Microbial residence time is a controlling parameter of the taxonomic composition and functional profile of microbial communities.

A remaining challenge within microbial ecology is to understand the determinants of richness and diversity observed in environmental microbial communities. In a range of systems, including activated sludge bioreactors, the microbial residence time (MRT) has been previously shown to shape the microbial community composition. However, the physiological and ecological mechanisms driving this influence have remained unclear. Here, this relationship is explored by analyzing an activated sludge system fed with municipal wastewater. Using a model designed in this study based on Monod-growth kinetics, longer MRTs were shown to increase the range of growth parameters that enable persistence, resulting in increased richness and diversity in the modeled community. In laboratory experiments, six sequencing batch reactors treating domestic wastewater were operated in parallel at MRTs between 1 and 15 days. The communities were characterized using both 16S ribosomal RNA and non-target messenger RNA sequencing (metatranscriptomic analysis), and model-predicted monotonic increases in richness were confirmed in both profiles. Accordingly, taxonomic Shannon diversity also increased with MRT. In contrast, the diversity in enzyme class annotations resulting from the metatranscriptomic analysis displayed a non-monotonic trend over the MRT gradient. Disproportionately high abundances of transcripts encoding for rarer enzymes occur at longer MRTs and lead to the disconnect between taxonomic and functional diversity profiles

The use of pure cultures as a means of understanding the performance of a mixed culture in the biodegradation of phenol

In an effort to gain a more fundamental understanding of the performance of mixed microbial cultures in the biodegradation of toxic organic chemicals, studies have been conducted using three phenol degrading species isolated from a municipal treatment plant. The rate of phenol degradation was investigated for each of the three pure phenol degrading species, and various combinations of the three species. A simple competitive model was used to predict the behavior of the mixed cultures by using the pure culture Monod rate constants. The model fit the growth data for total biomass very well, although (as with the pure culture experiments) the fit of the phenol degradation data was less accurate

Comparison of the Biodegradation of n-alkanes and Readily Biodegradable Substrates Using Open Mixed Culture under Aerobic, Anoxic and Anaerobic Conditions

Peer reviewedPublisher PD

A review of modeling approaches in activated sludge systems

The feasibility of using models to understand processes, predict and/or simulate, control, monitor and optimize WasteWater Treatment Plants (WWTPs) has been explored by a number of researchers. Mathematical modeling provides a powerful tool for design, operational assistance, forecast future behavior and control. A good model not only elucidates a better understanding of the complicated biological and chemical fundamentals but is also essential for process design, process start-up, dynamics predictions, process control and process optimization. This paper reviews developments and the application of different modeling approaches to wastewater treatment plants, especially activated sludge systems and processes therein in the last decade. In addition, we present an opinion on the wider wastewater treatment related research issues that need to be addressed through modeling.Key words: Mathematical modeling, water, wastewater, wastewater treatment plants, activated sludge systems

Generation of (synthetic) influent data for performing wastewater treatment modelling studies

The success of many modelling studies strongly depends on the availability of sufficiently long influent time series - the main disturbance of a typical wastewater treatment plant (WWTP) - representing the inherent natural variability at the plant inlet as accurately as possible. This is an important point since most modelling projects suffer from a lack of realistic data representing the influent wastewater dynamics. The objective of this paper is to show the advantages of creating synthetic data when performing modelling studies for WWTPs. This study reviews the different principles that influent generators can be based on, in order to create realistic influent time series. In addition, the paper summarizes the variables that those models can describe: influent flow rate, temperature and traditional/emerging pollution compounds, weather conditions (dry/wet) as well as their temporal resolution (from minutes to years). The importance of calibration/validation is addressed and the authors critically analyse the pros and cons of manual versus automatic and frequentistic vs Bayesian methods. The presentation will focus on potential engineering applications of influent generators, illustrating the different model concepts with case studies. The authors have significant experience using these types of tools and have worked on interesting case studies that they will share with the audience. Discussion with experts at the WWTmod seminar shall facilitate identifying critical knowledge gaps in current WWTP influent disturbance models. Finally, the outcome of these discussions will be used to define specific tasks that should be tackled in the near future to achieve more general acceptance and use of WWTP influent generators

Respirometric assessment of bacterial kinetics in algae-bacteria and activated sludge processes

Algae-bacteria (AB) consortia can be exploited for effective wastewater treatment, based on photosynthetic oxygenation to reduce energy requirements for aeration. While algal kinetics have been extensively evaluated, bacterial kinetics in AB systems are still based on parameters taken from the activated sludge models, lacking an experimental validation for AB consortia. A respirometric procedure was therefore proposed, to estimate bacterial kinetics in both activated sludge and AB, under different conditions of temperature, pH, dissolved oxygen, and substrate availability. Bacterial activities were differently influenced by operational/environmental conditions, suggesting that the adoption of typical activated sludge parameters could be inadequate for AB modelling. Indeed, respirometric results show that bacteria in AB consortia were adapted to a wider range of conditions, compared to activated sludge, confirming that a dedicated calibration of bacterial kinetics is essential for effectively modelling AB systems, and respirometry was proven to be a powerful and reliable tool to this purpose

Microbial Competition in Bioelectrochemical Systems

Bioelectrochemical systems(BESs)/ microbial fuel fells (MFCs) are a well-studied potential technology for bioremediation and decentralized wastewater treatment. However, progress has been somewhat stalled at the bench-scale. In well controlled experiments electron recovery is high. In natural environments, wastewaters are complex and anode-respiring bacteria can be outcompeted in the presence of competing microorganisms, leading to a loss in electron-recovery and power production. Furthermore, the cathode of the MFC plays a vital role in providing flexibility for treatment options but is an understudied part of MFCs. Modelling Intracellular Competition in a Denitrifying Biocathode: One potential MFC configuration uses an organic-oxidizing anode biofilm and a denitrifying cathode biofilm. However nitrite, a denitrification intermediate with environmental and public health impacts, has been reported to accumulate. In this study, before complete denitrification was achieved in a bench-scale, batch denitrifying cathode, nitrite concentrations reached 66.4 % ± 7.5 % of the initial nitrogen. Common environmental inhibitors such as insufficient electron donor, dissolved oxygen, insufficient carbon source, and pH, were considered as a cause of the accumulation. Improvement in these conditions did not mitigate nitrite accumulation. We present an activated sludge model with an integration of the Nernst-Monod model and indirect coupling of electrons (ASM-NICE) that effectively simulated the observed batch data, including nitrite-accumulation by coupling biocathodic electron transfer to intracellular electron mediators. The simulated half-saturation constants for mediated intracellular transfer of electrons during nitrate and nitrite reduction suggested a greater affinity for nitrate reduction when electrons are not limiting. The results imply that longer hydraulic retention times (HRTs) may be necessary for a denitrifying biocathode to ensure complete denitrification. These findings could play a role in designing full-scale MFC wastewater treatment systems to maximize total nitrogen removal. Experimental Evaluation of Responses of Anode-Respiring Communities to Nitrate: A poorly understood phenomenon with a potentially significant impact on electron recovery in MFCs is the role of competition between anode-respiring bacteria and microorganisms that use other electron acceptors. Nitrogen species are a major constituent of wastewater and nitrate can act as a competing electron acceptor in the anode. Studies investigating the impact of competition on population dynamics in mixed communities in the anode are lacking. Here, we investigated the impact of nitrate at different C/N ratios, 1.8, 3.7 and 7.4 mg-C/mg-N, on the electrochemical performance and the biofilm community in mixed-culture chemostat MFCs. The electrochemical performance of the MFC was not affected under electron donor non-limiting conditions, 7.4 mg-C/mg-N. At lower C/N ratios, electron donor limiting, electron recovery was significantly lower. The electrochemical performance recovered upon removal of nitrate at 3.7 mg-C/mg-N. Microbial community analysis showed a decrease of Deltaproteobacteria accompanied by an increase in Betaproteobacteria in response to nitrate at low C/N ratios, and no significant changes at 7.4 mg-C/mg-N. Transcriptional analysis showed increased transcription of nirK and nirS genes during nitrate flux suggesting that denitrification to N2 (and not facultative nitrate reduction by Geobacter spp.) might be the primary response to perturbation with nitrate. Modelling Interspecies Competition in the Anode of a Microbial Fuel Cell: MFCs offer great promise for simultaneous treatment of wastewater and energy recovery. Even though there have been extensive experimental studies of multi-species anode-respiring biofilms, models and process optimization studies have been scarce. The formulation and evaluation of models is a critical step in the application of MFCs to wastewater treatment and bioremediation. The purpose of this study was to formulate a model that could simulate the effect of influx of a competing electron acceptor such as nitrate on the anode biofilm community. A model was formulated considering two distinct communities of bacteria: an anode-respiring community (not capable of nitrate reduction) and a denitrifying community (not capable of anode-respiration). A competitive scenario involving the influx of acetate and nitrate at a C/N ratio of 1.8 mg-C/mg-N was used to calibrate the model using experimental data. Calibration results indicate that facultative reduction of nitrate by facultative anode-respiring bacteria could be an important factor playing a role in the robustness and resilience of the anode-biofilms to fluxes of nitrate. Sensitivity analyses revealed that the biofilm retention coefficient (biofilm detachment rate) and species-specific growth kinetic parameters could play a significant role in the robustness of anode communities to influx of nitrate. Further investigation of change in detachment rate in response to the presence of nitrate in bulk solution is required