Chemical, Rheological, and Physical Exploration of Gel-Like Behavior in Conditioning and Dewatering Processes

University Of Delaware, Newark DE

Investigators

Abstract

0229293 Dentel With over seven million dry tons of wastewater solids generated annually in the U.S., the costs of conditioning, dewatering, and disposal or reuse of these materials may cost in excess of $1 billion per year. Improvements in the conditioning and dewatering processes would reduce these costs and increase the number of feasible options for reuse. The physical and chemical origins of biosolids properties are not well understood, however, and thus it is not possible to clearly define the mechanisms through which chemicals may improve biological sludge dewaterability. The most common model of material properties invoked, for example, is that of rigid spherical particles bound by chains of linear polymers, but this model fails to describe some crucial aspects of biosolids behavior. Consequently, the selection of chemical types and doses for conditioning are by trial and error, and dewatering processes are empirically designed, selected, and operated. It is unlikely that we come close to optimum use of these processes, and the consequences are economically and environmentally costly. Recently, a quite different paradigm has been shown to be fruitful in describing some aspects of sludge conditioning and dewatering. Where sludges have typically been described as concentrated suspensions of rigid particles that are flocculated much as in water treatment coagulation, recent reports suggest the description of sludges as gels. Behavior of such systems has been accomplished in other fields such as polymer chemistry, and thus fundamental approaches are available. In particular, colligative properties of gels have been shown to explain certain aspects of sludge behavior in important respects like filtration and drying. An advantage of using these concepts is that they are grounded in molecular and thermodynamic principles, and thus may be linked to fundamental principles when describing chemical conditioning processes. This proposal presents four hypotheses that test the extent to which a gel model may assist in understanding sludge behavior. The hypotheses are based on measurable consequences of suspension behavior that will depend on whether colligative properties govern the material's response, or whether more structurally based properties must be invoked. Perhaps the most important shortcoming of a gel analogy is in the role of shear stress during filtration, which is essential in successful dewatering by belt filter presses and centrifuges. There is no obvious means by which a colligative model may account for this (although the effects of polymer breakage as an indirect factor will be assessed). However, even the newest analytical models of slurry filtration do not consider shear effects as a quantitative influence on a sludge's response to normal stresses. Beyond testing the efficacy of a gel or colligative model for describing sludges, it is also proposed to experimentally characterize shear effects during filtration, using rheometric and dynamic filtration devices. The experiments will not only delimit the gel-like aspects of sludge behavior, but extend the structural models into realms that are important in dewatering process applications. The experimental approach proposed is to combine methods classically used for sludge description (e.g. solids, electrophoresis, particle size analysis, filtration tests) with others that will describe gel behavior (e.g. rheometry, conductivity, swelling, charge density/titration) and use them on both synthetic and sampled sludges. The proposed research should improve our understanding of the nature of sludge and the specific reasons that it retains water so tenaciously. Through a consideration and possible synthesis of gel vs. particulate/structural descriptions of biological sludges, improved and quantitative descriptions of sludge flow, thickening, filtration, and even drying are likely to emerge. Ultimately, this will lead to sounder application of these practices in wastewater treatment facilities.