Wired Magazine has a short piece on a Living Machines installation at the Port of Oregon.  The idea is that, after settling (read: septic tank), the wastewater is naturally treated in a soil system with ornamental plants, put out in the lobby.  Disinfected effluent is used for flushing toilets, as well, making this a nice water reuse system.

This is a good approach, although it will only handle part of the flow from a typical large office building without a rather large area dedicated to the treatment facilities.  The Living Machines website goes into things a bit more.  They note some patents, but it’s doubtful whether proprietary methods are necessary to develop such a system.