Using 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.
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.