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.     

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.     

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。     

Investigation on effects of aggregate structure in water and wastewater treatment.

The fractal structure and particle size of flocs are generally recognized as the two most crucial physical properties having impact on the efficiency of operation of several unit processes in water and wastewater treatment. In this study, an experimental investigation is undertaken on the effect of aggregate structure in water and wastewater treatment in Hong Kong. The fractal dimension of the resulting aggregate is employed as a measure of the aggregate structure. Small angle light scattering technique is used here. Different amounts of polymers are mixed to bacterial suspensions and the resulting structures are examined. The addition of polymer may foster aggregate formation by neutralization of the bacterial surface charge and enhance inter-particle bridging. The aggregation behavior may affect the efficiency of certain water and wastewater treatment processes such as dewatering and coagulation. The impacts of aggregate structure on two representative processes, namely, ultra-filtration membrane fouling and pressure filter dewatering efficiency, are studied. It is found that the looser flocs yield a more porous cake and less tendency to foul whilst more porous filter cakes yield more ready biosolids dewatering.     

PDMDAAC协同处理地表水絮体分形维数研究

试验分别选用聚合铁(PFS)与聚合铝(PAC)处理地表原水,以聚二甲基二烯丙基氯化铵(PDMDAAC)为助凝剂,考察PDMDAAC助凝后絮体形态变化,絮体图像用显微摄像系统分析,计算絮凝体的分形维数。结果表明:在郑州市东风渠原水处理中,投加无机絮凝剂35 mg/L、PDMDAAC 1.0 mL/L,PDMDAAC助凝效果显著,浊度明显降低,絮体密实;协同PFS处理后絮体的一维分形维数由1.099 6增大到1.140 4,二维分形维数由1.520 9增大到1.544 3;协同PAC处理前后絮体的一维分形维数由1.038 0增大到1.046 8,二维分形维数由1.303 2增大到1.394 6;絮体大小和密实度可以从形态学方面进行量化,一维、二维分形维数均可以,二维分形维数更切合实际。     

Floc size distribution in a stirred suspension.

This paper focuses on the distribution of alumino-humic flocs found in a stirred suspension. Distributions were scaled using the transformation u = d/dL in which d is floc diameter and dL its arithmetic mean value and fitted by a gamma distribution. Flocs were treated as monofractal with solids mass concentration specified by C = A'rhos (d/do)D-3 in which A' is a packing coefficient, rhos the density of the floc solids, D the fractal dimension and do a reference size. It was shown that the overall solids concentration (M) complies with the dependence M proportional to NA'dDL-S(D) in which N is the number of flocs per unit volume and S(D) a distribution moment. Initial estimates of A' and D were obtained from analysis of floc sedimentation behaviour. From knowledge of the base parameters, the calculated value of M did not match the measured M and varied with shear. This was attributed to a kinematic influence on C over and beyond changes associated with the response of dL to shear. Issues of self-similarity were examined and it was concluded that distributions did not display strict self-similarity. Data are provided on the size distribution found in the flocculators of a treatment works at full scale.     

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.     

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.     

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％。     

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。     

基于微结构指标响应曲面法的固化盐渍土力学特性分析

固化土力学强度指标与固化土结构变化的关系是盐渍土特性研究中的重要问题。选取颗粒粒度分维D_(ps)、颗粒定向分维D_(di)、等效直径D_e为试验因素,利用响应曲面法进行试验。响应曲面模型可以对固化土的力学性能指标进行分析和预测,利用该模型研究石灰固化盐渍土的力学强度指标与最优微结构参数之间的关系,并进行了验证。结果表明,固化土微结构最优指标是粒度分维D_(ps)为0.8、颗粒定向分维D_(di)为0.93、等效直径D_e为1.75时,固化土无侧限抗压强度q达到1.636 MPa,与实测值仅相差0.038 MPa;黏聚力c为456.7 k Pa,与实测值完全一致;内摩擦角为39.6°,与实测值仅相差0.2°。该模型为探讨各类固化盐渍土物理化学指标、各掺加料比例及固化土微结构之间相互关系提供了借鉴和依据。     

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.     

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.     

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.     

Characterization of the activated sludge and of the operating conditions of six SBR treating dairy effluent and operated at industrial scale.

The sludge from six SBRs treating dairy effluent and located at same geographical location, in North East of France, were collected to study their characteristic behavior. The six plants were designed and constructed by the same manufacturer and are working under quite similar operating conditions. The objective of the study was to observe if any similarity existed in the characteristics of the sludge collected from the SBRs. The sludge was characterized for morphological properties (filament index, floc size), settling, compressibility, suspended solids (SS) concentration. The sludge from each plant was different from the others in most of the characteristics. One sludge out of six (sludge G) was completely different from the others with a very degraded structure and low discrete settling and compression. This reactor was not working fully satisfactorily with a too high COD at outlet, probably because this SBR was undergoing repetitive overloading linked to a very bad recovery of the whey by the cheese maker. The five other SBRs were working fully satisfactorily but the characteristics of the five sludges were quite different from one sludge to another. The size of the flocs seemed to be the only parameter measured which could be correlated to the settling characteristics of the sludge. The sludge characteristics and the parameter correlations were also compared with that of municipal activated sludge and were found to be very different.     

The assessment of different operating strategies for minimising activated sludge deflocculation under temperature transient conditions.

Three operating strategies were tested for decreasing activated sludge deflocculation due to temperature shifts from 30 degrees to 45 degrees C: magnesium sludge enrichment, increased sludge retention time (33 d), and spikes of an easily degradable substrate (methanol). The temperature shifts were conducted sequentially in 4 parallel lab-scale sequencing batch reactors (SBRs) treating kraft pulp mill effluent. Three SBRs operated at an SRT = 20 days, and in one of them the sludge was not manipulated, thus, serving as a reference SBR. The temperature shift was associated with decreased soluble chemical oxygen demand (SCOD) removals, decreased sludge settleability and substrate removal capacity, and increased effluent suspended solids (ESS) and turbidity levels. The shift also increased the sludge specific respiration rates and reduced the sludge substrate removal capacity. Sludge deflocculation was assessed as floc solubilisation (increased effluent SCOD levels) and floc fragmentation (increase in effluent solids smaller than 50 microm). Mg enrichment of the sludge and methanol spikes reduced the ESS levels (in 9 and 25%), and the three operating strategies decreased effluent turbidity (in 22-35%) compared to the maximum levels from the non-manipulated reactor (44 mg ESS/L). The stronger sludge floc structure achieved by magnesium enrichment and a high sludge age of 33 days was unsuccessful in significantly decreasing deflocculation. The mechanisms involved in sludge deflocculation require further fundamental research.     

Two-stage SBR for treatment of oil refinery wastewater.

A two-stage sequencing batch reactor (SBR) system was used for treatment of oily wastewater with COD and oil and grease (O&G) concentrations ranging from 1,722-7,826 mg/L and 5,365-13,350 mg/L, respectively. A suitable start-up protocol was developed using gradual increase in oily wastewater composition with methanol as the co-substrate. This strategy enabled a short acclimation period of 12 days for the sludge in the two-stage SBR to adapt to the oily wastewater. After acclimation, the 1st stage and 2nd stage SBRs were able to achieve COD removals of 47.0+/-2.4% and 95.3+/-0.5%, respectively. The 1st stage SBR was able to achieve 99.8+/-0.1% of O&G removal and effluent O&G from the 1st stage SBR was only 6+/-2 mg/L. The 2nd stage SBR was used to further remove COD in the effluent from the 1st stage SBR. The final effluent from the 2nd stage SBR had a COD concentration of 97+/-16 mg/L with no detectable O&G content. Thus, a two-stage SBR system was shown to be feasible for treating high strength oily wastewater to meet the local discharge standards.     

石英砂加载混凝工艺的DOM去除特征与混凝机理

为处理未纳入管网的市政污水,采用石英砂加载混凝工艺,考察石英砂的粒径及其投加量对处理效能的影响,分析了溶解性有机物(dissolved organic matters,DOM)的去除特征,并探讨了石英砂加载混凝机理。结果表明,与常规混凝相比,当石英砂的粒径和投加量分别为200目和 1.0 mg/L 时,石英砂加载混凝工艺对污水中浊度、COD的去除率分别为97.55%、59.2%,优于常规混凝;石英砂加载混凝工艺可明显去除芳香类、共轭双键、疏水性有机物,主要促进了腐殖质类物质的削减;石英砂通过架桥作用增加了絮体的密度,进而显著改善了絮体的沉降性能、强度;絮体的沉降性能、强度及再稳性能随着石英砂粒径的降低而升高。     

HYDRODYNAMICS OF FRACTAL FLOCS DURING SETTLING

The settling and hydrodynamic properties of 3-D fractal flocs in quiescent water are investigated with a numerical model based on the Lattice Boltzmann Method (LBM), with considering the settling velocity, hydrodynamic drag force and infra-floc flow. The comparisons of floc settling velocities and effective densities indicate that the numerical results present good agreements with observations in field and at laboratory. The results show that the drag force FD increases with the floc size df according to the relationship FD∝df3. Moreover, the intra-floc flow field and movement of the pore water provide a better understanding of the intra-floc flow from the microscopic viewpoint. The results also indicate that the lattice Boltzmann method is a promising approach to reveal the mechanisms of the flocculation in aquatic environments.