Several types of media filters are used as drainfield options when they are configured in a bottomless fashion to allow treated wastewater to infiltrate directly into the soil beneath the unit.
Essentially, their design looks quite similar to the single-pass media filter without the bottom liner or watertight sealed container. Media filters with an open bottom are specifically intended to disperse wastewater into the soil immediately under and adjacent to their footprint. Two media filters fall into this category: bottomless sand filters (BSFs) and bottomless peat filters (BPFs).
The service provider should pay careful attention to verifying that these systems are hydraulically disconnected from the stormwater that may be generated on a site and that the filter footprint is kept as dry as possible.