Critical evaluation for nitrogen removal performance of a stereotype activated sludge system under dynamic process conditions

Abstract

In recent years, with the increasing population and the effects of global warming, the design, construction, and operation of domestic and urban wastewater treatment plants are carried out considering the treatment steps that provide nutrient removal. The most suitable treatment alternative to remove nutrients from wastewater in terms of applicability and cost are determined to be Biological Nutrient Removal processes. Because of the need for biological nutrient removal, stress caused by the nutrients and organic matter on receiving water environments are reduced and active sludge systems gain more and more attention moving forward. As widely known, highly complex biological reactions occur in activated sludge systems and although stable state conditions are generally used to simplify design calculations, active sludge systems operate under dynamic conditions. This indicates that input wastewater characterization as well as the inlet flow, various environmental factors (temperature, precipitation, etc.) and operating conditions vary depending on time. Therefore, various modeling tools are used to understand the treatment system more efficiently. With the modelling tools, it is possible to comprehend system dynamics and determine the rehabilitation, refurbishment and expansion requirements of existing treatment plants, while for the new plants, plant design can be optimized considering modeling outputs. Additionally, data from pilot-scale reactors can be evaluated through models and used to predict full-scale plant performance. To reflect the actual conditions at wastewater treatment plants, process simulators which provide guidance on determining the design principles of wastewater treatment plants, creating automation scenarios, choosing equipment, and evaluating process performance for both wastewater and sludge units, are used. The main purpose of this thesis is to evaluate the use of oxidation ditch reactors in series in terms of nitrification and denitrification processes and to model the actual behavior of an Oxidation Ditch (OD) system operated by following the pre-denitrification principles using input wastewater data collected from an urban wastewater treatment plant in the Marmara Region (Istanbul, Turkey) under dynamic conditions. Sumo software was used to model and simulate the wastewater treatment plant under dynamic conditions and the treatment efficiency of the plant in terms of nitrogen removal was examined. This thesis mainly focuses on nitrogen removal under dynamic conditions in a municipal wastewater treatment plant that employs four oxidation ditches located upstream of Bio-P tanks and operated in series. Although simultaneous nitriding denitrification principles apply to plant configuration due to oxidation ditches, the treatment plant is operated as a conventional active sludge system and considers pre-denitrification principles, which the first oxidation ditch is operated under anoxic conditions. The second oxidation ditch in the plant is operated under anoxic and aerobic conditions by controlling the diffusers (on/off), while the remaining two oxidation ditches are continuously aerated by the diffusers located at the bottom of the tanks and operated under aerobic conditions. In this context, a dynamic simulation was carried out using Sumo software for the entire oxidation ditch system. Bio-P tanks and final sedimentation tanks were included in the model to ensure system integrity, but only the nitrogen removal efficiency of oxidation ditch reactors was examined within the scope of this thesis. Modeling and simulation results confirmed that the minimum nitrate production rate occurred in the first oxidation ditch due to lack of aerobic environment. It was also examined that the nitrate recirculated from the fourth oxidation ditch to the first oxidation ditch was consumed within this first reactor. Hence, transfer of recirculated nitrate to the second reactor does not occur. Additionally, it was confirmed by the modelling studies that nitrate is consumed within the first reactor only at rates of the recirculated nitrate. Even if the second OD reactor is operated under anoxic conditions to provide denitrification for the recirculated nitrate, the volume of the first oxidation ditch cannot be used efficiently, because the recirculated nitrate from the fourth OD to the first OD is very low due to simultaneous nitrification denitrification occurs in the remaining reactors. In addition, results confirmed that the highest nitrate consumption rate was achieved within the first reactor, while this is followed by the second, third and fourth reactors, respectively. Nitrate production and utilization rates were determined through model outputs, which were very close in the second oxidation ditch due to operating conditions and creating both anoxic and aerobic zones, while in the third and fourth reactors, the difference between these rates increases due to decreased anoxic volume. Considering the information obtained from the modeling studies, it can be stated that the system is divided into two parts as the first oxidation ditch reactor and the remaining tanks (OD-2, OD-3 and OD-4). This is because nitrate can be removed from wastewater in OD-1 reactor only at a rate and an amount of the recirculated nitrate, which is determined to be low due to simultaneous nitrification denitrification occurred within the remaining OD reactors. Hence, the first oxidation ditch reactor volume, operated under anoxic conditions to provide denitrification, is not used effectively, and does not fit for purpose. Therefore, it was recommended that the optimization of the system could be achieved by operating four oxidation ditches in parallel with the principles of simultaneous denitrification nitrification. In addition, it is envisaged that this will also provide flexibility to plant operators in case of maintenance works etc., and the treatment system can be operated without interruption even if one of the tanks is out of operation. It may also be beneficial to select simpler and more efficient treatment systems for the plant configurations to prevent such treatment complications in the future.