Sustainable airport cities

This TKI project is meant to showcase how different sectors can strengthen each other in the process of increasing the sustainability of their own sector – a win-win concept – through the application of innovative technologies originating in the water sector. The practical objective of the pilot project involves the recovery of phosphorus from Schiphol’s wastewater, so that it can be applied, without post-processing, as a fertiliser in the airport’s surrounding area.


A key component of this project involves selecting a technology, so that a number of different technologies were tested. One of the most important selection criteria was the quality, and therefore saleability, of the phosphorus recovered from Schiphol’s wastewater by the technology. In the end, a technology was chosen that produced struvite (NH4MgPO4.6H2O, or magnesium ammonium phosphate) in pellet form, because this bore the greatest resemblance to traditional fertilisers. However, the pellets produced were generally smaller (ca. 0.5 mm) than regular fertiliser pellets. During the pilot, about 700 kg of struvite was extracted from centrate (water from digested sludge) and from toilet wastewater from aircraft (faecal water).


In the past struvite from wastewater was considered a waste product, which excluded its useful practical application. However, given that phosphate ore is becoming increasingly scarce and that phosphorus is a finite resource, the government adjusted the legal and regulatory framework, thereby making the application of struvite possible. Among other instruments modified were the ‘Implementation Order of the Fertiliser Law’ and the ‘Implementation Regulations of the Fertiliser Law’.


The created struvite was analysed for its composition, contamination with pathogens and micropollutants. The crystalline component of the product extracted from centrate contains 96.8% struvite. The percentage for the product extracted from faecal water is 100%. The initial microbial measurements, using indicators, of the extracted struvite show that the product contains fewer pathogens than the materials it is made from. The product is however not pathogen-free. But since low concentrations of pathogen indicators were measured in the struvite, and humans and animals have little contact with it, the application of the product instead of animal manure does not represent an increased risk. The measured concentrations of micropollutants in struvite from centrate and faecal water are of the same order of magnitude, which is far below the legally maximum permitted values (Table 1 and Table 4 of Annex II of the Implementation Order of the Fertiliser Law).

On the basis of these results, we concluded that phosphorus recovery by means of struvite production at the Schiphol WWTP is feasible, even if on only a small scale. An important prerequisite for struvite recovery is the implementation of biological phosphorus removal at the WWTP. At the moment the main uncertainty concerns whether the phosphorus removal efficiency of 85% in the struvite reactor is feasible on a continuous basis. The cost savings resulting from the production of struvite affect the WWTP primarily, and are related to the reduction in the use of iron and the disposal of chemical sludge (iron phosphate).