Abstract: There are high content of phosphorus and COD, exceeding the standard in recirculating cooling water system，which is prone to cause scaling and corrosion of heat exchange equipment. Therefore，biological agents is used to improve the quality of recirculating cooling water. The results shows that the contents of COD，ammonia nitrogen, chloride ion and total phosphorus have decreased drastically，the removal rates reached 92%，95%，57% and 73% respectively. It is important that the discharge of sewage and phosphorus significantly decreased，and the proportion of make-up water decreased from 30% to 10%，a good economic and social benefit is achieved.

Key words: Coal tar refinery, Recirculating cooling water, Biological treatment