a filter for removal of biochemical oxygen demand and suspended solids utilizing multi - layer filtration media . the filter media includes plastic particles , sand , and garnet and is operated under an automatic control condition . fine filtering is accomplished by a sludge cake which develops at the upstream face of the filter media . for cleaning the filtration media , deposited sludge is removed from the filter bed by backwashing in a short time period at a high strength backwash flow rate .