AbstractThe reliability of the process of Ramanathan and Gaudy (Biotechnol Bioeng.,13, 125 (1971)) for the completely mixed activated‐sludge process holding the recycle cell concentration,XR, as a system constant with respect to step changes in hydraulic retention time was investigated. The experiments were run at initial dilution rates of ⅛, ⅙, ¼, and ½ hr−1treating a soft drink bottling wastewater. The influent substrate concentration was maintained at 1000 mg/liter chemical oxygen demand and the hydraulic recycle ratio at 0.3. The recycle sludge concentration was maintained at about 7000 mg/liter.It was found that the system could accommodate hydraulic shock loads up to 200% positive changes and down to 50%negative changes without disruption of the effluent quality. Shorter retention time of the range studied, from 2 to 8 hr, has the advantage of shorter response time with respect to the response of the concentration of biological solids in th