Settling Model Research Articles

Miller Compensation: Optimal Design for Operational Amplifiers with a Required Settling Time

Optimizing the settling response of an operational amplifier can be a serious design issue in today's low-power CMOS technologies. Several design challenges emerge when improving the linear and nonlinear responses of an amplifier. In this paper, we developed a settling model for use in design and optimization of two-stage Miller-compensated amplifiers. Using this model, the closed-form relations between settling time/settling error, gain-bandwidth product, noise, power and stability have been obtained. These relations are employed to form a settling-based design routine for Miller-compensated amplifiers. Simulation results in 0.18- $$\upmu $$ μ m CMOS validate the effectiveness of the proposed routine. In a design prototype, it predicts the settling time with an error less than 3 %. In another design example, the relationship between settling time and gain-bandwidth has been evaluated with an accuracy higher than 95 %. The proposed design routine is used to implement a 40 MS/s sample-and-hold amplifier. It achieves a settling time and signal-to-noise-plus-distortion ratio equal to 12.5 ns and 82 dB, respectively.

Settling velocities of multifractal flocs formed in chemical coagulation process

A number of different flocculation mechanisms are involved in the formation of chemical coagulation flocs. Consequently, two flocs with the same size may have been formed by different mechanisms of aggregation and therefore have different arrangement of primary particles. As a result, two flocs with the same size may have different masses or mass distributions and therefore, different settling velocities. Although the correct estimation of the floc mass and density is critical for the development of the floc settling model, none of the suggested floc settling models incorporate the information on mass distribution and variable density of flocs. A probability-based method is used to determine the floc fractal dimensions on floc images. The results demonstrated that flocs formed in lime softening coagulation are multifractal. The multifractal spectra indicated the existence of a multiple fractal dimensions as opposed to the unique box-counting dimension which is a morphology-based fractal dimensions typically introduced into the Stokes' Law. These fractal dimensions may provide information on the flocs' aggregation mechanism, floc's structure, and the distribution of mass inside the floc. More research is required to investigate how to utilize the information obtained from the multifractal spectra to incorporate the variable floc density and nonhomogeneous mass distribution of flocs into the floc settling models.

The morphological and settling properties of ANAMMOX granular sludge in high-rate reactors

Digital macro photography and settling tests were carried out to investigate the morphological and settling characteristics of ANAMMOX granules in a high-rate reactor. The ANAMMOX granules could be divided into settling and floating granules. The settling granules with an average diameter of 2.96±0.99 mm were smaller than the floating granules with an average diameter of 4.58±1.22 mm. A settling model was established and validated to correlate the settling velocity with the density (ρG), mass shape factor (ψmass), shape-correction factor (characterized by sphericity (Φ(')) or roundness (ξ)) and projected area equivalent sphere diameter (dP) of ANAMMOX granules. The sphericity was more suitable than the roundness for describing the settling behavior. The sensitivity of four parameters was in the order of ρG,ψmass, dP and Φ('). Based on the settling model, ANAMMOX granules with diameter of 1.75-4.00 mm were supposed to be optimal for the ANAMMOX process.

Dust deposits on La Graciosa Island (Canary Islands, Spain): Texture, mineralogy and a case study of recent dust plume transport

La Graciosa volcanic island evolved in the late Pleistocene–Holocene. It is situated along the northeast of the Canarian Archipelago, with a subaerial surface of 27km2. This island is located close to the Western Sahara, being 80km from the north-western African continental shelf and 145km from the coast. The island supports a very small permanent population. The climate is coastal-arid (116mm precipitation per annum). Regular occurrences of Saharan dust plumes (about 30% of the year), with a dust accumulation rate of 20g−2year−1, produce sedimentation of dust on the island of ca. 540tyear−1. Wind-blown sediments cover a large area (more than 52%) of the otherwise volcanic island, forming sandy beaches, sand sheets, nebkhas and other aeolian deposits. Re-distributed dust deposits often occur on the leeward sides of inter-volcano areas, inter-dune areas and in endorheic sediment traps. Many of these deposits are edaphized and carbonatized. Textural analysis of the dust deposit samples shows a variable proportion of silt (80–20%), clay (19–9%), and fine to very fine sand (71–4%). The general mineralogical composition of these sediments as measured by XRD and SEM-EDS is calcite (26%), illite (26%), quartz (11%), augite (10%), aragonite (6%), anorthite (8%), kaolinite (5%) and montmorillonite (3%). The mineralogy in different size fractions is very consistent; for example, quartz content decreases from 15% in the coarse silt fraction to 5% in the fine sand. A mineral and grain-size comparison with airborne dust collected on Gran Canaria Island was undertaken; close similarities were found in the two sample sets. Such closely-matched characteristics point to a similar origin for both airborne dust and dust deposits, in line with the Saharan plume dust that regularly traverses this archipelago. It is unusual to find so much quartz in the fine sand fraction of these aeolian dust deposits. Individual quartz grains with an intermediate axial length of ~160μm were identified by SEM-EDS. Previous investigators have found similar “giant” particles in long-range transported aeolian dust (Middleton et al., 2001), in contrast to the classic model for gravity settling of airborne dust particles. The debate on this subject remains open for discussion. We present evidence of long-range, wind-transported large mineral particles and a 2004 case study of uplift wind velocity vs. plume dust generation, as well as a transport efficiency model that can explain the existence of fine sand in the dust deposits on La Graciosa Island and in the airborne samplers on Gran Canaria Island. Recent Saharan dust shows that about 10% of similar coarse-grained particulate matter is also present.

Multiphase modeling of settling and suspension in anaerobic digester

Effective suspension and settling are critical for controlling biomass retention in a bioreactor. In this paper, a multi-fluid model with kinetic theory of granular flow (KTGF) was established to describe these phenomena in the biowaste particles flow in anaerobic digesters. Solid retention time (SRT) was added as a parameter into anaerobic digestion No.1 (ADM1) model to evaluate its effect on the biogas productivity. The model was experimentally validated in a liquid–gas–solid column reactor with gas and solid volume fraction and granular temperature as the major variables. The wastewater residence time distribution was also determined through modeling and measurement to evaluate the mixing pattern in the pilot column reactor. The effect of restitution coefficient on flow behavior of biowaste particles, particles settling and suspension were predicted. Settling and suspension processes of anaerobic digesters were simulated for lab and pilot-scale reactors with comparisons made for reactor configuration and geometry model, respectively. This study demonstrated that the multi-fluid model with KTGF could provide better understanding of impact of suspension and settling upon retaining biomass particles in the anaerobic digesters.

A computational study of the role of particle size standard deviation on the collision frequency in differential settling

The results of a simple computational model for differential settling are presented illustrating the significant role that particle size distribution plays in collision frequency and sedimentation rate of particles in a quiescent environment. The model tracks a large number of particles (order 105) with log-normally distributed diameters, as they settle at their Stokes settling velocities. Particle collisions are detected and result in larger particles that fall more rapidly. A number of simplifying assumptions are made in the model in order to avoid empirical correlations for phenomena such as collision efficiency and particle shape. These simplifying assumptions were needed to isolate and quantify the role of the particle size distribution. Simulated concentration profiles indicate that, even in the absence of collisions, the standard deviation (σD) of the particle size strongly influences the bulk mass settling rate as, for larger σD, more mass is concentrated in larger, faster falling particles. The collision frequency is also a strong function of σD. For a given mass concentration the collision frequency first increases linearly with increasing σD as greater variation in particle size leads to greater variation in particle velocity, and shorter times for particles to catch each other. For larger σD more mass is concentrated in larger particles, so, for a given mass concentration, there are fewer particles per unit volume, increasing the mean distance between the particles and reducing the collision frequency. The implications of these results for sedimentation measurement using optical attenuation techniques are discussed.

When are the People?: Temporality, Popular Sovereignty, and the U.S. Settler State

This essay examines the subject of anti-imperialism through the lens of settler postcolonial studies, an approach that immediately confounds sure distinctions between pro- and anti- imperialism. In the mid-1790s, during the Whiskey Rebellion, farmers in western Pennsylvania revolted against a federal government that used them to advance the course of empire while simultaneously treating them as colonists within the empire they labored and risked to create. Even the briefest sketch of this historical episode hints at the complexity of settler subjects’ imperial affiliations and attitudes. Technically, the whiskey rebels were both: in favor of those policies and customs that sanctioned the appropriation of Indian lands and against those that positioned white settlers as subalterns. My contention is that this popular uprising against the imperialist dictates of the U.S. government clarifies two competing visions of empire much more than it reveals opposing stances for and against. In this article I suggest that these competing visions of empire play out in the widespread debate over popular sovereignty, and my primary concern is how traditional notions of sovereignty were re-negotiated within the context of settler rights. Looking at the Whiskey Rebellion, including George Washington’s actual and symbolic role in suppressing the revolt, I argue that the last decade of the eighteenth century witnessed an uncomfortable shift from unitary to plural sovereignty. Made clear by documentary evidence from the rebellion, republican political culture shifted away from authority concentrated in a single individual to its anonymous dispersal far and wide. With this shift comes the dislocation of sovereign power, the diffusion of sovereignty in time as well as in space. This model of sovereignty, on which the decentralized settler model of empire thrived, would severely weaken the centralized federal model of empire.

Factors influencing the density of aerobic granular sludge

In the present study, the factors influencing density of granular sludge particles were evaluated. Granules consist of microbes, precipitates and of extracellular polymeric substance. The volume fractions of the bacterial layers were experimentally estimated by fluorescent in situ hybridisation staining. The volume fraction occupied by precipitates was determined by computed tomography scanning. PHREEQC was used to estimate potential formation of precipitates to determine a density of the inorganic fraction. Densities of bacteria were investigated by Percoll density centrifugation. The volume fractions were then coupled with the corresponding densities and the total density of a granule was calculated. The sensitivity of the density of the entire granule on the corresponding settling velocity was evaluated by changing the volume fractions of precipitates or bacteria in a settling model. Results from granules originating from a Nereda reactor for simultaneous phosphate COD and nitrogen removal revealed that phosphate-accumulating organisms (PAOs) had a higher density than glycogen-accumulating organisms leading to significantly higher settling velocities for PAO-dominated granules explaining earlier observations of the segregation of the granular sludge bed inside reactors. The model showed that a small increase in the volume fraction of precipitates (1-5%) strongly increased the granular density and thereby the settling velocity. For nitritation-anammox granular sludge, mainly granular diameter and not density differences are causing a segregation of the biomass in the bed.

Study on Vehicle Scheduling Algorithms for Logistics Operations Based on Cloud Computing and Neural Network

This paper studies on methods taken in optimizing the existing vehicle scheduling problems in logistic operations. This paper proposes a new vehicle scheduling algorithm based on Cloud Computing and Neural Network to meet quality requirements and customer satisfaction which cannot be done by applying algorithm based only on Cloud Computing. The new algorithm made a parameterized settling model, taking all the service quality and the user’s satisfaction into consideration, to calculate the overall satisfaction on various resources, and then assign them to meet every user’s needs, so as to achieve an equilibrium resource execution. The application of the improved neural network will accomplish the vehicle scheduling. The result of the test for the models above shows that the new algorithm can meet various user’s needs to improve their satisfaction, and meanwhile to improve the resource distribution rate as well as the system resources utilization frequency.

Predicting the settling velocity of flocs formed in water treatment using multiple fractal dimensions

Here we introduce a distribution of floc fractal dimensions as opposed to a single fractal dimension value into the floc settling velocity model developed in earlier studies. The distribution of fractal dimensions for a single floc size was assumed to cover a range from 1.9 to 3.0. This range was selected based on the theoretically determined fractal dimensions for diffusion-limited and cluster–cluster aggregation. These two aggregation mechanisms are involved in the formation of the lime softening flocs analyzed in this study. Fractal dimensions were generated under the assumption that a floc can have any value of normally distributed fractal dimensions ranging from 1.9–3.0. A range of settling velocities for a single floc size was calculated based on the distribution of fractal dimensions. The assumption of multiple fractal dimensions for a single floc size resulted in a non-unique relationship between the floc size and the floc settling velocity, i.e., several different settling velocities were calculated for one floc size. The settling velocities calculated according to the model ranged from 0 to 10 mm/s (average 2.22 mm/s) for the majority of flocs in the size range of 1–250 μm (average 125 μm). The experimentally measured settling velocities of flocs ranged from 0.1 to 7.1 mm/s (average 2.37 mm/s) for the flocs with equivalent diameters from 10 μm to 260 μm (average 124 μm). Experimentally determined floc settling velocities were predicted well by the floc settling model incorporating distributions of floc fractal dimensions calculated based on the knowledge of the mechanisms of aggregation, i.e., cluster–cluster aggregation and diffusion-limited aggregation.

Turbulent particle transport in streams: Can exponential settling be reconciled with fluid mechanics?

Most ecological studies of particle transport in streams that focus on fine particulate organic matter or benthic invertebrates use the Exponential Settling Model (ESM) to characterize the longitudinal pattern of particle settling on the bed. The ESM predicts that if particles are released into a stream, the proportion that have not yet settled will decline exponentially with transport time or distance and will be independent of the release elevation above the bed. To date, no credible basis in fluid mechanics has been established for this model, nor has it been rigorously tested against more-mechanistic alternative models. One alternative is the Local Exchange Model (LEM), which is a stochastic advection–diffusion model that includes both longitudinal and vertical spatial dimensions and is based on classical fluid mechanics. The LEM predicts that particle settling will be non-exponential in the near field but will become exponential in the far field, providing a new theoretical justification for far-field exponential settling that is based on plausible fluid mechanics. We review properties of the ESM and LEM and compare these with available empirical evidence. Most evidence supports the prediction of both models that settling will be exponential in the far field but contradicts the ESM's prediction that a single exponential distribution will hold for all transport times and distances.

EVALUATION OF DIFFERENT CONTROL STRATEGIES OF THE WASTE WATER TREATMENT PLANT BASED ON A MODIFIED ACTIVATED SLUDGE MODEL NO. 3

The first part of this work presents the development and implementation in the Benchmark Simulation Model No 1 (BSM1) of a modified Activated Sludge Model No 3 (ASM3). The enhanced ASM3 presented in this study has three modifications compared to the original ASM3. The first modification is the representation of the simultaneous heterotrophic biomass growth on the primary substrate and on the internal storage products. Second, nitrification is modeled as a two-step process with nitrite as an intermediate product, bringing an increased degree of complexity to the mathematical model. Third, the denitrification process is modeled as a three step process with nitrite and nitric oxide as intermediates. The nitric oxide is introduced in the model to account for the inhibition of some enzymes that are responsible for the growth of the heterotrophic bacteria under aerobic conditions. For a better representation of the real plant behavior, the secondary settler is modeled to be reactive. The built reactive settler model is the combination of the settler model described by Takacs in 1991 and the enhanced ASM3. The second part of this research consists of the investigation of five control strategies applied to the waste water treatment plant (WWTP). The control architectures studied in this research are multi-input-multi-output (MIMO) Model Predictive Controllers (MPC). The assessment of these strategies is made from three points of view: control performance, cost evaluation and quality of the effluent. The simulation results show that operational costs can be reduced using automatic control.

Sedimentation patterns caused by scallop dredging in a physically dynamic environment

Scallop dredging grounds in the Firth of Lorn, western Scotland, are juxtaposed with rocky reef habitats raising concerns that reef communities may be impacted by sediment disturbed by nearby scallop dredging. A particle-tracking model of sediment transport and settling is applied at two scales. In the near-field, a suspension of typical sand/gravel-dominated bed sediment is subjected to a steady current across the dredge track. In the far-field, silt particles, which may persist in suspension for multiple tidal cycles, are tracked in the context of a regional model of tidally-driven flow. The principal sedimentary risk to reef habitats is predicted to come from settling sand particles when dredge tracks approach within tens of metres of a reef. The cumulative effect of dredging at the relatively low intensities recorded in this region is not expected to have a significant long-term impact on suspended silt concentrations and settlement in this highly dispersive environment.

Modeling of Settling and Deposition of Suspended Organic Matter Considering Biodegradation

To design a model for the settling of Suspended Organic Matter (SOM), the decomposition rate of organic matter, and variations in the particle size and the density of SOM were studied in the course of decomposition. The decomposition rate of SOM is estimated using the analyzed results of SOM properties (i.e. particle density, particle size, IL, and resolvability). The Multi-G Model, which formulates the decomposition of organic matter derived from phytoplankton, was chosen to calculate the decomposition rate of SOM. The parameters of each fraction in the Multi-G Model were estimated on the basis of the correlation between the deposition age and the decomposition rate of SOM. In addition, it has become possible to determine the mean decomposition rate of organic matter using the C/N ratio. Temporal variations of the particle size and the particle density of SOM during decomposition were estimated from the C/N ratio, and approximate expressions for the variations were proposed. Based on these results, it is possible to calculate the settling velocity of SOM in considering the variation of its properties due to decomposition

Shall we upgrade one-dimensional secondary settler models used in WWTP simulators? – An assessment of model structure uncertainty and its propagation

In WWTP models, the accurate assessment of solids inventory in bioreactors equipped with solid-liquid separators, mostly described using one-dimensional (1-D) secondary settling tank (SST) models, is the most fundamental requirement of any calibration procedure. Scientific knowledge on characterising particulate organics in wastewater and on bacteria growth is well-established, whereas 1-D SST models and their impact on biomass concentration predictions are still poorly understood. A rigorous assessment of two 1-DSST models is thus presented: one based on hyperbolic (the widely used Takács-model) and one based on parabolic (the more recently presented Plósz-model) partial differential equations. The former model, using numerical approximation to yield realistic behaviour, is currently the most widely used by wastewater treatment process modellers. The latter is a convection-dispersion model that is solved in a numerically sound way. First, the explicit dispersion in the convection-dispersion model and the numerical dispersion for both SST models are calculated. Second, simulation results of effluent suspended solids concentration (XTSS,Eff), sludge recirculation stream (XTSS,RAS) and sludge blanket height (SBH) are used to demonstrate the distinct behaviour of the models. A thorough scenario analysis is carried out using SST feed flow rate, solids concentration, and overflow rate as degrees of freedom, spanning a broad loading spectrum. A comparison between the measurements and the simulation results demonstrates a considerably improved 1-D model realism using the convection-dispersion model in terms of SBH, XTSS,RAS and XTSS,Eff. Third, to assess the propagation of uncertainty derived from settler model structure to the biokinetic model, the impact of the SST model as sub-model in a plant-wide model on the general model performance is evaluated. A long-term simulation of a bulking event is conducted that spans temperature evolution throughout a summer/winter sequence. The model prediction in terms of nitrogen removal, solids inventory in the bioreactors and solids retention time as a function of the solids settling behaviour is investigated. It is found that the settler behaviour, simulated by the hyperbolic model, can introduce significant errors into the approximation of the solids retention time and thus solids inventory of the system. We demonstrate that these impacts can potentially cause deterioration of the predictive power of the biokinetic model, evidenced by an evaluation of the system's nitrogen removal efficiency. The convection-dispersion model exhibits superior behaviour, and the use of this type of model thus is highly recommended, especially bearing in mind future challenges, e.g., the explicit representation of uncertainty in WWTP models.

Modeling of Dispersion and Settling of Micron-Sized Dust Particles in Pressure Regulator Used in Oxygen-Enriched Environment

Abstract It is essential to maintain cleanliness in an oxygen system as many accidents occurring in oxygen-enriched environment indicate contamination as a major source of ignition. Computational fluid dynamics (CFD) analyses show that valve seats, T-joints, elbows and spots with complex geometries are possible locations for accumulation of the dust particles. It is important to understand the mechanisms of contaminant accumulation, the initiation of fire and its propagation in valves and regulators in order to suggest ways for their prevention. As contaminant dust particles flow with oxygen through valves and piping systems, the solid dust particles impact or rub with the pipe wall or with each other. This results in the building up of static electric charges in them. These charged dust particles may eventually accumulate and lead to electrostatic discharge, thus creating a potential cause of initiation of fire. The aim of the present work is to develop a mathematical model based on CFD, of the dispersion and settling of micron-sized charged dust particles in the pressure regulator, commonly used in oxygen application. A two-dimensional numerical model of flow, dust particle dispersion and particle settling, utilizing finite volume approach, has been developed on FLUENT 6.3.26 platform. The results show that there is a possibility of electrical discharge due to the settlement of charged dust particles if a regulator is attempted to be reopened within a short time of it being closed. Since it is practically impossible to make all oxygen systems free of contaminants at all times, it may be worthwhile to analyze ways about how to avoid ignition even in presence of large concentration of dust particles. The present study may give an insight into the possibility of ignition and offer suggestions to avoid it through appropriate system configuration and operation timings.

Application of fractal dimensions to study the structure of flocs formed in lime softening process

The use of fractal dimensions to study the internal structure and settling of flocs formed in lime softening process was investigated. Fractal dimensions of flocs were measured directly on floc images and indirectly from their settling velocity. An optical microscope with a motorized stage was used to measure the fractal dimensions of lime softening flocs directly on their images in 2 and 3D space. The directly determined fractal dimensions of the lime softening flocs were 1.11–1.25 for floc boundary, 1.82–1.99 for cross-sectional area and 2.6–2.99 for floc volume. The fractal dimension determined indirectly from the flocs settling rates was 1.87 that was different from the 3D fractal dimension determined directly on floc images. This discrepancy is due to the following incorrect assumptions used for fractal dimensions determined from floc settling rates: linear relationship between square settling velocity and floc size (Stokes’ Law), Euclidean relationship between floc size and volume, constant fractal dimensions and one primary particle size describing entire population of flocs. Floc settling model incorporating variable floc fractal dimensions as well as variable primary particle size was found to describe the settling velocity of large (>50 μm) lime softening flocs better than Stokes’ Law. Settling velocities of smaller flocs (<50 μm) could still be quite well predicted by Stokes’ Law. The variation of fractal dimensions with lime floc size in this study indicated that two mechanisms are involved in the formation of these flocs: cluster–cluster aggregation for small flocs (<50 μm) and diffusion-limited aggregation for large flocs (>50 μm). Therefore, the relationship between the floc fractal dimension and floc size appears to be determined by floc formation mechanisms.

Sedimentation model and analysis for differential settling of two - particle - size suspensions in the Stokes region

A model is developed for predicting the settling velocity in suspensions of particles of two different sizes based on experimental data for the settling rate of two-size suspensions in various liquids using particles of equal density. In these experiments, the retarding effect of the smaller particles on the settling velocities of the larger ones is taken into account. The model considers Steinour's fundamental equation and assumes a fixed arrangement of particles and constant velocity in a single-size suspension under creeping flow conditions and the flow spaces changing in shape and size with the volumetric concentration of solids. Based on the foregoing, a simplified model equation is proposed for the settling of particles in a suspension as a function of their free fall velocity and of the solids concentration. Different experimental conditions are used to show the effect of solids concentration, liquid density, and viscosity on the settling behaviour of the glass beads with low particle Reynolds numbers and bed porosity ranging from 0.62 to 0.95. The present study involves the lower and upper interface velocities of two-size suspensions of solids in liquids (concentrated and dilute suspensions) covering a wide range in the properties of the liquids and solids, and more specifically in solids concentration (Glass beads and Sand).

Numerical modelling of vertical suspended solids concentrations and irradiance in a turbid shallow system (Vaccares, Se France)

In shallow ecosystems, the short temporal variability of available underwater irradiance is considered a major process controlling submerged macrophytes development. Mechanistic models that estimate photosynthetically available radiation (PAR) in shallow ecosystems at very short time scales are needed for use in predicting submerged macrophyte growth and persistence. We coupled a 2D horizontal circulation model with, first, a 1D vertical numerical model of suspended solid (SS) re-suspension, diffusion and settling, and next, with a model of vertical extinction of irradiance, previously validated at the same site. The study site was the Vaccarès lagoon (France) where a large data set of high frequency bottom irradiance and SS concentration were available. SS and irradiance measurements were conducted at a vertical study station, monitored over a 6 month period (from December 1995 to May 1996) characterized by wide-ranging wind velocities (1.5–18 ms−1). In addition, grain-size analyses conducted over the whole lagoon, allowed adaptation of the 1D numerical model to the silt-sized (7 μm) and clay-sized (0.3 μm) fractions that prevail in the local sediment. First, model results showed that about 60% of the variance in bottom irradiance time series can be explained by our deterministic formulations, thus representing the same level of efficiency than those already obtained by a stochastic model previously developed with the same data set. Second, model results showed that the fit of the model to the field data (SS concentrations and bottom irradiance) depended mainly on storm occurrence and season (winter or spring). Finally, model results suggested that the underwater irradiance regime was controlled by seasonal succession of the horizontal circulation of turbid water in the lagoon, with increased solids concentrations in winter, followed by submerged canopy development and decreased solids concentrations in spring.

Dynamic simulation of secondary treatment processes using trickling filters in a sewage treatment works in Howrah, west Bengal, India

STOAT has been extensively used for the dynamic simulation of a trickling filter based Sewage Treatment Plant. Some alternative schemes have been suggested for the modernization of the existing sewage treatment works. Different schemes have been compared with respect to the removal of Total Suspended Solids (TSS), b-COD, ammonia, nitrates etc. in them. The humus tanks (settling tanks associated with trickling filters) were modeled using the Takacs settler model [I. Takacs, G.G. Patry and D. Nolasco. A dynamic model of the clarification–thickening process. Water Research. 25, 1263–1271, 1991]. The results emphasize that COD based models are preferred over the BOD based ones. A compact bio-film with variable film thickness was proposed for the new schemes suggested. The newly proposed Scheme 3 was found most effective for reducing the mean b-COD level to as low as 2.3 mg/l, while the mean TSS level was brought down to 0.5 mg/l as compared to the mean levels of TSS (42.0 mg/l) and b-COD (48.0 mg/l) in the existing plant.