From the baby 's bathwater to the dead rat washed down a curbside storm drain , from a slop sink at Gracie Mansion to a Washington Heights bodega bathroom , it all goes into the street sewers , which , in their intricate latticework , are laid out so that the sewage flows by gravity to one large main bound for a tunnel running under the East River to the plant on Wards Island , surrounded by Manhattan , Queens and the Bronx .