Activated Sludge Flocculation Research Articles

Effect of saltwater intrusion on activated sludge flocculation.

Saltwater addition negatively impacted the settling properties of the activated sludge in a sequencing batch reactor, which in turn caused the effluent quality and the dewatering properties to deteriorate. Experiments showed that increasing sodium ion concentrations above 1meq/L interfered with bio-flocculation by ion exchange of calcium ions displaced by sodium in mixed liquor flocs, causing increases in the sludge volume index, total suspended solids, and chemical oxygen demand concentrations of the final effluent and in the times of the time to filter test of the mixed liquor. Experiments were also conducted to reverse mixed liquor de-flocculation by adding calcium ions in wastewater with different sodium chloride concentrations and a positive relationship was found between calcium ion added and improved effluent quality. Calcium ion concentrations above 1.6meq/L proved to be beneficial for adhesion of microbial aggregates leading to the formation of stable flocs and improved sludge dewatering by reversing the ion exchange phenomenon observed with sodium concentrations above 1meq/L. Monovalent-to-divalent ratios did not provide predictive information on de-flocculation. Hence, individual ion concentrations in addition to monovalent-to-divalent cation ratios are important in the onset of de-flocculation effects while a system-specific concentration threshold was measured under which no adverse effects were observed. PRACTITIONER POINTS: The adverse effects of varying concentrations of sodium chloride on the biological process affecting bioflocculation and effluent quality. Impact of sodium on the health of the mixed liquor as well as on dewatering processes. Prediction of sea level rise due to climate change will impact utilities situated along the coast with saltwater intrusion problems. Management of dewatered flows containing saltwater from onsite construction Identify management practices to reduce the impact of saltwater on the wastewater treatment plant.

The Impact of Local Hydrodynamics on High-Rate Activated Sludge Flocculation in Laboratory and Full-Scale Reactors

High rate activated sludge (HRAS) processes have a high potential for carbon and energy recovery from sewage, yet they suffer frequently from poor settleability due to flocculation issues. The process of flocculation is generally optimized using jar tests. However, detailed jar hydrodynamics are often unknown, and average quantities are used, which can significantly differ from the local conditions. The presented work combined experimental and numerical data to investigate the impact of local hydrodynamics on HRAS flocculation for two different jar test configurations (i.e., radial vs. axial impellers at different impeller velocities) and compared the hydrodynamics in these jar tests to those in a representative section of a full scale reactor using computational fluid dynamics (CFD). The analysis showed that the flocculation performance was highly influenced by the impeller type and its speed. The axial impeller appeared to be more appropriate for floc formation over a range of impeller speeds as it produced a more homogeneous distribution of local velocity gradients compared to the radial impeller. In contrast, the radial impeller generated larger volumes (%) of high velocity gradients in which floc breakage may occur. Comparison to local velocity gradients in a full scale system showed that also here, high velocity gradients occurred in the region around the impeller, which might significantly hamper the HRAS flocculation process. As such, this study showed that a model based approach was necessary to translate lab scale results to full scale. These new insights can help improve future experimental setups and reactor design for improved HRAS flocculation.

Effects of Al3+ on pollutant removal and extracellular polymeric substances (EPS) under anaerobic, anoxic and oxic conditions

Aluminum ions produced by aluminum mining, electrolytic industry and aluminum-based coagulants can enter wastewater treatment plants and interact with activated sludge. They can subsequently contribute to the removal of suspended solids and affect activated sludge flocculation, as well as nitrogen and phosphorus removal. In this study, the effects of Al3+ on pollutant removal, sludge flocculation and the composition and structure of extracellular polymeric substances (EPS) were investigated under anaerobic, anoxic and oxic conditions. Results demonstrated that the highest chemical oxygen demand (COD) and total nitrogen (TN) removal efficiencies were detected for an Al3+ concentration of 10 mg/L. In addition, the maximal dehydrogenase activity and sludge flocculation were also observed at this level of Al3+. The highest removal efficiency of total phosphorus (TP) was achieved at an Al3+ concentration of 30 mg/L. The flocculability of sludge in the anoxic zone was consistently higher than that in the anaerobic and oxic zones. The addition of Al3+ promoted the secretion of EPS. Tryptophan-like fluorescence peaks were detected in each EPS layer in the absence of Al3+. At the Al3+ concentration of 10 mg/L, fulvic acid and tryptophan fluorescence peaks began to appear, while the majority of protein species and the highest microbial activity were also detected. Low Al3+ concentrations ( 10 mg/L) weakened microbial activity. Higher Al3+ concentrations (>30 mg/L) also inhibited the release of phosphorus in the anaerobic zone by reacting with PO43−.

Potential effects of suspended TiO2 nanoparticles on activated sludge floc properties in membrane bioreactors

With the rapid development and application of consumer products containing nanoparticles (NPs), especially titanium dioxide (TiO2) NPs, the potential effects of suspended NPs on wastewater treatment has been a concern over the recent years. This study investigated the potential effects of suspended TiO2 NPs on activated sludge flocculation properties in a membrane bioreactor (MBR). Results showed that suspended TiO2 NPs inhibited the viability of activated sludge flocs, and led to bacterial protein secretion for bacterial protection, causing an overall protein increase of soluble microbial products. Suspended TiO2 NPs also destabilized the activated sludge floc structure and reduced flocculation capacity of flocs, causing an over production of organic matter and resulting in a floc size decrease of over 50%. Suspended TiO2 NPs also caused a change in the phylogenetic distribution of bacterial community. Whereby, the dominant species in activated sludge was replaced from Comamonadaceae to Thiotrichaceae in 50 mg/L suspended TiO2 NPs.

Simulation of Floc Size Distribution in Flocculation of Activated Sludge Using Population Balance Model with Modified Expressions for the Aggregation and Breakage

The floc size distribution of activated sludge was simulated successfully by population balance model in the previous study (Population Balance Model and Calibration Method for Simulating the Time Evolution of Floc Size Distribution of Activated Sludge Flocculation. Desalination and Water Treatment, 67, 41‐50). However, nonignorable errors exist in the simulation for the volume percentage of large flocs. This paper describes the application of a modified population balance model in the simulation of the time evolution of floc size distribution in activated sludge flocculation process under shear‐induced conditions. It was found that the application of modified size dependent collision efficiency, modified breakage rate expression by assuming a maximum value, and binominal daughter‐particles distribution function could improve the population balance model for activated sludge flocculation and successfully predict the dynamic changes in volume percentage distribution and mean floc size of activated sludge under different shear conditions. The results demonstrate that the maximum breakage rate was independent on the velocity gradient, and both the collision efficiency and breakage rate coefficient show a power‐law relationship with the average velocity gradient; the former decreases while the latter increases with the rise of the average velocity gradient. These findings would help to understand the dynamics of activated sludge flocculation.

Effect of the Cationic Block Structure on the Characteristics of Sludge Flocs Formed by Charge Neutralization and Patching.

In this study, a template copolymer (TPAA) of (3-Acrylamidopropyl) trimethylammonium chloride (AATPAC) and acrylamide (AM) was successfully synthesized though ultrasonic-initiated template copolymerization (UTP), using sodium polyacrylate (PAAS) as a template. TPAA was characterized by an evident cationic microblock structure which was observed through the analyses of the reactivity ratio, Fourier transform infrared spectroscopy (FTIR), 1H (13C) nuclear magnetic resonance spectroscopy (1H (13C) NMR), and thermogravimetry/differential scanning calorimetry (TG/DSC). The introduction of the template could improve the monomer (AATPAC) reactivity ratio and increase the length and amount of AATPAC segments. This novel cationic microblock structure extremely enhanced the ability of charge neutralization, patching, and bridging, thus improving the activated sludge flocculation performance. The experiments of floc formation, breakage, and regrowth revealed that the cationic microblock structure in the copolymer resulted in large and compact flocs, and these flocs had a rapid regrowth when broken. Finally, the larger and more compact flocs contributed to the formation of more channels and voids, and therefore the specific resistance to filtration (SRF) reached a minimum.

Population balance model and calibration method for simulating the time evolution of floc size distribution of activated sludge flocculation

Population balance model and calibration method for simulating the time evolution of floc size distribution of activated sludge flocculation

Population balance modeling of activated sludge flocculation: Investigating the influence of Extracellular Polymeric Substances (EPS) content and zeta potential on flocculation dynamics

Abstract Modeling activated sludge flocculation via particle population balances has been used to investigate the floc size distribution (FSD). The impact of shear rate on the FSD has been stated in previous studies, but the quantitative effect of the Extracellular Polymeric Substance (EPS) content and zeta potential on the aggregation and breakage of activated sludge flocs need to be further investigated. This study defined the balance factor, i.e. the ratio of the breakage rate coefficient to the collision efficiency, to evaluate the combined effect of the EPS content and the zeta potential on the aggregation and breakage of activated sludge, and introduced a binomial breakage function for more probable daughter-particles distributions after the breakage of flocs. The flocculation dynamic parameters were found to be linearly related to the EPS content and the zeta potential. The flocculation balance factor decreased with the increase of EPS content and the decrease of absolute zeta potentials. Smaller flocculation balance factor means better flocculation ability of flocs, leading to larger mean-size flocs. The binomial breakage function parameter values varied from 0.79 to 1.78 for activated sludge flocs with different EPS content, from 0.87 to 1.19 (Al3+ addition) and from 0.96 to 1.93 (Ca2+ addition) for flocs with different zeta potentials, which suggests that the flocs might possess different breakage. Although the increase of EPS content is beneficial to the aggregation of larger flocs, the flocs with high EPS content are susceptible to large-scale fragmentation resulting in more smaller-daughter-particles. Lower absolute values of zeta potentials would promote flocculation, and the activated sludge flocs showed strong stability with Al3+ additions but weak stability with Ca2+ additions.

The Mechanism of Activated Sludge Flocculation under Al<sup>3+ </sup>Dosin

The mechanism governing activated sludge flocculation under Al3+ dosing was studied in this paper. Activated sludge was cultivated in sequencing batch reactors (SBR) at 22°C. Batch dosages of Al3+ were 0.00, 0.125, 0.5, 1 and 1.5meq/L respectively, and continuous dosage was 0.1meq/L. As batch dosage increased, the total interaction energy, zeta potential and turbidity tended to decline, which suggested that batch dosing promoted sludge flocculation. Under the equivalent dose, the zeta potential of continuous dosing was higher, while the LB-EPS content showed the opposite tendency and turbidity reduction was similar. Both batch and continuous dosing promoted flocculation performance: in terms of interaction energy, batch dosing was more effective; while in terms of EPS, it was on the contrary.

Modelling the Activated Sludge Flocculation Process Using Population Balance Model (PBM)

This paper describes the application of population balance models to activated sludge flocculation process. It presents the development and selection of appropriate expressions for aggregation and breakage kinetics within the population balance framework to describe the evolution of mean size and steady state distribution of flocs under shear conditions. A size and velocity gradient dependent collision efficiency is introduced into the aggregation expression. In the model, only 2 parameters need to be estimated: collision efficiency coefficient and the breakage frequency coefficient. They are obtained by the “best fit” with the experimental data, and keep unchanged under different shear condition for the same flocs. The modelling results indicate that the population balance models coupled with suitable aggregation and breakage kinetics is appropriate for describing activated sludge flocculation dynamics.

Towards mechanistic models for activated sludge flocculation under different conditions based on inverse problems

Experimental data of Ca-induced activated sludge flocculation under different conditions of temperature and dissolved oxygen are investigated in order to model the influence of changing physical and chemical factors. However, current kernel structures for collision frequency and efficiency are unable to describe activated sludge flocculation data. Therefore, an earlier developed methodology based on an inverse problem is applied, yielding empirical models, to find out how flocculation is affected by these different environmental conditions. This contribution shows the useful application of inverse problems to improve the understanding of complex aggregation mechanisms.

Solving the inverse problem for aggregation in activated sludge flocculation using a population balance framework

Many systems contain populations of individuals. Often, they are regarded as a lumped phase, which might, for some applications, lead to inadequate model predictive power. An alternative framework, Population Balance Models, has been used here to describe such a system, activated sludge flocculation in which particle size is the property one wants to model. An important problem to solve in population balance modelling is to determine the model structure that adequately describes experimentally obtained data on for instance, the time evolution of the floc size distribution. In this contribution, an alternative method based on solving the inverse problem is used to recover the model structure from the data. In this respect, the presence of similarity in the data simplifies the problem significantly. Similarity was found and the inverse problem could be solved. A forward simulation then confirmed the quality of the model structure to describe the experimental data.

Comparison of discretization methods to solve a population balance model of activated sludge flocculation including aggregation and breakage

Population balance models (PBMs) can be used to describe the evolution with time of distributions of properties of individuals. In this study a PBM for activated sludge flocculation including aggregation and breakage processes was investigated. The PBM is an integro-differential equation and does not have an analytical solution. A possibe method of solving the equation at relatively low computational cost is to use discretization. Two different discretization techniques, the fixed pivot and the moving pivot, were compared using geometric grids of different coarseness. Simulations were performed for three different processes: pure aggregation, pure breakage and combined aggregation – breakage. The results for pure aggregation showed that the fixed pivot overpredicts the large particle sizes when using coarse grids since grid refinement results in a clear downward trend. The predictions of the moving pivot technique show even lower predictions for the large particle sizes, with a slight upward trend for finer grids. This suggests that these predictions are closer to the pseudo-analytical solution (i.e. at infinitely fine grid). For the pure breakage case it was found that the moving pivot predictions collapsed onto one curve. Since a binary breakage case was studied, a fixed pivot with a grid with geometric factor 2 also collapsed onto that curve. Grid refinement for the fixed pivot case resulted in overestimations. Similar conclusions could be drawn for the combined aggregation – breakage case. Overall, the moving pivot is found to be superior since it produces more accurate predictions, even for much coarser grids. Despite the computational burden, the latter implies a lower computational load.

Population balance modelling of activated sludge flocculation: Investigating the size dependence of aggregation, breakage and collision efficiency

This paper describes the application of population balance models to activated sludge flocculation. It presents the development and selection of appropriate expressions for aggregation and breakage mechanisms within the population balance framework to describe the evolution of mean size and mass distribution of flocs under shear-induced conditions. To describe the flocculation process, 16 models with different size dependent aggregation and breakage expressions were compared and the kinetic parameters of aggregation rate constant and selection rate constant were extracted by fitting each model to the experimental data. Of the 16 models, the shear-induced aggregation and size-dependent selection model was found to best describe the experimental data, however there were some discrepancies between the model and experimental results at long experimental times. A size dependent collision efficiency was introduced into the aggregation expression and this improved the fitting of the model with the experimental data. However, the relationship between the kinetic parameters and shear rate did not follow expected physical relationships. Further improvements to the model were made by setting the aggregation rate constant proportional to shear rate and the selection rate size independent whilst still including the size-dependent collision efficiency. The aggregation rate constant, selection rate constant and critical size were extracted by fitting the model to the experimental data. This model was able to follow the change in mean size and evolution of mass and was used to predict other experimental data successfully. The modelling results indicated that the population balance model is a useful tool to describe the dynamics of activated sludge flocculation.

PBM and activated sludge flocculation: From experimental data to calibrated model

AbstractA new comprehensive calibration methodology for a population balance model (PBM) is presented using experimental size distribution data obtained on‐line during flocculation tests. It was found that the moving‐pivot discretization technique should be used to accurately solve the PBM. A new methodology for grid transformation and volume to number transformation, based on the sludge concentration and the densities of liquid, solids, and flocs, is presented. Model fits were performed on four different fitting variables. Volume‐based fitting (vol % and D[4, 3]) tends to fit the middle part of both the vol % and number distribution better than the tails of smaller and larger particle sizes. Severe underpredictions (up to a factor of 30) of the lower‐size classes are observed at steady state. Number‐based fitting [(weighted) number distribution] tends to fit the small‐size classes better, but still underpredicts them by a factor of 2 at steady state. © 2005 American Institute of Chemical Engineers AIChE J, 2005

PBM and Activated Sludge Flocculation: From Experimental Data to a Calibrated Model

A calibration methodology is presented for a Population Balance Model (PBM) using on-line flocculation data. The moving pivot discretisation technique is preferred to solve the PBM. A methodology for grid transformation and volume to number transformation is presented. Model fits on volume-based fitting variables fit the middle part of both the vol% and number distribution better, but under predict (30×) the smaller size classes. Model fits on number-based fitting variables fit the small size classes better, though still under predicting them (2×). For the application at hand, it is concluded that the model should be fitted to the number distribution. However, the Improved methodology pointed out that the model lacks flexibility to adequately describe the data

Modelling the effect of shear history on activated sludge flocculation

The aim of this paper is to investigate the effect of shear history on activated sludge flocculation dynamics and to model the observed relationships using population balances. Activated sludge flocs are exposed to dramatic changes in the shear rate within the treatment process, as they pass through localised high and low mixing intensities within the aeration basin and are cycled through the different unit operations of the treatment process. We will show that shear history is a key factor in determining floc size, and that the floc size varies irreversibly with changes in shear rate. A population balance model of the flocculation process is also introduced and evaluated.

Modelling the activated sludge flocculation process combining laser light diffraction particle sizing and population balance modelling (PBM)

A technique based on laser light diffraction is shown to be successful in collecting on-line experimental data. Time series of floc size distributions (FSD) under different shear rates (G) and calcium additions were collected. The steady state mass mean diameter decreased with increasing shear rate G and increased when calcium additions exceeded 8 mg/l. A so-called population balance model (PBM) was used to describe the experimental data. This kind of model describes both aggregation and breakage through birth and death terms. A discretised PBM was used since analytical solutions of the integro-partial differential equations are non-existing. Despite the complexity of the model, only 2 parameters need to be estimated; the aggregation rate and the breakage rate. The model seems, however, to lack flexibility. Also, the description of the floc size distribution (FSD) in time is not accurate.

Modelling activated sludge flocculation using population balances

Activated sludge flocculation was modelled using population balances. The model followed the dynamics of activated sludge flocculation providing a good approximation of the change in mean floc size with time. Increasing the average velocity gradient decreased the final floc size. The breakage rate coefficient and collision efficiency also varied with the average velocity gradient. A power law relationship was found for the increase in breakage rate coefficient with increasing average velocity gradient. Further investigation will be conducted to determine the relationship between the collision efficiency and particle size to provide a better approximation of dynamic changes in the floc size distribution during flocculation.

Activated sludge flocculation: direct determination of the effect of calcium ions

The effect of calcium on activated sludge flocculation dynamics is investigated using a unique experimental technique. The technique allows on-line analysis of the size of activated sludge flocs during flocculation and provides valuable insight into the mechanisms of flocculation. Activated sludge samples were firstly sonicated for 3 minutes at 50 W and then stirred at 100 rpm. The floc size was subsequently measured on-line using a Malvern Mastersizer/E. For concentrations of calcium less than 4 meq/L no significant increase in final floc size was observed even though an increase in the initial rate of change of floc size could be seen. Addition of calcium greater than 4 meq/L resulted in a dramatic increase in floc size. Results from this investigation support the theory that cations are involved in flocculation through cationic bridging, and will be used in ongoing investigations to model the flocculation process.