Biological and physical selectors for mobile biofilms, aerobic granules, and densified-biological flocs in continuously flowing wastewater treatment processes: A state-of-the-art review.

There have been significant advances in the use of biological and physical selectors for the intensification of continuously flowing biological wastewater treatment (WWT) processes. Biological selection allows for the development of large biological aggregates (e.g., mobile biofilm, aerobic granules, and densified biological flocs). Physical selection controls the solids residence times of large biological aggregates and ordinary biological flocs, and is usually accomplished using screens or hydrocyclones. Large biological aggregates can facilitate different biological transformations in a single reactor and enhance liquid and solids separation. Continuous-flow WWT processes incorporating biological and physical selectors offer benefits that can include reduced footprint, lower costs, and improved WWT process performance. Thus, it is expected that both interest in and application of these processes will increase significantly in the future. This review provides a comprehensive summary of biological and physical selectors and their design and operation.

MABR process development downstream of a carbon redirection unit: opportunities and challenges in nitrogen removal processes.

ABSTRACTCarbon redirection has become the desired option for sustainable and energy-efficient wastewater treatment due to its contribution to a circular economy. However, its impact on downstream processes such as nitrification and denitrification requires further investigation. This research characterizes the nitrogen removal performance, footprint, aeration mode, and microbial composition of a flow-through membrane aerated biofilm reactor (MABR) downstream of a chemically enhanced primary treatment (CEPT) carbon redirection unit. The batch and long-term studies demonstrated relatively higher nitrification rates than those reported using conventional primary treated wastewaters. The results indicated that reducing carbon in the liquid train positively impacted nitrification by achieving 87 ± 12% (1.4 ± 0.4 g/m2.d) ammonia removal with an effluent 2.5 ± 2.8 mg/L ammonia concentration at a short hydraulic retention time (HRT) of 2.5 h. Despite the lower (1.9 ± 1) soluble COD:N, up to 75 ± 25% (0.6 ± 0.4 g/m2.d) total nitrogen removal was achieved at 4 h HRT by implementing intermittent aeration. The batch tests using the developed biofilms showed nitrification (denitrification) capacity up to 11 ± 1.7 gNH4-N/m2.d (8.5 ± 0.5 gNO3-N/m2.d) and 2.7 ± 0.6 gNH4-N/m2.d (2 ± 0.3 gNO3-N/m2.d) corresponding to ammonia and nitrate concentrations ranging from 10-30 mg/L and 2-10 mg/L, respectively. Microbial analysis indicated that the nitrifiers such as Nitrosomonas and Nitrospira were the dominant species. The ammonia-oxidizing, nitrite-oxidizing, and denitrifying bacteria relative abundances were 10.3 ± 1.5%, 20.7 ± 1.7%, and 20.0 ± 2.8% under continuous aeration and 1.3 ± 0.07%, 1.8 ± 0.09%, and 40.5 ± 3.1% under intermittent aeration, supporting the observed ammonia and total nitrogen removal processes, respectively. Overall, the results demonstrated that MABR downstream of the CEPT behave differently; thus, design guides should be updated accordingly.

Available online sensors can be used to create fingerprints for MABRs that characterize biofilm limiting conditions and serve as soft sensors.

Membrane aerated biofilm reactors (MABRs) are a promising biological wastewater treatment technology, whose industrial applications have dramatically accelerated in the last five years. Increased popularity and fast industrial adaptation are coupled with increased needs to monitor, optimize, and control MABRs with available online sensors. Observations of commercial scale MABR installations have shown a distinctive and repetitive pattern relating oxygen purity in MABR exhaust gas to reactor ammonia concentrations. This provides an obvious opportunity for process monitoring and control which this paper investigates with the help of modeling. The relationship plots between the bulk ammonia concentration and the oxygen purity are defined as MABR fingerprint plots, which are described in the form of steady-state curves and dynamic trajectories. This study systematically investigated, analyzed, and explained the behaviors and connections of steady-state curves and dynamic trajectories with a MABR model in SUMO®, and proposed a hypothesis about utilizing the MABR fingerprint plots to characterize MABR system performance, identify the limiting factor of biofilms, and possibly develop a soft senor for MABR biofilm thickness monitoring and control.

Biodegradation of the endogenous residue of activated sludge in a membrane bioreactor with continuous or on-off aeration.

The goal of this study was to determine the effect of a long sludge retention time on the biodegradation of the endogenous residue in membrane digestion units receiving a daily feed of sludge and operated under either aerobic or intermittently aerated (22 h off-2 h on) conditions. The mixed liquor for these experiments was generated in a 10.4 day sludge retention time membrane bioreactor fed with a synthetic and completely biodegradable influent with acetate as the sole carbon source. It had uniform characteristics and consisted of only two components, heterotrophic biomass X(H) and endogenous residue X(E). Membrane digestion unit experiments were conducted for 80 days without any sludge wastage except for some sampling. The dynamic behaviour of generation and consumption of filtered organic digestion products was characterized in the membrane digestion unit systems using three pore filter sizes. Results from this investigation indicated that the colloidal matter with size between 0.04 µm and 0.45 µm was shown to contain a recalcitrant fraction possibly composed of polysaccharides bound to proteins which accumulated in the membrane digestion unit under both conditions. Modelling the membrane digestion unit results by considering a first-order decay of the endogenous residue allowed to determine values of the endogenous residue decay rate of 0.0065 and 0.0072 d(-1) under fully aerobic and intermittently aerated conditions, respectively. The effect of temperature on the endogenous decay rate was assessed for the intermittently aerated conditions in batch tests using thickened sludge from tests gave an endogenous decay rate constant of 0.0075 d(-1) at 20 °C and an Arrhenius temperature correction factor of 1.033.

Characterization of the heterotrophic biomass and the endogenous residue of activated sludge.

The activated sludge process generates an endogenous residue (X(E)) as a result of heterotrophic biomass decay (X(H)). A literature review yielded limited information on the differences between X(E) and X(H) in terms of chemical composition and content of extracellular polymeric substances (EPS). The objective of this project was to characterize the chemical composition (x, y, z, a, b and c in C(x)H(y)O(z)N(a)P(b)S(c)) of the endogenous and the active fractions and EPS of activated sludge from well designed experiments. To isolate X(H) and X(E) in this study, activated sludge was generated in a 200L pilot-scale aerobic membrane bioreactor (MBR) fed with a soluble and completely biodegradable synthetic influent of sodium acetate as the sole carbon source. This influent, which contained no influent unbiodegradable organic or inorganic particulate matter, allowed the generation of a sludge composed essentially of two fractions: heterotrophic biomass X(H) and an endogenous residue X(E), the nitrifying biomass being negligible. The endogenous decay rate and the active biomass fraction of the MBR sludge were determined in 21-day aerobic digestion batch tests by monitoring the VSS and OUR responses. Fractions of X(H) and X(E) were respectively 68% and 32% in run 1 (MBR at 5.2 day SRT) and 59% and 41% in run 2 (MBR at 10.4 day SRT). The endogenous residue was isolated by subjecting the MBR sludge to prolonged aerobic batch digestion for 3 weeks, and was characterized in terms of (a) elemental analysis for carbon, nitrogen, phosphorus and sulphur; and (b) content of EPS. The MBR sludge was characterized using the same procedures (a and b). Knowing the proportions of X(H) and X(E) in this sludge, it was possible to characterize X(H) by back calculation. Results from this investigation showed that the endogenous residue had a chemical composition different from that of the active biomass with a lower content of inorganic matter (1:4.2), of nitrogen (1:2.9), of phosphorus (1:5.3) and of sulphur (1:3.2) but a similar content of carbon (1:0.98). Based on these elemental analyses, chemical composition formulae for X(H) and X(E) were determined as CH(1.240)O(0.375)N(0.200)P(0.0172)S(0.0070) and CH(1.248)O(0.492)N(0.068)P(0.0032)S(0.0016), respectively. Data from EPS analyses also confirmed this difference in structure between X(E) and X(H) with an EPS content of 11-17% in X(E)versus 26-40% in X(H).

N2O emissions: modeling the effect of process configuration and diurnal loading patterns.

The objective of this research was to develop a mechanistic model for quantifying N2O emissions from activated sludge plants and demonstrate how this may be used to evaluate the effects of process configuration and diurnal loading patterns. The model describes the mechanistic link between the factors recognized to correlate positively with N2O emissions. The primary factors are the presence of ammonia and nitrite accumulation. Low dissolved oxygen concentrations also may be implicated through differential impacts on ammonia-oxidizing bacteria (AOB) versus nitrite-oxidizing bacteria (NOB) activity. Factors promoting N2O emissions at treatment plants are discussed below. The model was applied to data from laboratory and pilot-scale systems. From a practical standpoint, plant configuration (e.g., plug-flow versus complete-mix), influent loading patterns (and peak load), and certain operating strategies (e.g., handling of return streams) are all important in determining N2O emissions.

Uncertainty and variability in enhanced biological phosphorus removal (EBPR) stoichiometry: consequences for process modelling and optimization.

The overall potential for enhanced biological phosphorus removal (EBPR) in the activated sludge process is constrained by the availability of volatile fatty acids (VFAs). The efficiency with which polyphosphate accumulating organisms (PAOs) use these VFAs for P-removal, however, is determined by the stoichiometric ratios governing their anaerobic and aerobic metabolism. While changes in anaerobic stoichiometry due to environmental conditions do affect EBPR performance to a certain degree, model-based analyses indicate that variability in aerobic stoichiometry has the greatest impact. Long-term deterioration in EBPR performance in an experimental SBR system undergoing P-limitation can be predicted as the consequence of competition between PAOs and GAOs. However, the observed rapid decrease in P-release after the change in feed composition is not consistent with a gradual shift in population.

Predicting the degradability of waste activated sludge.

The objective of this study was to identify methods for estimating anaerobic digestibility of waste activated sludge (WAS). The WAS streams were generated in three sequencing batch reactors (SBRs) treating municipal wastewater. The wastewater and WAS properties were initially determined through simulation of SBR operation with BioWin (EnviroSim Associates Ltd., Flamborough, Ontario, Canada). Samples of WAS from the SBRs were subsequently characterized through respirometry and batch anaerobic digestion. Respirometry was an effective tool for characterizing the active fraction of WAS and could be a suitable technique for determining sludge composition for input to anaerobic models. Anaerobic digestion of the WAS revealed decreasing methane production and lower chemical oxygen demand removals as the SRT of the sludge increased. BioWin was capable of accurately describing the digestion of the WAS samples for typical digester SRTs. For extended digestion times (i.e., greater than 30 days), some degradation of the endogenous decay products was assumed to achieve accurate simulations for all sludge SRTs.

A time series model for influent temperature estimation: application to dynamic temperature modelling of an aerated lagoon.

Thirty-nine linear regression and time series models were built and calibrated for influent temperature (Ti) estimation at the primary aerated facultative lagoon in a municipal wastewater treatment plant. The models were based on mean daily ambient air temperature (Ta) and/or daily rainfall (P), and-optionally-wastewater temperature autoregression. The best fits were achieved with some time series models involving Ta and P, and Ti autoregression. The best-fit model was able to estimate influent temperature with a root-mean-square-error of 0.5 degrees C, and an R2 of 0.925, for the calibration period of 10.5 months. In addition, a dynamic lagoon-temperature (Tw) model from the literature was modified in its terms of solar radiation and aeration latent heat, and applied to the primary lagoon. The model was fed with the estimated influent temperature, and five model parameters were identified by calibration against 10.5-month Tw data. Dynamic lagoon-temperature estimation results were comparable to or better than other results of long-term simulations found in the literature. Sensitivity analyses were run on both models. Further validation with independent sets of data is needed for verification of the predictive capability of the models.

Dynamic modelling of nitrification in an aerated facultative lagoon.

Faced with the need to improve ammonia removal from lagoon wastewater treatment plants (WWTPs) operated in Quebec, Canada, mechanistic modelling has been proposed as a tool for explaining the seasonal nitrification phenomenon and to evaluate optimization and upgrade scenarios. A lagoon model that includes a modified activated sludge biokinetic model and that assumes completely mixed conditions in the water column and sediments has been applied to simulate 3 years of consecutive effluent data for a lagoon from the Drummondville WWTP. Successful prediction of results from this plant indicates that the seasonal nitrification is determined by temperature, dissolved oxygen (DO) concentrations, hydraulic retention time (HRT) of the water column and washout driven by a well-mixed water column. Results also indicate that sediments contribute to the ammonia load in the lagoon effluent, particularly in spring and early summer. Sensitivity analyses performed with the model indicate that the nitrification period could be prolonged by increasing DO concentrations in the lagoon and that bioaugmentation would be particularly effective in spring and early summer. Limitations of the model are discussed, as well as ways to improve the hydraulic model.