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ABSTRACT: BACKGROUND: The activated sludge process is one of the most widely used methods for treatment of wastewater and the microbial community composition in the sludge is important for the process operation. While the bacterial communities have been characterized in various activated sludge systems little is known about archaeal communities in activated sludge. The diversity and dynamics of the Archaea community in a full-scale activated sludge wastewater treatment plant were investigated by fluorescence in situ hybridization, terminal restriction fragment length polymorphism analysis and cloning and sequencing of 16S rRNA genes. RESULTS: The Archaea community was specialized and dominated by Methanosaeta-like species. During a 15 month period major changes in the community composition were only observed twice despite seasonal variations in environmental and operating conditions. Water temperature appeared to be the process parameter that affected the community composition the most. Several terminal restriction fragments also showed strong correlations with sludge properties and effluent water properties. The Archaea were estimated to make up 1.6-% of total cell numbers in the activated sludge and were present both as single cells and colonies of varying sizes. CONCLUSIONS: The results presented here show that Archaea can constitute a constant and integral part of the activated sludge and that it can therefore be useful to include Archaea in future studies of microbial communities in activated sludge.
BMC Microbiology 07/2012; 12(1):140. · 3.04 Impact Factor
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ABSTRACT: A comprehensive study of the dynamics in settling and flocculation properties at a full-scale activated sludge treatment plant with secondary settlers operated at their maximum capacity is presented. An extensive set of process data was analyzed to assess physical parameters affecting the capacity and performance of the secondary settlers. There was a large variation in the settling and compaction properties, and they changed quickly, as a result of the short sludge age. Even though the flocculation properties changed throughout the year, they were generally improved during the summer, with lower values of effluent turbidity and effluent suspended solids concentration. The shear sensitivity, which is a measure of floc stability, was higher during the winter, when the water temperatures were lower. The sludge floc morphology had, in general, a different internal structure, with more round microbial colonies during the warmer summer months and a more net-like open structure during the winter.
Water Environment Research 02/2010; 82(2):155-68. · 0.88 Impact Factor
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ABSTRACT: The variation in activated sludge floc composition, flocculation and settling properties was studied at a full scale plant over a 2-year period. A comprehensive set of process parameters was analysed and related to the floc properties to increase the understanding of the factors affecting floc formation. The composition of the activated sludge showed a seasonal change with higher concentrations of extractable extracellular polymeric substances (EPS) during the winter months. The protein content of the total sludge and EPS increased significantly during the winter. This coincided with higher effluent suspended solids concentrations and increased shear sensitivity of the sludge flocs. Only poor correlations between EPS contents and stirred sludge volume index (SSVI) could be observed. High iron concentrations in the sludge due to dosage of iron salt to precipitate phosphorus were found to have a negative impact on the settling and compaction properties of the sludge, whereas it had a positive impact on floc stability. Higher organic loading due to by-passed primary settlers leads to improved settling and compaction properties.
Water Research 09/2008; 42(16):4404-18. · 4.86 Impact Factor
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ABSTRACT: The efficiency of activated sludge treatment plants is dependent on the solid-liquid separation properties of the activated sludge. A critical parameter is the stability of the microbial flocs. Weak flocs deflocculate easily leaving increased concentrations of suspended solids in the effluent. The knowledge about how different bacteria are attached to the flocs and their influence on the bioflocculation is limited. In this study, the deflocculation of different phylogenetic groups of bacteria in activated sludge from a full scale plant was investigated. The experiments were carried out by using a shear method where the sludge flocs are deflocculated under controlled shear conditions. The degree of deflocculation was measured as increase in turbidity of the supernatant. Identification and quantification of the microbial community structure of both total activated sludge and deflocculated bacteria were conducted with group-specific gene probes for broad groups of bacteria (Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria and Actinobacteria) and fluorescence in situ hybridization (FISH). The microbial community structure of the deflocculated bacteria was different compared to the total activated sludge with a higher abundance of Gammaproteobacteria in the supernatant indicating that different groups of bacteria are bound with different strength to the floc surface. The results show that the bacteria in the outer parts of the flocs are relatively loosely bound to the floc matrix and can be easily eroded from the surface when exposed to shear.
Water Research 05/2008; 42(8-9):2300-8. · 4.86 Impact Factor
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ABSTRACT: Activated sludge flocs are known to deflocculate under short-term anaerobic conditions, but little is known about possible reflocculation under subsequent aerobic conditions. When activated sludge flocs from two wastewater treatment plants deflocculated under anaerobic conditions with well-defined shear conditions, they could be almost, but not completely, reflocculated by aeration for 1-2 h under the same shear conditions. If the biological activity was reduced by adding azide, chloramphenicol or by decreasing the temperature, no or only very little reflocculation took place. This indicated that the reflocculation was under direct or indirect microbial control. Only a small part of the reflocculation was due to improved flocculation properties obtained by oxidation of Fe(II) to Fe(III), which is a better flocculant. Fe(II) was produced under the anaerobic conditions by microbial iron reduction, and it was oxidized to Fe(III) within less than one hour after the aeration was started. However, by comparing two different sludges with different capabilities for iron reduction, iron oxidation and responses to substrate addition, it was found that the aerobic biological activity most likely was of greatest significance for the observed reflocculation and floc formation under aerobic conditions. This was further supported by adding organic substrates (glucose or ethanol) during the aerobic reflocculation phase, which promoted reflocculation. However, some substrates had the opposite effect (acetate and lactate), where a deterioration of the reflocculation was observed, probably due to different responses from different groups of microorganisms in the sludges.
Water Research 12/2004; 38(18):3909-19. · 4.86 Impact Factor
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ABSTRACT: In this article we present a study of the effects of external and internal mass transfer limitation of oxygen in a nitrifying system. The oxygen uptake rates (OUR) were measured on both a macro-scale with a respirometric reactor using off-gas analysis (Titrimetric and Off-Gas Analysis (TOGA) sensor) and on a micro-scale with microsensors. These two methods provide independent, accurate measurements of the reaction rates and concentration profiles around and in the granules. The TOGA sensor and microsensor measurements showed a significant external mass transfer effect at low dissolved oxygen (DO) concentrations in the bulk liquid while it was insignificant at higher DO concentrations. The oxygen distribution with anaerobic or anoxic conditions in the center clearly shows major mass transfer limitation in the aggregate interior. The large drop in DO concentration of 22-80% between the bulk liquid and aggregate surface demonstrates that the external mass transfer resistance is also highly important. The maximum OUR even for floccular biomass was only attained at much higher DO concentrations (approximately 8 mg/L) than typically used in such systems. For granules, the DO required for maximal activity was estimated to be >20 mg/L, clearly indicating the effects of the major external and internal mass transfer limitations on the overall biomass activity. Smaller aggregates had a larger volumetric OUR indicating that the granules may have a lower activity in the interior part of the aggregate.
Biotechnology and Bioengineering 05/2004; 86(4):445-57. · 3.95 Impact Factor
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ABSTRACT: Activated sludge samples from seven full-scale plants were investigated in order to determine the relationship between floc structure and floc stability. Floc stability was determined by shear sensitivity and floc strength. Floc structure was considered in terms of two size scales, the micro- and macrostructure. The microstructure refers to the organization of the floc components, such as the individual microorganisms. The macrostructure refers to the overall floc. The floc macrostructure was characterized by filament index, sludge volume index, size, and fractal dimension. It had a significant impact on floc stability. Large and open flocs with low fractal dimensions containing large number of filaments were more shear sensitive and had lower floc strength compared to small and dense flocs. Fluorescent in situ hybridization analysis indicated that the organization of the bacterial cells might also have an effect on the floc stability.
Water Research 10/2003; 37(15):3632-45. · 4.86 Impact Factor
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ABSTRACT: This paper examines the influence of the chemical constituents of activated sludge and extracted extracellular polymeric substances (EPS) on the surface properties, hydrophobicity, surface charge (SC) and flocculating ability (FA) of activated sludge flocs. Activated sludge samples from 7 different full-scale wastewater treatment plants were examined. Protein and humic substances were found to be the dominant polymeric compounds in the activated sludges and the extracted EPS, and they significantly affected the FA and surface properties, hydrophobicity and SC, of the sludge flocs. The polymeric compounds proteins, humic substances and carbohydrates in the sludge flocs and the extracted EPS contributed to the negative SC, but correlated negatively to the hydrophobicity of sludge flocs. The quantity of protein and carbohydrate within the sludge and the extracted EPS was correlated positively to the FA of the sludge flocs, while increased amounts of humic substances resulted in lower FA. In contrast, increased amounts of total extracted EPS had a negative correlation to FA. The results reveal that the quality and quantity of the polymeric compounds within the sludge flocs is more informative, with respect to understanding the mechanisms involved in flocculation, than if only the extracted EPS are considered. This is an important finding as it indicates that extracting EPS may be insufficient to characterise the EPS. This is due to the low extraction efficiency and difficulties involved in the separation of EPS from other organic compounds. Correlations were observed between the surface properties and FA of the sludge flocs. This confirms that the surface properties of the sludge flocs play an important role in the bioflocculation process but that also other interactions like polymer entanglement are important.
Water Research 06/2003; 37(9):2127-39. · 4.86 Impact Factor
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ABSTRACT: This paper presents a comprehensive study of sludge floc characteristics and their impact on compressibility and settleability of activated sludge in full scale wastewater treatment processes. The sludge flocs were characterised by morphological (floc size distribution, fractal dimension, filament index), physical (flocculating ability, viscosity, hydrophobicity and surface charge) and chemical (polymeric constituents and metal content) parameters. Compressibility and settleability were defined in terms of the sludge volume index (SVI) and zone settling velocity (ZSV). The floc morphological and physical properties had important influence on the sludge compressibility and settleability. Sludges containing large flocs and high quantities of filaments, corresponding to lower values of fractal dimension (Df), demonstrated poor compressibility and settleability. Sludge flocs with high flocculating ability had lower SVI and higher ZSV, whereas high values of hydrophobicity, negative surface charge and viscosity of the sludge flocs correlated to high SVI and low ZSV. The quantity of the polymeric compounds protein, humic substances and carbohydrate in the sludge and the extracted extracellular polymeric substances (EPS) had significant positive correlations with SVI. The ZSV was quantitatively independent of the polymeric constituents. High concentrations of the extracted EPS were related to poor compressibility and settleability. The cationic ions Ca, Mg, Al and Fe in the sludge improved significantly the sludge compressibility and settleability.
Chemical Engineering Journal.
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ABSTRACT: The influence of the microbial activity on the strength of activated sludge flocs was studied in short term experiments (0–3 h). Increased floc strength was generally obtained when the aerobic microbial activity was stimulated by adding substrate. Deflocculation was observed when the aerobic microbial activity was inhibited by (i) anaerobic conditions; (ii) addition of the metabolic inhibitors azide and chloramphenicol; and (iii) reduction of the temperature to 4°C. Furthermore, addition of nitrate as electron acceptor under anaerobic conditions partly prevented deflocculation from taking place. These results strongly suggested that microorganisms using oxygen and/or nitrate as electron acceptors were important for maintaining the floc strength. The increase in turbidity under deflocculation was well correlated with the number of bacteria and concentration of protein, humic substances and carbohydrates in the supernatant. However, only approximately 1–2% of the total amount of sludge deflocculated, so the deflocculation could be understood as an erosion of small particles from the larger flocs. The extent of deflocculation under anaerobic conditions could be enhanced by stimulation of the anaerobic biological activity. When anaerobic conditions prevailed, a microbial iron reduction immediately started with iron reduction rates of 4–150 μmol/gVS·h. Subsequently, a decrease in floc strength was observed which could also be observed when the iron-reducing bacterium Shewanella alga BrY was added to the activated sludge. Furthermore, the importance of Fe(III) for the floc strength was illustrated by removal of Fe(III) from the sludge matrix by adding sulphide, which resulted in strong deflocculation. Thus, the deflocculation observed could be either directly due to lack of aerobic microbial activity or indirectly due to change in the local physico-chemical conditions mediated by anaerobic microbial activity.
Colloids and Surfaces B: Biointerfaces.
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ABSTRACT: This study examined how the floc characteristics affect dewaterability of activated sludge. The floc properties were characterized by morphological parameters (floc size distribution, fractal dimension and filament index), physical properties (flocculating ability, surface charge, relative hydrophobicity and viscosity), and chemical constituents in sludge and extracted extracellular polymeric substances (EPS), including the polymeric compounds protein, humic substances, carbohydrates and the ions Ca2+, Mg2+, Fe3+ and Al3+. The dewaterability was defined in terms of the bound water content and capillary suction time (CST). The bound water and CST corresponded to a similar indication with respect to dewaterability of activated sludge. The floc physical parameters were the most important factors which effect significantly on the water binding ability of the sludge flocs. The morphological characteristics had relatively weak impact on the dewaterability. The polymeric components protein and carbohydrate had a significant contribution to enhance the water binding ability of the sludge flocs. The effect of humic substances in the sludge on the dewaterability was, however, insignificant. The CST had good statistical correlations with the polymeric constituents measured in both sludge and the extracted EPS, and the bound water was only correlated well with the individual polymers measured in the sludge. High concentration of Ca2+, Mg2+, Fe3+ and Al3+ had significant improvement for dewaterability.
Chemical Engineering Journal.
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ABSTRACT: Activated sludge deflocculated when it was exposed to anaerobic conditions and shear. The extent of deflocculation, measured as turbidity, varied with length of anaerobic period and sludge type (age). In order to restore the deflocculated sludge, oxygen was added, and subsequently, in less than 1 h, most of the deflocculated matter was reflocculated. A certain part of the deflocculated floc components remained, however, deflocculated even after several hours of aeration. As the sludge was exposed to repeated deflocculation–reflocculation conditions, the flocs were progressively damaged, and an accumulation of non-flocculated matter was observed. Measurements of the number of bacteria, the concentration of protein, humic substances and carbohydrate in the sludge and the supernatant indicated that the deflocculated material was mainly composed of bacteria and extracellular polymeric substances (EPS) attached to cells or floc fragments. The non-flocculated particles after reflocculation were enriched in bacteria and protein, compared to the average sludge composition. A reduced deflocculation was observed when nitrate acted as an electron acceptor (anoxic conditions), compared to anaerobic conditions. Increased shear and decreased temperature reinforced the deflocculation. Field experiments showed a temporal variation in floc strengths with the weakest flocs present in the winter period.
Water Research.
