Towards a better tinyhouse

Inventing to freedom?

Total greywater recycling system (greywater to potable)

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A water recycling system that recycled every last drop of water, or very nearly, at least would be pretty awesome. Of course once you get to the level where water can be added by the rain and the (clean) water can be easily disposed of, there are going to be some diminishing returns. Nevertheless, let’s think about it – it might turn out that it isn’t that much harder to recycle all the water vs most of it.

If it was small enough, you could take it anywhere and still be able to take a shower, do laundry, cook, etc. Off grid power systems are well developed and well known, but off grid water seems to get a lot less attention. The usual approach is just rainwater, maybe with “recycling” some of the water to flush the toilet – so not exactly portable, and requires a pretty big roof (I estimate minimum 60 M2 for a single person in my area).

When I started thinking about this, of course you start reading about it, and there are just so many systems that clean water to various degrees. There’s usually primary, secondary, and tertiary treatment in the context of sewage treatment plants. Tertiary can mean different things in different contexts, but in the context of sewage treatment it means treating it past basic particulate removal (primary), past digesting the material with bacteria to reduce the Biochemical oxygen demand (BOD, sort of a measure of the amount of stuff present that is food to bacteria) (secondary), then removing the nitrogen compounds and some of the phosphorus with more bacterial digestion, or by adding various chemicals to precipitate the phosphorus, flocculate the particles, adjust the PH, disinfect, etc. That’s nowhere near the level where you can drink it, but sewage treatment lingo ends there. Anything beyond that is still a tertiary treatment method.

Then, on the supply side, the legal rules vary for what can and can’t be used in various situations (toilet flushing, irrigation, etc.) depending on where you live. The reality based rules are going to be based, of course, on bacterial and chemical contaminants. In municipal recycling there is a lot of concern that the standards used for deciding how safe water is, and what it would be safe enough to use it for, can’t list every compound known to humankind, and of course toxicology data is pretty scarce for a lot – nay, most – of the compounds in municipal sewage water, that may or may not make it past the various treatment stages. The water standards used right now focus on chemicals that were traditionally found and could pose a risk, in the water, when it came from rivers etc. rather than straight from the sewage lines.

Nevertheless, science certainly has some handle on these contaminants, like personal care products and pharmaceuticals (PCPPs), and like said in a previous post, municipalities can recycle the water if they want to. There are some docs about all this in the archive file. It is mostly the PCPPs and toxic stuff from cleaning solutions and industry etc. that are the big deal. I still need to read up more about this.

Anyway, you can see that at the end on the day, it can make a lot of sense to take a contaminant-based approach, rather than classifying water in broad categories, but it’s just that there are so many different contaminants to consider. Plus, sometimes it makes a lot of sense to say something like “Activated carbon can remove organic compounds pretty effectively”, even though it’s capacity to remove stuff will be different for different organic chemicals, and different types of activated carbon. You can also test for broad categories of stuff much more easily that specific chemicals, and of course saying that sewage is harder to treat and more dangerous and therefore belongs in a different category that greywater makes some sense (though they are more similar that I thought, I can tell you that).

Fortunately, things are simplified in a home treatment system, where, as I mentioned in a previous post, you can separate the water streams, on both the supply and disposal side, and there are other safety-enhancing factors. The main problem from a practical standpoint of building a home treatment system, especially one small enough to fit in a tinyhouse comfortably, is getting the parts, and monitoring the water quality, though the need for the latter is reduced by separating water streams.

I’ll put the ideas for a practical system in a separate post…


Written by gregor

August 14, 2010 at 19:02

Posted in Uncategorized

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