Diffused Aeration Systems Research Articles

CFD simulation for comparative of hydrodynamic effects in biochemical reactors using population balance model with varied inlet gas distribution profiles

Abstract The operational efficiency of the airlift reactors relies significantly on the aeration and mixing provided by the inlet system. The diffused aeration system is the most energy-intensive component affecting the operation of the bioreactor, accounting for 45–75 % of the energy costs. This study presents a coupled CFD-PBM to investigate the collective impacts of multiple bubble diameters, variations in inlet gas distribution types, and flow rates on the hydrodynamic characteristics of bubble columns. The simulation results were validated through comprehensive comparisons with experimental data. The experimental data and simulations of the single bubble size model (SBSM) and multi-bubble size model (MBSM) were compared, proposing an enhanced inlet gas distribution type. The results indicate a close resemblance between the MBSM data and the experimental results, with an error margin not exceeding 5 %. Moreover, different flow rates were found to cause varying sensitivities in the bubble size distribution (BSD) within the column. Furthermore, the simulation results validate the similarity between lift coefficients and critical diameters to experiments and shed light on favorable conditions for reactor design. The key findings of this study encompass: (1) the use of MBSM can accurately predict the tower system characteristics; (2) the column circulation is intensified with small inlet bubble size and high gas velocity, which is favorable for chemical reactions and microbial aggregation to proceed; and (3) the BSD is not sensitive to the inlet gas distribution type at high flow rates.

Analyzing the effects of different patterns of diffuser layouts on air distribution and mixing quality in an aeration tank using CFD

A diffused aeration system in an activated sludge process is generally designed based on a limited number of typical diffuser patterns. There are instances where these patterns have shown various operational and process issues, resulting in poor process performance. To address these issues, bubble interactions and the mixing conditions must be factored into the design of alternative patterns. For this purpose, a validated full-scale CFD model was used to simulate five diffuser patterns: lateral, grid, line, stripe, and checkered patterns‎, the first two in operation, and the next three as designed patterns. The patterns were simulated for an aeration tank under identical functional conditions. The population balance method was incorporated into the model to analyze how various patterns affect bubble size distribution. It was found that changing the pattern of diffusers had a significant effect on bubble breakup/coalescence and the resulting bubble size distribution. The results showed a range of 6.1–22.9% for the coarse bubble percentage and 11.9–16% for the fine bubble percentage in the five simulated patterns. Moreover, evaluating flow circulation and velocity based on the average flow velocity data of the simulated patterns showed up to 30% variation, ranging from 0.29 to 0.375 m/s. It highlighted the great impact of diffuser patterns on bubble size and mixing quality as notable factors in oxygen transfer efficiency. Finally, the proposed patterns were compared and ranked based on air distribution, bubble size, and mixing quality criteria using the multi-criteria decision-making (MCDM) approach. It was concluded that the checkered pattern outperformed the proposed and existing ones.

Effects of Environmental Conditions and Bed Configuration on Oxygen Transfer Efficiency in Aerated Constructed Wetlands

This research evaluated the oxygen transfer efficiency in beds to be used as aerated constructed wetlands. The research methods included oxygen transfer efficiency evaluations in several bed configurations using diffused aeration systems. Experiments were conducted at two locations with different environmental conditions: a) Talca (Chile), 120 m above sea level (m.a.s.l.), 0.99 Atm and b) Cajicá (Colombia), 2550 m.a.s.l., 0.76 Atm. A column with only clean water and three bed configurations representing aerated constructed wetlands were evaluated. These configurations included: (a) coarse gravel, (b) coarse gravel with an empty core in the middle (inner container), and (c) fine gravel. Three airflow rates were evaluated: (a) low, 0.7 L/min; (b) medium, 2.5 L/min; and (c) high, 3.6 L/min. The overall oxygen mass transfer coefficient, standard oxygen transfer rate, and standard oxygen transfer efficiency were the variables calculated from the oxygen transfer evaluation tests. The research results indicated that in diffused aeration systems, oxygen transfer efficiency was negatively influenced by environmental conditions, particularly altitude, which limits the driving force for oxygen transfer into water. Furthermore, the results showed that the size of the gravel used in the bed is related to the oxygen transfer efficiency: the larger the gravel size, the higher the oxygen transfer, regardless of the altitude. Finally, research regarding oxygen transfer in aerated constructed wetlands has signaled the need for a standard procedure for aeration testing, and this work suggests a new methodology.

How elevation dictates technology selection in biological wastewater treatment

Most wastewater treatment facilities are built using procedures from previous designs which are predominantly from sites and regions not located at high elevation. Recognizing this limitation, we assessed the effects of elevation above sea level on the suitability of process configurations and technologies as well as their associated energy costs. Using the International Water Association (IWA) benchmark simulation model No. 2 (modified Ludzack-Ettinger process layout) as a reference, we simulated scenarios including different activated sludge process configurations, operating under different environmental and process conditions. In order to include a wide sample of environmental conditions, data on atmospheric pressure, wastewater temperature, air temperature, and relative humidity (for average, warmest, and coldest months) were collected from municipal wastewater treatment plants located at different latitudes and elevations. The results confirm that elevation is a driver against the selection of diffused aeration technologies. Aeration costs for aerobic wastewater treatment are highly influenced by local project conditions, particularly by elevation and wastewater temperature, which together influence the driving force for oxygen transfer into water. When the driving force is low, operating costs are high. Recommendations for designing treatment processes effectively, including diffused aeration systems operating at high elevations above sea level, are proposed.

Comprehensive investigation of SARS-CoV-2 fate in wastewater and finding the virus transfer and destruction route through conventional activated sludge and sequencing batch reactor

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA transmission route was thoroughly investigated in the hospital wastewater, sewage collection network, and wastewater treatment plants. Samples were taken on four occasions from December 2020 to April 2021. The performance of two different wastewater treatment processes of sequencing batch reactor (SBR) and conventional activated sludge (CAS) was studied for virus destruction. For this purpose, liquid phase, solid phase and bioaerosol samples were taken from different units of the investigated wastewater treatment plants (WWTPs). The results revealed that all untreated hospital wastewater samples were positive for SARS-CoV-2 RNA. The virus detection frequency increased when the number of hospitalized cases increased. Detection of viral RNA in the wastewater collection system exhibited higher load of virus in the generated wastewater in areas with poor socioeconomic conditions. Virus detection in the emitted bioaerosols in WWTPs showed that bioaerosols released from CAS with surface aeration contains SARS-CoV-2 RNA posing a potential threat to the working staff of the WWTPs. However, no viral RNA was detected in the bioaerosols of the SBR with diffused aeration system. Investigation of SARS-CoV-2 RNA in WWTPs showed high affinity of the virus to be accumulated in biosolids rather than transporting via liquid phase. Following the fate of virus in sludge revealed that it is completely destructed in anaerobic sludge treatment process. Therefore, based on the results of the present study, it can be concluded that receiving water resources could not be contaminated with virus, if the wastewater treatment processes work properly.

Upgrading of waste stabilization ponds using a low-cost small-scale fine bubble diffused aeration system.

Waste Stabilization Ponds (WSPs) are known for the economical treatment of wastewater, especially if low-cost land is available. In this research to overcome some common operational problems such as undesirable color changes in ponds, severe odor problems, and most importantly, deviations from the effluent standards, the performance of a novel installation of a small-scale fine bubble diffused aeration system in the inlet zone of the facultative pond has been investigated. The long-term operational data of the system in two wastewater treatment plants in the east of Iran demonstrated that this system can significantly improve the efficiency of the treatment plant in addition to eliminating the mentioned operational problems. Pre-aeration of the inlet zone of the facultative ponds (Birjand WSPs) by the aeration system consists of 250 fine bubble disk diffusers (12 inches in diameter) and one 22 kW roots blower showed that purple color and odor problem can eliminate after almost two weeks and organic matter removal efficiency increased from 58 ± 15% to about 85 ± 10% based on BOD5. Almost similar results were obtained from the WSPs of Neyshabur. Long-term experimental results showed this system can be used successfully to control the process and upgrade these natural and efficient treatment processes, especially in developing countries.

Prediction of standard aeration efficiency of a propeller diffused aeration system using response surface methodology and an artificial neural network

AbstractAeration experiments were conducted in a masonry tank to study the effects of operating parameters on the standard aeration efficiency (SAE) of a propeller diffused aeration (PDA) system. The operating parameters included the rotational speed of shaft (N), submergence depth (h), and propeller angle (α). The response surface methodology (RSM) and an artificial neural network (ANN) were used for modelling and optimizing the standard aeration efficiency (SAE) of a PDA system. The results of both approaches were compared for their modelling abilities in terms of coefficient of determination (R2), root mean square error (RMSE), and mean absolute error (MAE), computed from experimental and predicted data. ANN models were proved to be superior to RSM. The results indicate that for achieving the maximum standard aeration efficiency (SAE), N, h and α should be 1,000 rpm, 0.50 m, and 12°, respectively. The maximum SAE was found to be 1.711 kg O2/ kWh. Cross-validation results show that best approximation of the optimal values of input parameters for maximizing SAE is possible with a maximum deviation (absolute error) of ±15.2% between the model predicted and experimental values.

Evaluating the performance of fine bubble diffused aeration systems in cylindrical aeration tanks by fuzzy c-means algorithm.

In order to make a comprehensive evaluation of the performance of fine bubble diffused aeration systems in cylindrical aeration tanks, the following parameters are considered: distribution of DO concentration in the horizontal direction of the different water depth of an aeration tank, distribution of DO concentration in the vertical direction of the aeration tank, distribution of DO concentration in all the points of the aeration tank and ratio of total mass increment of DO in the aeration tank to total mass of oxygen in aeration air. The aeration characteristic criterion (ACC) is proposed to synthesize these parameters, and weighted sums of the similarity degrees derived from the extensions of fuzzy c-means (FCM) are used to construct ACC. The results show that compared with total volumetric oxygen transfer coefficient (KLα) and specific standard oxygen transfer efficiency (SSOTE), ACC is demonstrated to be more marked and sensitive for the performance evaluation of the fine bubble diffused aeration systems equipped with fine-pore aeration tubes. Moreover the performance of aeration systems equipped with different layouts of fine-pore aeration tubes is comparatively studied, and their performance from best to worst is ring-type diffuser > square-type diffuser > parallel-lines-type diffuser > cross-type diffuser.

Numerical modelling of oxygen mass transfer in diffused aeration systems: A CFD-PBM approach

The diffused aeration process is the most energy-intensive operation of bioreactor treatment, amounting to 45–75 % of the plant energy costs. To improve its efficiency, it is essential to measure the oxygen transfer rate from the aerators to wastewater. In this study, a multiphase mixture computational fluid dynamics (CFD) model is developed using k-ε turbulence closure equations along with a discrete population balance model (PBM) add-on with specific bubble classes, to predict the oxygen mass transfer. The transfer of oxygen species from air to water is modeled using the species transport model. The PBM is used to analyze the formation, growth, breakage, and coalescence of air bubbles. The validated model is then extended for sensitivity analysis for a diffused aeration system in a bench-scale aeration tank. Results show that, the volumetric oxygen mass transfer coefficient increases by 15 %, with a decrease of air bubble size by 10 %. The air bubbles have a wider distribution, with a larger diameter near the bottom of the bioreactor and a narrow distribution with a smaller bubble size at the top. Results show that, in the bioreactor, the dissolved oxygen concentration reaches the equilibrium or saturation value when the height by breadth ratio is 2.5 and does not increase further with increase in height of the water column. Also, the air bubble size of 6 mm was the efficient bubble size for a fixed airflow rate of 1.45 m3 h−1.

Supersaturated-oxygen aeration effects on a high-loaded membrane bioreactor (HL-MBR): Biological performance and microbial population dynamics

Conventional diffused aeration systems (such as fine-bubble diffusers) exhibit a poor oxygen transfer in wastewater treatment plants (WWTPs), particularly when operating at sludge concentrations higher than 15 g L−1. The supersaturated dissolved oxygen (SDOX) system has been proposed as an alternative for supplying dissolved oxygen (DO) at high mixed liquor suspended solids (MLSS) concentrations. The advantages introduced by such technology include the possibility of operating WWTPs at much higher than usual MLSS concentrations, increasing the treatment capacity of WWTPs. Recent studies have demonstrated that the SDOX system has higher oxygen transfer rates (OTRs) and oxygen transfer efficiencies (OTEs) relative to fine-bubble diffusers. However, it is unknown if the high-pressure conditions introduced by SDOX may possibly impact the biological performance of WWTPs. In this study, the effects of SDOX technology on the biological performance of a membrane bioreactor (MBR) were evaluated. The MBR was operated at an MLSS concentration of approximately 15 g L−1 in four phases as follows: (P1) with bubble diffusers, (P2) with an SDOX unit, (P3) with the bubble diffusers, and (P4) with the SDOX unit. The performance of the MBR was assessed by monitoring the sludge concentration, as well as changes in the particle size distribution (PSD), sludge activity, organic matter removal and nitrification performance, and changes in the microbial community within the MBR. The operational conditions exerted by the SDOX technology did not affect the concentration of active biomass during the study period. The biological performance of the MBR was not affected by the introduction of the SDOX technology. Finally, the microbial community was relatively stable although some variations at the family and genus level were evident during each of the study phases. Therefore, the SDOX system can be proposed as an alternative technology for DO supply in WWTPs increasing the overall treatment capacity.

Study of air–water interface generator as oxygen transfer enhancer in diffused aeration system

ABSTRACT An air–water interface generator is a proposed apparatus aiming to enhance oxygen transfer by increasing the contact area between air and water. It has been proved that this apparatus could increase the volumetric oxygen transfer coefficient in laboratory scale. Therefore, it is important to study about upscaling in order to apply this apparatus in an actual aeration tank. In this study, the apparatus was investigated for the effect of water volume and water depth on the oxygen transfer process. Referring to the experimental results, an air flow rate approximately 31.5–32.5 m3/hr per m3 of water could achieve satisfactory degrees of both oxygen transfer performance and efficiency. Moreover, an empirical model with ±40 % error was also proposed. From the model, the enhancement of volumetric oxygen transfer coefficient around 30 % could be estimated.

KKR and XPV Water Partners add to water quality platform

The water quality solutions platform that investment firm KKR and Canada's XPV Water Partners set up last year has acquired Environmental Dynamics International Inc (EDI), a US manufacturer of diffused aeration systems (see Filtration Industry Analyst, December 2019).

Oxygen transfer performance of a supersaturated oxygen aeration system (SDOX) evaluated at high biomass concentrations

Oxygen transfer in wastewater treatment is significantly influenced by the mixed liquor suspended solids (MLSS). The effect is more pronounced at MLSS concentrations higher than 20g L−1 when supplying air by conventional diffused aeration systems. The oxygen transfer performance of a supersaturated oxygenation technology (i.e., the supersaturated dissolved oxygen (SDOX) system) was evaluated in clean water and in activated sludge with MLSS concentrations from 4 to 40gL−1 as a promising technology for uncapping such limitation. The evaluation was carried out at the laboratory facilities of the faculty of food technology and biotechnology at the University of Zagreb. The sludge was collected from a full-scale conventional activated sludge (CAS) wastewater treatment plant (WWTP) operated at a solid retention time (SRT) of approximately 5 days. The evaluation was carried out using a laboratory-scale setup consisting of a bench-scale SDOX system (2.75L) supplying pure oxygen to a 5L biological reactor. The SDOX exhibited oxygen mass transfer rate coefficient (KLa) values (2.6h−1) in clean water lower than for fine bubble diffusers (11h−1). However, higher oxygen transfer rate (OTR) values and alpha factors (mass transfer ratio of process-water to clean-water) as a function of the MLSS concentration were observed. A standard oxygen transfer efficiency (SOTE) of approximately 100 % in clean water was reported. The SDOX technology can be presented as a promising alternative for supplying dissolved oxygen (DO) into mixed liquor solutions; particularly, at the high MLSS concentrations required by high-loaded membrane bioreactor (HL-MBR) systems and aerobic digesters.

Improvement of oxygen transfer by increasing contact area between gas and liquid using air–water interface generator

ABSTRACT In this current study, an apparatus called air–water interface generator was investigated for oxygen transfer enhancement by increasing a contact area between air and water. The effect of this apparatus and its optimal installation condition were investigated. The total volumetric oxygen transfer coefficient (kLaT ) in this study was divided into volumetric oxygen transfer for the bubble transfer (kLaB ), volumetric oxygen transfer for the inner interface transfer inside the apparatus (kLaI ), and volumetric oxygen transfer for the free water surface transfer (kLaS ). The experimental results show that the apparatus installed near the water surface could increase the oxygen transfer 18% due to the presence of an inner interface. Additionally, the effect of the number and the position of apparatus on bubble dispersion should also be taken into consideration since the bubble transfer makes the most significant contribution to total oxygen transfer in the diffused aeration system.

A study of optimum aeration efficiency of a lab‐scale air‐diffused system

AbstractUp to now, tremendous efforts have been devoted to modelling the oxygen transfer coefficient (kLa) for diffused aeration systems, while not considering the corresponding energy consumption. Enhancing kLa is favorable for an exemplary oxygenation process, but may come at the cost of greater energy withdrawal, an unwelcome tradeoff. Assessing the aeration efficiency (the rate of oxygen delivered per unit energy) reflects the overall effectiveness of an aeration process and guarantees a superior system performance. Presented here is a lab‐scale study that investigates the effect of the orifice diameter, the airflow rate and the water column on the aeration efficiency. Various combinations of the studied parameters were tested using a cylindrical tank with a single orifice for air injection. An optimal performance of the aeration efficiency was observed at an orifice diameter of 0.3 mm when tested under 0.91 m water column and an airflow rate of 0.05 SLPM. Furthermore, a new empirical formula of aeration efficiency was established with a high correlation index (R2 = 0.97) to allow preliminary prediction of aeration efficiency.

Study of Liquid Film Forming Apparatus (LFFA) Mechanisms in Terms of Oxygen Transfer and Bubble Hydrodynamic Parameters

Aeration system is extensively applied in aquaculture and waste water treatment. It provides oxygen for organism living and mixing while consumes colossal amounts of energy for operating. Hence, the improvement of aeration system is not only providing enough oxygen and mixing but also concerning to the energy saving. Liquid film forming apparatus (LFFA) is a simple equipment that itself does not consume any power. It can be installed in existing conventional aeration system without large-scale retrofitting. Laboratory scale experiment were performed in a 190-litre aeration tank. The different types of air diffuser providing different bubble aspects were installed at the bottom of the aeration tank as the conventional diffused aeration systems. The volumetric mass transfer coefficient (k_L a) of the aeration systems with LFFA are higher than the conventional systems notably. The mechanism of oxygen transfer in LFFA system can be summarized into 4 patterns: 1) Conventional mechanism, 2) Bubble collection mechanism, 3) Bubble recirculation mechanism and 4) Bubble-Liquid Foam mechanism. Then, the interfacial area (a) is improved comparing with the conventional diffused aeration system. The LFFA system should be operated with small bubble diameter generation ( 0.13 m/s) can be defined and provide a better understanding on oxygen transfer mechanism in LFFA system.

A New Control and Design of PEM Fuel Cell Powered Air Diffused Aeration System

Aeration of water by using PEM fuel cell power is not only a new application of the renewable energy, but also, it provides an affordable method to promote biodiversity in stagnant ponds and lakes. This paper presents a new design and control of PEM fuel cell powered by diffused air aeration system for a shrimp farm in Mersa Matruh in Egypt. Also Artificial intelligence (AI) techniques control is used to control the fuel cell output power by controlling input gases flow rate. Moreover the mathematical modeling and simulation of PEM fuel cell is introduced. A comparison study is applied between the performance of fuzzy logic control (FLC) and neural network control (NNC). The results show the effectiveness of NNC over FLC.

Fecal coliform removal from the effluent of UASB reactor through diffused aeration

Abstract Although UASB systems treating municipal wastewater generally remove fecal coliform (FC) by ∼ one order of magnitude, the effl uent still contains concentrations higher than the WHO standard for unrestricted irrigation. In this study, the effect of continuous diffused aeration on the removal of FC was examined. The diffused aeration system was operated at different hydraulic retention times (60, 30 and 15 min) and dissolved oxygen (DO) concentrations (5−6 and 1−2 mg/l). The removal of FC was affected by the applied HRT and DO. Optimum results were obtained at HRT and DO equal to 60 min and 5−6 mg/l respectively, with FC removal of approximately 2 log (97%). The mechanisms responsible for FC die-off were the pH and oxidation reduction potential (ORP) of the medium.

Hydrodynamic model based KLa estimation in Aeration tank equipped with diffused aeration system

Hydrodynamic model based KLa estimation in Aeration tank equipped with diffused aeration system

Assessment of diffuser pressure loss on WWTPs in Baden-Württemberg

Aeration of activated sludge is a critical treatment step for the operation of activated sludge plants. To achieve a cost effective treatment process, assessing and benchmarking of aeration system performance are important measures. A simple means of gauging the relative condition of a fine bubble diffused aeration system is to evaluate the pressure loss of the diffusers as oxygen transfer tests are rarely applied during the lifetime of an aeration system. This paper shows an assessment of fine bubble diffuser systems in Baden-Württemberg, Germany, based on the results of a questionnaire sent to 941 WWTPs. Apart from the results with regards to the diffuser pressure loss, this paper also presents information on the current state of diffuser technology such as types and materials as well as the diffuser cleaning methods used in Baden-Württemberg. The majority of the WWTPs were equipped with tube diffusers (71%) with 50% of all plants having EPDM membranes installed. Regular mechanical cleaning is the most common cleaning method followed by regular pressure release/air-bumping programs during operations. With regard to the diffuser pressure loss it was found that 50% of the evaluated plants had a diffuser pressure loss that was twice as high as measured for new diffusers.