Most marine toilets are flushed with raw water from sea, lake or river. This flushing water contains living organisms and it’s the demise of these little devils in the pipe work that begins the downward spiral; the resultant bacteria generate that awful sulphurous gas smell which you suck into the bog when you pump. The pipe work itself can become contaminated so that no amount of flushing will get rid of the smell.
At one time I handled maintenance for a fleet of charter boats and keeping the heads sweet was a big headache. I was persuaded that a major contributor to the odour was the fact that the translucent sanitation hoses let in sunlight which hastened the demise of the bugs and, thereby, the creation of the bacteria which caused the smell. I wrapped all my pipes in silver foil as a defence but found no real improvement and ended up changing all the pipes at the beginning of each season, and still had to deploy an array of disinfectants on a regular basis.
When I moved onto my boat full time and set off on my three year modest odyssey the problem was greatly alleviated by frequent and regular flushing. Unless you live aboard you simply can’t keep up the necessary flow.
The only boat of the seven I’ve owned not to suffer the odours was my GB32 trawler which had a fresh water flushing system and in-line deodorizer. But on a long distance cruising boat you simply can’t afford to flush freshwater down the bog, it’s way too precious for that.