The membrane biofouling is the most complicated and has relatively slow progress among all types of fouling. In recent years, many membrane biofouling control methods have been developed. Different from the physical or chemical methods, the biological-based strategies are not only more effective for membrane biofouling control, but also milder and more environment-friendly and, therefore, have been increasingly employed. This paper mainly focuses on the mechanism, unique advantages and development of biological-based control strategies for MBR membrane biofouling such as quorum quenching, uncoupling, flocculants and so on. The paper summarizes the up-to-date development of membrane biofouling control strategies, emphasizes the advantages and promising potential of biological-based ones, and points out the direction for future studies.

Rhodococcus wratislaviensis strain 9: An efficient p-nitrophenol degrader with a great potential for bioremediation

A Gram-positive bacterium, Rhodococcus wratislaviensis strain 9, was isolated from groundwater contaminated with nitrophenolics and trichloroethene following enrichment culture technique. The cells of strain 9 grown on LB broth (uninduced) degraded 720 μM p-nitrophenol (PNP) within 12 h, and utilized as a source of carbon and energy. Orthogonal experimental design analysis to determine optimal conditions for biodegradation of PNP showed that pH had a significant positive effect (P ≤ .05) on bacterial degradation of PNP, while glucose, di- and tri-nitrophenols exhibited significant negative effect. Cell-free extracts obtained from PNP-grown culture that contained 20 μg mL^-1 protein degraded 90% of 720 μM PNP within 5 h of incubation. Two-dimensional protein analysis revealed differential expression of the oxygenase component of PNP monooxygenase and an elongation factor Tu in PNP-grown cells, but not in those grown on glucose. The strain 9 remediated laboratory wastewater containing 900 μM PNP efficiently within 14 h, indicating its great potential in bioremediation of PNP-contaminated waters.

Exploiting extracellular polymeric substances (EPS) controlling strategies for performance enhancement of biological wastewater treatments: An overview

Extracellular polymeric substances (EPS) are present both outside of the cells and in the interior of microbial aggregates, and account for a main component in microbial aggregates. EPS can influence the properties and functions of microbial aggregates in biological wastewater treatment systems, and specifically EPS are involved in biofilm formation and stability, sludge behaviors as well as sequencing batch reactors (SBRs) granulation whereas they are also responsible for membrane fouling in membrane bioreactors (MBRs). EPS exhibit dual roles in biological wastewater treatments, and hence the control of available EPS can be expected to lead to changes in microbial aggregate properties, thereby improving system performance. In this review, current updated knowledge with regard to EPS basics including their formation mechanisms, important properties, key component functions as well as sub-fraction differentiation is given. EPS roles in biological wastewater treatments are also briefly summarized. Special emphasis is laid on EPS controlling strategies which would have the great potential in promoting microbial aggregates performance and in alleviating membrane fouling, including limitation strategies (inhibition of quorum sensing (QS) systems, regulation of environmental conditions, enzymatic degradation of key components, energy uncoupling etc.) and elevation strategies (enhancement of QS systems, addition of exogenous agents etc.). Those strategies have been confirmed to be feasible and promising to enhance system performance, and they would be a research niche that deserves further study.

Application of pseudo-emulsion-based hollow fiber strip dispersion for the extraction of p-nitrophenol from aqueous solutions

The extraction of p-nitrophenol (PNP) from aqueous solutions through a pseudo-emulsion hollow fiber strip dispersion (PEHFSD) system was conducted in a microporous hydrophobic polypropylene hollow fiber membrane contactor. For the optimization of the process variables, face-centered central composite design (FCCD) has been used. It was observed that initial feed concentration, carrier composition and stripping phase concentration were the three FCCD factors, which influenced the nitrophenol extraction. Using the optimized process conditions for the separation of PNP, experiments were also performed for the separation of other nitrophenols through PEHFSD system. By the FCCD design and analysis, almost 99% extraction of all three nitrophenols was achieved at optimum conditions. A mass transfer model was also developed and aqueous and membrane resistances were evaluated as 196.46 s cm(-1) and 50.14 s cm(-1), respectively.

Enhanced p-nitrophenol removal in a membrane-free bio-contact coupled bioelectrochemical system

In this study, a membrane-free bio-contact coupled bioelectrochemical system (BC-BES) was established for the enhanced reductive transformation of p-nitrophenol (PNP).

Changes in wastewater treatment performance and activated sludge properties of a membrane bioreactor at low temperature operation

The membrane bioreactor (MBR) activated sludge process is being applied more and more for wastewater treatment due to its high treatment efficiency and low space requirement. However, the usefulness of the MBR process in low-temperature zones is less studied than that under normal conditions. This study determined the effect of low temperature (∼13 °C) operation on MBR performance and activated sludge characteristics. When the wastewater temperature decreased from 22 °C to 13 °C, the average effluent COD concentration increased from (10 ± 5) to (25 ± 4) mg L(-1) and the nitrogen removal efficiency appeared not to be affected. The abundance and diversity of nitrifying bacteria such as Nitrosospira (ammonia-oxidizing bacteria) and Nitrospira (nitrite-oxidizing bacteria) in the activated sludge were reduced under low temperature exposure. The total biomass concentration decreased from about 10 000 mg COD L(-1) at room temperature to 8200 mg COD L(-1) at 13 °C at the same solid retention time. Furthermore, the sludge became bulking at 13 °C with a significant increase in the sludge volume index. The resultant sludge bulking was accompanied by accelerated membrane fouling resulting in a two-fold increase in the frequency of membrane cleaning. The results suggest that the performance of the MBR activated sludge process deteriorated at low wastewater temperatures even though the effluent water quality was still good enough for its applications in low temperature zones.

Influences of solid retention time, nitrification and microbial activity on the attenuation of pharmaceuticals and estrogens in membrane bioreactors

This study investigated the influences of solid retention time (SRT), nitrification, and microbial activity on the attenuation of pharmaceuticals and estrogens and the total estrogenic activity, using identical bench-scale membrane bioreactors. Phenacetine, acetaminophen, pentoxifylline, caffeine, bezafibrate, ibuprofen, fenoprofen, 17β-estradiol, and estrone were effectively attenuated even at short SRT (8 d). However, the attenuation efficiencies of gemfibrozil, ketoprofen, clofibric acid, and 17α-ethinylestradiol were dependent upon SRTs (20 and 80 d). Some acidic pharmaceuticals (gemfibrozil, diclofenac, bezafibrate, and ketoprofen) and 17α-ethinylestradiol were partially degraded by nitrification. Relatively high removal efficiencies were observed for 17β-estradiol and estrone (natural estrogens) compared to 17α-ethinylestradiol (synthetic estrogen) when nitrification was inhibited. Most of selected pharmaceuticals were not significantly attenuated under presumably abiotic conditions by adding sodium azide except phenacetine, acetaminophen, and caffeine. In this study, carbamazepine was found to be recalcitrant to biological wastewater treatment using membrane bioreactors regardless of the change of SRTs and microbial activity.