Like Theseus and the Minotaur, particles moving through a depth filter are caught within a labyrinth of channels, destined to never escape. Whether constructed of diatomaceous earth, polymers or even glass, depth filtration is based on trapping particles larger than the filter's porosity rating within a matrix of nearly infinite passageways. Certain filter media, like some pad filters, also exhibit a positive ZETA potential that attracts and holds particles exhibiting a certain charge. These two attributes allow depth filtration to retain a large number of solids for effective and economic filtrations. However, when solids are trapped in the depth of a filter, this also makes back flushing a moot point.