Peggy, The accepted way to connect a below the waterline sanitary system is to have vented loops in 1) the toilet intake between the toilet pump and toilet bowl; 2) between the holding tank waste discharge pump and the thru-hull (prevents leakage back through the waste discharge pump and into the holding tank - the holding tank would fill to the waterline); and 3) between the toilet discharge and thru-hull (when an option for discharging the toilet directly overboard is included). However, it seem to me that both the vented loops in cases (2) and (3) above could be eliminated (there are several advantages to eliminatinging vented loops) if the following is done: (1) Connect the toilet discharge directly to the holding tank. It's not as nice as pumping directly overboard when at sea, as one now must deal with the holding tank in all cases. However, the holding tank can still be pumped directly overboard as required and as legally allowed. If one intends to abide by the law, this minor inconvience should not really matter (when at sea, one can pump waste overboard in two steps instead of one, and when in restricted areas, the waste must go in the holding tank anyway). (2) Open the waste discharge thru-hull only when pumping waste directly overboard (as allowed by law) from the holding tank. When not pumping waste overboard from the holding tank, keep the discharge thru-hull closed, thus eliminating the leak back through the waste pump and into the holding tank. Of course, the vented loop at the toilet intake is still required to prevent flooding the toilet bowl (and the boat!). And the holding tank must include a line that allows pumping out at the deck (for restricted waters), which is incorporated into the system by placing a Y valve at the holding tank discharge that splits between the deck pump out line and the overboard discharge line. Connecting the toilet directly to the holding tank, thus eliminating the vented loop at the toilet discharge, means that the volume of waste water pumped into the holding tank with each flush is much less when sailing in restricted waters (the line to the holding tank is much shorter in length, thus less volume of waste water goes into the tank - this could be a waste water volume reduction of as much as 200 to 300%!). Therefore, instead of needing to pumnp out the holding tank every couple of days, you may be able to get a week or more out of it, depending on how you use your facility. If the holding tank is properly vented, and the "good bugs" are added to your waste in the holding tank, this should not cause a problem. Eliminating the vented loop at the holding tank's direct overboard discharge reduces the length of this line, and makes it less likely that waste will remain in the line after the holding tank is pumped dry. With a long (vented) discharge line, the only way to make sure that all the waste has been flushed out of the holding tank discharge line is to first pump the tank dry, then add additional water from the toilet, and then pump the holding tank dry again - right?
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I think Peggy would say --
First, I agree that you don’t need a vented loop between the holding tank and the overboard discharge. You have a seacock at the through hull that solves the problem of backflow when not pumping. Moreover, I like the simplicity of routing the toilet to the holding tank without a Y valve option to go directly overboard. However, if you’re a real cruiser and spend much time away from port, the convenience of leaving the toilet set to go overboard would be better than having to use power to pump the tank. What if you had no power? You would be up that creek without a pump. You still need the vented loop between the holding tank and the toilet to prevent backflow when healed on certain tacks. The joker valve can’t be trusted to hold back what could be a very nasty flood. I’m on a Texas lake and as of this year, our systems have to be modified to make it impossible to pump overboard. No more simply removing the handle from the seacock, now the plumbing has to be re-routed and disconnected from the through hulls. Therefore, I and a lot of other Texans have been pulling back the cushions and doing a lot of poking around in places not often seen. I now have two unused through hulls with seacocks for which I’m pondering a purpose. S/V Anodyne 36' Catalina
First, I agree that you don’t need a vented loop between the holding tank and the overboard discharge. You have a seacock at the through hull that solves the problem of backflow when not pumping. Moreover, I like the simplicity of routing the toilet to the holding tank without a Y valve option to go directly overboard. However, if you’re a real cruiser and spend much time away from port, the convenience of leaving the toilet set to go overboard would be better than having to use power to pump the tank. What if you had no power? You would be up that creek without a pump. You still need the vented loop between the holding tank and the toilet to prevent backflow when healed on certain tacks. The joker valve can’t be trusted to hold back what could be a very nasty flood. I’m on a Texas lake and as of this year, our systems have to be modified to make it impossible to pump overboard. No more simply removing the handle from the seacock, now the plumbing has to be re-routed and disconnected from the through hulls. Therefore, I and a lot of other Texans have been pulling back the cushions and doing a lot of poking around in places not often seen. I now have two unused through hulls with seacocks for which I’m pondering a purpose. S/V Anodyne 36' Catalina
Thru-hulls - Dennis
"I now have two unused through hulls with seacocks for which I’m pondering a purpose." Air conditioning?
"I now have two unused through hulls with seacocks for which I’m pondering a purpose." Air conditioning?
Weeellll...no 
If the tank or any part of it is below the waterline, there's no way to eliminate the vented loop in the tank "dump" line between the pump and the thru-hull...CG safety standards require it. And, flushing only into the tank may or may not eliminate the need for a loop, though not necessarily a vented loop, in the head discharge line...it depends on whether the inlet fitting on the tank is outboard or inboard toward the centerline of the boat. If it's outboard, even a partially full tank can run back toward the toilet. There's also a fallacy in your premise that flushing into the tank uses flush water than flushing overboard. The length of the head discharge has nothing to do with the amount of flush water, cuz no matter how long or short that line is, you're only gonna pump enough times to move the bowl contents all the way to the tank. Using the "dry" mode except to wet the bowl and rinse out the hose behind the waste will double or even triple the number of flushes a tank will hold. Any toilet that's working anywhere near original specs can move bowl contents at least 6' in the "dry" mode...and with a loop in the line immediately after the toilet, you should also get a lot of help from gravity between the top of the loop and the tank. So except for eliminating a thru-hull, you really don't gain anything by putting everything into the tank, but you do increase the risk that you won't be able use the toilet at all if the tank vent becomes blocked or the overboard discharge macerator or pump fails. Nice try though...shows you're thinking.
If the tank or any part of it is below the waterline, there's no way to eliminate the vented loop in the tank "dump" line between the pump and the thru-hull...CG safety standards require it. And, flushing only into the tank may or may not eliminate the need for a loop, though not necessarily a vented loop, in the head discharge line...it depends on whether the inlet fitting on the tank is outboard or inboard toward the centerline of the boat. If it's outboard, even a partially full tank can run back toward the toilet. There's also a fallacy in your premise that flushing into the tank uses flush water than flushing overboard. The length of the head discharge has nothing to do with the amount of flush water, cuz no matter how long or short that line is, you're only gonna pump enough times to move the bowl contents all the way to the tank. Using the "dry" mode except to wet the bowl and rinse out the hose behind the waste will double or even triple the number of flushes a tank will hold. Any toilet that's working anywhere near original specs can move bowl contents at least 6' in the "dry" mode...and with a loop in the line immediately after the toilet, you should also get a lot of help from gravity between the top of the loop and the tank. So except for eliminating a thru-hull, you really don't gain anything by putting everything into the tank, but you do increase the risk that you won't be able use the toilet at all if the tank vent becomes blocked or the overboard discharge macerator or pump fails. Nice try though...shows you're thinking.
Then if ....?
Dear Peggie: I have read many of your responses to varied questions. I have always wondered about your opinion of “dry” flushing passed a vented loop toward the holding tank. To me, that means you are pumping air instead of water. Am I right? When I imagine a transparent tube the same dimensions as a sanitary hose, I see bubbles bypassing the slug of “stuff” that I would like to see transferred to the holding tank. I imagine the volume of the hose from the toilet to the top of the loop. If I pump a liquid volume at least as great, I will probably cause the stuff to clear the loop. If not, some of the stuff will still be below the crest of the loop and further “dry” pumping will only send puffs of air bubbling up through this sludge. In addition, the weight of this sludge will be pressing against the joker valve. The joker valve is not designed to stand forever against this weight day after day while we are away earning the where-with-all to clean up this mess. OK, this isn’t really that much volume if the system is well designed. (if). Let me “fess up”, I know a little about using a gas to lift a liquid. But what I know about marine sanitation devices could easily be pumped overboard even in a restricted area. But…. When I’m finished and I’m pumping, I have a hard time imagining most of the stuff clearing the loop and actually going where it should. Eventually, with continued use, it will get there. It’s the volume between the toilet and the top of the loop. Yes, it will move on eventually. But when? Can pumping air really displace all the liquids and other stuff past the top of the loop? Stupidly yours, Dennis Thomas
Dear Peggie: I have read many of your responses to varied questions. I have always wondered about your opinion of “dry” flushing passed a vented loop toward the holding tank. To me, that means you are pumping air instead of water. Am I right? When I imagine a transparent tube the same dimensions as a sanitary hose, I see bubbles bypassing the slug of “stuff” that I would like to see transferred to the holding tank. I imagine the volume of the hose from the toilet to the top of the loop. If I pump a liquid volume at least as great, I will probably cause the stuff to clear the loop. If not, some of the stuff will still be below the crest of the loop and further “dry” pumping will only send puffs of air bubbling up through this sludge. In addition, the weight of this sludge will be pressing against the joker valve. The joker valve is not designed to stand forever against this weight day after day while we are away earning the where-with-all to clean up this mess. OK, this isn’t really that much volume if the system is well designed. (if). Let me “fess up”, I know a little about using a gas to lift a liquid. But what I know about marine sanitation devices could easily be pumped overboard even in a restricted area. But…. When I’m finished and I’m pumping, I have a hard time imagining most of the stuff clearing the loop and actually going where it should. Eventually, with continued use, it will get there. It’s the volume between the toilet and the top of the loop. Yes, it will move on eventually. But when? Can pumping air really displace all the liquids and other stuff past the top of the loop? Stupidly yours, Dennis Thomas
Air will push water...
...as long as the hose remains full--which is about 6'. And it doesn't require pumping any more times than pushing it with water. Maybe someone who's an expert in fluid dynamics will jump in and explain why.
...as long as the hose remains full--which is about 6'. And it doesn't require pumping any more times than pushing it with water. Maybe someone who's an expert in fluid dynamics will jump in and explain why.
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