Most of the soluble nitrogen (>99%) in the digestate is present as NH4 + (58). Concentrations above 1500 mg/L (reached with co-digestion) are usually avoided since they can inhibit the digestion process. One could expect that NH4 + concentration would be lower in digesters fed with sludge from EBPR plants due to the formation of struvite, but the pool of NH4 + is too big compared to PO4 3-and Mg 2+ to observe a significant difference (57).