The application of constant recycle solids concentration in completely mixed oxygen activated sludge process

Abstract

AbstractFor better operational control of the completely mixed oxygen activated sludge process (CMOAS), a study concerning the kinetics, performance, and operational stability of the Ramanathan‐Gaudy model was conducted. Short‐term experiments were conducted at various dilution rates (1/9, 1/6, 1/3, 1/1.5, and 1/1.0 hr−1) by using two recycle solids concentration values (5000 and 10,000 mg/liter). The influent substrate was an actual industrial organic wastewater (soft drink waste) and its concentration was maintained at 1000 mg/liter COD. The hydraulic recycle ratio, α, was maintained at 0.30.It was found that for CMOAS system with constant recycle cell concentration, a “steady state” with respect to reactor biological solids and effluent COD at different dilution rates could be attained. No appreciable dilute‐out of reactor biological solids and substrate was observed up to the dilution rate of 1 hr−1 for both systems of different XR (5000 and 10,000 mg/liter). For the system of XR = 5000 mg/liter, except the dilution rate of hr−1, the effluent filtrate COD was lower than 100 mg/liter, the aerator biological solids concentration was about 1550 mg/liter, and the COD removal efficiency was higher than 90% for all dilution rates. For the system of XR = 10,000 mg/liter, the effluent filtrate COD was lower than 71 mg/liter, the aerator biological solids concentration was about 2750 mg/liter, and the COD removal efficiency was higher than 90% throughout all the dilution rates selection in the present study. The value of the Sludge Volume Index (SVI) was the range of 37.0 to 58.5 and provided good settleability of sludge. The sludge yield was 0.53 for the system of XR = 5000 mg/liter and 0.57 for the system of XR = 10,000 mg/liter. The carbohydrate and the protein content of the cells were 10.1–21.6% and 35.6–50.6%, respectively.For predicting the reactor biological solid and effluent COD of the CMOAS system by using the Ramanathan‐Gaudy model, two sets of values for the biological kinetic constants should be considered since it provided the best fit of predicted values of the observed values. In the present study, μm = 0.4 hr−1, ks = 92 mg/liter for 1/3 ⩽ D ⩽ 1, and μm = 0.05 hr−1, ks = 11.1 mg/liter for 1/9 ⩽ D < 1/3 were used to calculate the predicted values of reactor biological solid and effluent filtrate COD.