Amsterdam’s sewage contains phosphate, an expensive and scarce fertiliser. Waternet has been commissioned by the Amstel, Gooi and Vecht Water Board to extract phosphate from sewage on a large scale. This results in annual cost savings of €400,000. The phosphate harvested from Amsterdam’s waste water can fertilise 10,000 football fields annually. There is also a special collection point for pure urine from popular musical venues and events with waterless urinals.
Phosphate stocks depleted
The phosphate harvested from waste water is converted into struvite (a fertiliser). Phosphate is an expensive fertiliser that is currently mined in countries like Morocco and China. These natural sources are expected to become depleted within 50 years. No crops can grow without phosphate and, given the growing world population, it is extremely important to reuse existing sources.
How does it work?
The end-product following the purification of sewage is sludge. Firstly, biogas is produced from this sludge. The phosphate is then extracted. This takes place in three large tanks. The sludge is first briefly aerated and then magnesium chloride (a harmless salt) is added. This results in the production of struvite, which is heavier than the sludge and thus sinks to the bottom. The struvite is drained from the bottom of the tanks, rinsed, and is then ready for use as a fertiliser.