Rainwater catchment system for flushing toilets

tcjohnssonMay 4, 2009

I'm building a modern multi-family complex in a very urban environment and have several large (1,250) gallon storage tanks planned to be installed under driveways. The buildings are 3 stories tall and incorporate green roofs (approx 20% coverage of entire roof). There are four roofs consisting of approx 500-650 sq ft each. Each rooftop is coupled to its own water storage tank approx 40 feet below the roof. I plan to install a pump in the tank to pump water to the toilets. The tank will have an overflow (to city storm water drains) and there will be an inlet w/ball valve for city water when the tank runs low (e.g. during dry spells).

We get roughly 25" of rain annually here so I calculate that the systems will save approx 7,500 gallons on the smaller roofs and 9,750 gallons of water on the larger roofs annually. The average person flushes their home toilets 5 times/day. I'm using dual-flush toilets which consume 1.6 gallons for #2s and .9 gallons for #1s. Assuming the average person will do two #2s and 3 #1s this comes to 6 gallons per day per person. The smaller units will have approx 3 people living in each one and the larger ones 4 people. This comes out to 6,570 gallons for the smaller units and 8,760 gallons for the larger ones... which matches the annual rainfall totals quite well. I realize that the summer months will be dryer but still, the tanks can hold 1,250 gallons so this should help buffer dry spells (a full tank will provide approx 2 months of flushing before going dry with no rain).

Has anyone here ever installed a system like this before? Some people tell me it's a glorious waste of time and money, not to mention the additional items that need to be maintained (pump, float valves, tank, etc). But it just seems like the right (and responsible) technology to integrate into a sustainable project. Am I nuts?

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I think it's a terrific idea. I think in some (or most) European countries nonpotable water is used for flushing toilets in urban settings. But I'm not sure where that water comes from, whether rain water from a roof collecting system or something else.

I remember reading a long time ago that most homes in Bermuda have water collecting systems on their roofs because water is scarce on the islands.

In the end, whether it makes sense for you to do it depends on the cost of construction and maintenance versus the savings in potable water not used, and how long the "payback" is, how many years of operating the water collecting system will it take before it begins to pay for itself. The big unknown is the changing price of potable water. It's sure to go up as the globe becomes more and more overpopulated and good clean water becomes scarcer.

The other factor you need to consider is how long you plan to own the property -- long enough to begin to reap the benefits of no-cost or nearly no-cost water for flushing? Since the system you are contemplating is probably rare, it is not likely to translate into an increase in resale value commensurate with the increased cost of construction.

These are the considerations that fall into the "narrow self-interest" category. In terms of the long-term general interest, your idea is admirable.

    Bookmark   May 5, 2009 at 6:49PM
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tcjohnsson, yes, youre a little nutty but thatÂs a good thing. The nutty ones make the world an interesting and fun place to live.

I like how you are thinking but it's full of many downsides in my opinion. First is having rain water going into the city sewer system is usually illegal. Water bills are based on the amount of water that goes through the water meter. Most of that water bill is for sewage treatment for the water you send back down the sewer. If you are dumping a bunch of rainwater into the sewer system, the city will be treating that water for free which I donÂt think they are going to want to do.

There are three classification of water. Tap water is classified as white water. Gray water comes from dishwashers, washing machines, showers or ground water. Black water is water thatÂs been in contact with fecal matter better known as sewage.

The water coming off the roof is definitely gray water and IÂm not so sure you want to be piping that into a residence. Roofs always have plenty of bird droppings, dirt from car exhaust, acid rain and many other contaminants. With your system, you will be pumping contaminated water into a residence. Even though itÂs in piping and a toilet, gray water can have odors and cause staining to plumbing fixture that might not be very pleasant. The water could also have some minerals that are not friendly to the pipes or the workings of the toilet.

I would think you would need some type of filtration and maybe even purification system to clean that water before you bring it into a residence. Now if you were talking about using this system for irrigation purposes, itÂs a winner but an expensive one and you would probably need bigger tanks.

I see youÂre in Hawaii with those active volcanoes. Volcanic ash washed down into a cistern can make it very difficult and expensive to clean out. You might need a way to shut down the collection system if you are near to an area with this ash.

Nice idea but I think the cons far outweigh the pros here. Not to mention that the cost to do this will be very chunky verses bringing in a simple city water line.

    Bookmark   May 5, 2009 at 9:22PM
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Agree with all what restoreguy says.

On money in my locale, you are talking about saving $21 in water costs per year for the smaller units. We pay another $30 for the sewer costs of that water.

Now as far as cost of the system. A conservative estimate is around $4000. I know it would actually cost more as I got an estimate for $13000 for a system for irrigation that had 3,400 gallons of underground storage. Your tanks would be cheaper and the piping would be cheaper but still - the control valves are not cheap.

So even at $4000 for the bargain basement system, it would take 200 years to reach payback. It would never last that long without significant maintenance and that neglects the time value of money.

In the US (at least mainland), 90% of our water use is wasted. I was just looking at sprinklers running despite rain this morning. We have zero issues with water that are not easily fixable in the mainland US and won't have issues for the foreseeable future. Parts of the world will but we will not.

Until we get serious with resource waste in this country, you are pissing into the wind .... and wasting resources yourself that could be used for other things that actually have real paybacks.... like insulation, solar hot water and sometimes, solar panels.

    Bookmark   May 7, 2009 at 8:14AM
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I think it is a good idea. With the proper filtering system you can use it to flush toilets and water the garden. Check out the planet green website. There is a show called Renovation nation that has shown many builds with people putting in rain catchmewnt systems. In the Alanta Ga. area they are promoting rain catchment as a way to ease up on the drought problems. There are compnies that sell the filtering equipment. I noticed in the new builds they labeled the pipes in the house with potable and non potable labels.

Go for it. water treatment and water bills are going up all the time. Whre I live in ohio they are raising the water and sewer rates to pay for sewer seperation. I am putting in 2 rain barrels this year to use the water for the garden.

    Bookmark   May 12, 2009 at 3:47PM
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Thanks for the responses guys.

haus_proud -

I do plan on keeping the property forever and pass it to my children (if I ever have any) when I die. Assuming steady inflation, it would take about 100 years or so to pay back the cost on these systems so this is obviously not being done for financial return. Unfortunately our city is run by inept morons that have no concept of promoting conservation. Sewer fees are based on 2,000 gallon chunks. The first 2,000 gallons per household is roughly $70 (water and sewer charges). The next 2,000 gallons is, now get this... $4.38. The next 2,000 gallons on top of the first 4,000 gallongs? Another $4.38.

My water-friendly units will likely consume about 2,500 gallons/mo (without rainwater going to toilets) so my water/sewer bill will be about $70. But I could TRIPLE my water consumption and pay about $80. Imagine that... reducing your consumption by 70% and reducing your water/sewer bill by a whopping 12%. Welcome to Hawaii politics.

restoreguy -

thanks, you woke me up. I'm located in Honolulu and while we don't have volcanoes nearby, we have other pollution to worry about. The project is located right next to a very busy 6-lane freeway, luckily upwind with our mostly prevalent tradewinds. You're probably right about the city not being keen on such a system, despite its obvious environmental benefits. Ironically, the person I spoke to at the city wastewater dept didn't know if it was or wasn't legal. So much help that was. The biggest concern is piping roofwater into porcelain toilets that can be sensitive to particulates. I could have some nasty looking toilets in short order. That wouldn't be good.

I'm looking at an irrigation-only system now. No water going into the home. Problem is that I don't think I will have enough plants to suck up all that water as the planting area around the property is very limited and I planned on serious xeriscaping throughout. The tanks are large when compared to the roof catchment area so I don't think larger tanks would make a difference. Any advice on an irrigation system? Should I just put the tanks underground and use them for storage of things other than water?

david_cary -

The cost is roughly $6,000 per system or about $24,000 for the entire set up. You're right about the money being better spent elsewhere but I already got those areas covered. The multi-family residence is 100% solar powered (13.5kW photovoltaic system + 320 gallon solar thermal system), utilizing high-SHGC windows, mechanically ventilated attic space, energy recovery ventilators in each room, high-SEER multi-split air conditioning, LED lighting, super-efficient appliances, green roof, etc, etc. Let's just say I'm trying to leave no stones unturned.

rjoh878646 -

I haven't checked out the website you referenced but I will shortly. I think the best option at this point is to use the tanks for irrigation purposes only. I don't think there is enough rainfall for both toilets AND irrigation. But the toilet consumption matched so nicely with the tank size and roof catchment area.

    Bookmark   May 17, 2009 at 7:10AM
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Many people in Australia are doing such a thing, if your layout would allow an above-ground water tank, you could simplify it greatly and have a gravity feed to the ground floor toilets, and maybe even not bother with upper floor, if you want to simplify.

The other benefit would be maintenance, remember with a tank particles will settle at the bottom so the water will be pretty clean. My parents used rainwater exclusively for years, however they were not next to a freeway.

Restoreguy did not read your post. I understood you said the overflow would go to the STORMWATER system, he read sewer system. You certainly wouldn't want stormwater going into the sewers.

You might want to read up on what's going on in Australia with rain water/grey water (something else you might consider) re-use, they do it a lot. Some bright spark had the idea of storing water in a modified eavestrough system for flushing toilets so it's all totally gravity-fed which is what you really want - the ecological footprint's a lot better than having to have pumps etc.

Irrigating using that water would make a lot of sense too.

    Bookmark   June 9, 2009 at 1:18AM
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The website below is the City of Austin's (TX) green building program rainwater collection information. Rainwater catchment systems are very popular in Austin and the surrounding areas and in some hill country locations rainwater is the ONLY source of house water. In our current house, we have a 1200 gallon tank that catches the rainwater off a modest 1100 sq. ft. roof and it easily fills up with a couple of inches of heavy rainfalls. My husband installed it himself with lots of buried underground PVC pipe, an old galvanized metal tank, and a Ditch Witch rental. With a small pump and solar panel trickle charging a 12V battery, it works great for irrigating the garden and fruit trees. Since that was successful, we decided to put in another catchment system in the house we are in the process of remodeling prior to moving in. At this house, however, we are using a catchment system for our house water and that added another layer of complexity for various reasons; size of house, number of downspouts, length of underground PVC, gutter and downspout screening, size of tanks (2 x 3000 gal.), multiple sediment filters plus a UV light for sterilization purposes, shed to house about equipment, etc. (You won't need a UV filter if the water isn't for potable uses.) Remember that for both these systems, you absolutely need a "first flush" mini-catchment system to divert the first water off your roof BEFORE it enters your main tank/cistern. The first rain will wash your roof of all the dust, bird droppings, leaves, and other detritus and you absolutely want to keep out of your main cistern because entering a cistern through a tiny manhole opening to clean out sediment is no fun. Therefore, you want to make sure to catch that first roof "flush" in a separate, smaller tank (that you empty or clean periodically) and not allow it to go into your cistern(s). There are various kits or plans for these types of "roof washers" or "first flush containment" systems available with different diverter options. Usually they use a ball valve that rises with the water in the smaller cleanout tank that will allow the subsequent rainwater to flow into your cistern. Some cleanout traps/tanks have a valve that slowly allow the first flush water to leak out in readiness for the next rainfall, but you still must plan on physically cleaning out the debris from the first flush tanks periodically. I can say that it is truly amazing the amount of rainwater one can gain from a catchment system. I believe it is possible, theoretically, to catch about 600 gallons per inch of rain for every 1000 s.f. of roof. I say theoretically because there will always be inefficiences in the system, still, it is quite amazing to go out during a rainstorm and watch the waterfall of rain flowing into your cistern (need a flashlight and a ladder, we have above-ground tanks). By the way, your cistern does not have to be immediately adjacent to your building. You can run pipe (above or below ground) to it from your downspout collection point(s) and as long as the opening into the cistern is LOWER than your gutters, it will fill up.

    Bookmark   June 10, 2009 at 11:43AM
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No, I read it correctly. If storm water is in the toilet tank and the toilet is flushed than storm water is going into the sewer which is a big no no.

    Bookmark   June 11, 2009 at 10:24PM
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I love how some people are always look at the cost effectiveness of something green. If we all did that it would be like living in the 80's forever. Many green projects currently have extended payback, but sometimes it just the RIGHT thing to do IF YOU CAN. I understand it can be a tough sell to the general public right now, but we need to change how we do things, otherwise future generations will suffer for our way of life.

    Bookmark   June 25, 2009 at 2:35PM
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Some of these posts concentrate on the COSTS and whether it's economically feasible. There are times that cost isn't necessarily the only consideration. You've already seen how the cost of oil has soared, which punishes the the less-able to pay. Fresh water is next. I applaud you for wanting to do your part in reducing the demand on fresh water.
I capture some rainwater which I use in my vegetable garden, but my system is pretty crude....a 35 gallon garbage can, a few paint buckets, a 120 gal cattle water tank.

    Bookmark   June 26, 2009 at 6:12AM
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Cost is a great equalizer. Too often people with an environmental "bent" forget that we live in a capitalist system where money means a lot. But lets be honest, many on the left are pretty hostile to capitalism.

His system may cost $24,000 because it uses 500 barrels of oil in the production of it. Is that green when the municipal water supply could give him 100 years of water for 1 barrel of oil?

Secondly cost is important because that $24,000 could be given to a low income family to say buy a Prius and scrap their FSP (aka SUV). Or beef up their insulation or replace an inefficient HVAC. See cost is important because it always can be compared to other things that can be done with the money.

While it isn't true that every dollar spent equals an equal amount of environmental degradation but it really can be used to compare different ways of minimizing the impact on the environment.

My personal situation would have given me the ability to put a geothermal system in the house we are building. In my climate and soil, it didn't make economic sense. The energy consultant came up with a $400 annual savings and the cost was around $30,000. Now - solar hot water does, better insulation does.... etc.

I do think factored into this is that the use of gasoline is so heavily subsidized as to pervert capitalism here. You really need to consider gas as costing about $10 a gallon.

    Bookmark   July 6, 2009 at 6:36AM
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