What I've learned from Peggie

[Roger Long]

1) A natural vent on a typical sailboat size holding tank will keep the contents sufficiently aerobic to control odor as long has a fairly straight run with a minimum of elbows and doesn’t angle up more than 45 degrees. 1” hose is the minimum size for good performance.

2) The nasty odors are heavier than air, thus vertical runs in the vent and a long distance from the tank to the through hull will trap them and prevent oxygen from reaching the contents.

3) A Sweet Tank system which pumps air into the bottom of the tank to both stir up the contents and provide oxygen is not necessary on tanks below about 30 gal capacity. It needs to run 24/7 so the electrical consumption is impractical for my boat anyway.


Peggie, have I got this right?

My boat was built before any of these things were known so I have berths, bulkheads, and joinerwork in the way. The only feasible vent run is tall and vertical with a lot of elbows. I’m using 1” hose but could only fit in a 3/4” through hull. It seems like there is a good chance I won’t get good air exchange.

(I’m using PVC for the parts of the vent and pump out runs I am going to enclose and hope to never see again. I’m keeping the hose to where I can replace it easily.)

Here’s what I would like to know now:

The amount of air exchange through the ideal vent seems like it would have to be much less than the Sweet Tank system and bottom stirring and bubbling is evidently not necessary on the 18 gallon tank I am installing.

Why would a small and much lower electrical draw air pump that simply injects into the top of the tank and keeps the air exchange going not improve the situation? It would basically just be overcoming the unavoidable deficiencies and restrictions in my vent system by adding a small amount of positive pressure to overcome the elbow restriction and “pooling” of the heavier gasses.

 

[sailor.fred]

Roger, I have tossed this exact idea around my mind for several years now. I am very interested in what Peggy may have to say about this. Seems like a reasonable idea to me as well.

 

[Ron20324]

Remember that when the boat is on a mooring or when out on the water, the tank's contents are agitated as she rocks. Also, using this movement to clean an empty tank by putting in soap and water is a no-brainer (that's why I know it).

 

[Peggie Hall HeadMistress]

What you didn't learn from Peggie

Is that your #s 1-3 are all generalities. And miss a couple of important points:

1. A natural vent on a typical sailboat size holding tank will keep the contents sufficiently aerobic to control odor as long has a fairly straight run with a minimum of elbows and doesn’t angle up more than 45 degrees.

And is also no longer than 5 feet.

2) The nasty odors are heavier than air, thus vertical runs in the vent and a long distance from the tank to the through hull will trap them and prevent oxygen from reaching the contents.

Not exactly...nothing TRAPS anything...it's actually much simpler than that: The gasses in the tank are heavier than air...so they don't rise. Fresh air, especially WARM air, doesn't fall...so air stays up, gasses stay down. No co-mingling can happen.

3) A Sweet Tank system which pumps air into the bottom of the tank to both stir up the contents and provide oxygen is not necessary on tanks below about 30 gal capacity.

RARELY IF EVER, not "not."


It needs to run 24/7 so the electrical consumption is impractical for my boat anyway.

The first half is true, but a small solar panel can supply all the power needed.

A computer muffin fan (or corrosion resistant equivalent) would work just fine to supply air to the tank that can't get enough any other way, but that requires TWO vent lines and the fan has to be an exhaust fan that PULLS air through the tank ...a fan that pushes air through it doesn't work as well.
Compare it to an attic or "whole house" fan in a house...it doesn't push air out open windows...it exchanges the air by pulling air in through open windows and out through the fan.

 

[Roger Long]

Compare it to an attic or "whole house" fan in a house...it doesn't push air out open windows...it exchanges the air by pulling air in through open windows and out through the fan.

But even a fairly tight house is full of air leaks and people are always opening doors and windows. If a holding tank were as vapor leaky as a house, the boat owner would certainly be posting a "Peggie, HELP!" thread here. If you push air into a tank, the only way it should be able to get out is through the vent.

The problem with the muffin fan sucking idea is that any stuff that gets into the suction vent in rough water or an overfill is, well, "shit hits the fan" comes to mind. A fan would also need a quite large diameter hose. I could never fit it into my boat. It would also have to terminate well inboard and be well protected from water because a duct that size would take on a lot of seawater. There would also be an electrical device that would have to be replaced at the first dousing.

The amount of air exchange in even the ideal passive vent situation has got to be pretty small. It's a single pipe so whatever goes out has to be replaced by air flowing the opposite way in the same pipe. It's a two lane road at rush hour with everyone trying to pass. If you were able to quantify the time of tank airspace exchange it would have to come out to being very, very low in terms of CFM. With different vapor densities all forces except slosh agitation are working against flow.

I would expect detailed tests and calculations to show that the perfect passive vent exchange rate in calm and moored conditions would not exceed what could be created with about a 1/4" hose and something like an aquarium bubbler pump if something like that could be obtained in 12 volts. You only have to introduce enough O2 to keep the bacteria happy and a corresponding amount of air will have to go out the tank.

It seems to me the air flow should go in the direction that will clean the hose if anything should get into it and a 1/4" hose will limit flow in any system malfunction or human error. I would mount the pump as high as possible, above the vent exit it possible.

 

[AnsleyS]

Peggy has been helping us for a long time

We bought our current boat 15 years ago. It is a Camper Nicholson 39 foot center cockpit ketch which was built in 1976. At that time both heads were plumbed straight over board. When we bought her I thought that we should do something with our effluent and through discussions with Peggy we put in a LectraSan on one head and a holding tank on the other.

Peggy advised me on the plumbing of the systems and I learned about not letting any effluent sit in hoses and of oxygen transfer in the holding tank. I put in two air vents into the tank, both with 1 inch hoses. One vent comes into the forward end of the tank. The vent goes to the top of the hull, just below the deck, and is covered by a clam shell vent cover which is installed backward to the usual mounting so that it acts as a wind intake. The other vent hose to the tank enteres the aft end of the tank. Its vent through hull is mounted just aft of the intake and the clam shell is mounted with the opening facing aft to act as a exhaust vent. This way there is a constant positive pressure ventilation through the tank when we are sailing or at anchor with any wind blowing. It is possible that we could get some sea water in through the vent if we were in very rough seas but the hose runs are vertical so it would run into the tank and not stop the air flow.

Peggy also helped me with the design of the plumbing so that there is no effluent sitting in any hose at any time. This included using a standpipe in the tank for the pump out hose and the placement and sloping of hoses so that they are all self draining.

The other valuable suggestion that I got was Peggy was to rinse and pump the heads with fresh water before leaving the boat for the week. A couple of gallons of fresh water are a small price to pay for non smelling heads. For fifteen years we have had no odors from our heads.

Of all the people who have advised me on my do it your self boat projects I most heartily thank and recommend our Head Mistress, Peggy!

 

[Roger Long]

I put in two air vents into the tank, both with 1 inch hoses. One vent comes into the forward end of the tank. The vent goes to the top of the hull, just below the deck, and is covered by a clam shell vent cover...

That sounds ideal. If only I had a way to get from there to there on my boat. I expect a lot of people will be in my position, at least on smaller boats.

 

[Peggie Hall HeadMistress]

The fallacies in your thinking, Roger...

But even a fairly tight house is full of air leaks...

If that's true of your house, I wouldn't want to pay your heating bill!

If you push air into a tank, the only way it should be able to get out is through the vent.

How will the air get into the tank without a second vent line? A fan in a single vent line becomes an obstruction in it, preventing the free exchange needed. And if there's a risk of effluent ruining a fan that's PULLING air through the system, how is there less risk if the fan is PUSHING air into the tank?

Besides, if the tank fittings are are where they should be, no amount of rocking and rolling can cause effluent to get into the fan unless your tank is so overfull that the vent is literally full.

Pulling is far more efficient than PUSHING. Does the exhaust fan over a stove push air down onto a stove or PULL smoke, steam and odors out? Does a bathroom vent fan push air into the room to remove odors and steam or pull them out? Do Nicro vent fans push air into the boat to provide ventilation to prevent mold and mildew...or do they pull air through the boat?

So WHY would you think it should work any differently in a tank???

There's NO reason you couldn't do essentially the same thing Ansley has done if you'd just STOP trying turn a very simple system into an over-engineered mess. It's your mind that's creating the barriers...it's only letting you see the hard ways to do things...which is gonna result in system that's only gonna be constant problems...and then you're gonna blame ME!

 

[thinwater]

If you push air into a tank, the only way it should be able to get out is through the vent.

The problem with the muffin fan sucking idea is that any stuff that gets into the suction vent in rough water or an overfill is, well, "shit hits the fan" comes to mind.

You're not so dumb. I've installed many fans in industrial settings; sometimes pulling is better, sometimes pushing. There is generally no differnce in mechanical efficiency; the benifit of the whole-house fan is that it gets to pull the warmest air and that it can pull all of the windows without ducting. In a tank there is no difference.

The same reasoning applies here and in industrial fans:
* If there are flamable gasses, push. In this case there is the potencial for methane and sulfides. Should the fan spark when it has been turned off for a while because a bearing has jamed leading to an over heat, you could need a new boat. I have used muffin fans in similar applications (draft on pilot scale biofilter) and I would figure it will fail ~ 1 year. This is a very minor but non-zero risk.
* If there are corrosives or condensation, push. H2S and organic acids cause serious sewer corrosion. Could shorten fan life.
* If there are mutiple inlets, pull. This is often the case in cooling towers, so most often they pull. However, it is hard on the motors and many push. Whole house fans, kitchen fan, bathroom fans.
* If there is something you are cleaning from the air, pulling after cleaning is accepted. Vacuums, bag houses, and kitchen fans.

Men working on sewers often pull (not always) but there is a specific reason; the safety watch doesn't want the sink coming up into their face, so they direct it away by pulling.

Another reason we often pull instead of push is the perception that it is a more even process, with less turbulence. If there are only 2 pipes going into a tank, the flow around each entrance is the same whether pushing or pulling. The laws of conservation of mass and momentum promise this. Seems counter intuitive, but it can be shown.

Push.

But surely a passive system is better.

 

[Roger Long]

There's NO reason you couldn't do essentially the same thing Ansley has done ....

You haven't seen my boat. I could get a convoluted run for a second deck vent that would end up right between my two main dorade vents and a skylight hatch. A sewage vent there, even one that is supposed to be an inlet seems like a very bad idea. A passive system can't be depended on to always go the same direction. Moving the Dorade vents, which are in the perfect location, is beyond the scope of anything I would undertake and would leave huge scars.

There certainly would be a second inlet for a powered system just as for a passive one.

Here you go, 15 bucks, 12 volt, and with two check valves to prevent back flow, available at Petco:

http://www.thewallaquarium.com/aquariumstore/index.php/biorb-and-biube-air-pump.html

 

[thinwater]

Here you go, 15 bucks, 12 volt, and with two check valves to prevent back flow, available at Petco:

http://www.thewallaquarium.com/aquariumstore/index.php/biorb-and-biube-air-pump.html

Thanks!

While I don't think I'll use this on the holding tank, we do keep an aquarium on the boat(works fine on a cat if you keep the level down a bit, great for the budding marine biologist) and this looks like an improvment!

 

[Roger Long]

Thanks!

Don't thank me too quickly. A little more digging indicates that they are 12 volt 50 / 60 HZ. I suspect that they are stepping down the voltage but it's still alternating so that they can use the same simple motor design as the 110 volt units. Probably won't work with a DC supply.

There are 12 volt DC bait well bubbler for under 40 bucks but they might be too much power draw and noise.

Here is a 4 watt 12 V DC pond bubbler: (Scroll to bottom)

http://www.backwoodssolar.com/catalog/appliances_dc.htm#AERATOR-BUBBLER

It's intended for solar power so a good fit for a boat. It would run on a small solar panel, about 10" x 10". It mentions installing a resistor to reduce air flow and power consumption. Since it is rated at 1 watt more than the Groco Sweetank pump, you would probably want to do this.

 

[Peggie Hall HeadMistress]

I'm washing my hands of this.

You can do whatever you want to do with it...and good luck!

 

[Roger Long]

There's NO reason you couldn't do essentially the same thing Ansley has done ...

Oops! I owe you an apology. I misread Ansley's post. I could do the back to back vents although my vent runs might be so long that I didn't get much air movement.

I've very reluctant to have clamshells at that location though since it's my docking side and the local dock I use a lot is high and right at that height. I'll have to give this some more thought.

Passive is obviously the best way to go. Even with my small and nearly blocked vent, I haven't had any significant odor problems up here in Maine. I may just see how it goes next winter down south. If Odorlos doesn't work, I can always install an air pump in an afternoon or spend some of the plentiful time I will have running a second vent. OTOH, everything is apart now and it would be easy to run that second vent.

Thanks for your help and pushing me to do it the right way.

 

[Peggie Hall HeadMistress]

The thru-hulls don't have to be very part as long as the fittings on the tank are far enough apart for air flow through the tank, not right next to each other. So put two in and put a bloomin' exhast fan that draws only miliamps (search Nicro). Any bubbler won't work unless there's piping inside the tank at the bottom to distribute air throughout the whole tank.

 

[thinwater]

The thru-hulls don't have to be very part as long as the fittings on the tank are far enough apart for air flow through the tank, not right next to each other. So put two in and put a bloomin' exhast fan that draws only miliamps (search Nicro).

No, I don't believe Nicro makes anything suitable, and I'm not at all certain that anyone else does, within the current rating needed. The US Coast guard specifically requires that such equipment in sanitary systems be ignition protected (this is not the engine section). Granted this is in a discussion of typeII systems, but I don't think they ever considered fans in holding tanks.

One more reason for a passive aproach.

Has anyone actually used non-protected fans? Ouch.

Title 33: Navigation and Navigable Waters
PART 159—MARINE SANITATION DEVICES
Subpart C—Design, Construction, and Testing


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§ 159.129 Safety: Ignition prevention test.

(a) Components of a device that are a potential ignition source in an explosive atmosphere must pass the test in paragraph (b) or (c) of this section or meet the requirements of paragraph (d) or have a specific warning in the instruction manual required by §159.57 that the device should not be installed in an explosive atmosphere.
(b) Components protected by vapor exclusion must be placed in a chamber filled with a rich mixture of gasoline or propane in air with the pressure being varied from 0 to 2 psig once an hour for 8 hours. Vapor readings must be taken in the void being protected and must indicate a leakage less than 20 percent of the lower explosive limit of the mixture in the chamber.
(c) Components providing ignition protection by means other than vapor exclusion must be fitted with an ignition source, such as a spark plug, and a means of injecting an explosive mixture of gasoline or propane and air into the void that protects the component. Connections must be made so as to minimize any additional volume added to the protected void by the apparatus delivering the explosive mixture. The component must be placed in a chamber filled with an explosive mixture and there must be no ignition of the explosive mixture surrounding the component when the following tests are conducted:
(1) Using any overload protection that is part of the device, the potential ignition source must be operated for one half hour at 110 percent of its rated voltage, one half hour at 50 percent of its rated voltage and one half hour at 100 percent of its rated voltage with the motor or armature locked, if the potential ignition source is a motor or part of a motor's electrical circuit.
(2) With the explosive mixture in the protected void, the test installed ignition source must be activated 50 times.
(3) The tests paragraphs (c) (1) and (2) of this section must be repeated with any plugs removed.
(d) Components that are certified as being intrinsically safe in accordance with the Instrument Society of America (RP 12.2) or explosion proof in accordance with the Underwriters Laboratories STD 698 in Class I, Group D hazardous locations (46 CFR 111.80–5(a)) need not be subjected to this testing.

 

[Roger Long]

Has anyone actually used non-protected fans? Ouch.

I think you are absolutely right about this.

Personally, I would rather experience something like a propane or gasoline explosion. Then, you're dead and someone else gets to clean up. With a holding tank explosion, you would probably survive and then have to deal with the aftermath. Can you imagine?

 

[thinwater]

I think you are absolutely right about this.

Personally, I would rather experience something like a propane or gasoline explosion. Then, you're dead and someone else gets to clean up. With a holding tank explosion, you would probably survive and then have to deal with the aftermath. Can you imagine?

That's when you sell the boat on e-bay.

 

[shemandr]

we do keep an aquarium on the boat

An aquarium. Really? You multi hull guys are a trip. Good on yer.

 

[thinwater]

An aquarium. Really? You multi hull guys are a trip. Good on yer.

One of those 3-gallon mini kits. We keep the water a bit lower (2 inches) and her cabin has a molded shelf with a high fiddle that holds it perfectly, even through 8-foot breaking seas (some other things did fly off the shelves). Can't be beat if you sail with kids. My daughter collects small fish and shrimp and crabs from tide pools and minnow traps. When we get to the next harbor, she's always looking for smaller kids to show it too.

She also has 2 saltwater aquariums in her bedroom at home, all stuff she has found.