Cistern under my house - bad idea?

Here in northern California we get essentially all our rain in the winter and none in summer. If I were ever to have the funds to build a home I’d like to make it water efficient. My idea is to have a grey-water cistern under the house or under the yard and an elevated water tank. A windmill would pump water from the cistern into the tank. The tank would supply water for flushing toilets, watering the yard, washing the car. Water from the kitchen and toilet would flow to the sewer as usual, but water from the bathroom sinks, showers, and washing machine and rain from the roof gutters would go into the cistern.

Please shoot me down and tell me why this is a really bad idea.

Putting used water in the tank would be a bad idea. Collecting rain water is fine and the traditional water used in a cistern. Grey water not good for plants either.

Water stored can have a problem with biological growth. Gray water would have a bigger problem with it.

Yes I know.

Two issues - first, how are you going to get the cistern in the basement? A lot simpler to put it in the yard, where you can use heavy equipment and any repairs or leakage issues can be fixed. Locate it away from the house and reduce the risk that your first indicator of leak is water in the basement.

Similarly, pump failure can also result in overflow into the basement. Depending on the sooil, would that water saturation also endanger the integrity of foundations?

Then the bio-growth issue- better to have an odor outdoors than indoors.

Finally - will your town allow it? If you are on the local water supply, they may restrict watering during droughts. “But I’m using grey water” you say. So what? If Fred down the street wants to have a nice lawn and flowers, and doesn’t mind paying for the water, he just runs the kitchen tap until the grey-water tank is full enough to water. How do you guarantee the grey water is legitimately and not wastefully greyed out? They MIGHT let you get away with a totally separate rainwater-only system, but still - how do you stop someone from running the garden hose into the tank at night? And how do neighbours know that they don’t have to report you because your system is legit?

Install a discrete water seepage system for watering; but even then the greenery will be a giveaway.

As to whether the water is sfe for plants - depends what you wash down the sink? If you are a heavy soap user, it may not be good; ditto detergents. Perhaps to clean the system out, you may have to drain the tank and hose it out from time to time - which means either carefully regulating use, or flooding the neighbours or storm sewers. Plus - what is grey water? Obviously not toilet waste; but do you wash cloth diapers in the sink (after initial cleaning) or anything else disgusting?

then you have to repipe the waste system to separate the grey (bath and sinks, aloundry…) from toilets. A lot of work.

The devil is in the details.

It is also worth checking local water right laws. I live in Colorado and legally I am not allowed to collect water that falls onto my land. Incidental collection is ok, like whatever may seep into the ground or is absorbed by grass and trees. But runoff, no. The rights to the water that falls on my land is owned by others (though I’m not actually sure who).

Likewise, it may be illegal to keep and reuse greywater. Your water supplier may require that greywater be returned. Sewer systems depend on a certain amount of water to ensure the proper flow of solid and semisolid wastes. And they may also have requirements to pass on the treated water to downstream rights holders. Hereis a reprint of the current California laws, but your local government can and like does have additional regulations.

Collecting rain from the roof in a cistern under the house is (or was, anyhow) very common in Bermuda.

Cistern under the house? Put the brethren down there as well!


I’d build the house over the cistern. No basement - they’re rare here anyway.

I’m sure all of this would be in violation of local ordinances. It’s just a fantasy. Though, unlike in Colorado, I don’t think there are any prohibitions here against capturing rainwater.

I guess soap and shampoo might be a bad idea in the cistern, though I hate the idea of sending that water into the sewer. Maybe there are some lawn-friendly soap products we could switch to.

If you have enough flat land, you can treat gray water using cattails. The water slowly flows down a wide shallow ditch filled with sand/gravel with cattails growing in it, and emerges at the far end clean enough for irrigation use, toilet flushing, etc.

I live in a dry climate. Typically under your house would be the sweet water cistern, which collects the rain water from the roof. Your toilet runs into a septic tank with adjoining soakaway tank. Showers and sinks run directly into the soakaway which is filled with large stones. Feeder pipes run underground from this into the garden where you’ve typically planted bananas, sugar cane etc. This is the only use of the grey water here. Builders advised me that putting the sweet tank (concrete) out of the ground would cause cracks - it likes the support of the earth around it. I use about 1,100 gallons a month from the government water and so most of it ends up in the garden.

I’ve set up my washing machine pipes so I can unhook it from the soakaway and direct the water to other parts of the garden.

Quite a few of the hotels here have small water treatment plants, but use the water for the grounds and not for toilet flushing etc. I’d say it’s going to be very expensive to deal with purifying the water to any degree.

I like Kevbo’s suggestion.

As someone already pointed out, the devil is in the details. It is not a bad idea per se.

In Venezuela, large cisterns under the house are the norm. My parents have what is essentially a complete basement that is a cistern and holds something like 20K gallons. They get water about 2-3 nights a week and hardly even notice except for when the pump is quiet when there is pressure from the street.

This is a cistern that holds tap water and is used throughout the entire house. No effort to recover it or be green about it. Just a way to deal with irregular water supply. No reason why it wouldn’t work with collected rain water.

The cistern needs to be cleaned (emptied and scrubbed clean) about every 10 years and there is no need to chlorinate it since it comes chlorinated from the street. Maybe if you are collecting rain water you would have to add some chlorine to it. Again, devil, details.

Definitely an idea worth exploring. We are on the planning stages of our future house and a large cistern to collect rain waters is definitely in the cards.

If you are building the house too, then you can do all the piping necessary to capture grey water. Sufficiently dilute, I’m not sure that even soap is that bad an idea. Perhaps you can add some acid or something to neutralize it. To get real fancy, you can have a pH meter controlling a gizmo that adds a dilute acid, ha ha. Even fancier is an electric valve to divert water depending on the load. (For really soapy, send to city drain.)

I wonder about even just grey water and cleanliness; what sort of aroma venting from it, how to get in to clean it, etc. Plus, during building is a bad time if the city inspector is anal and the bylaws make it difficult. They may still classify it as septic tank even if it is grey; definitely not allowed INSIDE the house.

If ever you need to do any work on it, outdoors is better unless freezing is a real problem. Even worse if you only have a foot or 3 of headroom in a crawlspace while getting in there. Plus ventilation - in any enclosed space, you want to be sure you don’t pass out and drown from the fumes while scrubbing it down or whatever. I recall an industrial accident where I worked where bad gas pooled in the bottom of the tank, and 2 guys drowned in serious acid while the 3rd went for help instead of climig down like the second guy.

My little victorian cottage was built about 135 yrs ago and there is the original cistern built under the house as part of the foundation, actually.

The home inspector we had in, when we purchased, said it would stand another 100yrs, easy, was surely water tight, and only needed a couple of pumps and an eavestrough redo, to set up. We priced it out to be under $2000. Not too bad.

Now we had no intention of introducing grey water, just rain water. For washing dishes, clothes, showers, watering lawns etc. We haven’t gotten around to it yet, but still plan to one day.

Just curious whether people get into the situaton where they disallow watering from rainwater cistern during drought “no watering” situations. After all, it’s hard to tell whether the water is realy rain water or came from the city system (or ground water pumped). The assumption would be that by the time a watering ban goes into effect, you should have no eain water left.