A method for concentration of phosphate compounds comprises dissolving (205) of sludge ash in hydrochloric acid. Insoluble residues are separated (210), thereby forming a first leach solution. A mole ratio of phosphorus to a sum of ferric iron and aluminum in the first leach solution is controlled (215) to be larger than 1. A base is added (220) to the first leach solution in an amount causing precipitation of phosphate compounds. The precipitated phosphate compounds are removed (225) from the first leach solution. Sulphuric acid is added (240) to the first leach solution, causing precipitation of sulphate compounds. The precipitated sulphate compounds are separated (245) from the first leach solution. At least a part of the leach solution is recycled (248) as the hydrochloric acid used for the dissolving of sludge ash. Further methods for processing the precipitated phosphate compounds are presented as well as arrangements for performing the methods.