Philadelphia's Water Department is serious about keeping stormwater out of the sanitary sewers. In many old neighborhoods like mine, one sewer serves both functions, so it is important to disconnect roof drains from the sewer if possible. The PWD subsidizes various kinds of capture and diversion devices including these planters made from cattle watering tubs. Each one holds 70 gallons, which drains slowly through the white pipe to a part of the yard that can absorb a lot of water. Overflow goes into the white pipe directly.