Handbook for the operation of water treatment works

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1. HANDBOOK FOR THE OPERATION OF WATER TREATMENT WORKS The Water Research Commission The Water Institute of Southern Africa Editor: Frik Schutte Water Utilisation…
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  • 1. HANDBOOK FOR THE OPERATION OF WATER TREATMENT WORKS The Water Research Commission The Water Institute of Southern Africa Editor: Frik Schutte Water Utilisation Division Department of Chemical Engineering University of Pretoria TT 265/06 MARCH 2006
  • 2. Obtainable from: Water Research Commission Private Bag X03 Gezina 0031 The publication of this report emanates from a project entitled: Development of a handbook for the operation of water treatment works (Consultancy No 559) DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. ISBN 1-77005-428-6 Printed in the Republic of South Africa Cover photographs courtesy of Rietvlei Water Treatment Works
  • 3. iii TABLE OF CONTENTS PURPOSE OF THE HANDBOOK....................................................................... 1 WHO SHOULD USE THE HANDBOOK ........................................................... 2 PART A: BACKGROUND 1. Introduction to water quality and treatment .................................................... 3 2. Drinking water quality..................................................................................... 25 3. Introduction to Water Chemistry..................................................................... 38 4. Water treatment calculations........................................................................... 58 PART B: TREATMENT PROCESSES 1. Coagulation-flocculation................................................................................. 70 2. Sedimentation and flotation............................................................................. 87 3. Sand filtration.................................................................................................. 98 4. Disinfection ..................................................................................................... 117 5. Chemical stabilisation ..................................................................................... 145 6. Fluoridation ..................................................................................................... 158 7. Residuals handling and treatment.................................................................... 164 8. Advanced processes......................................................................................... 172 PART C: MANAGEMENT AND CONTROL 1. General management concepts........................................................................ 193 2. Treatment plant control ................................................................................... 200 3. Maintenance and trouble shooting................................................................... 219 4. Safety............................................................................................................... 227 PART D: APPENDICES AND REFERENCES References ................................................................................................................ 232
  • 4. iv
  • 5. 1 PURPOSE OF THE HANDBOOK The treatment and supply of drinking water is a challenging task that includes many diverse aspects ranging from water quality management in catchments, operation and control of water treatment plants, distribution of treated water, community participation, project management, etc. The purpose of this handbook is to provide comprehensive information specifically on all aspects related to the treatment of water for domestic use. The focus is on the operational aspects of treatment plants and processes and not on process design. Sufficient background and process descriptions are provided to enable a proper understanding of the functioning of the different processes and on aspects such as the suitability of processes for different types of water and the limitations of different processes. The purpose of this Handbook is not to provide an instruction manual or task list for process controllers or operators to operate unit processes or a treatment plant. A handbook in the form of an illustrated operational guide is being prepared for the WRC for this purpose. The objective of this Handbook is to provide the plant supervisor and process controller with sufficient knowledge and insight to: Assess raw water quality as well as the quality of water from individual unit processes and the treatment plant as a whole to ensure that final water of the required quality is produced Understand the implications to consumers and other stakeholders if sub-standard water is produced and supplied Calculate and make adjustments to dosages and operating parameters in response to changes in raw water quality or other requirements Assess the performance of unit processes and the plant as a whole Identify potential causes of poor performance of unit processes Optimise the performance of unit processes and the plant as a whole Perform basic management tasks including water loss assessment and control and safety management The authors recognise that the majority of plant operators currently in operating positions might find the level of material in this Handbook too advanced and would encourage them to use the illustrated handbook mentioned above. However, the authors feel that wherever possible plant operators should be encouraged to improve their knowledge and qualifications and this Handbook should be useful for that purpose. The layout of this Handbook is such that some aspects are repeated in different sections. Many aspects covered in Part A (e.g. in Overview of Treatment Processes and in Water Chemistry) are discussed again in Part B (Treatment Processes). This duplication is beneficial to the reader since the focus of the different sections is different and the same material is presented from different perspectives.
  • 6. 2 This is a first attempt at developing a Handbook specifically for the operation of drinking water treatment plants to match the Handbook for the Operation of Wastewater Treatment Works. The initiatives of the Water Research Commission (WRC) and the Water Institute of Southern Africa (WISA) are recognised to have the Handbook developed. Some sections in this Handbook are similar to certain sections in the Wastewater Handbook because the subject material of the basic aspects is very similar. A number of reference books has been used in the compilation of the subject material and they are listed in the reference section. The study material has been compiled by experts in the field with many years of experience in the field of drinking water treatment. However, it is recognised that the Handbook may have shortcomings from the perspective of process controllers and operators who may have specific requirements not adequately addressed by the book. For this reason Handbook users are requested to submit any suggestions for improvement of future editions to the Water Research Commission or WISA or any of the authors. WHO SHOULD USE THE HANDBOOK? The level of presentation in this Handbook is on the BSc. and BTech. level, i.e. suitable for training of process controllers and treatment plant operators at tertiary training level. Some parts of the Handbook will be useful for in-house training of treatment plant operators and process controllers. The Handbook provides basic information on water treatment processes and water supply that should also be useful for other people involved in water treatment and water supply, including engineers and scientists. The groups who may find the Handbook useful include: Water treatment plant supervisors and managers Water treatment plant process controllers Engineers and scientists involved in water treatment and supply Educators and students
  • 7. 3 PART A: BACKGROUND CHAPTER A1: INTRODUCTION TO WATER QUALITY AND TREATMENT Frik Schutte INTRODUCTION The term “water quality” describes the physical, chemical and microbiological characteristics of water. These properties collectively determine the overall water quality and the fitness of the water for a specific use. These properties are either intrinsic to the water or are the result of substances that are dissolved or suspended in the water. Water quality is only meaningful when evaluated in relation to the use of the water. The reason is that water of a certain quality may be fit for a specific use, but completely unfit for another use. For example, water that is fit for human consumption may not be fit as boiler feed water because the dissolved inorganic salts that are acceptable in drinking water, are not tolerated in boiler feed water, since they may precipitate and cause blockages in the boiler equipment. Water that is fit for domestic use (drinking water) must comply with specific requirements. The most important requirement is that it must be safe to drink. Many raw water sources contain harmful micro-organisms or other substances in concentrations that make the water unsafe to drink or in other ways unfit for domestic use. These organisms and substances must be removed from the water by means of treatment processes to make the water fit for domestic use. In addition to the requirement that water must be safe to drink, water for domestic use must also be aesthetically pleasing (have a clean appearance, taste and odour) and it must furthermore be chemically stable (i.e. it must not cause corrosion or form deposits in pipes or fixtures such as geysers). The principal objective therefore of water treatment is to produce water that is fit for domestic use reliably and consistently from a raw water source at a cost that is reasonable to the consumers. A water treatment plant employs many individual treatment processes (sometimes called unit processes and unit operations) that are linked in a process train to produce water of the desired quality. In this chapter a brief overview is given of historic developments in water treatment, followed by a discussion of water quality aspects that are relevant to drinking water treatment. This is followed by an overview of the different treatment processes commonly used for water treatment and the processing of residuals. Each of these processes is discussed in detail in subsequent chapters in the Handbook.
  • 8. 4 HISTORIC DEVELOPMENTS The first references to clean water or ‘sweet water’ and for water to be good for use after passage over a certain number of stones date back about 3000 years to Biblical times. The Roman aqueducts are well-known later landmarks in Europe as testimony of conveying clean water to cities. By the eighteenth century the removal of particles from water by filtration was known as an effective way of clarifying water and the first municipal water filtration plant started operating in Scotland in 1832. However, the main objective at that time was simply to supply clear water because the germ theory and the knowledge that diseases could be spread by water was still unknown. It was only in 1855 that Dr John Snow, an epidemiologist showed empirically that a cholera outbreak in London was caused by drinking water contaminated by faecal wastes from a cholera patient. However, the concept of disinfection as a disease preventing measure and a practical disinfection process only developed much later. Pasteur demonstrated his germ theory only in the 1880’s and chlorination as treatment process was developed after 1905. By the early 1900’s the large increase in the number of water supply systems without proper treatment in the USA contributed to major outbreaks in water-borne diseases. However, it was only with the introduction of chlorine as a treatment process to disinfect water in 1908 that the spreading of diseases through contaminated water could be controlled. Chlorination was rapidly accepted as an essential part of water treatment and this resulted in a substantial decline in the number of deaths due to water-borne diseases. Research on coagulation-flocculation, sedimentation and filtration as basic water treatment processes during the early part of the previous century contributed to a better understanding of these processes and much improved performance. New processes were also developed during that time in Europe. The use of ozone for disinfection and taste- and colour enhancement was introduced early in the century in France and Germany. The most significant process development since the introduction of chlorine during the previous century was the development of synthetic membranes as treatment process. The first practical reverse osmosis membranes for the desalination of seawater were developed in the 1960’s. Later, other types of membranes were developed, including nanofiltration (NF), ultrafiltration (UF) and microfiltration (MF) membranes. These membranes find application in water treatment other than just desalination. For example, NF and UF membranes are used to replace some conventional treatment processes for removal of natural organic substances and micro-organisms from water. In South Africa water treatment and supply followed the pattern of the western world and in some aspects South Africa actually lead the way. For example, the concepts of water reclamation and reuse were pioneered in South Africa and Namibia. South Africa is also known as a world leader in the field of biological nutrient removal in advanced wastewater treatment. Sadly, the supply of clean and safe water in South Africa was until recently limited to formal municipalities. Most rural areas and many townships had no (or limited) water
  • 9. 5 supply and poor sanitation. In many rural areas in the country water is abstracted from a river, stream, borehole or well and consumed without any treatment or with only limited treatment. In these situations, the health of consumers is often at risk. Often the erection of a conventional treatment plant and the effective running of such processes may not be possible in many rural areas and alternative approaches have to be followed. The situation with respect to water supply has however, improved dramatically over the last number of years and it is a clear objective of government (and it is stated as a basic human right in the RSA Constitution) that every citizen must have access to clean water and basic sanitation. Many ambitious programmes are currently under way for water supply and sanitation in previously disadvantaged communities and in rural areas. The result of these developments is that the need for engineers and trained operators and process controllers has increased and special efforts are needed to provide appropriately trained people at all levels in the water treatment and supply industry. The main challenges facing the water industry today include: Deterioration in the quality of many raw water sources Removal of potentially harmful synthetic organic substances in water sources Removal of resistant micro-organisms from water Improved training of process controllers for new processes and process optimisation Demands for process integration and flexibility WATER QUALITY General aspects of water quality Water is a unique substance and one of its unique characteristics is its capacity to dissolve a variety of substances. As water moves through its cycle, called the hydrological cycle, comprising of rainfall, runoff, infiltration, impounding, use and evaporation, it comes into contact with many different substances that may be dissolved by the water to a greater or lesser extent or that may be suspended in the water. The type and amount of the dissolved substances together with suspended and colloidal substances (very small suspended particles) collectively determine the overall quality of the water and its fitness for domestic use. The types of contaminants or substances of concern that may occur in water sources vary over a wide spectrum and include inorganic salts, micro-organisms, clay particles and organic material. Those with similar characteristics that can be treated by the same type of treatment process are normally grouped together for design purposes and for general discussion. It is normally not possible to consider each individual substance of concern with the view to treatment. There are exceptions, however, for example the removal of a toxic substance from water is often specific for the particular substance.
  • 10. 6 The substances of concern in water can be categorised in different ways, e.g. as dissolved or suspended, as inorganic or organic, as macro or micro substances, as natural or synthetic substances, suspensions of micro-organisms etc. For the purpose of this discussion the characteristics of different groups of substances that are treated by he same types of processes will be considered briefly Dissolved substances Most substances are to a greater or lesser extent dissolved by water. Substances that are dissolved by water include gasses such as oxygen (O2), carbon dioxide (CO2) and ammonia (NH3), inorganic compounds such as sodium chloride (NaCl) and calcium sulphate (CaSO4) and organic substances such as humic acids and carbohydrates. Dissolved substances are generally more difficult to remove from water than suspended substances, since they must either be converted into the solid form by means of precipitation, or to the gas form by means of oxidation so that the gas can escape or be stripped from water. A further possibility to remove dissolved substances is by using advanced processes such as reverse osmosis or activated carbon adsorption. Suspended and colloidal substances In addition to the substances that are dissolved in water, some substances may not dissolve in water but remain in suspension as very small suspended or colloidal particles. Suspended solids are defined as solids that are relatively large and settle easily under quiescent conditions. Suspended solids are normally determined by filtering the suspended solids from a water sample of known mass, and determining the mass of the dried solids. Colloidal particles on the other hand are too small to settle and they also carry an electrical charge that prevents them from settling. They can actually remain in suspension for days without settling. A colloidal system is defined as a system in which particles in a finely divided state are dispersed in a continuous medium. Colloidal particles are not limited to any particular group of substances but are defined by size. The colloidal size range is generally regarded to extend in size from about 10 nanometer (nm) to 1 micrometer (μm). Colloidal particles impart undesirable properties to water: Turbidity is most often caused by inorganic clay minerals in surface water. Most turbidity particles are hydrophobic (water repelling) and range in size from 0.2 to 10 μm. Turbidity can be readily removed from water by coagulation-flocculation and separation. Colloidal organic substances, i.e. humic and fulvic acids with molecular mass ranging between 800 and 50000 Daltons (Mol Mass units) generally cause colour in natural water. Colloidal metal hydroxides (e.g. iron) also cause colour in water. Most of the particles responsible for colour are hydrophilic (water attractive) and more difficult to remove by coagulation than turbidity particles.
  • 11. 7 Bacteria, viruses and micro-algae are also colloidal in nature. They consist of polar organic molecules, are hydrated and hydrophilic. Certain complex organic compounds in treated industrial wastewater can also be considered as colloidal. Colloidal suspensions are stable and must be destabilised before it is possible to aggregate them into bigger floc particles that can be removed by sedimentation and filtration. Destabilisation is effected by coagulation, and aggregation by flocculation. These processes are discussed in detail in subs
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