Characterization of water distribution in sludges

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Virginia Tech

Knowledge of water distribution in sludges is important in developing a better understanding of the mechanisms by which various fractions of water are bound to the sludge matrix as well as the extent to which sludges may be dewatered in an economic manner. Various models have been proposed in the literature to describe the types or fractions of water present in sludges, both before and after dewatering. Likewise, there exist a variety of measurement techniques to quantify the fractions of water believed to be present in sludges.

The major objective of the research reported herein was an attempt to develop a more rational basis for defining the various fractions of water present in sludges, as well as to apply and develop feasible analytical techniques to characterize moisture distribution in sludges and determine if there was any correlation between these techniques. Secondary objectives included investigation of polymer addition, mechanical dewatering, and the effect of freeze-thaw cycles to different water fractions in sludge samples, and measuring or estimating various densities (bulk, floc, dry) and specific surface areas of sludge particles in an attempt to predict the dewatering performance of various sludge samples. The matrix of sludges included various water treatment plant/inorganic chemical sludges and biosolids.

The sludge moisture characterization techniques used or developed in this study were the thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dilatometric methods. The TGA method developed used the drying curve rate technique whereby changes in the drying (water removal) rates would provide information on moisture distribution in sludges. The DSC and dilatometric methods used the property of the free water fraction to freeze by -20° C, as compared to the bound water fraction that is thought not to freeze until well below -20°C.

It was found that there were uncertainties in the determination of different sub-fractions of sludge water. In an effort to better rationalize these various subfractions, it has been proposed in this study that water in sludges be considered to be in two major fractions: bound and unbound. For the TGA, the demarcation point between these two fractions was indicated by the onset of the first critical/inflection point on the drying rate curve. The DSC and dilatometric methods determined the amount of bound water by first measuring the amount of unbound water that froze by -20°C, and subtracting this value from the total water present in the dry solids analysis. While the DSC measured the calorimetric (enthalpic) changes, the dilatometer measured the expansion of the unbound water. The three DSC instruments evaluated in this study were not able to measure the rapid exothermic change of the freezing of unbound water, but was able to measure the amount of unbound water that melted. Freezing point depression (supercooling) phenomena was also observed for various samples using this technique. Due to these effects, the sludge moisture characterization method developed for the DSC measured the amount of unbound water from the endothermic changes upon melting.

The three different analytical techniques used to determine moisture in sludges were found to provide different quantities of the bound water fraction. Other sludge characteristics evaluated included the bulk, floc, and dry density values, as well as the specific surface area of dry sludge particles. Correlations were attempted to determine if there were any meaningful relationships in the results obtained.