You shall remove oil before ...
Published by Ademar Cesar Ferreira, H2O Engenharia Ltda - General Director
You shall remove oil before filtration, specially if you are using sand filter that works based in superficial filtration, otherwise you have to backwash your filter more frequently. free oil as you refer to is easy to be removed. In case you can not do it you can try to use a double media filter (anthracite and sand) .