The problem is not likely to be the tank itself. I haven't heard of a plastic tank permeating... though I do believe some of the flexible bladder tanks have.
The problem is likely one of two things:
- The hoses connected to the tank have permeated
- There was/is a leak and that has caused some of the sewage to leak out and innudate the area in sewage stench
I would recommend testing the hoses for permeation by soaking a cotton towel in really hot water and then applying to the hose for a few minutes. If the towel smells like sewage when you remove it, the hoses have been permeated with sewage and need to be replaced. Use Trident 101/102 hose for this per Peggie Hall's recommendation.
Then I would also recommend spraying the area down with Raritan CP, which is a bio-active cleaning solution that will eliminate the sources of the odor. Rinse well with water after letting the CP sit for the recommended period of time.
If the hoses are replaced and you clear the vent line to the holding tank, then you shouldn't have much problems in the way of odor. If you're serious about this, I would recommend upgrading the holding tank vent lines to 1" diameter hose and adding a second vent line to the tank, on the opposite end of the tank from the existing vent line, and plumb it to a through-hull just under the hull deck join. This will insure the tank is well ventilated and promote the growth of aerobic good bacteria rather than the anaerobic smelly producing bad bacteria.
One helpful recommendation would be to flush the head with FRESH WATER before leaving the boat for any period of time. The reason for this is one source of head smells is the sea life that comes in with the raw water. If it is left sitting in the head, pump and lines, it will die and start to decompose. If the head is being used regularly, it will end up in the holding tank before it dies... but if the head isn't being used for several days, it can cause a really nasty stench.
You need to rig the head intake line to do this. On my boat, the sink drain can either drain overboard or to the head intake line. If your boat's head sink has a drain through-hull and seacock that is below the waterline, the easiest way to do this is to re-route the head's intake line to a t-fitting in the sink's drain line, below the waterline. This would allow you to flush using raw water by leaving the seacock open and plugging the sink drain. It would also allow you to flush with fresh water by closing the seacock and filling the sink with water. It doesn't pose a contamination hazard, since the sink acts as a vacuum-break between the freshwater system and the head.
I wouldn't use a strong solution of bleach, since it can easily damage the parts in the head, like the gaskets, joker valve, pump seals, etc.. It will also kill off the good bacteria required to keep the holding tank from generating the nasty smells you want to avoid.
I've got a J/30 that we use primarily for ocean racing. We don't use the head except for occasional use during races by those who cannot go off the stern - and this is pumped overboard. I do not use, and never again want to use the holding tank.
I cannot remove the head and holding tank, but I want to totally eliminate all odor. I've replaced the head itself and most of the plumbing, but the tank itself and the general area still stinks.
I've tried filling/pumping it out, followed by putting in a bit of fresh water and commercial head additives, but after a while it still stinks.
What could I do to "decommission" the holding tank? I was considering filling it with a strong bleach/water mix and letting it sit for a week, then draining it and putting a small fan in place to circulate air through it till it is bone dry.
Woudl this work? Is this a bad idea? Any other good ideas out there?