The idea is to pass liquids contaminated with solids through media, trapping the unwanted solids in, then on, the media. The goal for the most economical filtration cost is to lengthen the filters cycle time. This maximizes the quantity of the unwanted solids per area of media to reduce media costs. Long cycles means the filtration process is working with fewer interruptions for media changes. This increases the capacity of a filter.
Smaller filters cost less and take up less space than larger filters.
As the unwanted solids build up on the media, the flow of the liquid is impeded. Unless the liquid is highly pressurized, the flow of the liquid through the filter will drop quickly and not satisfy the flow capacity required. Oberlin filters pressurize the liquid by pumping it into a chamber whose only outlet is through the unwanted solids on the media. The pressure is limited only by the size of the pump, not by atmospheric pressure. 30-40 psi is typical of an Oberlin filter.