Activated Sludge Flocs Research Articles

Entrapped elemental selenium nanoparticles affect physicochemical properties of selenium fed activated sludge

Selenite containing wastewaters can be treated in activated sludge systems, where the total selenium is removed from the wastewater by the formation of elemental selenium nanoparticles, which are trapped in the biomass. No studies have been carried out so far on the characterization of selenium fed activated sludge flocs, which is important for the development of this novel selenium removal process. This study showed that more than 94% of the trapped selenium in activated sludge flocs is in the form of elemental selenium, both as amorphous/monoclinic selenium nanospheres and trigonal selenium nanorods. The entrapment of the elemental selenium nanoparticles in the selenium fed activated sludge flocs leads to faster settling rates, higher hydrophilicity and poorer dewaterability compared to the control activated sludge (i.e., not fed with selenite). The selenium fed activated sludge showed a less negative surface charge density as compared to the control activated sludge. The presence of trapped elemental selenium nanoparticles further affected the spatial distribution of Al and Mg in the activated sludge flocs. This study demonstrated that the formation and subsequent trapping of elemental selenium nanoparticles in the activated sludge flocs affects their physicochemical properties.

Modeling Growth of Filaments and Floc Formers in Activated Sludge Flocs: Integrating the Effects of Kinetics and Diffusion

Filamentous bulking is the most common solid-separation problem in wastewater treatment systems. The phenomenon is attributed to competition between filamentous bacteria and floc-forming bacteria, a tradeoff between kinetic selection (KST) and substrate diffusion limitation (SDL). Previously, we explained our contradictory experimental results by both effects and developed a conceptual framework with three regions (bulking, intermediate, and non-bulking). However, within this purely qualitative framework, the range of substrate concentrations for each region was not determined. To test the framework and quantify the critical concentrations for the three regions, we here integrate KST and SDL into a model that simulates the growth rates of filaments and floc formers inside activated sludge flocs. Bulking and non-bulking sludges were produced in laboratory-scale reactors, and the kinetic model parameters were measured by respirometry. We then extended the model to verify our previous experimental results from sequencing batch reactors (SBRs). The effects of oxygen limitation and decay rates of floc formers and filaments on the modeled growth rates were also evaluated. In basic simulations, the model successfully determined the critical values of substrate concentrations in the framework. The results of the extended model were consistent with our experimental data, indicating that large flocs induce bulking whereas small flocs do not. Simulation results were more sensitive to the kinetic parameters than to the diffusion parameters, and the parameters more sensitively affected the growth of floc formers than filament growth.

Use of Optical Monitoring to Assess the Breakage of Activated Sludge Flocs

Optical monitoring with a charge-coupled device camera was used to assess the breakage of activated sludge flocs obtained from three different activated sludge plants: two municipal and one industrial. In this method, the samples were processed through the imaging unit and recycled back to a beaker using a centrifugal pump which causes the breakage of flocs together with hydrodynamics forces. Based on the image analysis results, the breakage models of the activated sludge flocs vary between the plants. The major breakage model in the two municipal plants was surface erosion whereas it was large-scale fragmentation in the industrial plant. A larger amount of filaments in the industrial plant most likely caused the large-scale fragmentation. Furthermore, the effect of the addition of a cationic polymer on the strength of activated sludge floc was studied in one sample. When the cationic polymer was used, the flocs started to grow at the start of the breakage process. However, they broke up at the end of the process and remained small, as found in flocs not exposed to any chemical treatment. Based on the results, the optical monitoring seems to be suitable for analyzing the breakage of flocs.

On-line optical monitoring of activated sludge floc morphology

An on-line device for the optical monitoring of activated sludge flocs was set up and tested in situ at a municipal wastewater treatment plant over eight months. A charge-coupled device camera was used to image the flocs and filaments, and an automated image analysis programme was used to analyse the morphological parameters of the flocs. Results were compared to those obtained from offline laboratory analysis for water quality indicators, such as suspended solids. The results showed that the activated sludge process has produced different levels of settling quality. Large and round flocs enabled good settling, whereas small and irregularly shaped flocs, as well as the large amount of filaments, led to poor settling. The image analysis results obtained from the on-line device indicated that the poor settling was most likely caused by filamentous bulking. Such bulking was most likely due to the compositional variations in incoming wastewater; the poorly settled sludge was characterised by much lower solids content, which could have been responsible for the growth of filamentous bacteria. The image analysis results also revealed that the changes in floc morphology occurred slowly, indicating the potential for optical monitoring to control and optimise the activated sludge process.

The role of lipids in activated sludge floc formation

Activated sludge is widely used to treat municipal and industrial wastewater globally and the formation of activated sludge flocculates (flocs) underpins the ability to separate sludge from treated water. Despite the importance of activated sludge flocs to human civilization there have been precious few attempts to rationally design fit for purpose flocs using a bottom-up approach based on a solid scientific foundation. Recently we have been developing experimental models for activated sludge floc formation based on the colonization and consumption of particulate organic matter (chitin and cellulose). In this study we lay the foundation for investigation of activated sludge floc formation based on biofilm formation around spheres of the lipid glycerol trioleate (GT) that form spontaneously when GT is introduced into activated sludge incubations. Sludge biomass was observed to associate tightly with the lipid spheres. An increase in extracellular lipase activity was associated with a decrease in size of the colonized lipid spheres over a 25 day incubation. Bacterial community composition shifted from predominantly Betaproteobacteria to Alphaproteobacteria in GT treated sludge. Four activated sludge bacteria were isolated from lipid spheres and two of them were shown to produce AHL like quorum sensing signal activity, suggesting quorum sensing may play a role in lipid spheres colonization and biodegradation in activated sludge. The development of this experimental model of activated sludge floc formation lays the foundation for rational production of flocs for wastewater treatment using lipids as floc nuclei and further development of the flocculate life-cycle concept.

The Effect of Anionic Surfactant Concentration on Activated Sludge Condition and Phosphate Release in Biological Treatment Plant

This paper discusses the influence of a wide range of anionic surfactant concentrations on activated sludge. Linear alkylbenzene sulphonate (LAS) was chosen as an example of a commonly used anionic surfactant. The fate of the surfactant during biological treatment of wastewater was tested. The effect of surfactant on glucose and starch removal was studied. It has been found that in the case of glucose the removal was independent of LAS concentration, while in the case of starch it was incomplete at high surfactant loads (above 15 mg·(g·dss) -1 ). The study established that surfactants can activate or inhibit microorganism activity, depending on surfactant concentration. LAS loads up to 3 mg·(g·dss) -1 positively stimulate the removal of COD, phosphorus release and the respirometric activity of the sludge. LAS loads higher than 15 mg·(g·dss) -1 inhibit respiration of activated sludge bacteria and decrease phosphorus removal. It also affects the morphology of activated sludge flocs, causing their fragmentation and lysis of protozoa cells.

POLEPSZENIE WŁASNOŚCI GRAWITACYJNYCH OSADU NADMIERNEGO PODDANEGO WSTĘPNEJ OBRÓBCE

The formation of sludge is an inevitable consequence of wastewater treatment processes. Their disposal and utilization requires knowledge on technology and engineering. The application of pretreatment processes/conditioning allows to obtain better mechanical properties of sludge. In the last decade a lot of research from around the world focused on new methods of conditioning of sludge can be noticed, i.e. The processes of disintegration, of which the destruction of the mechanical, chemical and biological. Despite different activities of each method (introduced energy, thermal phenomena, chemical reactions, mechanical), their common goal is the destruction of activated sludge floc structure and micro-organisms, which result in changes of properties in sediment and supernatant liquid. The influence of the disintegration of the microwave and freezing/thawing dry ice on selected properties of gravitational surplus activated sludge were presented. Characteristic parameters determined sludge sedimentation processes, i.e. the rate of descent and compaction density index sediment and sludge volume index and changes in the supernatant liquid. The study showed the efficacy of selected methods of sludge disintegration with regard to improving the properties of gravity and becoming a contribution to the determination of the effective methods of deposits’ preconditioning.

The role of lipids in activated sludge floc formation

The role of lipids in activated sludge floc formation

Modeling particle-size distribution dynamics in a shear-induced breakage process with an improved breakage kernel: Importance of the internal bonds

An improved aggregate breakage kernel was developed that accounts for the effects of both the internal bonding forces between particles within an aggregate and the fluid shear stress exerted on the aggregate. The ratio of the two opposite forces regulates the probability of aggregate breakage. Using the improved breakage kernel, together with the sectional modeling technique, the dynamics of particle breakage induced by fluid shear was well simulated. The results show that the internal bonding forces determine the strength of the aggregates, and the hydrophobic bonding forces are much stronger than van der Waals' forces for holding the aggregates against shear breakage. The simulations compared fairly well with the experimental results in terms of PSD evolution during the breakage of latex particle aggregates and activated sludge flocs. For the latex particle aggregates, van der Waals' forces apparently are the main internal bonding force between particles. However, for activated sludge flocs, the non-DLVO hydrophobic forces are shown to play an important role in maintaining a stronger structure of the flocs. (C) 2014 Elsevier B.V. All rights reserved.

Characteristics of Nitrous Oxide Emissions from Partial Nitrification Process Treating High Ammonium Wastewater

Nitrous oxide is a greenhouse gas, and biological nitrogen removal leads to nitrous oxide generation and emissions. In this study, the emission of nitrous oxide from partial nitrification process was investigated in two intermittently aerated SBRs (IASBRs). Activated sludge floc and aerobic granular sludge were feed into two IASBRs, respectively. In the steady state, partial nitrification was successfully achieved under intermittent aeration control strategy. Nitrous oxide emissions were 6.5% and 8.9% of the total influent nitrogen loading rate in IASBR1 and IASBR2, respectively. Nitrous oxide was mainly generated in non-aeration periods, but aeration period contributed to 91.8% and 90.6% of nitrous oxide emissions in two IASBRs, respectively. PHB can be used as the carbon source for heterotrophic denitrification, causing more nitrous oxide generated in IASBR2 which was seeded with aerobic granular sludge.

Variations of morphology of activated sludge flocs studied at full-scale wastewater treatment plants

Digital image analysis has been intensively developed over the last two decades including its application to describe morphology of activated sludge flocs. However, only few studies concerned the variation of flocs morphology with respect to the operational conditions, particularly oxido-reductive conditions, in a full-scale wastewater treatment plant (WWTP). In this work, morphology of activated sludge flocs was monitored over one year in two different full-scale WWTPs. The main aim of this study was to find the relationship between the operational parameters and morphology of sludge flocs. Simultaneously, the variations in floc size along activated sludge chamber were studied with respect to the oxido-reductive conditions. It was found that the sludge loading rate was one of the most important operational parameters influencing floc size. It was estimated that its values higher than 0.1 kg BOD5 kg TS−1 d−1 contributed to the decrease in floc size. Also, the oxido-reductive conditions influenced the floc size. It was statistically proved that flocs from the anaerobic zone were usually smaller than flocs from the anoxic or aerobic zones. Distribution of floc size in a full-scale WWTP usually could be described by a log-normal model.

Dynamic variation in physicochemical properties of activated sludge floc from different WWTPs and its influence on sludge dewaterability and settleability

Abstract Biosolid–liquid separation performance plays an important role in suspended solids removal and sludge thickening and dewatering process. Since the sludge is a highly complex colloidal system, the key factors affecting sludge dewatering and settling performance were not well understood. In this study, the dynamic variation in distribution and chemical composition of extracellular polymeric substances (EPS), floc size distribution, charge property and settling and dewatering performance was investigated at four full scale WWTPs over 100 days. It was found that the change in EPS distribution and composition from different WWTPs showed a very similar pattern with operating time. The temperature rather than wastewater composition had more significant influence on content and chemical constituents of different EPS fractions. Additionally, more EPS was extracted from activated sludge at lower temperature (

Effect of CuO nanoparticles on the production and composition of extracellular polymeric substances and physicochemical stability of activated sludge flocs

The effects of CuO nanoparticles (NPs) on the production and composition of extracellular polymeric substances (EPS) and the physicochemical stability of activated sludge were investigated. The results showed enhanced production of loosely bound extracellular polymeric substances (LB-EPS), protecting against nanotoxicity. Specifically, polysaccharide production increased by 89.7% compared to control upon exposure to CuO NPs (50mg/L). Fourier transform-infrared spectroscopy analysis revealed changes in the polysaccharide COC group and the carboxyl group of proteins in the EPS in the presence of CuO NPs. The sludge flocs were unstable after exposure to CuO NPs (50mg/L) because of excess LB-EPS. This also corresponded with decreased cell viability of the sludge flocs, as determined by the production of reactive oxygen species and the release of lactate dehydrogenase. These results are key to assessing the adverse effects of the CuO NPs on activated sludge in wastewater treatment plants.

Effect of Activated Sludge Retention Time, Operating Temperature, and Influent Phosphorus Deficiency on Floc Physicochemical Characteristics and UV Disinfection

Pilot-scale sequencing batch reactors (SBRs) were used to assess the effects of sludge retention time, temperature, and influent phosphorus level on floc physicochemical characteristics, effluent quality, and UV disinfection kinetics. Increasing the operating temperature from 12 to 22 °C caused an increase in the activated sludge flocs fractal dimension from 0.1 to 0.2, and improved the UV disinfection of final effluent. Influent phosphorus limitation, i.e., COD:N:P of 100:10:0.3, caused the formation of more spherical flocs with higher resistance to UV disinfection (by 1 log). However, influent phosphorus starvation, i.e., COD:N:P of 100:10:0.03, decreased the average floc size and sphericity, increased the final effluent turbidity, and lowered effluent UV dose demand. The findings provide useful information in terms of modifying wastewater treatment processes in the context of water reuse and improving UV disinfection efficiency.

A new classification paradigm of extracellular polymeric substances (EPS) in activated sludge: Separation and characterization of exopolymers between floc level and microcolony level

Extracellular polymeric substances (EPS) play a crucial role in the formation of activated sludge flocs. However, until now, the EPS are rather classified by the method used for extraction than by a theoretical consideration of their function and composition. In this paper, a new classification paradigm of EPS was proposed, which offered a novel approach to identify the role of EPS in the formation of activated sludge flocs. The current study gave an exploration to distinguish the EPS in the floc level (extra-microcolony polymers, EMPS) and in the microcolony level (extra-cellular polymers, ECPS). It was found that cation exchange resin treatment is efficient to disintegrate the flocs for EMPS extraction, however, inefficient to disaggregate the microcolonies for ECPS harvesting. A two-steps extraction strategy (cation exchange resin treatment followed by ultrasonication-high speed centrifugation treatment) was suggested to separate these two types of EPS in activated sludge flocs and the physicochemical characteristics of EMPS and ECPS were compared. The protein/polysaccharide ratio of ECPS was higher than that of EMPS and the molecular weight of proteins in EMPS and ECPS were found to be different. The ECPS contained higher molecular weight proteins and more hydrophobic substances than the EMPS contained. The result of excitation-emission matrix fluorescence spectroscopy analysis also showed that the EMPS and the ECPS have different fluorescent expressions and the components of EMPS were more diverse than that of ECPS. All results reported herein demonstrated that two different types of exopolymers exist in the activated sludge flocs and the inter-particle forces for aggregation of activated sludge flocs are not identical between the floc level and the microcolony level. It suggested that cation bridging interactions are more crucial in floc level flocculation, while the entanglement and hydrophobic interactions are more important in microcolony level cohesion.

Effects of mechanical disintegration of activated sludge on the activity of nitrifying and denitrifying bacteria and phosphorus accumulating organisms

The purpose of the study was to analyse the impact of hydrodynamic disintegration of thickened excess activated sludge, performed at different levels of energy density (70, 140 and 210 kJ/L), on the activity of microorganisms involved in nutrient removal from wastewater, i.e. nitrifiers, denitrifiers and phosphorus accumulating organisms (PAOs). Ammonium and nitrogen utilisation rates and phosphorus release rates for raw and disintegrated sludge were determined using batch tests. The experiment also included: 1) analysis of organic and nutrient compound release from activated sludge flocs, 2) determination of the sludge disintegration degree (DD), and 3) evaluation of respiratory activity of the biomass by using the oxygen uptake rate (OUR) batch test. It was shown that the activity degree of the examined groups of microorganisms depended on energy density and related sludge disintegration degree, and that inactivation of individual groups of microorganisms occurred at different values of DD. Least resistant to the destruction of activated sludge flocs turned out to be phosphorus accumulating organisms, while the most resistant were denitrifiers. A decrease of 20–40% in PAO activity was noted already at DD equal to 3–5%. The threshold values of DD, after crossing which the inactivation of nitrifiers and denitrifiers occurred, were equal to 8% and 10%, respectively. At lesser DD values an increase in the activity of these groups of microorganisms was observed, averaging 20.2–41.7% for nitrifiers and 9.98–36.3% for denitrifiers.

Simultaneous biological nutrient removal: a state-of-the-art review.

Simultaneous biological nutrient removal (SBNR) is the occurrence of biological nutrient removal (BNR) in systems that do not possess defined anaerobic and/or anoxic zones. A review of the relevant literature demonstrates that two mechanisms are primarily responsible for SBNR: (1) the bioreactor macro-environment and (2) the floc microenvironment. Complex hydraulic flow patterns exist in full-scale bioreactors that can result in the cycling of mixed liquor through the different environments needed for BNR. Diffusion resistance further allows oxygen-sufficient and oxygen-deficient zones to develop in activated sludge flocs if the external dissolved oxygen concentration is properly controlled. The diffusion of substrates between these zones allows BNR to occur. Long-term acclimation to the unique environmental conditions occurring in these systems results in the selection of microorganisms well adapted to the low dissolved oxygen concentrations occurring in them. The experience base for the design and operation of SBNR systems is expanding, thereby allowing their more widespread application, especially coupled with conventional mathematical modeling approaches. Computational fluid dynamics is an evolving tool to assist with the design and optimization of SBNR.

Investigation of the shape change of bio-flocs and its influence on mass transport using particle image velocimetry.

In this laboratory study, an advanced flow visualization technique - particle image velocimetry (PIV) - was employed to investigate the change of shape of activated sludge flocs in water and its influence on the material transport characteristics of the flocs. The continuous shape change of the bio-flocs that occurred within a very short period of time could be captured by the PIV system. The results demonstrate that the fluid turbulence caused the shift of parts of a floc from one side to the other in less than 200 ms. During the continuous shape change, the liquid within the floc was forced out of the floc, which was then refilled with the liquid from the surrounding flow. For the bio-flocs saturated with a tracer dye, it was shown that the dye could be released from the flocs at a faster rate when the flocs were swayed around in water. The experimental results indicate that frequent shape change of bio-flocs facilitates the exchange of fluid and materials between the floc interior and the surrounding water. This mass transfer mechanism can be more important than molecular diffusion and internal permeation to the function and behavior of particle aggregates, including bio-flocs, in natural waters and treatment systems.

A comparison between floc morphology and the effluent clarity at a full-scale activated sludge plant using optical monitoring

A charge-coupled device camera was used for the optical monitoring of activated sludge flocs and filaments, and the image analysis results were compared with the effluent clarity at a full-scale activated sludge plant during a three-month period. The study included a maintenance stoppage at the wastewater treatment plant, which was followed by a settling problem. Thus, the study presents the development of floc morphology from poor flocculation to good flocculation. In this case, the evolution of flocs was a slow process, and the optimum floc morphology was achieved before the purification results improved. To diagnose the cause of the settling problems using optical monitoring, four major factors were found to be relevant: the mean area of the flocs, the amount of small particles, the amount of filament and the shape parameters of the flocs. In this case, the settling problem was caused by dispersed growth based on the image analysis results. In conclusion, the method used has the potential for usefulness in the development of monitoring applications to predict plant performance and also to diagnose the causes of the settling problems.

Influences of d-tyrosine on the stability of activated sludge flocs

The sludge floc stability is essential for the solid/liquid separation in biological wastewater treatment. In this study, the effect of an exogenous d-tyrosine on the shear stability and surface characteristics of activated sludge flocs was investigated. Sludge flocs were found to be less stable in the addition of d-tyrosine. d-Tyrosine inhibited the production of extracellular polymeric substances (EPS) especially for the proteins. A high correlation coefficient was observed between the composition of EPS fraction and d-tyrosine content. In addition, the hydrophobicity of sludge flocs was reduced and the zeta potential was more negative with the content of d-tyrosine increased. A linear relationship between the extracellular polymeric substances and surface characteristics for sludge flocs indicated that the inhibited EPS production may be responsible for the instability of sludge upon the addition of d-tyrosine.