Fate and behavior of dissolved organic matter in a submerged anoxic-aerobic membrane bioreactor (MBR)

In this study, the production, composition, and characteristics of dissolved organic matter (DOM) in an anoxic-aerobic submerged membrane bioreactor (MBR) were investigated. The average concentrations of proteins and carbohydrates in the MBR aerobic stage were 3.96 ± 0.28 and 8.36 ± 0.89 mg/L, respectively. After membrane filtration, these values decreased to 2.9 ± 0.2 and 2.8 ± 0.2 mg/L, respectively. High performance size exclusion chromatograph (HP-SEC) analysis indicated a bimodal molecular weight (MW) distribution of DOMs, and that the intensities of all the peaks were reduced in the MBR effluent compared to the influent. Three-dimensional fluorescence excitation emission matrix (FEEM) indicated that fulvic and humic acid-like substances were the predominant DOMs in biological treatment processes. Precise identification and characterization of low-MW DOMs was carried out using gas chromatography-mass spectrometry (GC-MS). The GC-MS analysis indicated that the highest peak numbers (170) were found in the anoxic stage, and 54 (32%) compounds were identified with a similarity greater than 80%. Alkanes (28), esters (11), and aromatics (7) were the main compounds detected. DOMs exhibited both biodegradable and recalcitrant characteristics. There were noticeable differences in the low-MW DOMs present down the treatment process train in terms of numbers, concentrations, molecular weight, biodegradability, and recalcitrance

Formation mechanisms and control of biofouling in submerged MBRs

Membrane bioreactors (MBRs) are increasingly being applied in modern wastewater treatment plants. Biofouling of the MBRs represents a significant challenge in the application of membrane based technologies for water purification. Biofouling is the build-up of organic and biological cake-layers on the membranes that may block the membrane pores. The biofouling layers will reduce membrane permeability and increase the hydraulic resistance of the membrane. Thus, membrane fouling results in an increase of the trans-membrane pressure (TMP) when operated at a constant flux or a decreased flux when operated at a constant pressure. Therefore, it is necessary to identify the relative contribution of microbes and macromolecules to the biofouling process in order to develop appropriate strategies to reduce fouling and hence increase operational efficiency. Two identical laboratory-scale MBRs were operated at a low, constant flux (13 - 15 LMH) to treat artificial synthetic wastewater (TOC of 200 mg/L). The TMP was maintained at a low pressure (3 - 15 kPa), steady state for the first 80 - 87 d of operation and then was observed to increase exponentially from 15 to 90 kPa over 30 d. The biofouling layers on the hollow fiber membrane surfaces were observed to contain significant amounts of α-polysaccharides, β-polysaccharides, proteins and microorganisms that were always present on the hollow fiber membranes, even during the early stages of MBR operation when the TMP was still in the low pressure phase (Chapter 2). Quantitative image analysis indicated that each of these components on membrane correlated positively with the TMP increase, Pearson’s correlation coefficients 0.7 - 0.95. Among the four components, the proteins increased fastest when the TMP was rapidly increasing and comprised the greatest proportion of the individual components when TMP increased. This indicated that the production of proteins was more important than the two types of polysaccharides or the cells during the transition of TMP from the low to high TMP stage. Additionally, co-localization analysis revealed that approximately 50% of the EPS co-localized with 80 - 90% of the cells. The co-localization data indicated that the majority of the EPS components were closely associated with the cells, suggesting that the EPS components may be the byproducts of microorganisms on membrane rather than being randomly distributed on the membrane from the aqueous phase. Therefore, the formation of microbial biofilms on the membranes is the key driver of the biofouling process in MBRs. Thus, it is vital to develop methods to prohibit or reduce biofilm formation on the membranes in situ or to disperse the mature biofilms. As part of the development of novel strategies to control biofilms on membranes, it is also essential to determine if the biofilm is formed by a selected subset of microorganisms in the sludge community or if biofilm formation is a stochastic process. Therefore, the microbial biofilm community, including bacteria (Chapter 3) and fungi (Chapter 4), were investigated through 16S and ITS tagged pyrosequencing at different stages of the fouling process. At low TMP, the biofilms were most highly similar to the sludge and as the biofilm developed and the TMP increased, the biofilm communities diverged from the sludge. Ultimately the biofilm community appeared to be distinct from the sludge and the greatest differences were seen for the bacterial community. In contrast, the fungal communities were overall less diverse and showed only minor differences in relation to the sludge. This was mostly seen in subtle shifts in the percentage composition of individuals, while the community was still dominated by the same groups. It was noted that the correation between the richness of biofilm bacterial community and TMP increase was not evident in this study. Compared to the sludge community, the bacteria including Burkholderiales, Pseudomonadales and Rhizobiales and fungi including the Archaeosporales and Hypocreales were enriched in the biofilm, indicating these microorganisms were more fit when growing as biofilm compositions relative to the growth and competition in the planktonic sludge. Additionally, during the process of TMP increase, the Alphaproteobacteria, represented by Rhodospirillales, Sphingomonadales and Rhizobiales in this project, and the fungi including Saccharomycetales and Hypocreales became more dominant in the late stage biofilms, indicating these organisms may contribute more to the construction of late rather the early biofilm and may play an important role to the TMP increase in the biofouling process. These results indicated that the change of microbial community that occurred before the TMP jump may be the most important in its effect on biofouling process. It may be possible to target those organisms to ultimately delay their incorporation into the biofilm and hence delay the TMP jump. Finally, strategies based on nitric oxide (NO) induced biofilm dispersal were tested to control biofilm formation and TMP increase in the MBR (Chapter 5). The potential for NO to control biofilms in MBR systems was tested using two distinct approaches. The first was to disperse pre-established, mature biofilms that had developed during MBR operation and the second was to prevent biofilm accumulation by applying the NO from the beginning of the MBR operation. The results showed that treatment using the NO donor PROLI NONOate resulted in a 50% (in pre-established biofilm dispersal experiment) and 28.2% (in the biofilm prevention experiment) reduction of fouling resistance. The CLSM analysis showed that, in the biofilm prevention experiment, the NO treatment also resulted in a reduction of biofilm biomass, for both cells (66.7% reduction) as well as macromolecules (e.g. 37.7% reduction for proteins). Analysis of the community composition indicated that the bacterial Orders of Thiotrichales, Gemmatimonadales and Xanthomonadales and fungal Orders of Hypocreales and Glomerales were reduced in abundance after the PROLI NONOate treatment. Furthermore, the development of the community associated with the late stage biofilm was delayed, but not entirely prevented, as a consequence of the PROLI treatment. These results demonstrated that the NO donor PROLI NONOate had the potential to control biofouling in MBRs.DOCTOR OF PHILOSOPHY (SBS

Diversity of culturable aerobic denitrifying bacteria in the sediment, water and biofilms in Liangshui River of Beijing, China

Aerobic denitrification is a process reducing the nitrate into gaseous nitrogen forms in the presence of oxygen gas, which makes the nitrification and denitrification performed simultaneously. However, little was known on the diversity of the culturable aerobic denitrifying bacteria in the surface water system. In this study, 116 strains of aerobic denitrifying bacteria were isolated from the sediment, water and biofilm samples in Liangshui River of Beijing. These bacteria were classified into 14 genera based on the 16 S rDNA, such as Pseudomonas, Rheinheimera, and Gemmobacter. The Pseudomonas sp., represented by the Pseudomonas stutzeri, Pseudomonas mendocina and Pseudomonas putida, composed the major culturable aerobic denitrifiers of the river, followed by Ochrobactrum sp. and Rheinheimera sp. The PCA plot showed the unclassified Pseudomonas sp. and Rheinheimera pacifica preferred to inhabit in biofilm phase while one unclassified Ochrobactrum sp. and Pseudomonas resinovorans had higher abundance in the sediment. In the overlying water, the Pseudomonas stutzeri and Ochrobactrum rhizosphaerae were found to have higher abundance, indicating these aerobic denitrifiers had different habitat-preferable characteristics among the 3 phases of river system. The findings may help select the niche to isolate the aerobic denitrifiers and facilitate the bioaugmentation-based purification of the nitrate polluted surface water.Published versio

Identification of the WRKY Gene Family and Characterization of Stress-Responsive Genes in Taraxacum kok-saghyz Rodin

WRKY transcription factors present unusual research value because of their critical roles in plant physiological processes and stress responses. Taraxacum kok-saghyz Rodin (TKS) is a perennial herb of dandelion in the Asteraceae family. However, the research on TKS WRKY TFs is limited. In this study, 72 TKS WRKY TFs were identified and named. Further comparison of the core motifs and the structure of the WRKY motif was analyzed. These TFs were divided into three groups through phylogenetic analysis. Genes in the same group of TkWRKY usually exhibit a similar exon-intron structure and motif composition. In addition, virtually all the TKS WRKY genes contained several cis-elements related to stress response. Expression profiling of the TkWRKY genes was assessed using transcriptome data sets and Real-Time RT-PCR data in tissues during physiological development, under abiotic stress and hormonal treatments. For instance, the TkWRKY18, TkWRKY23, and TkWRKY38 genes were significantly upregulated during cold stress, whereas the TkWRKY21 gene was upregulated under heat-stress conditions. These results could provide a basis for further studies on the function of the TKS WRKY gene family and genetic amelioration of TKS germplasm

“Sea Anemone”-like CeFe Oxides for High-Efficient Phosphate Removal

The excessive release of phosphorus is a prime culprit for eutrophication and algal bloom in the aquatic environment, and there is always an urgent need to develop effective methods to deal with phosphorus pollution. Ce-based oxide is a type of compelling adsorbent for phosphate removal, and a self-templating strategy is used to construct high-performance Ce-based oxides for phosphate adsorption in this study. A “sea anemone”-like CeFe cyanometallate (CM) with a 3D microstructure is fabricated to provide a precursor for synthesizing CeFe-based oxides (CeFe-CM-T) by high-temperature pyrolysis. The as-prepared CeFe-CM-T maintains the “sea anemone” morphology well and has abundant micropores/mesopores, which render its superior phosphate adsorption capacity 1~2 orders of magnitude higher than that of the commercial CeO2 and Fe3O4 materials. Moreover, CeFe-CM-T shows high selectivity for phosphate removal when it co-exists with other anions and natural organic matter and exhibits excellent recycling performance. It demonstrates that both Ce3+ and Ce4+ are reserved in the oxides, where Ce3+ serves as the main active site for phosphate capture, which forms stable Ce-PO4 compounds via a ligand-exchange mechanism. Thus, the self-templating strategy using CM as a precursor is a potential method for synthesizing porous Ce-based oxides for phosphate removal

“Sea Anemone”-like CeFe Oxides for High-Efficient Phosphate Removal

The excessive release of phosphorus is a prime culprit for eutrophication and algal bloom in the aquatic environment, and there is always an urgent need to develop effective methods to deal with phosphorus pollution. Ce-based oxide is a type of compelling adsorbent for phosphate removal, and a self-templating strategy is used to construct high-performance Ce-based oxides for phosphate adsorption in this study. A “sea anemone”-like CeFe cyanometallate (CM) with a 3D microstructure is fabricated to provide a precursor for synthesizing CeFe-based oxides (CeFe-CM-T) by high-temperature pyrolysis. The as-prepared CeFe-CM-T maintains the “sea anemone” morphology well and has abundant micropores/mesopores, which render its superior phosphate adsorption capacity 1~2 orders of magnitude higher than that of the commercial CeO2 and Fe3O4 materials. Moreover, CeFe-CM-T shows high selectivity for phosphate removal when it co-exists with other anions and natural organic matter and exhibits excellent recycling performance. It demonstrates that both Ce3+ and Ce4+ are reserved in the oxides, where Ce3+ serves as the main active site for phosphate capture, which forms stable Ce-PO4 compounds via a ligand-exchange mechanism. Thus, the self-templating strategy using CM as a precursor is a potential method for synthesizing porous Ce-based oxides for phosphate removal

Characterization of the archaeal community fouling a membrane bioreactor

Biofilm formation, one of the primary causes of biofouling, results in reduced membrane flux or increased transmembrane pressure and thus represents a major impediment to the wider implementation of membrane bioreactor (MBR) technologies for water purification. Most studies have focused on the role of bacteria in membrane fouling as they are the most dominant and best studied organisms present in the MBR. In contrast, there is limited information on the role of the archaeal community in biofilm formation in MBRs. This study investigated the composition of the archaeal community during the process of biofouling in an MBR. The archaeal community was observed to have lower richness and diversity in the biofilm than the sludge during the establishment of biofilms at low transmembrane pressure (TMP). Clustering of the communities based on the Bray–Curtis similarity matrix indicated that a subset of the sludge archaeal community formed the initial biofilms. The archaeal community in the biofilm was mainly composed of Thermoprotei, Thermoplasmata, Thermococci, Methanopyri, Methanomicrobia and Halobacteria. Among them, the Thermoprotei and Thermoplasmata were present at higher relative proportions in the biofilms than they were in the sludge. Additionally, the Thermoprotei, Thermoplasmata and Thermococci were the dominant organisms detected in the initial biofilms at low TMP, while as the TMP increased, the Methanopyri, Methanomicrobia, Aciduliprofundum and Halobacteria were present at higher abundances in the biofilms at high TMP.Published versio

Succession of the bacterial community in the biofilms across the increasing TMP profile.

<p>The numbers in the plots represent the TMP values (kPa) when the samples were collected. The branches indicate the major bacterial phylotypes that contributed to the community differentiation. The same direction of the samples and branches means the branch-pointed bacteria has higher abundance in the samples.</p