This section is from the book "A Treatise On Beverages or The Complete Practical Bottler", by Charles Herman Sulz. Also available from Amazon: A Treatise On Beverages.
After animal charcoal has been used for some time it loses its absorbing and oxydizing capacity and must be renewed or regenerated. If carefully washed and separated from all soluble substances the animal charcoal holds absorbed in its pores, and if then subjected to an intense heat until the coal is red hot, which may be done in a suitable stove, oven or cylinder, in order to destroy the organic substances, the animal charcoal regains its power again. This process is called "regenerating," and it may undergo it from 25 to 30 times and regain its chemical properties, but finally gets "worn out" and is then a valuable manure. A better regenerating process is to boil the coal in soda lye, then extract it by means of diluted muriatic acid, carefully washing, and finally subjecting it to a red heat. But this regenerating process is only remunerative where exceptionally large quantities of animal charcoal, as in sugar refineries, are used; for a mineral-water establishment it proves impracticable.
Mere washing of charcoal by reversing the current or otherwise, application of hot water or steam, are entirely insufficient for regenerating the animal charcoal. Reburning only suffices.