A downflow sand filter used for treating wastewater which has degraded in function from its original design is at least in part rejuvenated by flowing air downwardly through the bed during time when flow of wastewater is ceased. The low velocity air flow helps dewater the bed and leads to the removal of accumulated organic material which has clogged the pores of the bed. In accord with different embodiments, air flow is alternated with water flow in particular ways, for instance, so the water flow is sufficient to create ponding or is sufficient to maintain moisture content within the bed.