Flocs in Water Treatment is the first of its kind - serving as a valuable aide-m�moire for scientists, process engineers and other professionals engaged in water treatment. The framework described in Flocs in Water Treatment can also be applied to aggregated solids found both in the natural environment, and within a broad range of industries. Flocs (aggregated solid matter) resulting from the combined influence of coagulation and flocculation play a vital role in solid-liquid separation processes. The design and operation of water treatment plants demands a proper understanding of the ways in which flocs affect treatment systems and how their properties can be manipulated to increase treatment efficiency.

Flocs in Water Treatment provides a comprehensive account of the ways in which flocs are formed, their characterization, and how they behave in practice. Flocs are complex entities, whose properties defy easy description and measurement. In spite of this, the authors provide a clear and discerning account of the current state of knowledge; this is rooted in science and draws on many disciplines. Based on their experiences in research and the workings of full scale treatment plants, the authors offer candid advice on tasks such as the measurement of floc properties and guidance on problems involving the use of chemicals for controlling floc properties within treatment systems.

Auszug. � Genehmigter Nachdruck. Alle Rechte vorbehalten.

Flocs in Water Treatment

By David H. Bache, Ross Gregory

IWA Publishing

Preface, viii, 
Section 1: Background Concepts, 1, 
Section 2: Characterisation, 61, 
Section 3: Flocs in Practice, 128, 
Appendix A: SI Units and Physical Constants, 261, 
Appendix B: Symbols, 263, 
Acknowledgements, 273, 
Index, 275,

CHAPTER 1

Section 1 Background Concepts

1.1.1 INTRODUCTION

Water quality perspective

The composition of water is moulded by its history. As rain passes through the atmosphere, it dissolves gases and traps aerosols. On reaching the earth's surface, it either evaporates or continues its flow in various guises of runoff. The energy associated with surface flow determines its erosive potential - resulting in the suspension of clays and a wide variety of particulate matter. In the form of sub-surface flows, there is greater contact time with crustal components, this allowing water's capacity as a solvent to increase. The gains depend on factors such as solubility and crustal abundance. One may see the appearance of elements such as Fe, Mn and As, as well as those which strongly influence the alkalinity and pH.

Beside the naturally occurring materials, there are contributions from industrial effluents, sewage and farmland runoff, each adding to the cocktail of materials which are present. Water will also contain a plethora of living matter ranging from microorganisms to small plants and animals along with their residues. Table 1.1 illustrates some common traits associated with different types of source.

While information of the type shown in Table 1.1 is useful for highlighting typical features of a water source, it lacks detail of the factors which affect its treatability. For this, one must look more closely at the nature of the materials held by water. For example, colour is strongly associated with humic acid and fulvic acid. These materials have different molecular weights and sizes and are not equivalent in terms of their response to coagulation and separation in conventional treatment. Coliforms can be considered as a specific type of particle which can be filtered out rather than being inactivated by a disinfectant. Turbidity is most strongly influenced by particles in the size range 0.1-1�m (Huber and Frost, 1998). Elements such as Mn, Fe and hardness may also show their presence in particulate form, depending on the pH. Particles are ubiquitous in natural waters and often exist in concentrations of 107 to 108 per litre (Stumm, 1992). Generally they occupy a continuous size range from molecular dimensions through to a few mm as evident in Figure 1.1.

The kinematics of particles and their behaviour in solid-liquid separation processes is strongly influenced by their size and surface properties. Figure 1.1 indicates that particles greater than a few �m can be removed by the combination of sedimentation and filtration, whereas for smaller sizes one either has to resort to different forms of membrane filtration or to promote aggregation so that separation technology for larger particles can be exploited. As particle sizes get smaller, their surface to volume ratio increases – implying that interfacial properties play an increasing role in particle dynamics.

1.1.2 Surface characteristics

The wide range of materials found in water would seem to imply a great diversity in the character of their interfaces. Fortunately there are many common patterns of behaviour which allow surface characteristics to be described in a systematic way (Stumm, 1992).

Atoms, molecules and ions exert forces on each other, a factor which does not stop at their interfaces. Many solid surfaces contain ionisable functional groups such as the hydroxyl (OH) and carboxyl (COOH) which serve as electron donors in surface coordination chemistry. The surface charge depends on the degree of ionisation, this being influenced by the pH of the suspending medium. Surface charges also arise from imperfections in the lattice structure and are modified by adsorption. The existence of surface charge gives rise to electric interactions which extend over much greater distances than chemical bonds. Fluctuations in the electromagnetic fields around atoms and molecules induce temporary dipoles whose interaction yield van der Waals forces. These attractive forces play an important role in adsorption and inter-particle bonding. As with electric interactions, van der Waals forces are long range, implying that they are significant over distances which are large relative to atomic and molecular dimensions. For reference purposes it is useful to note that a water molecule has a diameter of about 0.3 nm.

Beside interactions involving coordinative bonds and electric fields, particle behaviour is also influenced by surface-solvent interactions - surfaces being classed as hydrophilic and hydrophobic, or amphipathic, in which both classes of behaviour exist.

Hydrophilic materials are water soluble. The properties are conferred by the presence of polar and ionic groups and their affinity for water, with the consequence that the surface may become hydrated. Water molecules are polar, the charge separation creating a dipole. In bulk water, transient structures exist between water molecules because of hydrogen bonding – sometimes tending towards a hexagonal structure, such as found in ice crystals. When an ion is present, this alters the hydrogen bond network. A water molecule tends to reorient so that its polarised charge concentration faces the opposite charge of the ion. As the water molecules orient themselves toward the ion, they break the hydrogen bonds to their nearest neighbours (see Figure 1.2). The group of water molecules oriented about an ion is called a hydration shell. The orientation of molecules in the hydration shell results in a net charge on the outside of the shell, the charge having the same sign as the ion. Charges on the outside of the hydration shell tend to orient water molecules in their vicinity, leading to the formation of a second hydration shell.

If a particle surface is charged, it will tend to hydrate in similar same way to the ion with dipoles oriented towards or away from the surface depending on its charge. Under the influence of electric fields the ordering of water molecules close to the surface leads to higher values of dynamic viscosity and lower values of dielectric constant when compared with those found in bulk water.

When a surface has no polar or ionic groups or hydrogen bonding sites, there is no affinity for water and the surface is described as hydrophobic. Hydrophobic substances are sparingly soluble in water and tend to be adsorbed on relatively non-polar surfaces. The adsorption is not founded on attraction between the material and the surface. Instead, it stems from the natural tendency of water molecules to associate with each other rather than with a hydrophobic substance; this leads to a thermodynamic 'rejection' of a hydrophobic particle from the bulk water continuum.

Many organic substances, such as fatty acids and detergents contain both hydrophobic parts and hydrophilic groups. This amphipathic characteristic also applies to humic and fulvic acids because they contain various types of phenolic and carboxylic ionisable hydrophilic groups as well as aromatic and aliphatic moieties which impart hydrophobic properties. If the molecular weight of organic dipoles and organic ions is sufficiently high, the...

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