Method of recovering iron and/or phosphorus from sludge of waste water treatment plants, said sludge being obtained after precipitation by iron salts, wherein said method comprises separating sludge from waste water and submitting said sludge to a lactic fermentation to release a liquid phase where iron and phosphorus are dissolved. Lactic fermentation is performed with addition of a co-substrate rich in carbon, preferably rich in carbohydrate, in one single step of biological acidification or a sequencing biological acidification in two steps by first releasing phosphorus from the PAO contained in the sludge. Iron can then be recovered by means of a cationic exchange resin. Phosphorus can be recovered as struvite or calcium phosphate from the remaining solution substantially free from iron ions, after a precipitation step in presence of a magnesium or calcium source and a pH above 7.