Abstract: An enclosed, elongated structure for placement in the ground to filter and biologically pretreat wastewater after it leaves a septic tank and before it enters a drain field. The structure includes an upstream chamber or compartment connected to an inlet and containing a filter media such as stone and two or more baffles positioned crosswise to the direction of flow to direct the wastewater through multiple, sequential downward and upward flow paths through the stone for filtering and for anaerobic biological conversion of undesirable substances in the wastewater. A downstream chamber or compartment vented to atmosphere contains a filter media such as gravelly sand and receives the wastewater flow from the upstream chamber for further progressive filtering and for aerobic biological conversion of undesirable substances. Beneath the gravelly sand is a U-shaped perforated pipe for collecting the wastewater and directing it to an outlet leading to the drain field.