Gravity holding tank plumbing

[Ric]

I have decided to fit a gravity holding tank.

I intend to use it as little as possible to avoid smells etc. But there are occasions when it is too anti-social not to! Should I therefore connect a y-valve so that I can bypass the tank most of the time? Or are y-valves more trouble than they are worth?

Where is best to lead the tank vent? Is there a lot of persistent smell from the tank vent - i.e. is it worth leading it to the rear of the boat so it is downwind when at anchor?

I have read that it is best to fit a valve just below the tank to prevent solids sedimenting in the exit pipe - how much of a problem is this if I empty the tank fairly quickly after use?

 

[Talulah]

Most gravity tank installations do not have a Y valve to bypass it.
The Seacock is left shut when the holding tank is in use otherwise it is left open so the matter just passes straight through and out.
Common mistake is too small a vent pipe.

 

[gcrothers]

I have decided to fit a gravity holding tank.

I have read that it is best to fit a valve just below the tank to prevent solids sedimenting in the exit pipe - how much of a problem is this if I empty the tank fairly quickly after use?

I'd definitely install the extra valve at the bottom of the tank as well as the seacock.
I have recently had my gravity system block up (not pretty!!). I was unable to determine if it was the tank that had blocked or the hull outlet. An extra valve would have given me a lot more options to repair.

My air vent is aft and I haven't had problem with smells.
Based on my experience and numerous googled articles read over the last few week, I believe If you are depending solely on gravity you should use a 2" outlet. (mine is 1 1/4, so I'm now fitting a henderson mk5 pump inline).

garry

 

[garvellachs]

All in my opinion (and I have our holding tank out currently for cleaning): simple is best so if you can do without anything then do without it. I think the key to good odour control is the 38mm hose - use the best quality you can find - and lots of flushing when in use. I would feed the tank from the top so the feed hose from the heads can be flushed clean. Every city has someone who makes welded polythene tanks - needn't be expensive - the Wema access hatches are easy to use and effective.

 

[pvb]

I have decided to fit a gravity holding tank.

I intend to use it as little as possible to avoid smells etc. But there are occasions when it is too anti-social not to! Should I therefore connect a y-valve so that I can bypass the tank most of the time? Or are y-valves more trouble than they are worth?

Yes, I'd suggest plumbing it so that you can choose to either pump the toilet directly out to sea, or into the holding tank. My old boat had a gravity tank which I could bypass and pump direct to sea, but my new boat's toilet will only pump into the holding tank - I find that the tank fills up fairly rapidly.

Where is best to lead the tank vent? Is there a lot of persistent smell from the tank vent - i.e. is it worth leading it to the rear of the boat so it is downwind when at anchor?

I'd suggest you keep the vent pipe as short as possible. One purpose of the vent pipe is to allow air into the tank; with sufficient oxygen, the tank contents shouldn't get too smelly. I haven't ever noticed smells from the tank vents on my boats.

I have read that it is best to fit a valve just below the tank to prevent solids sedimenting in the exit pipe - how much of a problem is this if I empty the tank fairly quickly after use?

I don't think it is a problem at all, by the time solids have been through the pumping process, they're probably reduced to almost a slurry anyway. But again, keep the pipe as short as possible if you can.

 

[RichardS]

Having used boats with a diverter Y-valve and my current boat where the toilet always pumps directly into the top of the holding tank and there is a 2-inch outlet at the bottom with a Marelon-type plastic seacock, I would agree with those who say KISS and use the direct system. The top of the holding tank also vents to just above water level around the centre of the boat through a 1 inch pipe and smells are not an issue unless, of course, the tank is full and starts overflowing through the vent. The first "tinkling" sound when you pump the toilet is a sure sign that it's time to leave the bay!

I also have the advantage that the deck pump-out is directly above the holding tank outlet so it is easy to rod through the system with a broom handle if there is a blockage although has only ever been caused by people putting the wrong kind of paper down the toilet.

Richard

 

[prv]

Yes, I'd suggest plumbing it so that you can choose to either pump the toilet directly out to sea, or into the holding tank. My old boat had a gravity tank which I could bypass and pump direct to sea, but my new boat's toilet will only pump into the holding tank - I find that the tank fills up fairly rapidly.

If you don't need to hold the waste, surely you just leave the exit open and then it doesn't fill up?

Agree that a non-gravity tank should have a bypass, but in my opinion a gravity tank should not. It's one less thing to leak, and one less discontinuity in the pipe to snag a bit of hair/cloth/wet-wipe and start a blockage. A good gravity-tank install should have one continuous length of smooth-wall pipe from toilet outlet to tank - that's fewer joints than a non-tank installation because you don't need the anti-siphon, and should be almost impossible to block.

I'd suggest you keep the vent pipe as short as possible. One purpose of the vent pipe is to allow air into the tank; with sufficient oxygen, the tank contents shouldn't get too smelly. I haven't ever noticed smells from the tank vents on my boats.

Agree.

I don't think it is a problem at all, by the time solids have been through the pumping process, they're probably reduced to almost a slurry anyway.

They are, but I believe it's the paper that's the problem. Although it enters the tank in slurry form, if left it tends to settle to the bottom and clump together into a sort of papier-maché. The closed-off exit pipe provides an ideal sump for this to happen in, whereas if it just swills around in the bottom of the tank it tends to stay mixed up in enough water to stay mobile.

Probably not a problem if you empty the tank fairly frequently. If you were in America and using a marina pump-out a lot of the time, your exit pipe would have a chance to get seriously impacted

Pete

 

[Tranona]

Agree to go direct through tank with no Y valve. Useful to have a valve at the outlet to avoid having sludge building up at the outlet seacock. However, it means your tank will be mounted very high to allow space for the valve and fittings. Bottom entry from the loo with a stack pipe inside reduces the length of full hose. An inspection hatch immediately above the outlet aids rodding if you do get a blockage. There should not be smells from the vent. Large bore (at least 21mm) by the shortest route to just under the toerail.

 

[neil1967]

Surely the advantage of having the bypass, even in a gravity feed tank, is that liquid waste could be discharged directly into the sea, if appropriate, and solids could be diverted to the holding tank, thus increasing the time between emptying the holding tank, whether through discharge overboard or in a marina pumpout. That is what I am planning.....

 

[pvb]

Surely the advantage of having the bypass, even in a gravity feed tank, is that liquid waste could be discharged directly into the sea, if appropriate, and solids could be diverted to the holding tank, thus increasing the time between emptying the holding tank, whether through discharge overboard or in a marina pumpout. That is what I am planning.....

Exactly.

 

[pvb]

If you don't need to hold the waste, surely you just leave the exit open and then it doesn't fill up?

That only works until there comes a time when you want the tank to retain some waste. Once it's in the tank, you can't open the exit without it all going out to the sea. If you want to be selective about whether to pump the toilet to the tank (ie solids) or to the sea (ie liquids only), you have to have a bypass arrangement.

 

[RichardS]

Surely the advantage of having the bypass, even in a gravity feed tank, is that liquid waste could be discharged directly into the sea, if appropriate, and solids could be diverted to the holding tank, thus increasing the time between emptying the holding tank, whether through discharge overboard or in a marina pumpout. That is what I am planning.....

It's not very often that I read something completely new on these forums but that is it. :encouragement:

I like the concept of flipping a valve depending on whether you intend to do a Number 1 or a Number 2 but, in practice, I suspect it's not going to work. The concept of being in a bay and diverting the No. 1's directly into the sea along with the toilet paper (you do have women aboard sometimes presumably?) whilst people are swimming around is not going to go down too well.

I really like the concept though.

Richard

 

[Talulah]

If a bypass pipe then presumably you need two Y diverters?
1. To divert the contents to the tank or bypass.
2. For the skin fitting/seacock to accept the contents from the holding tank or from the bypass pipe. Either that or a second hull fitting.
(If not a divertor than certainly a Y junction.)

 

[prv]

That only works until there comes a time when you want the tank to retain some waste. Once it's in the tank, you can't open the exit without it all going out to the sea. If you want to be selective about whether to pump the toilet to the tank (ie solids) or to the sea (ie liquids only), you have to have a bypass arrangement.

True. I suppose I haven't come across that pattern of use - when we used to sail in the Med, we'd retain everything in harbour and in lunch-stop bays, then empty it all out en-route to the next.

I think if I was fitting a holding tank I'd keep it simple to start with, and only add a pee-diverter if I regularly found the tank filling up without a chance to empty it.

Pete

 

[prv]

If a bypass pipe then presumably you need two Y diverters?
1. To divert the contents to the tank or bypass.
2. For the skin fitting/seacock to accept the contents from the holding tank or from the bypass pipe. Either that or a second hull fitting.
(If not a divertor than certainly a Y junction.)

I think number 2 (ho ho) should be an open Y junction for simplicity, provided of course that the tank exit has its own valve. With a diverter you risk someone putting the top valve to "overboard" and leaving the lower one on "empty tank", then pumping the toilet against a closed valve and bursting something.

Pete

 

[neil1967]

I think number 2 (ho ho) should be an open Y junction for simplicity, provided of course that the tank exit has its own valve. With a diverter you risk someone putting the top valve to "overboard" and leaving the lower one on "empty tank", then pumping the toilet against a closed valve and bursting something.

Pete

Yes, we are planning on one diverter valve on the outlet from the toilet, and a Y connector immediately before the seacock on the skin fitting. To answer RichardS observation, although some people don't like the idea, we have a paper bin in the heads, so we don't put paper into the holding tank or overboard. We've never had a blocked holding tank on our yacht or any charter yachts we have used.

 

[blackbeard]

Surely the advantage of having the bypass, even in a gravity feed tank, is that liquid waste could be discharged directly into the sea, if appropriate, and solids could be diverted to the holding tank, thus increasing the time between emptying the holding tank, whether through discharge overboard or in a marina pumpout. That is what I am planning.....

I haven't tried this but I would worry that a build up of solid material, while the liquid goes through the bypass, will eventually lead to a build up of solid-ish material which will then produce a blockage.

Incidentally - on the topic of blockages - luxury kind-to-bum toilet paper has been implicated. You can get expensive specialised paper which dissolves easily in water and thus in theory doesn't cause a blockage; cheap and cheerful supermarket bargain cheapest will do the same at much less cost. Experiment by dropping a bit into a sink or bucket of water, it should dissolve into a mush. Kitchen roll is a guaranteed blockage (!) and I expect a hand wipe would do the same.

My tank holds 25 litres (all above water line of course) and, even being a bit careful with flushing water, equates to about 2 person days. OK for a short stop near a swimming beach or for midnight emergencies in marinas, but I sometimes wish it were a bit bigger. Apart from that, it's a simple installation (no bypass, it's effectively just a tank inserted in the pipe, plus air vent and access hatch of course) it works perfectly and has never given trouble (except with kitchen roll). If the tank is translucent plastic and kept reasonably clean, you can (with the aid of a torch) see exactly how full it is.

 

[Momac]

A holding tank is a relatively new thing for me.
Mine has no diverter valve so all toilet waste goes to the tank. When the tank is full it can be pumped out to sea or to the marina pump out to via a deck fitting.
Our policy is pee only and paper gets bagged -( Wilko nappy bags 35p for 300) - but accept that doesn't work for everyone. It works for us as we are almost invariably moored near shore facilities.
.
The outlet to sea, on my tank , is via a Jabsco macerator pump. I can imagine accumulated solids might not necessarily drain out too well under gravity . My pump had to be replaced as the survey found it to be corroded . I suspect inactivity was in part to blame.

.

 

[BabaYaga]

I have decided to fit a gravity holding tank.

I intend to use it as little as possible to avoid smells etc. But there are occasions when it is too anti-social not to! Should I therefore connect a y-valve so that I can bypass the tank most of the time? Or are y-valves more trouble than they are worth?

Where is best to lead the tank vent? Is there a lot of persistent smell from the tank vent - i.e. is it worth leading it to the rear of the boat so it is downwind when at anchor?

I have read that it is best to fit a valve just below the tank to prevent solids sedimenting in the exit pipe - how much of a problem is this if I empty the tank fairly quickly after use?

For reasons already given arrange so that you can bypass the tank. An Y-valve can be replaced by a T-piece and two ordinary valves (one of which is the seacock) if there is room, might be less expensive.

Never had any problems with smells from the vent, mine is dia 19mm. Keep the venthose short.

My tank is v-shaped and the 38mm exit is from the lowest spot. From there circa 200mm of hose before the ball valve - so in theory a perfect place for sedimentation of solids. In 35 years of use this has never happened.
So I think you will most likely be fine without a valve at the tank outlet. However, if you later want to arrange for pump out facility, this is the place for a T-piece and a hose up to the deck fitting. In that case I think a valve is a good thing, either at the bottom of the tank or between the T-piece and suction hose.

 

[macd]

I haven't tried this but I would worry that a build up of solid material, while the liquid goes through the bypass, will eventually lead to a build up of solid-ish material which will then produce a blockage.

Incidentally - on the topic of blockages - luxury kind-to-bum toilet paper has been implicated. You can get expensive specialised paper which dissolves easily in water and thus in theory doesn't cause a blockage; cheap and cheerful supermarket bargain cheapest will do the same at much less cost. Experiment by dropping a bit into a sink or bucket of water, it should dissolve into a mush. Kitchen roll is a guaranteed blockage (!) and I expect a hand wipe would do the same.

An alternative, of course, is "Nothing down here that hasn't passed through you first." Works for us, although others should do what works for them. It used to be a common thing to find disgusting strands of loo paper hanging from anchor rodes, although I can't say I've noticed this for some years.