If you suffer from heads that smell after leaving the boat for a few days (especially in warm weather) when pumping in flushing water, this is probably due to bacteria that thrive on stale seawater.
One simple fix if you have a hand basin nearby is to plumb in a simple bypass valve into the sink outlet. With a further T piece inserted into the heads inlet pipe you can then pump fresh water (maybe with some disinfectant) into the head when leaving the boat.
The following photo is hopefully reasonably self-explanatory of the valve bypass I have inserted in our SO39i.
The hand basin outlet was a 25mm (1”) internal diameter pipe and the heads inlet is a 19mm (3/4”) internal dia pipe, so I used a 25mm tee piece in the hand basin pipe a very short length of 25mm pipe then connected to the inline value that I had fitted with a 25mm barb and a 19mm barb . A length of 19mm pipe then connected to a 19mm T piece instead in the heads intake seacock.
The sketch below shows the general arrangement.
Once installed to use the system normally the bypass valve should be closed, allowing the Head and the basin to function as normal.
When you want to put fresh water into the head, close both the seacocks open the bypass value and fill the basin with fresh water.
Then you simply pump the Head to introduce the fresh water. I often add some disinfectant at this stage too.
An additional advantage on the SO39i is that it makes the head easier to use on the port tack as the seawater inlet tends to draw air when heeled, so this modification allows a fresh water flush if required.