AbstractThe coalescence phenomenon of drops in liquid/liquid systems is reviewed with particular focus on its technical relevance and application. Due to the complexity of coalescence, a comprehensive survey of the coalescence process and the numerous influencing factors is given. Subsequently, available experimental techniques with different levels of detail are summarized and compared. These techniques can be divided in simple settling tests for qualitative coalescence behavior investigations and gravity settler design, single-drop coalescence studies at flat interfaces as well as between droplets, and detailed film drainage analysis. To model the coalescence rate in liquid/liquid systems on a technical scale, the generic population balance framework is introduced. Additionally, different coalescence modeling approaches are reviewed with ascending level of detail from empirical correlations to comprehensive film drainage models and detailed computational fluid and particle dynamics.

Coalescence behavior in liquid-liquid dispersions is controlled by various parameters. Thus, data from different research groups can differ significantly. Dynamic coalescence processes were analyzed systematically with the EFCE standard test system toluene/water in two different research laboratories. After comparability was proven with standardized batch settling tests and drop rise velocity measurements, the influence of drop size ratio and varying NaCl and NaOH concentrations on coalescence probability was investigated. Since the ions led to a coalescence inhibition, no clear impact of drop size ratio could be observed. The analysis of drop contact showed longer contact times for increasing equivalent drop diameter. This promotes coalescence and corresponds to film drainage model.

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A B S T R A C TThis study aims at a better understanding of the effects of flocculants and adsorbents on membrane bioreactor (MBR) mixed liquor. A total of 12 different additives (metal salts, chitosans, starches, synthetic polymers and PAC) were tested with regards to their impact on particle size distribution in activated sludge. Of further interest was the shear stability and dewaterability of sheared sludge. This was tested in a range of shear rates dominating in MBR. For most additives, a significant effect on the capillary suction time (CST) was observed. Most additives formed aggregates that were stable in the tested shear range (0-4000 s À1 ). Nevertheless, only the tested chitosans and polymers were able to significantly increase the volume based particle size (up to 127%). In order to examine the long term effect of shearing on particle size three of the tested additives were surveyed in pilot plant experiments. Here the increase in particle size was only 17-18% for the tested polymers. In lab scale tests these polymers had caused an increase of approx. 50%.