الفهرس 
Introduction and Literature ReviewOnly 14 pages are availabe for public view
 |  | from | 190 |  |  |
| | | from | 190 | | |
Abstract
The global demand for sustainable development, universal and equitable access to safe and affordable drinking water has garnered a pace of attention in the last two decades. According to World Health Organization (WHO) reports, at least two billion people globally use contaminated drinking water sources for drinking. Contaminated water transmits diseases such diarrhea, cholera, dysentery, typhoid, and polio. It is estimated that by 2025, half of the world’s population will be living in water-stressed areas.
Numerous wastewater treatment processes and techniques have been utilized to reduce water pollution and improve drinking water quality. Understanding the behavior of microbial communities in biological processes has been recently the focus of many research studies. Efforts have been made to understand the functional characterization of microorganisms associated with the removal of organics, inorganics and nutrients from wastewater bulking and foaming in activated sludge.
The application of electric field to the bioreactors induces electrochemical mechanisms such as electrocoagulation that help not only in municipal wastewater treatment but also in the degradation of pollutants and removal of toxic and hazardous compounds in landfill leachate and industrial wastewaters. Therefore, in this thesis we designed an electrobioreactor which integrates the electrochemical technologies with biological treatment taking into consideration the effects of electricity on bacterial viability in such systems.
The main objective of the present research thesis work is to develop and optimize a conceptual model for the functional microbial communities in wastewater treatment electro- bioreactors. The conceptual model proposed is based on 16S rRNA gene high-throughput sequencing data analysis and bioreactor efficiency. The collective data clearly demonstrated that applying various electric currents affected the microbial community composition and stability as well as the reactor efficiency in terms of soluble chemical oxygen demand (sCOD), nitrogen (N) and phosphorous (P) removals for better effluent water quality.
Evaluation of the optimum current density 3, 5 and 7 A/m2, hydraulic retention time (6 – 75 h) and other operating conditions such as dissolved oxygen, electrical conductivity, and temperature of the wastewater electrobioreactors were conducted and discussed. The integration of serial passaging technique approach to wastewater electrobioreactors and characterizing the functional microbial communities to enhance sludge settleability were proposed.
Based on previous research study conducted in our laboratory, it was investigated that the impact of applying continuous and intermittent electric field at different current densities ranging from 5 – 20 A/m2 affected the bacterial counts in the reactors. We found that low current densities increased viable bacterial cells, while bacterial counts started to decrease at current densities higher than 10 A/m2. These experiments allowed us to conclude that
limited direct current (DC) that may enhance bacterial metabolism and cell growth. Accordingly, the optimum current densities used were 3, 5 and 7 A/m2.
The obtained results are summarized in the following:
1- The electro-bioreactors operated at current density of 3, 5 and 7 A/m2 and short HRT of 6 hours resulted in:
High removal efficiency of sCOD (95-98%), PO43-P (99.9%), and NH4+ - N (99.9%) in the effluent in comparison to the removal efficiency (94%, 49%, and 64%, respectively) in the control bioreactors. These results agree with previous findings that there is a significant removal of phosphorous and ammonia at all selected CDs at short HRT.
Appearance of high concentrations of NO3-N in the effluent (20.3±0.7, 19.6±1.3 and 12.2±3.9 mg/L, respectively), with the lowest levels observed in the control bioreactor (4.9±0.0 mg/L). These results suggest that nitrification through oxidizing ammonia was taking place and approaching completion in all the batches tested.
Enrichment of operational taxonomic units (OTUs) belonging to distinct functional bacterial families such as (Nitrospiraceae: 8.5, 12.5 and 12.6% relative abundance, respectively) and (Rhodocyclaceae: 8.1, 8.8 and 9.7% relative abundance, respectively), leading to efficient N-removal (>98%) and P-removal (>98%).
A strong inverse relationship between current density and the abundance of Enterobacteriaceae was observed. The relative abundance was 5.3, 3.4 and 1.5% at 3, 5 and 7 A/m2, respectively. Abundance of Enterobacteriaceae in wastewater treatment plant effluents is an indicator of the degree of effluent quality.
BIO-ENV trend correlation analysis (using QIIME v 1.9.0) was performed to investigate what physiochemical or operating condition best correlated with the variability in bacterial abundance and diversity from each sample. The variables best explained weighted Unifrac distance between all samples were best correlated with incorporation of CD, PO43--P, NH4+- N and NO3--N (correlation = 0.6, 0.6, 0.485 and 0.284, respectively).
2- Operating the bioreactors and electro-bioreactors at short HRT (6, 10 and 16 h), long HRT (24, 50 and 75 h) and current density of 3 A/m2 resulted in:
HRT of 6, 24, 50 and 75 h resulted in high sCOD removal efficiency (96 – 98%) and lower removal (92%) at HRT 10 and 16 h. sCOD concentrations in the effluents were 79.1±0.9, 173±1.0, 177±1.0, 27.05±0.95, 58.95±1.85 and 70.15±0.35 mg/L at the HRT of 6, 10, 16, 24, 50 and 75 h, respectively. HRT of 6, 10 and 16 had 99% removal efficiency with PO43-
-P concentrations in the effluent of 0.02±0.0, 0.003±0.001 and 0.04±0.0 mg/L, respectively. The removal efficiency of NH4+ - N increased from 61.7% at HRT of 6 h to 99% at the other HRTs tested.
The analysis of the raw Illumina MiSeq sequencing data (using QIIME v 1.9.0) and β- diversity using UPGMA clustering revealed that the bacterial communities in the samples
could be clustered into two main groups containing: (i) Samples at long HRT of 24, 50 and 75 h. (ii) Samples at short HRT of 6, 10 and 16 h, followed by separation in the branches showed that the presence or absence of an electric current was the next most important factor.
Stimulating the growth of bacterial communities such as Lactococcus, Pseudomonas and Flavobacterium. Significant high relative abundance of those genera at HRT of 6 h (5.8, 4.3 and 3.7%, respectively) was detected. Nitrifying bacteria such as Nitrospira sp. were observed to be stimulated at HRT of 6, 10 and 16 h with relative abundance (21.5, 22.3 and 26.9%, respectively). Nitrospira have been recognized as the numerically dominant nitrite- oxidizing bacterial genus primarily responsible for the second step of aerobic nitrification.
These are the first results to describe effects of varying HRT on microbial community structure in wastewater electro-bioreactors.
3- The introduction of serial passaging, as a new approach to electro-bioreactors operated at 3 and 7 A/m2 over 15 days to enhance sludge settling and assessing the functional characterization of the microbial community structure resulted in:
The passaging days 9, 12 and 13 had the highest sCOD removal of 97% then slightly decreased to 91% by day 15, while the highest sCOD removal was 97 – 98% at CD of 3 and 7 A/m2 on days 12 and 15. The decline observed on day 15 could be due to the dispersion of the activated sludge flocs which causes turbidity resulting in high sCOD concentration in the effluent; these results are in agreement with previous findings which confirms that sludge settling is an important factor affecting organic removal. Efficient removal of PO43-
-P reaching 99% in all reactors during the whole serial passaging period.
pH was initially 9.2±0.2 in all reactors tested during the serial passaging period. At CDs of 3 and 7 A/m2, pH decreased by 11-14% to 8.2±0.3 and 7.9±0.2 after 5 days of passaging then by approximately 6% to 7.8±0.1 by day 9 then stabilized in the range (6.9±0.1 – 7.0±0.1) from day 12 to 15 at CDs of 3 and 7 A/m2.
Assessment of the microbial communities based on heatmaps were constructed which depends on functionally interesting bacterial families affiliated with sludge bulking and foaming (Comamonadaceae, Flavobacteriaceae, Verrucomicrobiaceaea and Pseudomonadaceae) and pathogenic microflora (Neisseriaceae, Enterobacteriaceae and Streptococcaceae), which clearly differentiated the behavior of these bacteria when it is serially passaged.
Comamonadaceae (22,959 ± 85 and 26,504 ± 105) was initially increased in abundance with highest OTU counts by day 9 at 3 A/m2 and day 12 at 7 A/m2 then decreased and stabilized. Flavobacteriaceae presented with highest OTU counts (3527 ± 20 and 538 ± 7) by day 9 at 3 and 7 A/m2, respectively. Verrucomicrobiaceae and Streptococcaceae were depleted through the 15-days period while Pseudomonadaceae had the highest OTU count
(4847 ± 28 and 11,301 ± 40, respectively) by day 6 then depleted by the end of serial passaging at 3 and 7 A/m2.
This approach represented a strategy that can be employed to solve sludge bulking and foaming problems, leading to improvement in sludge settleability and a more efficient wastewater treatment process. These results are the first to describe the effects of current density on microbial community structures in serially passaged sludge.
4- The conceptual model based on 16S rRNA gene was developed and demonstrated a simplified representation of the behavior of microbial communities in electro- bioreactors operated at different CDs (3, 5, and 7 A/m2 – represented as 3-EBR, 5-EBR and 7-EBR, respectively) and HRTs (short: 6,10 and 16 h; and long: 24,50 and 75 h – represented as EBRshort and EBRlong, respectively) was proposed. The conceptual genetic model developed here in this thesis is based on all the bioinformatics, and statistically significant data analyzed. It displays correlations and relationships for how bioreactors and electrobioreactors operate and includes the substrates to be removed from wastewater and the associated distinct functional bacteria. The model illustrated briefly for example the following points:
Illustration of the enrichment of functional bacterial species Neisseriaceae and sCOD removal was high (95-98%) in both 3-EBR and 7-EBR. Pseudomonas species were depleted in 7-EBR and enriched in 3-EBRlong. The correlation of Flavobacterium, Lactococcus and Vogesella enrichment in 3-EBRshort is presented. P removal (97%) was directly correlated with the abundance of Rhodocyclaceae, Dechloromonas, Rhodobacteraceae and Rhodobacter.
A clarification of certain types of bacterial such as Sulfur reducing bacteria, Thiobacillus and Thiobacteriales were presented. Thiobacillus species were enriched in 3-EBR and 7- EBR, while Thiobacteriales favored 7-EBR only. Both species favored 3-EBRshort operated at HRT of 6 h.
Antibiotic resistance associated bacterial species Enterobacter were depleted except for 3- EBRlong while beneficial bacterial species Bdellovibrio were enriched at all short HRTs. It is possible that enrichment of Bdellovibrio, which are predators of Gram-negative microbes, might explain the decrease in Enterobacter species, but that cannot be determined conclusively from the data collected.
This model is a first attempt at analyzing data from our results and other previous studies to provide guidance for future studies of the structure and function of microbial communities exposed to electric field.
The thesis research results suggested a hypothetical illustration of the assumed mechanisms of nutrient removal in electro-bioreactors. A schematic illustration of manipulation of the operating conditions that are tailored to favor functional bacteria relevant to specific types of wastewater is proposed.