[osolemio]

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 ...