Abstract: The cost and benefit concepts involved in liquid-chlorine-based disinfection experiments for effluents from sewage plants are analyzed,and corresponding calculation methods and parameters of cost-benefit analysis presented.Put Lucun Sewage Plant,Wuxi as an example,the colon bacillus killing rate is about 98% with chlorine concentration of 30 mg/L available and an acting period of 30 min; the total cost for Lucun Sewage Plant is about 7.3×10⁶ yuan/a,while the total benefit is about 19.61×10⁶ yuan/a,including 7.67×10⁶ yuan/a of health benefit gained from reducing intestine-based contagious disease and 11.94×10⁶ yuan/a of environmental benefit gained from reducing lake pollution and blue algae reproduction.The benefit-cost ratio of the re-disinfecting technology is about 2.7∶1.