Water Tank Into Waste Tank

[smitch6]

I have 2 large water tanks on my boat,

1 is right beside the toilet i want to install a proper toilet with a waste tank

what i want to do is convert that water tank into a waste tank

it has an inlet at the top which is a normal water inlet - this will need to be converted to a pump out point.

it has an overflow near the top that goes to the outside - this will need to be converted to the waste in,

it also has an outlet near the bottom which was where the water came out, this will need to be sealed.

it is a plastic tek-tank

how do i do the conversions?

i have been told i need a dip pipe for the pump out?

which i gather is a long tube that goes nearly to the bottom of the tank?

can this be made with normal plumbing rigid pipe? as if it's flexible won't it move?

thanks

[JanetAnne]

You do need a modification to the out pipe that will allow the tank to be sucked empty from the bottom. Sucking it from the top just ain't going to work!

From the top of the tank you also need a vent to outside the hull of the same diameter as the pump out pipe to allow air back in at the same rate. Forget this bit and you can suck the seals out of the toilet.

Apart from that you are pretty much there.

[Regulo]

The air vent doesn't have to be the same diameter as the suck out pipe. Mine are 1", definitely not the same diameter as the suck out pipe. A dip pipe will work, but it's more liable to blockage (in my opinion) than a side outlet as low as possible, or even from the bottom underneath, if you can get one there. Are plastic tanks OK for waste? I don't know, but someone here will.

[Regulo]

Another thought - how does the tank sit height-wise in relation to your proposed toilet? It will be tricky if it's anything other than lower.

[ExSurveyor]

2 minutes ago, Regulo said:

Another thought - how does the tank sit height-wise in relation to your proposed toilet? It will be tricky if it's anything other than lower.

Unless it is pumped ?

I have a manual pump toilet with the waste tank base level with the pump.

Hopefully it must have some sort of one way valve.

[Regulo]

Yes, there's usually a one way valve involved, at least with electric pumped toilets there is. Shouldn't be a problem, as long as the pipework can be arranged to suit. If the waste inlet to the tank is near the top, even if that is higher than the pump, there will only be what's in the pipe to worry about. 

[smitch6]

it is the same level as the toilet

i have been told it will be ok as the hand pumped one that i will be getting will pump it ok

will just have to make sure it's pumped a few times to push it through the pipe

[ranworthbreeze]

2 hours ago, smitch6 said:

it is the same level as the toilet

i have been told it will be ok as the hand pumped one that i will be getting will pump it ok

will just have to make sure it's pumped a few times to push it through the pipe

Hello smitch6,

Just make sure that you use smooth walled pipe rather than any ribbed pipe or you may find yourself replacing it within a few years.

Regards

Alan

[annv]

Hi Smitch You will need to cut a hole in top of tank for hand access to tighten nuts on hose tails, you then need to cut two 1 1/2 inch holes in the top this is for suction pipe and the other for venting during pump out both going to deck fitting the suction hole needs to have a length of rigid pipe going to the lowest part of tank,  copper or plastic, you will then need a air vent hole in top of tank to outside with a mesh hull fitting,   the supply from the toilet needs to go in the top of tank dia to suit toilet outlet if toilet is lower then top of tank you may get a flow back that's why top of tank should be below toilet outlet if not!!!!, the hose material needs to be vapor proof LeeSan made hose from  ASAP, with double stainless jubilee clips on ALL hose connections. A contents gauge is useful to stop stress,One way valves are not good on toilet waste pipes as any pip or hard matter will/can lift valve seating un like clean water.  best of luck John

[MauriceMynah]

I think the bottom line ( if you will excuse the expression ) is that you will do better to have a tank made for the job rather than modify one made for a different purpose.

Do you have enough space to keep the two water tanks on board as water tanks ?

[smitch6]

no and this tank is ideally situated and a perfect size also to be used as the waste tank

i can re-use the fittings as there is 1 at the bottom - perfect for pumping out 50/50 as to wether top or bottom pumpout

there is 2 top outlets, vent and toilet in.

the only thing i will have to add is a sender gauge which will be easy enough.

as far as i am aware all fittings are 1 1/2" tails pre-fitted at the factory.

i will obviously fit a shut-off valve at the bottom as close to the tank as possible

[Smoggy]

If a tec-tank you can send it back and get them to fit extra threaded bosses where you need them (at a price of course), I believe they weld them in ultrasonically.

Check the thickness of the tank wall as they usually specify 3/8" thick for a holding tank.

[smitch6]

just spoke to tek-tank, each tank has a unique serial number that they can track and tell me all about it.

so i'll take a photo of it and they can they tell me all the fittings and how to adapt them.....

[ZimbiIV]

I certainly wouldn't fancy doing that job...………………………………..the other way round with the tanks,  Waste to water!!!

paul

[marshman]

I must admit, whilst it is probably possible, I would be a bit hesitant and when it has been done it will be difficult to clear it properly.

After many years I can still get my tank pretty clean but it helps being GRP and angled so most of the solids and crud gets down to one corner which is easily pumped - angled hoses down the hole cleat the rest.  (TMI? ) Not sure how easy that will be to achieve in an old fuel tank!

[smitch6]

who said it was an old fuel tank?

it's a purpose built plastic tank

[marshman]

Fair enough.

[dnks34]

Assuming your tank is plastic I was advised when fitting a plastic holding tank a few years back that the vent was better to be 1 inch 1/2 diameter to reduce the risk of it imploding if a blockage occured during the pumpout, I was told aluminium tanks are not so prone to this so a narrower vent pipe could be used.  

I would also fit the pump out feed to the lowest connection on the tank, fill from the top and pump from the bottom ive never seen it done the other way around. 

 

[Regulo]

I'm not disputing the above facts as told, but I can't see how a blockage in the sucky-out line could implode the tank. Surely if the pipe were blocked, there would be no negative pressure in the tank?

[grendel]

until the blockage suddenly cleared I guess.

[dnks34]

I think it may be to do with the risk of a narrow vent pipe becoming blocked, then the tank (more so if plastic) could end up imploding under the suction.  

I have seen an imploded tank before on a sports type cruiser and it caused the owner a few headaches, that was an aluminium tank so Im not quite sure what went wrong! 

[annv]

LeeSan insiste  that the pump out vent pipe should be the same dia as pump out pipe to prevent imbalance of pressure in tank, and to allow flushing while pumping/sucking out contents, the small dia vent pipe with screen is to allow pressure to equalize in tank when using toilet and to stop tank pressurizing do to fermentation  gases from contents, John