Yara said..
Since the original thread on holding tanks has gone off on wild personal issues, I thought I would add some information.
Just for a bit of background, I earn my living from process equipment for sludge handling.
The first rule in large-scale sludge handling is "never hold sludges". Clearly where we have a holding tank, we are breaking that rule.
The next rule is that if you want to minimise odours, you need to keep the sludge aerobic, meaning keeping some oxygen available for the bugs (microscopic creatures), that will be in the seawater and sludge, and dining on your sewage. If you're sailing, the constant motion of the boat will stir up the surface of the tank, and provide some aeration. Unfortunately, this won't happen if you are tied up to a marina or a calm mooring. Composting toilets have a mixing handle to provide some aeration. Not really practical in a holding tank.
The last important rule is, if there is no stirring of the tank, sludge will settle in the bottom. In large-scale plants, we have scrapers to move the sludge towards the outlet. The sludge settling is what I believe is the problem in SouthAce's original post.
So what to do? The best practical solution appears to be to arrange the inlet piping such that the incoming waste stirs up the bottom of the tank. The incoming waste will contain some oxygen as well. When discharging at sea, you could continuously flush the toilet to assist in the discharge.
A more complicated solution would be blowing air into the bottom of the tank, or miniature aerator mixers, but that would only be for retired municipal engineers with nothing else to do. Another practical, but complicated, solution would be a recirculating pump, dropping some of the feed back into the tank at a high level in order to get some aeration and mixing.
My conclusion-get a composting toilet.
Many thanks Yarra for starting afresh ( maybe not the best discription ) it is going to be an on going topic as marine pollution becomes more in focus.
I read a post of Peggy Halls suggesting that adding nitrate to the holding tank has an oxygenating effect, some chemical reaction that your probley more aware of than I.
My issue was with the commercially avalable activated carbon filter. ( had no idea how much s**t would hit the fan ) Should get the chance this weekend to put a diy filter together.
P.S. Composted would be the way to go especially if you have the same system at home
