There have been a variety of improvements made to the head. Improvements submitted as a project can be opened by all users. Improvements submitted as a Tech Note can only be accessed by C34IA members.
The Head Mistress aka Peggy Hall is the expert on marine heads and holding tank systems. Her book is available at most marine stores or online. She moderates a forum at www.catalinaowners.com select forum, then expert forum, Marine Sanitation. Here is the link http://www.sailboatowners.com/forums/menunew.tpl?fno=31
Why does the Head Smell FAQ 
Vent Filter I just finished building a vent filter for the holding tank. The breakthrough thought came from my wife, while we were discussing the possibilities, and I asked what the filter medium could be. She immediately suggested activated charcoal, which makes perfect sense, and I was soon off to Hechingers for parts. I built a U-tube structure with 2" PVC sanitary piping and 3/4" elbow end fittings to connect into the vent tubing where it rises up past the electrical panel in the aft-most cuddy on the port side. The U is about 12" long, and 8" wide, and is filled with a mixture of straight activated charcoal and something called "Ammocarb". These came from a pet store, where they are sold in bulk to people who own fish. They are used to filter out the same noxious gases in fish tanks. I have never seen the commercial product, and didn't know how big the absorber bed is, so I figured that a bed that was 2" in diameter by about 30" long should be fine. If not, I can extend it by adding some more loops. The U also has a side tap so that I can empty and refill it when the charcoal is exhausted. The whole thing cost about $20 in parts,and about $10 for the charcoal. And I can re-fill it very cheaply. I filled one leg with Ammocarb, and the other with straight charcoal. It works very well, and we now don't have to worry about stinking up a whole raft of boats any more! (Ralph Caruso)