Air Flotation Process Research Articles

Effect of bubble hydrodynamic and chemical dosage on treatment of oily wastewater by Induced Air Flotation (IAF) process

The objective of this work is to study the treatment of oily wastewater containing anionic surfactant at Critical Micelle Concentration (CMC) by the Modified Induced Air Flotation (MIAF) process. MIAF, which is the combined process between the IAF process and the coagulation process, was applied and also compared with the Induced Air Flotation (IAF) and coagulation processes. The study has shown that the removal efficiency, considered in terms of COD, was related to the alum dosage, pH value and gas flow rate. Moreover, the interfacial area ( a) obtained experimentally from the bubble hydrodynamic parameters (bubble size, bubble rising velocity, bubble formation frequency) and the velocity gradient ( G) have been proven to be the important parameter for controlling the flotation process efficiency and operation cost. The simple proposed correlation, based on the a/ G ratio, provides relatively a good coincidence between the experimental and predicted values of treatment efficiencies in this study.

Effects of ozone in treating drinking water by DAF system

The Dissolved Air Flotation (DAF) process is very effective in removing low density floc particles for drinking water treatment. It is especially well known in removing turbidity and algae by flotation process. The micro-bubbles in the DAF process pick up small flocs, thereby turbidity materials and floated algae are removed efficiently. When air is substituted by ozone in the DAF process, the removal rates of water quality parameters are highly enhanced including turbidity and algae. The turbidity removal rate is enhanced by 1.1% while the removal rate of THMFP is increased by 11.7%. Other parameters such as TOC, and microbial agents are also removed greatly by using ozone instead of air in DAF system.

Low energy ballasted flotation

A novel process which involves the replacement or supplementation of bubbles in the dissolved air flotation process with low density beads is presented. The work comprised a series of bench-scale flotation trials treating three commonly encountered algal species ( Microcystis, Melosira and Chlorella) that were removed in a flotation cell configured as either: conventional dissolved air flotation (DAF); ballasted flotation using low density 70 μm glass beads with a density of 100 kg m −3; or a hybrid process of ballasted flotation combined with conventional DAF. Results indicated that the bead only system was capable of achieving better residual turbidity than standard DAF at bead concentrations of 500 mg L −1. Addition of beads in combination with standard DAF reduced turbidity further to even lower residual turbidity levels. Algae removal was improved when glass beads were dosed, but removal was dependent on algal species. Microcystis was removed by 97% for bead only systems and this removal did not change significantly with the addition of air bubbles. Melosira was the next best removed algae with bead only dosed systems giving similar removals to that achieved by standard DAF using a 10% air recycle ratio (81 and 76% removal respectively). Chlorella was the least well removed algae by bead only systems (63% removal). However, removal was rapidly improved to 86% by the addition of air bubbles using only a 2% recycle ratio. Energy estimations suggested that at least a 50% energy reduction could be achieved using the process offering a potential route for future development of low energy separation processes for algae removal.

Treatment of meat industry wastewater using dissolved air flotation and advanced oxidation processes monitored by GC–MS and LC–MS

The meat processing industry generates a large amount of wastewater which represents a serious problem due to their high levels of organic matter which demands effective and high cost treatments. In the present study the effectiveness of the wastewater treatment by dissolved air flotation (DAF) followed by advanced oxidation processes (AOPs) using photo-peroxidation (H 2O 2/UV) and photo-Fenton reactions were evaluated in laboratory scale. The primary treatment was carried out in a DAF system, using optimal dosages of ferric sulfate and coagulation aids. The efficiency of these processes was evaluated through the following parameters: COD, BOD 5, color, turbidity, total solids, fixed solids and volatile solids, both before and after the DAF and AOP treatments. The elimination efficiency of pollutants was monitored by gas chromatography–mass spectrometry (GC–MS) and liquid chromatography–mass spectrometry (LC–MS) analysis. The results had shown that the DAF treatment efficiently reduced the COD, color, turbidity and total solids contents. In addition, the advanced oxidation process increased the organic matter removal of the treated samples. Among the AOPs, the photo-Fenton reaction reached the best overall results. These results show that the DAF process followed by an AOP process might be efficient for meat wastewater treatment, intended or not to water reuse purposes.

Rise velocity verification of bubble-floc agglomerates using population balance in the DAF process

The dimension of the separation zone in the dissolved air flotation (DAF) process is practically determined by the rise velocity of the bubble-floc agglomerates: flocs attached to several bubbles. To improve the flotation velocity and particle removal efficiency in the DAF process, many researchers have tried to attach as many bubbles as possible to flocs. In this study, the rise velocity of bubble-floc agglomerates, considered to be the most important factor in designing the separation zone of the flotation tank in the DAF process, was investigated on the basis of comparison between theoretical and observed results. The observed rise velocity measured by a particle image analyser (PIA) and the predicted value, however, did not show similarity, in contrast to what we had expected. Furthermore, the simulated results using population balance to predict the maximum number of attached bubbles on a floc were too impractical to accept under the practical condition of the surface of the floc if there was no change of bubble size. These findings led us to suggest that there were three possible causes which could conceivably explain the observations. It was suggested that the differences between predicted and observed values could be attributed to one or more of the possible causes.

Algae Harvesting With a High Rate Water Processing Technology: Dissolved Air Flotation

Algae Harvesting With a High Rate Water Processing Technology: Dissolved Air FlotationThis paper discusses possible algae harvesting techniques relevant to the algae-to-biofuels industry. Advantages and disadvantages of each technique are discussed. Emphasis is placed on the dissolved air flotation process because it still remains the most effective means of separating a relatively low concentration of algae from a large body of water. Some of the important design features of...Author(s)John BratbyDenny ParkerSourceProceedings of the Water Environment FederationSubjectSession 120 - Algae TechnologiesDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2009ISSN1938-6478SICI1938-6478(20090101)2009:7L.7937;1-DOI10.2175/193864709793900294Volume / Issue2009 / 7Content sourceWEFTECFirst / last page(s)7937 - 7954Copyright2009Word count237Subject keywordsalgaealgae harvestingalgae removalalgae thickeningflotationdissolved-air flotationcoagulation

Micro-bubble size distribution measurements by laser diffraction technique

The size distribution of microbubbles generated for the dissolved air flotation process could be assessed on a fast and reliable way by the application of laser diffraction technique. The bubble generation step could therefore be optimized, in order to decrease the size of the bubbles. Increasing air saturation pressure had a positive effect on bubble size, up to 400 kPa; higher pressures, however, had no effect. Surfactants (Flotigam and sodium oleate) significantly inhibited bubble coalescence, and higher concentrations of the reagents decrease the bubble size distribution, up to the critical micelle concentration. The mode of conditioning also had an important effect on bubble size improvement, by adding the surfactant into the pressure vessel. Overall median bubble size decreased 35% over the size distribution obtained without addition of reagent and conditioning.

A Case Study of the DAF-based Drinking Water Treatment Plant in Korea

Since 2003, a full-scale dissolved air flotation (DAF) process has been operated by the Korea Water Resources Corporation (K-Water) in the Songjeon drinking water treatment plant (SWTP). The SWPT was designed with an adaptable operation mode so that it is able to produce safe and stable drinking water, even when the raw water is in very poor condition. The adaptable operation mode is able to buffer the dramatic change in the characteristics of the raw water. During the service period of 2003 to 2006, the SWTP has shown a constantly sound performance for the treatment of high turbid water (64–430 NTU), yielding a significantly low level of turbidity (DAF treated water, 0.15 ∼ 1.16 NTU; anthracite filtered water, 0.02–0.09 NTU). In terms of the DAF process, this work focused on suggesting some practical solutions that would cope with several difficult problems that occurred in the DAF-based drinking water treatment plant. These problems included the unexpected high turbidity occurring during the heavy rainfall season, the scraper problem due to the shortage of feed water to the DAF process, and the increase of turbidity due to the use of powder activated carbon (PAC) prior to the DAF process. In addition, from a cost-effective perspective, the relationship between the recycle ratio and the operating cost was investigated in the DAF process under two different recycle ratio conditions.

Simultaneous Removal of Algae and Their Secondary Algal Metabolites from Water by Hybrid System of DAF and PAC Adsorption

The feasibility of a hybrid system consisting of powdered activated carbon (PAC) adsorption and dissolved air flotation (DAF) processes was examined for the simultaneous removal of algae (anabaena and mycrocystis) and their secondary algal metabolites (2‐methylisoboneol and geosmin). Before studying the hybrid system, adsorption equilibrium and kinetics of organics (2‐methylisoboneol and geosmin) produced from algae on three powdered activated carbons (wood‐based, coal‐based, coconut‐based) were studied. The flotation efficiency of algae and PAC in DAF process was evaluated with zeta potential measurements. Interestingly, we found that the agglomerate of bubble and PAC particle can be successfully floated by DAF. In addition, the simultaneous removal of algae and organics (i.e., secondary algal metabolites) dissolved in water can be achieved by using the hybrid system of adsorption/DAF processes.

DISSOLVED AIR FLOTATION (DAF) FOR PRIMARY AND TERTIARY TREATMENT OF MUNICIPAL WASTEWATERS

ABSTRACT Tertiary treatment of municipal wastewater by dissolved air flotation was studied on a pilot‐scale. The effects of coagulant dose, flocculation pattern, dispersion water recycle ratio and hydraulic surface load on process performance were evaluated. The treatment of primary effluents by dissolved air flotation was investigated to assess the suitability of this process for the treatment of heavily polluted effluents and wastewater treatment plant by‐passes. The tertiary dissolved air flotation process typically achieved 90–99 % reductions in the numbers of enteric microbes (total coliforms, enterococci and F‐RNA coliphages). The average reductions of total phosphorus and chemical oxygen demand were 55–81 % and 28–39 %, respectively. Increasing the polyaluminium chloride coagulant dose from 2 to 10 mgAl3+ l−1 and the dispersion water recycle ratio from 11 to 22 % improved the efficiency of the process. Changes in the flocculation conditions (range of G‐values 10–55 s−1; retention time 4–8 min) and hydraulic surface load (5 or 10 m h−1) did not clearly affect the process efficiency. The dissolved air flotation process decreased the numbers of enteric microbes and reduced total phosphorus, chemical oxygen demand and suspended solids from the primary treated wastewaters on average by 98–99.8%, 90 %, 47% and 77 %, respectively. The dissolved air flotation process was demonstrated to be a suitable method for efficient tertiary treatment of wastewaters, as well as for the elimination of peak pollution loads or by‐pass wastewaters during the treatment plant overloading situations.

Effect of water treatment processes on Cryptosporidium infectivity

Conventional water treatment processes have the ability to remove Cryptosporidium oocysts through coagulation, flocculation, sedimentation and filtration, provided there is efficient management of plant performance. The potential exists for the breakthrough of oocysts through the treatment train. The effect of the water treatment chemical aluminium sulphate (alum) on Cryptosporidium oocyst infectivity has been assessed using an assay that combines cell culture and real-time polymerase chain reaction techniques. The infectivity of fresh and temperature-aged oocysts (stored up to 6 months at 4 or 15 °C) was unaffected by exposure to a range of doses of alum in standard jar test procedures and dissolved air flotation processes and subsequent exposure to chlorine or chloramine. Removal efficiencies and infectivity measures are important in determining risk to public health and will reflect the ability of water treatment plants to act as a barrier to these pathogens.

Effects of floc and bubble size on the efficiency of the dissolved air flotation (DAF) process

Dissolved air flotation (DAF) is a method for removing particles from water using micro bubbles instead of settlement. The process has proved to be successful and, since the 1960s, accepted as an alternative to the conventional sedimentation process for water and wastewater treatment. However, limited research into the process, especially the fundamental characteristics of bubbles and particles, has been carried out. The single collector collision model is not capable of determining the effects of particular characteristics, such as the size and surface charge of bubbles and particles. Han has published a set of modeling results after calculating the collision efficiency between bubbles and particles by trajectory analysis. His major conclusion was that collision efficiency is maximum when the bubbles and particles are nearly the same size but have opposite charge. However, experimental verification of this conclusion has not been carried out yet. This paper describes a new method for measuring the size of particles and bubbles developed using computational image analysis. DAF efficiency is influenced by the effect of the recycle ratio on various average floc sizes. The larger the recycle ratio, the higher the DAF efficiency at the same pressure and particle size. The treatment efficiency is also affected by the saturation pressure, because the bubble size and bubble volume concentration are controlled by the pressure. The highest efficiency is obtained when the floc size is larger than the bubble size. These results, namely that the highest collision efficiency occurs when the particles and bubbles are about the same size, are more in accordance with the trajectory model than with the white water collector model, which implies that the larger the particles, the higher is the collision efficiency.

Effects of Al3 + and hydraulic characteristics on the removal and behaviour of particles in dissolved air flotation

The removal efficiency of the dissolved air flotation (DAF) process to separate particles from water and wastewater depends on the size and zeta potential of bubbles and particles, the solution and operating conditions, hydraulic characteristics, etc. The effects of aluminium ions and turbulent flow-produced when air-saturated water was spouted into the reactor in the DAF process, on removal and, particle behaviour were on investigated. When bubble size was similar to particle size (10–50 μm), the maximum removal efficiency was 92% in a Kaolin solution of 10−3 M Al3 + without pre-treatment for flocculation process, and, as time passed, the floc size was observed to increase at a pH of 8, which was the condition of high removal efficiency as seen through image analysis. When the air-saturated water was spouted into the reactor, the size of particle at p.z.c. (point of zero charge) seemed to increase to form a floc due to collision effects caused by turbulent flow. Consequently, floc formation by turbulent flow in the reactor seemed to positively affect removal efficiency.

Dissolved air flotation of polishing wastewater from semiconductor manufacturer

The feasibility of the dissolved air flotation (DAF) process in treating chemical mechanical polishing (CMP) wastewater was evaluated in this study. Wastewater from a local semiconductor manufacturer was sampled and characterised. Nano-sized silica (77.6 nm) with turbidity of 130 +/- 3 NTU was found in the slightly alkaline wastewater with traces of other pollutants. Experimental results indicated removal efficiency of particles, measured as suspended particle or turbidity, increased with increasing concentration of cationic collector cetyltrimethyl ammonium bromide (CTAB). When CTAB concentration was 30 mg/L, pH of 6.5 +/- 0.1 and recycle ratio of 30%, very effective removal of particles (> 98%) was observed in saturation pressure range of 4 to 6 kg/cm2, and the reaction proceeded faster under higher pressure. Similarly, the reaction was faster under the higher recycle ratio, while final removal efficiency improved slightly as the recycle ratio increased from 20 to 40%. An insignificant effect of pH on treatment efficiency was found as pH varied from 4.5 to 8.5. The presence of activator, Al3+ and Fe3+, enhanced the system performance. It is proposed that CTAB adsorbs on silica particles in polishing wastewater through electrostatic interaction and makes particles more hydrophobic. The increase in hydrophobicity results in more effective bubble-particle collisions. In addition, flocculation of silica particles through bridging effect of collector was found; it is believed that flocculation of particles also contributed to flotation. Better attachment between gas bubble and solid, higher buoyancy and higher air to solid ratio all lead to effective flotation.

Removal of clay and blue-green algae particles through zeta potential and particle size distribution in the dissolved air flotation process

The raw water characteristics of a water treatment plant in Korea are mainly dependent on two major factors: the clay particles attributed to rainfall and blue-green algae in reservoirs. In this work, zeta potential and particle size distributions of clay and algae particles, which are the important parameters affecting their removal efficiency, were measured to investigate the behavior and removal characteristics of particles under various conditions. The results showed that the zeta potential of blue-green algae was more sensitive to treatment conditions than clay, and it fluctuated highly with coagulant dosage, suggesting that the control of zeta potential is important for effective removal of algae particles. On the other hand, the range of particle size distribution that remained from the preliminary sedimentation tank was generally smaller than for flotation. However, the zeta potential of the remaining particles was either close to the isoelectric point or positive, and the particles were not so hard to remove for that reason. In the final analysis, for simultaneous removal of clay and algae particles, a sufficient zeta potential difference must be formed not only for algae particles but also for small clay particles from the sedimentation tank in the dissolved air flotation process.

Removal of fluoride from semiconductor wastewater by electrocoagulation–flotation

This work employs an anodic surfactant, sodium dodecyl sulfate (SDS), to improve the flotation performance of the electrocoagulation-flotation (ECF) process to treated fluoride containing semiconductor wastewater following calcium precipitation. The dissolved fluoride ions and CaF(2) particles in the wastewater after calcium precipitation were effectively removed in the ECF process simultaneously. The dosage of SDS required for ECF was much less than those for dispersed air flotation (DiAF) or dissolved air flotation (DAF) processes because the CaF(2) particles can be collected by hydro-fluoro-aluminum flocs in ECF. Thus, SDS only served as a frother to make the bubbles tiny and stable in the ECF defluoridation process. The interference of co-existing anions can be overcome by increasing the dosage of calcium ions and SDS. The optimum initial acidity for ECF is close to the initial fluoride concentration after calcium precipitation; the amount of SS removed dropped rapidly if the initial acidity exceeded the optimal value because the surface charge of the hydro-fluoro-aluminum particles increased. The initial acidity of the wastewater after calcium precipitation can be modified by changing the [Ca(OH)(2)]/[Ca(2+)](T) ratio and the appropriate ratio is approximately given by the acid dissociation constant of hydrofluoric acid and the initial pH of the wastewater before calcium precipitation.

The future for electrocoagulation as a localised water treatment technology

Electrocoagulation is an electrochemical method of treating polluted water whereby sacrificial anodes corrode to release active coagulant precursors (usually aluminium or iron cations) into solution. Accompanying electrolytic reactions evolve gas (usually as hydrogen bubbles) at the cathode. Electrocoagulation has a long history as a water treatment technology having been employed to remove a wide range of pollutants. However electrocoagulation has never become accepted as a ‘mainstream’ water treatment technology. The lack of a systematic approach to electrocoagulation reactor design/operation and the issue of electrode reliability (particularly passivation of the electrodes over time) have limited its implementation. However recent technical improvements combined with a growing need for small-scale decentralised water treatment facilities have led to a re-evaluation of electrocoagulation. Starting with a review of electrocoagulation reactor design/operation, this article examines and identifies a conceptual framework for electrocoagulation that focuses on the interactions between electrochemistry, coagulation and flotation. In addition detailed experimental data are provided from a batch reactor system removing suspended solids together with a mathematical analysis based on the ‘white water’ model for the dissolved air flotation process. Current density is identified as the key operational parameter influencing which pollutant removal mechanism dominates. The conclusion is drawn that electrocoagulation has a future as a decentralised water treatment technology. A conceptual framework is presented for future research directed towards a more mechanistic understanding of the process.

Dissolved Air Flotation Process for Algae Removal

Sedimentation is the most widely used primary clarification stage before filtration for the production of potable water. Raw water that is algae laden is difficult to clarify by sedimentation, as gravitational forces are largely ineffective due to algae stability. Algae form low-density particles that exist in a colloidal state and have a tendency to float hence causing solid-liquid separation problems in the sedimentation stage. Additionally, their low turbidity, soft and high color nature produces very light flocs which settle very slowly. This study investigated the performance of the Dissolved Air Flotation (DAF) process as an alternative to sedimentation for algae removal in surface water treatment in Kenya. Batch DAF experiments were carried out in the laboratory using algae laden surface water samples collected from the river and laboratory cultured algae water samples. The performance and effectiveness of the process were assessed for algae and turbidity removal and color improvement. Experimental results showed that clarification of surface water laden with different types of algae such as blue-green, green, diatom and dinoflagellates was effective. At optimum coagulant dosage, algae removal from 70 percent to 90 percent, turbidity reduction of over 80 percent and color improvement of about 75 percent was achieved. Journal of Civil Engineering Research and Practice Vol.1(2) 2004: 27-38

Factorial evaluation of operational variables of a DAF process to improve PHC removal efficiency

A dissolved air flotation (DAF) process is used for removing oil from oil—water emulsions. Operational conditions that increase shearing and disturbance of liquid during the operation of large-scale DAF systems can significantly reduce oil removal efficiency. Objectives of this study were to evaluate the individual and interactive effects of the operational variables of the DAF process on removal efficiency of petroleum hydrocarbons from water sources contaminated with fuel oils. A series of batch and continuous experiments (utilizing full pressurization and effluent recirculation) were conducted using a 60-L DAF system which could be operated either in batch or continuous modes. The experiments were conducted as a factorial design to evaluate both the individual effects and the interactions of the operational variables which included oil concentration, detention time, water type (brackish and pond), coagulant use, and operational mode. The factorial analysis showed that for the batch mode of operation, oil concentration, detention time, coagulant use, and water type had a significant effect on PHC removal. However, for the continuous DAF runs, the only variable that was significant at the 95% confidence level was detention time. Coagulant use did not have a significant effect on PHC removal efficiency for the continuous runs due to shearing of the flocs.

Modelling of the dispersed air flotation process applied to dairy wastewater treatment

The aim of this work was to develop a mathematical model for a continuously operating flotation tank to provide the scale-up and optimization of the separation system. The fluid dynamic of the tank was assumed to be a perfect mixture and the flotation kinetics to be a first-order reaction, as suggested by the batch operation. The experiments were carried out in a continuously operating 60 L tank, used in the treatment of a synthetic dairy waste at its isoelectric point. A mathematical model that describes the behavior of a continuously operating flotation tank used in the treatment of a synthetic dairy waste at its isoelectric point is not only proposed, but also validated with experimental data.