15. Composting Toilet Design-Build


In 1998, as part of a passive-solar super-insulated farm house I designed and built for organic farmers in western MA, I designed and incorporated the state’s first approved site-built composting toilet. I am happy to report that it has been performing well for more than a dozen years, turning both human waste and kitchen scraps into a nutrient-rich soil amendment.

NorthWest with Compost Chamber Hatches & Roof Vents

NorthEast Entry

To design this two-throned, two-compartment toilet, I had to research human waste volumes. This is what I found:


Human Waste Output

1/2 lb. feces / person – day   180 lbs. / person – year      3 cf / person-year

1 qt. urine / person – day     80 gal. / person – year      10 cf / person-year


Waste Volume Reduction

70% weight reduction through composting (20% of the mass of feces is bacteria)

solid waste volume reduces to 1 cf / person-year

(government sources suggest 2 cf / person-year for storage capacity)


Aerobic Composting

feces is 65%-80% moisture, 10%-20% ash, 10%-20% solubles, 5%-10% nitrogen, 20% bacteria

aerobic composting requires moisture content of 50-60%

aerobic composting requires fecal content of no more than 25% by volume

minimum 3x as much absorbent carbonaceous material must be added


Moisture Removal

(septic codes typically require no liquid discharge from composting toilets)

50% RH air @ 1 cfm can remove .38 qt/day

evaporation of 1 qt/day x 4 persons requires 10 cfm @ 50% RH

Supplemental heat (e.g. solar), or mechanical ventilation (in-line electric fan) will assist evaporation.  Thermophilic decomposition of waste is unlikely to produce sufficient stack effect movement of air to evaporate excess liquids and evacuate odors.


Quantities for 4 people for two years (MA Title V requires 2 years of storage capacity):

feces:  .5 lbs (.15 lbs dry)  x 4 x 730 days         =    438 lbs.

urine: 1 qt. (.15 lbs solids) x 4 x 730 days         =    438 lbs.

sawdust and toilet paper: (1 lb/day)                   =    730lbs.

1606 lbs.

@ 50 lbs/cf of finished compost                        =    32 cf

(1 cf / person-year reduced waste plus 3 cf carbon) x 4 persons x 2 years = 32 cf

Safety margin of 2:1 would require compost chambers of 64 cf total capacity

Utilizing two 64 cf chambers allows 2 full years of decomposition in each chamber while other chamber is in use.

                         

Compost chambers were built of dry-stacked, surface-bonded (inside & out) concrete block, sealed on the inside with two coats of UGL DryLok latex masonry sealer. They have a 14″ liquid reservoir on the bottom.

                                           

Chambers were insulated with 4″ of XPS foam board (R-20) and fitted with 12″ waste chutes, fly-catcher funnels and vents to bring indoor heated air down over the liquid chambers and evacuate moisture and odors through vent stacks through the roof.

                                            

The liquid chambers are isolated by plastic milk crates with heavy plastic mesh to separate solids and allow excess liquids to drain to the reservoir for evaporation. Insulated hatch doors are protected with plastic mesh on the inside.

6″ vent stacks through roof are capped with turbines to accelerate discharge, but passive stack-effect venting was not sufficient, so in-line fans were added to the main stacks operated by speed-control switches set on lowest speed.

Indoor “throne” with two seats (for alternating years), operable screened vent inlets, mason jar fly-catchers and boxed vent stacks behind.

The compost chambers were sized for a full year’s output for a family of four, but because composting is occurring during use, each chamber took 1½ years to fill, which allowed a similar 1½ year dormancy period for composting & decontamination before emptying.

IMG_8122Of course, a much simpler method to perform a similar function of turning human waste into soil is the traditional outhouse, in which the “soil” is captured in buckets to be emptied into adjacent compost piles (two piles allows one to be dormant).

IMG_8119IMG_8113

Outhouse Blues

By Juanita Nelson

Winter

Well, I went out to the country to live the simple life,
Get away form all that concrete and avoid some of that strife,
Get off the backs of poor folks, stop supporting Uncle Sam
In all that stuff he’s puttin’ down, like bombing Vietnam
Oh, but it ain’t easy, ‘specially on a chilly night
When I beat it to the outhouse with my trusty dim flashlight —
The seat is absolutely frigid, not a BTU of heat…
That’s when I think the simple life is not for us elite.

Well, I try to grow my own food, competing with the bugs,
I even make my own soap and my own ceramic mugs.
I figure that the less I buy, the less I compromise
With Standard Oil and ITT and those other gouging guys.
Oh, but it ain’t easy to leave my cozy bed
To make it with my flashlight to that air-conditioned shed
When the seat’s so cold it takes away that freedom ecstasy,
That’s when I fear the simple life maybe wasn’t meant for me.

Well, I cook my food on a wood stove and heat with wood also,
Though when my parents left the South I said, “This has got to go,”
But I figure that the best way to say all folks are my kin
Is try to live so I don’t take nobody’s pound of skin.
Oh, but it ain’t easy, when it’s rainy and there’s mud
To put on my old bathrobe and walk out in that crud;
I look out through the open door and see a distant star
And sometimes think this simple life is taking things too far.

But then I get to thinkin’, if we’re ever gonna see
The end of that old con game the change has got to start with me.
Quit wheelin’ and quit dealin’ to be a leader in any band,
And it appears the best way is to get back to the land.
If I produce my own needs I know what’s going’ down,
I’m not quite so footsy with those Wall Street pimps in town.
‘Cause let me tell you something, though it may not be good news,
If some folks win you better know somebody’s got to lose.

So I guess I’ll have to cast my lot with those who’re optin’ out.
And even though on freezing nights I will have my naggin’ doubts,
Long as I talk the line I do and spout my way out views
I’ll keep on usin’ the outhouse and singin’ the outhouse blues.

nelson_oldercouple

Wally & Juanita Nelson at their Hand-Built House on Woolman Hill MA

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If you need project consultation or design services, contact me directly at

HouseWright (at) Ponds-Edge (dot) net.

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by Robert Riversong: may be reproduced only with author attribution for non-commercial purposes and a link to this page
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6 Responses to 15. Composting Toilet Design-Build

  1. Fred Heier says:

    Like your composting toilet. Do you sell plans along with a supply list for needed supplies for a DIY builder

    • Riversong says:

      I have a set of schematics, including the plan and elevation layout (and materials list) for the blockwork, and a floor plan and two detailed and labeled cross-sections that I can send in either paper or digital format, along with human solid & liquid waste data, for $100.

      Obviously the user would be responsible for determining the legality of a home-made composting toilet in their jurisdiction and for the quality of construction and usability.

      If you don’t have considerable building expertise, I would not recommend tackling such a sophisticated toilet design as this one.

      Those interested can contact me at HouseWright (at) Ponds-edge (dot) net.

      • Jon Moonsamy says:

        Hi this is Jon in rural Namibia. Am helping a family in the village to install a toilet for a family of 10. This is a community upliftment project. Would you please consider donating your design to contribute towards this cause or perhaps consider a subsidized fee.

      • Riversong says:

        Jon,
        I posted my design so that anyone could use it, but it was designed for inside a cold-climate superinsulated house and may not be appropriate for another climate or another culture. It also required an inline exhaust fan on the vent “chimney” for each chamber in order to eliminate odors, insects and excess liquid accumulation (you might want to incorporate a urine separation system). Here are the construction drawings: [sent by email]

  2. Arthur Atkinson says:

    Robert, Thanks for all your work figuring out low energy housing over the decades and thanks for posting all the results online. Are there any changes you or the owners of your site-built composting toilet would recommend? Any problems the owners have had?

    • Riversong says:

      The only problems were the ones alluded to in the article and anticipated in advance for easy remediation. Excessive liquid accumulation, some odor infiltrating into the toilet room, and tiny baby flies exiting through the intake grills – all were eliminated by the installation of in-line fans, which were already wired to a switch box in the toilet room for variable-speed controls (the lowest speed was sufficient to eliminate all three problems).

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