Study of hydrodynamics and residence time distribution of activated sludge process

Abstract

Water is an important commodity for manufacturing and process Industries. The enormous amount of fresh water used by these Industries generates tons of wastewater that not only pollutes the environment, but also causes irreversible damage to the ecological system. Industries require highly efficient and competent water treatment for responsible handling and management of the effluent generated. Activated sludge process (ASP) is widely used for treatment of wastewater produced by process industries, especially in pulp and paper industry. The hydraulic performance of ASP is an essential factor as it can directly affect the wastewater treatment efficiency in the reactor. The hydrodynamics of wastewater treatment reactors can be conveniently studied by residence time distribution (RTD) technique. RTD study was carried out on an industrial scale ASP system of an effluent treatment plant in an integrated pulp and paper industry using radiotracer Iodine-131. The system consisted of two stage aeration tank and a secondary clarifier connected in series. The primary objective of the investigation was to measure actual mean hydraulic retention times (MHRTs) and analyze the hydraulic performance of the complete ASP and individual system (aeration tank-1, secondary clarifier-1, aeration tank-2 and a secondary clarifier-2). Two sets RTD experiments were performed on the industrial scale ASP. The measured RTD data was treated to remove experimental errors and noises. Several treatment methods like dead-time correction, background correction, radioactive decay correction and tail correction were applied to the raw data before it can be used for further assessment. The experimental MHRTs were estimated for all the reactors. The analysis of RTD curves indicated small fraction of bypassing stream (3%) in the aeration tank-1. The dead volume in the aeration tank-1 and the secondary clarifier-1 was found and estimated to be 2.34% and 4.6%, respectively. In order to obtain detailed information about flow structure of wastewater within the aeration, secondary clarifier and complete ASP, the measured RTD data was modeled using suitable and representative mathematical models. The modeling of the measured RTD data of the aeration tank-1 revealed that the hydraulic behavior of the aeration tank-1 could be represented by two CSTRs with a moderate degree of the back-mixing between them. In the secondary clarifier-1, it was also found to be operating normally without and significant malfunctioning. A simple axial dispersion model was found suitable to describe the flow behavior of the secondary clarifier-1. Based on the prior information and results of the modeling of the individual systems, a compartment model was proposed for the entire activated sludge processing system. The compartment model was a combination of tank-in-series with back-mixing component connected in series with an axial dispersion model component along with a recycle line, which was a well representation of the system. Convolution method was applied to model the system with an imperfect impulse radiotracer input to aeration tank-2 and secondary clarifier-2. The treated RTD curves were further simulated using suitable hydraulically representative mathematical models and detailed flow patterns in the reactors were deciphered. The aeration tank-2 was fitted with tank-in-series model and was found that the reactor was working approximately as a complete mixing tank. The aeration tank-2 was working efficiently in the absence of any dead zones or bypassing. The secondary clarifier-2 was simulated with a simple axial dispersion model, with Peclet number equals to 10, which signifies that the clarifier was acting precisely as a plug flow reactor. It was impractical to change the operating parameters in a full-scale industrial system. Hence, a pilot scale ASP was constructed to study the effect of MHRT and sludge recycle ratio, on the hydrodynamic efficiency and biological treatment efficiency of the system. RTD study of the pilot scale ASP treating the pulp & paper mill effluent has been performed using LiCl as tracer. The hydraulic performance and treatment efficiency of the aeration tank and ASP at different operating parameters like residence time, recycle rate was investigated. The residence time of aeration tank was varied at 14 h, 16 h, 20 h and 24 h and operated with no sludge recycle. For each set of MHRT the sludge recycle rate from the bottom of the clarifier to the aeration tank was adjusted to 10% and 20% of the main flow and RTD experiment was performed to examine the effect of sludge recycling on the hydrodynamics of the reactor. The flow anomalies were identified and based on the experimental data empirical models were suggested to interpret the hydrodynamics of the reactors using compartment modeling technique. The analysis of the RTD curves and the compartment models indicated bypassing stream for aeration tank operating at 14 h MHRT, but disappeared at higher MHRTs. Increase in back-mixing ratio was observed as the MHRT of the tank was increased. The model with recycle line or bypassing line. Based on the modeling of aeration tank, hydrodynamic model was proposed for ASP and the model block consisted of the tank-in-series with backmixing model component connected in series with axial dispersion model component. The maximum fraction of dead zone estimated in the complete ASP was approximately 7%. It was found that the dead zone fraction increased by approximate 20 - 25% with increase in recycle rate. The fraction of the stagnant zone was found well below experimental limits for all performed experiments. The wastewater treatment parameters were also evaluated. The substrate removal of 91% for COD and 96% for BOD were observed for the ASP working at a hydraulic mean hydraulic residence time 39 hr MHRT with a 20% recycling of activated sludge, which was well below the discharge limit set by the pollution control board. The results of the investigation shows that tracer technique is suitable for evaluating performance of complex ASP. The hydrodynamic study can provide valuable information to assess the actual behaviour of the reactors and the extent of deviation from the expected behaviour. The study can also help industries to take corrective measures to improve reactor performance to achieve optimum operation and maximum treatment efficiency of the effluent treatment plant. Figure 1 shows the schematic of the overall thesis work.