Water tank vacuum systems

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neb

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#295254 16-Mar-2022 16:09

This was very indirectly referenced in this thread, for the 2000L tank at the Casa we're looking at getting some sort of drain-from-the-bottom overflow system to deal with the fact that the roof areas it's fed from get lots of leaves and debris and even with a leaf screen it's eventually going to build up sludge at the bottom. The obvious solution is a TankVac but they want around $500 for a few bits of PVC while you can get something with equivalent functionality (although not exactly the same thing) at Bunnings for $39.95. The main difference is that the TankVac has a cutoff mechanism to stop the siphoning action before it drains the tank while the Rainharvesting one doesn't seem to have anything.

Does anyone have any experience with a drain-from-the-bottom system, and how do you deal with the siphoning issue? And as a follow-on, anyone know of any installers in Orkland that don't want to sell you a tank as part of the deal? The plumber who did other parts of the house said it wasn't really something he knows much about, and a few tank places I called want to sell and install the whole tank.

pih

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#2887417 16-Mar-2022 22:02

Not sure about those systems, but the RainHarvesting one does mention an anti-syphon vent in the elbow on this page. As for installers, look for a rural plumber - they will all have experience plumbing up a tank without needing to sell you one. I can recommend Habitat Heating in northwest Auckland, if that's not too far. He connected our new tank a year ago and did a great job at a reasonable price.

Ge0rge

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#2887425 16-Mar-2022 22:14

I did diy for one of my 25,000l tanks.

Couple of 90° elbows and a length of pvc pipe. Cut some saw-teeth around one end of the pipe that went to the bottom of the tank - allows it to sit on the bottom but not suck itself to the bottom. 90° bend at the overflow level, short length out of the tank and then another 90° bend to take it down to the soak pit. Just below the bend on the inside I drilled a couple of 10mm holes.

With the roof area we have, it fills rather fast. Once over the level of the short length, it begins to syphon without issue. The 10mm holes make very little difference to how much water/sediment gets picked up from the bottom. Once the water level gets below the 10mm holes, air rushes in and breaks the syphon - rain and repeat.

Noting the diy you've posted about in the past @neb, you'd have this squared away in an arvo.

neb

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#2887427 16-Mar-2022 22:17

Yeah, I'd dug a bit deeper and saw that in the install docs, there's a tiny hole in the elbow that allows air in when the water level drops below a certain point and stops the tank from siphoning dry, although I'll probably add an air gap at the outlet somewhere in case it gets blocked. Thanks for the Habitat link, I'll give him a call tomorrow, Kaukapakapa is about 30-odd minutes from here so not too far.

neb

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#2887436 16-Mar-2022 22:43

Ge0rge: Couple of 90° elbows and a length of pvc pipe. Cut some saw-teeth around one end of the pipe that went to the bottom of the tank - allows it to sit on the bottom but not suck itself to the bottom. 90° bend at the overflow level, short length out of the tank and then another 90° bend to take it down to the soak pit. Just below the bend on the inside I drilled a couple of 10mm holes.

Thanks, that sounds pretty much like a DIY version of what Bunnings is selling. It has to hook into the stormwater system which can't handle the volume of runoff and fills up during heavy rains so it may need backflow protection from the stormwater network into the tank, and I've been told it needs seismic restraints (2 tons of water in a stack 2m high and only 0.9m wide) so I was planning to leave this one to the pros to make sure we don't get dinged at any upcoming building inspection.

nickb800

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#2887469 17-Mar-2022 08:09

The benefit to the tankvac system is in the siphon valve. It means that you get a full bore flow draining the tank, which creates a strong vacuum to move sediment. A basic overflow like the Bunnings option will only drain at the rate that water is being added to the tank, which may not be enough to move heavier sediment particles, except perhaps for short periods during intense rainfalls. You can see in the tank vac video that the draining cycle carries on after they have stopped adding water to the tank - this suggests that the valve waits until the tank water level is above the top of the overflow, then drains it down to below the bottom of the overflow.

Not saying that tankvac is necessarily worth the extra cost for your application, just part of making a fully informed decision. If levels permit, you may find that a sediment trap (basically a sump) before the tank will reduce the amount of sediment making its way to the tank such that you don't really need a tankvac. Also, I'd consider replicating the design of the tankvac with a lateral running across the floor of the tank with holes drilled in the sides, to suck sediment from across the floor of the tank. I'm not sure how effective a single point overflow intake would be on the floor.

Finally, what's the tank water going to be used for - as in, how important is it for the water to be clean? If it's for garden taps, then doesn't matter so much (other than avoiding tank cleaning every few years). If it's for emergency drinking water, then why use rain water? A 2000L tank would only cost $3 to fill up with mains water, then chuck a bottle of chlorogene in there, and replace annually.

Bung

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#2887525 17-Mar-2022 10:35

nickb800:If levels permit, you may find that a sediment trap (basically a sump) before the tank will reduce the amount of sediment making its way to the tank such that you don't really need a tankvac.

We have a small tank installed by 1st owner just to make tea and rinse her hair. It has a charged pipe into it and the horizontal section acts as a sump trapping most of the sediment. It was a pain to empty as it wasn't done in pipe that had any inspection fittings. I've replaced it in 100mm.

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neb

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#2887895 17-Mar-2022 20:51

nickb800:
You can see in the tank vac video that the draining cycle carries on after they have stopped adding water to the tank - this suggests that the valve waits until the tank water level is above the top of the overflow, then drains it down to below the bottom of the overflow.

[...]

Finally, what's the tank water going to be used for - as in, how important is it for the water to be clean?

The Bunnings one should do the same thing, there's a hole drilled halfway down the elbow to break the vacuum once the water level drops a bit below the top of the overflow, so it shouldn't be exclusive to the Tank Vac.

For use, it's purely rainwater harvesting for the garden so no need for any special treatment.

I've pinged the guy in Kaukapakapa, I'll report back on any useful results.

nickb800

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#2887948 18-Mar-2022 06:57

neb:
nickb800:

You can see in the tank vac video that the draining cycle carries on after they have stopped adding water to the tank - this suggests that the valve waits until the tank water level is above the top of the overflow, then drains it down to below the bottom of the overflow.

The Bunnings one should do the same thing, there's a hole drilled halfway down the elbow to break the vacuum once the water level drops a bit below the top of the overflow, so it shouldn't be exclusive to the Tank Vac. For use, it's purely rainwater harvesting for the garden so no need for any special treatment. I've pinged the guy in Kaukapakapa, I'll report back on any useful results.

It's not so much about breaking the vaccum - you're right that only a simple hole is needed to prevent a vacuum from completely draining the tank. But the tankvac valve lets the tank 'overfill' to get a stronger flow when it drains. A simple series of elbows will only allow the overflow to flow at the same rate that water is flowing in, which most of the time won't be enough to move sediment.

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#2887963 18-Mar-2022 07:58

Shirly any siphon system will suck at a rate limited by the pipework from the time the water gets to the top until it gets to the vacuum break. You just have to decide how much capacity you can spend.

nickb800

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#2887988 18-Mar-2022 08:32

Bung: Shirly any siphon system will suck at a rate limited by the pipework from the time the water gets to the top until it gets to the vacuum break. You just have to decide how much capacity you can spend.

Yes, but with a simple pipe system you're not going to actually going to get a siphon very often, because that would require the water level in the tank getting higher than the top of the siphon to start the siphon. This would only happen when you have an intense rainfall flowing into the tank faster than it can run out of the overflow by gravity. Most tanks seem to have similarly sized intakes and overflows (e.g. 100mm inflow, 90mm overflow) so this wouldn't happen very often. If you had (say) 24 hours of light rain flowing into an already full tank then you would just get 24 hours of a dribble out of the overflow with a simple piped overflow (like the Bunnings option) and therefore wouldn't move much sediment from the bottom of the tank. With the tank vac, their proprietary valve would block the overflow until the tank water level is above the top of the siphon, then release it to enable a full siphon flow. You can see this in the video on the front page of the tankvac website - he tops up the tank with a hose, then pulls the hose out of the tank, then you see a full siphon flow draining gunk out of the tank (admittedly the gunk is probably added for theatrical/sales effect)

neb

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#2888345 18-Mar-2022 16:09

nickb800:
It's not so much about breaking the vaccum - you're right that only a simple hole is needed to prevent a vacuum from completely draining the tank. But the tankvac valve lets the tank 'overfill' to get a stronger flow when it drains. A simple series of elbows will only allow the overflow to flow at the same rate that water is flowing in, which most of the time won't be enough to move sediment.

Ah, so that's what the secret sauce is! Good to know, because I probably won't need it in that case, firstly because we had four hundred-year rain events just last year alone (so much rain that the street drains can't keep up and water pours down the driveway), it'll definitely get a ton of water pouring through, and secondly because it's not drinking water, all I'm really concerned with is keeping the outlet area free of gunk and with at least some movement into the siphon located next to the outlet it should be good enough.

The fact that it's less than a tenth the cost of the Tank Vac also helps :-).