PURPOSE: To provide a sewage treating agent capable of carrying out solid-liquid separation, granulation and hardening of sludge smoothly which is obtained by adding water-granulated slag in powder form to cement, nonionic polymer powder and anionic polymer powder.

CONSTITUTION: A sewage treating agent is abtained by adding water-granulated slag in powder form to cement (e.g. portland cement), nonionic polymer powder (e.g. PVA of M.W. above four million) and anionic polymer powder (e.g. polyacrylamide partial hydrolysate of M.W. above six million). The nonionic and anionic polymers are preferably contained in the sewage treating agent in a concentration of 0.2W0.8wt% and 0.7W1.2wt%, respectively. Water-granulated slag is preferably 0.5W3 times by wt. of cement. This sewage treating agent is used in a concentration of 0.05W1.5wt% per sewage.