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.