In soils the solid matrix is frequently dominated by sand, while the water content may be <25%; various levels of debris can be found depending on the history of the site, but rarely exceed 10%. River, harbor, and canal sediments contain mostly water (frequently >60% to 70%), while the fine fraction (<63 pm) dominates the solids if dredging takes place in the upper sediment layers. Industrial and municipal sludges are mostly very moist (>95% water) and have a large content of organics (>60%) (INES, 1997). Industrial sludges mostly originate directly from corrosion and wear from equipment or water treatment units on the site.
Disregarding the heterogeneous nature of the waste, the contaminant behavior is largely determined by the fines (Werther and Wilichowski, 1990), because submicron particles such as humic-clay structures and clay agglomerates have an extremely high adsorption capacity (Brady, 1984). The solid waste, therefore, basically contains a contaminated fine fraction, a less contaminated sand-gravel fraction, cleaner debris, and a contaminated water phase.
In Figure 11.1 the line shows a typical particle size distribution for soils having a dominant fraction of fines (the step between 10 and 20 pm) and a dominant sand fraction (the step between 150 and 250 pm).
The bars show the measured mineral oil concentration as distributed over the fractions. For efficient processing of this waste, only the fractions <150-200 pm should be treated.
In general one can state that, to develop an appropriate recycling technology, the particle features of the solids have to match the type of process operation. For bio-processing this implies integration of the separation technology and bioreactors in a sequence of operation in which the clean fractions are removed from the feed before entering the reactor (Kleijntjens et al., 1999).
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