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Development of a model for activated sludge aeration systems: Linking air supply, distribution, and demand
Abstract
During the design of a water resource recovery facility, it is becoming industry practice to use simulation software to assist with process design. Aeration is one of the key components of the activated sludge process, and is one of the most important aspects of modelling wastewater treatment systems. However, aeration systems are typically not modelled in detail in most wastewater treatment process modelling studies. A comprehensive dynamic aeration system model has been developed that captures both air supply and demand. The model includes sub-models for blowers, pipes, fittings, and valves. An extended diffuser model predicts both oxygen transfer efficiency within an aeration basin and pressure drop across the diffusers. The aeration system model allows engineers to analyse aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, controller design and tuning, and energy costs. This enables engineers to trouble-shoot the entire aeration system including process, equipment and controls. It also allows much more realistic design of these highly complex systems.
... The oxygen transfer efficiency is higher at lower airflow rates per diffuser (Leu et al. 2009;Rosso et al. 2005) because low airflow rates lead to less expansion of the membranes and therefore smaller bubbles with higher specific surface area (Baquero-Rodríguez et al. 2018). A higher diffuser density leads to higher oxygen transfer efficiency (Schraa et al. 2017) because of fewer opportunities for vertical water circulation that reduce the retention time of the rising air bubbles in the tank. A higher number of diffusers will generally lead to lower airflow rate per diffuser and therefore higher oxygen transfer efficiency. ...
... The interdependent relations between the aeration diffuser system and the biological treatment process can be investigated with existing models, combining models such as Activated Sludge Model No. 1 (ASM1) (Henze et al. 2000) with oxygen transfer models (Amaral et al. 2017;Arnell 2016;Juan-Garcia et al. 2018;Schraa et al. 2017). In this way, the link between the electricity usage and the effluent water quality can be systematically evaluated. ...
... where A and m = constants. The thus fitted SOTE data were used, as part of the simulations, to calculate the airflow rate into each aerated tank as a function of the oxygen demand determined in the process model as otherwise described elsewhere (Supplemental Materials; Schraa et al. 2017). Each tank was considered with a specific number of uniformly distributed diffusers in it, with an oxygen transfer performance according to the input SOTE profile. ...
The influence of aeration diffuser system design on electricity usage, effluent water quality, and life-cycle cost in biological wastewater treatment was investigated. A plant-wide model was implemented, and simulations were carried out with different process configurations and aeration systems. Model-aided design of new aeration diffuser systems could significantly decrease electricity usage and life-cycle cost while at the same time avoiding negative effects on the treatment performance. The optimum distribution of diffuser systems in tanks in series was found to be influenced by process configuration, volumetric loading rate, temperature, and the internal recirculation flow rate. Compared with a conventional design approach, increasing the number of diffusers, up to a critical point, led to higher energy efficiency and lower life-cycle cost. This was despite an increasing limitation of the minimum airflow rate, leading to dissolved oxygen levels significantly exceeding control targets. Aeration systems optimized by simulations were found to, independently of process configuration, exhibit 20% lower electricity usage and 16%–18% lower life-cycle costs compared with systems designed based on a more conventional approach typically applied in practice.The influence of aeration diffuser system design on electricity usage, effluent water quality, and life-cycle cost in biological wastewater treatment was investigated. A plant-wide model was implemented, and simulations were carried out with different process configurations and aeration systems. Model-aided design of new aeration diffuser systems could significantly decrease electricity usage and life-cycle cost while at the same time avoiding negative effects on the treatment performance. The optimum distribution of diffuser systems in tanks in series was found to be influenced by process configuration, volumetric loading rate, temperature, and the internal recirculation flow rate. Compared with a conventional design approach, increasing the number of diffusers, up to a critical point, led to higher energy efficiency and lower life-cycle cost. This was despite an increasing limitation of the minimum airflow rate, leading to dissolved oxygen levels significantly exceeding control targets. Aeration systems optimized by simulations were found to, independently of process configuration, exhibit 20% lower electricity usage and 16%–18% lower life-cycle costs compared with systems designed based on a more conventional approach typically applied in practice.
... For the air supply system and the oxygen transfer, the topics have been intensively studied. For instance, with respect to model-ling the aeration system and its energy consumption, Schraa et al. (2017) developed a fully dynamic model around the aeration header network to simulate the air distribution, and evaluated its limitations with various optimisation options and influent loadings [3]. Juan-Garcia et al. (2018) modified their study to the plant controlling systems by integrating a biokinetic model having oxygen uptake phenomena [4]. ...
... For the air supply system and the oxygen transfer, the topics have been intensively studied. For instance, with respect to model-ling the aeration system and its energy consumption, Schraa et al. (2017) developed a fully dynamic model around the aeration header network to simulate the air distribution, and evaluated its limitations with various optimisation options and influent loadings [3]. Juan-Garcia et al. (2018) modified their study to the plant controlling systems by integrating a biokinetic model having oxygen uptake phenomena [4]. ...
... Kjeldahl Nitrogen (TKN) with thumb rules to adapt the parameters to the plant design. For example, Schraa et al. (2017) and US Environmental Protection Agency (USEPA) selected 150~200 mg-BOD/L and 30~40 mg-N/L as the default BOD 5 concentration and total TKN concentration respectively [3]. With respect to the oxygen demand in design-daily-maximum, the guidelines suggest calculating the load in proportion to the inflow rate of the design-dailyaverage. ...
Ascertaining peak oxygen demand is crucial for plant designers to determine blower capacities of wastewater treatment plants in planning phase. To obtain this technical information without cumbersome influent sampling and analysis, a set of field-test activated sludge reactors equipped with DO and nitrate-N sensors was installed at 3 sites and continuously operated for a couple of months in each field. Under the controlled aeration and hydraulics of the reactors, the hourly influent oxygen demands were back-calculated as biodegradable constituents using the IWA-Activated Sludge Model #1. The daily maximum concentrations (rounded to last for 1-hour) of biodegradable organics and nitrogen were ranged between 45~258 mg-COD/L and 10.4~32.3 mg-N/L in Site #1; 119~244 mg-COD/L and 28.3~38.7 mg-N/L in Site #2; 194~552 mg-COD/L and 30.2~51.7 mg-N/L in Site #3 respectively. The marginal blower capacities to maintain at least 1.0 mg-O2/L of DO in the daily maximum oxygen demand were estimated based on the datasets using the statistical method, Extreme Value Distribution analysis. To maintain the DO concentration for 99 days out of 100 days of the plant operations, the blower capacity was supposed to be designed as high as 1.4~2.2 times than those of the blower calculated from the daily average concentration.
... Dynamic modelling of the wastewater treatment processes has been the topic of various research [9,[13][14][15][16][17][18][19][20][21][22][23][34][35][36][37], and usually with the aim of incorporating control systems leading to improvements in the performance, or with the purpose of designing (or redesigning) WWTPs [9,36]. In [36,37], mathematical models are developed by applying material and energy balances to a process tank, and similar methods are presented in [34,35] where the aeration tank and settler are modelled in Matlab Simulink. ...
... This is particularly true for systems where multiple aeration tanks are maintained by a single air supply unit, which will be investigated in this work. Various control strategies have been established for continuously aerated reactors (DO set-point at 2-3 mg/L) [19,31,44]. In industrial practice, the process controller usually consists of simple feedback loops, such as bang-bang control ref. [45] or Proportional Integral (PI) controllers that regulates airflow and valve position Refs. ...
... To account for pressure losses in the main supply pipe due to friction between the air and the pipe surface, friction losses are implemented in the airflow model. Friction losses in the supply pipe can be calculated based on the Darcy-Weisbach equation, applying the methodology described in [19,51]. The Darcy-Weisbach equation expresses the dynamic pressure difference from CV A to CV B , P A|B , due to friction in the supply pipe as: ...
Aiming at reducing their emissions, wastewater treatment plants (WWTP) seek to reduce their energy consumption, where a large amount is used for the aeration. The case plant, Grindsted WWTP uses an alternating aeration strategy, with a common air supply system facilitating the process in four aeration tanks and thus making optimisation challenging. In this work, a nonlinear model of the air supply system is designed, in which multiple key parameters are estimated by data-driven optimization. Subsequently, a model-based control strategy for scheduling of compressors and desired airflow is proposed, to save energy without compromising the aeration performance. The strategy is based upon partly static- partly dynamic models of the compressors, describing their efficiency in terms of system head and volumetric airflow rate. The simulation study uses real plant data and shows great potential for improvement of energy efficiency, regardless of the aeration pattern in any of the four process tanks, and furthermore contributes to a reduction in compressor restarts per day. The proposed method is applicable to other WWTP with multiple compressors in the air supply system, as this study is conducted using first principle models validated on data from the daily operation.
... The inlet and outlet pressure of the storage volumes is used to calculate the mass flow rates between them. A subsequent integrated model was proposed in Schraa et al. (2017), which includes a more detailed representation of sitespecific equipment such as blowers, valves and diffusers. This has allowed describing more realistically the interactions occurring between the aeration system (oxygen supply side), the biological reactors (oxygen demand side), and the control system. ...
... The secondary settling tanks have been modeled using a spatially discretized layered clarifier model that accounts for concentration driven settling behavior (Tak acs et al., 1991). The air supply distribution was modeled similarly to other studies with equipment components such as the blower battery, piping system, valves and diffusers (Amaral et al., 2017;Juan-García et al., 2018;Schraa et al., 2017). As thoroughly presented in Schraa et al. (2015) and Schraa et al. (2017), the air supply system equipment sub-models are linked together by using physical gas models block diagrams, which, although use a different simulation environment, are based on the same physical equations. ...
... The air supply distribution was modeled similarly to other studies with equipment components such as the blower battery, piping system, valves and diffusers (Amaral et al., 2017;Juan-García et al., 2018;Schraa et al., 2017). As thoroughly presented in Schraa et al. (2015) and Schraa et al. (2017), the air supply system equipment sub-models are linked together by using physical gas models block diagrams, which, although use a different simulation environment, are based on the same physical equations. However, specific equipment curves, such as blowers, diffusers and valves, were alternatively modeled by implementing directly the manufacturer curves into the Simulink model by using a 2-D look-up table block. ...
Diffused aeration is the most implemented method for oxygen transfer in municipal activated sludge systems and governs the economics of the entire treatment process. Empirical observations are typically used to regulate airflow distribution through the adjustment of manual valves. However, due to the associated degrees of freedom, the identification of a combination of manual valves that optimizes all performance criteria is a complex task. For the first time a multi-criteria optimization algorithm was used to minimize effluent constituents and energy use by parametrizing manual valves positions. Data from a full-scale facility in conjunction with specific model assumptions were used to develop a base-case facility consisting of a detailed air supply model, a bio-kinetic model and a clarification model. Compared to the base-case condition, trade-offs analysis showed potential energy savings of up to 13.6% and improvement of effluent quality for NH4+ (up to 68.5%) and NOx (up to 81.6%). Based on two different tariff structures of a local power utility, maximum costs savings of 12800 USD mo-1 to 19000 USD mo-1 were estimated compared to baseline condition.
... As it was shown in Figure 1, the relationship between the disc position and flow is nonlinear. To address valves' non-linearity, many researchers [2][3][4][5][6][7][8] proposed to use the first principle approach for valve modeling. Tang et al. [9] used a semi-empirical approach to model pneumatic valves' characteristics. ...
... As it was shown in Figure 1, the relationship between the disc position and nonlinear. To address valves' non-linearity, many researchers [2][3][4][5][6][7][8] proposed to first principle approach for valve modeling. Tang et al. [9] used a semi-empirical a to model pneumatic valves' characteristics. ...
Control of airflow of activated sludge systems has significant challenges due to the non-linearity of the control element (butterfly valve). To overcome this challenge, some valve manufacturers developed valves with linear characteristics. However, these valves are 10–100 times more expensive than butterfly valves. By developing models for butterfly valves installed characteristics and utilizing these models for real-time airflow control, the authors of this paper aimed to achieve the same accuracy of control using butterfly valves as achieved using valves with linear characteristics. Several approaches were tested to model the installed valve’s characteristics, such as a formal mathematical model utilizing Simscape/Matlab software, a semi-empirical model, and several machine learning methods (MLM), including regression,support vector machine, Gaussian process, decision tree, and deep learning. Several versions of the airflow-valve position models were developed using each machine learning method listed above. The one with the smallest forecast error was selected for field testing at the 55.5×103 m3/day 12 MGD City of Chico activated sludge system. Field testing of the formal mathematical model, semi-empirical model, and the regularized gradient boosting machine model (the best among MLMs) showed that the regularized gradient boosting machine model (RGBMM) provided the best accuracy. The use of the RGBMMs in airflow control loops since 2019 at the City of Chico wastewater treatment plant showed that these models are robust and accurate (2.9% median error).
... However, the efficiency of oxygen transfer is to be evaluated considering head loss associated with the small-sized orifice required to produce small bubbles. The high oxygen transfer efficiency at the cost of large pressure drop resulting in high compression cost makes aeration an energy-intensive process (Schraa et al., 2017). The mechanism of the diffused aerator is shown in Fig. 7. Diffused aerators have certain advantages over mechanical aerators such as better mixing from the bottom of the tank to the water surface, preventing sedimentation, higher SAE, greater adjustment level due to the use of control valves, low maintenance requirement, and they are suitable for deeper applications (Schraa et al., 2017). ...
... The high oxygen transfer efficiency at the cost of large pressure drop resulting in high compression cost makes aeration an energy-intensive process (Schraa et al., 2017). The mechanism of the diffused aerator is shown in Fig. 7. Diffused aerators have certain advantages over mechanical aerators such as better mixing from the bottom of the tank to the water surface, preventing sedimentation, higher SAE, greater adjustment level due to the use of control valves, low maintenance requirement, and they are suitable for deeper applications (Schraa et al., 2017). However, diffused aerators are more complex to be implemented and have greater upfront costs because of complexity, installation, and the number of equipment. ...
The industrial revolution has created severe water contamination issues due to the release of toxic chemicals into water channels. This crisis has become worst in underdeveloped countries due to the lack of resources and very little implementation of water management policies. The economical water treatment technologies are requisite to resolve water pollution and associated problems. The article deals with two water systems-wastewater and clean drinking water systems, highlights associated problems and discusses the potential instigating factors. Cleaner, sustainable, and energy-efficient solutions for the discussed hitches have been proposed. Aeration technologies being the most energy-intensive operation of the wastewater treatment, have been discussed with their limitations. Mechanical and diffused aerators provide lower standard aeration efficiency while combined aerators are cost-intensive and have limited oxygen mass transfer efficiency, though higher than the former ones. An efficient way of reducing the energy efficiency of the aeration has been proposed by controlling bubble size using a cost-effective technique-Fluidic Oscillator. Therefore, a fluidic oscillator-based aeration system along with the micro-bubbles generation phenomenon and their complexities have been critically deliberated. Purification techniques for drinking water systems have been discussed with a particular focus on sterilization of water through chlorination, chlorine-based compounds, UV radiations, Ultrasonication, and Ozonation. Ozonation using plasma micro-reactor for ozone and reactive species generation for wastewater treatment has been critically discussed. Recent developments in plasma ozonation have been included along with their limitations. Solar integrated cold plasma ozonation has been given special attention as advanced economical water treatment techniques. A decentralized system based on an advanced oxidation reactor capable of producing and dosing oxidation species such as hydroxyl radicals simultaneously in wastewater in-situ has been proposed to provide a clean drinking water supply. A perspective and an integrated strategy have been presented to enhance the efficiency of existing wastewater and water treatment techniques.
... Hence, fully dynamic models can be used to find solutions for energy and process optimizations that are more realistic and tailored to a specific facility. In this study, the advanced model developed and implemented in SIMBA# by Schraa et al. (2016) is used to carry out a model-based process performance and energy audit of the Girona WRRF (Spain), with the aim of reducing energy consumption while maintaining effluent quality. The Girona WRRF's 2013 energy consumption record shows that 63% of the plant's energy consumption is due to aeration. ...
... After assessing the current performance, three different optimisation scenarios have been evaluated, including three different temperature variations and a stress test in the form of an ammonia peak. This is the first published application of the aeration system model library developed by Schraa et al. (2016) at a full-scale WRRF. ...
A model-based treatment performance and energy audit of the Girona WRRF in Spain was conducted using advanced and fully dynamic air supply models. The focus of the study was on the aeration system, which represents 63% of the plant's energy consumption. A dynamic model consisting of a process, a realistic controller, and a very detailed air supply model, was developed and calibrated to be used as the baseline for testing various scenarios. Results show that the implementation of an ammonia-based controller and a redistribution of the diffusers led to energy savings between 12 and 21%, depending on wastewater temperature. In addition, the model demonstrated that the current blower is too large, which causes an intermittent behaviour, endangering the equipment and shortening its lifetime , plus limiting the minimum air-supply. The applied aeration system models enable engineers to identify bottlenecks by modelling equipment constraints (e.g. blower turn-down). Ignoring the air supply side in an assessment could result in an overestimation of energy savings or treatment performance and consequently in non-optimal control solutions or equipment selection. INTRODUCTION One of the largest urban consumers of energy are water resource recovery facilities (WRRFs, formerly wastewater treatment plants). Within a WRRF, aeration energy consumption typically accounts for 50% of the facility's total operating costs (Olsson, 2012). WRRFs are therefore important targets for reducing municipal energy demands. Energy audits, which identify opportunities to reduce energy use, are typically based on the average energy consumption of a facility; they include benchmarking with similar plants or comparison against standard performance indicators. However, significant saving potential lies in the variability of wastewater treatment and the resulting process dynamics. Current energy audits should therefore include dynamic models that integrate water and sludge lines and their respective energy consumption.
... Such process models have a wide range of offline and on-line applications. Offline applications range from support and verification of plant design to optimisation [10] of treatment processes, control concepts [11], energy consumption [12] and mechanical designs [13] (e.g. blower selection). ...
State-of-the-art modelling tools and dynamic simulations have become important tools for planning and operational decision making in the environmental sector, including wastewater treatment plants. Due to increasing regulatory requirements (energy savings, treatment performance , GHG footprint), the practical application of these instruments is becoming more challenging. AI methods could be a solution to support users in the application of domain-specific modelling and simulation tools. This contribution presents first steps towards the integration of the AI methods Bayesian Networks (BN) and Artificial Neural Networks (ANN) into a modelling and simulation tool for urban waste water systems, including example applications. Zusammenfassung: State-of-the-art Modellierungswerk-zeuge und dynamische Simulationen sind zu wichtigen Instrumenten für die Planung und betriebliche Entscheidungsfindung im Umweltsektor, einschließlich Kläranlagen, geworden. Aufgrund steigender gesetzlicher Anforderungen (Energieeinsparungen, Reinigungsleistung, Treibhausgasbilanz) wird die praktische Anwendung dieser Instrumente immer anspruchsvoller. KI-Methoden
... The problem of high energy consumption applies to essentially every plant using activated sludge, hence the constant attempts to introduce possibly intelligent aeration control techniques. There are different strategies to improve control: some are based on a mathematical model of the aeration process [12,13], others (mostly the newer ones) use artificial intelligence techniques to build specific control algorithms [5]. The application of artificial neural networks in the calculation of complex phenomena, such as activated sludge process, requires a large amount of data at the stage of network training. ...
Due to the difficulties in implementing other methods of removing organic compounds and nitrogen from wastewater, municipal wastewater treatment plants use classical processes (nitrification and denitrification) that require large energy expenditure on aeration. The problem of high energy consumption concerns every treatment plant using aerobic activated sludge, hence the constant attempts to introduce possibly intelligent aeration control techniques. In this study, a short-term (hourly) forecast of oxygen concentration in the aeration chamber was calculated under the conditions of changing values of wastewater flow and pollutant concentrations as well as active aeration control according to an unchanging algorithm. Artificial neural networks were used to calculate the forecast. It is shown that an accurate prediction can be obtained by using different sets of input data but depending on what data we choose, the neural network required to obtain a good result has a more or less complex structure. The resulting prediction can be applied as part of a system for detecting abnormal situations and for preventing excessive energy consumption through unnecessary over-oxygenation of activated sludge.
... The oxygen mass transfer characteristics in the aeration tank can be affected by a number of hydrodynamic parameters, including gas flow rate, aeration tank capacity, and bubble size [12]. Currently, to reduce aeration cost and maximize nitrification efficiency, intermittent aeration, use of simulation software, Enhanced Biological Phosphorus Removal (EBPR) method development, modified paddlewheel, and superoxygenation systems are applied and used [13,14,15]. Understanding the factors influencing oxygen transport provides useful information about the applicability of the developed technology, energy efficiency, and nitrification process. ...
In biological wastewater treatment, the oxygen supply in an aeration tank is the most important factor for removing organic pollutants, but it takes a large amount of electricity to generate the oxygen supply required. The Jetventurimixer (JVM) is a device that applies Bernoulli’s principle, and the difference in flow rate pressure through the impeller is generated by the rotational force. Due to this physical mechanism, this device can supply oxygen in the atmosphere to the bioreactor without additional power. In this study, the JVM-based aeration process was developed for more efficient water treatment that demands lower energy. Parameters were measured for validating the efficiency and lower power demands, including the oxygen mass transfer characteristics and power efficiency. The results indicated that all parameters related to the oxygen mass transfer characteristics were advanced in performance by more than 200 % compared to those of the conventional air diffuser. In the case of power efficiency, it was confirmed that performance was 153–176 % higher. Therefore, it was confirmed that the JVM provides high-efficiency and low-energy benefits to the aeration process and, based on these advantages, the developed system seems to require further studies and validation for application to the water treatment system.
... The inclusion of aeration dynamics on process optimization has been largely overlooked . The integration of daily and seasonal oxygen transfer fluctuations can improve performance diagnostics (i.e., over-aeration and imbalanced flows) (Jiang et al., 2017;Reifsnyder et al., 2020;Schraa et al., 2017). Moreover, energy costs can vary significantly depending on plant size, location, time of the day, season, and so on, and so a detailed description of the site-specific tariffstructures is fundamental to perform accurate economic evaluations (Aymerich et al., 2015). ...
Aeration systems often lack the efficiency to maintain a desired residual dissolved oxygen (DO) concentration in the tank in part because little consideration is given to the dynamic daily and seasonal loading conditions. Although advanced aeration controllers exist, the majority of plants have DO set points typically based on common practice and literature values rather than site‐specific conditions, which can result in DO set points higher than those necessary to meet treatment objectives. DO set point reduction strategies have primarily been proposed through either static or dynamic simulations. In this study, the substantial improvements associated with DO set point reduction are demonstrated at full scale. A yearlong characterization of full‐scale aeration dynamics captured the effect of diurnal and seasonal fluctuations on oxygen transfer and energy demand and so facilitated the estimation of the potential savings of DO reduction strategies. Full‐scale validation provided direct evidence of DO reduction strategies inducing an overall enhancement of oxygen transfer efficiency along the different bioreactors, while confirming that energy savings as high as 20% were feasible. This study quantifies the influence of oxygen transfer efficiency on operating choices and site‐specific conditions (control strategy, loading conditions, and influent flow variability).
Practitioner points
We quantified the energy reduction and cost savings associated with a DO reduction in an aeration tank.
For each 0.2 mg/L of DO decreased, the average power demand reduction per unit water treated exceeded 17%.
Field measurements of dynamic alpha values eliminate the uncertainty in estimating aeration energy and cost savings from DO variations.
... In addition, different factors influencing SSOTE were investigated. The relationship between SSOTE and specific properties of the aeration system and tank geometry is repeatedly tried to describe with (empirical) equations (EPA 1989;DeMoyer et al. 2001;Gillot et al. 2005;Schraa et al. 2017). To integrate the correlation of these parameters into simulation tools can improve current modelling capabilities of the entire wastewater treatment process (Nolasco et al. 2018). ...
We summarized the experience from three decades of oxygen transfer testing and aeration research at Technical University of Darmstadt to validate the oxygen transfer efficiency of modern fine-bubble diffusers. 306 oxygen transfer tests in clean water of 65 different fine-bubble diffusers, carried out in the same test tank under identical test conditions, were analysed and compared with previous results. As a result, we could show that the performance of fine-bubble aeration systems has increased by 17% over the last three decades. Therefore, modern well designed and operated aeration systems can achieve SSOTE values between 8.5 and 9.8% · m−1. Additionally, a comparison of various diffuser types and diffuser densities was done. Based on the new results an exemplary cost/benefit analyses for a 100,000 PE WWTP shows the calculation of an optimized diffuser density with respect to investment and operating costs.
... Due to seasonal and daily control of air supply to the aeration tanks, it is possible to save about 30-40% of the power consumption, which becomes achievable when using air blowers with a range of performance control more than 50-100% [3,4]. The method of estimating the dynamics of technological loads fluctuation (by flows and concentrations) plays a key role in the formation of initial data [5,6]. ...
The object of analytical research are controlled air blower units of wastewater treatment plants (WWTPs) for wastewater utilities with pneumatic aeration systems for biological processes, based on oxygen consumption. The goal of analytical research is accounting and estimation values of temperature, relative humidity, atmospheric pressure for energy consumption by controlled air blowers. The system of equations is proposed for the joint solution: 1st equation - according to the Guide to Meteorological Instruments and Methods of Observation; 2nd equation - modification for determining the energy consumption for the blower unit P wire in kW. These form the analysed mathematical model with the following results of study. Fluctuations in the full range of operating temperatures leads to a change in power consumption in the range –14.9 to 19.9%. Fluctuation of atmospheric pressure in the full range leads to a change in power consumption in the range from –3.4 to 3.7%. Relative humidity affects power consumption only in combination with high temperature (between 20-40°C the maximum influence of relative humidity is 0.6-3.3%). The combined influence of all three parameters in the full range is from –9.2 to 11.2%. All presented data are supplied also for the range of average values.
... plant design, aeration system, air flow rate (AFR), reactor hydraulics) and environmental conditions (influent wastewater oxygen demand and pollutant concentrations) [11][12][13]. Aeration is a critical research topic for wastewater treatment technologies involving the injection of air or oxygen such as activated sludge and membrane bioreactor technologies [12][13][14][15][16], biofilters [17], anaerobic ammonia oxidation (ANNAMOX) systems [18], granular sludge reactors [19] and aerated wetlands [7,20] as aeration is the main contributor of associated operational costs [21]. However, research on aeration in wetlands, especially oxygen transfer, has not evolved to the same extent as for activated sludge and membrane bioreactor technologies. ...
Aerated treatment wetlands are an increasingly recognized nature-based technology for wastewater treatment that relies heavily on mechanical aeration. Although aeration-mediated oxygen transfer into the wastewater can be impeded by wastewater pollutants, little is known about the link between the volumetric oxygen mass transfer coefficient kLa and the organic carbon concentration of the wastewater in aerated wetlands. In this study, oxygen transfer experiments were carried out in a lab-scale gravel column using clean water and wastewater from a pilot-scale horizontal flow (HF) aerated wetland treating domestic sewage. The α-factor, which describes the ratio of the volumetric oxygen mass transfer coefficient kLa in wastewater to clean water, was reduced by increasing soluble chemical oxygen demand (CODs). The derived regression equation α=1.066-1.372E-3mgCODsL-1 was incorporated into a numerical process model to simulate the impact of the reduced oxygen transfer on a hypothetical HF aerated wetland. The simulations revealed that α and treatment efficacy for nitrogen were substantially reduced by CODs at low aeration (kLa of 1 h⁻¹) and high influent wastewater strength (CODs of 300 mg L⁻¹). At the same kLa and influent CODs concentration, longitudinal gradients of α and concentrations for dissolved oxygen (DO), NH4-N and NOx-N in the simulated wetland were shifted up to 21% of wetland length downstream. These effects decreased with increasing kLa and were found to be negligible at kLa > 3 h⁻¹, which corresponds to an air flow rate of approximately 400 L m⁻² h⁻¹. Following this, higher organic carbon concentrations can reduce oxygen transfer in HF aerated wetland systems, thus resulting in decreased treatment efficacy.
... A diffused-air aeration system moves air into contact with water that leads to a sufficient dissolved oxygen (DO) level for the microorganisms present in an aerobic reactor. Keeping the DO at an optimal level requires high energy supply, which qualifies DO as a key parameter in the aeration tank system (Schraa et al., 2017). ...
In this work, effects of anionic surfactant on the oxygen transfer is investigated in a cylindrical reactor. For the first time, a systematic study in a wide surfactant concentration range, below and above the critical micellar concentration, is shown. A maximum reduction in the volumetric mass transfer coefficient (KLa) was observed for a surfactant concentration of 0.07 mM, before the CMC. Results are discussed based on the molecular interactions among water, air bubbles, and surfactant molecules. To better understand the experimental findings, a mathematical model was developed based on the estimated volumetric mass transfer coefficient. Predictions agree very well with the experimental results and point to a maximum reduction of 41% in the KLa, regardless of the inlet airflow rate in the reactor. We believe that our model can be used to predict future experimental results. In addition, this work can also be useful to better design an aerobic system.
... In diffusive aeration systems, the dissolved oxygen (DO) in bulk solution and the oxygen transfer coefficient (K L a) of diffusers are used to compile the DO-K L a feedback control loop for minimizing aeration energy [7]. Commercial process simulators are usually used to assist the design and optimization of aeration control, and accurate aeration models of diffusive aeration system are also available for engineers [9]. Diffusive aeration control can achieve energy savings ranging from 10% to 25% of total energy [10-13]. ...
Oxidation ditches are popularly used in rural areas and decentralized treatment facilities where energy deficiency is of concern. Aeration control technologies are well established for diffusion systems in order to improve energy efficiency, but there are still challenges in their application in oxidation ditches because surface aerators have unique characteristics with respect to oxygen transfer and energy consumption. In this paper, an integral energy model was proposed to include the energy, aeration, and fluidic effects of surface aerators, by which the energy for aeration of each aerator can be estimated using online data. Two types of rotating disks with different diameters (1800 mm and 1400 mm) were monitored in situ to estimate the model parameters. Furthermore, a feedforward–feedback loop control strategy was proposed using the concept of energy analysis and optimization. The simplified control system was implemented in a full-scale Orbal oxidation ditch, achieving an approximately 10% saving in full-process energy consumption. The cost–benefit analysis and carbon emission assessment confirmed the economic feasibility and environmental contribution of the control system. The energy model can help process designers and operators to better understand and optimally control the aeration process in oxidation ditches.
... The model combines the existing biofilm and sequencing batch reactor (SBR) models in SIMBA# (see ifak, 2017 for details). The new variable volume biofilm reactor model includes the detailed aeration system modeling capabilities discussed by Schraa et al. (2015Schraa et al. ( , 2016 and also allows aeration from the surface of the biofilm media to facilitate the modeling of membrane-aerated biofilm reactors (MABRs). ...
... The large changes in a factor experienced are of maximum interest for process designers, as they directly determine the process efficiency and concurrently the aeration power requirements. However, to date, constant a factors are the most common practice in process design and modelling, thus promoting inaccuracies in the oxygen provision and the constant need for model recalibration to compensate for a lack in structure (Amaral et al., 2017;Plano et al., 2011) Although many plants have efficiently deployed automatic control to address daily dynamics, many of them did not accurately account for the compounding effect on air requirements of decreasing OTE and a factors at peak loading, which led to the inefficient design of blowers and the concurrent incapacity to optimize the controllers Schraa et al., 2017). Therefore, many plants systematically discharge more air than required as they simply tend to use a conservative blower specification to compensate for the low a factors during peak loading periods. ...
... The large changes in a factor experienced are of maximum interest for process designers, as they directly determine the process efficiency and concurrently the aeration power requirements. However, to date, constant a factors are the most common practice in process design and modelling, thus promoting inaccuracies in the oxygen provision and the constant need for model recalibration to compensate for a lack in structure (Amaral et al., 2017;Plano et al., 2011) Although many plants have efficiently deployed automatic control to address daily dynamics, many of them did not accurately account for the compounding effect on air requirements of decreasing OTE and a factors at peak loading, which led to the inefficient design of blowers and the concurrent incapacity to optimize the controllers Schraa et al., 2017). Therefore, many plants systematically discharge more air than required as they simply tend to use a conservative blower specification to compensate for the low a factors during peak loading periods. ...
Due to the importance of wastewater aeration in meeting treatment requirements and due to its elevated energy intensity, it is important to describe the real nature of an aeration system to improve design and specification, performance prediction, energy consumption, and process sustainability. Because organic loadings drive aeration efficiency to its lowest value when the oxygen demand (energy) is the highest, the implications of considering their dynamic nature on energy costs are of utmost importance. A dynamic model aimed at identifying conservation opportunities is presented. The model developed describes the correlation between the COD concentration and the α factor in activated sludge. Using the proposed model, the aeration efficiency is calculated as a function of the organic loading (i.e. COD). This results in predictions of oxygen transfer values that are more realistic than the traditional method of assuming constant α values.
... The large changes in a factor experienced are of maximum interest for process designers, as they directly determine the process efficiency and concurrently the aeration power requirements. However, to date, constant a factors are the most common practice in process design and modelling, thus promoting inaccuracies in the oxygen provision and the constant need for model recalibration to compensate for a lack in structure (Amaral et al., 2017;Plano et al., 2011) Although many plants have efficiently deployed automatic control to address daily dynamics, many of them did not accurately account for the compounding effect on air requirements of decreasing OTE and a factors at peak loading, which led to the inefficient design of blowers and the concurrent incapacity to optimize the controllers Schraa et al., 2017). Therefore, many plants systematically discharge more air than required as they simply tend to use a conservative blower specification to compensate for the low a factors during peak loading periods. ...
The aeration process is the largest energy consumer in wastewater treatment plants (WWTP), and the optimization of the process based on computational models can offer significant savings for the plant. Recent theoretical developments have revealed that many of the parameters commonly assumed as constants in aeration modelling have a dynamic nature; however, there still lacks a universal way to model these factors accurately. This work proposed a new framework and optimal time scale for building hybrid machine learning-mechanistic oxygen transfer rate models. The data used for the modelling were from existing sensors in a full-scale WWTP supplemented by a two-month field sampling campaign for collecting 15 min of dynamic data, including off-gas air fraction. Two machine learning (ML) models were built: the first set was a novel hybrid ML-mechanistic model developed to estimate the off-gas aeration fraction from wastewater parameters and calculate dynamic alpha per the off-gas method. The second ML model directly estimated dynamic alpha from wastewater process and operation parameters. The performance of the ML models was compared with recalibrated published regression-based models. Results showed that the hybrid ML-mechanistic alpha model (NSE = 0.96 and RMSE = 0.03) were more accurate than the direct ML alpha (NSE = 0.67 and RMSE = 0.07) and recalibrated regression (NSE < 0 and RMSE = 0.18 to 0.28) models, and therefore could be efficiently applied to predict dynamic alpha factor of an activated sludge plant. In addition to typical surfactant indicators such as MLSS and COD, other parameters like temperature and aeration ammonia concentration were substantial in dynamic off-gas and alpha estimation.
Energy demand of wastewater treatment plants (WWTPs) is counted as an important energy consumer especially in developed and developing countries. Environmental concerns and pressure on freshwater resources have caused to implement energy-intensive wastewater treatment processes and thus highly energy demanding WWTPs. Although wastewater itself holds a substantial amount of chemical and thermal energy, conventional management practices are not capable of recovery this energy content. Since there has been a paradigm shift in wastewater treatment, wastewater is considered as an important energy source. Several new applications have been developed to recover energy content of wastewater as much as possible. The recent developments in materials and equipment also promise a significant opportunity to improve energy balance of WWTPs. Adopting new measures and enhancing energy recovery show that WWTPs do not have to be energy consumers no longer, instead they can be energy-neutral and even energy-positive systems.
Energy costs in the wastewater industry are increasing due to increasing trends in electricity rates and more stringent requirements for effluent quality. Wastewater aeration process is typically the largest energy consumer of the treatment plant and the optimization of the aeration process can offer significant savings for the wastewater treatment plants (WWTPs). Utilization of dynamic models can offer optimization solutions for improving the energy efficiency and process performance. In this work a simplified modelling approach emphasizing the control valves and the blowers is tested by developing aeration system models for two Finnish WWTPs. The developed model requires calibration of only a single parameter and the results from the simulations showed that reasonable estimations of the aeration systems energy demand could be made with a limited knowledge on the details of the physical system. The promising results highlight the strong influence of the control valve positioning to the whole system and indicate that airflow distribution along the system could be estimated simply from the positioning of the valves. The presented modelling approach allows the comparison between different blower and control valve alternatives during operation and for the process upgrades and offers prospect for improving the aeration operation control strategies. HIGHLIGHTS
An aeration system model based on valve positions was developed.;
The model only requires calibration of one parameter.;
Energy consumption can be estimated with only a limited information about the physical aeration system.;
Valve positions impact the system power consumption and the air distribution between and within aeration grids.;
Including valve positions opens possibilities to improve the aeration control strategies.;
Future energy systems must mainly generate electricity from renewable resources. To deal with the fluctuating availability of wind and solar power, new versatile electricity markets and sustainable solutions concentrating on energy flexibility are needed. In this research, we investigated the potential of energy flexibility achieved through demand-side response for the wastewater treatment plant of the Benchmark Simulation Model 1. First, seven control strategies were simulated and assessed. Next, the flexibility calls were identified, two energy flexibility scenarios were defined and incorporated into the model, and the control strategies were evaluated anew. In this research, the effluent ammonia concentration needed to be maintained within the limits for as long as possible. Strategy 5, which controlled ammonia in Tank 5 at a low value and did not control any nitrate in Tank 2, of Scenario 1, which was characterized by an undetermined on/off aeration cycle, was then found to be the best. Although this control strategy led to high total energy consumption, the percentage of time during which aeration was nearly suspended was one of the highest. This work proposes a methodology that will be useful to plant operators who should soon reduce energy consumption during spikes in electricity prices.
Current practice to enhance resilience in Water Resource Recovery Facilities (WRRFs) is to ensure redundancy or back-up for most critical equipment (e.g. pumps or blowers). Model-based assessment allows evaluation of different strategies for quantitatively and efficiently enhancing resilience and justifying the allocation of resources. The goal of this study is to provide guidance for the development of tailored deterministic models of full-scale WRRFs. A framework for model-based resilience assessment is proposed that provides guidance on data collection, model selection, model calibration and scenario analysis. The framework is embedded into the Good Modelling Practice (GMP) Unified Protocol, providing a new application for resilience assessment and an initial set of stressors for WRRFs. The usefulness of the framework is illustrated through a resilience assessment of the WRRF of Girona against power outage. Results show that, for the Girona facility, limited energy back-up can cause non-compliance of WRRF discharge limits in the case of a blower power shut-down of 6h, and around 12h when the blower shut-down is also combined with a shut-down of the recirculation pumps. The best option to enhance resilience would be increasing the power back-up by 218%, which allows the plant to run with recirculation pumps and blowers at minimum capacity. In such a case, resilience can be further enhanced by manipulating the air supply valves to optimise the air distribution, to balance oxygen needs in each reactor with the overall system pressure. We conclude that, with industry consensus on what is considered an acceptable level of resilience, a framework for resilience assessment would be a useful tool to enhance the resilience of our current water infrastructure. Further research is needed to establish if the permit structure should accommodate levels of functionality to account for stress events.
In Germany, the EU and worldwide, the demands placed on wastewater treatment plants in terms of energy efficiency, cleaning performance, operational reliability and minimization of operating costs continue to grow. This results in an increasing demand for automation solutions with high quality requirements that are well integrated with process engineering and equipment specification. Integrated planning of automation technology in combination with mechanical equipment and process engineering requires the use of simulation tools in planning. A once existing simulation model of a plant can then not only be used for planning and optimization, but also for further questions about the life cycle of the plant. The automation concept simulated during the planning can be used, for example, as a precise requirement specification for the programming of the automation. A powerful application scenario is the virtual commissioning of the automation system.
Gas-liquid mass transfer in wastewater treatment processes has received considerable attention over the last decades both from academia and industry. Indeed, improvements in modelling gas-liquid-mass transfer can bring huge benefits in terms of reaction rates, plant energy expenditure, acid-base equilibria and greenhouse gas emissions. Despite these efforts, there is still no universally valid correlation between the design and operating parameters of a wastewater treatment plant and the gas-liquid mass transfer coefficients. That is why the current practice for oxygen mass transfer modelling is to apply overly simplified models, which come with multiple assumptions that are not valid for most applications. To deal with these complexities, correction factors were introduced over time. The most uncertain of them is the α-factor. To build fundamental gas-liquid mass transfer knowledge more advanced modelling paradigms have been applied more recently. Yet, these come with a high level of complexity making them impractical for rapid process design and optimisation in an industrial setting. However, the knowledge gained from these more advanced models can help in improving the way the α-factor and thus gas-liquid mass transfer coefficient should be applied. That is why the presented work aims at clarifying the current state-of-the-art in gas-liquid mass transfer modelling of oxygen and other gases, but also to direct academic research efforts towards the needs of the industrial practitioners.
Ammonia-based aeration control (ABAC) is a cascade control concept for controlling total ammonia nitrogen (NHx-N) in the activated sludge process. Its main goals are to tailor the aeration intensity to the NHx-N loading and to maintain consistent nitrification, to meet effluent limits but minimize energy consumption. One limitation to ABAC is that the solids retention time (SRT) control strategy used at a water resource recovery facility (WRRF) may not be consistent with the goals of ABAC. ABAC-SRT control is a strategy for aligning the goals of ammonia-based aeration control and SRT control. A supervisory controller is used to ensure that the SRT is always optimal for ABAC. The methodology has the potential to reduce aeration energy consumption by over 30% as compared to traditional dissolved oxygen (DO) control. Practical implementation aspects are highlighted for implementation at full scale, such as proper selection of the set point for the supervisory controller, proper calculation of the rate of change in sludge inventory, using a mixed liquor suspended solids (MLSS) controller, and tuning of the controllers. In conclusion, ABAC-SRT is a promising approach for coordinated control of SRT, total ammonia nitrogen, and dissolved oxygen in the activated sludge process that balances both treatment performance and energy savings.
The wastewater industry is currently facing dramatic changes, shifting away from energy-intensive wastewater treatment towards low-energy, sustainable technologies capable of achieving energy positive operation and resource recovery. The latter will shift the focus of the wastewater industry to how one could manage and extract resources from the wastewater, as opposed to the conventional paradigm of treatment. Debatable questions arise: Can the more complex models be calibrated, or will additional unknowns be introduced? After almost 30 years using well-known International Water Association (IWA) models, should the community move to other components, processes, or model structures like 'black box' models, computational fluid dynamics techniques, etc.? Can new data sources - e.g. on-line sensor data, chemical and molecular analyses, new analytical techniques, off-gas analysis - keep up with the increasing process complexity? Are different methods for data management, data reconciliation, and fault detection mature enough for coping with such a large amount of information? Are the available calibration techniques able to cope with such complex models? This paper describes the thoughts and opinions collected during the closing session of the 6th IWA/WEF Water Resource Recovery Modelling Seminar 2018. It presents a concerted and collective effort by individuals from many different sectors of the wastewater industry to offer past and present insights, as well as an outlook into the future of wastewater modelling.
This paper introduces the application of a fully dynamic air distribution model integrated with a biokinetic process model and a detailed process control model. By using a fully dynamic air distribution model, it is possible to understand the relationships between aeration equipment, control algorithms, process performance, and energy consumption, thus leading to a significantly more realistic prediction of water resource recovery facility (WRRF) performance. Consequently, this leads to an improved design of aeration control strategies and equipment. A model-based audit has been performed for the Girona WRRF with the goal of providing a more objective evaluation of energy reduction strategies. Currently, the Girona plant uses dissolved oxygen control and has been manually optimised for energy consumption. Results from a detailed integrated model show that the implementation of an ammonia-based aeration controller, a redistribution of the diffusers, and the installation of a smaller blower lead to energy savings between 12 and 21%, depending on wastewater temperature. The model supported the development of control strategies that counter the effects of current equipment limitations, such as tapered diffuser distribution, or over-sized blowers. The latter causes an intermittent aeration pattern with blowers switching on and off, increasing wear of the equipment.
Aeration is the largest energy consumer in most water and resource recovery facilities, which is why oxygen transfer optimization is fundamental to improve energy efficiency. Although oxygen transfer is strongly influenced by the bubble size distribution dynamics, most aeration efficiency models currently do not include this influence explicitly. In few cases, they assume a single average bubble size. The motivation of this work is to investigate this knowledge gap, i.e. a more accurate calculation of the impact of bubble size distribution dynamics on oxygen transfer. Experiments were performed to study bubble size distribution dynamics along the height of a bubble column at different air flow rates for both tap water and solutions that mimic the viscosity of activated sludge at different sludge concentrations. Results show that bubble size is highly dynamic in space and time since it is affected by the viscosity of the liquid and hydrodynamics. Consequently, oxygen transfer also has a dynamic character. The concept of a constant overall volumetric oxygen transfer coefficient, KLa, can thus be improved. A new modeling approach to determine the KLa locally based on measurements of the bubble size distribution dynamics is introduced as an alternative. This way, the KLa for the entire column is calculated and compared to the one measured by a traditional method. Results are in good agreement for tap water. The modeled KLa based on the new approach slightly overestimates the experimental KLa for solutions that mimic the viscosity of activated sludge. The difference appears to be lower when the air flow rate increases. This work can be considered as a first step towards more accurate and rigorous mechanistic aeration efficiency models which are based on in-depth mechanism knowledge. This is key for oxygen transfer optimization and consequently energy savings.
Aeration is an essential component of aerobic biological wastewater treatment and is the largest energy consumer at most water resource recovery facilities. Most modelling studies neglect the inherent complexity of the aeration systems used. Typically, the blowers, air piping, and diffusers are not modelled in detail, completely mixed reactors in a series are used to represent plug-flow reactors, and empirical correlations are used to describe the impact of operating conditions on bubble formation and transport, and oxygen transfer from the bubbles to the bulk liquid. However, the mechanisms involved are very complex in nature and require significant research efforts. This contribution highlights why and where there is a need for more detail in the different aspects of the aeration system and compiles recent efforts to develop physical models of the entire aeration system (blower, valves, air piping and diffusers), as well as adding rigour to the oxygen transfer efficiency modelling (impact of viscosity, bubble size distribution, shear and hydrodynamics). As a result of these model extensions, more realistic predictions of dissolved oxygen profiles and energy consumption have been achieved. Finally, the current needs for further model development are highlighted.
Direct solutions of pipe flow problems are not possible because of the implicit form of Colebrook-White equation which expresses the hydraulic resistancee of commercial pipes. The three basic and major problems encountered in hydraulic engineering practice are the determination of pipe diameter, the discharge and the head loss. The solution of these problems on conventional lines involves many trials and tedious computations. Some research workers have proposed graphical solutions which have their own inherent limitations. Reported herein are explicit and accurate equations for pipe diameter and head loss and a closed form solution for the discharge through the pipe, based on Colebrook-White equation. These explicit equations can also be utilized with advantage in optimization studies of pipelines and water distribution systems.
This review covers automatic control of continuous aeration systems in municipal wastewater treatment plants. The review focuses on published research in the 21st century and describes research into various methods to decide and control the dissolved oxygen (DO) concentration and to control the aerobic volume with special focus on plants with nitrogen removal. Important aspects of control system implementation and success are discussed, together with a critical review of published research on the topic. With respect to DO control and determination, the strategies used for control span from modifications and developments of conventional control methods which have been explored since the 1970s, to advanced control such as model-based predictive and optimal controllers. The review is supplemented with a summary of comparisons between control strategies evaluated in full-scale, pilot-scale and in simulations.
Aeration consumes about 60% of the total energy use of
a wastewater treatment plant (WWTP) and therefore is a major
contributor to its carbon footprint. Introducing advanced process
control can help plants to reduce their carbon footprint and at the
same time improve effluent quality through making available unused
capacity for denitrification, if the ammonia concentration is below
a certain set-point. Monitoring and control concepts are cost-saving
alternatives to the extension of reactor volume. However, they also
involve the risk of violation of the effluent limits due to measuring errors,
unsuitable control concepts or inadequate implementation of the
monitoring and control system. Dynamic simulation is a suitable tool
to analyze the plant and to design tailored measuring and control
systems. During this work, extensive data collection, modeling and fullscale
implementation of aeration control algorithms were carried out at
three conventional activated sludge plants with fixed pre-denitrification
and nitrification reactor zones. Full-scale energy savings in the range of
16–20% could be achieved together with an increase of total nitrogen
removal of 40%.
The immense environmental challenges facing the world now and in years to come can only be met through marshalling the talents of the best environmental engineers and scientists, and through the use of innovative, cost-effective solutions. Written by three leading aeration experts, Aeration: Principles and Practice, covers the principles and practice of aeration, a unit process critical to the performance of activated sludge treatment and to the budget of wastewater plants. This reference presents the state of the art in aeration, using examples from a variety of facilities in the USA and Europe. The authors investigate conventional and deep-tank aeration systems for BOD removal and nitrification, as well as high-purity oxygen systems. Operating and capital costs, as well as energy use data are presented together with design information allowing the adaptation of new aeration technologies to plants of diverse size. Both practitioners and advanced students of wastewater management will appreciate the detailed presentation of oxygen transfer principles, especially as they are illustrated in numerous applications. With the information presented in this book engineers and managers will be able to understand and monitor the efficiency of existing aeration systems and to develop strategies for process improvement.
During the design of a water resource recovery facility (WRRF), it is becoming industry practice for process engineers to use simulation software to assist with the design of the plant and its aeration system. The aeration process is one of the key components of the activated sludge process, and as such, is one of the most important aspects of modeling wastewater treatment systems. A comprehensive aeration system model has been developed in SIMBA# that can model both the air supply and demand. The model includes sub-models for centrifugal and positive displacement blowers, pipes and fittings, valves, and diffusers. Both compressible and incompressible flow can be modelled. Oxygen transfer within aeration tanks is also included as part of the overall model. The aeration system model allows engineers to analyze aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, and controller design and tuning. This will allow more detailed system-wide testing before commissioning.
During the design of a water resource recovery facility (WRRF), it is becoming industry practice for process engineers to use simulation software to assist with the design of the plant and its aeration system. The aeration process is one of the key components of the activated sludge process, and as such, is one of the most important aspects of modeling wastewater treatment systems. A comprehensive aeration system model has been developed in SIMBA# that can model both the air supply and demand. The model includes sub-models for centrifugal and positive displacement blowers, pipes and fittings, valves, and diffusers. Both compressible and incompressible flow can be modelled. Oxygen transfer within aeration tanks is also included as part of the overall model. The aeration system model allows engineers to analyze aeration systems as a whole to determine biological air requirements, blower performance, air distribution, control valve impacts, and controller design and tuning. This will allow more detailed system-wide testing before commissioning.
The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.
EPANET 2-User's Manual
- L A Rossman
Rossman, L. A. EPANET 2-User's Manual, National Risk
Management Research Laboratory Office of Research and
Development, US Environmental Protection Agency,
Cincinnati, OH 45268.
The Effect of Membrane Diffuser Design and Geometry on Fine-Bubble Performance. Technical White Paper, US Filter - Envirex Diffused Aeration Products
- R D Sproull
- M L Doyle
- L B Ratzlow
Sproull, R. D., Doyle, M. L. & Ratzlow, L. B. The Effect of
Membrane Diffuser Design and Geometry on Fine-Bubble
Performance. Technical White Paper, US Filter -Envirex
Diffused Aeration Products.
- Mueller