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Constructed wetlands for greywater

Constructed wetland diagramAlthough greywater will probably continue to flow to a mulch basin in the winter, you risk drowning dormant trees.  Instead, the system that looks most efficient for greywater treatment during temperate winters is a constructed wetland.

In general, it's much more sanitary to filter greywater through soil than to allow the water to pool on the surface, so most constructed wetlands for treating greywater are more like bogs than like ponds --- in other words, there's no standing water.  Greywater enters the system through a drain at the top, then percolates down through gravel and sand until clean water flows out the bottom (possibly into a pond).

Large constructed wetlandIn cold climates, Create an Oasis with Greywater recommends planning about 1.5 to 3 square feet of wetland per gallon of greywater produced each day.  For best results, the width should be about half the length, ensuring that water has to travel a good distance before treatment is complete.  Since I estimate we use about 20 gallons of water per day at the kitchen sink, we'd need a roughly six foot by ten foot wetland to delete our swampy drain out back.

Constructed wetlands get around the issue of waterlogged soil by using aquatic plants that are well suited to the environment.  Ludwig doesn't suggest any particular species (noting that selections are region specific), but I suspect cattails, horsetails, sedges, water plantain, and sweet flags would do well in our region since they live in nearby swampy ground.  Perhaps we could even cut the leaves once a year to create mulch or deep bedding for the chickens?

Tub wetlandsFixing our greywater system isn't a top priority, but I suspect it'll make the list sometime in the next year or two since our new porch brings traffic to an area that used to be hidden by weeds.  I'm going to hang onto Create an Oasis with Greywater to refer back to once we hit the planning stage for that project, and meanwhile I highly recommend the book for anyone who wants to build a sustainable greywater treatment system.  Be sure to leave a comment if you've created a greywater system different from the ones described in this week's lunchtime series --- I'm always interested to see other DIY options.

Start a no-till garden, try out chickens, and learn to cook with seasonal produce in Weekend Homesteader.



This post is part of our Create an Oasis with Greywater lunchtime series.  Read all of the entries:





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Anna - does he cover how to build a house to make future greywater systems easier to install? We have house building in our future, and probably want a dual system for several reasons - with greywater to be added later.
Comment by De Fri Aug 17 13:13:21 2012

You don't have to go and get plants for your wetland, they will find their way there in time, hitching a ride in bird droppings, wind, or other mysterious ways. You can jump start the process by going to a local wetland and taking a shovel of soil home to yours. For my drain basin, I just put a shovel of a local swamp soil in the middle and cattails sprung out of it. I imagine that many local bacteria and fungus hitched a ride as well.

Comment by Eric in Japan Fri Aug 17 18:44:11 2012

When looking at the sketch, I wonder how you are to keep the surge basin/grease trap from freezing in the winter?

Unless you bury the whole thing beneath the frost line, which sounds a bit extreme. :-)

And 20 gallons/dag seems like a lot. My average use over a whole year is around 11 gallons/dag, and that includes showers, flushing the toilet and running the washing machine, not just the kitchen sink.

Comment by Roland_Smith Fri Aug 17 20:49:20 2012

De --- I skimmed the fancier sections, so I'm afraid I can't answer that in depth. But he does go into plumbing for dual systems that can be swapped over to septic, etc. It's definitely easier to build from scratch, figuring out what system you want first, than to retrofit. (Although you may have to build the system to code, in that case, which might be a nightmare. There's a whole separate book by the same author that covers that.)

Eric --- I really liked that part of your post about (not) seeding your system, and I think I'll follow suit!

Roland --- We wouldn't be constructing one with a surge trap. I just used that sketch from the internet because I liked it otherwise and was too lazy to draw my own. :-) Surge traps are actually a bad idea in greywater according to the author of this book because they hold water too long, which makes it go anaerobic (thus, creating bad smells).

The twenty gallons I estimated is mostly for dishes and washing vegetables. I was just guessing wildly, but I think our sink holds about five gallons in each basin. (Although, now that I think of it, I suspect we only fill each basin about a third of the way most of the time? Maybe my estimate is way off. :-) )

Comment by anna Fri Aug 17 22:28:43 2012
i know that the gravel used for a homemade rainwater filter needs to be cleaned regularly. does the gravel in this need to be cleaned regularly as well?
Comment by robin Mon Dec 24 08:12:03 2012
Robin --- Excellent question. I think you're right that you'd need to dig out the gravel and wash it or replace it at intervals, which does make a system with gravel look much less enticing.
Comment by anna Mon Dec 24 11:55:54 2012