Biomedical subjects

I G Droppo

Publications and source records attributed to I G Droppo.

9 recordsLinked to original sources

PubMed2006

Effect of solids retention time on structure and characteristics of sludge flocs in sequencing batch reactors.

The effect of solids retention time (SRT) (4-20 d) on sludge floc structure, size distribution and morphology in laboratory-scale sequencing batch reactors receiving a glucose-based synthetic wastewater was studied using image analysis in a long-term experiment over one year. Floc size distribution (>10 microm) could be characterized by a log-normal model for no bulking situations, but a bi-modal distribution of floc size was observed for modest bulking situations. In each operating cycle of the SBRs, the variation in food /microorganisms ratio (0.03-1.0) had no significant influence on floc size distribution and morphology. The results from a long-term study over one year showed that no clear relationship existed between SRT and median floc size based on frequency. However, sludge flocs at the lower SRTs (4-9 d) were much more irregular and more variable in size with time than those at higher SRTs (16 and 20 d). The level of effluent-suspended solids at lower SRTs was higher than that at higher SRTs.

Bioreactors↗

PubMed2002

Interparticle interactions affecting the stability of sludge flocs.

Interparticle interactions affecting the stability of sludge flocs taken from laboratory-scale sequencing batch reactors at different solids retention times (SRTs) were investigated in batch experiments by varying the pH, ionic strength, cation valence, and urea and ethylenediaminetetraacetate concentrations of suspending solutions. The ultrastructure of sludge floc surfaces was observed by transmission electron microscopy. Changes in dissociation constants of sludge flocs under different conditions indicated that ionic interactions and hydrogen bonds held flocs together and compensated for the negative influence of electrostatic interactions on the stability of sludge flocs. Ionic interactions and hydrogen bonds were two dominant forces that maintained the stability of sludge flocs at lower SRTs; other mechanisms, such as physical enmeshment and van der Waals and/or hydrophobic interactions, were more important in controling the stability of sludge flocs at higher SRTs. Sludge flocs at higher SRTs (16 and 20 days) were physically more stable than those at lower SRTs (4 and 9 days). A conceptual model of floc structure, based on interparticle interactions, for describing the stability of sludge flocs is proposed. The floc matrix is proposed to consist of two physically distinct regions that are defined by the arrangement of extracellular polymeric substances (EPS). These are likely to be differentially affected by the agents applied to manipulate interparticle forces. Thus, the heterogeneity in the packing of and the type of EPS reflects the stability of the floc.

Electrochemistry↗

PubMed2002

Flocculation/aggregation of cohesive sediments in the urban continuum: implications for stormwater management.

The urban continuum, as it applies to sediments and associated contaminants, represents the area over/through which sediments are conveyed from a depositional or eroded surface to a treatment system and/or receiving water body. This study has focused on the changing physical characteristics of the sediment, with an emphasis on flocculation/aggregation, as it progresses through the urban continuum. The sediments of the urban continuum are found to change from an unflocculated state on the street, to a flocculated state in the surface runoff to a very large floc form in the sewer system. The high organic content in the sewers contributes to the large floc size. The structure of the flocs and the flow regime of the receiving water will dictate the fate of the sediment following a combined sewer overflow. Probability distributions fitted to the distributions of each sediment type (compartment) confirmed significant differences in the sediment population sizes. Bulk and individual particle settling velocity experiments also revealed substantial differences between compartments. Sewer flocs were found to be of low density, with high porosity, water and organic content and with settling velocities which increase with floc size.

Cities↗

PubMed2002

Effect of solids retention time on floc structure.

Correlative microscopy was applied to study the influence of solids retention time on activated sludge floc structure. Conventional optical microscopy revealed flocs at lower SRTs (4 and 9 days) to be irregular in shape while flocs at higher SRTs (16 and 20 days) had a more spherical and compact structure. Flocs were examined by environmental scanning electron microscopy (ESEM) and by transmission electron microscopy (TEM) coupled with energy-dispersive spectroscopy (EDS). Distinctive differences in floc structure and the arrangement of EPS were revealed. Flocs from higher SRTs were less hydrated and were found to possess a dense EPS layer that covers much of the surface. Extracellular osmiophilic granules present in these flocs indicate that the cells at the higher SRT may produce more lipid-like material. This EPS layer appears to decrease the floc surface roughness and protects the interior cells from disruption by changes in the external environment. Sludge flocs at higher SRTs were found to be physically more stable than those at lower SRTs.

Bioreactors↗

PubMed2001

The effect of depositional history on contaminated bed sediment stability.

Experiments were conducted in an annular flume using a commercially available kaolinite clay as well as contaminated bed sediment from Hamilton Harbour (Ontario) to assess their stability against erosion. Critical shear stress for erosion was measured under different conditions of bed formation (quiescently deposited beds and shear deposited beds) as well as with and without the presence of a biostabilized bed. Results suggest that a biostabilized bed and a bed formed under a flowing condition, similar to a river scenario, will be more resistant against erosion than will a non-biostabilized bed and a bed formed under quiescent conditions. Up to three cycles of erosion and flocculation/deposition were observed to occur within one experiment. These results suggest that the depositional history and biostabilization of river bed sediments need to be seriously considered within sediment and contaminant transport models if meaningful estimates of sediment and contaminant source, fate and effect are to be generated and used for the management of our aquatic ecosystems.

Animals↗

PubMed2001

Surface properties of sludge and their role in bioflocculation and settleability.

The influence of sludge retention time (SRT) on the extracellular polymeric substances (EPS) and physicochemical properties (hydrophobicity and surface charge) of sludge was studied using laboratory-scale sequencing batch reactors (SBRs) fed a synthetic wastewater containing glucose and inorganic salts. Sludge surfaces were more hydrophobic (larger contact angle) and less negatively charged at higher SRTs (16 and 20 d) than at lower SRTs (4 and 9 d). The ratio of proteins to carbohydrates within the EPS of the sludges increased as the SRT increased from 4 to 12 d corresponding to the changes in the physicochemical properties of the sludge. The protein:carbohydrate ratio remained constant at SRTs of 16 and 20 d. A transition in sludge properties appeared to occur between the upper range of low- (9 d) and lower range of high-SRTs. The total EPS content, however, was independent of the SRT. A higher sludge volume index (SVI), an indication of poorer settleability or compression, was associated with a larger amount of total EPS but no significant correlation between SVI and the surface properties of sludge was observed. A more hydrophobic and less negatively charged surface corresponded to lower levels of ESS. These results indicate that it is the surface properties, hydrophobicity, surface charge and composition of EPS, of sludge, rather than the quantity of EPS, that govern bioflocculation. In contrast, the EPS content is more important in controlling the settleability of sludge.

Carbohydrates↗

PubMed2001

Sequential erosion/deposition experiments--demonstrating the effects of depositional history on sediment erosion.

Experiments on the erosion of a bed of kaolinite were carried out in a rotating circular flume. Each experiment was carried out using the stratified bed which resulted from the previous experiment. Changes in suspended sediment concentrations during the experiments were explained by the history of the deposition. The sequence of experiments showed how the rate of erosion and the amount eroded reflected the structure of the bed and that of the individual flocs which created it. Results suggest that modelling of sediment/contaminant transport needs to account for the manner in which deposition took place.

Chemical Precipitation↗

PubMed1996

Distribution of lead, copper and zinc in size-fractionated river bed sediment in two agricultural catchments of southern Ontario, Canada.

Metal (Pb, Cu and Zn) partitioning in six separated sediment size fractions (<8, 8-12, 12-19, 19-31, 31-42, 42-60 microm) of river bed sediment have been analyzed by sequential extraction. The concentrations of some major elements (Si, Al, Ca, Mg, K, Na, Fe, Mn and P), and organic and inorganic C were determined to correlate the elemental composition of the sediment with metal speciation and grain size. Results show that Zn and Pb concentrations increase with decreasing grain size. For Big Creek and Big Otter Creek, respectively, the highest concentrations of Zn (326 and 230 mg kg(-1)) and Pb (158 and 67 mg kg(-1)) were found in the smallest (<8 microm) fraction, whereas the Cu levels (619 and 1281 mg kg(-1)) were most abundant in the second smallest (8-12 microm) fraction. The major accumulative phases for Cu, Zn and Pb were carbonates, Fe/Mn oxides and organic matter, but the relative importance of each phase varied for individual metals and grain sizes. The extraction data show increasing potential bioavailability of metals with decreasing grain size. Estimates of metal yields in the study catchments suggest that over 80% of the metal yield in sediment smaller than 63 microm is associated with solids smaller than 31 microm.

Journal Article↗

PubMed1996

Floc stabilization for multiple microscopic techniques.

A nondestructive stabilization technique for the characterization of microbial flocs which permits the application of correlative microscopic techniques is described. Flocs embedded in agarose are retained in a porous, resilient medium which allows for the transport, staining, washing, and subsampling of the flocculated material directly within a plankton chamber with minimal or no destructive forces. A single agarose disc can be subdivided into numerous sections for analysis by several microscope types and associated techniques.

Journal Article↗