Rancid exterior black water odor

We have a new 2016 Beneteau Swift 30. Initially, unbeknownst to us, it had a saltwater head which we converted to fresh water to avoid interior odors.
We are still getting rancid odor out the vent despite using every chemical imaginable. It’s so bad our neighbors rightfully complain when we flush the head.
Are there any other Beneteau or Trawler owners that have encountered this issue and have suggestions for a cure?

Greetings,
Mr. b. DITCH THE CHEMICALS. They're killing your work force. Sorry but what you want in your holding tank is a LIVE colony of microorganisms (bugs) that will digest your digestibles thus resulting in NO odor at all. Please try to do all you can to keep the microscopic denizens of your holding tank happy. No sh*t.

You may have to increase the diameter or routing of your tank vent as well. Your next pump out, rinse/flush your tank AND the lines really well with fresh water. You may have to "inoculate" the tank with "bugs" initially. Ms. Potty Lady (Headmistress) will be surely able to suggest the optimum procedure and materials to follow and use.

It's a common problem in most boats. [FONT=&quot]Sewage—all organic material--contains both aerobic (needs oxygen) and anaerobic bacteria (functions in the absence of oxygen); but only the anaerobic bacteria produce foul-smelling gasses! Both aerobic and anaerobic bacteria break down organic matter, but when organic matter breaks down aerobically, it converts to CO2, which is an odorless gas, and water. So, as long as there is a sufficient supply of oxygen to the tank, and an aerobic treatment is added to aid that which naturally occurs in sewage, the aerobic bacteria thrive and overpower the anaerobic bacteria, and the system cannot produce odor.

[/FONT] [FONT=&quot]So the key to odor control in most tanks is the vent line; it must allow a free exchange of fresh air with the carbon dioxide generated by the sewage. Boat builders, boat owners and boat yard personnel who install holding tanks have always viewed the vent line only as a source of enough air to allow the tank to be pumped out without collapsing and as an exhaust for methane (many even mistakenly believe that methane—which in fact is odorless—to be the source of odor.). Some take the attitude that tanks must inevitably stink, so the thing to do is run that vent line as far from people areas—cockpits, sun decks, etc.—as possible, or make the line as small as possible, or install a filter in it. All of the above actually create the very problem you want to solve.

[FONT=&quot]If you'd like to send me a p[FONT=&quot]rivate message [/FONT]I'll be glad [FONT=&quot]to [FONT=&quot]help you sort out the specifics of what you need to do to modify the [FONT=&quot]tank venting to [/FONT]increase the amount [FONT=&quot]of air exchange needed to c[FONT=&quot]reate the [FONT=&quot]a[/FONT]erobic conditions in the tank[FONT=&quot] [FONT=&quot]needed to prevent [FONT=&quot]odor. Fwiw, it[FONT=&quot] was the desperate need to solve exactly the same problem on my first [FONT=&quot]boat with a holding tank that [FONT=&quot]started me on t[FONT=&quot]he path to a career in marine [FONT=&quot]san[FONT=&quot]itation [FONT=&quot]30 years ago.

[FONT=&quot]Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein[/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][/FONT][FONT=&quot]
[/FONT]

Here is one other thing that might be going on: we have a forward head that flushes directly overboard. We use it for “liquid”wastes. When we arrive back in the river. It takes a few weeks and repeated flushes for the hydrogen sulphide aroma to disappear on flushing. I’ve traced the aroma to the incoming raw water line. You get saltwater organisms growing in the line and they die and decompose when in freshwater. We find it took a while for these remains to fall off the sides of the lines and to flush through. If you are using the same lines and waste tank, it might take a while for the smells to dissipate.

Buy a new “vent line filter”.
All it takes is to overfill your black tank once, and the vent line filter fills up with sewage.
Simple fix that I would try first.

...You get saltwater organisms growing in the line and they die and decompose when in freshwater.

They decompose in salt or fresh water. Fresh water can sometimes be pretty skanky too! But most likely the source of your odor isn't micro-organisms, but the remains of not-so-micro sea or freshwater animal and/or vegetable life that gets trapped and dies in the inlet line and the channel in the rim of the bowl. If it were anything else, all the sea water would be gone in a few flushes..it wouldn't hang around for weeks unless the "remains" remained too. A strainer on your intake line would most like solve the problem.

Buy a new “vent line filter”.All it takes is to overfill your black tank once, and the vent line filter fills up with sewage.

A tank level indicator prevents that problem.

One concept that has not been discussed is the use of a large diameter sea water line to feed the head that is made from copper tubing.

Copper seems to slow or stop the goodies alive in fresh or seawater from growing.

If the water in your area is so bad it dissolves copper you could always install rubber over the tubing for a belt and suspenders job.

bwalz

Are you 100.00% sure your three way valve is sending all head material to the holding tank?