Decomisioning Holding tank question

[hellosailor]

Dang, Wheels. You just gotta associate with a better class of people. You know, the kind who's ---- don't stink.<G>

Those vents certainly sound adequate...although I'm not sure what "adequate" scales up to for a thousand litres. I guess capping everything and checking to see if that tank holds air pressure is the only way you're going to confirm there's a leak. And then, if you can only find it...

It sounds like you have decided to go with fixing the original problem rather than creating new ones. Good choice.
One simple check before you move on to the hard ones. Is there any chance your vents exit anywhere near a dorade, or portlight that could allow the smell to get back into the boat? If not, I would recommend flushing the tank out completely to get it as clean as possible. Try the pressure test. I would also make sure all of the plumbing is 100%, including the line from the head to the tank. If all is in order, and the pressure test shows no leaks, you have a problem with a fixture. I would check the sinks first. Although you have a trap, when the boat heels, the water in the trap can easily drain into the tank, This would allow gas to exit through the sink drain. The set up you have, is really not desireable, and I would strongly recommend finding another outlet for your gray water. I suspect if you were to eliminate the gray water lines from the tank, the smell would go away. If, none of this works, do you have sufficient access to install smaller tanks inside the holding tank? Cutting the top out, and installing a plastic holding tank, and a seperate gray water tank would be a very good way to modify your system. If the access is limited, you can put several smaller tanks, or waste tank bladders.
If there is anyway to post a couple of pictures of the tank area, it would help suggest some options.

[Amgine]

Even if you're giving plenty of ventilation to the surface of the tank, that doesn't mean there is any oxygen in the lower dregs of the tank contents.

You have a huge tank. You need to think about it as a sewage pool. Sewage pools are aerated or mixed constantly to move oxygenated water to the bottom of the pool. Sailing in rough weather every day would probably do the same thing, due to the baffle, but you wouldn't be able to live below while the aerobic bacteria bloomed enough to handle things.

I'd again suggest you talk with a sewage treatment expert to get an idea of what you might need to do to keep your tank aerated. If your area has cruise ships or large shipyards there will be marine experts to ask questions of. If not, talk to the city sewage experts.

[Whimsical]

Just a thought
If you find the source and can't fix it or just give up. Put a bladder tank in the side of the tank that is accessable then use the remaining volume of the tank for a grey water holding tank that can be used as flush water for the loo. Fresh water can be added to maintain enough water for flushing purposes.
Mike

[Alan Wheeler]

Thanks for the advice so far guys.
Amgine, I appreciate your thoughts on tank size, but do realise I am not using ALL the tank. Probably only 100ltrs at the most. I tried measuring more accurately today. The odd angles make it hard to work out exactly. But the re-calculation has brought my tank size down to maybe more like a 1000ltrs. Still waaaay too big. I also blocked the vents today and then filled the tank with fresh water. As it filled, I poured very soapy detergent all over the area's I could get too and looked for bubbles. but sadly I could not see aything. But I could certainly smell it.
I took another look at the plate on the tank that is under the galley and with some minor timber cutting, it just might be possible to get the lid off. So I may re-look at that. If I can gain access to inside the tank, I could then get it realy clean and coat the walls with some form of waterproof tank membrane or compleatly glass it, either which should solve the problem. Plus that also means I could divide the tanks and turn one into extra fresh water holding and have a smaller blackwater holding tank on the side all the current plumbing is running into.

[Amgine]

but as far as I'm aware you cannot get the build of smell you're reporting without anaerobic activitiy. Therefore...

[Alan Wheeler]

Yeah but even if it was much less ummm... "agressive" to the nose, I would rather not have it inside the boat. I would like my boat to smell lovely and fresh.

[Amgine]

Believe it or not, most of them don't smell much if at all. I believe San Francisco's treatement plant is a very popular park, iirc.

[ssullivan]

I hope you get it soon, Wheels. It's a long, tough battle. I'm weary for you. Just words of support.

[Alan Wheeler]

Thanks Sean. I am posing the Lectrasan question just to cover and broaden my possible scenario's, choices, decisions and what ever else.

[Alan Wheeler]

I have been a busy wee beaver, head down bum up in the bilge working on the holding tank. I think I may have fixed it. It's taken a week, but I may have it beat.
Firstly I flushed the tank through with fresh water. Then I filled the tank right to the top and in fact overflowing. I had leaks everywhere and some where not that small. I was shocked. In the top outer edges of the tank, steel plates had been fitted. They were embedded into the Cement. These attached to floor bearers to support the floor over top. It looks like they were not plastered very well around the plates. Drilling down into the cement at a plate would reveal a void below. Each was a simple fix of pouring in a slurry of cement till the void would not take anymore. Move on a few inches and do it again till no more void remained. Once that was done, a few weaps became evident in some other places. I used a special concrete sealant made by Cemix called Penetron. It follows hair line cracks and cracks up to .4mm wide. It reacts with the cement and creates crystals that bond the cement and fill the cracks making them waterproof. As water continues to come in contact with it over time, it continues to react sealing the crack. I think I have them all bar one I need to get to in the morning. It is in a difficult place to reach, but I should be able to get it. Then it will be a case of trying the tank and see if the smell has been stopped.
Interestingly I found a similar problem with my fresh water tank. It had been leaking water down into the bilge when it was filled to the top each time. I had never realised. So I have been working on that as well. I have probably been losing 1/3rd my water supply in leaks.

[seafox]

And you always blamed Dawns long showers

[hellosailor]

Sounds like you found one of the reasons why ferro boats are held in suspicion by so many folks, Wheels.

[Benny]

Sounds promising Wheels - you used water - if you want to take it a step further, close off your big vent and pump air through the small vent to see if it's pushed out anywhere else. Even a small leak can allow ordors to escape the tank/hoses/fixtures. I had the same prob's as you with hidden hoses and I found the culprit using the above method - it was a long screw fasteneing a battery hold down strap through the locker bottom under which the hose ran - this was after I replaced the holding tank and fittings, the pump out hose, the vent hose and the shorter aft head hose - the longer forward head hose which was fairly new was the problem and I hadn't replac3ed it. Under normal conditrions, no seapage or smell would emit but if the head was pumped, the raised pressure in the hose as the effluent was pushed to the tank would cause a slight seapage and we all know it does'nt take much of that stuff to stink up the boat.

Good luck with the problem

[btrayfors]

Wheels,

Congratulations on finding the leaks! You may well be on the path to a solution.

When you have a spare moment, I'd also think long and hard about the size of your holding tank. Even if it's 1,000 liters instead of the 2,000 you originally thought, it's still MUCH TOO BIG. I simply can't imagine the thought of 250 gals of waste slushing around in a seaway. And, if the tank doesn't have proper baffles, the hydrostatic pressure created could be enormous...enough, perhaps, to render your repairs insufficient.

What about just installing a reasonably-sized holding tank, cleaning the big tank as best you can and treating it with microbes and, maybe, coffee grounds, and either leaving it empty or fill it with water as needed for ballast? In other words, just forget about the huge waste tank.

Good luck with whatever you decide to do...

Bill