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1.
Water Res ; 82: 14-24, 2015 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-25959073

RESUMO

Biological wastewater treatment removes organic materials, nitrogen, and phosphorus from wastewater using microbial biomass (activated sludge, biofilm, granules) which is separated from the liquid in a clarifier or by a membrane. Part of this biomass (excess sludge) is transported to digesters for bioenergy production and then dewatered, it is dewatered directly, often by using belt filters or decanter centrifuges before further handling, or it is dewatered by sludge mineralization beds. Sludge is generally difficult to dewater, but great variations in dewaterability are observed for sludges from different wastewater treatment plants as a consequence of differences in plant design and physical-chemical factors. This review gives an overview of key parameters affecting sludge dewatering, i.e. filtration and consolidation. The best dewaterability is observed for activated sludge that contains strong, compact flocs without single cells and dissolved extracellular polymeric substances. Polyvalent ions such as calcium ions improve floc strength and dewaterability, whereas sodium ions (e.g. from road salt, sea water intrusion, and industry) reduce dewaterability because flocs disintegrate at high conductivity. Dewaterability dramatically decreases at high pH due to floc disintegration. Storage under anaerobic conditions lowers dewaterability. High shear levels destroy the flocs and reduce dewaterability. Thus, pumping and mixing should be gentle and in pipes without sharp bends.


Assuntos
Esgotos/química , Eliminação de Resíduos Líquidos/métodos , Filtração , Floculação , Água/química
2.
Chemosphere ; 117: 144-50, 2014 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-25014564

RESUMO

Fouling by free extracellular polymeric substances was studied in an enhanced biological phosphorus removal-membrane bioreactor. It was demonstrated that the free extracellular polymeric substances, primarily consisting of humic-like substances, were adsorbed to the membrane used in the enhanced biological phosphorus removal-membrane bioreactor plant. Infrared analyses indicated the presence of the humic-like substances on the membrane's active surface after filtration of the free extracellular polymeric substances suspension. Scanning electron microscopy showed the presence of a gel layer on the membrane surface after filtration of the free extracellular polymeric substances suspension. The gel layer caused a significant decline in water flux. This layer was not entirely removed by a backwashing, and the membrane's water flux could not be re-established. The membrane used in the enhanced biological phosphorus removal-membrane bioreactor plant showed infrared spectra similar to that fouled by the free extracellular polymeric substances suspension in the laboratory. Thus, the results of this study show the importance of humic-like substances in irreversible fouling of enhanced biological phosphorus removal-membrane bioreactor systems.


Assuntos
Reatores Biológicos , Géis/análise , Substâncias Húmicas/análise , Polímeros/metabolismo , Eliminação de Resíduos Líquidos , Adsorção , Permeabilidade , Fósforo/metabolismo
3.
Water Res ; 47(17): 6719-30, 2013 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-24094729

RESUMO

In the conventional activated sludge process, a number of important parameters determining the efficiency of settling and dewatering are often linked to specific groups of bacteria in the sludge--namely floc size, residual turbidity, shear sensitivity and composition of extracellular polymeric substances (EPS). In membrane bioreactors (MBRs) the nature of solids separation at the membrane has much in common with sludge dewaterability but less is known about the effect of specific microbial groups on the sludge characteristics that affect this process. In this study, six full-scale MBR plants were investigated to identify correlations between sludge filterability, sludge characteristics, and microbial community structure. The microbial community structure was described by quantitative fluorescence in situ hybridization and sludge filterability by a low-pressure filtration method. A strong correlation between the degree of flocculation (ratio between floc size and residual turbidity) and sludge filterability at low pressure was found. A good balance between EPS and cations in the sludge correlated with good flocculation, relatively large sludge flocs, and low amounts of small particles and single cells in the bulk phase (measured as residual turbidity), all leading to a good filterability. Floc properties could also be linked to the microbial community structure. Bacterial species forming strong microcolonies such as Nitrospira and Accumulibacter were present in plants with good flocculation and filtration properties, while few strong microcolonies and many filamentous bacteria in the plants correlated with poor flocculation and filtration problems. In conclusion this study extends the hitherto accepted perception that plant operation affects floc properties which affects fouling. Additionally, plant operation also affects species composition, which affects floc properties and in the end fouling propensity.


Assuntos
Bactérias/crescimento & desenvolvimento , Reatores Biológicos/microbiologia , Cidades , Filtração , Esgotos/microbiologia , Águas Residuárias/microbiologia , Purificação da Água , Biodegradação Ambiental , Floculação , Membranas Artificiais , Fósforo/isolamento & purificação
4.
Water Res ; 46(16): 4999-5008, 2012 Oct 15.
Artigo em Inglês | MEDLINE | ID: mdl-22819870

RESUMO

A drainage/filtration apparatus was developed for automatically determining sedimentation velocity and dewatering rate. Pressure-step testing was used to study filter cake compressibility, resistance, and swelling. Activated sludge was analysed, and the data indicate that the sludge is highly compressible even at low pressures (10 kPa). Furthermore, compressed sludge cakes swell if the pressure is released. Hence, the average specific cake resistance decreases if the pressure is released, though the resistance is higher after the compression cycle than before. Sludge must be dewatered under low pressure, because higher pressure only compresses the cake and does not improve the dewatering rate.


Assuntos
Dessecação/métodos , Filtração/métodos , Esgotos/química , Eliminação de Resíduos Líquidos/métodos , Filtração/instrumentação , Fluoresceína , Pressão , Vácuo , Eliminação de Resíduos Líquidos/instrumentação , Água/química
5.
Water Res ; 46(14): 4330-8, 2012 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-22748327

RESUMO

Fouling is inevitable in membrane bioreactors (MBRs) due to the complex nature of activated sludge, which contains a broad variety of potential foulants. Filter cakes that build up from sludge particles are traditionally highly compressible due to both the deformation of the individual sludge particles and the rearrangement of these particles in the cake. However, this phenomenon has been little examined in studies of fouling mechanisms in MBR systems. This study examines the properties of the cake layer, modeling the cake buildup and specific cake resistance (α), including compressibility, in terms of pressure-dependent α. The changes in fouling resistance during transmembrane pressure (TMP)-step filtration in an MBR setup were simulated using an empirical pressure dependence of the specific cake resistance and a simple mass balance model. The total change in fouling resistance in each TMP step could be divided into an initial rapid change in specific cake resistance due to filter cake compression followed by simple cake buildup. By including cake compression in this simple model, the model fitted the data with high precision. We demonstrated that compressibility should be considered when describing cake fouling in MBRs.


Assuntos
Incrustação Biológica , Reatores Biológicos , Filtração/instrumentação , Filtração/métodos , Membranas Artificiais , Modelos Teóricos , Pressão , Algoritmos , Simulação por Computador
6.
Water Res ; 45(19): 6453-60, 2011 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-22000719

RESUMO

Sludge-drying reed beds can be a cost-effective and sustainable solution to surplus activated sludge dewatering and mineralization, especially for small wastewater treatment plants. However, the simplicity as well as low energy and monitoring requirements of this technology are often counterbalanced by frequent operational problems consisting of slow and insufficient dewatering, poor vegetation growth, odor, and overall poor mineralization of the sludge residues. The main reason is that the general rules for facility design and operation are based on empirical experience rather than on the actual and current sludge parameters. In this study a new method for the assessment of activated sludge drainage properties has been applied to determine the reasons behind operational problems faced by the operators of reed bed facility accepting surplus activated sludge from two wastewater treatment plants in Esbjerg, Denmark. The importance of sludge quality monitoring as well as the damaging effect of shear forces, oxygen depletion, and long-distance sludge transportation were demonstrated. Finally, more general guidelines for reed bed facility design and operation are given, based on experimental data from seven full-scale plants.


Assuntos
Drenagem Sanitária , Poaceae/química , Esgotos/química , Purificação da Água/instrumentação , Purificação da Água/métodos , Simulação por Computador , Coleta de Dados , Dinamarca , Geografia , Oxigênio/análise , Fatores de Tempo , Eliminação de Resíduos Líquidos
7.
J Colloid Interface Sci ; 356(2): 681-9, 2011 Apr 15.
Artigo em Inglês | MEDLINE | ID: mdl-21310425

RESUMO

Flocculation is an important process in separation science, but only few methods are available for on-line evaluation of the process. Recently, it has been shown that dielectric spectroscopy can be used to characterise the flocculation process of hard polystyrene particles. As many "real life" suspensions contain particles covered with a porous layer of organic material, it is of interest to investigate the potential of dielectric spectroscopy to characterise the flocculation of such suspensions as well. In this paper dielectric spectroscopy is used to investigate the flocculation process of core-shell particles. The flocculation process is characterised using a photometric dispersion analyser as a reference method, and the results are compared to the dielectric dispersions measured by dielectric spectroscopy. It is found that the use of the relaxation time of the dielectric dispersion for an evaluation of the flocculation process is commensurate with the use of the photometric dispersion analyser. Furthermore, the magnitude of the dielectric dispersion is observed to decrease as the charge of the core-shell particles is neutralised. Dielectric spectroscopy is thus found to have potential as an on-line flocculation monitor.

8.
Water Res ; 45(5): 1941-50, 2011 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-21239036

RESUMO

A laboratory scale setup was used for characterization of gravitational drainage of waste activated sludge. The aim of the study was to assess how time of drainage and cake dry matter depended on volumetric load, SS content and sludge floc properties. It was demonstrated that activated sludge forms compressible cakes, even at the low pressures found in gravitational drainage. The values of specific cake resistance were two to three orders of magnitude lower than those obtained in pressure filtration. Despite the compressible nature of sludge, key macroscopic parameters such as time of drainage and cake solid content showed simple functional dependency of the volumetric load and SS of a given sludge. This suggests that the proposed method may be applied for design purposes without the use of extensive numerical modeling. The possibilities for application of this new technique are, among others, the estimation of sludge drainability prior to mechanical dewatering on a belt filter, or the application of surplus sludge on reed beds, as well as adjustments of sludge loading, concentration or sludge pre-treatment in order to optimize the drainage process.


Assuntos
Algoritmos , Esgotos/química , Eliminação de Resíduos Líquidos/métodos , Dessecação , Filtração/métodos , Floculação , Gravitação , Reologia , Resistência ao Cisalhamento , Fatores de Tempo , Purificação da Água/métodos
9.
Phys Chem Chem Phys ; 13(3): 1182-8, 2011 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-21072404

RESUMO

Interactions between methyl tert-butyl ether (MTBE) and water have been investigated by scanning calorimetry, isothermal titration calorimetry, densitometry, IR-spectroscopy, and gas chromatography. The solubilization of MTBE in water at 25 °C at infinite dilution has ΔH° = -17.0 ± 0.6 kJ mol(-1); ΔS° = -80 ± 2 J mol(-1) K(-1); ΔC(p) = +332 ± 15 J mol(-1) K(-1); ΔV° = -18 ± 2 cm(3) mol(-1). The signs of these thermodynamic functions are consistent with hydrophobic interactions. The occurrence of hydrophobic interaction is further substantiated as IR absorption spectra of MTBE-water mixtures show that MTBE strengthens the hydrogen bond network of water. Solubilization of MTBE in water is exothermic whereas solubilization of water in MTBE is endothermic with ΔH° = +5.3 ± 0.6 kJ mol(-1). The negative mixing volume is explained by a large negative contribution due to size differences between water and MTBE and by a positive contribution due to changes in the water structure around MTBE. Henry's law constants, K(H), were determined from vapor pressure measurements of mixtures equilibrated at different temperatures. A van't Hoff analysis of K(H) gave ΔH(H)° = 50 ± 1 kJ mol(-1) and ΔS(H)° = 166 ± 5 J mol(-1) K(-1) for the solution to gas transfer. MTBE is excluded from the ice phase water upon freezing MTBE-water mixtures.


Assuntos
Éteres Metílicos/química , Água/química , Varredura Diferencial de Calorimetria , Cromatografia Gasosa , Densitometria , Ligação de Hidrogênio , Interações Hidrofóbicas e Hidrofílicas , Espectrofotometria Infravermelho , Termodinâmica
10.
J Colloid Interface Sci ; 340(1): 46-52, 2009 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-19751938

RESUMO

The use of dielectric spectroscopy as a monitor for coagulation processes was investigated. Hydrophobically modified poly(acrylic-acid) polymers were used as model macromolecules and coagulated with barium ions. The coagulation process was quantified using a photometric dispersion analyser, thereby serving as a point of reference for the dielectric spectroscopy. It was found that the hydrophobic modification increased the dosage of barium needed to obtain complete coagulation, whereas the dosage required to initiate coagulation was lowered. The coagulation of the polymer samples caused the relaxation time of the measured dielectric dispersion to increase, and this parameter was found to be a good indicator of the formation of polymer aggregates. The magnitude of the dielectric dispersion decreased as a function of barium dosage, but when coagulation was initiated an increase was observed. The observed agreement between the onset of coagulation and the changes in the dielectric dispersion shows the potential use of dielectric spectroscopy for the characterisation of coagulation processes.


Assuntos
Resinas Acrílicas/química , Bário/química , Precipitação Química , Análise Espectral/métodos , Condutividade Elétrica , Interações Hidrofóbicas e Hidrofílicas , Íons/química
11.
J Colloid Interface Sci ; 331(1): 113-21, 2009 Mar 01.
Artigo em Inglês | MEDLINE | ID: mdl-19058811

RESUMO

The flocculation of colloidal suspensions using synthetic polymeric flocculants is an important operation in separation processes. Optimizing flocculant use requires insight into the underlying mechanisms governing flocculation. As most existing methods for the online characterization of flocculation processes can only be used on dilute suspensions, new methods applicable at high solid content levels are of interest. This study used dielectric spectroscopy to investigate the mechanisms involved in the flocculation of polystyrene particles with three different cationic polymers. We observed that the relaxation time of the dielectric dispersion increased as particle flocculation was initiated. Reduction of particle charge due to polymer addition was found to reduce the magnitude of the dielectric dispersion, whereas the formation of aggregates increased it. This resulted in decreasing magnitude when charge reduction was the primary cause of flocculation, whereas the magnitude increased when the particles were flocculated by bridging.

12.
Water Res ; 43(3): 773-83, 2009 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-19081594

RESUMO

Pig production is concentrated in large farms, increasing the need to export excess nutrients, so manure separation would be useful to concentrate the nutrients. We examined the physicochemical properties of pig manure pertinent to flocculation and separation. Manures from three farms were stored for 3 months at 13 degrees C. The organic pools in the manure did not vary significantly during storage, so microbial metabolism was low and storage was of minor importance to separation. The manure contained highly charged particles and surface charges did not vary between the manures. This implies that the polymer doses required for flocculation can be determined directly from the manure dry matter content and that highly charged, high-molecular-weight cationic polymers can be used. Phosphorus was mainly found in the particulate fraction as struvite and was retained in the solids fraction at high pH. Thus, pH adjustment can control the amount of dissolved phosphorus in manure.


Assuntos
Esterco/análise , Sus scrofa , Animais , Tampões (Química) , Cálcio/análise , Coloides , Ácidos Graxos Voláteis/análise , Floculação , Ferro/análise , Magnésio/análise , Compostos de Magnésio/química , Esterco/microbiologia , Nitrogênio/análise , Compostos Orgânicos , Fosfatos/análise , Fosfatos/química , Potenciometria , Compostos de Amônio Quaternário/análise , Estruvita , Propriedades de Superfície
13.
J Colloid Interface Sci ; 327(2): 362-9, 2008 Nov 15.
Artigo em Inglês | MEDLINE | ID: mdl-18804775

RESUMO

The flocculation of colloidal suspensions is an important unit operation in many industries, as it greatly improves the performance of solid separation processes. The number of available techniques for evaluating flocculation processes on line is limited, and most of these are only functional in dilute suspensions. Thus, techniques usable for flocculation characterization in high-solids suspensions are desirable. This study investigates the use of dielectric spectroscopy to monitor the flocculation of polystyrene particles with a cationic polymer. The frequency-dependent permittivity is modeled and the model parameters are used to describe the particle aggregation. The results show a peak in the modeled time constants of the dielectric relaxation at the onset of flocculation. Further, the adsorption of polymeric flocculant onto the particle surface results in a reduction in particle charge, evident as a decrease in the magnitude of the dielectric dispersion. The use of dielectric spectroscopy is found to be valuable for assessing flocculation processes in high-solids suspensions, as changes in parameters such as floc size and charge can be detected.


Assuntos
Poliestirenos/química , Poliestirenos/síntese química , Análise Espectral/métodos , Cátions/química , Eletroquímica , Floculação , Modelos Químicos , Tamanho da Partícula , Polímeros/química , Polímeros/metabolismo
14.
Water Res ; 42(10-11): 2449-56, 2008 May.
Artigo em Inglês | MEDLINE | ID: mdl-18313718

RESUMO

The potential of multi-channel fluorescence as an in-line monitor of the activated sludge process has been investigated. Humic substances are known to be present as an important constituent in most activated sludges, and it was tested whether humic substances influenced the fluorescence signals from protein and NADH/NADPH. A negative correlation between the humic acid (HA) signal and the signals from both protein and NADH/NADPH was found. This phenomenon was investigated in detail by studying the interaction between a standard HA (Janssen) and a standard protein (bovine serum albumin, BSA). When mixing HA and BSA, the fluorescence signal from BSA declined or even disappeared. The quenching of the fluorescence from BSA was shown to relate to an adsorption of HA onto the protein as demonstrated by capillary electrophoresis. This quenching phenomenon was also observed in activated sludge, which was spiked with HA. As 250 mg HA/L was added to activated sludge, the fluorescence signal from both protein and NADH/NADPH was reduced by 20-30%. The results indicate that it is difficult to carry out quantitative analyses of the components in activated sludge by multi-channel fluorescence, and that variations in HA can affect the signals representing the biomass and the biological activity.


Assuntos
Substâncias Húmicas , Esgotos , Animais , Bovinos , Eletroforese Capilar , Fluorescência , Soroalbumina Bovina , Espectrometria de Fluorescência , Purificação da Água
15.
Water Res ; 38(18): 3909-19, 2004 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-15380981

RESUMO

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.


Assuntos
Bactérias Aeróbias/fisiologia , Esgotos/microbiologia , Eliminação de Resíduos Líquidos/métodos , Aderência Bacteriana , Floculação , Ferro/química
16.
Water Res ; 36(12): 2931-40, 2002 Jul.
Artigo em Inglês | MEDLINE | ID: mdl-12171389

RESUMO

The presence of micron-sized primary particles is critical in many solid/liquid separation processes, and characterisation of stability with respect to the degree of dispersion is therefore relevant. Characterisation of floc strength so far has mainly dealt with the size of flocs and not addressed quantification with respect to primary particles. In the present work floc strength was quantified in terms of the shear sensitivity (kss) as a standardised parameter based on the recent adhesion-erosion model (AE-model). The shear sensitivity quantifies the degree of dispersion for low total solids contents and intermediate turbulent shear rates. It reflects the affinity of adhesion/erosion of primary particles to and from flocs and thus provides a means of characterising the interaction energy between the sludge colloids.


Assuntos
Coloides/química , Esgotos/química , Floculação , Modelos Químicos , Nefelometria e Turbidimetria , Resistência ao Cisalhamento , Soluções/química , Suspensões/química , Termodinâmica
17.
Water Res ; 36(10): 2451-62, 2002 May.
Artigo em Inglês | MEDLINE | ID: mdl-12153011

RESUMO

An investigation was carried out for a variety of different sewage sludges in order to establish correlations between sludge composition, structure and dewatering properties. Results indicated that the fraction of extracellular polymeric substances (EPS) in sludges was the most important parameter with respect to sludge structure. With high EPS contents, sludges had a lower shear sensitivity and lower degree of dispersion. This in turn lead to better filterability in terms of low resistance to filtration (SRF). The floc stabilising role of EPS components was not consistent with DLVO-theory, as the zeta-potential increased with increased EPS content due to increased EPS charge content. This indicates that polymer entanglement is a key factor to stable floc structure. This does not rule out the possible change in dispersion due to changed electrostatic repulsion for a given EPS content. While EPS had a good effect on floc stability and filterability, the cake dry matter content decreased with large EPS contents. This could be due to an osmotic pressure related to the polymer charge quantity, or it could be caused by water entrapment in the floc structures. A high degree of sludge dispersion increased the cake dry matter content in filtration. This mechanism is, however, impractical due to high SRF and not important to conditioned sludge. In practice, dewatering also includes sludge expression. Taking this into account, osmosis related to EPS charges is likely to be increasingly important (increasing the negative effect of EPS content on cake dry matter).


Assuntos
Esgotos/química , Eliminação de Resíduos Líquidos/métodos , Filtração , Floculação , Osmose , Polímeros/química , Água/química
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