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Chrisc · 11-04-2007, 18:46

I think the smelly holding tank question has been done to death, but, If I could ask a couple of questions and propose a solution...
My holding tank smells. This is because (I think) I bunged it into the forepeak of my boat without properly thinking the installation through. Consequently, I only have a one inch vent line from the tank that winds its bendy way up to the deck, so the tank itself is not well ventilated. If I were to put in a 2 inch vent vertically as recommended it would come through the middle of my double berth. Whilst this would not interfere with sleeping, it would not be beneficial in maintaining a harmonious relationship with my beloved. I understand from my research that using chemicals in a holding tank to deal to odours is just masking the problem after it has already occurred, and that the real solution to odour control is to provide air flow through the tank so that the aerobic bacteria can thrive. Does anyone have experience of this, ie, does airflow through the tank cure the problem? It it does, what is required to have a happy tank, 2 or 3 air changes per hour, or 2 or 3 air changes per day??
My toilet is connected to the tank via a 2'' sanitation hose that is 4' long, dead straight and level and enters the holding tank at the top. I propose to put a 'tee' in this line where it passes through the bulkhead (about 1' from the toilet) and take a vertical 2" pipe up through the deck and connect it to one of those little solar vents. The theory is that this would provide forced draft ventilation of the tank, with the incoming air exiting via the existing 1" vent line.
Before I start hacking into my boat and it systems, I would appreciate any feedback as to whether this is a daft idea or not.

Chris

delmarrey · 11-04-2007, 20:21

Yeah! There is a lot of info about holding tanks in the archives. Just go to the SEARCH and type in holding tank. You'll get a lot of threads.

As for the air line, 1" is plenty. The purpose of the air line is to allow the gases to excape so you don't have an expansion/explosion (build up of methane). It allows the flamable gases to vent out. Had a friend on a 36" powerboat that had his blow up. One bad mess!

The smell can be controled by not storing for very long and to use fresh water when ever possible. Salt water has little creachers that die and rot.

Any way do the SEARCH.............................._/)

Richhh · 11-04-2007, 21:47

You might want to consider forced aerobic fermentation such as from a small 12vdc air pump delivering to a poly tube thats terminally connected to a porous stone located in the bottom of the tank. The finer the 'bubbles' the more efficient the mass transfer, etc.
Wouldnt take much mass flow of air, you'd just have to overcome the static height of the 'contents'. A few pounds per hour (liters/min) of air would do it ... an air pump the size of what's used in aquariums tanks would do or that which is used in 'bait wells' (but you'll need consider the static 'head' pressure or 'height' of 'contents'). This would assure aerobic fermentation which has vastly much less 'stink' than anerobic fermentation; plus you'd get a small 'sweep volume' of air to purge the CO2, H2S, the mercaptans/putrenes etc. rather than letting the gases passively diffuse through a 'vent'.
If an air-purge system still was too stinky for your admiral a carbon (granules) filled canister on the vent line would help but would need to be regenerated often.
Ive seen such pumped air systems at boat shows .... just a technology transfer from the (bio-pharm) fermentation industry. But unlike the constant process of that industry the amount of purge air you would need would be regulated based on what you ate before it ultimately went into the tank.

cal40john · 11-04-2007, 22:21

I have done the two vent system suggested by Peggy Hall. They are the standard size used, 3/4?. I put one vent in the front and one in the back midline on the tank that I installed. The aft vent tube is the original from a previous owner and is long and has bends.

(At least I fixed the part where it immediately went down and then back up to an on deck vent. On the old holding tank with the vent outlet on the side of the tank, when the boat would heel, a slug of liquid would go into the vent and down to the low point, the next flush would spout that onto the deck. The vent is no longer on the deck as well.)

Anyway back to this story. The front vent goes to the hull on the opposite side from the back vent. I have no holding tank odor in the boat. I have had the tank go anaerobic only twice on me since I installed it. Both times the boat hadn't been used in a month and it was summer, so it was hot and the dock is well sheltered so in the summer with very low winds there is not much chance at air flow. As I have kids, the tank is never completely empty. The tank going anaerobic has been discovered by using the head at the dock and having the smell emanate from the outlet outside of the boat. If I actually use the boat on a regular basis in the summer and get away from the docks to get any airflow at all, it works well.

John

**Alan Wheeler** · 12-04-2007, 00:37

Hi chris, I assume you have a small tank if it is in the forepeak. Therefore I would also imagine you emtpy it daily. So a small breather of 3/4 to maybe 1" is all you will need.
You know my saga, well at one stage I had a 1 1/2" and a 3/4" into the big tank. It was set up that it blew in the one and out the other to allow good airflow. It did nothing for the smell. I don't think the smell was anerobic made. It was just simply sewerage smell. It doesn't matter what you do with it, you aren't going to make crap smell sweet....or not smell at all. The best thing to do is to eliminate any chances of odour escaping from that tank and have the breather go somewhere outside well away from any air intakes back into the boat. I would be cautiouse of the line from the toilet to the tank being horizontal, was that what you meant??? You want an angle down to the tank so as matter in the hose drains to the tank. You do not want anything traped in that hose. It will permeate even sanitation hose.

Chrisc · 12-04-2007, 01:15

Yes, our tank is only 60 litre capacity and we empty it daily. I have no outlet from the toilet to the skinfitting anymore as that required a diverter valve which was another pain in the bum - leaking,sticking etc. So now we pump the toilet directly to the tank, and empty the tank via a macerator pump.
The line from the toilet to the tank is horizontal and I know its not the best so we try to completely evacuate the hose by a lot of dry pumping at the end of the flush. When used daily, we have very little or no odour from the tank - the stink only starts if the system has been idle for two or three days, and the smell is definately not poop but sulpherus, so am assuming that its an anerobic problem from a lack of ventilation. With regard to my origional query, is there any reason why I cannot put a forced draft vent pipe tapped into the toilet to holding tank line?

Chris

**GordMay** · 12-04-2007, 01:18

HOLDING TANK VENTILATION:

Ensure that the holding tank vent line has a constant rise from the top of the tank to the vent fitting. Otherwise low spots may fill with sewage and act as a "P" trap, effectively plugging the vent.
The vent line should be as short as possible, and connected to the uppermost point of the top surface of the tank if they are to operate effectively. The exterior vent fitting should be located above the holding tank and above the waterline at all angles of heel. It should also be no less than 24 inches away from any opening into an accommodation space.

Overfilling the holding tank can force sewage into the tank vent line. A clogged vent line will be ineffective and can make it difficult, if not impossible, to flush or pumpout the tank, due to to overpressurizing - may even cause causing leakage or rupture.

A small 12-volt in-line fan (ie: “Lil Stanker” Lil' Stanker™ In-Line Deodorizer Fans) can be attached to your vent pipe, continuously ventilating the holding tank.

See the diagrams Sewage Holding Tanks Systems (Appendix 3.0),
from the Canadian Construction Standards for small vessels:
http://www.tc.gc.ca/MarineSafety/tp/...-Appendix3.pdf

Construction Standards for Small Vessels (TP 1332 E)
Construction Standards for Small Vessels (2004) | TP 1332 | Marine Safety

**Alan Wheeler** · 12-04-2007, 01:51

Chris, you sure that smell ain't coming from the toilet bowl itself?? If you are leaving the boat for a couple of days or more, you need to pour fresh water into the bowl. It is the water in teh bowl that makes that sulpherouse smell and will travel through the boat.

Benny · 12-04-2007, 05:52

I wouldn't put a T fitting in the waste line to act as a vent because when you 'pump' the head, it will force the contents of the hose up that 2" vent hose as well - not good. It is not like a house system which works on gravity where you have a vent pipe connected to the main stack.

Your vent line can be installed other than straight up through your berth - I could see a good cartoon made from that arrangement - as long as it is always in an upward angle it will vent properly and not allow standing liquid. Due to sailing and tank contents sloshing around, you may wish to give the vent a clean out at the dock by connecting one of those inflatable toy air pumps to the deck end of the vent and pump some air through to the tank. This should clean out any clogs that may have occured due to boat motion while underway.

**GordMay** · 12-04-2007, 06:15

Quote:

Originally Posted by Benny

... Due to sailing and tank contents sloshing around, you may wish to give the vent a clean out at the dock by connecting one of those inflatable toy air pumps to the deck end of the vent and pump some air through to the tank. This should clean out any clogs that may have occurred due to boat motion while underway.

It’s a good idea to clear & clean your vent fittings.
When dockside, I’ve always flushed my holding tanks (& WC) with copious amounts of fresh water. I overfill the last tank rinse, so that clean water drains out the vent fitting.

Benny · 12-04-2007, 06:23

Yes Gord this works as well - hopefully you have only done this right after a pump out

Brent Swain · 25-04-2007, 18:09

I'd switch to the air head composter head. They are overpriced but easy to build. Make the vent pipe high and add an extension on it when in a marina.
Brent

mkinney716 · 26-02-2020, 14:16

Here is another question.
I purchased a steel boat with two holding tanks that have not
Been touched for at least 6 years. I haven’t opened either one yet but have a fear that everything has dried out and I will have to clean it all out. Other than taking a hammer and chisel, what do you think is best way to attack this. Is there a simple way to bring the contents back to a liquid state or do I just have to break down and chisel it all out? Holding tanks are steel and have 4” square plate they the hoses enter and leave.

Muaddib1116 · 26-02-2020, 14:26

Way to revive a 13 year old thread! I think it might be easier to clean it out dry, if you have adequate access. A 4" access hatch is pretty small. If the tank has been vented and drying out for 6 years, it may be pretty easy to handle with gloves, a mask, and a scraper. But you may need to cut a bigger access hatch. You might want to check if it's actually dried out first.