Evaluation of floc strength by morphological analysis and PDA online monitoring.

This paper established a method for the evaluation of floc strength by morphological analysis and PDA online monitoring. Theoretically, the binding force of a floc can be expressed as B=k(1)d(2Df/3), where k(1) is a coefficient, d is floc size and D(f) is the fractal dimension of the floc. In order to calculate the binding force under a given flocculation condition, a jar test was conducted and d and D(f) were measured by image analysis. A shear force was exerted on the grown flocs by introducing the flocculated suspension through a transparent tube where the velocity gradient value of the flow could be accurately controlled. As the tube was connected with a particle dispersion analyzer (PDA), the condition of floc breakage was online monitored and the critical condition was identified by analyzing the PDA outputs (FI curves). The binding force coefficient k(1) could thus be determined, and the binding force B which represented the floc strength was evaluated. The validity of this method was proved by a series of experiments using aluminium sulphate as coagulant for the flocculation of humic substances.     

Formation and breakage of flocs using dual polymers.

The effect of shear on the formation and break-up of flocs generated using cationic, anionic and non-ionic polymers, and dual combinations of these, has been investigated using conventional jar test procedure and by continuous optical monitoring. The breakage of flocs was followed at a high stirring speed, corresponding to average shear rate (G) of approximately 520 s(-1). Most of the breakage occurred within a few seconds of increasing the shear rate. After each breakage, the stirring rate was reduced to the original value of 50 rpm (G approximately 23 s(-1)) to allow the flocs to regrow. For cationic-anionic polymers case, the regrowth of flocs was fully reversible and the breakage factors were smallest indicating highest floc strength. In contrast, flocs formed using non-ionic polymer together with anionic or cationic did not produce as strong flocs. It is found that the sequence of polymer addition is not very significant for floc formation and reformation when the cationic-anionic dual system is used.     

Nutrient conditions and reactor configuration influence floc size distribution and settling properties

Floc formation and settleability is critical for effective solid-liquid separation in many wastewater treatment processes. This study aimed to investigate the relationship between particle size distribution and nutrient conditions in different bioreactor configurations. Size distribution profiles of flocs that formed in continuous (B1), continuous with clarifier and return sludge (B2) and SBR (B3) reactors were investigated in parallel under identical nutrient conditions. An eight-fold dilution of the influent COD of a synthetic dairy processing wastewater resulted in a 'feast and famine' regime that triggered significant effects on the biomass and flocculation characteristics. Floc size analysis of reactor MLSS revealed a shift in floc sizes when reactors were fed with the minimum (famine) COD wastewater feed (0.61 g L(-1)). Increasing floc size distributions were detected for all reactors during the minimum COD feed although different size patterns were observed for different reactor configurations. These increases corresponded with variations in aggregation and EPS quantities. The SBR yielded comparatively larger flocs when operated under both COD feeds as indicated by d(0.9) values (90% of particles ≤ d in size). Overall the results indicated that floc formation and floc size are mediated by nutrient concentrations and represents an important step towards improved solid-liquid separation.     

Temperature effects on flocculation, using different coagulants.

Temperature is known to affect flocculation and filter performance. Jar tests have been conducted in the laboratory, using a photometric dispersion analyser (PDA) to assess the effects of temperature on floc formation, breakage and reformation. Alum, ferric sulphate and three polyaluminium chloride (PACI) coagulants have been investigated for temperatures ranging between 6 and 29 degrees C for a suspension of kaolin clay in London tap water. Results confirm that floc formation is slower at lower temperatures for all coagulants. A commercial PACl product, PAX XL 19, produces the largest flocs for all temperatures; and alum the smallest. Increasing the shear rate results in floc breakage in all cases and the flocs never reform to their original size. This effect is most notable for temperatures around 15 degrees C. Breakage, in terms of floc size reduction, is greater for higher temperatures, suggesting a weaker floc. Recovery after increased shear is greater at lower temperatures implying that floc break-up is more reversible for lower temperatures.     

Particles under stress

The complex nature of particulate matter in natural water resources and in waste waters is characterized by the heterogeneous distribution of particle size, shape, density, and shear strength. Among these parameters, floc strength is most important in the last stages of flocculation. Experimental data on floc strength based on different methods are assessed and correlated with shear gradients in different aquatic environments. The analysis of turbulent motion reveals that the energy which affects particle agglomerates is only a small portion of the totally dissipated energy. Among the different flow fields in turbulent motion, converging/diverging flow cause strain forces which prove to be critical with respect to floc rupture. Model calculations of the surface forces on ellipsoidal particles in pure shear and strain flow fields and corresponding experiments confirm the importance of converging flow. A comparison of modeled and measured rupture forces allows to establish relationships between floc size, strain and shear rates and resulting surface forces leading to probable floc break-up. Flocs of appreciable size (200 - 2000 µm) prepared for settling are likely to be ruptured under moderate velocity gradients occurring in flocculation tanks, whereas smaller agglomerates (< 200 µm) may withstand strain forces much higher than found under practical conditions. An example of model application shows the particle stress in the entrance to porous media filters where typically high strain gradients may easily lead to a break-up of flocs larger than 200 µm.     

Flocculation of Fine-Grained Lake Sediments Due to a Uniform Shear Stress

Experiments were performed to investigate the effects of fluid shear on the flocculation of fine-grained lake sediments in fresh water. In these experiments, a Couette viscometer was used to apply a uniform shear stress to a sediment suspension. The sediments were from the Detroit River inlet of Lake Erie. They were prepared such that the initial (unflocculated) size distribution contained approximately 90% of its mass in particles less than 10 μm in diameter with the average diameter being about 3.5 μm. Experiments were performed at shear stresses of 1, 2, and 4 dynes/cm² and sediment concentrations of 50, 100, 400, and 800 mg/L, values which are characteristic of those found in the Great Lakes. Data in the form of floc size distribution as a function of time were obtained. For steady-state conditions, the median diameters of the flocs formed were typically 20 to 100 μm depending on shear stress and sediment concentration. Quantitative results for the decrease in the steady-state floc size with increasing shear stress and with increasing sediment concentration were obtained. The times required for flocculation to occur under different conditions were also determined and were typically on the order of 1 hour.     

The duplicity of floc strength.

The breakage of flocs is dependent upon the strength of the bonds holding the aggregate together. The present work describes the breakage and re-growth behaviour of three different types of floc, these were: 1) coagulant precipitate flocs, 2) turbidity flocs and 3) organic matter flocs. Floc aggregates were exposed to increased levels of shear on a conventional jar tester and the sizes of the flocs were observed dynamically using a laser diffraction instrument. The organic flocs showed most resistance to breakage across the whole range of shears under investigation. The dynamic procedure provided detailed information on particle size distributions (PSD). Large and small scale degradation events could be identified from analysis of the PSD data. All of the flocs under investigation showed little re-growth potential after breakage. The precipitate and organic flocs showed slightly better re-aggregation of the small floc sizes.     

Characterization of micro-flocs of NOM coagulated by PACl, alum and polysilicate-iron in terms of molecular weight and floc size.

Micro-flocs of NOM coagulated by polyaluminium chloride (PACl), alum and polysilicate-iron (PSI) were characterized by flocs size, HPSEC-based molecular weight and the captured content of coagulants-based aluminium and iron. Changes in floc composition with respect to the mass ratios of captured NOM to Al and Fe were examined. Lowering water pH to optimum levels was found to be capable of removing small NOM constituents that are generally difficult to be precipitated at neutral pH levels. For PACl and PSI, the distribution of micro flocs (0.1-5.0 microm) reached steady stage after rapid mixing for 30 seconds, with NOM being found existent within the non-coagulated fraction (d<0.1 microm) and the coagulated fraction with floc sizes above 5.0 microm (d >5.0 microm). For alum, however, the existence of NOM inside intermediate floc fractions of d = 0.1-1.0 microm, 1.0-3.0microm and 3.0-5.0 microm was confirmed.     

絮凝体的分形特性研究

试验现象及电镜照片研究表明,絮凝体表面和内部具有高度不规则性,絮凝体的结构及其形成过程具有分形特征。从絮凝体的密度、孔隙率,絮凝体的粒径分布,絮凝体的强度、沉速,絮凝条件与分数维的关系并结合黄河泥沙试验,概述了絮凝体的分形特性及结构特征,同时对影响絮凝体分形特性的因素进行了分析。     

The effects of starvation on physical characteristics of flocs in SBR for treating saline wastewater.

This study focused on the effects of starvation on physical characteristics of flocs in SBR for treating saline wastewater. Feeding was stopped for 5 and 15 days. A time response of the floc to these starvation periods was monitored as well as the removal efficiencies of pollutants. Correlation between the physical characteristics of flocs and settling of sludge was conducted. As the starvation periods were increased, there was a shift in the floc size distribution from a high proportion of large flocs to the development of small size flocs. The fractal dimension of flocs also decreased, as starvation periods were increased. From the results, the effect of starvation on SBR treating saline wastewater can be ordered as follows: COD Mn removal < floc size and fractal dimension < T-N removal < T-P removal.     

The relationship between the structure of activated sludge flocs and the sorption of hydrophobic pollutants

The relationships between the structure of minimally perturbed activated sludge flocs and the sorption of organic contaminants were studied. Sorption, settling velocity, size distributions, floc structure and EPS composition were all examined. The results show significant removal of selected halogenated hydrocarbons and polycyclic aromatic hydrocarbons by biosorption to activated sludge flocs. However, statistically significant effects on the settling or size of the flocs caused by this sorption were not observed. The addition of chromium (Cr(III)) metal ions to the biomass caused observable changes in the floc structure and decreased ruthenium red binding to the acidic polysaccharides of the floc matrix. At low concentrations (0.6 mg/1), chromium caused an increase in the sorption of organic compounds to flocs, suggesting that changes in the floc structure can be induced, which can have an impact on the sorption of pollutants to the flocs.     

The role of carbohydrate and protein parts of extracellular polymeric substances on the dewaterability of biological sludges.

There are many factors affecting the biological sludge dewaterability such as particle size distribution, floc structure, extracellular polymeric substances (EPS), etc. In this research, the role of the protein and carbohydrate parts of EPS (EPS(carbohydrate), and EPS(protein)) on the dewaterability of biological sludges was investigated. The sludge EPS composition was altered by feeding the sludges of same origin, in different reactors, with synthetic media having carbon to nitrogen (C/N) ratios of 8, 19 and 30 (in terms of COD/NH3-N), respectively. EPS in sludge samples were extracted by a cation exchange resin (CER). The characteristics of EPS were investigated by analytical methods and by using FT-IR spectroscopy. The dewaterability of the sludges was determined in terms of filterability and compactibility. Filterability, as filterability constant (X), and compactibility, as cake solids concentration, of sludges were determined by using the capillary suction time (CST) test and the centrifugation, respectively. The floc structure of sludge samples was also observed microscopically. Filterability and compactibility of the sludge samples were improved considerably with the increasing carbohydrate part and the decreasing protein part of the sludge EPS. EPS(protein) was inversely related to the cake solids concentration, which might be explained by the water holding capacity of EPS(protein). Filterability and compactibility of sludges improved by the increase of the size and strength of the flocs.     

Bacterial composition of activated sludge--importance for floc and sludge properties.

Activated sludge flocs consist of numerous constituents which, together with other factors, are responsible for floc structure and floc properties. These properties largely determine the sludge properties such as flocculation, settling and dewaterability. In this paper we briefly review the present knowledge about the role of bacteria in relation to floc and sludge properties, and we present a new approach to investigate the identity and function of the bacteria in the activated sludge flocs. The approach includes identification of the important bacteria and a characterization of their physiological and functional properties. It is carried out by use of culture-independent molecular biological methods linked with other methods to study the physiology and function, maintaining a single cell resolution. Using this approach it was found that floc-forming properties differed among the various bacterial groups, e.g. that different microcolony-forming bacteria had very different sensitivities to shear and that some of them deflocculated under anaerobic conditions. In our opinion, the approach to combine identity with functional analysis of the dominant bacteria in activated sludge by in situ methods is a very promising way to investigate correlations between presence of specific bacteria, and floc and sludge properties that are of interest.     

Influence of floc structure on membrane permeability in the coagulation-MF process.

The relation between floc structure and membrane permeability was studied in a coagulation-MF hybrid process. The floc structure changed with operating parameters in the coagulation process and was quantified with fractal dimension (dF). The concentration ratio between suspended colloids and injected coagulant had an essential effect on dF of coagulated flocs. Larger flocs with low fractal dimension were produced for ALT (aluminum ion concentration dosed/suspended particle concentration) between 0.4 and 0.8. Flocs maintained stable characteristics at the coagulation period of over 20 minutes. Membrane permeability was improved with coagulated flocs of lower fractal dimension, which tend to have higher porosity and aggregate relatively loosely. These more porous flocs reduce specific resistance of coagulated flocs. The relation between membrane filterability and fractal dimension of flocs was explored in a submerged MF hybrid system as well as in a batch unstirred cell filtration.     

微污染水源水的强化絮凝处理

利用微涡旋控制理论对南方某水厂传统絮凝网格进行了强化絮凝改造。运行结果表明,基于对剪切力与粘性力控制的强化絮凝网格处理微污染水源水具有一定优势:絮凝能耗小,出水矾花密实(分形系数大),矾花粒径大,对应的沉后水浊度降低0.1～0.3 NTU,沉后水、砂滤水藻类和有机物的去除率分别增加2％～5％。     

Monitoring floc formation and breakage.

Monitoring of flocculation processes is of great practical importance and can give very useful information on the action of different coagulants under various conditions. It is shown that a continuous optical monitoring technique provides a convenient means of following the formation, break-up and regrowth of flocs under standard mixing and stirring conditions. Examples of results using this approach are given, with alum and PACl coagulants as well as a cationic polyelectrolyte. PACl products give larger flocs than with alum, but they show about the same relative degree of breakage when the shear rate is increased. With all of these coagulants, floc breakage is not fully reversible; i.e. flocs only re-grow to a limited extent when the shear rate is reduced. This is in marked contrast to the action of a cationic polyelectrolyte, where floc breakage is almost completely reversible. Some possible reasons for these observations are discussed, but, so far, there is no adequate model available.     

Investigating the effect of storm events on the particle size distribution in a combined sewer simulator.

The changes in particle size of sewer sediment particles rapidly eroded from a previously deposited sediment bed are described, using a rotating annular flume as a laboratory scale sewer simulator. This is the first time that particle size distributions of eroded sewer sediments from a previously deposited sediment bed have been monitored in such a controlled experimental environment. Sediments from Loenen, The Netherlands and Dundee, UK were used to form deposits in the base of the annular flume (WL Delft Netherlands) with varying conditions for consolidation in order to investigate the effect of changing consolidation time, temperature and sediment type on the amount and size of particles eroded from a bed under conditions of increasing shear. The median size of the eroded particles did not change significantly with temperature, although the eroded suspended solids concentration was greater for the higher temperature under the same shear stresses, indicating a weaker bed deposit. An increase in consolidation time caused an increase in median size of eroded solids at higher bed shear stresses, and this was accompanied by higher suspended solids concentrations. As the shear stress increased, the solids eroded from the bed developed under a longer consolidation time (56 hours) tended towards a broad unimodal distribution, whilst the size distribution of solids eroded from beds developed under shorter consolidation times (18 or 42 hours) retained a bi- or tri-modal distribution. Using different types of sediment in the flume had a marked effect on the size of particles eroded.     

Enhanced sludge conditioning by enzyme pre-treatment: comparison of laboratory and pilot scale dewatering results.

The effect of enzyme pre-treatment on dewaterability of anaerobically digested sludge was investigated at both laboratory and pilot scale. Our results revealed a significant increase in cake solid content (27% cake solids compared to 18% without enzyme pre-treatment), using an enzyme dose of only 20 mg/L. In order to assess practical application, enzyme pre-treatment was applied at the Wilmington, Delaware (U.S.) wastewater treatment plant, using a pilot-scale centrifuge. However, the efficiency reached in laboratory scale could not be obtained in pilot scale, where the final cake solids content did not exceed 20%. Centrifuge and belt filter press (simulated by Crown Press) dewatering were compared in terms of the process efficiencies in the absence and presence of enzyme pre-treatment. Possible factors that might cause the differences were tested by experimental and statistical comparisons. Results indicated that the higher shear applied in centrifugation is responsible for the lack of improved cake solids. The network strength of sludge determined by rheological measurements revealed that enzymatic treatment weakens the gel structure of the sludge floc through the hydrolysis of extracellular polymeric substances; this allows improved dewatering by filtration processes, but leads to floc deterioration when subjected to high shear during centrifugation.     

长江口不同河段表层细颗粒泥沙絮凝特性

利用长江口的现场观测资料分析河口不同区段表层细颗粒泥沙絮凝特性及其影响因素。结果表明:在河口上段水域,流速对絮凝表现为破坏作用,盐度是"制约"条件,流速和浊度对絮团影响明显,絮团平均粒径约为40.69mm,是分散粒径的4倍左右;口门最大浑浊带水域絮团峰值出现在憩流或者中等流速处,高盐度抑制、低盐度促进絮团生长,小潮时流速影响最明显、大潮时则是盐度,絮团平均粒径小潮为28.54mm、大潮35.17mm,是分散粒径2~3倍左右;北港口外,絮团生长或者破碎频繁,流速表现出促进作用,盐度有抑制作用,盐度对絮团影响最大,絮团平均粒径约为110.36mm,是分散粒径的18倍左右。     

Bioflocculent algal-bacterial biomass improves low-cost wastewater treatment.

An innovative technology for the biological treatment of wastewater in regions with sufficient solar radiation based on the simultaneous growth and degradation processes of algal and bacterial biomass is presented. The aim of the work is the improvement of pond technology through the formation of stable algae-bacteria aggregates, which a) permit a simple separation of the algal biomass by gravity sedimentation, b) enable a high removal efficiency for organic carbon and nutrients, and c) are independent in terms of oxygen provision through algal photosynthesis. Algae-bacteria aggregates could be developed with a suitable algal species (Chlorella vulgaris, Strain Hamburg) as a 'model organism' in a wastewater environment. The morphology of algal-bacterial flocs is similar to activated sludge flocs. They are stable and settle quickly. Floc size ranged between 400 and 800 microm. Results of our experiments with an artificially irradiated lab-scale system, operated in continuous flow mode, revealed that even at a relatively short hydraulic detention time of two days, a high elimination capacity of 9.96 g N m(-2) d(-1) and 0.87g Pm(-2) d(-1) can be achieved. Recent investigations confirmed that floc formation of unicellular algae and wastewater bacteria also could be developed and maintained in a pilot-scale system with a water depth of 0.5 m.