Struvite

Through a basic precipitation reaction, the majority of phosphorus in urine can be crystallised into a white, odourless powder: struvite (MgNH₄PO₄•6H₂O), sometimes also called Magnesium Ammonium Phosphate Hexahydrate (M-A-P). Struvite is a bioavailable, slow-release fertiliser; it is compact and can be stored, transported and applied easily, and does not smell (see the picture below). Through struvite recovery, over 90% of phosphate can easily be removed from urine.

Urine contains phosphate (PO₄) and ammonium (NH₄). When magnesium (Mg) is added to urine, phosphate, ammonium and magnesium bind and form struvite (MgNH₄PO₄•6H₂O), which can then be filtered out, collected, and dried into a powder (see the figure below).

Various magnesium-containing substances can induce the precipitation of struvite- each with different qualities and costs. Bittern, the liquid, which remains after salt (NaCl) extraction from seawater, is highly concentrated with magnesium (and other minerals). Therefore, in coastal areas where salt is produced, this is an ideal source of magnesium. Furthermore, because it is a liquid, the bittern is easily dosed and readily mixed with urine.

Magnesite rock (MgCO₃) can also be used as precipitant if it is treated properly. The rock powder is heated in a calcination kiln (analogous to lime calcination) to produce magnesium oxide (MgO), which dissolves in urine (untreated MgCO₃ will not dissolve). If no local magnesium source is available, other magnesium compounds (e.g. magnesium chloride, MgCl₂.6H₂O or magnesium sulphate, MgSO₄•7H₂O), may be purchased, though they are likely to be more expensive.

A struvite reactor can be easily built, using locally available materials and skills (see the figure below). The main tank can be constructed with conventional plastic containers (e.g. polyethylene, polypropylene, polyvinylchloride) or galvanised steel sheets. The stirring mechanism, as well as the supporting structure should be assembled from steel, which is coated or galvanised for rust proofing. Pipes and fittings should be made of plastic to prevent corrosion.

The reactor consists of a stirring mechanism, which is fitted inside the tank; a nylon cloth filter bag hangs below a valve to allow the main reactor to be drained (see the figure below). To start the process, the collected urine and magnesium are mixed for 10 minutes in the reaction tank. The valve is opened and the suspension is then drained into the filter bag. The filter bag retains the struvite while the effluent passes through. The filter bag is air dried for one to two days, after which point the struvite is ready to use. In field experiments, this type of reactor was able to recover over 90 % of the phosphate contained in the urine. Because struvite also precipitates naturally from urine, any precipitate in the collection system should be incorporated into the final product, in order to maximise nutrient recovery.

Struvite can be applied to fields just like any other fertiliser. In a simple granulation drum (see the figure below), the powder can be transformed into granules. In granular form, a fertiliser is easier to apply and does not cake in moist environments. Struvite performs comparably to diammonium phosphate (DAP) fertiliser and has the following advantages:

• Bio-available: The nutrients in struvite can be readily absorbed by the plant.
• Slow-release: Due to its low solubility, struvite guarantees a slow but steady nutrient supply.
• Highly pure: contaminants (e.g. pharmaceuticals or heavy metals), which may be present in the urine, do not precipitate with the struvite.

After the struvite is filtered out of solution, the same quantity of urine remains; this effluent still contains high concentrations of nutrients, particularly nitrogen and potassium. The effluent may be used in drip-irrigation for fertigation (fertigation is the combination of fertilisation and irrigation). Further nutrient recovery technologies are being developed to extract the remaining nutrients and complement struvite precipitation.