A process and an apparatus for efficiently recovering magnesium ammonium phosphate (MAP) crystals of high purity and good quality in the technique of removing phosphorus and the like as MAP crystals from wastewater containing high concentration organic substance, phosphorus and nitrogen such as a digested supernatant liquor of human waste sewage and septic tank sludge, a digested liquor of sludge, chemical plant wastewater, and simultaneously recovering the MAP crystals. The sludge treatment process includes treating a sludge mixed liquor formed in the biological treatment system for organic wastewater in an anaerobic digestion tank to effect the digestion of sludge, simultaneously adding a magnesium source to the anaerobic digestion tank to allow crystals particles of MAP to form and grow in the anaerobic digestion tank, withdrawing a sludge mixed liquor containing the crystals of MAP from the anaerobic digestion tank, separating and recovering solids containing MAP crystal particles from the withdrawn sludge mixed liquor, and returning part of the sludge mixed liquor after separation and recovery of MAP crystal particles to the anaerobic digestion tank.