Modeling and simulation of an optimal unified tank model for aeration-sedimentation processes in wastewater treatment plants
Introduction
Wastewater treatment (WWT) is a very complex and detailed process. In a full scaled WWT process operation, mathematical relations between the chemical, physical and biological processes are not easy to model [1]. Recently, many researchers in interdisciplinary areas carried out studies for the treatment of wastewater and its removal without harming the nature. The design and the specific application of an activated sludge WWT plant (WWTP) is mainly focused on removal of biological nitrogen, biological phosphorus, and organic carbon substances [2]. A WWTP is a combination of complicated processes and it requires execution of several steps for efficacy of WWT process including establishment, operation and maintenance. Existing studies focus more on decentralized systems in order to decrease the initial cost despite the fact that decentralized plants may have drawbacks regarding both operation and maintenance steps in WWTP. For efficient operation and maintenance, qualified labor and proper control systems are needed [3]. There has to be an equilibrium between operation and energy consumptions, as well as safety and environmental concerns.
The activated sludge mainly consists of a mixture of wastewater and bacteria population, which is kept moving in water by stirring or aeration [4]. Aeration process is the biggest cause of energy consumption, which constitutes approximately 60 % of the total consumed energy in a WWTP [5]. The role of aeration is important for two aspects. First, oxygen is a main component for bacteria and other types of microorganisms to live, grow, and multiply. Second, it supports mixing sludge with the sewage where air is bubbled into the aeration tanks through wastewater that contains dissolved organic substances [6,7]. The living bacteria consume the dissolved organic materials, which are the pollutants in the waste [6,8,9]. During nitrification, the amount of DO concentration has a crucial role in the reduction of ammonium and ammonia nitrogen (SNH) and nitrate (SN) concentrations which are more difficult to keep below the legally determined limits [10]. For that reason, DO control and optimization has been a target of a various researches recently [[11], [12], [13]]. The aim of the secondary clarifier is to obtain the purified water (effluent) from the activated sludge through sedimentation. The secondary clarifier and the aeration tank together can be considered as a single process from a process control viewpoint [14].
Different types of reactors such as; Batch Reactor, Complete mix Reactor, Plug flow Reactor, Suspended Reactor, the Moving Bed Biofilm Reactor (MBBR) and Membrane Bioreactor (MBR) are used in secondary treatment of WWT process [[15], [16], [17], [18]]. MBR technology is an essential recent wastewater treatment technology for micropollutant removal, sustainability, and operating cost assessment. System details for MBR on both aspects of efficiency and the operational cost involved were also demonstrated [19]. The mathematical modeling studies on MBR technologies demonstrate the advantages and stability of operation [20,21].
The fill and draw system in the aeration process of a WWTP, called Sequential Batch Reactor (SBR), was first designed and used at Corpus Christi, Texas in 1969 for large-scale industrial WWT [[22], [23], [24]]. With the improvements in computers and electronically controlled valves, SBR has been dramatically improved and widely used in the biological treatment of industrial and domestic wastewater containing organic compounds in recent decades [[24], [25], [26]]. SBR plants which are properly controlled have many advantages making them a competitive alternative to classical activated sludge systems [[27], [28], [29]]. The SBR process saves more than 60 % of the operating costs compared with a conventional activated sludge process, with the capacity to treat a similar amount of wastewater in a cycle [24].
Computer-aided design has become part of optimization activities [[30], [31], [32]], and computer simulations offer a faster and more efficient way to design WWT processes [33] with extensive mathematical models expressing the processes running in the background [[34], [35], [36], [37]]. In this paper, a mathematical model of unified tank in aeration system is offered and simulated to investigate the performance of the system. Additionally, in order to improve the performance of an automated unified tank plant, we need to find the optimized parameters in the control loop. There are many optimization techniques including the metaheuristic approaches that have been widely used recently, many of which have been inspired by biological systems and animals behavior [38]. The spiral path inspired by the motion of feeding humpback whales is called the bubble-net feeding maneuver and it was first mathematically modelled with the purpose of obtaining optimization solutions [39,40]. The resulting optimization algorithm is named as whale optimization algorithm (WOA) [[41], [42], [43], [44]], which is one of the recently developed heuristic optimization methods in use [45]. The WOA method is used in this study to obtain minimized cost for production or maximized efficiency of a unified tank model in aeration system for WWTPs.
To predict the real behavior of activated sludge process, biological mathematical modeling studies were initialized with ASM1 (Activated Sludge Model 1) [46], in which carbon oxidation, nitrification and denitrification are considered. ASM1 describes the fractionation of the mixed liquor based on 13 components: particulate and soluble substrate. In the second model, ASM2 [47], the solution of biological and chemical phosphorus removal are considered. These models are based on Chemical Oxygen Demand (COD) is used to identify organic pollution. ASM2d [48] is an extended model which considers the nitrification capacity of phosphorus accumulating organism. For ASM3, another essential parameter is also considered which is the total organic carbon [49,50]. International Water Association (IWA) summarized the most popular mathematical descriptions of biological processes at composed models as ASM1, ASM2, ASM2d, ASM3 [51]. In all of these models, Biological Oxygen Demand (BOD) and COD are the most widely used parameters for characterizing the organic carbon content in wastewater.
The Unified Tank Model (UTM), unlike the conventional systems, involves a conical tank configuration used for both aeration and sedimentation processes. Along with the physical charging and discharging; aerobic, anaerobic and sedimentation phases are alternatively modeled in UTM, which is used for modeling the activated sludge process in a unified tank that includes both biological reactions and solid–liquid separation periodically. Besides the advantages of SBR such as flexibility of operation, low cost and small footprint technology [52], the UTM offers a sequence of successive phases that can be adapted to the existing plants with low-cost modifications. This provides the required combination for the growth conditions in removing contaminants with different types of microorganisms from wastewater.
In this study, an optimized aeration system model in the form of UTM is proposed that will help obtain a more homogeneous and efficient aeration solution based on DO optimization for the UTM obtained via WOA. The mathematical model of the conventional structure with two separate tanks is presented. The UTM mathematical model is derived according to the existing studies on conventional WWTPs. The dynamic model is adapted to the UTM and numerical simulations are presented. The functional specifications, and the objectives of the process (purpose, accuracy, boundaries, time scale, etc.) have been clarified. For physical charging and discharging durations of the unified tank volume, some deterministic models have been used including finite element approaches for settling process. The aeration system has been modelled with electrical analogy to obtain a more realistic model. For biological processes, the experimental researches and models have also been presented. Computer simulations of UTM have been applied to the Oğuzeli WWTP in Gaziantep province of Turkey. These results are used for validation and comparison in this study.
The rest of the paper is organized as follows: In Section 2, the problem statement with the materials and methods are presented. In Section 3, the UTM design and modeling with the improvements of proposed WOA approach are discussed and the physical model structure, aeration process with WOA, and improvements in sedimentation are explained. In Section 4, the Oğuzeli WWTP and influent-effluent characteristics are introduced. In Section 5, simulation results are given and performances presented in graphical form are discussed. Conclusive remarks and future studies are presented in Section 6.
Section snippets
Mathematical modeling
WWTPs are fundamentally hydraulic systems. The main governing equation for any hydraulic system is the volume balance equation, which is formulated in terms of the difference between the charge and the discharge as shown in Fig. 1.
The activated sludge models focus on food-microorganism ratio is [8,9,49];
The activated-sludge process is aimed to remove substances that have a demand for oxygen from the system. The metabolic reactions of synthesis-respiration and nitrification
Design of unified tank model
The conventional system consists of two interconnected water tanks, two submersible pumps, and blower pumps. Since the amount of Return Activated Sludge (RAS) remains constant in UTM, the effect of pump used for sludge return can be neglected in the modeling. Influent water characteristics are the inputs while the treated water overflow from the conic tank and waste sludge produced after the process are the outputs. Treated water is discharged via several level taps. Discharging cycle starts
Description of the plant
Oğuzeli WWTP is located at Gaziantep province in Turkey, projected for 40,000 people with a treatment capacity of 8000 /day [72]. The annual rates in the year of 2019 for influent and effluent flows are depicted in Fig. 11a. Annual distribution of influent-effluent characteristics are shown in graphics of Fig. 11.
Due to seasonal variations in weather conditions, some fluctuations can be observed as shown in Fig. 12, which indicates Dissolved Oxygen (DO) concentrations in oxidation pool during
Conclusion
In this paper, a novel design for aeration-sedimentation processes in WWTPs is proposed and investigated. The UTM design for both aeration and sedimentation phases is obtained through optimization, as an alternative for the conventional approach where two separate tanks are used for aeration and sedimentation processes. In this design, the aeration tank is embedded within the main tank with an Archimedean spiral geometry. An optimization problem is defined to find optimal locations of the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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