Limiting Solids Flux Research Articles

Bed density and bubble void fraction variation in a fluidized bed stripper

This study was targeted at understanding the local flow and flooding behaviors of fluidized bed strippers. Beddensity, pressure fluctuations andbubble void fraction were measured in a 0.6 m diameter stripper fordisk and donut, as well as gratinginternals using FCC catalyst particles.The gratingstripper had a higher bed density andoperatedsmoothly without flooding over a wider range of gas and solids flows than the disk and donut stripper. Flooding in thedisk and donut stripperwas a sudden rather than a gradual occurrence when a limiting solids flux was exceeded.Increasing fines composition and addingholes to the disk and donut traysmitigatedflooding to some extent.The gratings stripper had flattened parabolic radial bubble void fraction profiles whereas the disk and donut stripper had characteristic M-shape profiles. The results hereare expected to be valuable for the operation and design of gas-solids fluidized bed strippers.

Optimization Methodology for High COD Nutrient-Limited Wastewaters Treatment Using BAS Process

Optimization of biofilm activated sludge (BAS) process via mathematical modelling is an entangle activity since economic, environmental objective and technical decision must be considered. This paper presents a methodology to optimize the operational conditions of BAS process in four steps by combining dynamic simulation techniques with non-linear optimization methods and with operative decision-making criteria. Two set of variables are separately prioritized in the methodology: essential variables related to physical operation to enforce established process performance, and refinement variables related to biological processes that can generate risks of bulking, pin-point floc and rising sludge. The proposed optimization strategy is applied for the treatment of high COD wastewater under nutrient limitation using an integrated mathematical model for COD removal that include predation, hydrolysis and a simplified approach to the limiting solids flux theory in the secondary clarifier in order to facilitate the convergence of the optimization solver. The methodology is implemented in a full-scale wastewater treatment plant for a cellulose and viscose fibre mill obtaining (i) improvement of the effluent quality index (Kg pollution/m3) up to 62% and, (ii) decrease the operating cost index (€/m3) of the process up to 30% respect the regular working operational conditions of the plant. The proposed procedure can be also applied to other biological treatments treating high COD nutrient-limited industrial wastewater such as from textile and winery production among others.

LIMITING SOLIDS FLUX ANALYSIS: A STUDY CASE COMPARING 1-D AND 2-D MODELS

LIMITING SOLIDS FLUX ANALYSIS: A STUDY CASE COMPARING 1-D AND 2-D MODELSThis paper presents a limiting solids flux analysis for a typical secondary settling tank (SST) of a trickling filter/solids contact (TF/SC) wastewater treatment plant (WWTP). For this clarifier, the permissible solids loading rate (SLR) was found using three different approaches: (1) applying a 1-D solids-flux analysis with the Vesilind equation for the zone settling properties, (2) applying a...Author(s)J. AlexSourceProceedings of the Water Environment FederationSubjectSession 31: Municipal Wastewater Treatment Processes: Primary and Secondary ClarifiersDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2005ISSN1938-6478SICI1938-6478(20050101)2005:13L.2562;1-DOI10.2175/193864705783865136Volume / Issue2005 / 13Content sourceWEFTECFirst / last page(s)2562 - 2584Copyright2005Word count225

Empirical equations for the limiting solids flux of final clarifiers.

Using the solids flux theory, two quantities (i.e., the dimensionless limiting solids flux [G = kGL/v(o)] and the dimensionless underflow velocity [U = FR/(Av(o))] are introduced for a final clarifier based on the Vesilind equation of sludge zone-settling velocity (v = v(o) exp[-kX], where X is the suspended solids concentration, v(o) and k are empirical coefficients, and GL, FR, and A are the limiting solids flux, recycle flowrate, and surface area of the settling tank, respectively). In this paper, the dimensionless limiting solids flux is shown to be a function of the dimensionless underflow velocity only and is accurately represented by empirical equations. The limiting solids flux can, thus, be easily calculated by using the empirical equations without resorting to tedious numerical procedures. Furthermore, using the dimensionless quantities (G and U), two final clarifier equations that express the relationship among suspended solids concentration, surface area of the clarifier, recycle flowrate, and wastewater flowrate are derived. Their applications to final clarifier analysis and to determining required surface area of final clarifier are illustrated.

Activated sludge ballasting in batch tests

The ballasting performance of various mineral materials was investigated using batch settling tests. These tests revealed the complexity of the ballasting phenomenon; both the sludge and the mineral material affected the ballasting performance. Differences between the materials could be localized using the Tukey test, but no correlation between the ballasting performance and the density or the particle size distribution was found. It was assumed that the interaction mechanism between the activated sludge and the mineral material, and hence the surface characteristics, are crucial for the ballasting action. A set of settling tests was done with activated sludges and mineral materials with known surface characteristics. The linear regression analysis produced a model (r 2=0.85) for ballasted settling where the important variables were sludge volatile fraction; mineral material dosage, density, specific surface area, cation exchange capacity; sludge suspended solids, mineral material particle size, and hydrophobicity. A batch pilot study was performed with calcium carbonate and the simulation program efor version 2.2, 1993 was used to simulate the effect of the ballasting agent on sludge settling in connection with a higher inflow and higher sludge concentration than was possible to achieve in the pilot plant. efor version 2.2, 1993 simulations indicate a possibility to increase the inflow by 67%, and the limiting solids flux by 155% when calculated as kgSS/m 2h and 43% when calculated as kgVSS/m 2h. This study gives a clear indication of the possibility of improving the sludge settling by ballasting.

Use of dynamic models to study secondary clarifier performance

Efficient separation of solids and supernatant is crucial in an activated sludge system since performance of the system depends on the return of active biological solids. A simple discrete linear dynamic model is applied to secondary clarifier performance. Dynamic modelling could accurately predict movement of the sludge blanket height in secondary clarifiers and also indicate the level of the current limiting solids flux. Change in model parameters may indicate either altered sludge composition and settling characteristics, or disturbances in the sedimentation process.

Use of dynamic models to study secondary clarifier performance

Efficient separation of solids and supernatant is crucial in an activated sludge system since performance of the system depends on the return of active biological solids. A simple discrete linear dynamic model is applied to secondary clarifier performance. Dynamic modelling could accurately predict movement of the sludge blanket height in secondary clarifiers and also indicate the level of the current limiting solids flux. Change in model parameters may indicate either altered sludge composition and settling characteristics, or disturbances in the sedimentation process.

On-line estimations of settling capacity in secondary clarifier

In a biological nitrogen removal system, efficient performance of the secondary clarifier is essential since the system depends on a high concentration of active biological solids being available in the biological reactor. Estimations of the current capacity of the clarifier can be made from observing maximum feasible solids flux to the clarifier which does not cause escape of sludge. The aim of this work is to demonstrate a method to establish maximum feasible solids flux, the limiting solids flux, from on-line operational data. In this method the solids flux into the clarifier is calculated from on-line measurements of effluent flow and of suspended solids in the biological reactor, and a known value of return sludge flow. From relations between solids flux and sludge blanket height, the value of limiting flux is estimated. This method is based on the assumption that the movement of sludge blanket height is directly related to changes in the solids flux. When the limiting solids flux is estimated from on-line measurements a measurement of the capacity of the clarifier is retrieved. The suggested method also gives an early warning to prevent sludge escape from the clarifier.

On-line estimations of settling capacity in secondary clarifier

In a biological nitrogen removal system, efficient performance of the secondary clarifier is essential since the system depends on a high concentration of active biological solids being available in the biological reactor. Estimations of the current capacity of the clarifier can be made from observing maximum feasible solids flux to the clarifier which does not cause escape of sludge. The aim of this work is to demonstrate a method to establish maximum feasible solids flux, the limiting solids flux, from on-line operational data. In this method the solids flux into the clarifier is calculated from on-line measurements of effluent flow and of suspended solids in the biological reactor, and a known value of return sludge flow. From relations between solids flux and sludge blanket height, the value of limiting flux is estimated. This method is based on the assumption that the movement of sludge blanket height is directly related to changes in the solids flux. When the limiting solids flux is estimated from on-line measurements a measurement of the capacity of the clarifier is retrieved. The suggested method also gives an early warning to prevent sludge escape from the clarifier.

Dynamic simulation of sludge blanket movements in a full-scale rectangular sedimentation basin

Bromma sewage treatment plant in Stockholm is the second largest plant in Stockholm and will in the near future have requirements for nitrogen removal. This means that a higher sludge age must be used in the aeration basin. This may be accomplished by an increase of the sludge concentration up to values until the limiting solids flux is exceeded. Measurement of the sludge blanket level is a possibility for better control of the sedimentation basin. Different measurements were performed to evaluate the main factors influencing the level. Dynamic simulation studies were performed at Bromma sewage treatment plant in Stockholm of the sludge blanket level and the return sludge concentration in a full-scale sedimentation basin. The simulations were performed with the help of a Danish simulation package, EFOR (1992), in which both reactions in the aeration basin (mainly based on the IAWPRC model) and separation processes in the sedimentation basin (both clarification and thickening) can be studied. The thickening model is based on the solids flux theory and the Vesilind formula (1979). Different methods were compared for determination and use of characteristic parameters in the Vesilind formula.

Dynamic simulation of sludge blanket movements in a full- scale rectangular sedimentation basin

Bromma sewage treatment plant in Stockholm is the second largest plant in Stockholm and will in the near future have requirements for nitrogen removal. This means that a higher sludge age must be used in the aeration basin. This may be accomplished by an increase of the sludge concentration up to values until the limiting solids flux is exceeded. Measurement of the sludge blanket level is a possibility for better control of the sedimentation basin. Different measurements were performed to evaluate the main factors influencing the level.Dynamic simulation studies were performed at Bromma sewage treatment plant in Stockholm of the sludge blanket level and the return sludge concentration in a full-scale sedimentation basin. The simulations were performed with the help of a Danish simulation package, EFOR (1992), in which both reactions in the aeration basin (mainly based on the IAWPRC model) and separation processes in the sedimentation basin (both clarification and thickening) can be studied. The thickening model is based on the solids flux theory and the Vesilind formula (1979). Different methods were compared for determination and use of characteristic parameters in the Vesiind formula.

Experimental study of the dynamics of a vertical continuous thickener—I

The dynamic behaviour of a continuous thickener at stepwise changes of solids load has been studied at thickening of unflocculated barium sulphate. Trace amounts of the radioactive isotope 131Ba made it possible to apply a radioactive measuring technique. On the basis of steady state conditions at optimal load a model describing the dynamic behaviour has been formulated. The model shows agreement with the experimental results. An analytical expression, giving the true capacity limiting solids flux as a function of solids concentration at a known relationship between solids load and underflow discharge concentration, has been derived. Calculated concentrations of the capacity limiting fluxes show agreement with those experimentally determined.