Settling Curves Research Articles

Sedimentation kinetics of kaolinitic and illitic clay: Impact of cation valence, ionic concentration, and water content

Investigating the sedimentation of clay in saline water was extensively studied because of its wide applications in engineering. This study investigated the impact of cation valence, ionic concentration, and water content on sedimentation behavior of kaolinitic and illitic clays through batch sedimentation tests for the comprehensive understanding of clay particle association and elapsed time for the transition of settling regimes from initial hindered settling to zone settling. The quantitative representation of observed settling curves was achieved using four fitted parameters in batch sedimentation model, which represented the shape of settling curves and the boundary of the initial hindered, zone, and consolidation settling regimes. In addition, the obtained settling velocity from the settling curves provided the fraction of clay particles and the size of clay flocs. The observed settling curves, trends of fitted parameters, and the estimated size of clay flocs shown in this study implied that the type of clay particle association (face-to-face aggregation or edge-to-face flocculation), settling velocity, final interface height, and boundary between initial hindered and zone settling are function of clay mineralogy, water content, cation valence, and ionic concentration.

Analysis of the Sludge Settling Behavior of the Kibendera Waste Stabilization Ponds in Ruiru, Kenya

Waste Stabilization Ponds (WSP) provide both cost-effective and high-performance wastewater treatment benefits in the tropical regions. The 3-month sludge settling behavior in the Kibendera WSP in Ruiru, Kenya, was studied between July and September 2022. The study aimed at determining the sludge volume index (SVI) and hindered settling velocity during the dry weather flow regime. Based on these experimental data, Vesilind Model was used to characterize the settling behavior in the anaerobic and primary facultative ponds. Whereas Standard Methods for the examination of water and wastewater were used to determine the total suspended solids (TSS) concentrations in the ponds, batch column settling tests were used to generate the batch settling curves for each pond. Generally, good settling behavior characterized by SVI values of less than 150 mL/g was observed during the study period. Highest settling velocities of 1.16 m/hr and 1.22 m/hr were recorded in the anaerobic and primary facultative ponds respectively for TSS concentrations of 0.6 g/L and 0.416 g/L. Optimal maximum velocity (<i>V_max</i>) values of 3.42 m/hr and 1.78 m/hr were observed in the anaerobic and primary facultative ponds respectively. The corresponding optimal model parameters (n) describing the measured data accurately were 0.47 L/g and 0.51 L/g respectively. The model parameters so obtained may be useful in characterizing the sedimentation behavior in the WSP during the dry weather flow regimes.

The influence of aeration rate on the sorption of emerging pharmaceuticals in activated sludge

ABSTRACT The sorption of pharmaceuticals on activated sludge during the wastewater treatment process has been widely studied and considered one of the main mechanisms for the removal of these micropollutants from domestic sewage. Understanding the removal mechanism is important to reduce the environmental risk associated with these compounds. To the best of our knowledge, no data are reporting the influence of the aeration rate and, consequently, of the physicochemical properties of the sludge flocs, on the sorption of pharmaceutical compounds. In this context, the influence of the aeration rate (2, 5, and 8 L min−1) on the physical properties of the sludge and the sorption of two emerging pharmaceuticals, 17-alpha-ethynylestradiol (EE2) and diclofenac (DCF), was evaluated. The pharmaceuticals were analyzed by Solid Phase Extraction and Liquid Chromatography, and the sludge by Laser Particle Size Analyzer and Settling Curves. As a result, higher sorption for 17-alpha-ethinylestradiol (78-96%) in comparison to diclofenac (23-43%) was observed, corroborating the greater hydrophobicity of EE2. Higher pharmaceuticals removal rates were observed for the highest aeration (10.02 µgEE2 gSST−1 and 3.99 µgDCF gSST−1) in comparison to the lowest one (7.81 µgEE2 gSST−1 and 2.58 µgDCF gSST−1), what can be attributed to structural and surface changes in flocs. Smaller and more dispersed flocs were observed when aeration was increased (104.4 µm for 8 L min−1 and 63.8 µm for 2 L min−1). The results suggest that the increase in aeration seems to be promising for the removal of pharmaceuticals by sorption in sewage sludge, especially for the hydrophobic ones.

Experimental investigation of sedimentation behaviour using cerium oxalate particles in oxalic‐nitric acid medium

AbstractSettling behaviour of cerium oxalate particles in the solution was investigated by batch settling tests. The settling curves were experimentally generated for different overall solid fractions (OSF), initial dispersion height (ht) and batch cylinder diameter (d). Settling curves were analysed to obtain bulk settling velocity, and it was observed that bulk settling velocity decreases with increase in OSF and remains unchanged with change in cylinder diameter. Bulk settling velocity was found more or less same with increase in initial suspension height as long as the OSF was maintained constant. The particle settling time is delayed at higher OSF due to dense particle population as the movement of one particle is affected by the presence of neighbouring particle. Regression analysis of the settling curve was performed to identify suitable mathematical model. Two parameter based model was found to represent the settling curve better in comparison to three and five parameter based models. Applicability of dispersion number for characterizing the solid–liquid suspension was checked, and it was found that the dispersion number was not unique for the solid‐liquid system where it was influenced by the operating and dimensional parameters.

Preliminary study to determine the maximum settling velocity and model parameter of Cu nanoparticle by settling method

Nanoparticle suspensions also known as nanofluids are often polydisperse and tend to settle with time. Its settling process is very challenging and quite complexs to understand. In this study, Copper (Cu)nanoparticles were dispersed in base fluid (water) and the settling behaviour were examined to obtain the settlement parameter. A series of experiments adopted from sludge settling methods were conducted and the results were analysed. Based on the experiments, the settling curve was successfully established.

A novel approach to model the batch sedimentation and estimate the settling velocity, solid volume fraction, and floc size of kaolinite in concentrated solutions

The settling curve is commonly used for studying the sedimentation process of a suspension. This paper introduces a mathematical equation (S-model) to simulate the settling curve of a suspension and offers new approaches to calculate the settling velocity, solid volume fraction, and flocs size of suspended particles in dilute range during sedimentation. The proposed S-model was evaluated by both experimental data and literature test data which is shown to accurately represent different types of settling behaviors and provide closer simulation than the previous published models. The proposed S-model also enhanced the representation of the hindered settling and consolidation portions of a settling curve through the introduction of two model parameters that controlling the deflection points of the sedimentation and consolidation. In addition, Computational Fluid Dynamics (CFD) simulation was conducted which indicated that the proposed S-model is a robust tool in modeling and characterizing the sedimentation behaviors of batch sedimentation. The findings from this research may provide new approach in modeling the sedimentation behavior of clay suspensions and bring broad impacts to engineering applications, such as biological secondary treatment process, mineral processing, water purification, sedimentation, soil modification, and ground improvement.

Enhanced settling in activated sludge: design and operation considerations.

Settling of activated sludge particles has long been the key to successfully achieving secondary treatment. While soluble products can be converted to particulate components via microbial reactions in the activated sludge process, it is the subsequent removal of these particulate components that is the key to achieving ultimate water quality criteria. An understanding of the operating parameters for selecting good settling activated sludge particles was first documented in the 1970s and 1980s. An understanding of the growth pressures that can be imposed on filamentous organisms, and the impacts of selector zones in general, allowed the design and operation of activated sludge processes to routinely achieve good sludge settleability. More recently, research has identified what could be the next evolution in flocculant growth, with the growing interest in aerobic granular sludge. Aerobic granular sludge is purported to provide superior settling properties, and many of the growth pressures identified for aerobic granular sludge are also present in activated sludge systems. These enhanced settling sludge systems are gaining significant interest, but the factors leading to enhanced sludge settleability could be present in historical and existing systems. Three facilities were evaluated that exhibited enhanced settleability (i.e. sludge volume indices of less than 70 mL/g the majority of the time) to determine how these enhanced settling sludges compare to typical settling curves from the literature. The enhanced settling sludge facilities exhibit key differences related to surface overflow rate, return activated sludge (RAS) pumping requirements, and sensitivity to solids concentration that are critical for developing effective settling designs for enhanced settling sludge facilities. As more facilities aim to achieve enhanced settling sludge for intensification of infrastructure, it will be important to carefully consider historic settling curves and to develop site-specific settling criteria when possible.

Changes in flocculation patterns of cohesive sediments after an iron ore mining dam failure

In November 2015, a tailing dam failure dumped approximately 32 million m3 of very fine iron-rich tailing material into the Doce River watershed, Southeast Brazil. A highly concentrated tailing slurry travelled 660 km down to the river mouth, reaching the Atlantic Ocean. The changes in flocculation patterns of cohesive sediments after dam failure and the deposition of tailing material are investigated. A series of experiments were performed using sediments collected from inside the iron ore mining dam, from a side bank inside the Doce River (after failure), and from the inner continental shelf before and after the tailing deposition. The experiments were performed in settling columns with an optical backscatter sensor and a high resolution digital camera, using only the samples mud fraction (<63 μm). Comparing settling curves, settling velocities and floc sizes showed that the flocculation and settling behavior of cohesive sediments has changed to a new sort of particles of a more homogeneous size and delayed settlement. Sediment from inside the mining dam was almost entirely composed of heavy minerals with fast settling velocity and very small size, but a fraction of it was very similar to the sediment from the side bank of the river: small size with very slow settling velocities and delayed deposition. Sediments from the continental shelf after the tailing deposition differed from sediments before the failure in the presence of heavy minerals, delayed settlement and a narrower range of floc size. We conclude that the tailing deposition increased the residence time and dispersion of mud fraction sediments in the marine water column due to deposition of a finer, porous and/or metallic coating material.

Influence of shape and surface charge on the sedimentation of spheroidal, cubic and rectangular cuboid particles

This study investigated the complex settling behaviour of colloidal particles with varied surface charge and shape factors, of specific relevance to nuclear waste processing. Caesium phosphomolybdate (CPM), zirconium molybdate (ZM) and zirconium citromolybdate (ZMCA) were firstly synthesised, producing spheroidal, cubic and rectangular cuboidal morphologies respectively, and compared to agglomerated titania. While zeta-potential measurements indicated all simulant particles attained low isoelectric points, surface group leaching rendered suspensions very acidic, with CPM around its IEP, and ZM/ZMCA stable and positively charged. In sedimentation tests at various concentrations in water and 2M HNO3, CPM and titania were found to settle with extremely high hindered settling exponents, consistent with aggregated structures. Exponents for ZM and ZMCA in water were both also well above values for spherical particles; however, this was assumed to be due to heightened drag effects from relative shape factors, rather than aggregation. ZMCA in particular showed a very high exponent of ~11.4, due to the propensity for the rod-like particles to settle in a flat conformation. For ZM in acid, double layer compression from the high electrolyte aggregated the dispersions, correlating to a significant increase in the settling exponent. An extended Stokes relationship was additionally used to understand theoretical limits of shape and aggregation on particle size prediction from hindered settling curves. Importantly, calculated sizes were consistent for the stable ZM and ZMCA in water, despite their non-sphericity and enhanced drag. The relationship failed however with agglomerated suspensions, highlighting its application as a general stability test for sedimenting dispersions.

Batch settling curve registration via image data modeling

To this day, obtaining reliable characterization of sludge settling properties remains a challenging and time-consuming task. Without such assessments however, optimal design and operation of secondary settling tanks is challenging and conservative approaches will remain necessary. With this study, we show that automated sludge blanket height registration and zone settling velocity estimation is possible thanks to analysis of images taken during batch settling experiments. The experimental setup is particularly interesting for practical applications as it consists of off-the-shelf components only, no moving parts are required, and the software is released publicly. Furthermore, the proposed multivariate shape constrained spline model for image analysis appears to be a promising method for reliable sludge blanket height profile registration.

Determination of the Settling Rate of Aggregates Using the Ultrasound Method during Paraffinic Froth Treatment

This paper presents results of a study on the settling rate of aggregates formed during paraffinic treatment of bitumen froth. Experiments were performed at temperatures ranging from 30 to 90 °C and various solvent/bitumen ratios, using n-pentane, isopentane, and n-hexane as paraffinic solvents. Characteristic settling curves of diluted bitumen froth were obtained by two independent methods: visual observation and an ultrasound technique. The ultrasound study showed that the hindered settling zone interface can be accurately tracked in place using an ultrasound velocity profiler. Problems associated with determining the settling rate by fitting a linear function to the initial part of the settling curve of the upper interface at higher temperatures were avoided by applying the so-called lower interface method. Settling rates determined using the upper and lower interface methods showed good agreement, confirming interchangeability of the methods.

A new approach for obtaining settling velocity in a thickener using statistical regression: A case study

In this research work, the parameters affecting the settling velocity within the thickeners were studied by introducing an equivalent shape factor. Several thickener feed samples of different densities including copper, lead and zinc, and coal were prepared. The settling tests were performed on the samples, and the corresponding settling curves were plotted. Using the linear regression analysis, the Chein's equation was fitted to the experimental data in order to obtain the equivalent shape factors for the different minerals. Moreover, the relations between the equivalent shape factors and the settling parameters were investigated. The R-squared values for the fits proved the capability of the Chein's equation to fit well on the experimental data (0.96R 2 0.99 for the copper sample, 0.96R 2 0.98 for the lead and zinc sample, and 0.93R 2 0.97 for the coal sample). The results obtained showed that the equivalent shape factor generally decreased with increase in the initial solid concentration. The equivalent shape factor for the coal sample in the samples with concentrations higher than 25% was negative but close to zero. This unusual behavior was explained by discussing the effects of the flocculants on the formation of the network structure in these samples. Determination of the equivalent shape factors would be an invaluable achievement in mineral processing. The researchers can simply put these factors into the Chein's equation to calculate the settling velocity of the minerals instead of performing expensive and time-consuming experiments.

Practical identifiability and uncertainty analysis of the one-dimensional hindered-compression continuous settling model

The practical application of the one-dimension hindered-compression settling models remains a challenge, since the model calibration strongly depends on experimental observations with limited information. In this study, the identifiability of parameter subsets of the hindered-compression models is evaluated for various experimental layouts. Global sensitivity analysis is used to preliminarily select the influential parameters which can be reasonably estimated, while the identifiability analysis of parameter subsets is conducted based on the local sensitivity functions and collinearity measures. The batch settling curve observations are informative for calibrating hindered parameters, and to determine the compression parameters, the concentration profile observations may need to be collected. For different experimental layouts, at least three parameters are identifiable, and the number of identifiable parameters can potentially increase to five, if both batch settling curve and concentration observations are available. The parameter subset identifiability is sensitive to the choice of initial parameter values, and determining the initial values of hindered parameters and gel concentration by measuring the hindered settling velocities and the top concentration of the static sediment respectively allows efficient reduction of the sensitivity. Parameter subset estimates are sensitive to the values of fixed parameters, and reliable estimation of identifiable parameter subsets is possible to significantly decrease model prediction uncertainties.

A settling curve modeling method for quantitative description of the dispersion stability of carbon nanotubes in aquatic environments.

Understanding the aggregation and deposition behavior of carbon nanotubes (CNTs) is of great significance in terms of their fate and transport in the environment. Attachment efficiency is a widely used index for well-dispersed CNT solutions. However, in natural waters, CNTs are usually heterogeneous in particle size. The attachment efficiency method is not applicable to such systems. Describing the dispersion stability of CNTs in natural aquatic systems is still a challenge. In this work, a settling curve modeling (SCM) method was developed for the description of the aggregation and deposition behavior of CNTs in aqueous solutions. The effects of water chemistry (natural organic matter, pH, and ionic strength) on the aggregation and deposition behavior of pristine and surface-functionalized multi-walled carbon nanotubes (MWCNTs) were systematically studied to evaluate the reliability of the SCM method. The results showed that, as compared to particle size and optical density, the centrifugal sedimentation rate constant (ks) from the settling curve profile is a practical, useful and reliable index for the description of heterogeneous CNT suspensions. The SCM method was successfully applied to MWCNT in three natural waters. The constituents in water, especially organic matter, determine the dispersion stability of MWCNTs in natural water bodies.

Particle settling characteristics of alcohol precipitation mixture of Paeoniae Radix rubra extract and models establishment of settling velocity

To understand the particle settling characteristics of alcohol precipitation mixture of Paeoniae Radix rubra extract and establish models of the sedimentation rate. Focusing on the particle settling characteristics such as particle settling curve, particle settling velocity (PSV), particle volume index (PVI), the particle settling process of alcohol precipitation was investigated. The effect of three key process factors on the settling process was discussed and mathematical models for describing the particle settling velocity were developed. Controlling of higher final alcohol concentration, higher density of Paeoniae Radix rubra extract, or lower initial alcohol concentration, was conducive to settlement of alcohol precipitation particles. In the constant speed phase, an empirical calculation formula of v(0) was established, with both the variables PSV and PVI (v(0)=-0.236PSV+0.022PVI+7.521). Another developed model was applied to predict the settling velocity in decelerated phase and the simulation was very good [v=k(1-n(1)X)(4)exp(-n(2)X)/X]. The results of this work will contribute to a better control and optimization of alcohol precipitation process, and help to implementation of accuracy control in the manufacture of botanical medicines.

Towards Improved 1-D Settler Modelling: Calibration of the Bürger Model and Case Study

Recently, Burger et al. (2011) developed a new 1-D SST model which allows for more realistic predictions of the sludge settling behaviour than traditional 1-D models used to date. However, the addition of a compression function in this new 1-D model complicates the model calibration. This study aims to report advances in the calibration of this novel 1-D model. Data of the evolution of the sludge blanket height during batch settling experiments were collected at different initial solids concentrations. Based on the linear slopes of the batch settling curves the hindered settling velocity functions by Vesilind (1968) and Takacs et al. (1991) were calibrated. Although both settling velocity functions gave a good fit to the experimental data, very large confidence intervals were found for the parameters of the settling velocity by Takacs. Global sensitivity analysis showed that it is not possible to find a unique set of parameter values for the settling function by Takacs based on experimental data of the hindered settling velocity. Subsequently, the calibrated Vesilind settling velocity was implemented in the 1-D model by Burger et al. (2011) and the parameters of the additional compression function were calibrated by fitting the model by Burger et al. (2011) to the batch settling curves. Simulation results showed that while the 1-D model by Takacs et al. (1991) underpredicted the experimental data of sludge blanket heights, the model by Burger et al. (2011) was able to predict the experimental data far more accurately. However, a global sensitivity analysis showed that no unique optimum for the combined set of hindered and compression parameters could be found. (Less)

A new approach to define batch settling curves for analyzing the sedimentation characteristics

The results of batch settling tests (BST) are used to investigate settling behavior of solids suspension, which contribute to sizing thickeners. Conventional methods in analyzing BST on the basis of visual and graphical procedures lead to sub-optimally sized and selected thickeners. A computational approach based on quantitative analysis of BST can be beneficial. About 300 settling experiments were performed by varying conditions, including solids concentration, type and dosage of chemical aids. Solid samples were collected from iron, copper, coal, lead and zinc tailings and feed streams. Settling curves based on experimental data considering extreme limits were generated and analyzed. Therefore, a mathematical model, h(t), is introduced to define batch settling curves. Furthermore, it is shown that, on settling velocity curves a maximum value is likely to occur (except in extreme conditions such as very high or very low solids concentration suspensions or extremely high dosage of flocculant). In addition, to compare batch settling curves quantitatively, an index, Ii, based on parameters which can be obtained from the model h(t), is developed. The proposed model and index can simply be utilized in a computerized approach of settling curves analysis.

Numerical Study on the Effect of Twin-Tube Shield Excavation on Adjacent Pipelines

Based on Dalian subway line 2 from Chun-guang street station to Xiang-gong street station,the three dimensional finite element model was established using FLAC3D software , the shield excavated surface against the pressure, the stress release, the shield tail escape and grouting. The numerical result indicated that the pipeline displacement increases gradually with the advance of the tunnel excavation. When one-sided tunnel excavation is carried out, the largest displacement is located at the tunnel axis, the settling curve basically conforms to the normal distribution curve with the unimodal characteristic. The excavation of right-side tunnel is disadvantageous to the left-side tunnel. The analysis indicated that the pipeline is in a secure state. The work in this paper provided theoretical basis and the practical guidance to this project.

Enhancement of Sludge Settling with Chemical Additives

Solids in aeration tank effluents contribute to the total organic content and suspended solids concentration, and violates disposal and reuse standards. Therefore, improving the settling properties of sludge helps produce an effluent with a quality that conforms to effluent standards, assuming all other conditions for good settling in the sedimentation tank are favorable. In this paper, seven chemicals that are all cationic polyelectrolytes were tested individually as additives to improve the settling properties of aeration tank effluent from a full-scale wastewater treatment plant. Pilot settling columns designed and constructed at a local wastewater treatment plant were used in this project. Batch settling tests were performed on fresh activated sludge samples that were collected before the secondary sedimentation tank and mixed with different concentrations of the additives without cumulative additions. Each additive was accessed based on its particular sludge settling properties, that is, the shape of the settling curves, the values of the hindered zone settling rate or zone settling velocity, and the stirred sludge volume index. Each additive result was compared with the corresponding properties of a plain sludge sample collected on the same day and used as a control. Substantial and dissimilar improvements were obtained and are presented in the form of graphs and values using 92 settling runs. Shorter lag-times, quicker descending speed of sludge-water interface, higher descending distance, and lower stirred sludge volume indices values were attained with the use of these additives.

Unscented Kalman filter approach to tracking a moving interfacial boundary in sedimentation processes using three-dimensional electrical impedance tomography

The monitoring of solid-fluid suspensions under the influence of gravity is widely used in industrial processes. By considering sedimentation layers with different electrical properties, non-invasive methods such as electrical impedance tomography (EIT) can be used to estimate the settling curves and velocities. In recent EIT studies, the problem of estimating the locations of phase interfaces and phase conductivities has been treated as a nonlinear state estimation problem and the extended Kalman filter (EKF) has been successfully applied. However, the EKF is based on a Gaussian assumption and requires a linearized measurement model. The linearization (or derivation of the Jacobian) is possible when there are no discontinuities in the system. Furthermore, having a complex phase interface representation makes derivation of the Jacobian a tedious task. Therefore, in this paper, we explore the unscented Kalman filter (UKF) as an alternative approach for estimating phase interfaces and conductivities in sedimentation processes. The UKF uses a nonlinear measurement model and is therefore more accurate. In order to justify the proposed approach, extensive numerical experiments have been performed and a comparative analysis with the EKF is provided.