Rich, Bill - air flow?

[Roger Long]

I should be able to answer this question but I haven't done any ventilation design or other fluid flow stuff in decades when I stopped doing vessel systems design to focus on the big picture stuff. As they say, I had people for that.

Anyway if Bill, Rich, or any other engineer types would like to take a swing at this question I would appreciate it.

I'm satisfied from discussions with Peggie and someone who has such a system installed in their boat that a vent set up like this will keep a 20 - 30 gallon holding tank sufficiently aerobic to prevent objectionable odors.

The vent hoses are 1" dia. by 5 feet long with clamshells facing foreward on one and aft on the other and the through hulls just below the rail in the hull side. The openings are standard through hulls with one elbow inside. Clamshell opening area would be about the same as the vent hose cross section.

Here's my question: What would the probable average flow rate through the system be in typical cruising conditions? I know it will be a wide range but even the number for 8 knots of wind while anchored with one clamshell facing forward and one aft would give me a handle on it.

I could have done this 30 years ago, figuring the boundary losses of the hull surface etc. but
that was a long time ago.

Rich may know a more direct approach to the problem, how many cfm of new air does it take to keep 10 gallons of holding tank bacteria happy at 80 degrees? In a tank under 30 gallons, I'm assuming that vessel motion will keep things sufficiently mixed up.

 

[catsailor]

I should be able to answer this question but I haven't done any ventilation design or other fluid flow stuff in decades when I stopped doing vessel systems design to focus on the big picture stuff. As they say, I had people for that.

Anyway if Bill, Rich, or any other engineer types would like to take a swing at this question I would appreciate it.

I'm satisfied from discussions with Peggie and someone who has such a system installed in their boat that a vent set up like this will keep a 20 - 30 gallon holding tank sufficiently aerobic to prevent objectionable odors.

The vent hoses are 1" dia. by 5 feet long with clamshells facing foreward on one and aft on the other and the through hulls just below the rail in the hull side. The openings are standard through hulls with one elbow inside. Clamshell opening area would be about the same as the vent hose cross section.

Here's my question: What would the probable average flow rate through the system be in typical cruising conditions? I know it will be a wide range but even the number for 8 knots of wind while anchored with one clamshell facing forward and one aft would give me a handle on it.

I could have done this 30 years ago, figuring the boundary losses of the hull surface etc. but
that was a long time ago.

Rich may know a more direct approach to the problem, how many cfm of new air does it take to keep 10 gallons of holding tank bacteria happy at 80 degrees? In a tank under 30 gallons, I'm assuming that vessel motion will keep things sufficiently mixed up.

Air flow I'm not so good on, but I do biological waste treatment design.

Many variables, but assuming there are only a few pounds of actual dry-basis solids in the tank and it biodegrades over a period of perhaps 10 days, we are talking about a maximum of 10 ft3/day. That is assuming perfect O2 transfer, which isn't even close to true, but we don't need to be perfectly aerobic either. The actual requirement is probably much less.

This isn't much, which is why Peggy tells us that a well designed single pipe system will do it. I've got to agree with her that any rational 2-pipe system with well spaced tank entries should be very adequate.

 

[RichH]

Although there are specific mass transfer calculations ... crap tanks are not 'controlled' so any 'air padding' of the holding tank is only a 'good guess' to keep it aerobic.

Sparging (air injection) into the bottom of the tank is by far the best method as it keeps a constant rate to affect proper mass transfer of oxygen - just a few cc's per minute AND it 'sweeps' out any aerobic blanketing gases. A simple small aquarium piston pump delivering to a porous stone etc. at the bottom of the tank will be magnitudes better.
Also the use of fresh flush water, instead of seawater will also vastly affect the mass transfer of oxygen .... the 'salts' in seawater greatly depress oxygen saturation.

Oxygen saturation decreases as the tank contents increase in temperature - got insulation for summertime use? ... a good place to pour discarded 'melted ice water' is into the holding tank. If the holding tank is adjacent to the engine ... then definitely a need to insulate the tank!!!!

The heart of the problem is that H2S and other higher density gases will 'blanket' directly over the fluid, thus preventing/retarding the mass transfer of air into the 'soup'. So unless you have a positive displacement (pumped) air flow, the amount of air needed (a few cc's per minute for the typical holding tank) will be a 'shot in the dark' with a 'convective' or naturally aspirated tank.

Of course 'angling the cowls' (one forward, one aft facing) 'may' accommodate enough flow, but you really need to have a 'clean' piping system with minimal resistance .... or prepare to do some ACTUAL (acfm) calcs to correct for any pressure drops.
Just a few scant cc's (in acfm, not scfm) per minute should be enough for a good 'sweep volume'.

A very small aquarium air pump (running on 12vdc) is probably the best solution.
A small 12vdc timing motor (a few rpm) coupled to a small plastic piston pump (or home made 'peristaltic' pump using soft 'silastic' tubing) would be the cheapest.
ATC (if still in business) probably has such timing motors in stock for a few $$.
A 10 ml plastic syringe with 2 intravenous tube check-valves could probably suffice as your 'piston pump' - got any contacts in a hospital?

When Im not on my boat, I simply open/remove the 'waste' deck fitting on my 'short hosed' holding tank if I dont have the opportunity to 'pump'; plus, KO and fresh water flushes seem to minimize the 'putrenes' venting from the tank.

 

[Roger Long]

we are talking about a maximum of 10 ft3/day.

So, .007 cfm, is that about right?

 

[Roger Long]

Just a few scant cc's (in acfm, not scfm) per minute should be enough for a good 'sweep volume'.

That's the way it's beginning to look to me. I don't have flow specs on the Groco Sweet Tank system but, based on the three watts and the fact that it's for larger tanks and one size has to fit a lot of situations, I'm suspecting that it's over sized. Bigger is better when it comes to poop control and the stirring of stuff at the bottom of the tank is a noble objective.

What I'm trying to get a handle on is the minimum that you can get away with, just as you are suggesting, for a situation where a two pipe, minimum restriction, vent run isn't practical.

Thanks for the pump ideas. Worth looking into.

 

[RichH]

d. Bigger is better when it comes to poop control and the stirring of stuff at the bottom of the tank is a noble objective.

No no no .... aggressively sparging into most fermentation situations and you risk:
1. "foam-out",
2. reduced capacity of the tankage ... a lot of air becomes mixed/aereated within the solution.
Smaller is better as you want to keep the contents 'quiet', not induce rapid fermentation; and the task is only to gently 'sweep out' / diffuse the anaerobic heavier density gases and 'volatile putrenes'.

If you change the fermentation kinetics by high volume sparging and the rate of fermentation accelerates, then turn both the cowls rearward and enjoy the 'added thrust'.

 

[Roger Long]

No no no ....

I was kidding. But, do you think I'm on the right track that a 3 watt air pump is considerable overkill, even it it's not producing a foam volcano?

3 watts is too much for my minimalist approach to electrical production and consumption. Zero would be ideal but I really, really, don't want to have cowls on the side of my boat at the necessary location for some practical reasons. If I can determine that the electrical requirements for pumping air to an 18 gallon holding tank can be gotten down to about the equivalent of the power generated by one of those Nicro solar vents with battery and could find a pump of about the same power draw, it might be a good alternative.

Introducing the air under the liquid is clearly best but increases the pressure head considerably. It looks from the success of passive systems that just keeping air flowing through the tank will work. It might not on land but boat motion helps with stratification.

I'm not looking for optimum or best here but trying to get a handle on the minimum that I can get away with.

 

[Peggie Hall HeadMistress]

Roger this is the only system that can possibly work for you

bumper dumper

All you need to install it is a trailer hitch on the transom.

 

[RichH]

You already know what is going to happen to 'cowls' when the boat is over on its ear in a slop-sea.
As regards static head... I reemphasize the need for 'positive displacement' (piston or peristaltic, etc.) pump, then the need for overcoming the static head to purge/clear the influent line of liquid is minimal or moot.
Lastly you dont need to run such a pump 'all the time' to keep a steady state mass transfer and sweep volume going; for power savings I'd opt for an intermittent on/off via a 555 type timing control chip etc. maybe on 5-10% of the time.

And I hate to say, the timing adjustment could be trimmed by your 'olfactory senses' and depending on what you ate the previous day: Eggs & Asparagus at dinner = constant run, breakfast of cereal and toast = forget it, Dinner at a Siamese Restaurant = will probably foam out all by itself, etc.

The 'bumper dumper' isnt such a bad idea ... we keep a stock of "Wag Bags". Great when the damn holding tank is full. The hydroxymethylcellulose (a starch) in the bag soaks up all the 'fluids'; best of all 'no stink'.
http://www.amazon.com/Phillips-12-pack-WAG™-Bag-Kit/dp/B0018ZLHLC

 

[All U Get]

bumper dumper

All you need to install it is a trailer hitch on the transom.

I like that, nice and simple. While traveling along the shoreline, my wife and I were making note of different navigational aids. While passing a bouy with a whistle my wife suggested we could fashion something quieter that would vent out the cabin. Every wave would pump in/out a volume of air, 0 amps. My list for the project included a bellows, bowling ball, dorade, duck tape, and a parrot. I also rented Apollo 13 for inspiration. I'm sure you could come up with something as simple. :stirthepot:
All U Get

 

[Bill Roosa]

Hey, what the heck is going on here. Someone is designing a poo trap and I didn't get a personal invite??? I'm not feeling the, oh wait, my name is on the title.
A great idea, how to pump air into liquid without using electricity.
AllUGet has the idea, you know the contents are moving around due to boat motion so use the motion to stir the tank contents and move the air. a vertical baffle in the center starting from the top and going to the bottom with largish holes in the bottom half will cause a piston action in the air space once the holes are covered. The flow around the baffles will agitate the poo and keep the O2 levels up.
That still leaves the long periods at anchor with no significant boat motion solution to find. I'm thinking crew drills to "rock the boat" Alternately you could install the baffle so it will move with an external handle and mount it to a hinge (need a locking means so it can be secured underway). Then just stir the contents occasionally while at anchor. A 20-30 gal tank is not going to need much in the way of maintenance.
Are we considering doing any volume reduction via evaporation???? I'm thinking using the same pumping concept to move a working fluid (antifreeze) through a tube that catches the sun and gets warm then transfers that warmth to the poo. Given all the stiring of the contents I'd not worry about O2 saturation dropping due to increased temp of the effluent. Course you can only take this so far before you are building a composting toilet and doing the "once a year" clean out.
They have composting toilets for boats now BTW.

 

[Roger Long]

my wife suggested we could fashion something quieter that would vent ....

Smart wife. Keep her.

I brought up this idea a couple of years ago when I first learned of the importance of air flow in a holding tank. It was poo pooed because it wouldn't provide the bottom stirring or air volume of the Groco system.

I see this a lot on production items. One size has to fit a lot of situations. Products mostly compete in the market place on specs (look at the games that are played with things like stereo speaker power ratings). There are a lot of pressures to make a product the best or at least look like the best. Since everything has a price and everything on a boat is a compromise, just good enough is often better than "best".

It's clear from the success of passive systems, and substantiated above, that very little air volume is needed and, at least in a boat, it can be introduced and exit from the top of the tank. Such a system clearly would loose out in a side by side sniff comparison with a Sweet Tank set up but, if no one is saying "Yuck" and shifting their seating position, it's working well enough.

Anyway, for a 30'ish size sailboat tank, it seems that we are looking for about a 2" cube of air per minute at most. Not trying to bubble it up through the liquid means very low pressure head. A fairly compact bellows with a weight on it would do this:

Very simple and inexpensive to make out of molded plastic in China.

 

[Roger Long]

Here is a 10 buck positive displacement pump from the fish store.

Way too loud but pumps .032 CFM (by test), about 4.5 x what appears to be required, with a single 1.5 volt battery. Next question: how much of a solar panel would it take to get it to run at quarter speed?

 

[RichH]

Exactly what Im referring.

Whats the rating of the motor? Could probably be rpm controlled by a simple variable resistor.

Power requirements ... all depends on the frictional characteristics, etc. of the pump and the starting torque values of the motor. Ultimately you'd replace with a small motor from a Hobby Shop ... an N-scale motor, etc. (easy to change the 'brushes').

Id still consider a cheapo 555-type chip timer to make it automatically 'intermittent'.

 

[Roger Long]

Could probably be rpm controlled by a simple variable resistor.

I had a 10 ohm resistor kicking around. That slows it down just a bit more than seems right but it stalls at dead center and sometimes doesn't start. It's quiet enough when running at that speed that it might work with some sound shielding.

Anyway, it's sufficient for playing with. A lot of the noise is from the connecting rod pins having a lot of slop and clattering. Higher quality would quiet it down a lot. Replacing the motor with a rack and pinion driven by a swinging weight is a fun idea. It pumps equally well running in either direction because the valving is set up like a diaphragm pump.

It won't pump very slowly because of air leakage around the piston. It runs at about half speed with the outlet blocked.

 

[catsailor]

I agree with Rich's volume reasoning for a bubbler; very small is better. I was considering air flow across the top, allowing for all of the poor mixing issues Rich brought up.

I still dislike adding another mechanical electric-consuming bit to a small boat, though.

Another thing bothers me. Anaerobic and anoxic systems produce very little solids. That is why even a whole home septic systems can go years without cleaning and why sewage plants use anaerobic digestors to compact the the sludge the aerobic systems produce. There is some benifit to adding just enough air to allow aerobic organism enough air to oxidize the gases the anaerobic organism produce, without encouraging the aerobic organisms to start producing settleable biomass (aerobic biomass is more prone to settling than aerobic biomass.

Simply put, I wonder if adding too much air could increase solids settling problems. I don't know about this case, but I know this is true in sewage plants.

Less is better. Also consider just a few hours per day. Treatment units are not always aerated continuously.

 

[Roger Long]

Report from Dr. Frankenstein's Lab

The solar cell from this 10 buck Home Depot fence light will run the air pump on a partly cloudy Maine mid afternoon. It doesn't quite have enough umph to start reliably after a cloud goes across the sun however. Two cells in parallel should run it fine.

http://www.rogerlongboats.com/videos/AirPump.wmv

The battery pack and photo cell are cut out of the circuit. The logic is wrong because I'd rather have it running during the day than at night when it's quiet. Charging the batteries and running the pump might be possible with two solar cells and the right circuit board but it sounds from Catsailers' post that running it only part of the time is better anyway.

I still dislike adding another mechanical electric-consuming bit to a small boat, though.

I agree 100% which is why I'm playing with the solar angle. I doubt I'll actually put this on my boat. Things were fine with a single small vent even partially blocked. It could be different down in warmer waters though. It's something to play with while there is too much melting snow around the boat to go back to work adding reinforcement to some of the tabbing (ugh!).

It will be easy to add a connection for this with a Uniseal. I'll get it figured out enough that I can add it next winter easily if it proves necessary.

As I start to learn how little air is needed, I'm beginning to be pretty sure that a boat which is moving frequently and not spending long periods sitting at a quiet marina dock will pump enough air in and out of a single 3/4" vent with motion to keep the bugs happy.

 

[catsailor]

Interesting....

I'm going to keep following this one. In fact, I've got a solar fan over my tank that is not needed and doesn't work. If it is only the fan and not the cells....

 

[RichH]

Interesting....

I'm going to keep following this one. In fact, I've got a solar fan over my tank that is not needed and doesn't work. If it is only the fan and not the cells....

I didnt know they made explosion proof solar fans !!!!! :-0

Consider the offgassing from raw sewage: sulfur, H2S, SOx, methane, mercaptans, putrenes. I would expect a Solar fan to last about 15 minutes in such an atmosphere; and when the methane (if at explosive concentrations) contacts the open frame motor and if the brushes are arcing ..... should just about hit orbital velocity after 'lift off'. As for the holding tank you may not have to ventilate the 'remnants' after that. »8-O

 

[catsailor]

I didnt know they made explosion proof solar fans !!!!! :-0

Consider the offgassing from raw sewage: sulfur, H2S, SOx, methane, mercaptans, putrenes. I would expect a Solar fan to last about 15 minutes in such an atmosphere; and when the methane (if at explosive concentrations) contacts the open frame motor and if the brushes are arcing ..... should just about hit orbital velocity after 'lift off'. As for the holding tank you may not have to ventilate the 'remnants' after that. »8-O

I didn't explain myself. What I'm considering is that the panels are mounted already mounted on the deck and all I would have to do is connect the small air pump Roger is playing with, a 15-minute job. I was the one that pointed out the weaknesses of solar fans in this application and that they do not meet CG requirements for this application. Roger and I already joked about the probable resale value of a boat that had suffered a holding tank explosion.

Sorry I scared you.