The effect of polymeric flocculants on floc strength and filter performance.

Polymeric flocculants are widely used throughout the water industry as flocculant aids, they are known to increase floc density and aid settlement in the clarification stage of the water treatment process. In this research, polymeric flocculants were used to improve floc strength prior to filtration on a dissolved air flotation (DAF) plant in an attempt to prevent filter breakthrough. A modified jar test procedure using a PDA (photometric dispersion analyser) optical flocculation monitor was developed in order to evaluate the system floc strength. Filtration trials were carried out on a pilot filter rig situated on a surface water treatment works in Yorkshire. The filter feed originated from the main plant filter channel. Filter performance was assessed by continuous online monitoring of effluent particle counts, turbidity and headloss over the period of the filter run. Results indicated that low doses of polymeric flocculants had a beneficial effect on filtered water quality, as measured by particle counts, turbidity, UV254 absorption and dissolved organic carbon (DOC). Polymeric flocculants also had the effect of extending filter run length. The modified jar test results indicated that the flocculants used improved the floc strength and enhanced reflocculation of the micro flocs present after the flotation process.     

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.     

黏性泥沙絮团强度的试验研究综述

絮凝是河口地区水沙交换过程中极为重要的一环,动水絮凝过程可概括为:颗粒碰撞—有效碰撞黏结—絮团重构—絮团破裂—絮团再形成,最终絮团达到动态平衡状态,这是絮团强度在紊动剪切作用下的结果。从絮团强度的试验研究角度回顾了国内外研究进展,此方面研究多围绕絮凝的影响因素与絮团强度的特征参数之间的定性规律、经验或半经验定量关系的角度展开,采用不同的观测装置、设备和技术,以期对絮凝动力学得到更为深入的认识。     

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.     

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.     

Simultaneous determination of activated sludge floc size distribution by different techniques.

The activated sludge floc size distribution (FSD) is investigated by using different techniques and the results are compared against each other in order to gain insight into the FSD characteristics, as well as to detect the limitations of each measurement technique. The experimental set-up consisted of three devices coupled in series: a MastersizerS, an automated image analysis system (IMAN) and a CIS-100. The latter instrument has two measurement channels, based on time of transition (TOT), and image analysis (SHAPE) principles. In order to minimise the variability between successive measurements, the activated sludge samples collected from a pilot-scale SBR were flocculated until steady state was achieved. The results show that the MastersizerS and SHAPE devices yield similar volume weighted FSD. In contrast, the IMAN overestimated the floc size and TOT frequently showed a bimodal distribution. The number distributions from TOT and SHAPE were in agreement, while those generated by the MastersizerS were mainly located in the submicron range and those of IMAN corresponded to larger sizes. The experimental distributions show a good fit to the log-normal model. It is shown that the measurement principle is of utmost importance and results transformation may lead to data misinterpretation.     

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.     

Modeling the activated sludge floc microenvironment

Integrated modeling of activated sludge systems will only be complete when settling characteristics can be determined within the model. These characteristics are fundamentally related to the size, shape and porosity of both the sludge flocs and their component microcolonies. Microcolony porosity is of particular interest since it may both influence, and be influenced by, the substrate and electron acceptor gradients that establish competitive microenvironments. A model was developed to explore the relationship between biomass growth, death, and hydrolysis; soluble component diffusion and microcolony porosity. A key element of the model is the assumed distribution of daughter cells following replication. Preliminary results indicate rapid progression toward minimum porosity with relatively slow expansion of the microcolony surface. Although little is documented on porosity variation within flocs, these results contrast with observations for biofilms and may emphasize the difference between the mechanisms involved in the structural development of microcolonies and larger scale aggregates.     

The impact of reduced phosphorus levels on microbial floc properties during biological treatment of pulp and paper wastewaters.

The effect of limiting P in activated sludge was investigated in laboratory scale sequencing batch reactors (SBRs) fed effluent from a container board mill. Floc characterization included measurement of hydrophobicity, surface charge, and analysis of extracellular polymeric substances (EPS). Reactor performance was assessed by monitoring COD and inorganic P removal, MLSS, and sludge settleability (SVI and batch settling flux) over a period of eight months. Control reactors (BOD:N:P of 100:5:1) were compared to reactors run under P-limited conditions (100:5:0.3; 100:5:0.1). Reactor performance at lower temperatures (14 degrees C; control = 26 degrees C) was also studied to assess the impact of P-limitation. Changes in floc structure and the composition of EPS occurred within 1 to 3 days following a reduction in P levels. There was an insignificant increase in SVI; however, gravitational settling velocity and batch settling flux values for low P floc were consistently higher than for floc generated under control conditions. Lower temperatures (14 degrees C) resulted in a deterioration in floc settling properties at a BOD:N:P of 100:5:1. This impact on settling was significantly reduced at a BOD:P of 100:0.1. Reducing P concentrations in the wastewater treatment system has the potential to improve sludge settleability and reduce final P discharges.     

Effect of phosphorus limitation on microbial floc structure and gene expression in activated sludge.

The effect of limiting phosphorus (P) in activated sludge was investigated in laboratory-scale sequencing batch reactors (SBRs). Correlative microscopy revealed that P-limitation (COD:N:P = 100:5:0.05) leads to morphological changes in floc structure and the composition of extracellular polymeric substances (EPS). This was found to be accompanied by expression of quorum-sensing in an acyl homoserine lactone bioassay. Differential gene expression in relation to P-limitation was examined in a global profile using the Affymetrix Escherichia coli antisense genomic microarray. Three separate experiments were conducted where the impact of P-limitation was examined under batch conditions and in SBRs at stable operating conditions and within 3-7 days following a down-shift in P. Significant changes in open reading frames (ORF) and intergenic regions based on the E. coli microarray were observed. Several genes associated with cell structure, including slt, wbbH, fimH, amB, rfaJ and slp were found to be expressed. Quorum regulated genes were also found to be expressed including psiF which is known to be induced by P-starvation (92% confidence level; 1.45 log ratio).     

非饱和黄土的结构强度与抗剪强度

本文分析了非饱和黄土强度的组成和结构强度的来源，给出了结构强度的确定方法，研究了结构强度的变化规律，结果表明：非饱和黄土的结构强度可用其天然结构破坏后所丧失的强度衡量，结构强度与初始含水量间具有幂函数关系；结构强度与不稳定凝聚力具有良好的线性关系；提出了非饱和黄土结构强度终止含水量的概念；给出了非饱和黄土抗剪强度的计算公式。     

图像分析粘性细颗粒泥沙絮体孔隙初探

以细颗粒含量较高的滇池底泥为例,利用SEM图像分析法和统计学方法,研究了粘性细颗粒泥沙絮体孔隙大小、形状及孔径分布,同时探讨了絮体孔隙大小及孔径分布与絮体沉速之间的关系.实验结果表明:粘性细颗粒泥沙絮体孔隙一般属于毛细管孔隙,孔隙投影面的形状近圆形;自然状态下絮体孔隙孔径近似呈对数正态分布;加入絮凝剂后,呈对数偏态分布;絮体沉速与絮体孔隙的大小和孔径分布具有较强的相关性.     

絮凝对三峡水库泥沙沉降的影响

现场与室内实测的泥沙级配差异证实了三峡水库中存在泥沙絮凝现象,絮凝会加速水库泥沙淤积速率。为了确定絮凝对泥沙沉降速率的影响,现场采集了三峡水库原样泥沙,基于三峡水库水体环境,采用室内静水沉降试验对三峡水库中泥沙絮凝沉降规律进行了研究。研究结果表明,当试验采用的初始含沙量大于0.3 kg/m3时,部分单颗粒泥沙会聚集形成絮团,且泥沙絮凝程度以及沉速增加的幅度与水体中含沙量呈正比。参与絮团形成的单颗粒泥沙粒径均位于0.022 mm以下,小于临界粒径的泥沙占全沙的83%;采用絮凝因数和物质沉降通量因数分别表征絮凝对泥沙沉降的影响程度,这两种因数都随含沙量增加而增加,含沙量在1.5 kg/m3以下时其最大值分别为5.03和1.66,表明絮凝对三峡库区汛期的泥沙淤积有可能造成较大影响。     

泥沙絮团结构的试验研究综述

承接《泥沙研究》2015(5)黏性泥沙絮团强度的试验综述,絮团结构与絮团强度密切相关,是动水条件下表现出的絮团强度的微观解释。絮团结构,即絮团内部链接键的数量、强度和链接方式。总结了以往絮凝试验研究中对于絮团结构的量化描述以及相应研究进展。     

Effects of floc and bubble size on the efficiency of the dissolved air flotation (DAF) process.

Dissolved air flotation (DAF) is a method for removing particles from water using micro bubbles instead of settlement. The process has proved to be successful and, since the 1960s, accepted as an alternative to the conventional sedimentation process for water and wastewater treatment. However, limited research into the process, especially the fundamental characteristics of bubbles and particles, has been carried out. The single collector collision model is not capable of determining the effects of particular characteristics, such as the size and surface charge of bubbles and particles. Han has published a set of modeling results after calculating the collision efficiency between bubbles and particles by trajectory analysis. His major conclusion was that collision efficiency is maximum when the bubbles and particles are nearly the same size but have opposite charge. However, experimental verification of this conclusion has not been carried out yet. This paper describes a new method for measuring the size of particles and bubbles developed using computational image analysis. DAF efficiency is influenced by the effect of the recycle ratio on various average floc sizes. The larger the recycle ratio, the higher the DAF efficiency at the same pressure and particle size. The treatment efficiency is also affected by the saturation pressure, because the bubble size and bubble volume concentration are controlled by the pressure. The highest efficiency is obtained when the floc size is larger than the bubble size. These results, namely that the highest collision efficiency occurs when the particles and bubbles are about the same size, are more in accordance with the trajectory model than with the white water collector model, which implies that the larger the particles, the higher is the collision efficiency.     

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.     

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.     

碾压混凝土抗拉强度的规律分析

本文分析了碾压混凝土抗拉强度的影响因素,并比较劈拉强度与轴拉强度的异同,在此基础上搜集劈拉强度、轴拉强度与抗压强度的大量工程室内试验数据,对数据进行分析,得出初步结论:劈拉强度或轴拉强度的平均增长率与龄期呈对数关系;抗压强度与劈拉强度或轴拉强度、劈拉强度与轴拉强度的关系均为线性,5个相关方程在文中分别列出。