On a recent This Old House - recent as in last season - they were rehabbing a house built on top of a rocky hill with almost no room for a leach field. Instead they had about a 15x5x3 tank set up in the basement for treatment of grey and black water. It had bacteria, filters and pumps. The water at the far end was potable, as the tech drank from a bottle allegedly filled from the system. It was over my head on how it worked, but this seemed pretty cool for an area with arid conditions. Worst case, I'd use it to water the garden.