Towards a better tinyhouse

Inventing to freedom?

Greywater to ‘potable’ or nearly potable water recycling system

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The mobile condo blog, has a post up on their greywater system.  I’ve left a comment, but I want to make some less on-topic remarks on my own blog here, now that I have been reminded of the topic.

The system shown has a supply of rainwater available from the relatively large roof, so it does not need to recycle all the water.  Recycling even 50% of the water might be enough.  (Edit: apparently this is wrong, the system is intended to recycle most of the water, but in most climates, by my calculations, I seem t remember you might get away with 50% recycle rate) This simplifies things in many ways compared with a total water recycling system.  First of all, you don’t worry about certain dissolved contaminants accumulating in the system.  Second of all, you have some free water with which to backflush the particulate filters (although an air scoured/cross flow filter like used in membrane bioreactors could be even better, and as I mentioned in a previous post, obtaining a suitable membrane or two might be doable,) since you were going to throw it away anyway.  Plus, the rainwater can be used as drinking water, and the system’s excess water could used for evaporative cooling or watering plants.

You could also use it to flush the toilet, though you might have too much of it to use it all. 

BTW, I just want to take a moment to gripe here about the definition of “potable.”  Usually shower, sink, water for dishwashing, and drinking are all lumped together in this one category. When it comes to bacteria and other pathogens, obviously you want the water for all those applications to be pretty much free of any pathogens.  But when it comes to chemical contaminants, this categorization makes no sense at all.   Suppose you accidentally ingest 20 ml of water every day as a result of the shower and other non-drinking water, but still human-contact related, water uses.  Most people drink 1.5 liters a day or so of water, so that’s 75 times as much water.  Clearly what you care about is the total amount ingested from all these sources, and given the difficulty of removing chemical contaminants, it would not make sense to clean the non-drinking water so well.  It is okay to shower in water you wouldn’t drink, people do it all the time.  You can shower or swim in seawater and untreated river water.  On a municipal scale this might make sense, because it becomes pretty cheap to remove contaminants, probably cheaper than a dual-supply plumbing system.

But, as far as I can tell, almost all municipalities and regions in developed countries have explicit rules against using “wastewater,” including greywater, for any “potable” use.  So that’s why there are no commercial systems available that clean the water up to a suitable level for showering etc.

Lastly, I want to mention something I read about in a NASA paper, which is the possibility of segmenting the different water sources a bit more finely. We already see 3 categories of supply water here in this post: Toilet flushing, showering and other human contact but not drinking, and drinking water.  A fourth is obvious: irrigation, which can be done with untreated (not stored for any length of time) greywater.  There are also many categories of wastewater, shower water and water that comes from the kitchen not the same (esp. if you insist on putting food down the drain), sometimes these are referred to as “light grey” and “dark grey” (laundry is usually “dark grey” too.)

But you know what?  It makes much more sense to look directly at the different types and amounts of contaminants in every different water stream.  Laundry machine water, for instance, might have more and different types of detergent in it, than might kitchen water.  And water that comes from the dishwasher in a kitchen will not be the same as the water from the sink, and water from the sink is going to be a lot cleaner if you do not put food down the drain on purpose.

So if you are going to design a small scale treatment, supply, or recycling system, it might make sense to think this way.  Anyone reading this might be aware of the problems associated with mixing greywater and blackwater.  If you do that, now you pretty much have to treat all the water to the same high degree as the original blackwater before you can use it or dispose of it.  That makes the system a lot bigger and more expensive, but the same principle could be applied to other water streams, whether on the supply side or the wastewater side.

On the other hand, having a separate treatment system for all these streams might not make sense, you may be able to combine them at certain points to simplify the system.


Written by gregor

August 10, 2010 at 16:13

Posted in Uncategorized

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