The goal of this paper is to focus on the effect of fluid flow, solution chemistry and surface morphology of fibrous media (in terms of roughness and charge heterogeneity) on colloid removal. A theoretical framework on the role of flow velocity on particle deposition to cylindrical substrate is presented. Existing theories related to particle attachment onto cylindrical substrates and on the effect of surface morphology on better predictability of particle adhesion have been modified. At higher flow velocities, the theory predicted the detachment of larger sized particles from the collector surface owing to the increased lift forces. Experimental studies confirmed removal of 3 particles at higher flow velocities. The effect of solution chemistry (in terms of concentration and ionic strength) on the particle adhesion has been studied. The ionic strength improved the attachment of the microspheres, despite the presence of high energy barriers as predicted by DLVO calculations. A new model incorporating the surface roughness and charge heterogeneity was developed that predicted a reduction in the energy barriers in otherwise unfavorable conditions of adhesion, and hence could explain the adhesion more accurately than DLVO alone.
A systematic study was carried out to investigate the attachment of inorganic particles to fabric media in the presence of divalent cations and the dissolved organics in water, on filter media, or on both. The presence of organics (humic acid, HA) reduced the attachment of inorganic particles on the fabric. Specific attachment trends were observed for the inorganic particles–fibre system. For [CaCl2] < 500 ppm, the particle attachment was lower in the presence of 10 ppm of HA compared to its absence. At [CaCl2] ≥ 500 ppm, the attachment in the presence or absence of HA was similar, suggesting that the attachment was independent of the presence of HA. It was also found that the particle attachment to the fabric was lower when HA was present in water compared to when present on the fabric, suggesting that the attachment behaviour of inorganic particles was dependent on water chemistry (i.e. presence of calcium ions and organics in water), which also altered the surface properties of filter media. The removal trends were explained on the basis of particle aggregation, surface charge and Derjaguin–Landau–Verwey–Overbeek (DLVO) theory.
Biofilm plays an important role in controlling the transport of colloids in a porous media. Biofilms are formed when micro-organisms come in contact with substrates, and are able to attach and grow with availability of nutrients. The microorganisms get embedded in a matrix of the substrate and extracellular polymeric substances which are responsible for the morphology, physico-chemical properties, structure and coherence of the biofilm. In this study, the effect of biofilm and its aging on colloid removal was studied on a glass bead column. Oocysts, polystyrene microspheres and inorganic colloids were used as colloidal particles. Pseudomonas aeruginosa was used as a model biofilm-forming microorganism. Presence of biofilm significantly enhanced colloid removal in the column. After 3 weeks, almost complete colloid removal was observed. The formation of biofilm was confirmed by various physical characterization techniques. During the extended aging study, biofilm sloughed off under shear stress. The loss of biofilm was higher during the early stage of its growth, and subsequently slowed down probably due to the formation of a more rigid biofilm. This research indicates that biofilm formation, maturation and sloughing-off play a critical role in colloid removal through porous media.
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