To retain high cell densities in reactor, microbes can be grown as biofilm attached to carrier ma­terial. Planktonic cells can be retained in the fermentation broth by installing solid/liquid sepa­rators such as membranous ultra-filtration units, spiral wound filtration systems, hollow fibres, cell-recycling membranes and centrifuges [214—216]. The concentrations of solvents from gas fermentation rarely exceed 6% [w/v] so a cost — and energy — efficient product recovery process is required. Furthermore, acetogens also exhibit low resistance towards solvents like ethanol [217, 218] and butanol [219, 220] so an in situ/online product recovery system can en­hance solvent productivity by decreasing solvent concentrations (and hence toxicity) in the fer­mentation broth. Distillation has been the traditional method of product recovery but the associated high energy costs have led to the development of alternative methods such as liq­uid-liquid extraction, pervaporation, perstraction, and gas stripping [24, 221].