Methane fermentation treatment of solids yields a digested sludge containing nitrogen and phosphorus in high concentrations. For the purposes of phosphorus load reduction of the water treatment system, anti-scaling of methane fermentation tanks and their surrounding apparatus, and phosphorus resource recovery, a system for positive crystallization and recovery of magnesium ammonium phosphate (MAP) from the digested sludge was devised. This system consists of a crystallization process, a separation process using the hydrocyclone and a low temperature drying process. The optimum operational conditions for MAP and the MAP drying conditions for a digested sludge containing solids at a concentration of several per cent were clarified, and a pilot-scale plant having a treatment capacity of 2m(3)/h was built and subjected to 16-month demonstration tests. For the separation of MAP using the hydrocyclone, it is necessary to suppress nucleation in the crystallization process. Based on the laboratory tests, it was found that MAP nucleation is suppressed and the crystal growth becomes predominant when a good fluidized condition of MAP is maintained in the reactor, the reactor is operated at a low supersaturation degree, and a suitable MAP crystallization loading per charged seed crystal amount is maintained. It was also found that, in the drying process, MAP can maintain its form without transition when the drying temperature is set below 70 degrees C. In the demonstration tests of the series of processes, the PO(4)-P removal efficiency in the crystallization process was above 85%, showing that the reactions proceeded well. Moreover, only about 10% of the reacted phosphorus was nucleated; that is, about 90% of reacted phosphorus grew on the seed crystal surfaces, and as a result, MAP could be separated well from the digested sludge. MAP recovered from the drying process had a moisture content below 0.1%, a purity of 92.6%, and a hazardous metal content below the standard values prescribed by the Fertilizer Regulation Act, and therefore, it could be confirmed that MAP has a high quality as a fertilizer product.