Handbook on Particle Separation Processes

Handbook on Particle Separation Processes

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Product Details

ISBN-13: 9781843392774
Publisher: IWA Publishing
Publication date: 03/19/2012
Pages: 288
Product dimensions: 6.12(w) x 9.25(h) x 0.75(d)

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CHAPTER 1

Introduction

A. van Nieuwenhuijzen and J. van der Graaf

1.1 INTRODUCTION

Particles in water play an important role in all kinds of water quality and treatment issues. Since the early beginnings of centralised water production and treatment, the main goal of water purification was primarily the removal of water turbidity in order to produce clear water free from visible particles. Although the connection between hazardous organisms in water and diseases caused by water consumption was only proven in the second half of the 19th century (John Snow c.s, see Figure 1.1), experience learned human civilisation to treat water especially from surface water sources by removing the turbidity. Sedimentation in ponds and large pots, filtration through porous materials and textiles and other means of water treatment were applied in smaller communities and single households. With population growth and settlements of larger cities, water treatment and disease control were often neglected. Especially in medieval civilizations the idea that diseases would originate from (drinking) water was lost to a great extent. Only the discovery and recognition of pathogenic micro-organisms in water and their distinct effects on humans and animals during several water-born epidemics in (the first mega) cities in the World, created by an explosive growth due to industrialisation around 1850, brought the idea of hygiene in daily life back to the population. Pushed by medical and pharmaceutical specialists, water filtration in central treatment plants, water boiling in households and after 1900 also centralised water disinfection were introduced to produce safe drinking water. The necessity of controlled discharge of particle-rich human wastewater and treatment of it was recognised as well. Of course, particle removal and the transfer of soluble material into biological particles play an important role also in wastewater treatment. Around 1940, so called primary treatment was commonly applied consisting of simple sedimentation facilities to remove about 50% of the particles contained in raw sewage. Later, biological treatment was introduced where biological flocculation and the removal of the activated sludge is an important solids separation step for small colloidal particle fractions.

1.2 THIS HANDBOOK

Advances in particle removal in water and wastewater treatment are still important developments in understanding and optimising treatment processes and concepts. In 2008 a major summer course on this topic was organized by Delft University and UNESCO-IHE Delft, more than 30 years after the first International Summer Course on Particle Separation, in 1977 in Cambridge. The Academic Summer School on Advances in Particle Separation in Water and Wastewater Treatment was a selective symposium and workshop for invited lecturers and participants only, organised under supervision of the IWA Specialist Group Particle Separation. The purpose of the Summer School was to exchange knowledge and expertise from a selected group of honourable experts with a long track record in the field of particles and particle separation in water and wastewater treatment to emerging new and young specialist in this area. Since the older generation of experts is more and more retiring, information exchange is essential to provide future experts with available expertise.

The IWA Handbook on Particle Separation Processes (Van Nieuwenhuijzen and Van der Graaf, 2010) provides an overview of the latest developments on particle separation in water and wastewater treatment. This book has been edited from the presentations and workshops held at the Academic Summer School Particle Separation in Water and Wastewater Treatment. The purpose of the Handbook is to provide knowledge and expertise from a selected group of international experts with a wealth of experience in the field of particles and particle separation in water and wastewater treatment. The book contains material ranging from Methods and Instrumentation for Particle Analysis and Characterisation (by Prof. Markus Boller), over Natural Organic Matter: Particles, Colloids and Macromolecules (by Prof. Gary Amy), NOM Removal Technologies (by Prof. Hallvard 0degaard), Several Physical Chemical Treatment Technologies (Prof. Yoshima Watanabe), Filtration Characteristics for Effluent Treatment by Prof. Jaap van der Graaf and Flotation for Particles Removal by Prof. Mooyoung Han. Three chapters are dedicated to some new promising particle removal developments. Other topics like Modelling Particle Removal (Prof. Desmond Lawler), Particle Separation in Drinking Water Treatment (Prof. Rolf Gimbel) and Nanoparticles in Water and Wastewater (Prof. Mark Wiesner) are presented in other gremials and publications.

1.3 FOCUS ON PARTICLES

It becomes obvious that the removal of particles from water and wastewater is crucial for safe potable water production and efficient wastewater treatment. If particles are present in a water source, it is the primary purpose of all purification techniques to eliminate or inactivate the particles and with them also eventual hygienic hazards.

For several reasons particles represent undesired pollutants in most product waters. Apart from the mass of suspended matter as often used bulk parameter, many other quality indicators are strongly associated with particles such as hygienic contaminants and adsorbed chemicals. On one hand, particles may negatively interfere in various treatment processes and supply systems, on the other hand particulate matter in the form of biomass is a necessary prerequisite in many water treatment schemes. The removal of particulate matter was and will be one of the most crucial steps in water and wastewater treatment. In order to understand the behaviour of particles in water and to develop and design efficient treatment facilities, the characteristics of particles has to be known on the basis of individual solids and of whole particle populations. In water treatment, particles are of extremely heterogeneous nature with respect to size, density, shape, chemical composition, shear strength, surface charge, etc. which represent information that is in most cases not available. This book aims at gathering knowledge on particle characterisation by presenting research studies including results with innovative new instruments and methods.

In order to understand the role of particles in water quality evaluation and water purification and wastewater treatment processes, the particles and their behaviour in aqueous systems have to be known and characterized.

1.4 OCCURRENCE OF PARTICLES IN WATER

Solids in water are of very different origin and appear in a large variety of sizes, shapes, chemical composition, etc. An incomplete list of particles in water is shown below and illustrates the complex nature of aqueous solids:

(1) Domestic Wastewater

Coarse solids: gravel, sand, faeces, paper, hair, cotton, wood pieces, plastic pieces (e.g. ear sticks), cigarette-ends;

Fine solids: faeces, road dust, atmospheric solids deposits, aggregates of micro-organisms, single micro-organisms, worm eggs, viruses, biological debris, clay minerals, chemical precipitates, nano-size particles;

(2) Industrial wastewater

‐ Great variety according to production and treatment processes cellulose fibres, asbestos fibres, incineration ashes, precipitation products, pigments, emulsified oil droplets, polymers, coal dust, silicates, metals, blood, milk;

(3) River water

‐ Gravel, sand, silt, clay, algae, decay products of plants, protozoa, bacteria, viruses;

(4) Lake water

‐ Phytoplankton, zooplankton, detritus, precipitation products, bacteria, viruses, protozoa;

(5) Groundwater/Spring water

‐ Precipitation products (Fe, Mn, Ca), soil colloids, bacteria, viruses, protozoa;

(6) Potable water

- Nano-size particles, bacteria, viruses, corrosion products, calcite particles, particles of natural organic matter (NOM).

While most of the above description do not give much information on particle characteristics, detailed analysis of particle numbers include the evaluation of the particle size. A coarse indication of the size range of some well known particle classes is given in Figure 1.2 and Figure 1.3.

It also becomes clear that the standard procedure for solids analysis by filtering water samples with 0.45 pm membrane filters is not an accurate procedure to define solid and dissolved matter. There is still suspended matter in the size range below 0.45 pm which would have to be clearly classified as particulate.

In Figure 1.2 the breakdown of constituents in typical municipal wastewater (in the Netherlands) is presented; it finally shows the divisions into four fractions: settleable, unsettleable, colloidal and dissolved.

Markus Boller presents in chapter 2 an overview on possible techniques for the measurement and characterisation of particles in water. Especially the submicron particles need very precise attention.

In chapter 3 the natural organic matter (NOM) is introduced by Gary Amy; the various analytical techniques finally give information on the selection of suitable removal processes.

1.4 PARTICLE SEPARATION PROCESSES

In most treatment trains for water treatment, particle separation is the first treatment step. There are different alternatives for solids separation and their beneficial application depends strongly on the quality of the water or more precisely on the characteristics of the particle suspension to be treated. The processes most widely applied in water treatment are:

‐ sedimentation

‐ flocculation

‐ flotation

‐ granular media filtration

‐ contact filtration

‐ screening, straining

‐ membrane filtration

‐ NOM-removal technologies.

Among these processes flocculation is not really a solids separation process but it helps to improve solids separation considerably by particle agglomeration. A primary factor for the decision which processes are suitable under which conditions, is the solids concentration in terms of mass quantity, volume or number concentrations. On the other hand, the particle size is an important parameter which determines the removal mechanisms which are best to be promoted in order to achieve optimal solids separation performance. Particle size fractionation of waters can be of interest for selection of an appropriate separation technology. Figure 1.4 presents a standardised method for particle size characterisation developed by the author.

1.4.1 Removal of particles >30 µm

Particles >30 µm can be removed by screening processes such as microstrainers. The smaller range of mesh sizes is in the order of 8 µm. Also a granular media filter may remove particles >20 µm completely. The combination rapid filter/slow sand filter can reach a particle removal efficiency of 100% for particles >10 µm. Screens and granular filters are subject to clogging and need to be cleaned regularly. At higher concentrations, sedimentation for particles with a higher density and flotation for particles with a low density are more suited. Filters and screens may be used below concentrations in the order of 50 mgTSS/l. At higher concentrations sedimentation or flotation are necessary.

1.4.2 Removal of particles between 0.5 µm and 30 µm

Many important particles in surface waters are in the size range between 0.5 and 30 µm such as bacteria, blue and green algae, diatoms, and partly also inorganic particles. In this size range orthokinetic flocculation (particle transport by shear) has to be applied in order to agglomerate the particles to larger flocs and thus enable removal by sedimentation, flotation or filtration. A combination with sedimentation or flotation is suitable at concentrations above 50 mgTSS/l or filtration or contact filtration at concentrations below this value. Also membrane filters such as micro- and ultra filters may cope with this particle class. In full scale, membrane filters are usually protected against too large particles by screening filters with openings in the order of 20 µm (Amy, 2004).

1.4.3 Removal of particles <0.5 µm

The smaller the particles in the nano-size range, the more difficult it is to describe their behavior in water. Viruses and some types of inorganic precipitates such as calcite and iron (hydr)-oxides typically belong to this size range. From particle analysis in this size range, it becomes clear that the number of particles is increasing with decreasing size. Particle numbers of nano-sized colloids in natural waters may easily reach 108 to 108/ml. In addition to the mentioned colloids, decay products of suspended organic material are present in large quantities. In future also an increasing number of synthetic nano-particles in wastewater, natural water and drinking water have to be expected.

The aggregation of submicron particles is relatively fast if their surface chemistry is suited and if their concentration is high enough (>108/ml). The transport of submicron particles for aggregation is brought about by Brownian motion, also know as perikinetic flocculation. Often the agglomerates are still small and cannot be removed by sedimentation or filtration, further agglomeration with the help of orthokinetic flocculation, contact filtration or sludge blanket clarifiers is necessary.

The particle class <0.5 µm can efficiently be removed by ultra filtration. Again pre-filters with openings of 10 to 20 µm are applied in order to protect the membranes. In view of the small pore size of ultra filters in the order of 0.01 µm, viruses and other colloids may be removed completely by ultra filter membranes. Ultra filters may cope with particle concentrations up to more than 1000 mgTSS/l. However, membrane fouling caused by accompanying organic material such as polysaccharides, proteins and humic substances can limit the application of ultra filters to much lower concentrations in the order of 100 mgTSS/l and lower.

1.4.4 Flocculation

This paragraph summarizes the researches on the flocculation conducted by Tambo and Watanabe, and on the inclusion of flocculation in the monolith ceramic membrane filtration conducted by the authors as described in more detail in chapter 5 of this book. The floc density function describes the quantitative relationship between the size and effective (buoyant) density of flocs. The exponent Kρ in the function is related to the fractal dimension (D) for the aggregates formed in Cluster-Cluster Aggregation (CCA) as D = 3 - Kρ. The Kρ is a function of the ALT ratio and has the numerical value of 1.00 and 1.25 for the ALT ratio of around 1/100 and 1/20, respectively. These values coincide with the D value determined for the reaction and diffusion limited case (2.05 and 1.75), respectively, determined in the field of the Fractal Physics.

The authors have also clarified the characteristics unique to monolith ceramic membrane with pre-coagulation by referring to the behaviour of micro-particles. The region exists in the monolith channel with the optimum G and GC0T value for good flocculation. The flocculation of micro-particles reduces the membrane fouling. If pre-coagulation and chemically enhanced backwashing are included in the monolith ceramic membrane filtration system, extremely high filtration flux is possible for a long operation period.

1.4.5 NOM-removal

Natural organic matter (NOM - with its main constituent humic substances), has several negative influences in water that is to be used for water supply and needs therefore to be removed (Boller, Odegaard). A better understanding of NOM character and its removal by various treatment methods is essential to improve treated water quality meeting increasingly stringent standards. In order to have a better insight into the types of organic compounds present before and after different water treatment processes several characterization techniques have been developed worldwide. These have provided considerable knowledge in understanding the impact of NOM on treatment processes. The characterization techniques differ considerably in terms of analytical approach, NOM fractionations or components analyzed, time and skills required, costs, and the form of the output or results (whether it can be interpreted easily and used by the treatment plant operators).

Comparative analysis of different methods of characterization of NOM has clearly shown that there is no single method which can fully reveal its characteristics that are important for water treatment practice. It is obvious that use of combinations of different methods would be required for proper analysis of the fate of different fractions of NOM during different treatment processes. However, the methods of characterization to be applied under given conditions depend on the source of NOM and treatment methods applied.

(Continues…)



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Table of Contents

Contents: Introduction A. van Nieuwenhuijzen and J. van der Graaf Characterization of Aquatic Particles M. Boller and R. Kaegi Characterization Profiling of NOM- as a Basis for Treatment Process Selection and Performance Monitoring G. Amy, S. Sharma, S. Salinas Rodriguez, S. Baghoth and S. Maeng Technologies for the removal of natural organic matter H. Ødegaard, S. Østerhus, E. Melin and B. Eikebrokk Advanced Physical Chemical Treatment by Flocculation Y. Watanabe Dissolved Air Flotation M.Y. Han Characterising the Membrane Filtration Process of Wastewater J. van der Graaf, S. Geilvoet and J. Roorda Enhanced Flocculation/ Sedimentation Process by a Jet-Mixed Separator Y. Watanabe Particle Behaviour and Removal in a Rainwater Storage Tank and Suggestions for Operation J.S. Mun and M.Y. Han Direct Membrane Filtration of Wastewater A. Ravazinni, A.F. van Nieuwenhuijzen and J.H.J.M. van der Graaf

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