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Old 01-12-12, 08:51 AM   #1
osolemio
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Question Rain water to Drinking water

Where I live, rain water harvested water may only be used for toilet flushing, laundry, irrigation of non-edibles, cleaning and so on. And that's it, no showering, dishwasher etc.

I have such a rainwater harvesting system now, which is working fine except ... I produce way more water than I can use by flushing toilets and doing laundry.

I want to expand the use of rain water to as much as I can, apart from drinking and cooking purposes. That means all hot water and all normal taps, as I will keep a few dedicated cold water taps with straight city water.

There are systems to clean water by a series of filters, usually one or more particle filters, then an active carbon filter followed by a UV filter.

I am considering the option of having a pump and a UVC filter installed directly inside the underground rainwater tank, where a small pump runs continuously and exposes the water in the rain water tank to UVC light. The water in the tank is constantly around 40F, there is no sunlight at all, so the level of bacteria growth is an absolute minimum. The technique of having a pump and a UV filter is often used in garden fish ponds to keep water clear and clean.


I think it makes more sense to have the UV light in the rainwater tank, not in the filter in the house, as the cleaning function of the UV filter is pointless all the while there is no flow of water. I realize that since the water exposed to UVC light is returned into the same tank, it is not a 100% guaranteed method, but I am convinced it will keep the level of bacteria to an absolute minimum. The intake for the pump will be from the far end of the rainwater tank, and the outlet just next to where the rainwater system sucks its water from.

The pump and the filter will use less than 30W, but since it will be running year-round, it does create some impact on the electricity use. I will have a solar PV installed this summer, so it will not be the greatest issue though.

Do I still need a UV filter inside the house, downstream of the particle and carbon filter?

Although the water is not going to be used directly for cooking or drinking, I still want to make sure it is free of bacteria, as it will be used for showering (can be inhaled in the steam), in the dishwasher (food/drink will be consumed by the cleaned items).

I need all your expertise in this project, so I can make a system which is not prohibitive in investment and running cost. It must work in the simplest way possible, be reliable and just do its job.

One day, I will stand in the shower, using water harvested from the roof, heated by the sun. I cannot wait, really, but I am not there yet.

Please help me on my way by offering your opinions and ideas.

Thank you.

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Last edited by osolemio; 01-12-12 at 08:54 AM.. Reason: Clarification, bad wording corrected
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Old 01-12-12, 10:27 AM   #2
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So the only argument I can see for keeping it after the filter is this:
Without any chlorine in the water the filter medium is an ideal breading ground for bacteria, having a UV system post filter will concentrate the dose to the water you are using. What about this?
Then you are getting your infinite dose, and the water being used has just been sent through the system. I forgot to draw the filters before the intake of the UV.
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Old 01-12-12, 03:42 PM   #3
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Quote:
Originally Posted by pinhead View Post
So the only argument I can see for keeping it after the filter is this:
Without any chlorine in the water the filter medium is an ideal breading ground for bacteria, having a UV system post filter will concentrate the dose to the water you are using. What about this?
Then you are getting your infinite dose, and the water being used has just been sent through the system. I forgot to draw the filters before the intake of the UV.
I am not sure I understand your drawing. No, in fact, I am sure I don't understand it Lower right is the UV filter, the big tank is the rainwater tank?

You are right, it is a dilemma. The UV filter will be doing most bang for the buck in the tank, keeping the source of the water itself bacteria and virus free (not entirely, but keeping it to a minimum) - before they grow too much. However, the downstream filters could contain bacteria, and no matter how clean the water in the tank is, it could be infected as it pass the carbon filter.

So ideally, if money was not an issue, there would be a UV filter in both the tank, and trailing the particle/carbon filters, although this is an extra cost.

I am still hunting the best solution. Sometimes there is something more simple, which is better, cheaper, smarter, than the complicated solution. It must, however, solve several problems in order to work.

Pinhead, I hope you could elaborate more on your drawing, thanks
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Old 01-12-12, 04:23 PM   #4
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I don't have an answer for you on turning stored rainwater into potable water in an efficient manner. It seems storing non-chlorinated water that won't be boiled prior to drinking is the challenge. If it were my water and I already had my toilet and laundry taken care of, I'd use it in the summer for gardening but that I realize that doesn't help when you are outside the growing season. Most of my water usage outside of laundry(which is a pretty big user of cold water for me) is hot water usage with showers, cooking, dishwasher, and washing hands consuming heated water. Once you've plumbed into the hot water you've got most of your usage covered. I'm not sure if its worth doing or not, where I am it isn't because the cost of pressurizing it costs me more than what it costs me to pay my city to pump it out of the ground(US$2 per 1000 gallons or 3785 liters) and I use less than 1000 gallons a month. I guess it depends on what your motivations are. Good luck though.
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Old 01-12-12, 04:24 PM   #5
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my old house got it's water from a surface spring so it was always treated as contaminated. I took the water from the spring to a 3000 gallon tank, from there it went to a 200ish gallon tank that had a circulating pump and ozone generator on it. The pump just kept the water moving and sucked the ozone into the tank. This should have killed everything at that point. After that it went through a pressure pump then a filter and finally into the house.

That circulating pump ran 24/7...
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Old 01-12-12, 04:29 PM   #6
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Yeah, as I said I forgot to draw the filters, they would be on the red line. Then into the UV system, then pumped back into the tank. Where ever your water is going to go would be the top blue line, exiting right after the UV system.
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Old 01-12-12, 06:36 PM   #7
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I don't understand why you can't use your harvested water to irrigate your vegetables? Are there other issues with your rainwater besides bacteria? Industrial pollution or?

Are there any corrosion or electrical safety issues associated with placing the UV sterilizer in the tank?

It's interesting to read about what you're doing on the other side of the world, I think that here in Canada we are truly fortunate to have so much easily accessible, clean, safe, and inexpensive water.

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Old 01-12-12, 08:50 PM   #8
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Quote:
Originally Posted by osolemio View Post
Where I live, rain water harvested water may only be used for toilet flushing, laundry, irrigation of non-edibles
Is it a legal reason that you cannot use the water for vegetables?

The first thing that I would use rain water for is watering my garden vegetables and I could not see any reason that you should not be able to use it for that purpose.
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Old 01-13-12, 01:48 AM   #9
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Thank you for your answers so far, I really appreciate you guys taking your time to offer your input here. Sorry for this long post - I have added BOLD highlights to make it more readable ...

I know that different problems are solved in different ways, around the world, and we can learn great stuff from each other. Not only about how to do things in a smart way, but even more so, how NOT to do them.

By browsing the internet, including Ecorenovator, I found the technology of using drain water heat recovery, which I have yet to hear of in Europe. For more than a year, we have had a Canadian "Power Pipe" installed, and it raises the temperature of the city water going to the hot water buffer tank, by up to 30F during showering (less during other water uses). On that account alone, we save a lot of energy in a passive way (the sewer rats are not happy, though, they are freezing their tails off!).

The economics in this is quite interesting, compared to places like Canada. We do have enough water underground - so far -so that most of the city water is actually spring water and not treated surface water. But the widespread use of pesticides over a long time is closing more and more springs due to increasing values of nasties.

Using a gallon or more water each time we flush the toilet, with pristine drinking grade water seems a waste, all the while we have increasing problems with overflowing sewers and flooding from rain water. Just like solar power, it is all about flattening peaks and filling lows. With rain water harvesting, water is saved during peaks and subsequently released on a daily, low and predictable rate. The peaks and lows are the eternal challenge when dealing with our dwelling in changing weathers. Rain water is a triple win-win-win. The only losers are the people making money off flooded basements and other flooding damage, as less flooding is less work for these people.

Water here has added taxes for various purposes, and I think we pay about 30 USD pr 1000 USG, maybe even more (and it depends on where you are connected, as it varies locally). But the price is high enough to let it be viable for these solutions, depending on installation and running costs, obviously. I am using about 20,000 USG pr year, although this figure has now split between my own harvested rain water, and city water.

I still have excess rain water, so I want to use more rain water and less city water, if I can. We get a lot of limestone, and the washing machine is very happy now, running on rain water. But the shower and sinks, still using city water, needs weekly cleaning for limestone deposits. Not so with rain water! One sink is actually already on rain water, and it is permanently clean and sparkling, no visible deposits at all.

Actually, now I am not sure about the vegetable issue, maybe I am thinking of some other water source, or even organic fertilizer. Forget that part! It does not matter to me anyway, as this is a city house with a minute back yard (20x20'!), and no vegetables here. In fact, the garden is so small it is just large enough to contain the 2000 gallon rainwater tank and two seep down overflows (which struggle to keep up at times).

The tank is made of some kind of PE, and I asked the factory about UV light. They give a 25 year warranty, and this is based on NO UV light at all (since it is underground). The PE of this tank is meant to be underground, and thus the tank is not UV proof.

But I can use the same principle they use in garden ponds some places - circulate the water through a sealed UV light housing, then back into the tank.

These UV lights do NOT like to be turned on and off too much, apparently that degrades their life quite a bit. Also, I believe they are not up at full strength the instant you turn them on. If it wasn't for these two properties, I could have it turn on and off with the supply pump to the system, so it only ran as needed.

But because it has to be on 24/7, the yearly use is several hundred kWh, depending on what lamp is installed. With electricity here being around .4 USD/kWh, it amounts to quite a bit. It will be offset by electric PVs installed hopefully during 2012, but I could use those kWh for something else.

A powerful, but instant-on UV-C light, which is not harmed by switching on an off many times a day, would be optimum. When it has to be on 24/7, not only does it cost me over 100 USD a year, but 99% of the time it will just sit and beam the same small amount of water, to no use. It just seems so overkill and pointless.

The other solution, about having UV light in the tank (through a pump, not submerged directly), makes a lot of sense in a different way. A very low flow pump, using only little electricity, will be operating 24/7, with a quite low power lamp. Electricity usage will be less than half of the other solution, and the lamp will be treating new water continuously.

One drawback is that the water is taken from the same tank it is pumped back into, although I would take it from the far end of the tank, and let it go back into the tank just where the house pump takes the water from. Another drawback is pollution downstream of the rain water tank, especially possible growth in the carbon filter.

Ideally, by in-tank UV light treatment, the water should then be quite close to bacteria and virus free - but it is not guaranteed. I like the thought of the UV light being productive all the time, and not just by water flow. I also like the thought that the tank, which is the only place where the water is not first-in-first-out, is the place where bacteria and virus potentially could multiply over time. This should really be where the UV light is most effective - to kill the nasties initially, BEFORE they even start to gang up against me! It seems suboptimal to first grow a farm of bacteria in the tank, only to take them out later. Why not kill them before they even grow?

There is no light at all in the tank, and at a temperature around 40F year round, it is not really optimum conditions for bacteria growth. But that does not guarantee there isn't any. A dead bird on the roof would definitely add some nasties to the water.

The last point is that even if a circulating pump in the tank, with a UV filter, could keep it bacteria free, the water would still need to travel into the house, through particle filters and then active carbon. If these filters are once infested with bacteria, it does not matter how clean the water in the tank is. I read that carbon filters are good for catching odor and many different chemicals and other items which pass through the other filters, but also that bacteria have no problems living in them.

Does anyone know of a UV-C light source, which is NOT harmed by flicking on/off on a daily basis, AND, is up to full power near-instantly? If that light would still last ~9000 hours, it would mean 10-20 years of operation, and much less electricity use. Even if it was less efficient when it is on, the fact that it only runs about 50 times less than a 24/7 light means it would use far less energy.

On-demand UV-C filtering would make quite a difference ...
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Old 01-13-12, 06:00 AM   #10
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Artical Here
But that system is a little too big for your application. I would be surprised if this ever makes it to a household size before LED technology is ready.

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