Abstract: Process for preparing sludge for dewatering in a tertiary line precipitation treating system in which raw sewage has been dosed with calcium hydroxide to remove phosphates. A source of carbon dioxide is introduced into the treating system to convert calcium hydroxide salts and other salts to the more readily dewatered carbonate form, and to convert magnesium hydroxide salts to the bicarbonate form to facilitate removal of magnesium from the system. Carbon dioxide may be introduced in gaseous form, an available source of which is the exhaust gases from sludge incineration following dewatering thereof. Alternatively, sulfuric acid or hydrochloric acid or other suitable chemical reactant may be introduced into the system to neutralize the higher pH of the sludge. Conversion of the sludge to the carbonate form is effected in conjunction with the production of carbon dioxide during the neutralization reaction effected by the chemical reactants introduced into the system.