Ian G. Droppo

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.

Flocculation of suspended sediment in rivers of southeastern Canada

Abstract Flocculation has been studied extensively within marine science and sanitary engineering and the controlling processes are generally well known. Less research, however, has focused on flocculation within the freshwater river environment. Consequently the occurrence, importance and controlling mechanisms are poorly understood. We examine the suspended sediment characteristics of six rivers in southeastern Canada in order to determine the nature and significance of flocculation in finegrained suspended sediment transport and the factors that may influence it. While flocs may not always comprise the majority of particles in transport, they always represent more than 90% of the total volume of sediment transported. Independent variables that may influence flocculation, such as temperature, pH, major ions, dissolved and particulate organic carbon, suspended solids concentration and bacteria were measured in conjunction with floc size for one of the rivers. Particulate organic carbon concentration, suspended solids concentration and attached bacteria may be important controlling factors of riverine flocculation.

The state of suspended sediment in the freshwater fluvial environment: a method of analysis

Abstract Primary particles in rivers are frequently and perhaps characteristically transported as larger flocculated particles. Typically floc size has been determined by destructive, indirect laboratory methods or by laborious microscopic measurements. A unique direct observation digitizing method of floc characteristic analysis was developed and applied to suspended solids from Sixteen-Mile Creek in Southern Ontario. Suspended sediment particles in the creek are strongly flocculated, and the flocs are relatively stable within the realms of the sampling and analytical method. The digitizing method allowed for direct observation of floc structures which is impossible with conventional methods of sediment sizing. The method provides reasonable results with good precision on floc equivalent spherical diameter, surface area, perimeter and circularity. It is limited, however, by its labour intensive nature, possibility of sediment overlap, the individual investigators criteria and by the photographic and microscopic instruments and techniques used. Other sizing techniques, or the use of the digitizing method by other investigators, may produce different results.

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.

Influence of antecedent conditions on critical shear stress of bed sediments

Experiments were conducted in an annular flume using kaolinite clay and bed sediment collected within an industrial boat slip of Hamilton Harbour to assess their stability and transport characteristics. The critical shear stresses required to erode the sediment were measured under different conditions for bed formation. It was discovered that the way in which the bed deposited (i.e. deposited under quiescent conditions or under shear) had a strong influence on the stability of the bed. The critical shear stress for beds deposited under shear was up to eight times larger than that for beds deposited under quiescent conditions. The results suggest that modeling efforts which do not take into account the history of the bed formation may seriously underestimate the bed strength and as such, can result in erroneous predictions of sediment and contaminant source, fate and effect.

Impact of river transport characteristics on contaminant sampling error and design.

Within monitoring programs, loading errors are generally associated with the inadequate delineation of the temporal variance of discharge and of the parameter(s) of interest. Often little consideration is given to the impact of additional transport characteristics on contaminant sampling error and design. Detailed examination of five transport characteristics at a single river cross-section emphasizesthe importance of understanding the complete transport/loading regime of a sampling station, defining the required end products of the monitoring program, and defining the accuracy required to meet specific program needs before implementing or evaluating a monitoring program

Structure, Stability, and Transformation of Contaminated Lacustrine Surface Fine-grained Laminae

An in situ annular flume (Sea Carousel) was deployed in Hamilton Harbour, Lake Ontario, to assess the structure, stability, and transformation of lacustrine surface fine-grained laminae (SFGL). Such laminae typify depositional lacustrine environments and occur at the sediment-water interface. The critical erosion thresholds, erosion rates, and internal friction coefficients were determined for natural undisturbed and physically disturbed sites in order to assess changes in bed stability. Sediment cores, taken at each site, were analyzed for bulk density using a CT scanner in order to provide an intercomparison of our results. Flocs and aggregates, pumped from the flume during erosion experiments, were analyzed for morphological characteristics using conventional optical microscopy and transmission electron microscopy to evaluate the mechanism and development of the SFGL and its structure. A general three-layer model is developed which depicts the organic flocs of the SFGL (Layer 1) compressing within a collapse zone (Layer 2) to form a consolidated bed (Layer 3). The SFGL was observed to be porous, of low density, and of high water content which possessed yield resistance due, in part, to binding by extracellular polymeric substances secreted by a colonized microbial population. Transmission electron microscopy observations showed an active biological community in the sediment promoting biostabilization. Time series of erosion thresholds and friction coefficients of the disturbed bed showed that the SFGL reconsolidated rapidly and increased in shear strength. The artificial disturbance of the sediment resulted in incorporation of the SFGL and the collapse zone with the consolidated bed through mixing. This stimulated the process of consolidation chiefly through removal of gas and breakdown of the organic fibril network. Reconsolidation began with the formation of a collapse zone at the surface of the disturbed sediment. High friction coefficients and an increasing density with depth in the new surface layer indicated that rapid consolidation occurred after disturbance.

Suspended Solids, Trace Metal and PAH Concentrations and Loadings from Coal Pile Runoff to Hamilton Harbour, Ontario

Abstract The Kenilworth boatslip has been identified as a “hotspot” within the Hamilton Harbour Area of Concern because of highly contaminated bottom sediments. There are a variety of sources potentially contributing to this sediment contamination, including runoff from industrial coal piles. The objective of this study was to assess the contaminant characteristics associated with coal pile runoff and identify potential impacts that the runoff might have on sediment quality. Flow-proportioned samples of the coal pile runoff were collected from June through October, 1996 and analyzed for total suspended solids, trace metals, and PAHs. Loadings for selected metals and PAHs were determined using a volumetric approach that considered mean contaminant concentrations and measured or modeled flow volumes. The suspended solids and total trace metal concentrations (Al, Fe, Mn, V, Cd, Cr, Cu, Pb, Zn) often exceeded Canadian Water Quality Guidelines for the Protection of Aquatic Life. Concentrations of fluoranthene, phenanthrene, pyrene, and chrysene exceeded the provincial “Severe Effect Level” for sediment, although the bioavailability of these PAHs should be examined more thoroughly. In general, suspended solids concentration and coal pile volume were significantly correlated with the concentrations of trace metals in the coal pile runoff, while hydrometeorologic variables such as rainfall intensity, rainfall depth, and runoff volume were weakly correlated with metals concentrations. The ratios of various PAHs to naphthalene suggested that coal pile runoff could be an important contributing source to bed sediment contamination. Given the concentrations and loadings of solids, metals, and PAHs and the relatively low volume of runoff, it would be prudent to treat the coal pile discharge as a remedial action for the harbor.

Effects of ultrasound on suspended particles in municipal wastewater.

The objective of this research is to explore the fundamental characteristics of how particles in wastewater respond to ultrasound, with an aim to improve wastewater disinfection. Particles of a predetermined size fraction and concentration were treated with varying doses of ultrasound at 20.3 kHz. Ultrasonic power transfer to the fluid was measured using calorimetry or acoustical measurements. Image analysis particle counting was used to measure the size distribution of particles before and after ultrasound treatment. The influence of three parameters: particle origin (raw wastewater or from the aeration basin of the activated sludge process), particle concentration, and particle size on the percentage of particle breakage after ultrasound treatment was compared. It was found that raw wastewater and aeration basin particles of the same size fraction (90-106 microm) responded to ultrasound in a similar way. Particle breakage was not affected by changes in particle concentration from 100 to 400 particles per mL. Larger wastewater particles (90-250 microm) were more susceptible to breakage than smaller ones (38-63 microm diameter). The percentage of particle breakage increased linearly with a logarithmic increase in the ultrasound energy density, that is the ultrasound energy delivered per unit volume of the sample (R(2)=0.48-0.91). An expression that predicts the percent of particles broken as a function of ultrasound energy density is provided.

Physical and Ecological Controls on Freshwater Floc Trace Metal Dynamics

Significantly higher concentrations of Ag, As, Cu, Co, Ni, and Pb are found in suspended floc compared to surficial bed sediments for a freshwater beach in Lake Ontario. Contrasting observed element-specific bed sediment metal partitioning patterns, floc sequestration for all elements is dominated by one substrate: amorphous oxyhydroxides. More specifically, floc metal scavenging is controlled by floc biogeochemical architecture. Floc organics, largely living microbial cells and associated exopolymeric substances (EPS), act as scaffolds for the collection and/or templating of amorphous Fe oxyhydroxides. While interactions between floc organics and amorphous Fe oxyhydroxides affected floc sorption behavior, specific element affinities and competition for these limited substrates was important for overall floc partitioning. Further, assessment of metal dynamics during stormy conditions indicated energy-regime driven shifts in floc and bed sediment partitioning that were specifically linked to the exchange of floc and bed sedimentary materials. These novel results demonstrate that the microbial nature of floc formation exerts an important control on floc metal dynamics distinguishable from surficial bed sediments and that hydrologic energy-regime is an important factor to consider in overall floc metal behavior, especially in beach environments.