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Limits to Circular

Industry Matters Newsletter 16 June 2022

Mark E. Jones, PhD

I’ve frequently tested my limits and my wife’s patience in my quest to be more sustainable. Pulling something out of the trash she’s just thrown away because it is recyclable always gets at least an eyeroll. Recent reporting prompted me to explore a new type of recycling. Reducing reliance on industrially produced fertilizers seemed like an idea worth exploring to me. Only half of my two-person household agreed. My investigation reaffirms that just because you can do something, it doesn’t mean you should.

Reading about two University of Michigan professors “pee for peonies” urine diversion to fertilize plants struck me as impractical as I read it.  Not one comfortable with rash decisions, I decided to do a little research before dooming the idea.

Preparing for urine diversion to fertilze.

Nitrogen, phosphate and potassium are necessary plant nutrients.  The three numbers on the bags of fertilizer tell the percentages of nitrogen, phosphate as equivalent P₂O₅, and potassium as K₂O.  These three nutrients are also found in human urine.  Urine contains most of the nutrients we excrete, more than 70% of the nitrogen,  in excess of 50% of the phosphorus and about 90% of the potassium. Nitrogen, phosphorus, and potassium all  have natural cycles, cycles we’ve disrupted.  Modern agriculture depends on industrial inputs of all three key nutrients. 
 
Phosphorus is supplied to agriculture as phosphate from mining.  Same for potassium or potash.  Nitrogen is a chemical product, supplied as ammonia made from air.  All three key nutrients are the products of energy intensive processing.  By most accounts, ammonia production is the largest CO₂ emitter from the chemical industry.  The environmental concern doesn’t stop at production.  Fertilizer lost to runoff damages ecosystems.  Nitrogen oxides released to the air are potent greenhouse gases.  Modern wastewater treatment lets nutrients escape.  We end up literally pissing the key nutrients away.  The result is damage to watersheds. 
 
Fertilizers are globally traded commodities.  Russia and Belarus are major suppliers, now subject to embargoes.  Together the two countries account for about 37% of potash production, about 10% of ammonia and about 1% of phosphate.  They represent a larger fraction of the export market, causing concerns about shortages in many countries.      
 
Urine can, when recycled, address fertilizer production, supply chain issues, and damaging loss to the environment.  Urea is the second largest component of human urine after water, making up to 2.5% by some accounts.  Urea is an effective fertilizer, able to replace industrially produced ammonia.  Other nutrients are at lower levels, with potassium being about 0.6% and phosphate at 0.1%.   Life-cycle assessments demonstrate benefit of urine as fertilizer, but ignore the capital intensity and logistical issues. 
 
Urine comes with a bit of a yuck factor.  It isn’t completely warranted.  Thanks to the osmotic filtering in the kidneys, urine from a healthy human, while not sterile, doesn’t contain many dangerous micro-organisms.  Collecting and fertilizing diluted urine is touted by many.  Love and Wigginton, the intrepid Michigan professors, are not the first to recognize the potential in urine.  Community collection was demonstrated and shown beneficial in hay production, as one example.  “Pee for peonies” uses imported Vermont urine, leveraging an existing program and highlighting the logistical challenges of local collection.
 
My testing was decidedly less involved.  Urine diversion via a juice bottle proved easy enough, at least for half of my household.  In round numbers, I produced about 1.7 L of urine a day.  Focusing only on the urea, necessary for greening my lawn, I was collecting about 25 grams a day, using one of the higher, peer reviewed concentration estimates.  Dog urine, known to damage plants, and human urine are similar in composition.  To be safe, I diluted my urine 10:1 before using it around the yard.  I can buy a 50-pound bag of urea for $35.  At that price, my urine collecting efforts are worth about 4¢ a day in fertilizer.  I avoid a couple of cents of water use through avoided flushing.  I’m saving less than a dime a day.
 
My yard’s yearly fertilizer needs could be met if all household urine for a year were diverted.  Storage for at least 500 liters during the winter is not a pleasant proposition, something necessary given year-round production and seasonal use.  Urine releases ammonia when it degrades, causing a stink.  Treatment with either acid or base stabilizes the urine.  Some say sterilization is also required.  Urine storage and stabilization, even at the household scale, just doesn’t make economic sense and it is a lot of unpleasant work.
 
Ideally, the nutrients in my urine would be recycled to produce food I consume.  Those who have looked at the infrastructure requirements, from diverting toilets to the energy required to concentrate the urine to an economically applied fertilizer, conclude it is impractical.  Urine diversion can be done and is environmentally responsible, yet is difficult to implement economically at scale.
 
Recycle of urine, while certainly possible, has yet to reach the threshold of practical.  I also can’t tell any difference in my yard.  Fertilized and unfertilized areas are identical after my short test.  I’ve ceased peeing in bottles and returned to flushing.  Eye-rolling is now back to pre-diversion levels.