## 1 Introduction  
Worldwide environmental problems are becoming more and more acute. It is known, for example, that many specific industrial wastewater streams with large flows contain toxic metals in concentrations up to 500 mg/L, which have to be removed prior to water recycling, indirect discharge into the sewage system or direct discharge into surface waters [1]. When discharged directly into rivers, polluted wastewater poses a great risk to the aquatic ecosystem, whilst discharge into the sewage system negatively affects bio-sludge activity and leads to contamination of the excess sludge to be disposed of. As a result of the standards specified in the Water Resources Act, industry takes precautions against these risks by treating dangerous components in a partial stream, _i.e._, before being mixed with other types of wastewater. For centuries, water has been a manufacturing tool that industry has taken for granted, because it is inexpensive and plentiful. However, population growth, globalization, and climate change are shepherding in a new water-constrained era. Good, clean water just cannot be replaced and it is becoming harder to come by [2]. Therefore, research was focused, among other things, on the development of highly selective bonding agents, including the biosorbents, with fast reaction kinetics for the removal of toxic substances. The latter may led to a better use of the capacity of the bonding agents, resulting in smaller units and low residual concentration of pollutants in the treated water streams, to comply with individual standards for water re-use or discharge. The ability of microorganisms to remove metal ions is a well-known phenomenon; biosorption was called the process that makes use of dead biomass (in comparison to bioaccumulation), a separation technique usually applied to fermentation wastes or byproducts. This process has been recently reviewed in depth [3]. Several biomass types were investigated, such as bacteria (_Streptomyces clavuligerus_ or _Actinomycetes AK61/JL322_), yeast (_Saccharomyces cerevisiae_ or _carlsbergensis_) fungi (_Penicillium Chrysogenum_), as well as biomass from a biological wastewater treatment plant after anaerobic digestion and grape stalks (a by-product of the winery industry). As an example, the use of _Aeromonas caviae_ biomass is given in the following (_i.e._, in the kinetics study) for the removal of cadmium and/or chromate ions; this microorganism is a Gram-negative bacterium, isolated from raw water wells near Thessaloniki [4,5]. An advantage of the proposed operation is that its basic unit processes are rather conventional and widely applied in the field [6]. Biosorbents can be regenerated for multiple reuse, offering in addition and meanwhile the metal recovery possibility from concentrated wash solutions, _i.e._, by electrochemical methods. Non-living biomass showed generally greater metal binding capacities than the respective living bacterium. From a comparison between biosorption and other metal separation methods, as filtration, centrifugation, _etc._, it was found that in terms of the removal efficiency and applicability at lower (acidic) pH values, the former was favored [2,7]. In the latter, biosorption was studied for a mixture of copper, zinc and nickel. A common feature in biotechnology is of course the presence of suspended particles, living or disrupted microorganisms (generally, having low density), which are difficult or time-consuming to remove by filtration and on the other hand, centrifugation is apparently more expensive. The possibility of applying downstream biomass flotation has been extensively reviewed in this volume; observed recoveries were reaching almost 100% [8]. The efficiency of combined \"biosorptive flotation\" process has been proved for the treatment of toxic metals' mixture, even when applying a multi-cyclebiosorption-flotation operation [2]. A flowsheet of the proposed (two-stage countercurrent) biosorptive flotation process was presented [9].