Dave wrote:
First, if it's fairly well-known that air into the holding tank is
the key to odor-free operation, why doesn't one see more holding
tanks with two vent lines instead of one?

Tank locations don't always make two vents better than one. And, at
least as when it comes to OEM installations, cost. And, two vents
aren't necessarily better than one. A short straight vent 1" vent line
can keep a tank just as odor-free as two "standard" size 5/8"
vents...maybe even do a better job.

When it comes to venting a tank, there isn't a "one size fits all"
solution for every installation. 10 vents couldn't make some tanks
aerobic enough to prevent odor..tanks buried way low on a deep keel, or
the centerline of a large boat...tanks much deeper than about 20"...most
bladders... For some tanks, the only solution is an aeration system.

I'm also wondering whether a single vent tank might be improved by
putting a T fitting at the tank outlet and running a second vent hose
to the other side of the boat from where the current outlet runs.

That won't work...air flow would just be across the top of the tee
instead of going through the tank.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://www.seaworthy.com/store/custo...0&cat=6&page=1
http://shop.sailboatowners.com/books...ku=90&cat=1304