Emerging Models of Nitrogen and Carbon Cycling in Engineered Wastewater Treatment Processes

The engineered nitrogen cycle provides a rich framework to study the structure, function and interactions within mixed microbial communities. The knowledge objtained from such studies also allows us to harness the potential of such communities towards achieving multiple goals including the production of clean water, treatment of drinking water and the synthesis of commodity chemicals and fuels, among others. Within the spectrum of engineered nitrogen cycling processes, autotrophic biological nitrogen removal (BNR) offers an energy and resource efficient alternate to conventionally followed approaches. The successful implementation of autotrophic BNR processes is contingent upon the selective retention of aerobic and anaerobic ammonia oxidizing organisms over nitrite oxidizing organisms. While significant work has been conducted examining the microbial ecology, metabolism and modeling aspects relating to autotrophic BNR processes, the focus has mainly been on the nitrogen cycle. Discussions on organic and inorganic carbon (the preferred substrate for several communities in these processes) are somewhat uncommon. In this work, we first consider the impact of organic and inorganic carbon supply as a driver for interactions amongst different communities present in autotrophic nitrogen removal processes. The metabolic basis for some of these interactions is then evaluated based on a more fundamental look at select members of such communities. Further, the interplay between conventionaly understood protagonists of the microbial N-cycle and some newly discovered bacteria, including those catalyzing complete ammonia oxidation (comammox) is also discussed

Strategies of Nitrosomonas europaea 19718 to counter low dissolved oxygen and high nitrite concentrations

<p>Abstract</p> <p>Background</p> <p><it>Nitrosomonas europaea </it>is a widely studied chemolithoautotrophic ammonia oxidizing bacterium. While significant work exists on the ammonia oxidation pathway of <it>N. europaea</it>, its responses to factors such as dissolved oxygen limitation or sufficiency or exposure to high nitrite concentrations, particularly at the functional gene transcription level are relatively sparse. The principal goal of this study was to investigate responses at the whole-cell activity and gene transcript levels in <it>N. europaea </it>19718 batch cultures, which were cultivated at different dissolved oxygen and nitrite concentrations. Transcription of genes coding for principal metabolic pathways including ammonia oxidation (<it>amoA</it>), hydroxylamine oxidation (<it>hao</it>), nitrite reduction (<it>nirK</it>) and nitric oxide reduction (<it>norB</it>) were quantitatively measured during batch growth, at a range of DO concentrations (0.5, 1.5 and 3.0 mg O₂/L). Measurements were also conducted during growth at 1.5 mg O₂/L in the presence of 280 mg-N/L of externally added nitrite.</p> <p>Results</p> <p>Several wide ranging responses to DO limitation and nitrite toxicity were observed in <it>N. europaea </it>batch cultures. In contrast to our initial hypothesis, exponential phase mRNA concentrations of both <it>amoA </it>and <it>hao </it>increased with decreasing DO concentrations, suggesting a mechanism to metabolize ammonia and hydroxylamine more effectively under DO limitation. Batch growth in the presence of 280 mg nitrite-N/L resulted in elevated exponential phase <it>nirK </it>and <it>norB </it>mRNA concentrations, potentially to promote utilization of nitrite as an electron acceptor and to detoxify nitrite. This response was in keeping with our initial hypothesis and congruent with similar responses in heterotrophic denitrifying bacteria. Stationary phase responses were distinct from exponential phase responses in most cases, suggesting a strong impact of ammonia availability and metabolism on responses to DO limitation and nitrite toxicity. In general, whole-cell responses to DO limitation or nitrite toxicity, such as sOUR or nitrite reduction to nitric oxide (NO) did not parallel the corresponding mRNA (<it>nirK</it>) profiles, suggesting differences between the gene transcription and enzyme translation or activity levels.</p> <p>Conclusions</p> <p>The results of this study show that <it>N. europaea </it>possesses specific mechanisms to cope with growth under low DO concentrations and high nitrite concentrations. These mechanisms are additionally influenced by the physiological growth state of <it>N. europaea </it>cultures and are possibly geared to enable more efficient substrate utilization or nitrite detoxification.</p

Resilience and limitations of MFC anodic community when exposed to antibacterial agents

This study evaluates the fate of certain bactericidal agents introduced into microbial fuel cell (MFC) cascades and the response of the microbial community. We tested the response of functioning urine fed MFC cascades using two very different bactericidal agents: a common antibiotic (Ampicillin, 5 g/L) and a disinfectant (Chloroxylenol 4.8 g/L) in concentrations of up to 100 times higher than the usual dose. Results of power generation showed that the established bacteria community was able to withstand high concentrations of ampicillin with good recovery after 24 h of minor decline. However, power generation was adversely affected by the introduction of chloroxylenol, resulting in a 99% loss of power generation. Ampicillin was completely degraded within the MFC cascade (>99.99%), while chloroxylenol remained largely unaffected. Analysis of the microbial community before the addition of the bactericidal agents showed a significant bacterial diversity with at least 35 genera detected within the cascade. Microbial community analysis after ampicillin treatment showed the loss of a small number of bacterial communities and proportional fluctuations of specific strains within the individual MFCs community. On the other hand, there was a significant shift in the bacterial community after chloroxylenol treatment coupled with the loss of at least 13 bacterial genera across the cascade

Metatranscriptomic Investigation of Adaptation in NO and N2O Production From a Lab-Scale Nitrification Process Upon Repeated Exposure to Anoxic–Aerobic Cycling

The molecular mechanisms of microbial adaptation to repeated anoxic–aerobic cycling were investigated by integrating whole community gene expression (metatranscriptomics) and physiological responses, including the production of nitric (NO) and nitrous (N2O) oxides. Anoxic–aerobic cycling was imposed for 17 days in a lab-scale full-nitrification mixed culture system. Prior to cycling, NO and N2O levels were sustained at 0.097 ± 0.006 and 0.054 ± 0.019 ppmv, respectively. Once the anoxic–aerobic cycling was initiated, peak emissions were highest on the first day (9.8 and 1.3 ppmv, respectively). By the end of day 17, NO production returned to pre-cycling levels (a peak of 0.12 ± 0.007 ppmv), while N2O production reached a new baseline (a peak of 0.32 ± 0.05 ppmv), one order of magnitude higher than steady-state conditions. Concurrently, post-cycling transcription of norBQ and nosZ returned to pre-cycling levels after an initial 5.7- and 9.5-fold increase, while nirK remained significantly expressed (1.6-fold) for the duration of and after cycling conditions. The imbalance in nirK and nosZ mRNA abundance coupled with continuous conversion of NO to N2O might explain the elevated post-cycling baseline for N2O. Metatranscriptomic investigation notably indicated possible NO production by NOB under anoxic–aerobic cycling through a significant increase in nirK expression. Opposing effects on AOB (down-regulation) and NOB (up-regulation) CO2 fixation were observed, suggesting that nitrifying bacteria are differently impacted by anoxic–aerobic cycling. Genes encoding the terminal oxidase of the electron transport chain (ccoNP, coxBC) were the most significantly transcribed, highlighting a hitherto unexplored pathway to manage high electron fluxes resulting from increased ammonia oxidation rates, and leading to overall, increased NO and N2O production. In sum, this study identified underlying metabolic processes and mechanisms contributing to NO and N2O production through a systems-level interrogation, which revealed the differential ability of specific microbial groups to adapt to sustained operational conditions in engineered biological nitrogen removal processes

Development of efficient electroactive biofilm in urine-fed microbial fuel cell cascades for bioelectricity generation

The Microbial fuel cell (MFC) technology harnesses the potential of some naturally occurring bacteria for electricity generation. Digested sludge is commonly used as the inoculum to initiate the process. There are, however, health hazards and practical issues associated with the use of digested sludge depending on its origin as well as the location for system deployment. This work reports the development of an efficient electroactive bacterial community within ceramic-based MFCs fed with human urine in the absence of sludge inoculum. The results show the development of a uniform bacterial community with power output levels equal to or higher than those generated from MFCs inoculated with sludge. In this case, the power generation begins within 2 days of the experimental set-up, compared to about 5 days in some sludge-inoculated MFCs, thus significantly reducing the start-up time. The metagenomics analysis of the successfully formed electroactive biofilm (EAB) shows significant shifts between the microbial ecology of the feeding material (fresh urine) and the developed anodic biofilm. A total of 21 bacteria genera were detected in the urine feedstock whilst up to 35 different genera were recorded in the developed biofilm. Members of Pseudomonas (18%) and Anaerolineaceae (17%) dominate the bacterial community of the fresh urine feed while members of Burkholderiaceae (up to 50%) and Tissierella (up to 29%) dominate the anodic EAB. These results highlight a significant shift in the bacterial community of the feedstock towards a selection and adaptation required for the various electrochemical reactions essential for survival through power generation

Implementation and process analysis of pilot scale multi-phase anaerobic fermentation and digestion of faecal sludge in Ghana [version 1; referees: 2 approved]

Background.  In Ghana, faecal sludge (FS) from on-site sanitation facilities is often discharged untreated into the environment, leading to significant insults to environmental and human health. Anaerobic digestion offers an attractive pathway for FS treatment with the concomitant production of energy in the form of methane. Another innovative option includes separating digestion into acidogenesis (production of volatile fatty acids (VFA)) and methanogenesis (production of methane), which could ultimately facilitate the production of an array of biofuels and biochemicals from the VFA. This work describes the development, implementation and modeling based analysis of a novel multiphase anaerobic fermentation-digestion process aimed at FS treatment in Kumasi, Ghana.  Methods.  A pilot-scale anaerobic fermentation process was implemented at the Kumasi Metropolitan Assembly’s Oti Sanitary Landfill Site at Adanse Dompoase.  The process consisted of six 10 m3 reactors in series, which were inoculated with bovine rumen and fed with fecal sludge obtained from public toilets.  The performance of the fermentation process was characterized in terms of both aqueous and gaseous variables representing the conversion of influent organic carbon to VFA as well as CH4.  Using the operating data, the first-ever process model for FS fermentation and digestion was developed and calibrated, based on the activated sludge model framework. Results and Conclusions.  This work represents one of the first systematic efforts at integrated FS characterization and process modeling to enable anaerobic fermentation and digestion of FS. It is shown that owing to pre-fermentation of FS in public septage holding tanks, one could employ significantly smaller digesters (lower capital costs) or increased loading capabilities for FS conversion to biogas or VFA. Further, using the first-ever calibrated process model for FS fermentation and digestion presented herein, we expect improved and more mechanistically informed development and application of different process designs and configurations for global FS management practice