Struvite recovery from urine in Nepal (STUN)
In many developing countries, nutrient recovery via urine separation can address both the need for a safe, domestic supply of fertilizer and the problem of inadequate sanitation. The STUN project (Struvite recovery from Urine in Nepal) – operated in co-operation with UN-HABITAT − examines whether it is technically and economically feasible to convert source-separated urine into a dry fertilizer in Nepal.
Struvite (MgNH4PO4∙6 H2O) is
a phosphate mineral with a high fertilizer value. Together with calcium
phosphate, struvite precipitates spontaneously during the storage of urine. However,
only 30 to 50% of the phosphate precipitates spontaneously due to the limited
availability of magnesium and calcium. By adding more magnesium to urine and
mixing the two in a reactor, all of the remaining phosphate in urine can be
recovered as struvite.
The struvite reactor features a cylindrical, galvanized steel tank with a tapered bottom. A nylon or cotton bag for struvite filtration can be connected at the bottom outlet (photo: Bastian Etter)
A feasibility study in the town of Siddhipur (near Kathmandu)
confirmed that there is a high potential for struvite production and use in Nepal.
Urine can be collected in EcoSan toilets (urine-diverting dry toilets promoted
throughout Nepal): the acceptance of a urine-derived fertilizer is high and
there is a strong demand for fertilizer. Projected costs for struvite
production depend strongly on the price of the magnesium source. The cost
calculations, however, should not only include the financial value of the
struvite fertilizer but also environmental and hygienic benefits.
Dried struvite granules (the diameter of the 1 rupee coin is 20 mm) (photo: Bastian Etter)
To prove the technical feasibility of struvite production in Nepal, reactors at different scales have been built and are operated with locally available materials. Bittern – a waste product from salt production – was found to be an excellent source of magnesium. More than 90% of the phosphorus in solution could be recovered in the form of struvite. After filtration struvite was dried and pelletized with low cost equipment. About 2 kg of struvite could be produced from 1000 L of urine, not taking into account struvite and calcium phosphate that precipitates spontaneously in the urine collection tank.