Meditation cabin water system - off grid

[Jeremy Harris]

An acquaintance has heard about my borehole supply trials and tribulations and came over this afternoon to ask my advice about a drinking water supply she has in her "meditation cabin". Basically this is a tiny wooden shed deep in some woodland, where she goes to meditate. She has to hike in, as there's no nearby road or track, so everything that goes in there has to be hand carried. Some years ago someone set up a drinking water supply for her, to save her having to carry water in (this lady is probably around 70 years old, at a guess). The diagram below is pretty much what she described to me today, and she has some problems with it she'd like to try and fix:

The system works using a solar panel driving a small 12V pressure set pump, with a pressure switch, that's located down in the valley below the cabin. This draws water from a small terracotta cistern that catches water from a tiny spring, little more than a trickle. The solar panel acts as a cover over the cistern, together with some stone walls around it. Whenever there's enough sun the pump runs and pressurises the bladder tank in the cabin, then shuts off when it's up to pressure. The cabin can be empty for weeks, and then used for a few days, and she says there is always enough water (it's only a drinking water supply, I think). To make the water safe to drink it passes through a couple of big blue pre-filters, then a Doulton ceramic filter to take out the bugs.

The problem is that the pre-filters clog randomly, they can last for six months, but sometimes she goes up there and they've clogged after a week. Carrying spare filters in is a nuisance, she tells me, and she asked if I could come up with a system that did away with the need to change filters.

I'm afraid I don't know the specs of any of the existing kit, but do know that the buried feed pipe is about the size of the ladies little finger, so my guess is that it's 1/2" at the biggest, probably either 10mm or 12mm MDPE, so the 12V pump is most likely a very low capacity one, maybe one like those used on small boats. She's never had a problem with the pump or the solar panel system, both of which have worked well for years. She does clean out the cistern from time to time, but I suspect that it sometimes picks up extra sediment if it's been raining, and that may well be what sometimes clogs the pre-filters quickly.

As it happens I have an unused 100 litre (~26 gallon) pressure tank, and I showed this to her today and she thinks she has room for it, If I can put it high up where her sleeping platform is in the roof space. So, what I was thinking of doing was getting a small filter tank, something like a 6" x 35", with a standard head, riser and screen, put in a gravel bed and 2/3rds fill it with turbidex. I'd then use the second pressure tank purely for filter manual backwash, like this:

The idea is that when the sun shines both pressure tanks get charged with filtered water, with the check valves isolating them from each other. Her drinking water supply pressure tank will always be full of pre-filtered water, so the Doulton filter should work as it does at the moment. She reckons that tank holds more than enough water for the length of time she stays there, so that bit should work fine, I think. Before she leaves the cabin, she needs to open the drain valve outside and open the backflush valve, which will discharge a few gallons at a fairly high pressure through the turbidex filter to clean it. She can then close both the backflush valve and the outside drain valve and go away and let the system recharge, ready for her next visit.

Apart from the fact that I'll probably have to be the one who lugs all this kit through the woods and install it, it seems to me to be a workable solution. The main advantage is that she won't need to keep buying big blue pre-filters, so the cost of a small filter tank, some turbidex and a few fittings should soon be recovered.

Can anyone see any major problems with this idea, please?

 

[Reach4]

turbidex says "Backwash rate: 14 - 18 gpm/ ft2" http://www.turbidex.com/parameters.htm
That will have to figure into your calculations as well as the drawdown capacity.

On the Big Blue filters,

1. what size are the elements? If it is 4.5 x 10, you can replace the blue part with a 4.5 x 20 without doing further plumbing. The black part would be the same. That should double the life, but the housings are twice as heavy when full of water.
2. Which filter cartridge clogs first. While 25 and 5 is a pretty good match to typical conditions, it might not match all. If there is sand, a pre-filter that is easily washed out with no element could be good. Note that you should not use cellulose elements on a non-chlorinated system; use polypropylene.

 

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[Jeremy Harris]

Thanks again.
Right now I don't know for sure exactly what the lady has up there, I'm planning on walking up there this weekend to take a look. From her description it sounds like she already has 20" filters, as she described them as being longer than the Doulton filter, and I'm pretty sure that the Doulton only comes in a 10" housing. My guess (just from knowing a bit about the soil around the woods up on that hill top) is that there's no sand up there (unlike where I am a few hundred feet lower at the bottom of the valley), as the spring will be coming from a chalk aquifer, probably with clay above it. My guess is that it's fine clay that washes in after rain, and it may well be that the cistern could be improved with a settling chamber and weir to help keep sediment out.

I'm not sure she knows which cartridge clogs first, she was just told to change them if the flow slowed down by the chap who fitted the system for her (who's sadly no longer around). From her description, I'm pretty sure she's using wound polyprop filters, but I'll check at the weekend.

My back of the envelope calcs suggest that a 6 x 35 housing with turbidex would need a backwash rate of around 2.8 to 3 US gals/min, so if I was to lower the pre-charge in the 100 litre (~26 US gal) pressure tank to, say, 1.5 bar (~22 psi) and if the pump can pressurise it to 50 psi or so (I'll check at the weekend, but I think these little boat pumps can go to 100psi, less the static head up the hill), then I should have around 12 US gals or more of water available to back flush the filter with. That would give a backflush time of around 4 to 5 mins, which may be enough to clean it, especially as the filter will only have handled around 20 to 30 US gals of water between back flushes, at a guess (I don't know how big the present pressure tank is, but the lady thought it was about the same size as the spare 100 litre one I have sitting in my garage).

It's probably way off the typical sort of use that this filter media is specced for, but, to be honest, it's a pretty wacky set up! The thing that has sort of convinced me that this might work is the low usage between backwashes, as I can't see the turbidex getting that heavily loaded. The existing filters may well handle a dozen or more tank fills before getting clogged, so will be dealing with a lot more sediment, whereas it's no problem to clean the filter every time she visits, reducing the filter load a lot.

Despite this being a bit wacky, I like the idea. The chap that put the original system together did a good job in finding a way to get good drinking water up on that hill, and came up with an ingenious solution. I feel I owe it to him to try and see if I can improve it a little bit, and save this lady having to lug filters up there, when I guess all she really wants to do is sit in the woodland and meditate, rather than have hassles with a water supply.

 

[Jeremy Harris]

I went up into the woods this morning to take a look at the set up this lady has. I'm really impressed with the workmanship of the chap who built it (who I now know was her late husband). It's such an interesting system that I thought a description here might help and perhaps give people some ideas if they want to do something similar.

There is a stone shelter over the source of the spring in the valley below and the spring has been diverted by a short length of PVC pipe, maybe 6" in diameter, into a large terracotta flower pot (I guess around 15" or so in diameter, that is set at a slight angle, so waste water always flows over the same edge. There is a piece of slate that partially covers the top, wedged under the edge of the feed pipe, so the water runs over this before down into the pot and can't fall directly on to the pump pick up pipe screen. The feed pipe for the pump has been fitted in what had been the drainage hole for the flower pot, enlarged a bit to take a tank fitting. The feed pipe comes about 2/3rds of the way up the pot and has a stainless mesh screen on it, that looked nice and clean. The chap who built it thought it through, as when the water is flowing fast it will tend to run across the bit of slate and out over the edge of the pot, hopefully taking any debris with it. The water in the pot is the slower moving stuff that trickles off the bottom of the slate.

The pump is a really nice, bronze, boat pump, rated at 100 psi and fitted with a pressure switch, set to 75 psi, as near as I could tell. It is housed in a waterproof locked box, together with a very big capacitor (presumably to take the motor starting current surge). The capacitor looks very much like those used to help stabilise supplies in big car audio systems. There are two solar panels, 18V maximum, wired in parallel and fitted to a galvanised steel frame, that is also the roof of the stone shelter. The roof is hinged to get at everything inside and padlocked shut normally. There is a voltage regulator (Australian made) that is connected between the solar panel and the capacitor and pump in the waterproof box. I wish I'd taken a camera with me, as this set up was just so nicely put together, it was a real work of art.

The pipe running from the pump is 15mm PEX (roughly the same as our old 1/2" copper pipe), running through armoured flexible conduit and neatly buried all the way to the hut and comes up inside the floor of the hut. Inside the hut there is a 100 litre (~26 US gal) vertical pressure tank, then there are two 20" long big blue filters in series fitted on the wall, one's a 25µ, one's a 5µ, just as the lady described. These are piped with another length of insulated 15mm PEX pipe to a Daulton Sterasyl filter that is connected directly to the drinking water tap, over a table with a cut out for a small bowl. I guess it was arranged like this so that the virtually sterilised water coming out of the Doulton filter ran via a very short (as in no) pipe to the tap, to both minimise the chance of stale water in the pipe and make cleaning the filter easy (as it's over the bowl).

The cabin is small, at a guess around 10ft x 10ft, but is triangular in section, like a steeply pitched roof. It only has limited standing headroom at one side, as there is a mezzanine sleeping platform and storage space running along one roof pitch. There looks to be room up at the end of that platform to take another pressure tank as a back flush water source, and the whole thing is massively over-engineered, so weight isn't going to be an issue. The hut is also nicely insulated, the whole inside is left with the aluminium foil of the insulation board showing, which makes it light inside even though there are only two small circular (from a boat I think) portholes high in each gable end.

I put a tyre gauge on the pressure tank and it read 55 psi, and must have been full, as the sun was out and the pump was off. That tallies with the hut being around 40ft (at a guess) above the spring and the pump being set to 75 psi.

I'm waiting to hear from a supplier of turbidex here, as to whether they can sell me 1/2ft³ (which is still more than I need). I can get a 6" x 35" vessel with head, riser and screen easily enough (they are common here for window washing DI resin systems), and the valves are no problem, even getting flow control valves so I can set the backwash to "just" enough to raise the bed and clean the turbidex, but limit it so I don't blow the stuff out of the flush pipe, are easy enough things to get. Getting turbidex or micro-Z seems near-impossible here, though. I've spent hours searching the web for suppliers in the UK, and they are rarer than hen's teeth. I envy you guys over the other side of the pond where you're spoilt for choice with suppliers!

The lady is keen for me to get on and modify her system to a back-washable pre-filter system, and having seen how awkward it is for her to change the existing big blue filters I can understand why. I don't think there's any point in trying to improve the filtering down at the pump end, as that looks as good an arrangement as it's possible to get, with a weir to let the debris out and a good depth of settling tank (which looked pretty clean to me). I'm now convinced that a relatively small (say 22" to 24" bed depth, which I know is right on the lower limit) turbidex or similar filter, that will flow at around 1 1/2 US gal a minute absolute maximum (that's the most the pump can deliver against no head, probably in reality it's lower than this pretty much all the time) will do the job, especially as the lady can easily manually backwash the filter every time she leaves the hut for a few days.

Anyway, I'll post an update in a few weeks time as to whether this system works when I can finally get the parts. I'll have my work cut out trying to match the neat workmanship of the gentleman who built the hut and water system though, it really is a work of art. I really admire the skills of craftsmen who make even the things few will ever see as well finished as if they are in a showroom. I fear this sort of craftsmanship is a dying art, though.

 

[Jeremy Harris]

Yea God's I'm struggling with this!

All the calcs say this SHOULD work, but can I buy the parts to build it? Can I heck as like............

It looks like I'm going to have to buy this stuff through the back door, as the filter people just don't want to know, so I've ordered a pressure tank with feed and riser from a company selling them for window cleaning systems (persuaded them to sell me an empty tank, with no resin) and I've managed to buy a cu ft of Turbidex, more than twice what I need, but I guess it'll come in handy some day (although it's expensive stuff to just have lying around unused). I also managed to get some flow control valves here (had t hunt for them, they are another rare commodity this side of the Atlantic), so I can set the backwash flow rate to be in the right range no matter what the backwash pressure tank charges too, to give me a bit of leeway in adjusting the tank pressure and hence stored volume.

With luck I may get the parts by the end of the week and can haul it all up to the ladies little cabin in the woods next weekend and set it up. I'm looking forward to this, as an interesting little sideline. There are so few people that know anything at all about private water suppliers, filtration and treatment over here that I'm tempted to set up a free advice service, based on all the stuff I've had to learn over the past year or so setting up our borehole system. I'm sure if more people understood that this stuff isn't really rocket science they'd go for it, as we have hundreds of disused wells and boreholes that were all decommissioned when the government laid on a mains water supply across pretty much the whole country. People are only now beginning to realise that this mains water is relatively expensive (around 0.5 US cent per US gallon) and often not of high quality (it frequently has a LOT of chlorine in it and across most of Southern England the mains water is hard, 150 to 200mg/litre or more CaCO3 is typical).

I'll post another update when I get the system set up and tested, just so you all know whether it works or not!

 

[Reach4]

There are so few people that know anything at all about private water suppliers, filtration and treatment over here that I'm tempted to set up a free advice service, based on all the stuff I've had to learn over the past year or so setting up our borehole system. I'm sure if more people understood that this stuff isn't really rocket science they'd go for it, as we have hundreds of disused wells and boreholes that were all decommissioned when the government laid on a mains water supply across pretty much the whole country.

Interesting stuff.

You may not want to be too high profile, since monopolies don't like competition. You might end up with a meter on your well.