The phosphorus in the liquid fraction of manure or digestate (soluble phosphate) can be precipitated by adding magnesium. This creates magnesium ammonium phosphate, also called struvite. Struvite is a slow releasing P-fertilizer. Currently, the use of phosphorus recovery is mainly limited on waste water. Nonetheless, a lot of research is being done on phosphorus recovery from manure and digestate.
For example, in the EU project BioEcoSim, a technique was researched by Fraunhofer IGB that would make it possible to extract a maximum of phosphorus from pig manure and digestate, in addition to nitrogen.
Phosphorus recovery takes place by acidification of the raw manure slurry, whereby organically bound phosphorus goes becomes soluble (H3PO4).
After separation, the liquid fraction contains a larger proportion of phosphorus than usual and thus a low phosphorus solid fraction is produced.
After addition of a base to the liquid fraction, phosphorus salts precipitate, which are removed from the liquid fraction by precipitation.
After the project was finished, the project partners started to build a mobile unit for phosphorous recovery from manure. The pilot installations of the BioEcoSim project (100 L/h) was scaled up to 1 m³/h. The set-up of two containers made transportation to different farms possible. This allowed demonstration of the technique at different places but also optimization of the technique for different types of manure. This mobile unit is still available for testing.
Meanwhile, the BioEcoSim technology was sold to Suez, a wastewater treatment and waste management giant, who will distribute the technology in Europe.
A first full-scale plant implementing the BioEcoSim technique on manure is planned to be ready for use by the end of 2018 in Northern Germany. This location, close to the border with The Netherlands, also coping with a manure surplus, is not coincidentally.
Meanwhile, Wageningen University is also investigating a similar technique (the Re-P-eat system), where after separation of raw manure the solid fraction is acidified.
The liquid solution contains the soluble phosphate which precipitates as phosphorus salts after the addition of a base.
This technique is therefore distinguished by the application of acid to the solid fraction (Re-P-eat) in contrast to application to the raw manure (BioEcoSim).
More information about the research at Wageningen University can be found in a this report.