bacteria bed
noun
a layer of sand or gravel used to expose sewage effluent, in its final stages, to air and the action of microorganismsCompare filter bed (def. 1)
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012