Compost with a Capital 'P'

A last spring, we wrote about a few ways we reduce the water used by our toilet.  Unfortunately, most of those methods still result in the loss of the valuable nutrients in our urine.  We've read a lot of anecdotes about using urine in a compost pile and how great it is for 'activating' the pile (e.g., page 149 of the Rodale Book of Composting).  But the assumption always seems to be that there is already a 'pile' to which the urine can be added.  What about all those budding homesteaders who might want to recover these nutrients, but don't have a place for a compost pile?  What about all those skinflints like us, who want to make use of all of the urine, not just enough to 'activate' a compost pile?  What about especially territorial people with a compulsion to deter would-be compost thieves by marking their compost as theirs alone?

It's possible to dilute the urine and use it to water plants, but because of urine's high nitrogen and salt content, we've got to be careful about using it that way too frequently, especially on potted plants.  (Although there is an interesting story about a sort of in situ urine composting experiment in Mexico City.) Similarly, if we had a fishless aquaponics setup, we could convert it to be pee-powered, but that would also be maxed out before long.  It's obviously possible to distribute it around the yard and count on rain to dilute it before we have to pee in the same place twice, but again, what about those poor blokes with an apartment and no yard?  (Don't apply the 'distribution' principle to the back deck--your downstairs neighbors will not forgive you no matter how many plates of cookies you bring them.)

In the interest of full disclosure, we confess that we now have enough space for full-size compost bins.  Can you guess which side has some of the chicken leftovers in it?

That got us wondering what would be the minimum-size compost pile that is able to absorb 100% of our pee, or, alternatively, how frequently would we have to make a new pile for a given pile size (e.g., a five-gallon bucket).  And now that we've spent way too much time playing with engineering calculations (this post was originally going to come out on Sunday, but we got lost somewhere in Nerdland...), we figured it was time to wrap up our theoretical composting, summarize the results here, and start practicing.

It turns out that a human produces (normally) 0.8 - 2 liters per day, of which around 95% is water, and the other 5% is made up of nitrogenous waste products like urea and creatine, and some inorganic substances like potassium and phosphorus.  The C/N ratio is on the order of 0.8:1.  The very high moisture content and very low C/N ratio make it difficult to balance a compost pile with a high fraction of urine.

A proper compost pile will have a moisture level of 50-60% and a C/N ratio of around 30:1.  Higher moisture contents limit the access of air to the pile and can cause anaerobic conditions, which lead to stinkiness.  The organisms that turn the raw materials into finished compost need a C/N ratio of about 25:1--too much higher and the pile takes forever to break down (and doesn't get hot enough to kill weed seeds or plant pathogens), too much lower and the excess N lets the bacteria make the pile too hot and they die, which in turn also leads to anaerobic conditions and stinkiness.  It's worth noting that C/N ratios closer to 35:1 will preserve more of the N in the finished product.

So, the golden question remains: how much urine can I add to various materials to make an optimal compost pile?  The calculations we need to do can be found here, but we wanted to (potentially) check out a lot of different combinations and have the ratios automatically calculated. We needed a spreadsheet.

We took the most recent version of Cornell University's compost spreadsheet and tricked it out a little to make it more user friendly (in our minds, anyway).  We imported a table of different materials one might want to compost (also from Cornell), and filled in some of the missing data with other info from the Internets. (Urine wasn't in the table? Seriously?)  We also automated the section on the first sheet to select materials from a dropdown list based on the table we imported (except for a few odd materials like pharmaceutical waste--for those you're on your own), and the moisture, carbon, and nitrogen contents are automatically populated.

We also added a feature to specify the volume of the pile desired, and added an 'error' box to let Excel numerically solve for the most optimal mixture of the ingredients selected.  So, even if it's not possible to get exactly the target moisture content and C/N ratio with the ingredients selected, you can use Excel's 'Solver' function to see how close you can get. (One caveat is that you might have to try multiple starting points when optimizing because numerical solvers can get stuck at local minima--i.e., there can be more than one composition that 'minimizes' the error.)  The spreadsheet can be downloaded here.

Ok, then.  What did we find?  Well, as we hinted above, we found that it's very difficult to balance a compost pile with a high urine content because of its high moisture content and low C/N ratio.  We were most interested in balancing it with just dry leaves or just wood shavings, since those are the ingredients of which we currently have an abundance.  But, if we balance the C/N ratio, the moisture content is too high.  And if we balance the moisture content, the C/N ratio is too high.

The first look--attempting to balance compost ratios with just urine and a high-carbon ingredient.  The 'Actual' moisture content and C/N ratios are the optimized compositions based on error minimization from Excel's Solver function. Note that oat straw gets the closest of the high-carbon materials.  So to those folks advocating straw bale urinals (and here), we say, 'touché!'  (Also, in hindsight, maybe labeling the second ingredient '#2' wasn't such a good idea for this post...)

However, if we add urine, a high-carbon material, and some food scraps, like would normally be found in a standard backyard compost pile, we can get a pretty good balance.  With the two main high-carbon resources on our homestead, we can mix up a five-gallon bucket with about a half-gallon of urine.  If we had oat straw, we could get up to 0.66 gallons of urine per bucket.

Second attempt, with the third ingredient being food scraps.  All of the high-carbon materials will work for some combination.  (Fresh) grass clippings still don't work because they're on the same side of the target C:N ratio as the urine and the food scraps. 

 
The upshot is that, if we wanted to keep a five-gallon bucket in the garage and do all our peeing there, we'd need a fresh five-gallon bucket every 1-2 days.  If we do half our peeing away from the home (e.g., at work, on the neighbor's bushes, etc.), we could extend that to 2-4 days, or about two buckets per week.  That would still add up quickly.  The other problem is that the compost won't get very hot (unless we insulate it really well) because a five-gallon bucket is too small.  That in turn means that the materials will take longer to break down, and we would rapidly amass a small army of five-gallon buckets full of ingredients about which our neighbors and guests probably wouldn't want to know, but would probably ask.  (Sorry for the bad news, apartment dwellers.)

On the other hand, if we had a compost bin at the recommended standard size of one cubic yard (202 gallons), we could handle 20 gallons of urine in the pile, which would keep us covered for 6-12 weeks. That's the same time frame as experienced compost chefs say will be required for finished compost with biweekly instead of bi-daily turning of the pile.  Thus, in theory anyway, a regular-sized compost bin should be able to supply one person with all the pee absorption they need.

In reality, that size pile can probably do more than that for a couple reasons.  First, we're adding the ingredients gradually over time, meaning that the early additions start breaking down and losing volume before the last batch is even a twinkle in our water bottle's eye.

Second, we haven't accounted for evaporation of water or nitrogen (as ammonia, NH₃), which can be significant.  The evaporation rate depends on a lot of things, including temperature, atmospheric pressure (to a small extent), relative humidity, wind speed, and surface area.  There are a number of models one can use to predict the evaporation rate for a given set of environmental conditions, but just for fun (nerd fun, anyway), we calculated the evaporation rate we'd expect just by diffusion (based on Example 1.2-2 here), the simplified model developed by Irving Langmuir for a different purpose, and the model NOAA uses to estimate evaporation rates of chemical spills.

Of the three, the diffusion model goes too slow, the Irving model seems way too fast, and the NOAA model seems about right, based on our intuition and love for the baby bear in everything.  (The calculations are also included in the spreadsheet if you are interested in channeling your inner nerd and/or checking our work.)  From the NOAA model, we would expect to lose about 0.5 pounds of water every 10 days from a five-gallon bucket (at a temperature of 68 °F and 45% relative humidity).  By contrast, these same conditions would lose upwards of four pounds of water per day from a 3' x 3' x 3' cubic pile, if the five sides not facing the ground are all included.  That assumes a very modest wind speed of 0.7 m/s, which is in the range that one sees inside a house just from natural convection (e.g. near a cold window or warm radiator).  Also, 68 °F is a pretty modest temperature for a compost pile, so the actual water loss rate is probably even higher.  (Heavy evaporative losses are also common in industrial compost setups.)  Rain on the compost pile washes some of the urine away into the surrounding area, which spreads out the urine burden even more.

Bottom line is, a standard 3' x 3' x 3' compost pile is conservatively about the right size to handle all the urine from one person.  If you want to calculate for your own needs, scale from there according to volume, number of people, and personal risk tolerance.


Do you compost your own pee?  How big a compost pile do you use?  What's your setup?  Let us know in the comments section below!