Dissolved Air Flotation System Research Articles

Comparative life cycle assessment (LCA) of pre-treatment technologies for desalination in South Africa

In the context of South Africa’s water scarcity, desalination has emerged as a possible solution for coastal areas. However, the quality of the intake water for desalination is often problematic, prompting the need for pre-treatment. The aim of this study was to conduct a comparative environmental life cycle assessment (LCA) on 4 seawater filtration systems intended for the pre-treatment of a reverse osmosis desalination project. These systems were implemented in a pilot trial and are based on modern water treatment technologies, namely, granular filtration (pressure driven and gravity driven), dissolved air flotation (DAF), and ultrafiltration (UF). For all 4 systems, data were collected for both the construction and operation phases, and LCAs were performed, resulting in environmental scores that allow for comparison based on the pre-treatment of 1 kL of seawater of the same quality. The SimaPro LCA tool and the ReCiPe midpoint method were used and environmental scores were calculated for 18 impact categories, including climate change, acidification, toxicity, eutrophication, resource depletion, etc. This methodology also allowed the identification of the highest environmental burdens/scores within each system. The most significant finding is that local electricity consumption is responsible for the greatest proportion of environmental impacts. Thus, the systems consuming more energy for operating equipment such as blowers, pumps, and mixers were found to have the highest environmental burdens. Hence, the DAF system has the highest environmental scores for most impacts, followed by the single-phase gravity filtration system, then the two-phase partial pressure filtration system and finally the UF system. Therefore, focus should shift towards energy optimisation of process units, especially the rotary ones, as well as energy mitigation and recovery strategies. The use of renewable energy for pre-treatment should also be considered locally.

Numerical investigation on mixing behaviour in the contact zone of an industrial‐scale dissolved air flotation tank

AbstractThe mixing behaviour of the wastewater and the recycle bubbly water in the contact zone could affect the separation performance of dissolved air flotation systems. In this study, the mixing characteristics in the contact zone of an industrial‐scale dissolved air flotation tank were investigated using the computational fluid dynamics method. At first, the influence of the turbulence model on the simulation results was clarified. It is demonstrated that the realizable k‐ε model could predict the gas holdup distribution with better accuracy, and the maximum relative deviation between the predicted gas holdup and the experimental results is only 10%. The influence of the geometry and operating parameters on the mixing process was then studied numerically. It is found that the mixing performance in the contact zone could be improved by decreasing the height position of the nozzle inlet in a certain range. The reason is that microbubbles disperse better in the bottom region and a double circulation flow state forms in the contact zone when the nozzle inlet is installed at a low height. Additionally, the mixing process could be further enhanced by decreasing the diameter of the nozzle inlet. As to the operating conditions, increasing the recycle ratio could not only increase the gas holdup but also improve the mixing performance in the contact zone. When the recycle flow rate remains constant, increasing the wastewater flow rate will lead to a decrease in gas holdup and deterioration of the mixing effect.

Application of a simplified model for assessing particle removal in dissolved air flotation (DAF) systems: Experimental verification at laboratory and full-scale level

Particle-bubble collisions in dissolved air flotation (DAF) systems play a crucial role in the removal of total suspended solids (TSS). DAF particle-bubble collision models incorporate factors such as particle diameters, charge and density, bubble diameters, and collision factors. The challenge lies in accounting for the wide range of particle and bubble sizes and obtaining complex model inputs. To address this, a simplified model for TSS removal in DAF units was established using low-cost laboratory measurements, including particle size distribution and density. Additionally, microbubble diameter profiles were derived from bubble velocities using particle image velocimetry software (PIV). Six independent variables, encompassing influent particle characteristics (such as particle size distribution and density) and DAF running characteristics (temperature, contact zone detention time, inflow and recycle flows), were employed in the simplified model. The model's accuracy was evaluated using a laboratory-scale DAF system with two different influents: Delft canal water and anaerobic sludge. The predicted TSS removal from the simplified model aligned well with the laboratory-scale DAF results, yielding removal efficiencies of 68 ± 1 % and 77 ± 3 % for Delft canal water and anaerobic sludge, respectively. Furthermore, when the simplified model was applied to two full-scale DAF systems, it successfully identified an underperforming system (DAF2) with a TSS removal efficiency of 91 %, contrasting with the theoretical removal model-predicted efficiency of 98 %. This study highlights the utility of combining bubble size distribution measured by PIVlab and particle size distribution obtained using FIJI-ImageJ as an economical and efficient approach to acquiring the necessary inputs for predicting TSS removal in DAF systems.

Characterization of Microbubbles Generated in a Venturi Tube via Image Processing: Effect of Operating Parameters

Context: This research developed a dissolved air flotation system using a Venturi tube to produce microbubbles. The Venturi tube replaces the saturation tank and the pressure-reducing valve of conventional systems. Method: The system has both suction and injection air inlets, regulates the recirculation flow of the liquid to the tank, and provides a high hydraulic load in a reduced size. Counting and measuring the microbubbles produced via digital image processing helps to characterize the system's performance. Results: The system with air suction produces smaller bubbles than that with air injection. A higher liquid recirculation pressure produces more bubbles and reduces their size in the case of air suction. Conclusions: In air injection, the change in flow rate influences the size of the microbubbles. Air injection and recirculation pressure do not influence the number of bubbles generated.

Water quality characteristics and reuse potential using adsorption as a post-treatment option for a full-scale hydrocyclone, coagulation, flocculation, and dissolved air flotation system.

This study established a full-scale hybrid water treatment system combining a hydrocyclone, coagulation, flocculation, and dissolved air flotation unit (HCFD) and evaluated its performance in treating anthropogenically impacted lake water. The HCFD system offered the stable and efficient treatment of fluctuating influent loadings, meeting most of the highest water reclamation quality criteria except for that of organic matter. Adsorption was subsequently examined as a post-treatment process for the HCFD effluent, which has not been examined in many previous studies. As the adsorbent for the post-treatment, pine bark, a locally available agricultural waste feedstock, was modified using H2O2 to maximize its adsorption capacity. The surface modification increased its adsorption capacity for organic matter by 53-112%. The HCFD system in conjunction with the synthesized adsorbent thus demonstrated the ability to meet the highest standards for all water quality parameters, highlighting their synergistic potential for enhancement of water treatment. Liquid chromatography-organic carbon detection and Fourier transform infrared analysis were then employed to determine the mechanisms involved in the removal of specific contaminants using the HCFD system and post-adsorption unit. While the HCFD system successfully eliminated particulate and colloidal matter (e.g., phosphorous and biopolymers with a high molecular weight) using centrifugal and floating separation with the aid of two complementary polymers, the post-adsorption unit effectively adsorbed small-sized dissolved substances (e.g., low molecular weight acids and building blocks) via surface functional groups (-CH, -OH, -CH2, C=O, C=C, and C=O) using van der Waals forces, hydrogen bonding, and π-π or n-π interactions.

Microalgae harvesting using flocculation and dissolved air flotation: Selecting the right vessel for lab-scale experiments

Flocculation combined with dissolved air flotation (DAF) is a promising technology for harvesting microalgae; therefore, optimisation of flocculant–DAF operating conditions are frequently explored in laboratory experiments. DAF systems have jars of differing volumes, height to diameter ratios, shapes and materials used to manufacture the jars; thus, the harvesting efficiency (η) may differ between these jars. The aim was to systematically compare η between different types of benchtop DAF jars. Evaluation of 30 different types of DAF jars revealed that η was not influenced by the volume of the jars, but was impacted by the height to diameter ratio, with optimal η at a ratio ranging between 1.6 and 2.05. There was no difference in η between cylindrical and cuboid jars, but jars made of hydrophobic (polypropylene) plastic resulted in a lower η. Overall, these results are useful to guide the design of lab-scale DAF microalgae harvesting experiments.

Abattoir Wastewater Treatment Plants in India: Understanding and Performance Evaluation

Slaughterhouse plants, especially meat and poultry products (MPPs), generate significant quantities of wastewater during the slaughtering process and the cleaning of machinery. The handling and final disposal of wastewater from the slaughterhouse is a public and environmental health issue due to its pollution potential. The monitoring of three full-scale buffalo meat-based abattoir effluent treatment plants (ETPs) to remove organic matter and nutrients was conducted at three separate sites in India. Rotary and static screens, an equalization tank, a dissolved air flotation (DAF) system, aeration tanks, and clarifiers were the component unit operations/processes of the wastewater treatment systems. All the treatment plants were operating at an average flow rate of 254 m3/d Meem Agro (designed capacity 500 m3/d); 427 m3/d Al Noor (designed capacity 500 m3/d), and 353 m3/d International Agro Foods (IAF) plant (designed capacity 750 m3/d) for the treatment of slaughterhouse wastewater. The Al Noor and Meem Agro’s ETP consisted of a DAF unit working at the surface-loading rates of 5.7 and 1.48 m3/m2 h, respectively, applied during the analysis to the DAF system, resulting in 46.5 and 56.8% efficiencies of O&G and SS removal. The ETP of Al Noor and Meem Agro and IAF was loaded at an organic loading rate of 276 kg COD/day, 746 kg COD/day, and 629 kg COD/day, respectively. Performance of ETPs showed that the efficiency of total COD, total BOD, and TSS removal was 97, 98, and 99%; 94, 94, and 93%, and 95, 98, and 99% for Al Noor, Meem Agro, and IAF plants, respectively. The treatment plants’ efficiencies in all three plants for the removal of organic matter surpassed the effluent discharge level (as per Indian standards). Still, it is concluded that the ETPs need to be upgraded by incorporating the advanced wastewater treatment methods for the removal of nutrient concentration from treated effluents.

Evaluating dissolved air flotation for oil/water separation using a hybridized coagulant of ferric chloride and chitosan

Dissolved air flotation (DAF) is a separation technology widely used in the industry to reduce oil concentrations in wastewater. However, there are some issues with the existing DAF system, including high chemical cost, environmental pollution by chemical residual in the effluent, and generation of non-biodegradable sludge. In this study, an environmentally friendly and cost-effective DAF process was proposed and tested for separating emulsified oil from water. A hybrid coagulant/flocculant system with ferric chloride as a coagulant and chitosan as a bio-degradable flocculant was used to enhance the efficiency of oil/water separation. The findings of this study showed that the hybrid configuration led to (i) 37% reduction in chemical costs (ii) 90% reduction in the use of non-biodegradable ferric chloride and (iii) 50% reduction in flocculator footprint when compared to individual coagulants. This proposed dosing strategy also has a broader operating pH range between 5 and 9 and wider applicability for both fresh and high salinity water. The results from this study contribute to the wider, environment-friendly, and cost-effective application of air flotation for oil/water separation.

Mono- and multi-diameter approaches to predict stratified flow structure by means of CFD simulations in DAF systems

This paper presents a Computational Fluid Dynamics (CFD) model of a pilot scale dissolved air flotation (DAF) tank. A Multiphase Mixture model was used to analyse the influence of bubble sizes on the formation of a stratified flow structure. Critical bubble diameter is defined as the size of the bubble that implies the equality of the bubble rising velocity and flow downward velocity in the separation zone (SZ). The fact as to whether using air bubble sizes which are greater or less than the critical diameter value significantly affects the air content, flow structure and the limit of the whitewater blanket inside the SZ is assessed. The study was carried out using two approaches, namely, mono- and multi-diameter. The results obtained via the mono-diameter approach proved to be closely in line with experimental data when air concentration in the SZ had almost, but not quite, a constant value. However, it failed to predict the case of the progressive decrease in air below half of SZ height. A combined effect of bubbles with different rising speed was required to reproduce a smooth air profile curve, as measured experimentally. In this context, a multi-diameter approach is deemed to be a suitable method for reproducing the stratified structure. In addition, this approach offers the chance to study bubble size distribution inside the SZ domain.

Experimental study to make gas bubbles as proxies for oil droplets to test AUV detection of oil plumes

To prepare gas bubble plumes as proxies for oil in autonomous underwater vehicle field experiments, experiments were conducted to compare three microbubble generators, including a dissolved air flotation system, a microbubble diffuser, and a Nikuni Karyu Turbo Mixer (KTM) pump, based on sizes of bubbles and their practicability to be used in field experiments. The sizes of bubbles were measured with a Laser in-situ Scattering and Transmissometry sensor. A Ping360 sonar with a frequency of 750 kHz was tested to assess its ability to capture the generated bubbles. Among the three systems, the KTM pump generated the most stable bubbles with average diameters smaller than 100 µm. Detailed experiments were carried out in a towing tank to further test the size distribution of bubbles generated by the KTM pump. Smaller bubbles were found to remain at higher concentrations at the depth of bubble release, as larger bubbles having higher rise velocities surfaced quickly without staying in the water column. The bubble sizes were cross-validated with the measured residence time of bubbles. The bubble generating system based on the KTM pump is planned to be applied in future field experiments.

Advanced wastewater treatment process using algal photo-bioreactor associated with dissolved-air flotation system: A pilot-scale demonstration

This research explores the feasibility of using an algal photo-bioreactor as an integrated algal-bacterial treatment system combined with a dissolved air flotation (DAF) system for the deduction of COD, BOD5, TSS, TN and TP from primary treated wastewater (PTW). Isolated algae species of Anomoeoneis, Scenedesmus, Anabaena and Spirulina at fixed hydraulic retention time (HRT) of 16 h were used to conduct batch experiments to determine optimum conditions of pH, temperature, light intensity and mixing rate on the removal rates of contaminants. The optimum removal rates were studied at neutral pH, 25 °C, light intensity of 90 μmol m−2S−1 and 100 rpm mixing rate. A scaled-up pilot plant treatment system was designed, constructed and operated to evaluate the system performance to treat (0.1 L/min) of primary treated wastewater disposed of ZENIN WWTP-Giza, Egypt. The design criteria of the algal photo-bioreactor were acquired from the batch investigations. The plant was operated for 10 continuous days and the analytical analysis was performed twice a day at a fixed time. The DAF system was used to isolate the infiltrated algae from the system for cultivation and reuse purposes. The achieved results revealed the superior COD, BOD5, TSS, TN and TP removal percent reached in average 56.4%, 61%, 59.2%, 48.3% and 51.7% at 12 pm and 49.7%, 54.9%, 52.4%, 39% and 40.57% at 12 am, respectively. All data gained from experimental studies demonstrated the efficiency of using algal photo-bioreactor combined with DAF system as an effective wastewater treatment technique.

A two-stage dissolved air flotation saturator configuration for significant microbubble improvement

ABSTRACT The presence of suspended contaminants in water and wastewater, such as algae, colloids, fats and oil, necessitates the use of systems such as dissolved air flotation (DAF) for their removal. In the current study, a novel setup has been proposed for bubble enhancement. An industrial scale (pilot) DAF system was tested at saturator pressures of 3–7 atm and flow rates of 5–20 L/min in three different configurations, namely, empty, packed, and the innovative two-stage (TS) configuration. In the TS system, after the nucleation of micro bubbles, the water is returned to the saturator to undergo pressurization for a second time before it is passed through the nozzle once more and is released. The results show that the highest volume of released air as well as the smallest microbubbles are seen in the TS configuration, followed by packed mode, with the empty configuration showing the least favourable results. Moreover, the bubbles produced at the lowest residence time and pressure (3 atm) with the novel setup are better than the bubbles produced by the standard configuration, even with pressures as high as 7 atm. Thus, the novel TS setup can allow for significantly lower energy requirements and lower capital costs. For real-world application of the TS system, the feed for the saturator could be extracted from within or near the contact zone, i.e. where the bubbles are released in the DAF tank.

Numerical simulation on the effects of bubble size and internal structure on flow behavior in a DAF tank: A comparative study of CFD and CFD-PBM approach

Although dissolved air flotation (DAF) plays a crucial role in wastewater treatment, fluid dynamics in the contact zone (CZ) of a DAF tank are still considered black-box processes, which hampers DAF's optimal design and operation. In this article, we perform a modeling study to further unravel the governing mechanisms occurring in a DAF tank. A computational fluid dynamics (CFD) model coupled with a generic population balance model (PBM) for the DAF system was developed in this work. CFD-PBM results showed that the eddy capture is the dominating mechanism for the bubble coalescence in the most volume of the CZ. Simultaneously, the velocity gradient mechanism and turbulent induced mechanism also play a key role in bubble coalescence in the regions with drastic flow transition and in the nozzle downstream. CFD-PBM simulations reveal that increasing the recirculation rate is better than increasing the volume fraction in the smaller recirculated flow to achieve higher bubble number density and smaller bubble size if the same amount of gas flow rate is injected. The baffle with proper transverse corrugate can equalize and reduce the total coalescence rate. By establishing a synergy between fluid dynamics and bubble size to achieve the best CZ efficiency, the generic CFD-PBM approach developed in this study has laid a solid basis for the optimal design and operation of DAF systems.

Empirical modeling of turbidity removal in a dissolved air flotation system: application of artificial neural networks

Abstract Dissolved air flotation (DAF) is a physical separation process that uses air microbubbles to remove suspended material dispersed in a liquid phase. Even though DAF is considered a well-established unit operation, modeling it is difficult due to the complexity of the phenomena involved, resulting in conceptual models with no practical application. Thereby, the objective of this work was to evaluate empirical modeling efficiency in predicting the turbidity removal dynamic using artificial neural networks applied to a DAF prototype. For the study of the neural network input variables, a two-level, full-factorial design was utilized to verify the statistical significance of the saturation pressure and the saturated water flow rate in relation to the turbidity removal. Using a time-delay recurrent neural network architecture, two empirical models were proposed to simulate the dynamic behavior of the turbidity removal promoted by the DAF prototype. The real-time model provided good predictions with R = 0.9717 and MSE = 1.0482, and the simulation model was also able to predict the process behavior presenting performance criteria equal to R = 0.9475 and MSE = 1.8640.

Large batch bench-scale dissolved air flotation system for simulating full-scale turbidity removal

ABSTRACT One of the expected outcomes of global warming is increased algal and cyanobacterial blooms. Based on its ability to separate algal particles, dissolved air flotation (DAF) is considered as a climate change adaptation technology for water treatment. The feasibility of DAF treatment is often assessed using DAF jar tests; however, they are not particularly good at predicting a full-scale DAF system’s turbidity removals. Therefore, our group has developed a more reliable larger-diameter/larger-volume batch apparatus (LB-DAF), which was optimized by comparison with a full-scale DAF plant treating a low turbidity, highly coloured river water (SUVA ∼ 4.3). The objective of this study was to verify that the LB-DAF was capable of simulating full-scale DAF systems treating two significantly different waters. One was water from a large eutrophic bay in Lake Ontario (SUVA ∼2.6) and the second was a river water (SUVA ∼3.5). The turbidity removals achieved by the full-scale DAF systems treating these waters were compared with those for the LB-DAF tests conducted using different flocculation velocity gradients, saturated water pressures, recycle ratios and water depth to diameter ratios. The LB-DAF tests are good predictors of the full-scale DAF turbidity removals, the average difference for the two waters tested were 2% and 6%. The LB-DAF natural organic matter (NOM) removals for both waters differed by less than 1% from that measured at the corresponding treatment plants. In addition, as in our previous LB-DAF study, varying the different LB-DAF operational variables did not have a significant impact on turbidity and NOM removals.

Interactions between flocs and bubbles in the separation zone of dissolved air flotation system

The interaction between flocs and bubbles is crucial to achieve separation efficiency in the separation zone of the dissolved air flotation process. In this study, a micro-scale observation system was established to investigate the interaction between flocs and bubbles in the separation zone of the system. Four periodic interactions were observed-collision, adhesion, coalescence, and desorption (CACD). Small flocs achieved higher unit buoyancy (buoyancy per surface area) than that of large flocs. High collision probability can be obtained between bubbles and small flocs according to Stokes' law and Reynolds experiments. Simulation using Fluent indicated that low-pressure drag acting on small flocs resulted in a high probability of collision. Therefore, small flocs can capture bubbles and accumulate high enough buoyancy in a short time due to the high collision probability and low buoyancy required for small flocs to float. Moreover, the small flocs have a limited number of bubbles on the surface within such a short time, leading to a low probability of desorption and coalescence. The practical verification also show that small flocs are favorable for the high separation efficiency of the flotation process.

Study of 3D-Printed Pressure Release Nozzle for Microbubble Formation in Full-Scale Dissolved Air Flotation (DAF)

Pressure-release equipment for the formation of microbubbles is an important part of any DAF system. In this study, 3D printing was employed to produce new nozzles, followed by testing to find the optimum pressure and flowrate. The flowrate through the nozzle affected both the empty bed residence time (EBRT) of the saturator and also the nucleation of bubbles. With the increase of flow rate, the total number of bubbles was seen to increase. This is a significant observation meaning that the enhanced precipitation of bubbles outweighs the reduced EBRT. This fact can improve equipment sizing and reduce CAPEX. Meanwhile, DAF is a technology that can help extend membrane life and reduce operational issues in seawater reverse osmosis (SWRO). So for further validation, a full-scale DAF was built and tested for SWRO pretreatment on the coast of the Persian Gulf. The system was tested with/without coagulation as well as with/without filtration media embedded inside the DAF tank. The results showed that the use of coagulation alongside DAF reduced the turbidity by more than 60% while using a DAF with embedded filtration could reduce the turbidity to less than 1 NTU. In comparison to common media filters, the proposed setup showed better performance.

Design of a mobile dissolved air flotation system with high rate for the treatment of liquid radioactive waste

The global trend towards sustainable development with low carbon emissions has encouraged the expansion of nuclear energy. The nuclear reactors, under normal operating conditions or radiological emergency, generate different types of radioactive waste, mainly liquids and solids that require efficient and mobile treatment systems. This work presents the design proposal of a mobile dissolved air flotation system (mDAF) with a high rate for the treatment of liquid radioactive waste generated in the attention and response to radiological post-emergencies of nuclear power plants or environmental emergency responses associated with oil spills. The evidence obtained showed that the design used allows to reach, under the conditions of the experiment tested, removal levels higher than 95 % in the concentrations of oil waste, SO4−2, NO3-, PO3-3, Ni and Cr. Removal levels of approximately 94 % and 75 % for the total Co and 60Co, respectively, were achieved. It was possible to demonstrate the feasibility of designing a mDAF of low cost with enough versatility to treat radioactive liquid waste generated in a radiological post-emergency.

Numerical simulation of dissolved air flotation using a lattice Boltzmann method.

In this paper, the behavior of a bubble and droplet rising in a system, namely, a dissolved air flotation system, is investigated under different conditions. A lattice Boltzmann model which is based on the Cahn-Hilliard equations for ternary flows is implemented. This model can handle high density and viscosity ratios, remove parasitic currents, and capture partial and total spreading conditions. Two classical problems, such as spreading of a liquid lens and the Rayleigh-Taylor instability are used to determine the accuracy of the model. As a practical application, three-component flow in a tank is studied and the dynamics of bubble and droplet under different conditions is investigated. We then concentrate on the dimensionless average velocity and locations of bubble and droplet at different density ratios, viscosity ratios, and diameter ratios. Also, total spreading and partial spreading conditions are compared. The numerical results are justifiable physically and show the ability of this model to simulate three-component flows.

Effect of salinity on air dissolution, size distribution of microbubbles, and hydrodynamics of a dissolved air flotation (DAF) system

In the present study, air dissolution, size distribution and hydrodynamics of microbubbles in a dissolved air flotation system were experimentally investigated. The amount of dissolved air was calculated by Henry’s laws of gas dissolution and compared with measured values from the liquid displacement technique. The measured air concentrations were 23.2 and 10.1 mg/l for freshwater and saline water (30 g/L), respectively, which were much lower than those calculated by Henry’s equations. Shadow imaging technique was applied to measure the size distributions of microbubbles for saline and freshwater. Whilst the size of microbubbles for freshwater were between 40–250 μm, the corresponding values for the saline water were smaller and varied between 20–160 μm. Particle image velocimetry (PIV) was also applied to map flow patterns and velocities of microbubbles for saline and freshwater. The results indicated that the flow patterns were mostly affected by the flow itself rather than the salinity and the presence of microbubbles since there was not a noticeable change between single-phase (flow without microbubbles) and two-phase flows. While the range of the velocities was between 0–64 mm/s for the freshwater, it did not exceed 48 mm/s for the saline water.