Odorless Fertilizer from Household Urine

103_9206Using an analysis of urine and  some basic arithmetic,  I realized yesterday how many useful compounds we unfortunately flush down the toilet.  Based on the fact that an individual’s daily output of urine contains about 3 g of potassium ion, 2.5 grams(g)  of phosphate,  16 g of urea, 3 g of amino acids, a gram of ammonia,  a city of 3 million like Montreal, wastes 75 000 kg of potential fertilizer every day, none of which is recovered by sewage treatment.

Luckily there is a solution. At the elevated pH of aged urine, which will also have more ammonium, the latter ion and phosphate will not dissolve in the presence of magnesium ion. If we add more Mg²+ we can create far more of a precipitate known as struvite. The product is odorless, free of heavy metal contaminants and releases ammonium and phosphate ions slowly into the soil, whose pH is lower than that of aged urine.  Over a temperature range of 25 to  35 oC, 169 to 213 mg of struvite dissolve per liter at pH = 7. The Swiss Federal Institute of Aquatic Science is actively involved in building STUN reactors to recover struvite, but I had to try it myself on a small scale.

The easiest way to do it at home is to use Epsom salts. Every 500 L of urine can potentially produce 1 kg of struvite.  The typical trip to the washroom produces 0.150 L  of urine, which proportionally can yield  0.3 g of MgNH4PO4.6H­2O (struvite) or 0.3/245 = 0.00245 moles. Epsom salts are pure hydrated magnesium sulfate MgSO4.7H2O (246g/mole), and a one to one molar ratio, implies that 0.603 g of  the magnesium salt are needed.

With time, the urine’s pH climbs because urea, urine’s 2nd most common ingredient, breaks down into ammonia, which dissolves in urine’s primary ingredient to produce ammonium hydroxide(NH4OH). At an elevated pH, MgNH4PO4.6H­2O  is poorly soluble, which makes recovery possible. Hoping to eventually form larger crystals, I added the Epsom salts while  the urine was fresh so that the struvite would precipitate gradually. As expected no crystals were apparent without the formation of ammonium.

Struvite crystals, six days after the addition of Epsom salts to fresh urine. The white crystals are at the lower edge of the mason jar.
Filtered struvite crystals, seven days after the start of the experiment.

It took between 15 and 34 hours of aging  to get the initial crystals, a time that was probably elongated by the fact that the mason jar of urine was kept in a cold storage room. More crystals formed during the week. I checked the pH of the urine 7 days after first adding the epsom salts, and it had gone alkaline.

To make sure I was not getting magnesium sulfate instead of struvite crystallizing (unlikely since it’s soluble in ocean water), I added baking soda to a magnesium sulfate solution and obtained no precipitate. For an additional control I added baking soda, magnesium sulfate to filtered aged urine and again obtained no crystals.

The cost of Epsom salt is about $ 3.00/kg .  One kg of Epsom salt will produce 0.3/0.603 = 0.5 kg of struvite; so it would cost $6 to make a kilogram of  struvite, which exceeds the cost of commercial fertilizer , and the latter also has the advantage of including plant-required K+. Magnesium carbonate, is cheaper, and can be more easily converted into soluble MgO, but the process releases carbon dioxide into the environment. So the most ecological and economical source of Mg2+  would be bittern, a magnesium-rich liquid that remains after salt (NaCl) is extracted from seawater.

You may be unaware, as I was, that struvite is found in a type of kidney stones. Unlike oxalate or uric acid stones, the struvite -variety sometimes form after frequent urinary infections. The bacteria responsible for the infection hydrolyzes urea to ammonium, raising the pH and mirroring the precipitating effect used in the recovery of struvite from aged urine.


 Other Sources:
  http://oatao.univ-toulouse.fr/4645/1/Hanhoun_4645.pdf ( a detailed study of the solubility and thermodynamics of struvite )


Ammonium Nitrate and Ignorance: A Deadly Combination

Ammonium nitrate, NH4NO3, was used as a fertilizer before people realized that it was potentially explosive. And to this day people who should no better are still unaware of its dangers. Between 1911 and 1921, a German company BASF was producing a mixture of ammonium sulfate and ammonium nitrate in the town of Oppau . The latter easily attracts water, which turns the whole concoction into a hard plaster-like substance. Since the fertilizer was stored in large silos, it was difficult to package the product without first breaking through the hard crust. Someone had the idea of using small amounts of dynamite to convert the ammonium nitrate back into powder form. Because the ammonium sulfate is not explosive, the technique worked without incident for years, but because the Germans were running out of sulfur, they were gradually producing less ammonium sulfate while increasing the concentration of ammonium nitrate. On September 21, 1921, the dynamite not only broke through the crust, it decomposed 450 000 kg of NH4NO3, causing a massive explosion that killed nearly 600 people and injuring 2000. Eighty percent of the homes in Oppau were destroyed, leaving 6500 homeless.

The equation representing the explosion is

2 NH4NO3(s)  2 N2(g) + 4 H2O(g)+ O2(g)

Two solid molecules produce a total of seven gaseous molecules, causing the sudden increase in pressure responsible for the explosive character.

But upon close inspection, the reaction is bizarre. How could an oxidizing agent like nitrate(NO3) produce an even stronger electron-thief like oxygen? What’s going on?

The equation actually represents an overall reaction, which hides the short-lived sequence of ammonium nitrate’s decomposition.

First NH4NO contains both a reducing agent(NH4+) and an oxidizing agent(NO3). With the proper activation energy, the fertilizer will decompose into dinitrogen monoxide (nitrous oxide or laughing gas) and water.

NH4NO3(s)N2O(g) + 2 H2O(g)

Although the following is not be the actual mechanism, its electron transfer can be rationalized as follows:

H2O + 2 NH4+ N2O(g) + 8e- + 10H+                 (reduction half reaction)

2 NO3 + 10 H+   + 8e- N2O(g) + 5 H2O      (oxidation half reaction)

(1) Overall : 2 NH4NO3(s)2 N2O(g) + 4 H2O(g)

 In fact, this is how laughing gas is produced. But when temperature exceeds 240oC, N2O(g) decomposes

(2) 2 N2O(g) 2 N2(g) + O2(g)

If you combine (1) and (2) , the net equation is

2 NH4NO3(s)2 N2(g) + 4 H2O(g)+ O2(g)

texasSince Oppau, there have been more ammonium nitrate disasters including the one in Texas City, the deadliest ever . It occurred aboard a ship carrying the same fertilizer, which was stacked, further increasing pressure for the gaseous products. One of the explosions projected an anchor and a ship’s propeller a mile from the source. Over 500 people were killed and over 5000 injured.

westExplosionThe most recent NH4NO3(s) accident occurred last year(2013) also in Texas, in a town called West. It produced 14 casualties and injured 226 as the blast broke windows 7 miles from the site.
A year later according to the US Chemical Safety Board’s Preliminary Report the fertilizer was not stored in an area with sprinklers, and the volunteer firemen who arrived on the scene did not realize ammonium nitrate is explosive. The West community itself was unaware of the storage area’s potential dangers and had no emergency plan in place.


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