The bottom solids return pipe 42 operates most of the time as a packed moving bed but can also be at a semi-fluidized or conventionally fluidized state. In the LSCFB waste water treatment system of the present invention, the circulation rate of the solid particles 18 may be controlled by a butterfly valve 70 located in the bottom return pipe The splitting of the feed stream to the riser into two streams 60 and 62 with the second stream mobilizing the particles in the bottom section of the riser forms a further hydraulic valve that can also control the circulation rate of the solid particles There are other possible types of hydraulic valves such as loop seal etc.
Either a mechanical valve or a hydraulic valve or both can be used to control the flow rate of the solid particles Normally, a mechanical valve is preferred over a hydraulic valve because the mechanical valve usually provides a higher and more stable pressure drop across the return pipe 42 and therefore makes the system more stable and also makes it easier to maintain a pressure balance between the two columns. The auxiliary liquid stream 62 , if used together with the mechanical valve, provides additional control of the circulation rate of the solid particles The recycle stream 20 from the effluent 44 as above described enters at the bottom of the bed 10 and travels, together with the injected gas mostly likely air stream, in a countercurrent relationship.
The clarifier 76 at the top of the first fluidized bed 10 separates out the entrained particles 18 and sloughed sludge formed during the process, and returns them back to bed 10 , before the gas and fluid exiting the column. The clarifier 76 also periodically discharge the sloughed sludge through a port or ports The fluid exiting from column 10 at 44 is the effluent, part of which is released as treated water and part of which recycled back to the bottom of the downer column 10 to sustain the operation of the process.
The second fluidizing fluid 28 which is a combination of raw waste water feed 72 plus the recycle stream 74 from the liquid-solid separator 46 at the top of the riser column 12 and the particles 18 along with some of the nitrified effluent from the aerobic zone from pipe 42 travel in co-current relationship upward through the bed 12 and then enter liquid-solid separator 46 such as a settler having a fluid outlet 48 through which most of the second fluidizing fluid 28 is removed and an outlet 50 for the particles 18 and some of the fluid from the fluidized bed 12 located at the bottom open to the upper connecting pipe Solid particles from the separator 46 , together with some entrained liquid, flows down via the upper connection pipe 19 to the inlet 17 in the downer column If additional gas is injected to the bottom 26 of the riser column 12 , the gas will travel co-currently upwards, together with the fluid, to fluidize the solid particles.
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Such gas will be separated out in the top separator The application of the present invention will be further described below in relation to wastewater treatment, but it may be also used in other applications, for example effluent treatment from a host of industrial processes. The present invention will now be illustrated using the following non-limiting example. In connection with FIG. An alternative is to have the anoxic process in the upper part of the riser and the anaerobic in the lower part of the riser. In this case, however, most of the nitrified effluent from the aerobic zone needs to be fed half way in the riser to the bottom of the anoxic zone.
The fluid containing the raw wastewater, effluent recycle stream 74 , the previously nitrified wastewater and the solid particles loaded with microbes, flow co-currently up the riser 12 while contacting each other. The riser column 12 operates in the liquid-solid circulating fluidization regime and provides excellent interfacial mass transfer between the liquid and the activated sludge, thus significantly enhancing the process intensity. In the lower section of the riser 12 , anoxic conditions that are conducive to denitrification prevail. The carbon source for denitrification in the bottom half of the riser can either be a portion of the wastewater feed or the decaying biomass sloughed from the downer bed biofilm, or both.
Then, wastewater flows further upwards into the anaerobic zone maintained in the upper half of the riser, where stored phosphorus is released and volatile fatty acids are stored for further degradation in the aerobic zone. The elimination of nitrates upstream of the anaerobic zone will greatly enhance phosphorus release and subsequent uptake in the aerobic downer bed It is important to note that since liquid-solid separation and subsequent solid recycle to the downer bed 10 occurs immediately after the anaerobic phase, enhanced phosphorus uptake in the aerobic downer bed will also likely take place.
In the liquid-solid separator 46 at the top of the riser 12 , the solid particles loaded with microbes are separated out from the liquid stream and, along with some effluent liquid also from the liquid-solid separator 46 , are introduced into the top of the downer bed 10 through the upper connecting pipe Treated wastewater that comes with the recirculated solids from the riser 12 is joined at the top inlet 17 of the downer bed 10 by recycled effluent from downer column 10 flowing up through the downer bed Part 20 of the combined stream equivalent to approximately times of the wastewater feedrate is recycled into the downer bed 10 and the remaining part 44 exits the system as treated effluent.
The recycled liquid 20 is also used to fluidize the downer bed 10 so that it is essential to introduce it into the downer bed 10 from the bottom, near the injection port for air or oxygen containing gas. By provision of air in the downer bed 10 and control of air flowrate, and by control of the recycled liquid flowrate, the aerobic zone can be well maintained in the downer bed 10 under the conventional gas-liquid-solid three-phase fluidization regime.
The solids attached with biomass flow by gravity to the bottom of the downer bed 10 , and then into the riser bottom through the bottom inclined connection pipe Some of the sloughed sludge formed during the process is periodically discharged from the aerobic zone through outlet 45 in clarifier The system can be designed such that the fluid flow from the downer bed to the bottom of the riser 12 is of an appropriate recycle flowrate e. Another alternative is to feed the wastewater halfway in the riser to the anaerobic zone, as shown by the dotted line in both FIGS.
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In such a case, a portion of the effluent from the riser needs to be recycled to the bottom of the riser, to fluidize the bottom anoxic portion of the riser. In addition to the advantages of the fixed-film processes with respect to sludge settle-ability and accordingly to the reduced size of the clarifiers for liquid-solid separation, the LSCFB re-circulates attachment media with high settling velocities, such that only a small liquid-solid separation tank is needed instead of a separate clarifier.
Furthermore, due to fluidization, the BNR-LSCFB can handle both soluble and particulate-laden wastewater, thus possibly eliminating the need for primary clarifiers. More specifically, the BNR-LSCFB system disclosed herein offers the following advantages over the widely employed conventional activated sludge and the more recently developed suspended growth BNR systems. In addition, while this fixed-film BNR LSCFB process is applicable to any municipal wastewater treatment application, it is particularly advantageous for treatment of wastes from industries that very often have space limitations and special needs.
For example, this technology can be used to treat specific concentrated streams of industrial wastewater as well as to provide end-of-pipe treatment to the overall wastewater. The other potential principal application of the technology would be small communities and new developments, which have limited financial resources, given the drastically reduced cost, space, and energy requirements. No nitrite accumulation could be observed either in the anoxic bed or in the aerobic bed.
The BNR-LSCFB appears to be not only an excellent alternative for conventional activated sludge type BNR technologies but also capable of processing much higher loadings and suitable for industrial applications. It will be appreciated that the apparatus and process of the present invention are not limited to having the aerobic section in the downer column 10 and the anoxic and anaerobic sections in the riser column 12 , and that these may be reversed so that the aerobic section with gas injection may be located in the riser column 12 and the anoxic and anaerobic sections may be in the downer column These terms are not to be interpreted to exclude the presence of other features, steps or components.
The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiments illustrated.
It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents. Year of fee payment : 4. Year of fee payment : 8. Year of fee payment : Biological nutrient removal BNR in municipal wastewater treatment to remove carbonaceous substrates, nutrients and phosphorus, has recently become increasingly popular worldwide due to increasingly stringent regulations.
Biological fluidized bed BFB technology, which could be potentially used for BNR processes, can provide some advantages such as high efficiency and compact structure. This present invention incorporates the fixed-film biological fluidized bed technology with the biological nutrient removal in a liquid-solid circulating fluidized bed, which has achieved the simultaneous elimination of organic carbon, nitrogen and phosphorus, in a very efficient manner and with very compact space requirements.
The new BNR-LSCFB system is not only an excellent alternative for conventional activated sludge type BNR technologies but is also capable of processing much higher loadings and suitable for industrial applications. This design increased the pressure drop across the bottom solids return pipe 42 and makes the system more stable. The recycle stream 20 from the effluent 44 as above described enters at the bottom of the bed 10 and travels, together with the injected gas mostly likely air stream, in a countercurrent relationship to the particles 18 through the downer column 10 and leaves at the top of column 10 as indicated at Applications of the Present Invention The application of the present invention will be further described below in relation to wastewater treatment, but it may be also used in other applications, for example effluent treatment from a host of industrial processes.
Pollut, Control Fed. Control Fed. Control, 82, 5 Cooper, P. Karamanev and Amarjeet S. Liang, W. Edition P, , Water Pollution Control Fed. Phosphate removal by crystallization in a fluidized bed.
Phosphate removal in real anaerobic supernatants: Modelling and performance of a fluidized bed reactor. A gas-liquid-solid circulating fluidized bed system, comprising: a first fluidized bed;. The gas-liquid-solid circulating fluidized bed system according to claim 1 wherein said gas injection means is connected to said first fluidized bed, and wherein said first fluidized bed includes an aerobic zone for biodegrading wastewater predominately by the immobilized bacteria in the presence of oxygen. The gas-liquid-solid circulating fluidized bed system according to claim 2 wherein said second fluidized bed includes an anoxic zone for denitrification, and an anaerobic zone for phosphorus release.
The gas-liquid-solid circulating fluidized bed system according to claim 2 wherein said gas injected into the first fluidized bed contains oxygen. The gas-liquid-solid circulating fluidized bed system according to claim 1 wherein said gas injection means is connected to said second fluidized bed, and wherein said second fluidized bed includes an aerobic zone for biodegrading wastewater predominantly by the immobilized bacteria in the presence of oxygen.
The gas-liquid-solid circulating fluidized bed system according to claim 5 wherein said first fluidized bed includes an anoxic zone for denitrification and an anaerobic zone for phosphorus release. The gas-liquid-solid circulating fluidized bed system according to claim 5 wherein said gas injected into the second fluidized bed contains oxygen. The gas-liquid-solid circulating fluidized bed system according to claim 1 wherein said first means connecting is adapted to form a first hydraulic seal between said first and second fluidized beds, and wherein said second means connecting is adapted to form a second hydraulic seal between said first and second fluidized beds.
The gas-liquid-solid circulating fluidized bed system according to claim 8 wherein said first hydraulic seal is a first moving packed bed, and wherein said second hydraulic seal is a second moving packed bed. The gas-liquid-solid circulating fluidized bed system according to claim 1 wherein said first fluidized bed is a counter-current fluidized bed.
The gas-liquid-solid circulating fluidized bed system according to claim 1 wherein said second fluidized bed is a riser bed operated in a circulating fluidization regime. The gas-liquid-solid circulating fluidized bed system according to claim 1 wherein said first and second liquid fluidized beds are substantially vertical columns. The gas-liquid-solid circulating fluidized bed system as defined in claim 12 wherein said first end of said first fluidized bed is the top end, said second end of said first fluidized bed is the bottom end, said one end of the second fluidized bed is the bottom end and said other end of said second fluidized bed is the top end.
This is cer- experiments where all inputs are as well characterized as pos- tainly a factor to be considered. However, Gidaspow and Lu sible. The CFD model uses empirical drag laws to calculate the , and Lettieri et al. In the two former cases, good agree- the column diameter to minimize wall effects. The Plexiglas column has a 0. Properties of the liquid and solids appear in Table 2. Pressure Fluidized bed ports 3. In general, accounting for time-averaged turbulent behaviour and turbu- Distributor lent interactions between phases can make simulation predic- tions more realistic for beds operating at high Reynolds num- ber.
However, unless an appropriate turbulence model with the Liquid flow correct empirical constants and closures is chosen, the model meter Liquid pump predictions may be less consistent with experimental data than the turbulence-free model. Conservation of mass and momentum provide the governing Fig. Schematic of experimental system. A set of constitutive equations closes the governing equations. The dense solid phase containing inelastic spherical particles 4. The relevant equations are listed in Table 3.
Mass transfer between the phases is ignored. External body, lift and virtual mass forces are assumed to be negligible in the momentum equations. Volume percentage [-] The transfer of forces between the liquid and solid phases is described by empirical drag laws based on both Wen and Yu and Gidaspow et al.
The latter combines the 2. The equation of Lun et al. It describes the resistance of an emulsion to com- pression or expansion. The kinetic part of the granular viscosity and the granular conductivity are both obtained from relation- Fig. Particle size distribution of spherical glass beads. The radial distribution function of Ding and Gidaspow takes into account the proba- bility of particles colliding with each other when the granular the solids were spherical glass beads of 1.
Voidages were estimated from pressure drop measure- ble precision mode. A Pentium 4 CPU running on 3. The simula- perimental set-up are provided by Lee et al. The density and maximum packing density of the To determine the overall bed voidage since the upper bed particles were determined with a graduated cylinder.
A time step of 0.
Gas-Liquid-Solid Fluidization Engineering
Hulme et al. The focus of this section is a parametric study of tal value and stable in time. We begin with a base case ferent transient behaviour. Detailed settings for the base case appear ble. Decreasing the time step further to 0. Seemingly the simulations show less realistic physical 4. Mesh behaviour when using excessively small time steps. The relation between the time step and mesh size is discussed below.
Jack T. Schematic representation of base case settings and parametric studies performed with this setting. Bold underlined values are base case settings. The rectangular region simulated ferent convergence criteria.
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At 4 s the simulation results are is strictly symmetric about a central axis. As expected, a stricter convergence convergence criterion. In Fig. The dimensionless Experimental value Courant number 0. Keeping the Courant number low decreases oscillations, 0. Gobin Time [s] et al. As noted above, a smaller time step increases the relative error Fig.
Comparison of overall bed voidage as a function of time for different and therefore low Courant numbers also give inaccurate re- mesh resolutions. Hence in our work, the Courant number was best in the range 0. The drag laws of Wen and Yu Time [s] and Gidaspow et al. Effect of time step TS on overall bed voidage. Therefore, the Wen and Yu relation was used for all further 4.
Summary of mesh, time step and convergence criterion studies. The difference in voidage was negli- step and convergence criteria. These three are inter-related. However, if the time step becomes all subsequent simulations. Therefore, choosing a smaller time step can increase the relative error. At the other extreme, if the time Limtrakul et al. A larger cell results in a smaller tracking. Effect of convergence criteria on overall bed voidage as a function of Fig.
Effect of Courant number on overall bed voidage as a function of time. Distributor geometry cause of computation time restrictions. A perforated plate distributor was simulated tuating component of solids velocity, sampled at 2 Hz. Since the in comparison with a perfectly uniform entry. The distrib- region simulated was symmetrical about a vertical axis, values utor in our experimental work on an axisymmetric column on the left and right sides were averaged. Just below the perforated circles as shown in Fig.
Consequently, it was assumed to minimize non-uniformities such as jets upstream of the dis- that the distributor had nine holes, and each hole was meshed tributor holes. To convert the two-dimensional distributor plate with three cells. Winslow smoothing was applied to the top grid to create a more uniform grid.
This book provides a comprehensive mechanistic interpretation of the transport phenomena involved in various basic modes of gas-liquid-solid fluidization. These modes include, for example, those for three-phase fluidized beds, slurry columns, turbulent contact absorbers, and three-phase fluidized beds, slurry columns, turbulent contact absorbers, and three-phase transport. It summarizes the empirical correlations useful for predicting transport properties for each mode of of operation. Gas-Liquid-Solid Fluidization Engineering provides a comprehensive account of the state-of-the-art applications of the three-phase fluidization systems that are important in both small-and large-scale operations.
These applications include fermentation, biological wastewater treatment, flue gas desulfurization and particulates removal, and resid hydrotreating. This book highlights the industrial implications of these applications. In addition, it discusses information gaps and future directions for research in this field.
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