The effect of variable yield strength and variable fractal dimension on flocculation of cohesive sediment

A new formulation for floc yield strength of cohesive sediment is theoretically derived and incorporated into a flocculation model based on variable fractal dimension. The new flocculation model is validated with existing data on the temporal evolution of floc size measured in the laboratory. Comparing with existing flocculation models using a constant yield strength, it is found that new flocculation model based on variable yield strength and variable fractal dimension is superior in predicting the temporal evolution of floc size. It is also demonstrated that the present model results are very similar to that using an empirical formulation of variable yield strength suggested by Sonntag and Russel (1987. Structure and breakup of floccs subjected to fluid stressses. II. Theory. J. Colloid Interface Sci. 115(2), 378-389) when the empirical coefficient is specified according to our theoretical value. Hence, it is concluded that the new variable yield strength formulation derived in this study and the variable fractal dimension are effective in improving the prediction of flocculation process.     

A two-class population balance equation yielding bimodal flocculation of marine or estuarine sediments

Bimodal flocculation of marine and estuarine sediments describes the aggregation and breakage process in which dense microflocs and floppy macroflocs change their relative mass fraction and develop a bimodal floc size distribution. To simulate bimodal flocculation of such sediments, a Two-Class Population Balance Equation (TCPBE), which includes both size-fixed microflocs and size-varying macroflocs, was developed. The new TCPBE was tested by a model-data fitting analysis with experimental data from 1-D column tests, in comparison with the simple Single-Class PBE (SCPBE) and the elaborate Multi-Class PBE (MCPBE). Results showed that the TCPBE was the simplest model that is capable of simulating the major aspects of the bimodal flocculation of marine and estuarine sediments. Therefore, the TCPBE can be implemented in a large-scale multi-dimensional flocculation model with least computational cost and used as a prototypic model for researchers to investigate complicated cohesive sediment transport in marine and estuarine environments. Incorporating additional biological and physicochemical aspects into the TCPBE flocculation process is straight-forward also.     

逆流共聚气浮水处理工艺研究

逆流共聚气浮水处理工艺相对于传统的气浮,沉淀处理工艺有很大的优越性,一方面絮凝过程在逆流共聚气浮反应柱中进行,溶气回流水释放的微气泡参与到悬浮颗粒物的絮凝反应中而有助于形成体积重量小而结构牢固的絮体;另一方面反应柱中的微絮体在气泡的生成过程中充当了“核”的作用,有助于溶气水中气泡的迅速并提高气泡与絮体的碰撞粘附效率,同时反应柱中可形成稳定的气泡－絮体共聚悬浮层,有利于拦截随水流下行的絮体与上升的微气泡,提高了处理效率。     

Electrocoagulation versus chemical coagulation: coagulation/flocculation mechanisms and resulting floc characteristics

Electrocoagulation (EC) and chemical coagulation (CC) are employed in water treatment for particle removal. Although both are used for similar purposes, they differ in their dosing method - in EC the coagulant is added by electrolytic oxidation of an appropriate anode material, while in CC dissolution of a chemical coagulant is used. These different methods in fact induce different chemical environments, which should impact coagulation/flocculation mechanisms and subsequent floc formation. Hence, the process implications when choosing which to apply should be significant. This study elucidates differences in coagulation/flocculation mechanisms in EC versus CC and their subsequent effect on floc growth kinetics and structural evolution. A buffered kaolin suspension served as a representative solution that underwent EC and CC by applying aluminum via additive dosing regime in batch mode. In EC an aluminum anode generated the active species while in CC, commercial alum was used. Aluminum equivalent doses were applied, at initial pH values of 5, 6.5 and 8, while samples were taken over pre-determined time intervals, and analyzed for pH, particle size distribution, ζ potential, and structural properties. EC generated fragile flocs, compared to CC, over a wider pH range, at a substantially higher growth rate, that were prone to restructuring and compaction. The results suggest that the flocculation mechanism governing EC in sweep floc conditions is of Diffusion Limited Cluster Aggregation (DCLA) nature, versus a Reaction Limited Cluster Aggregation (RLCA) type in CC. The implications of these differences are discussed.     

A two-state microbial growth kinetics model

This study considers the possibility that dispersed bacteria and bacteria aggregated in floc possess different physiological states and that aggregation of dispersed cells to form floc particles is a reversible rate process. A suitable kinetic model of flocculation is proposed, in which the floc phase and dispersed phase bacteria exhibit different growth rates. Transient responses to changes in dilution rate and inlet concentration are investigated. Results are compared with experimental observations. Variation in observed yield factor and hysteresis of specific growth rate are observed.     

Low technology water purification by bentonite clay flocculation as performed in Sudanese villages. Virological examinations

Viruses were removed from various types of water by flocculation with natural bentonite clays from the banks of the Nile as well as with pure bentonite. The flocculation technique employed corresponds to that used to clarify Nile water in Sudanese villages. If proper flocculation occurred 3 to 4 log₁₀ units of virus could be removed. Some of the conditions for floc formation were examined.     

Lysis as a factor in sludge flocculation

A tentative relationship was established between the levels of extractable exopolymers and flocculation in activated sludge growing on a carbon limited system.

Deflocculation occurred in the logarithmic growth phase and flocculation predominated in the endogenous phase. There appeared to be a relationship between lysis of sludge and flocculation, as a decrease in cell dry weight accompanied by the release of polymers, e.g. nucleic acids, resulted in an increase in flocculation.

This relationship was further investigated using selected components released on lysis, i.e. DNA, which was shown to promote significant levels of flocculation when added to various microbial systems—a significant increase in the rate of flocculation in the case of addition to sludge and in increase in the rate and degree of flocculation when added to individual bacterial cultures. Thus DNA can act as a floc agent and can also survive breakdown or depolymerization. Thus polymers released on lysis of cells can enhance flocculation.     

Shear-induced flocculation: The evolution of floc structure and the shape of the size distribution at steady state

The flocculation of polystyrene particles in a stirred tank was studied at various shear rates (63–129 s⁻¹) and aluminum sulfate, Al₂(SO₄)₃ 16H₂O, flocculant concentrations. The competition between coagulation and fragmentation during shear-induced flocculation determined the equilibrium or steady state particle (floc) structure and size distribution. The evolution of the floc structure with time was monitored by image analysis of digitized floc images. The average floc structure became less open or irregular as the floc size distribution attained steady state as a result of shear-induced breakage/restructuring. At high alum (flocculant) concentrations, the steady state floc size distribution appeared to be self-preserving with respect to shear rate. In contrast, at lower flocculant concentrations, the steady state floc size distribution narrowed with increasing shear rate as the large tail of the distribution was pushed to smaller particle sizes by shear-induced fragmentation.     

A study of titanium sulfate flocculation for water treatment

There are limited studies available on titanium salt flocculation. In this research, coagulation experiments of titanium sulfate were conducted using both distilled water and kaolin clay suspension. Results showed that titanium sulfate flocculation was most effective in the pH range 4-6, and negligible concentrations of titanium were found in the well-flocculated water. The floc isoelectric point (IEP) was found to be near pH 5. Measurements showed that the titanium flocs possessed greater density, diameter and settling velocity than the aluminum flocs. The titanium flocs were composed of TiO(OH)₂, which would change from the amorphous phase into anatase titanium dioxide under elevated temperatures. Floc images showed the structural similarity of titanium and aluminum flocs. Laboratory results and a pilot experiment showed that titanium sulfate could be an alternative coagulant for water and wastewater treatment.     

Relationship between floc composition and flocculation and settling properties studied at a full scale activated sludge plant

The variation in activated sludge floc composition, flocculation and settling properties was studied at a full scale plant over a 2-year period. A comprehensive set of process parameters was analysed and related to the floc properties to increase the understanding of the factors affecting floc formation. The composition of the activated sludge showed a seasonal change with higher concentrations of extractable extracellular polymeric substances (EPS) during the winter months. The protein content of the total sludge and EPS increased significantly during the winter. This coincided with higher effluent suspended solids concentrations and increased shear sensitivity of the sludge flocs. Only poor correlations between EPS contents and stirred sludge volume index (SSVI) could be observed. High iron concentrations in the sludge due to dosage of iron salt to precipitate phosphorus were found to have a negative impact on the settling and compaction properties of the sludge, whereas it had a positive impact on floc stability. Higher organic loading due to by-passed primary settlers leads to improved settling and compaction properties.     

Optimising natural organic matter removal from low turbidity waters by controlled pH adjustment of aluminium coagulation

Effective removal of soluble natural organic matter (NOM) from low-turbidity waters can be achieved by aluminium coagulation provided due attention is paid to micro-floc formation (coagulation) and macro-floc development (flocculation). This work provides guidelines and an explanation of how pH should be controlled to maximise soluble NOM removal while at the same time making full use of pH-influencing chemicals (acid, alum, and lime). For low-turbidity waters, essential floc nucleating sites can be provided by the lime that is used for pH and alkalinity adjustment, provided that the lime is added in sufficient quantity and at a point where it retains some of its particulate nature, i.e. immediately after coagulant addition. Adjusting the pH downwards to between 4 and 5 prior to coagulant addition allows the formation of soluble NOM-aluminium complexes that link to each other, thereby forming large insoluble bridged complexes (micro-flocs), that also act as nuclei for macro-floc development (flocculation).     

吸附架桥机理主导下APAM的多级絮凝效能

对吸附架桥机理主导下阴离子聚丙烯酰胺(APAM)的絮凝过程进行了研究,通过改变絮凝剂投加工况,对比分析常规絮凝与多级絮凝在污染物去除效果、絮体性能、絮体生长动力学与污泥调理能耗等方面的差异。结果表明,相同投药量下,两级絮凝的出水浊度低于三级絮凝和常规絮凝,两级絮凝在最少的APAM投加量(2 mg/L)下达到最低的出水浊度(19.53 NTU);与常规絮凝相比,两级絮凝的絮体成长速率、平均粒径和沉降速率分别增加12.67%、30μm、36.74%。两级絮凝在投加间隔为240 s、投配比为1∶1条件下絮凝效能最优,出水浊度为15.34 NTU,絮体沉降速率为1.1 NTU/s,絮体密度达到1.123 4 g/cm³。絮体破碎再絮凝过程中,两级絮凝与常规絮凝破碎后均能恢复至破碎前水平,但破碎后均出现不可逆的絮体结构破损,粒径在0~100μm的絮体颗粒增多,粒径>400μm的絮体减少,破碎后两级絮凝的絮体强度因子(68.15%)高于常规絮凝(41.63%),两级絮凝的絮体强度和抗破碎剪切能力更高。在剩余污泥调理方面,两级絮凝产生的污泥只需要投加40mg/L的APAM就可以达到最低的滤饼含水率(75.5%)。因此,两级絮凝可以显著提升除浊效能与絮体性能,是强化絮凝的发展方向。     

Application of fractal flocculation and vertical transport model to aquatic sol-sediment systems

In estuarine and coastal environments, flocculation occurs between particles of different fractal dimensions and of different densities. Questions remain concerning the level of detail required to model particle flocculation and settling in these heterogeneous systems. This paper compares the goodness of fit between two flocculation models, using measured time series particle size distribution data collected from clay, colloidal silica, emulsified crude oil, clay-crude oil, and silica-crude oil systems. The coalesced sphere (CS) flocculation model includes the effects of heterogeneous particle size and density; the modified coalesced fractal sphere (mCFS) model adds effects due to heterogeneous fractal dimension. Goodness of fit was quantified using values of a minimized objective function, the mean of the sum of the square of the relative residuals (MSSRR). For nearly all tested experimental conditions, MSSRR values varied less than 5% between the CS and mCFS flocculation models. Additionally, collision efficiency values for single-particle-type (alpha(HOMOO)) and dual-particle-type (alpha(HETT)) systems were obtained through parameter regression using the CS and mCFS models. Using the mCFS model, estimated fractal dimension (D) values obtained for clay and clay-oil systems were between 2.6 and 3.0, lower than that postulated by the CS model but higher than that estimated experimentally by the particle concentration technique. The Stokes settling velocity of a clay aggregate of a given mass is reduced with decreased fractal dimension. This results in clay-oil flocculation occurring faster than floc sedimentation in the studied hydrodynamic range. Thus, the mCFS model provides insights to the fate of spilled oil in inland and coastal waters.     

Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge

Laboratory experiments on the activated sludge (AS) process were carried out to investigate the influence of microbial extracellular polymeric substances (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), on biomass flocculation, sludge settlement and dewaterability. The heat EPS extraction method was modified to include a mild step and a harsh step for extracting the LB-EPS and TB-EPS, respectively, from the sludge suspension. Six lab-scale AS reactors were used to grow AS with different carbon sources of glucose and sodium acetate, and different sludge retention times (SRTs) of 5, 10 and 20 days. The variation in the bioreactor condition produced sludge with different abundances of EPS and different flocculation and separation characteristics. The sludge that was fed on glucose had more EPS than the sludge that was fed on acetate. For any of the feeding substrates, the sludge had a nearly consistent TB-EPS value regardless of the SRT, and an LB-EPS content that decreased with the SRT. The acetate-fed sludge performed better than the glucose-fed sludge in terms of bioflocculation, sludge sedimentation and compression, and sludge dewaterability. The sludge flocculation and separation improved considerably as the SRT lengthened. The results demonstrate that the LB-EPS had a negative effect on bioflocculation and sludge-water separation. The parameters for the performance of sludge-water separation were much more closely correlated with the amount of LB-EPS than with the amount of TB-EPS. It is argued that although EPS is essential to sludge floc formation, excessive EPS in the form of LB-EPS could weaken cell attachment and the floc structure, resulting in poor bioflocculation, greater cell erosion and retarded sludge-water separation.     

Kinetics of pneumatic flocculation

A mathematical model has been established to describe the kinetics of pneumatic flocculation in terms of the process variables: energy of agitation, volume of turbid water, temperature, water viscosity, amount of synthetic turbidity, flocculating time, amount of air flow, depth of water column and size of orifice. The mathematical model tested for both normal and taper types of flocculation. The experimental data vindicated the model relationship between the concentration ratio and the rate of air flow for the air flow rate above 50 cm³ min⁻¹. Accordingly, the developed mathematical model was then linearised.     

Evaluation of the flocculation performance of carboxymethyl chitosan-graft-polyacrylamide, a novel amphoteric chemically bonded composite flocculant

In the present work, a novel amphoteric chemically bonded composite flocculant (carboxymethyl chitosan-graft-polyacrylamide, denoted as CMC-g-PAM) was successfully prepared and used to flocculate the kaolin suspension. The flocculation performance of CMC-g-PAM in acidic, neutral, and alkaline conditions was systematically evaluated by light scattering in combination with fractal theory, as well as by traditional turbidity and zeta potential measurements. Based on the experimental facts from in situ size and fractal dimension measurements, different flocculation mechanisms play key roles at various pH levels, resulting in substantially varied flocculation kinetic processes under three pH conditions. In acidic condition, patching was the main mechanism involved in the opposite zeta potential between CMC-g-PAM and the kaolin suspension. A flat configuration was favored when the polymeric flocculant was adsorbed onto the particle surface, leading to a slower initial floc growth rate but larger and denser flocs. Bridging was the dominant mechanism in neutral and alkaline conditions. A faster initial rate of bridging resulted in smaller and more open floc structures. A rearrangement process in neutral pH subsequently led to more compact flocs, whereas no restructuration of flocs occurred in alkaline conditions because of the electrostatic repulsion of the same negative charges on the flocculant and particles.     

A novel method for on-line evaluation of floc size in coagulation process

Chemical coagulation is a simple and widely used water treatment process. A jar test based on the residual turbidity in the treated water was used to evaluate the optimal conditions for floc formation. However, the final residual turbidity does not show up variation of turbidity and floc formation during the flocculation process. Hence, a nephelometric turbidimeter method based on on-line monitoring was devised to determine the floc size variance during flocculation. A nephelometric turbidimeter coupled with a data acquisition unit was used to measure turbidity every second at 3 cm below the water surface during the coagulation process. Laboratory results indicated that this new instrument was capable of recording floc agglomeration during slow mixing very accurately. The standard deviation (SD) of the measured turbidity was proportional to the square root of the floc size; a greater SD indicated larger floc sizes. Hence, in addition to monitoring turbidity, the nephelometric turbidity meter is also a valuable tool to study the floc agglomeration process and variations in the resulting floc size. This method is simple and effective; it contributes significantly to the selection of coagulant and optimal flocculation conditions to improve water treatment.     

Fluid shear influences on the performance of hydraulic flocculation systems

Gravity driven hydraulic flocculators that operate in the absence of reliable electric power are better suited to meet the water treatment needs of green communities, resource-poor communities, and developing countries than conventional mechanical flocculators. However, current understanding regarding the proper design and operation of hydraulic flocculation systems is insufficient. Of particular interest is the optimal fluid shear level needed to produce low turbidity water. A hydraulic tube flocculator was used to study how fluid shear levels affect the settling properties of a flocculated alum-kaolin suspension. A Flocculation Residual Turbidity Analyzer (FReTA) was used to quantitatively compare the sedimentation velocity distributions and the post-sedimentation residual turbidities of the flocculated suspensions to see how they were affected by varying fluid shear, G, and hydraulic residence time, θ, while holding collision potential, Gθ, constant. Results show that floc breakup occurred at all velocity gradients evaluated. High floc settling velocities were correlated with low residual turbidities, both of which were optimized at low fluid shear levels and long fluid residence times. This study shows that, for hydraulic flocculation systems under the conditions described in this paper, low turbidity water is produced when fluid shear is kept at a minimum. Use of the product Gθ for design of laminar flow tube flocculators is insufficient if residual turbidity is used as the metric for performance. At any Gθ within the range tested in this study, best performance is obtained when G is small and θ is long.     

Repeated phosphate removal from recirculating aquaculture system using cyanobacterium remediation and chitosan flocculation

Unicellular cyanobacterium Synechocystis sp. was used for phosphate removal in a recirculating aquaculture system. The cell harvesting was performed using chitosan solution in this study. The parameters (i.e. cell density, pH of cell suspension and chitosan concentration) affecting the flocculation efficiency of chitosan were investigated. With the optimal condition, the repeated flocculation for phosphate removal in a photobioreactor was demonstrated. The results show that the flocculation efficiency of chitosan depends on cell density, pH of cell suspension and chitosan concentration. The optimal flocculation process could be accomplished by adjusting the pH to 7.2 before adding 20 mg/L chitosan followed by pH adjustment to 7.5. With single inoculation, the sequential process of phosphate removal using cyanobacterial uptake followed by cell flocculation using chitosan with the optimal condition in the photobioreactor was successfully achieved for 12 cycles with water from a recirculating fish tank.     

Physical aspect of flocculation process—III: Flocculation process in a continuous flow flocculator with a back-mix flow

The performance of the multi-stage complete mixing flow flocculator was investigated using the authors' model in which the flocculation process was divided into two categories. One is the floc growth process which creates settlable flocs. Another is the adsorptive decrease of micro-flocs by the above flocs.The combination of these two processes enable an evaluation of the percentage removal of the raw water suspension in a sedimentation tank and filter with respect to various combinations of detertion time, agitation intensity and the compartment number of the flocculator, physical characteristics of flocs and overflow rate of the sedimentation basin.