Flotation efficiency of activated sludge flocs using population balance model in dissolved air flotation

The activated sludge process is one of the most frequently used processes for biological wastewater treatment. Conventional gravity sedimentation (CGS), which is widely used as a secondary clarifier in activated sludge processes, has a routine problem due to floating tendency, called bulking, caused by filamentous microorganisms. Dissolved air flotation (DAF) has been applied as potential alternative to CGS as a secondary clarifier. A series of experiments were performed to measure physico-chemical characteristics and removal efficiency of activated sludge flocs. The removal efficiency of flocs corresponding in lag and exponential growth phases was lower, while that of flocs both in stationary and endogenous phases considerably increased. The rise velocity of floc/bubble agglomerates was calculated by using a population balance (PB) model explaining the distribution of floc/bubble agglomerates. The experimental results of flotation efficiency showed a similar tendency with the results predicted by PB model for the rise velocity and distribution of floc/bubble agglomerates. It was found from our study that the DAF process was very effective as a secondary clarifier in the activated sludge process.     

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

Abstract

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.     

逆向-同向流气浮工艺构建与运行特性

针对传统溶气气浮（DAF）工艺中气泡对絮体颗粒捕集、黏附效率低,泡絮体黏附不稳定等问题,基于泡絮碰撞黏附理论,研发了集逆向流与同向流于一体的DAF工艺（CCDAF）。该工艺溶气水分两次投加,接触室分为碰撞接触室和黏附接触室。试验结果表明,CCDAF工艺显著提高了泡絮黏附效率和泡絮体稳定性,对浊度、藻类平均去除率达到96.4%、96.50%,出水颗粒物以2~7 μm粒径为主。工艺主要去除大分子、疏水性有机物,COD_Mn、UV₂₅₄、DOC平均去除率分别达到37.6%、46.3%和32.11%,CCDAF比同向流及逆向流DAF除污染效能更加显著。泡絮黏附机理分析表明,CCDAF工艺逆向流碰撞区主导作用机制为碰撞黏附及共聚作用,同向流接触区为碰撞黏附及网捕、包卷和架桥作用。     

Comparison of Dissolved Air Flotation and Sedimentation in Treatment of Typical North China Source Water

The treatment of typical north China water by dissolved air flotation (DAF) and sedimentation process was examined. A pilot plant with a water treatment capacity of 120 m3/d constructed in the Jieyuan Water Treatment Plant (JWTP) of Tianjin, China, was utilized for the comparison of the two processes. The results show that during the pilot test, DAF process can remove particles and organic mater more efficiently than sedimentation process. The removal rate for turbidity by DAF process is 5.5% higher than that by sedimentation in normal turbidity period, and 40% higher in low turbidity period, it is 5%~10% higher for removals of algae, total organic carbon (TOC), trihalomethane formation potential (THMFP) and bacteria in all periods. The removal rates for turbidity, TOC, THMFP, algae and bacteria by DAF process can reach 95%, 30%, 20%, 94% and 97% respectively. From the results of the pilot test, it can be concluded that DAF is a feasible clarification process, especially for source water with low turbidity and high algal blooming.     

The effect of different types of micro‐bubbles on the performance of the coagulation flotation process for coke waste‐water

BACKGROUND: Comparison experiments were carried out with three kinds of micro‐bubbles on the coagulation flotation process treatment of coke waste‐water under optimum coagulation conditions obtained from zeta potential measurement. RESULTS: Micro‐bubble flotation with ozone showed the best performance. The ozone micro‐bubbles exhibited high absolute zeta potential values, creating repulsion forces thus avoiding the coalescence of bubbles as well as creating attractive interaction between bubbles and particles in the waste‐water. Moreover, the fluorescence intensity of three micro‐bubble samples showed that the ozone micro‐bubble system produced the most hydroxyl radicals, which contributed to the degradation of organic material in the coke waste‐water. Compared with either air micro‐bubble flotation and oxygen micro‐bubble flotation processes, pyridine removal efficiency of the ozone micro‐bubble flotation process was, respectively, 4.5 and 1.7 times higher, and benzene removal efficiency 3.6 and 1.5 times higher. Finally, drainage models and oxygen diffusion models of the three kinds of micro‐bubble water samples verified the long persistence of the three kinds of micro‐bubbles in the water. CONCLUSION: The application of ozone micro‐bubble technology in coagulation processes may provide an efficient and cost‐effective approach to the treatment of waste‐water containing refractory organic compounds. Copyright © 2011 Society of Chemical Industry     

Micro-Bubble Flotation of Freshwater Algae: A Comparative Study of Differing Shapes and Sizes

This study investigates the effects of the morphology (shape and size) of differing algae species on the removal efficiency by the dissolved air flotation (DAF) process. Three selected shapes in this research are spherical (Microcystis sp.), oval (Chlamydomonas angulosa), and filamentous algae (Phormidium sp.), cultured in the laboratory under certain cultivated conditions. Physical properties of three differing algae were understood as morphology, size distribution, and zeta potential in various pHs. Under pH conditions of 4.5?7, all algae species showed negative charges: ?10 mV for filamentous shape, ? ?20 mV for oval shape, and ? ?40 mV for spherical shape. The size of these shapes ordered increasingly as spherical < oval < filamentous algae. In order to separate algae from water, positively charged bubbles were generated by adding a coagulant (as Al³⁺) and were applied at 10%, 20%, and 30% recycling ratio. Assessment of algae removal efficiency was shown by the number of the cells, total organic carbon (TOC), dissolved organic carbon (DOC), and the morphological changes (i.e., shape or size distribution). In terms of the number of cell, filamentous algae showed the greatest removal, i.e., 86% removal at 30% recycling ratio and 5 mg/L of Al³⁺ addition. For TOC and DOC removal perspectives, spherical algae showed the greatest removal above 80%. These results, therefore, confirm that the shapes and sizes of algae affect the removal process of bubble technology.     

Estimation of air concentration in dissolved air flotation (DAF) systems using a simple predictive tool

Dissolved air flotation (DAF) has been used for several decades in water treatment systems. It is a unit operation for the separation of solid and semisolid (floc) particles from a liquid phase that has been used for the clarification of potable water. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank or basin. The increased dissolved air concentration in water at elevated pressure is the fundamental principle that allows the formation of microbubbles. In this article a simple predictive tool is developed to estimate air saturated concentration in dissolved air flotation (DAF) systems. Estimations are found to be in excellent agreement with reported data in the literature with average absolute deviation being around 0.5%. The tool developed in this study can be of immense practical value for process engineers to have a quick check of air concentration in dissolved air flotation systems (DAF) at various conditions without opting for any experimental trials. In particular, water practitioners and process engineers would find the proposed approach to be user-friendly with transparent calculations involving no complex expressions.     

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

Abstract

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.     

Operating and scale‐up factors for the electrolytic removal of algae from eutrophied lakewater

Electrolytic removal of algae was conducted in batch and continuous reactors to investigate operating factors affecting removal efficiency and to explore engineering relationships which could be useful for operation and scale‐up. The system integrated both electro‐flocculation and electro‐flotation mechanisms by using polyvalent metal anodes and inert metal cathodes. Batch reactor studies confirmed that high electrical input power or higher electrical current achieved higher and faster removal efficiencies. Natural liquid circulation was observed during electrolytic operation and increased with higher electrical power. However, a small degree of external mixing may be useful at lower electrical power input. Electro‐flotation alone could not achieve complete algae removal (maximum efficiency 40–50%), and showed the importance of algal floc formation for the complete removal of algae. In continuous electrolysis experiments, the ratio of the volumetric current intensity (amperes dm^?3) and the chlorophyll a loading (mg dm^?3 h^?1) was found to be a useful operating and scale‐up factor to balance high algal removal efficiency with minimum release of excess aluminum. This ratio was eventually found to be just the charge dose or the amount of coulombs required to remove a unit mass of chlorophyll a. The optimum charge dose was determined and used to relate the operating current and electrolysis time of a continuous process. © 2002 Society of Chemical Industry     

On the structure of collision and detachment frequencies in flotation models

New explicit analytical expressions are obtained for both the collision frequency and the bubble/particle detachment frequency which enter flotation separation models. The expression for the collision frequency takes into account both the particle settling velocity and the bubble rise velocity while that for the detachment frequency is motivated by analogous results for floc disruption in a turbulent flow field. In all the cases considered, it is shown that the inclusion of the particle settling velocity increases the collision frequency by a factor of approximately 1.5 and that the most significant factor affecting the collision frequency is the bubble radius.     

Strategy for a Scale‐up Correlation in a Dissolved Air Flotation Chamber

Beginning with a laboratory‐scale physical model, a scale‐up correlation for a pilot unit project was determined based on the analysis of dynamic similarity correlations involving the predominant phenomena of a dissolved air flotation (DAF) chamber. The implantation costs of pilot units require special strategies due to the lack of correlations of this type, as novel flotation methods have been considered strictly from an economic standpoint. With the aid of computational fluid dynamics and videos of microbubble and floc flow, inertia and gravity were identified as the predominant phenomena in a DAF chamber. The strategy described herein is simple and reduces the likelihood of future risks in scale‐up investments.     

Evaluation of Model Simulation and Field Experiment on Collision-Attachment Efficiency for Operational Factors in Dissolved Air Flotation Process

This study conducted a series of tests and simulations to estimate collision-attachment efficiency using a trajectory analysis method, to evaluate model sensitivity due to major impact factors in designing and operating dissolved air flotation (DAF) for the various types of wastewaters, and to find the possibility of model application in determining the flotation efficiency of DAF. Collision-attachment efficiency between a bubble and a particle in the model is described by hydrodynamics related to stream function in Stokes’ flow and surface forces based on a classic DLVO theory, encoded using Matlab language software, and performed numerical analysis using the 5th Runge-Kutta method. Model simulation examined the effects of two major factors, size and zeta potential, on collision-attachment efficiency during flotation and field experiment evaluated to model results. Both results from the simulation and field experiment indicated that the larger particles presented the better collision and removal efficiency and the effect of particle zeta potential on collision-attachment efficiencies was increased as the particle zeta potential increased from the negative domain to the positive domain. Therefore, the collision-attachment efficiency model using the trajectory analysis method may be used in estimating the flotation efficiency and in explaining its characteristics.     

Distribution of clay minerals in the process streams produced by the extraction of bitumen from Athabasca oil sands

This study investigates the affinity of clay minerals in oil sands for the water‐continuous tailings and hydrocarbon‐continuous froth streams produced from the extraction of bitumen from oil sands. Clay minerals in oil sands processing impact bitumen flotation in separation vessels, emulsion formation during froth treatment, and fine tailings behaviour. X‐ray diffraction of oriented clay slides and random powder samples were used to quantify the clay minerals in the oil sands ore and process streams. Particle size distribution and clay activity balances were also conducted around the extraction process. The degree of partitioning during the conditioning and flotation stages in a batch extractor was determined by the surface properties of the clay minerals present. The water‐continuous tailings stream was further separated into fine and coarse tailings fractions through sedimentation. The bulk of the clay minerals reported to the fine tailings stream. Illite and mixed layered illite‐smectite partitioned less to the hydrocarbon‐continuous froth than kaolinite. Also, the illite‐smectite in the froth stream appeared to be different from the illite‐smectite in the water continuous streams.     

The Potential for Using Bubble Modification Chemicals in Dissolved Air Flotation for Algae Removal

Abstract

This paper investigates the potential for using surface modified bubbles in the treatment of algae using dissolved air flotation (DAF) instead of upstream coagulation and flocculation. Bubble modification is attempted by adding either metal coagulant, surfactant or polymers direct to the saturator. In this way, the chemical characteristics most suitable for removing small algae cells using this technique are examined. Optimum removal using metal coagulant, aluminium sulphate, was 60%; however, both a decrease in the magnitude of the zeta potential and microfloc generation occurred concurrently, thus accounting for the improved removal. In contrast, there was no change in system zeta potential and no microfloc generation when using cationic surfactant cetyltrimethyl-ammonium bromide (CTAB), for which 63% removal was achieved. An average of 95% removal was achieved using the cationic polymer, PolyDADMAC, with no change to system zeta potential. The results therefore confirm that there is a potential for adapting the conventional DAF process to operate without upstream coagulation and flocculation. A chemical with both a hydrophobic component in addition to a high molecular weight, hydrophilic, highly charge component is advised for the process.     

Flotation of blue-green algae using methylated egg ovalbumin

The removal of blue-green algae by dispersed gas flotation was conducted. Methylated ovalbumin (MeOA) was used as frother and flocculant, which is a biodegradable substance. The continuous flotation experiments were conducted at different feed mass flow rate of the blue-green algae cells and MeOA. The operating variables were the mass flow rate of blue-green algae cell and MeOA, the initial concentration of the cells and MeOA, and superficial gas velocity. The results showed that the mass flow rate of MeOA was the most dominant variable affected by the removal efficiency and that the removal efficiency achieved ca. 0.85 when a ratio of the mass flow rate of MeOA and that of the cells was over 0.3. A proposed flotation model considering the adsorptions of MeOA to the cells, MeOA to bubble surface and the cells bearing with MeOA to bubble surface was applied to explain the experimental removal efficiency. The experimental and the calculated removal efficiency were within error 19%, indicating that the proposed model was valid fundamentally.     

DAF+MSBR联合法处理煤焦化废水实验研究

为了使焦化废水NH3-N能够达标排放,采用气浮法+改良式序列间歇反应器(DAF+MSBR)联合法处理榆林某焦化厂焦化废水。结果表明,进水CODcr平均质量浓度为1490 mg/L,NH3-N平均质量浓度为200 mg/L,挥发酚平均质量浓度为176 mg/L,CN平均质量浓度为17 mg/L,SS平均质量浓度为580 mg/L,DAF处理时间3 h,MSBR处理时间9 h。经DAF+MSBR法处理后,出水CODcr浓度为74.3 mg/L,NH3-N为10 mg/L,挥发酚为0.25 mg/L,CN为0.09 mg/L,SS为30 mg/L,去除率分别为93.8%、95%、99.8%、99.4%、95%,达到《炼焦化学工业污染物排放标准》排放标准。     

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.     

Picobubble Enhanced Fine Coal Flotation

Abstract

Froth flotation is widely used in the coal industry to clean ?28 mesh fine coal. A successful recovery of particles by flotation depends on efficient particle‐bubble collision and attachment with minimal subsequent particle detachment from bubble. Flotation is effective in a narrow size range beyond which the flotation efficiency drops drastically. It is now known that the low flotation recovery of particles in the finest size fractions is mainly due to a low probability of bubble‐particle collision while the main reason for poor coarse particle flotation recovery is the high probability of detachment. A fundamental analysis has shown that use of picobubbles can significantly improve the flotation recovery of particles in a wide range of size by increasing the probability of collision and attachment and reducing the probability of detachment.

A specially designed column with a picobubble generator has been developed for enhanced recovery of fine coal particles. Picobubbles were produced based on the hydrodynamic cavitation principle. They are characterized by a size distribution that is mostly below 1 µm and adhere preferentially to the hydrophobic surfaces. The presence of picobubbles increases the probability of collision and attachment and decreases the probability of detachment, thus enhancing flotation recovery. Experimental results with the Coalberg seam coal in West Virginia, U.S.A. have shown that the use of picobubbles in a 2″ column flotation increased fine coal recovery by 10–30%, depending on the feed rate, collector dosage, and other flotation conditions. Picobubbles also acted as a secondary collector and reduced the collector dosage by one third to one half.     

Simultaneous removal of phosphate and phenanthrene by alum aided with low amounts of surfactant‐modified bentonite

BACKGROUND: Organobentonite is efficient for organic pollution removal; however, it is infrequently applied in waste‐water treatment engineering because of its high cost of modification and the difficulty of separation from treated water. This study investigated a new process for simultaneously removing phosphate and organic pollutants by flocculation with the aid of organobentonite. RESULTS: The results showed that low amounts of surfactant‐modified organobentonite could cheaply enhance simultaneous phosphate removal and phenanthrene removal. During the process, some of the alum cations were exchanged to the inner surface of the lamellar bentonite and the mechanism of phosphate adsorption on alum was changed to ligand exchange, which improved phosphate adsorption. Moreover, the coagulant helped clay sedimentation, and settlement speed of the floc itself was accelerated. It was found that the optimal removal efficiency was obtained at an alum concentration of 0.2 mmol L^?1 and pH 5.3, with more than 90% phosphate and 98% phenanthrene removed and residual turbidity kept below 10 NTU. CONCLUSION: Results showed that the proposed process was suitable for simultaneously removing non‐biodegradable organic pollutants such as phenanthrene and phosphate from water after the bioprocess in waste‐water treatment. Copyright © 2009 Society of Chemical Industry     

Effect of Biosurfactant Addition in a Pilot Scale Dissolved Air Flotation System

Effluent production of oily water type has generated many environmental problems for several industries. The use of flotation as a separation process of oily waters has been described, although it has been sometimes criticized due to the toxicity of collectors. The development and use of biodegradable surfactants may enhance the further acceptance of this separation technology. This work investigated the removal of the emulsified oil products in water by dissolved air flotation (DAF) with and without the use of a biosurfactant, in pilot scale. The biosurfactant added considerable value to the process, increasing from 80.0% to 98.0% the separation efficiency.