from Nicolas Kalogerakis writing in Microbial Bioremediation of Non-metals: Current Research:

Although in-situ bioremediation technologies for the treatment of contaminated soils are economically attractive, ex-situ approaches are more often used for surface contaminated soils (typical depths less than 5 m) since they allow a much tighter control of the bioremediation process and provide better estimates of the residual contamination at the end of the treatment period. Ex-situ bioremediation is the method of choice for hot spot treatment if they are reasonably accessible. In this chapter, the classical ex-situ technologies (Landfarming, Composting, Biopiling and Slurry-phase bioremediation) are presented with a few examples of innovative modifications that enhance their productivity and/or effectiveness. Ex-situ bioremediation typically refers to the methods applied for the remediation of excavated contaminated soils. Besides slurry-phase bioremediation where the soil is mixed with water and other nutrients in mechanically agitated bioreactors, ex-situ bioremediation includes solid phase bioremediation that covers the techniques of landfarming and the various forms of composting, namely windrows, biopiles and in-vessel composting. In the following sections, the above methods are presented focusing on design principles and on experience gathered from their application.

Further reading: Microbial Bioremediation of Non-metals: Current Research