The biogas plant of Groot Zevert Vergisting (GZV) in the Netherlands produces biogas out of pig manure and residues from food and feed processing. Disposal of digestate is problematic due to the surplus of manure in the region of the plant. GZV therefore decided to invest in the development and implementation of the first RePeat (Recovery of P to eat) installation to extract phosphorus from the solid fraction of digestate. This RePeat installation has been built and has beens fully operational since February 2020. This is a full scale installation which produces a calcium phosphate and an organic soil improver with a low P content from the solid fraction of digestate. The soil improver can be used to increase the organic matter content of sandy soils in the region of the plant. Alternatively, it can be upgraded into a peat replacement for the potting soil industry. The calcium phosphate can be used as a raw material for fertilizer production.
The installation has a treatment capacity of 17 kilo tonnes of solid fraction of digestate per year. This is an amount equivalent to 140 kilo tonnes of unseparated digestate. The solid fraction is obtained from the liquid digestate by means of a decanter. The liquid fraction of the digestate is further processed into NK concentrate and clean water. The RePeat process separates the P from the organic matter through extraction with water and sulphuric acid followed by precipitation of released P through addition of calcium hydroxide or magnesium hydroxide. The extraction is performed in two sequential leaching steps during which 70–90% of the total P is removed from the initial solid fraction. The precipitated calcium phosphate is recovered in a settling tank. Process water is continuously reused in the process. Thereby a waste stream is prevented.
In the following months the installation will be further optimized in terms of chemicals addition, process conditions and product quality. Also, tests will be done to compare the performance of different bases, resulting in different types of P fertilisers such as calcium phosphate and magnesium ammonium phosphate (struvite).