Gtavity drained holding tanks proned to block?

[BabaYaga]

In this thread from the other day

http://www.ybw.com/forums/showthread.php?482576-Holding-tank-sea-cock-failure&p=6128396#post6128396

and in many others the problem of holding tank blockages are discussed. It appears to be a fairly common thing. Many forumites witness of having to "rod out" from below or from above, through a suitably positioned deck fitting..
I get the impression, but could be wrong, that this problem is associated only or mostly with gravity drained tanks.
On my boat the holding tank is under the fore peak bunks, with the tank outlet well below the water line. For discharge to the sea, a diaphragm pump is used (another option is emptying at a pump out station, through a deck fitting). In 35 years of use I have never had a holding tank blockage.
Could it be that gravity drained tanks are more prone to block? If so, what could the mechanism be? My own thinking is that a length of vertical exit pipe would more easily get clogged by dense matter compared to a length of more or less horizontal exit pipe typical of a tank at or below water level.

 

[Tranona]

Yes it is a potential drawback, but once you are aware of it you take precautions to prevent it, the most obvious are not to pump excess paper in, and not leave it for any length of time with matter in it. Advantage is extreme simplicity compared with a pumped system.

 

[pvb]

I don't think it makes a big difference; both systems involve pipes, and any blockages are inevitably in pipes. We've always used the "Greek toilet" system on our boats, and avoid getting any toilet paper in the holding tank. No problems so far in 25 years.

 

[vyv_cox]

We have had a gravity holding tank for something like 10 years now and a small wrap-around pumped one for five years before that. There has never been a blockage in either. The action of the joker valve breaks up any solids passing into the tank, whether poo or paper, and unless these are allowed to settle for an appreciable time it is difficult to imagine how they could block a large outlet.

We clean out the tank at the end of most seasons, mostly using a hose pipe with a small nozzle. Over a season we find that a thick layer of carbonate deposit develops on the floor and walls of the tank. By banging on the outside of the tank and using a pressure hose, large flakes of deposit are removed and sometimes these block the outlet, needing a poke with a stick to break them up, when they pass through the outlet. Maybe the regular cleaning that we carry out prevents these flakes from forming during the season?

 

[BabaYaga]

Advantage is extreme simplicity compared with a pumped system.

I agree a gravity system is simpler, at least in theory.
Adding to the practicalities is also back filling of discharge water into the toilet bowl and the need to keep the joker valve clean. A problem also often mentioned in this forum. Clearly an issue more associated with gravity systems than with pumped systems, where toilets, pipe work and tanks tend to be located more or less at the same level.

We clean out the tank at the end of most seasons, mostly using a hose pipe with a small nozzle. Over a season we find that a thick layer of carbonate deposit develops on the floor and walls of the tank. By banging on the outside of the tank and using a pressure hose, large flakes of deposit are removed and sometimes these block the outlet, needing a poke with a stick to break them up, when they pass through the outlet. Maybe the regular cleaning that we carry out prevents these flakes from forming during the season?

I have had my stainless steel tank out for cleaning only on a few occasions and yes, there has been some scaling on the inside (with the potential to cause blockages if it came loose, I guess).
Still, this has never happened. The scaling is also much less severe than what you have found. Perhaps it depends on in which waters you sail (salinity, minerals etc).

Thanks all for your input.

 

[RichardS]

We seem to be the exception that we put all the paper down the toilet with gravity holding tank and have never had a blockage. However, use of anything other than proper soft toliet tissue leads to disaster. :ambivalence:

Richard

 

[vyv_cox]

I have had my stainless steel tank out for cleaning only on a few occasions and yes, there has been some scaling on the inside (with the potential to cause blockages if it came loose, I guess).
Still, this has never happened. The scaling is also much less severe than what you have found. Perhaps it depends on in which waters you sail (salinity, minerals etc).

Thanks all for your input.

Our boat is normally in constant use for half the year, so maybe a matter of throughput. Also, one of the downsides of a holding tank is that if the level of flushing required to keep the hose clear is always carried out the holding tank would be full in a day. Our Jabsco Lite pumps a relatively small flush volume, good for the holding tank capacity but less good for creating deposits. A compromise that requires cleaning out regularly but allows us three to four days at anchor or in ports without emptying.

 

[vyv_cox]

I agree a gravity system is simpler, at least in theory.
Adding to the practicalities is also back filling of discharge water into the toilet bowl and the need to keep the joker valve clean. A problem also often mentioned in this forum. Clearly an issue more associated with gravity systems than with pumped systems, where toilets, pipe work and tanks tend to be located more or less at the same level.

I don't think so. The joker valve becomes fouled due to the mixture of seawater and urine in the upward leg of the loop. In our case at least this is exactly the same as it was before I installed the holding tank.

 

[Norman_E]

I don't think so. The joker valve becomes fouled due to the mixture of seawater and urine in the upward leg of the loop. In our case at least this is exactly the same as it was before I installed the holding tank.

That was the main reason I changed to fresh water flushing.

 

[Tranona]

I agree a gravity system is simpler, at least in theory.
Adding to the practicalities is also back filling of discharge water into the toilet bowl and the need to keep the joker valve clean. A problem also often mentioned in this forum. Clearly an issue more associated with gravity systems than with pumped systems, where toilets, pipe work and tanks tend to be located more or less at the same level.

Fail to see how backfilling could occur if the system is designed properly, nor create problems with the joker valve.

The discharge should either go in the top of the tank therefore creating a loop or in the bottom with a discharge stack inside. The only way there can be backflow is if the tank is full, and that would happen with a pumped system anyway unless you had a shut off valve.

Understanding how a system works and operating it accordingly avoids any problems.

 

[pvb]

Fail to see how backfilling could occur if the system is designed properly, nor create problems with the joker valve.

The discharge should either go in the top of the tank therefore creating a loop or in the bottom with a discharge stack inside. The only way there can be backflow is if the tank is full, and that would happen with a pumped system anyway unless you had a shut off valve.

Understanding how a system works and operating it accordingly avoids any problems.

The holding tank in my Bavaria is a top entry, so there's around a metre of vertical pipe from the toilet outlet to the top of the tank. This pipe is always full of liquid, so any failure of the joker valve to seal will result in backfilling.

 

[Tranona]

The holding tank in my Bavaria is a top entry, so there's around a metre of vertical pipe from the toilet outlet to the top of the tank. This pipe is always full of liquid, so any failure of the joker valve to seal will result in backfilling.

But that is the same as any looped outlet pipe, not specific to a gravity holding tank. Should only be seawater anyway not sewage.

 

[johnalison]

Like RichardS, we have a gravity tank and no problems related to this. The tank, of 60l, drains directly down through a "seacock" type valve and joins the main drain at a T junction before continuing to the ocean. We take no specific precautions other than pumping plenty of water in. After draining, we pump more water into the tank, and if practicable, let it slosh around while sailing to let it clean the tank before draining.

 

[pvb]

But that is the same as any looped outlet pipe, not specific to a gravity holding tank. Should only be seawater anyway not sewage.

I'm sorry, I was simply explaining how it happens, as you said you failed to see how backfilling could occur.

 

[Pavalijo]

Just a few days ago somebody told me that flushing fabric softener into the tank will soften and dislodge any dry cruddy bits!
Is this correct? I had never heard that one before!!

 

[ex-Gladys]

Just a few days ago somebody told me that flushing fabric softener into the tank will soften and dislodge any dry cruddy bits!
Is this correct? I had never heard that one before!!

Nooooo!! Look at the inside of the dispenser drawer in the washing machine! Fabric softener is a marketing artifice and would have no impact on loo crud!!!

 

[RupertW]

But that is the same as any looped outlet pipe, not specific to a gravity holding tank. Should only be seawater anyway not sewage.

Not if you are using the holding tank, ie with the outlet seacock closed. If you clear the pipe of sewage from joker valve to the tank then you will fill the tank very quickly. So as the aim is to completely clear the toilet bowl but no more the pipe up to the tank will be sewage. Otherwise the number of days in an anchorage with a few people using the loo would be very limited.

 

[Tranona]

Not if you are using the holding tank, ie with the outlet seacock closed. If you clear the pipe of sewage from joker valve to the tank then you will fill the tank very quickly. So as the aim is to completely clear the toilet bowl but no more the pipe up to the tank will be sewage. Otherwise the number of days in an anchorage with a few people using the loo would be very limited.

That is your choice. It is maybe a disadvantage of a gravity tank, but knowing this you must have confidence in your joker valve. Exactly the same thing happens if you don't have a holding tank and leave sewage in the pipe from the valve to the top of the loop.

 

[Robin]

We seem to be the exception that we put all the paper down the toilet with gravity holding tank and have never had a blockage. However, use of anything other than proper soft toliet tissue leads to disaster. :ambivalence:

Richard

Our Heads specialist ( loo brain surgeon) told us only to use supermarket own brand eco -friendly type toilet paper as it dissolves in seconds and at all costs stay well away from super soft/comfy, multiple padded types. Worked perfectly for us at home or on two different boats ( one macerator pumped, one gravity system) AND it is cheaper. Just make sure you file finger nails well regularly. :ambivalence:

 

[RupertW]

That is your choice. It is maybe a disadvantage of a gravity tank, but knowing this you must have confidence in your joker valve. Exactly the same thing happens if you don't have a holding tank and leave sewage in the pipe from the valve to the top of the loop.

I agree and the choice makes sense to us as the disadvantage seems to be a smell and a half filled bowl every other year, followed by a 15 minute joker valve change after clearing the pipe of any sewage at all.