Britt-Marie Wilén

The influence of key chemical constituents in activated sludge on surface and flocculating properties.

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.

A comprehensive insight into floc characteristics and their impact on compressibility and settleability of activated sludge

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 (D-f), 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

The effect of dissolved oxygen concentration on the structure, size and size distribution of activated sludge flocs

The variation in activated sludge floc structure, size and size distribution were studied for different dissolved oxygen (DO) concentrations in pilot scale completely mixed reactors. The size distribution by volume for flocs larger than about 10 mm fitted well to log-normal distribution functions. No clear relationship between DO concentration and average floc diameter could be found; there was only a trend towards larger flocs at higher DO concentrations. Lower DO concentrations (0.5-2.0 mg/l) produced sludge with poorer settling properties and higher turbidities of the effluent than higher DO concentrations (2.0-5.0 mg/l). The main reason to the deteriorated settling properties were excessive growth of filamentous bacteria and the formation of porous flocs. Alternating oxic and anoxic conditions (1-4 hours) did not effect the settling properties to a large extent. The turbidity increased dramatically during the anoxic period and decreased during the oxic period. The supernatant was also analysed with a particle analyser. During alternating oxic/anoxic conditions, the proportion of smaller flocs (2-20 mm) increased gradually during the anoxic periods. Directly after that the oxygen supply was turned on, the number of small flocs decreased. The flocs in the supernatant had after 20-60 minutes settling normally a diameter up to 70-80 mm. In most of the measurements, more than 80% of the flocs were smaller than 2 mm. The size distribution of small flocs could best be fitted to power functions. The number of flocs in the supernatant could relatively well be related to the turbidity.

Impacts of structural characteristics on activated sludge floc stability

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.

Influence of microbial activity on the stability of activated sludge flocs

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 mmol/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.

Anaerobic deflocculation and aerobic reflocculation of activated sludge

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.

Relationship between floc composition and flocculation and settling properties studied at a full scale activated sludge plant

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.

Microbial community structure in activated sludge floc analysed by fluorescence in situ hybridization and its relation to floc stability.

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.

Structure and composition of biofilm communities in a moving bed biofilm reactor for nitritation-anammox at low temperatures.

It is a challenge to apply anaerobic ammonium oxidation (anammox) for nitrogen removal from wastewater at low temperatures. Maintenance of anammox- and aerobic ammonia oxidizing bacteria (AOB) and suppression of nitrite oxidizing bacteria (NOB) are key issues. In this work, a nitritation-anammox moving bed biofilm pilot reactor was operated at 19-10°C for 300 d. Nitrogen removal was decreasing, but stable, at 19-13°C. At 10°C removal became unstable. Quantitative PCR, fluorescence in situ hybridization and gene sequencing showed that no major microbial community changes were observed with decreased temperature. Anammox bacteria dominated the biofilm (0.9-1.2 × 10(14) 16S rRNA copies m(-2)). Most anammox bacteria were similar to Brocadia sp. 40, but another smaller Brocadia population was present near the biofilm-water interface, where also the AOB community (Nitrosomonas) was concentrated in thin layers (1.8-5.3 × 10(12) amoA copies m(-2)). NOB (Nitrobacter, Nitrospira) were always present at low concentrations (<1.3 × 10(11) 16S rRNA copies m(-2)).

The Choice of PCR Primers Has Great Impact on Assessments of Bacterial Community Diversity and Dynamics in a Wastewater Treatment Plant

Assessments of bacterial community diversity and dynamics are fundamental for the understanding of microbial ecology as well as biotechnological applications. We show that the choice of PCR primers has great impact on the results of analyses of diversity and dynamics using gene libraries and DNA fingerprinting. Two universal primer pairs targeting the 16S rRNA gene, 27F&1492R and 63F&M1387R, were compared and evaluated by analyzing the bacterial community in the activated sludge of a large-scale wastewater treatment plant. The two primer pairs targeted distinct parts of the bacterial community, none encompassing the other, both with similar richness. Had only one primer pair been used, very different conclusions had been drawn regarding dominant phylogenetic and putative functional groups. With 27F&1492R, Betaproteobacteria would have been determined to be the dominating taxa while 63F&M1387R would have described Alphaproteobacteria as the most common taxa. Microscopy and fluorescence in situ hybridization analysis showed that both Alphaproteobacteria and Betaproteobacteria were abundant in the activated sludge, confirming that the two primer pairs target two different fractions of the bacterial community. Furthermore, terminal restriction fragment polymorphism analyses of a series of four activated sludge samples showed that the two primer pairs would have resulted in different conclusions about community stability and the factors contributing to changes in community composition. In conclusion, different PCR primer pairs, although considered universal, target different ranges of bacteria and will thus show the diversity and dynamics of different fractions of the bacterial community in the analyzed sample. We also show that while a database search can serve as an indicator of how universal a primer pair is, an experimental assessment is necessary to evaluate the suitability for a specific environmental sample.