Activated Sludge Flocs Research Articles

Unique episymbiotic relationship between Candidatus Patescibacteria and Zoogloea in activated sludge flocs at a municipal wastewater treatment plant.

Candidatus Patescibacteria, also known as candidate phyla radiation (CPR), including the class-level uncultured clade JAEDAM01 (formerly a subclass of Gracilibacteria/GN02/BD1-5), are ubiquitous in activated sludge. However, their characteristics and relationships with other organisms are largely unknown. They are believed to be episymbiotic, endosymbiotic or predatory. Despite our understanding of their limited metabolic capacity, their precise roles remain elusive due to the difficulty in cultivating and identifying them. In previous research, we successfully recovered high-quality metagenome-assembled genomes (MAGs), including a member of JAEDAM01 from activated sludge flocs. In this study, we designed new probes to visualize the targeted JAEDAM01-associated MAG HHAS10 and identified its host using fluorescence in situ hybridization (FISH). The FISH observations revealed that JAEDAM01 HHAS10-like cells were located within dense clusters of Zoogloea, and the fluorescence brightness of zoogloeal cells decreased in the vicinity of the CPR cells. The Zoogloea MAGs possessed genes related to extracellular polymeric substance biosynthesis, floc formation and nutrient removal, including a polyhydroxyalkanoate (PHA) accumulation pathway. The JAEDAM01 MAG HHAS10 possessed genes associated with type IV pili, competence protein EC and PHA degradation, suggesting a Zoogloea-dependent lifestyle in activated sludge flocs. These findings indicate a new symbiotic relationship between JAEDAM01 and Zoogloea.

Real-time monitoring of activated sludge flocs via enhanced mask region-based Convolutional Neural networks

The functionality of activated sludge in wastewater treatment processes depends largely on the structural and microbial composition of its flocs, which are complex assemblages of microorganisms and their secretions. However, monitoring these flocs in real-time and consistently has been challenging due to the lack of suitable technologies and analytical methods. Here we present a laboratory setup capable of capturing instantaneous microscopic images of activated sludge, along with algorithms to interpret these images. To improve floc identification, an advanced Mask R-CNN-based segmentation that integrates a Dual Attention Network (DANet) with an enhanced Feature Pyramid Network (FPN) was used to enhance feature extraction and segmentation accuracy. Additionally, our novel PointRend module meticulously refines the contours of boundaries, significantly minimising pixel inaccuracies. Impressively, our approach achieved a floc detection accuracy of >95%. This development marks a significant advancement in real-time sludge monitoring, offering essential insights for optimising wastewater treatment operations proactively.

Effects of hydrogen pressure on stabilization with improved denitrification in a hydrogen-based membrane biofilm reactor

Although start-up is a critical procedure in biofilm-based processes, systematic research aimed at improving start-up efficiency in hydrogen-based membrane biofilm reactors (H2-MBfRs) is lacking. In this study, we investigated the effects of H2 pressure on the stabilized denitrification performance of H2-MBfRs during start-up. Following inoculation with anoxic sludge, the biomass was acclimated to various H2 pressures. The increase in denitrifying microbial activity under high H2 pressures (i.e., H2 flux) shortened the denitrification lag time and reduced NO3–N removal flux, indicating an improved acclimation efficiency. The acclimated biomass was attached to membranes under different H2 pressures. High H2 pressures impeded biomass attachment, possibly because of bubble-less evaporation. Reducing the H2 pressure appeared to be necessary for improving the biomass attachment. However, this measure decreased previously acclimated denitrification activity. Therefore, the loss of both acclimation and biomass adhesion must be considered during the attachment phase. The highest removal flux (0.63 e- eq/m2-d) was obtained after just 3.5 d of acclimated biomass attachment (i.e., activated sludge floc) at 1.6 bar. During Phase 3, the initial performance was further improved to 0.89 e- eq/m2-d by gradually increasing the H2 pressure. These results provide a start-up strategy for improving the initial performance of H2-MBfRs.

New insight into the granule formation in the reactor for enhanced biological phosphorus removal.

While granulated activated sludge exhibits high productivity, the processes of granule formation are incompletely studied. The processes of granule formation and succession of communities were investigated in a laboratory sequencing batch reactor (SBR) under conditions for enhanced biological phosphorus removal (EBPR) using microbiological and molecular techniques. Active consumption of acetate, primarily by the phosphate-accumulating organisms (PAO), commenced at day 150 of cultivation. This was indicated by the high ratio of molar P-released/acetate uptake (0.73-0.77 P-mol/C-mol), characteristic of PAO. During this period, two types of granule-like aggregates formed spontaneously out of the activated sludge flocs. The aggregates differed in morphology and microbial taxonomic composition. While both aggregate types contained phosphorus-enriched bacterial cells, PAO prevailed in those of morphotype I, and glycogen-accumulating organisms (GAOs) were predominant in the aggregates of morphotype II. After 250 days, the elimination of the morphotype II aggregates from the reactor was observed. The subsequent selection of the community was associated with the development of the morphotype I aggregates, in which the relative abundance of PAO increased significantly, resulting in higher efficiency of phosphorus removal. Metagenomic analysis revealed a predominance of the organisms closely related to Candidatus Accumulibacter IС and IIС and of Ca. Accumulibacter IIB among the PAO. Based on the content of the genes of the key metabolic pathways, the genomes of potential PAO belonging to the genera Amaricoccus, Azonexus, Thauera, Zoogloea, Pinisolibacter, and Siculibacillus were selected. The patterns of physicochemical processes and the microbiome structure associated with granule formation and succession of the microbial communities were revealed.

QUANTITATIVE DETERMINATION OF THE STRENGTH OF ACTIVATED SLUDGE FLOCKS IN BIOLOGICAL TREATMENT TECHNOLOGIES

The performance of sludge and the effectiveness of wastewater treatment in various biological treatment facilities are influenced by the morphological features of activated sludge flocs. Currently, these features are assessed qualitatively through visual inspection, without statistical processing of the data. The article presents the results of quantitative determination of the strength of activated sludge flocs. A computerized method has been created to quantify the morphological characteristics of activated sludge flocs by analyzing sludge micrographs using the ImageJ software. Color and contrast indices were used to quantify the strength of the flocs. Photographs of activated sludge from the work of D. Eikelboom were used as standards for determining strength. In the calculations, we used photographs by D. Eikelboom taken at a magnification of 300 times (100 pixels) and 150 times (50 pixels). A laboratory study of the effect of Mg ions on the properties of activated sludge flocs was carried out when controlling the morphology of the flakes using the developed computerized technique. Examining the impact of Mg ions on activated sludge flocs revealed that higher magnesium concentrations in the sludge liquid result in increased linear dimensions (by 59%), floc area (by 81%) and floc volume (by 275%), but concurrently lead to a decrease in the strength of activated sludge flakes (by 21%). The obtained results in general indicated the positive effect of magnesium ion on the sedimentation properties of sludge in the aerotank-settlement system. A scale for quantitative determination of the strength of activated sludge flocs has been constructed. The developed computerized method enhances the precision and detail of visual evaluations of the technological attributes of activated sludge flocs, enabling the detection of even the smallest changes in the morphological characteristics across different wastewater treatment technologies. Keywords: activated sludge, flocs, concentration of magnesium, strength, technological characteristics.

Bulking of activated sludge in the biological treatment of municipal and industrial wastewater, due to the massive development of filamentous bacteria Type 021N

A research of the causes and consequences of bulking of activated sludge was conducted at existing biological wastewater treatment plants (municipal and local at an industrial enterprise). The filamentous bacterium that was caused the bulking of the activated sludge in the studied treatment plants was identified as Type 021N. It was established that the studied filamentous bulking causes a significant deterioration of the technological characteristics of activated sludge flocs and it is accompanied by suppression of the nitrification process. To suppress the bulking of activated sludge two methods of suppressing the inflow of hydrogen sulfide into biological treatment plants were successfully used: blowing from wastewater at municipal treatment plants and inhibiting its formation at local treatment plants in the settling and laminar flow zones.

Tracing morphological characteristics of activated sludge flocs by using a digital microscope and their effects on sludge dewatering and settling

ABSTRACT In wastewater treatment by the activated sludge (AS) process, settleability and dewaterability of AS are key issues that are directly related to the treated water quality and sludge treatment costs. Several studies investigated the relationship between the shape of AS flocs and their settling/dewatering property. To quantify the floc morphology, it is imperative to attach a camera to a microscope or move the stage manually. Hence, labour and equipment costs may increase. In this study, by combining a digital microscope and an automatic stage, more than 100 magnified floc images were rapidly obtained from one AS sample dropped on a slide glass, and shape parameters were collectively calculated using an analysis software. During 1-year monitoring of four wastewater treatment plants in Sapporo City (Hokkaido, Japan), the morphological parameters and extracellular polymeric substances (EPS) quantity/quality of AS were analyzed on the basis of their correlation to the time to filtration (TTF) and sludge volume index (SVI), which are indicators for describing the dewatering and settling properties of AS, respectively. In one plant, larger, denser, and smoother flocs tended to contain less EPS and exhibited better sludge dewaterability. In another plant, larger, denser, and smoother flocs were considered to contribute to better settlement. Especially, an equivalent high-density floc diameter and the ratio of mixed liquor suspended solids (MLSS) concentration to the total floc area were commonly suggested to explain AS dewaterability and settleability.

Microalgal-bacterial aerobic granular sludge for old leachate treatment: Development, performance, and lipid production

A lab-scale photo bio sequencing batch reactor (PSBR) was used to develop microalgal-bacterial aerobic granular sludge (MB-AGS) for old leachate treatment by seeding activated sludge, while indigenous microalgae were allowed to grow naturally. Upon 100 days of experiment, mature MB-AGS was successfully developed with an average diameter of 2.5 ± 1.5 mm and excellent settling properties, with a settling velocity of 39 ± 24 m/h. The biochemical oxygen demand (BOD), chemical oxygen demand (COD), ammonia (NH4+-N), total nitrogen (TN), and total phosphorus (TP) removal efficiencies were 80 ± 14%, 95 ± 2%, 88 ± 14%, 56 ± 16%, and 57 ± 14%, respectively. The shift in microbial populations were also examined, in which the abundant EPSs producing bacteria in the system could be one of the reasons behind the successful transformation of activated sludge flocs to mature MB-AGS. Furthermore, lipid was able to be extracted from the MB-AGS, in which the lipid production was 9.35 ± 1.1 mg/g-SS, accounting 0.56 ± 0.07 mg/L.d of lipid productivity.

Bioaugmentation of a structural extracellular polymeric substances (EPS) producer to improve activated sludge bioflocculation: lessons learned

Abstract Structural extracellular polymeric substances (EPS) contribute to the bioflocculation performance of activated sludge systems. This research investigates the potential of bioaugmentation of a structural EPS producer, Azoarcus communis, as a bioflocculation improvement or remediation approach. An antibiotic-resistant and fluorescent protein-producing mutant was constructed to monitor the survival, persistence, and location of the augmented strain in the membrane bioreactor. Preliminary batch tests against a kaolin clay model system and deflocculated sludge revealed the flocculation potential of this strain. Morphological image analysis and fluorescence microscopy suggest that most of the bacteria augmented in suspension were initially attached to the sludge flocs with, however, only a limited fraction getting incorporated within the activated sludge floc biomass. This limited bioaugmentation prevented assessing its impact on bioflocculation and might be explained by metazoan and protozoan grazing, together with competition with indigenous organisms and sub-optimal growth conditions in the reactor for the engineered strain.

Biomass types of nitrate-reducing denitrifiers to differentiate the hybrid community allocation of partial denitrification (PD) and anammox: IFAS vs MBBR

Partial denitrification (PD) and anaerobic ammonia oxidation (anammox) are modern treatment processes that can be combined to provide energy- and resource-efficient nitrogen removal methods; however, these processes are challenging to implement as a one-stage hybrid system. The coupling forms to achieve steady long-term PD/anammox operation need to be investigated. In this study, two reactors were operated for 300 days at 25 ± 1℃, in which one was inoculated with PD biofilms and anammox biofilms in a moving bed biofilm reactor (MBBR), and the other was inoculated with PD flocs and anammox biofilms in an integrated fixed-film activated sludge (IFAS) reactor. The results showed that the IFAS reactor had suprerior nitrogen removal performance, with the effluent total nitrogen (TN) concentration in the MBBR and IFAS of 5.17 ± 0.94 mg∙L−1 and 1.52 ± 0.75 mg∙L−1, respectively. Illumina MiSeq sequencing showed that Thauera and OLB8 were the dominant nitrate-reducing denitrifiers in PD biofilms in MBBR and flocs in IFAS, respectively. Since Thauera tended to selectively immigrate to the anammox biofilms, nitrate-reducing denitrifiers and anammox bacteria (AnAOB) coexisted in the anammox biofilms in MBBR. This decreased the PD activity (486.14 ± 23.04 mg∙(L∙d)−1 to 249.66 ± 26.90 mg∙(L∙d)−1) in the PD biofilms. On the contrary, nitrate-reducing denitrifiers and AnAOB were separately allocated in flocs and anammox biofilms in the IFAS reactor. The retention of PD biomass and superior mass transfer performance of the activated sludge flocs maintained a higher PD activity (2193.20 ± 26.62 mg∙(L∙d)−1), which provided sufficient NO2−-N for AnAOB and was beneficial to the growth of AnAOB. Hence, IFAS may be a potentially superior coupling mode for the PD/anammox process for engineering applications.

The mechanism of extracellular polymeric substances in the formation of activated sludge flocs

In this study, activated sludge (AS) presented high aggregation ability of 56.9% to form flocs. To elucidate the effects of extracellular polymeric substances (EPS) on the AS flocs formation, four EPS fractions, including soluble EPS (S-EPS), loosely bound EPS (LB-EPS), ionic tightly bound EPS (ionic TB-EPS) and hydrophobic tightly bound EPS (hydrophobic TB-EPS), were extracted and characterized. The ionic TB-EPS were the largest fraction, accounting for 51.3% of the total EPS. By extracting EPS fractions, it was shown that ionic TB-EPS followed by hydrophobic TB-EPS played positive contribution to the formation of AS flocs, while both S-EPS and LB-EPS had negative effects. Furthermore, chemical reagents (Urea, SDS and EDTA) assays, zeta potential and spectroscopic assays were conducted to explore the role of hydrogen bonds, electrostatic interaction and hydrophobic interaction in the formation of AS flocs. The result showed that ionic TB-EPS contained sufficient cation-binding sites, such as -OH and CO-O- groups, and promoted AS aggregation through ion bridging. Hydrophobic TB-EPS contained hydrophobic groups and promoted AS aggregation through hydrophobic action. A high proportion of uronic acids resulted in high surface negative charge, which might be the reason that LB-EPS and S-EPS inhibited AS aggregation through electrostatic repulsion. From the perspective of molecule structure, the proteins in ionic TB-EPS had a loose structure, which fully exposed the inner cation-binding sites. This might be the critical reason why ion bridging acted as the main force for the formation of AS flocs. This study provided a new sight for the extraction of EPS fractions and expanded the understanding of EPS in the formation of AS flocs.

Nitrification kinetics, N2O emission, and energy use in intermittently aerated hybrid reactor under different organic loading rates

This study investigated the impact of intermittent aeration strategies and reduction in the reactor’s organic and nitrogen loading rates on the course of particular stages of the nitrification process, as well as energy consumption and N2O emissions in a hybrid reactor with nitrification/denitrification. Each of the analysed series revealed the greatest ammonia oxidation activity in activated sludge flocs. The highest activity of nitrite nitrogen oxidation was demonstrated in the case of biofilm. A reduction in the reactor’s organic and nitrogen loading rate value had a greater effect on changes in the activity of ammonia-oxidizing bacteria than nitrite-oxidizing bacteria. In a system where the operation of air pumps was controlled through switching them and off according to the adopted ratio between non-aerated and aerated sub-phase times and the assumed oxygen concentration, a reduction in the duration of aerated sub-phases caused no decrease in energy use for aeration. Lower N2O emission was recorded when the reactor operated with a longer duration of aerated sub-phases.

Alleviating the adverse impacts of high influent ammonium on an activated sludge system by adding Fe salts and ballasted media

High influent ammonium often leads to sludge disintegration and solubilization in activated sludge systems and thereby deteriorates pollutant removal, due to the presence of free ammonium at high concentrations. The effects of adding Fe(II)/Fe(III) salts and ballasted media (magnetic powder, PAC and quartz sand) into activated sludge systems to alleviate the adverse impacts imposed by high influent ammonium were investigated in lab-scale continuously operated SBRs. When influent ammonium was approximately 500 mg N/L, the beneficial effects of Fe salts and ballasted media addition on pollutant removal were not significant, although they showed elevated aggregation of activated sludge flocs as reflected by the values of SV30 and MLVSS. When the influent ammonium concentration was increased to as high as 700 mg N/L, the pollutant removal and sludge properties in all the reactors deteriorated. Fe salts and ballasted media addition alleviated such deterioration by retaining sludge settleability and relatively stable biomass and resulted in better pollutant removal compared to no additive addition. Microbial analyses also revealed that the abundance of flocculation bacteria (Zoogloea and Haliscomenobacter) and nitrifiers (Nitrosomonas and Nitrosopira) were higher in Fe salts or ballasted media-supplemented reactors, which correlated well with the maintenance of sludge settleability and nitrification under high ammonium stress. When the influent ammonium concentration was increased to approximately 900 mg N/L, rapid collapse occurred in all reactors regardless of whether Fe salts and ballasted media were added, indicating that activated sludge systems cannot withstand such a high influent ammonium concentration. According to the continuous monitoring of pollutants removal, nitrification, and changes in sludge properties, the degree of alleviating high ammonium stress follows the order (from high to low) of magnetic powder, PAC, Fe(II), Fe(III), and quartz sand. According to an operation cost analysis, Fe(II) had the least additional cost (0.4–0.5%). The additional costs of MP and PAC were same (4–5.33%). Fe(II) was recommended based on an overall balance in technical and cost aspects.

Evaluation of the efficiency of the mixing system of the laboratory SBR-type reactor

Requirements for the continuous improvement of the quality of wastewater discharged to a receiving water body have led to the development of many devices used in biological treatment. Current research in water and wastewater solutions often focuses on finding energy-efficient solutions that have the least negative impact on the environment while reducing operating costs. Treatment systems commonly use the properties of microorganism structures in the form of activated sludge flocs to remove organic and nutrient compounds contained in wastewater. An important part of this solution is the use of a mixing and aeration system in the appropriate configuration. In a wastewater treatment plant with a biological part operating with SBR technology, many types of mixing equipment are used. This article presents the use of an innovative mixing system in sequential batch bioreactors in which activated sludge is the process medium.

A hindered settling velocity model related to the fractal dimension and activated sludge flocs characteristics: Application to a sludge with a previous fragmentation and flocculation process

The modelling of the activated sludge hindered settling velocity as a function of the flocs characteristics and the incorporation of these models to the secondary settling tanks simulation is a hardly studied subject. Commonly used empirical models cannot describe the changes in the settling velocity caused by changes in the flocs characteristics. In this paper, a model for the hindered settling velocity as a function of the fractal dimension and other flocs characteristics is proposed. The model was used to describe the settling velocity after a flocculation process originated by activated sludge fragmentation. The model reproduces the observed abrupt decrease of hindered settling velocity in a small range of suspended solids concentration. It also enables to relate the flocs characteristics and the settling velocity to the fragmentation and sludge aggregation mechanisms.

Diversity and Ecophysiology of the Genus OLB8 and Other Abundant Uncultured Saprospiraceae Genera in Global Wastewater Treatment Systems.

The Saprospiraceae family within the phylum Bacteroidota is commonly present and highly abundant in wastewater treatment plants (WWTPs) worldwide, but little is known about its role. In this study, we used MiDAS 4 global survey with samples from 30 countries to analyze the abundance and distribution of members of Saprospiraceae. Phylogenomics were used to delineate five new genera from a set of 31 high-quality metagenome-assembled genomes from Danish WWTPs. Newly designed probes for fluorescence in situ hybridization (FISH) revealed rod-shaped morphologies for all genera analyzed, including OLB8, present mostly inside the activated sludge flocs. The genomes revealed potential metabolic capabilities for the degradation of polysaccharides, proteins, and other complex molecules; partial denitrification; and storage of intracellular polymers (glycogen, polyphosphate, and polyhydroxyalkanoates). FISH in combination with Raman microspectroscopy confirmed the presence of intracellular glycogen in Candidatus Brachybacter, Candidatus Parvibacillus calidus (both from the former genus OLB8), and Candidatus Opimibacter, and the presence of polyhydroxyalkanoates in Candidatus Defluviibacterium haderslevense and Candidatus Vicinibacter. These results provide the first overview of the most abundant novel Saprospiraceae genera present in WWTPs across the world and their potential involvement in nutrient removal and the degradation of macromolecules.

On the mechanisms for aerobic granulation - model based evaluation

In this study a mathematical framework was developed to describe aerobic granulation based on 6 main mechanisms: microbial selection, selective wasting, maximizing transport of substrate into the biofilm, selective feeding, substrate type and breakage. A numerical model was developed using four main components; a 1D convection/dispersion model to describe the flow dynamics in a reactor, a reaction/diffusion model describing the essential conversions for granule growth, a setting model to track granules during settling and feeding, and a population model containing up to 100,000 clusters of granules to model the stochastic behaviour of the granulation process. With this approach the model can explain the dynamics of the granulation process observed in practice. This includes the presence of a lag phase and a granulation phase. Selective feeding was identified as an important mechanism that was not yet reported in literature. When aerobic granules are grown from activated sludge flocs, a lag phase occurs, in which not many granules are formed, followed by a granulation phase in which granules rapidly appear. The ratio of granule forming to non-granule forming substrate together with the feast/famine ratio determine if the transition from the lag phase to the granulation phase is successful. The efficiency of selective wasting and selective feeding both determine the rate of this transition. Brake-up of large granules into smaller well settling particles was shown to be an important source for new granules. The granulation process was found to be the combined result from all 6 mechanisms and if conditions for either one are not optimal, other mechanisms can, to some extent, compensate. This model provides a theoretical framework to analyse the different relevant mechanisms for aerobic granular sludge formation and can form the basis for a comprehensive model that includes detailed nutrient removal aspects.

Assessment of the Impact of Selected Industrial Wastewater on the Nitrification Process in Short-Term Tests.

Many chemical compounds can inhibit the nitrification process, especially organic compounds used in the chemical industry. This results in a decrease in the nitrification intensity or even a complete termination of this process. As the technological design of the selected municipal and industrial wastewater treatment plant (WWTP) assumed the dephosphation process, without taking into account nitrification, it was necessary to reduce the concentration of ammonium nitrogen in the treated sewage supplied to the Vistula River. Therefore, the aim of the research was to determine the inhibition of nitrification in the activated sludge method under the influence of industrial wastewater from the production of various organic compounds and to select the most toxic wastewater in relation to nitrifiers. The assessment of nitrification inhibition was carried out on the basis of the method of short-term (4-h) impact of the tested sewage on nitrifying bacteria in the activated sludge. The research covered nine different types of chemical sewage, including wastewater from the production of synthetic rubbers, styrene plastics, adhesives, solvents and emulsifiers. The nitrification process was inhibited to the highest degree by wastewater from the production of styrene-butadiene rubbers (72%). Only wastewater from the production of methacrylate (polymethyl methacrylate) had the lowest degree of inhibition: 16%. These wastewaters also have a toxic effect on the entire biocenosis and adversely affect the structure of activated sludge flocs. The attempts to filter toxic wastewater through the ash basins significantly relieved the inhibition of nitrification.

Segmentation of Activated Sludge Flocs in Microscopic Images for Monitoring Wastewater Treatment

The proposed work describes an approach for segmentation of activated sludge flocs from the microscopic images for monitoring wastewater treatment. The morphological features of flocs (microbial aggregates) and filaments are related to the state of an activated sludge wastewater treatment plant and must be monitored for proper functioning. Hence, image processing and analysis could be a time‐saving monitoring tool. To address this challenge, we propose a novel framework involving a multiphase edge detection algorithm based on information theory. The proposed framework is evaluated and scrutinized critically considering the artifacts found in the photographs tested. To evaluate the segmentations, gold approximation of estimated truth images is created. In addition, the performance was subjectively evaluated for its potential for segmenting activated sludge images. Experimental results show that the proposed framework exhibits the good results and demonstrates its effectiveness.

Correlation of cells with proteins in extracellular polymeric substance matrix of bioaggregates

The bioaggregates for wastewater treatment are made of cells and the extracellular polymeric substances (EPS) secreted by the cells. This study performed multiple fluorescence staining technique and confocal laser scanning microscope (CLSM) observations to probe the spatial distributions of cells, proteins, and α-polysaccharides in the activated sludge flocs and the aerobic granule. The pair correlation analysis revealed that the cells have high affinity to proteins instead of to polysaccharides. Based on the patterns of the pair correlation functions, the sludge floc was shown to be a matrix reinforced by α-polysaccharides with embedded microaggregates of cells and proteins of size of 2–3 cell diameters.