: New ecolabels for textile products and tighter restrictions on wastewater discharges are forcing textile wet processors to reuse process water and chemicals. This challenge has prompted intensive research in new advanced treatment technologies, some of which currently making their way to full-scale installations. These comprise polishing treatments such as Ðltration, chemical oxidation and specialized Ñocculation techniques and pre-treatment steps including anaerobic digestion, Ðxed-Ðlm bioreactors, FentonÏs reagent oxidation, electrolysis, or foam Ñotation. Though several of these new technologies are promising in terms of cost and performance, they all su †er limitations which require further research and/or need broader validation. A segment of the research deals with the separate handling of speciÐc sub-streams such as dyebath effluents to which membrane Ðltration is sometimes applied. The main limitation of this approach is the treatment of the concentrate stream. The spectrum of available technologies may, in the future, be further broadened to include oxidation, specialized bio-sorptive processes, solvent extracfungi/H 2 O 2 -driven tion, or photocatalysis.1998 SCI (

The biological clogging of natural porous media, often in conjunction with physical or chemical clogging, is encountered under a wide range of conditions. Wastewater disposal, artificial groundwater recharge, in situ bioremediation of contaminated aquifers, construction of water reservoirs, or secondary oil recovery are all affected by this process. The present review provides an overview of the techniques that are used to study clogging in the laboratory, or to monitor it in field applications. After a brief survey of the clogging patterns most commonly observed in practice, and of a number of physical and chemical causes of clogging, the various mechanisms by which microorganisms clog soils and other natural porous media are analyzed in detail. A critical assessment is also provided of the few mathematical models that have been developed in the last few years to describe the biological clogging process. The overall conclusion of the review is that although information is available on several aspects of the biological clogging of natural porous media, further research is required to predict its extent quantitatively in a given situation. This is particularly true in cases that involve complicating factors such as predation or competition among organisms.

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Bacterial reductions of the saturated hydraulic conductivity, Ks, of natural porous media have been traditionally associated with the development of anaerobic conditions and the production of large amounts of extracellular polymers by the bacteria. Various researchers have also reported that these reductions occur predominantly at or very near the surfaces of injection of nutrients within the porous media. Attempts to describe mathematically the resulting clogging process have, in the past, been based on the assumption that bacterial cells form impermeable biofilms uniformly covering pore walls. A series of percolation experiments was carried out to determine the extent to which an obligately aerobic bacterial strain, Arthrobacter sp., is able to clog permeameters filled with fine sand. A second objective was to elucidate the mechanism(s) responsible for this process. The experimental results indicate that strictly aerobic bacteria are able to reduce Ks by up to four orders of magnitude. Rapid reductions in Ks are associated with the formation of a bacterial mat at the inlet boundary of the sand columns. When the colonization of the inlet is prevented, clogging proceeds within the bulk of the sand at a noticeably slower rate. Under O2‐ or glucose‐limited growth conditions, this decrease in Ks within the sand does not appear on scanning electron micrographs to be caused by exopolymers, which are entirely absent, but rather seems to be due to the presence of large aggregates of cells that form local plugs within the pores. Under conditions of severe N limitation, the same mechanism seems to be largely responsible for the observed clogging, in spite of the production by the cells of extracellular substances, visible under light microscopy and on scanning electron micrographs. In all cases, the coverage of the solid surfaces by the bacterial cells is sparse and heterogeneous, contrary to the basic tenets of the biofilm model.

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