What valve can I use to redirect drain water between two 2" PVC pipes?
Situation is as follows: A 2" PVC pipe from a sink (or tub, whatever)
comes into basement and I want to be able to select whether it goes
into a 4" septic drain pipe, or an alternate 4" gray water drain.
Presumably I have to split it off into a T and put two huge 2" shut-
off valves, one on each side. Is this right? The only valves I know of
are expensive brass shut-off valves with red handles that cost around
$35. Is there a cheaper plasticky valve?

If your local home center doesn't carry large PVC valves, you could order
them for about $17 each from:

http://www.plumbingsupply.com/pvcvalve.html

They even have a PVC 3-way diverter valve, though it's $50 and cheaper to
go with two seperate valves.

I ordered some brass fittings from plumbing supply several years ago, and
they provided relatively good service.

Take care,

Anthony