Presence and activity of ammonia-oxidising bacteria detected amongst the overall bacterial diversity along a physico-chemical gradient of a nitrifying wastewater treatment plant

Wastewater microbial diversity versus molecular analysis at a glance: a mini-review

Microorganisms play a vital role in biological wastewater treatment by converting organic and toxic materials into harmless substances. Understanding microbial communities' structure, taxonomy, phylogeny, and metabolic activities is essential to improve these processes. Molecular microbial ecology employs molecular techniques to study community profiles and phylogenetic information since culture-dependent approaches have limitations in providing a comprehensive understanding of microbial diversity in a system. Genomic advancements such as DNA hybridization, microarray analysis, sequencing, and reverse sample genome probing have enabled the detailed characterization of microbial communities in wastewater treatment facilities. This mini-review summarizes the current state of knowledge on the diversity of microorganisms in wastewater treatment plants, emphasizing critical microbial processes such as nitrogen and phosphorus removal.

Effects of ammonia nitrogen and organic carbon availability on microbial community structure and ecological interactions in a full-scale partial nitritation and anammox (PN/A) system

Nutrient availability significantly impacts microbial biosynthesis, cell growth, and cell cycle progression. In this study, a full-scale plug-flow partial nitritation/anammox (PN/A) system was used to investigate variations in the microbial community structure in both immobilized carriers and flocs, as well as a gradual decrease in nutrient availability from upstream to downstream. We found that reduced ammonia nitrogen (from 150.4 to 30.6 mg/L) and organic carbon (from 415.7 to 342.8 mg/L) availability significantly lowered microbial diversity and altered microbial communities in biofilms other than flocs from upstream to downstream. The abundance of all anammox bacteria increased by 1.97 times, from 3.25 × 1010 to 6.40 × 1010 copies per gram of wet sludge, in the biofilm core microbiome. Furthermore, from upstream to downstream, taxa with lower ribosomal RNA operon copy numbers were consistently enriched in both biofilm and floc communities, indicating that slow-growing microorganisms are more likely to be enriched in low-nutrient environments. Rare taxa with a relative abundance of less than 0.1% exhibited unique metabolic functions, including amino acid, carbohydrate, cofactor, and vitamin metabolisms, which was inferred by PICRUST2 and persisted across the nutrient gradient in both the biofilm and floc communities. Despite their low abundance, they may play important roles in mediating the stability and function of the PN/A system. Overall, the results demonstrate the impact of a naturally formed ammonia nitrogen and organic carbon gradient in a full-scale plug-flow PN/A installation on nutrient availability and its effects on microbial diversity, community composition, and microbial interactions, which expands our fundamental understanding of this energy-efficient and promising biotechnology for treating high-strength ammonium wastewater.

Use of functional gene expression and respirometry to study wastewater nitrification activity after exposure to low doses of copper

Autotrophic nitrification in biological nitrogen removal systems has been shown to be sensitive to the presence of heavy metals in wastewater treatment plants. Using transcriptase-quantitative polymerase chain reaction (RT-qPCR) data, we examined the effect of copper on the relative expression of functional genes (i.e., amoA, hao, nirK, and norB) involved in redox nitrogen transformation in batch enrichment cultures obtained from a nitrifying bioreactor operated as a continuous reactor (24-h hydraulic retention time). 16S ribosomal RNA (rRNA) gene next-generation sequencing showed that Nitrosomonas-like populations represented 60-70% of the bacterial community, while other nitrifiers represented <5%. We observed a strong correspondence between the relative expression of amoA and hao and ammonia removal in the bioreactor. There were no considerable changes in the transcript levels of amoA, hao, nirK, and norB for nitrifying samples exposed to copper dosages ranging from 0.01 to 10 mg/L for a period of 12 h. Similar results were obtained when ammonia oxidation activity was measured via specific oxygen uptake rate (sOUR). The lack of nitrification inhibition by copper at doses lower than 10 mg/L may be attributed to the role of copper as cofactor for ammonia monooxygenase or to the sub-inhibitory concentrations of copper used in this study. Overall, these results demonstrate the use of molecular methods combined with conventional respirometry assays to better understand the response of wastewater nitrifying systems to the presence of copper.

Quantitative response of nitrifying and denitrifying communities to environmental variables in a full-scale membrane bioreactor

The abundance and transcription levels of specific gene markers of total bacteria, ammonia-oxidizing Betaproteobacteria, nitrite-oxidizing bacteria (Nitrospira-like) and denitrifiers (N2O-reducers) were analyzed using quantitative PCR (qPCR) and reverse-transcription qPCR during 9 months in a full-scale membrane bioreactor treating urban wastewater. A stable community of N-removal key players was developed; however, the abundance of active populations experienced sharper shifts, demonstrating their fast adaptation to changing conditions. Despite constituting a small percentage of the total bacterial community, the larger abundances of active populations of nitrifiers explained the high N-removal accomplished by the MBR. Multivariate analyses revealed that temperature, accumulation of volatile suspended solids in the sludge, BOD5, NH4(+) concentration and C/N ratio of the wastewater contributed significantly (23-38%) to explain changes in the abundance of nitrifiers and denitrifiers. However, each targeted group showed different responses to shifts in these parameters, evidencing the complexity of the balance among them for successful biological N-removal.

Exploring the links between population dynamics of total and active bacteria and the variables influencing a full-scale membrane bioreactor (MBR)

Long-term dynamics of total and active bacterial populations in a full-scale membrane bioreactor (MBR) treating urban wastewater were monitored during nine months by temperature-gradient gel electrophoresis (TGGE) of partial 16S-rRNA genes, amplified from community DNA and RNA templates. The bacterial community, dominated by Alphaproteobacteria, displayed the required characteristics for a successful and steady contaminant removal under real operating conditions. The evolution of population dynamics showed that a fully-stable microbial community was not developed even after technical stabilization and steady performance of the MBR were achieved. Non-metric multidimensional scaling and BIO-ENV demonstrated that the trends of the populations were often mostly explained by temperature, followed by the concentration of volatile suspended solids and C/N ratio of the influent. These variables were mainly responsible for triggering the shifts between functionally redundant populations. These conclusions contribute to the prediction of the complex profiles of adaptation and response of bacterial populations under changing conditions.

Reverse-transcriptional gene expression of anammox and ammonia-oxidizing archaea and bacteria in soybean and rice paddy soils of Northeast China

The relative gene expression of hydrazine oxidoreductase encoding gene (hzo) for anaerobic ammonium oxidizing bacteria (anammox) and ammonia monooxygenase encoding gene (amoA) for both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in Sanjiang Plain soybean and rice paddy soils of Northeast China was investigated by using real-time reverse-transcriptional quantitative PCR. Metabolically active populations of anammox, AOA, and AOB in rice paddy soils were evident by the presence and successful quantification of hzo mRNA and amoA mRNA genes. The expression ratio of amoA gene for both AOA and AOB varied between soybean soils and different rice paddy soils while the expression of hzo gene for anammox was detectable only in rice paddy soils by showing a diverse relative expression ratio in each soil sample. Gene expression of both archaeal and bacterial amoA genes in rice paddy soils differed among the three sampling depths, but that of hzo was not. Both archaeal and bacterial amoA genes showed an increase trend of expression level with continuation of rice paddy cultivation, but the low expression ratio of hzo gene indicated a relatively small contribution of anammox in overall removal of inorganic nitrogen through N2 even under anoxic and high nitrogen input in agriculture. Bacterial amoA gene from two soybean fields and three rice paddy fields were also analyzed for community composition by denaturing gradient gel electrophoresis fingerprint. Community shift was observed between soybean and paddy fields and within each of them. The consistent occurrence of three bands 5, 6, and 7 in all samples showed their high adaptability for both arid cultivation and continuous rice paddy cultivation. Our data suggest that AOA and AOB are playing a more important role in nitrogen transformation in agricultural soils in oxic or anoxic environment and anammox bacteria may also contribute but in a less extent to N transformation in these agricultural soils under anoxic condition.

Low-dissolved-oxygen nitrifying systems exploit ammonia-oxidizing bacteria with unusually high yields

In wastewater treatment plants, nitrifying systems are usually operated with elevated levels of aeration to avoid nitrification failures. This approach contributes significantly to operational costs and the carbon footprint of nitrifying wastewater treatment processes. In this study, we tested the effect of aeration rate on nitrification by correlating ammonia oxidation rates with the structure of the ammonia-oxidizing bacterial (AOB) community and AOB abundance in four parallel continuous-flow reactors operated for 43 days. Two of the reactors were supplied with a constant airflow rate of 0.1 liter/min, while in the other two units the airflow rate was fixed at 4 liters/min. Complete nitrification was achieved in all configurations, though the dissolved oxygen (DO) concentration was only 0.5 ± 0.3 mg/liter in the low-aeration units. The data suggest that efficient performance in the low-DO units resulted from elevated AOB levels in the reactors and/or putative development of a mixotrophic AOB community. Denaturing gel electrophoresis and cloning of AOB 16S rRNA gene fragments followed by sequencing revealed that the AOB community in the low-DO systems was a subset of the community in the high-DO systems. However, in both configurations the dominant species belonged to the Nitrosomonas oligotropha lineage. Overall, the results demonstrated that complete nitrification can be achieved at low aeration in lab-scale reactors. If these findings could be extended to full-scale plants, it would be possible to minimize the operational costs and greenhouse gas emissions without risk of nitrification failure.

Kinetic parameters and inhibition response of ammonia- and nitrite-oxidizing bacteria in membrane bioreactors and conventional activated sludge processes

Ammonium and nitrite oxidizing biomasses (AOB and NOB) were investigated in parallel pilot plants: a membrane bioreactor (MBR) and a conventional activated sludge process (CASP) fed with domestic wastewater. The kinetics of AOB and NOB were monitored through titrimetric tests. The maximum specific growth rate of the AOB (micro(max,AOB)) was affected by the solids' retention time (SRT) maintained during the start up: by varying the start up SRT from 20 d to 8 d, micro(max,AOB) in the CASP varied from 0.45 d(-1) +/- 0.04 to 0.72 d(-1) +/- 0.2 respectively; the mean value of micro(max,AOB) in the MBR samples (always maintained at SRT = 20 d) was in the range 0.45-0.49 d(-1). The endogenous decay coefficients of the NOB and AOB and the maximum specific growth rates of the NOB were similar in both MBR and CASP. Inhibition tests with different concentrations of allylthiourea (ATU) were carried out on samples from both activated sludge systems: the MBR sludge exhibited higher sensitivity to a low ATU concentration; however, the maximum nitrification activity recovered more rapidly than the CASP sludge.

A full-scale biological treatment system application in the treated wastewater of pharmaceutical industrial park

A full-scale combined biological system is used for the treatment of treated wastewater discharged from a pharmaceutical industrial park. This treated water is rich in NH(4)(+)-N (average in 86.4 mg/L), low in COD/NH(4)(+)-N (average in 3.4) and low in BOD(5)/COD ratio (average in 0.24) with pH varying from 7.16 to 7.78. The final effluent of the combined treatment process was stably below 100mg/L COD and 20mg/L NH(4)(+)-N, separately, with organic loading rate of 4954 kg COD/d and 92.5 kg NH(4)(+)-N/d. It is found that the BOD(5)/COD ratio could be raised from 0.24 to 0.35, and the production of total VFAs account for 9.57% of the total COD via the treatment of hydrolysis/acidification. MBBR and oxidation ditch represent 35.4% and 60.7% of NH(4)(+)-N removal, 30.2% and 61.5% of COD removal, separately, of the total treatment process. PCR-DGGE is used for microbial community analysis of MBBR and oxidation ditch.

The application of molecular techniques to the study of wastewater treatment systems

Wastewater treatment systems tend to be engineered to select for a few functional microbial groups that may be organized in various spatial structures such as activated sludge flocs, biofilm or granules and represented by single coherent phylogenic groups such as ammonia-oxidizing bacteria (AOB) and polyphosphate-accumulating organisms (PAO). In order to monitor and control engineered microbial structure in wastewater treatment systems, it is necessary to understand the relationships between the microbial community structure and the process performance. This review focuses on bacterial communities in wastewater treatment processes, the quantity of microorganisms and structure of microbial consortia in wastewater treatment bioreactors. The review shows that the application of molecular techniques in studies of engineered environmental systems has increased our insight into the vast diversity and interaction of microorganisms present in wastewater treatment systems.