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Wastewater Treatment Advanced Processes and Technologies

Wastewater Treatment Advanced Processes and Technologies edited by

D. G. Rao R. Senthilkumar J. Anthony Byrne S. Feroz

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

Co-published by IWA Publishing, Alliance House, 12 Caxton Street, London SW1H 0QS, UK Tel. +44 (0)20 7654 5500, Fax +44 (0)20 7654 5555 [email protected] www.iwapublishing.com ISBN13: 978-178040-034-1

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2013 by Taylor & Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Version Date: 20120501 International Standard Book Number-13: 978-1-4398-6045-8 (eBook - PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www. copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-7508400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

Contents Preface..................................................................................................................... vii Contributors.............................................................................................................xi 1. Introduction......................................................................................................1 D. G. Rao, R. Senthilkumar, J. A. Byrne, and S. Feroz 2. Solar Photo-Fenton as Advanced Oxidation Technology for Water Reclamation.................................................................................. 11 Sixto Malato Rodríguez, Nikolaus Klamerth, Isabel Oller Alberola, and Ana Zapata Sierra 3. Solar Photocatalytic Treatment of Wastewater........................................ 37 J. A. Byrne and P. Fernández-Ibáñez 4. Advanced Oxidation Processes: Basics and Applications..................... 61 Rakshit Ameta, Anil Kumar, P. B. Punjabi, and Suresh C. Ameta 5. Impinging-Jet Ozone Bubble Column Reactors.................................... 107 Mahad S. Baawain 6. Biological Treatment of Wastewaters: Recent Trends and Advancements...................................................................................... 137 K. Vijayaraghavan 7. Removal of Heavy Metals by Seaweeds in Wastewater Treatment................................................................................................163 R. Senthilkumar, M. Velan, and S. Feroz 8. Microbial Treatment of Heavy Metals, Oil, and Radioactive Contamination in Wastewaters................................................................ 185 Sourish Karmakar, Arka Pravo Kundu, Kanika Kundu, and Subir Kundu 9. Anaerobic Wastewater Treatment in Tapered Fluidized Bed Reactor................................................................................................... 211 R. Parthiban 10. Treatment of Effluent Waters in Food Processing Industries............. 239 D. G. Rao, N. Meyyappan, and S. Feroz

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Contents

11. Removal of Lower-Molecular-Weight Substances from Water and Wastewater: Challenges and Solutions........................................... 275 V. Jegatheesan, J. Virkutyte, L. Shu, J. Allen, Y. Wang, E. Searston, Z. P. Xu, J. Naylor, S. Pinchon, C. Teil, D. Navaratna, and H. K. Shon 12. Treatment and Reuse Potential of Graywater from Urban Households in Oman.................................................................................. 319 Mushtaque Ahmed, Abdullah Al-Buloshi, and Ahmed Al-Maskary 13. Anaerobic Fixed Bed Reactor for Treatment of Industrial Wastewater.................................................................................................... 335 Joseph V. Thanikal

Preface The importance of wastewater treatment in the modern industrial world is very high in view of the fact that more than 97%, dormant in polar regions, of the available water is saline (in seas and oceans) and 2% of the freshwater is unavailable for human consumption. Thus, very little quantity of water is available for human consumption. The world population is increasing, and the per capita water consumption is also increasing day by day, which lays a heavy burden on science, technology, and engineering to meet the challenges of water treatment and supply in the future. Economic and social growth cannot be ensured without industrialization, which is in turn a culprit in spoiling the available water resources due to the generation of large quantities of wastewater. It is paradoxical but true. To add another dimension to the existing problem is the increased day-by-day legislative restrictions that are being imposed by various governments all over the world in view of the safety and health concerns of the citizens. Urbanization with overconcern for hygiene also generates huge quantities of wastewater that is known as graywater. It comes from household kitchens, toilets, and restaurants. The graywater from kitchens and restaurants is not toxic but is not suitable for human consumption. In the present complex scenario, the only alternative is to treat the available wastewater to make it as clean as possible. The treated water may not be exactly suitable for potable purpose, but can at least be used for various other purposes, viz., recycling partly for industrial purposes, steam generation, or gardening and agriculture. The treatment of wastewater is complicated because of the heterogeneous nature of the water streams coming from the various domestic and industrial sources. The industrial sources are as diverse as drugs and pharmaceutics, pesticides, food processing, fermentation, vaccines manufacturing nuclear processing, and metallurgical and animal processing industries. The pollutants generated can be physical, chemical, and biological in nature, and they can be toxic or nontoxic. Hence, the treatment methods are also varied in nature in order to process the diverse effluent wastewaters coming from various sources. This book is an honest attempt to present important concepts, technologies, and issues in this direction by various experts in the field of wastewater treatment. The treatment methods cover various process industries and utilize various technologies for the purpose. Chapters 2–4 deal with advanced oxidation processes including processes based on Fenton and photo-Fenton, ozonolysis, photocatalysis, and sonolysis. Various types of reactors used in wastewater treatment are dealt with in Chapters 5, 9, and 13. Microbial treatment methods, in general, for wastewater treatment are described in Chapter 6, whereas those used in various process industries are covered in Chapter 8. vii

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Preface

Effluent treatment methods, usually practiced in food processing industries, are comprehensively dealt with in Chapter 10. Removal of low-molecularweight substances from wastewater is a challenging task, and hence special methods for their removal are needed, which are all described in Chapter 11. Seaweeds are good adsorbents and may be applied in wastewater treatment for the removal of toxic substances (Chapter 7). The treatment of graywater needs a special attention in view of its increasing magnitude. Chapter 12 describes such treatment methods with a case study of the Muscat municipality. A special concept of central effluent treatment plants (CETPs) is gaining prominence in the treatment and release of wastewater from small-scale processing units into municipal water lines, after meeting the stringent legislative requirements. It is dealt with in the introductory chapter (Chapter 1). All efforts have been made by the editors and authors to judiciously blend most of the treatment processes and technologies in one single book in order to make the diverse subject matter as comprehensible as possible. It is, indeed, difficult to make it concise with the whole gamut of advanced processes and technologies in a single book of this nature; hence, enthusiastic readers are advised to consult the original references for complete understanding of any process or technology. This book is ideally suited for researchers and professionals working in the area of wastewater treatment. Each chapter is specific in its own way and, hence, may cater to the requirements of professionals interested in that area. The bibliography given at the end of each chapter would act as a guide for comprehensive information in that particular area. Hence, most of the chapters end with a comprehensive list of literature references. At the very outset, we would like to thank all our contributing authors, who have done an excellent job in drafting and delivering the chapters. The success of this publication is largely due to them. We would also like to extend our sincere thanks to the staff of the editorial and publication department of CRC Press, who have been very helpful and cooperative throughout the preparation of this material and have been largely responsible for the book in its present form. We thank all the authors, publishers, and industries whose works have been referred to and who have extended the copyright permissions to utilize their published information in this book in some form or the other. We would like to extend our sincere thanks to the executives and management of Caledonian College of Engineering, Muscat (Sultanate of Oman), and to the staff of the University of Ulster (United Kingdom), for their encouragement and support for this work. We also thank our families, who had largely extended their moral support during the last 2 years while preparing (editing) this book. This publication is a sincere effort made by us to put in a nutshell the vast subject matter of wastewater treatment, which is so vital in the twenty-first century. We are aware of the fact that this book may not be holistic in its approach; but still we feel we are richly rewarded if the publication meets at least partly the requirements of researchers, professionals, and young

Preface

ix

students working in the area of wastewater treatment. Since this book is an edited version of the works of so many authors in the field, we are afraid that there may be some mistakes or omissions. We request the readers to kindly bring them to the notice of the editors (e-mail addresses enclosed) by contacting us with their views and positive criticisms for the overall improvement of the book. D. G. Rao R. Senthilkumar J. Anthony Byrne S. Feroz

Contributors Mushtaque Ahmed College of Agricultural and Marine Sciences Sultan Qaboos University Al-Khod, Muscat, Sultanate of Oman Abdullah Al-Buloshi College of Agricultural and Marine Sciences Sultan Qaboos University Al-Khod, Muscat, Sultanate of Oman Ahmed Al-Maskary College of Agricultural and Marine Sciences Sultan Qaboos University Al-Khod, Muscat, Sultanate of Oman Isabel Oller Alberola Plataforma Solar de Almería Carretera Senés Tabernas, Spain J. Allen School of Engineering Deakin University Geelong, Australia Rakshit Ameta Department of Pure and Applied Chemistry University of Kota Kota, India

Suresh C. Ameta Department of Chemistry M.L. Sukhadia University Udaipur, India Mahad S. Baawain Department of Civil and Architectural Engineering Sultan Qaboos University Al-Khod, Muscat, Sultanate of Oman J. A. Byrne Nanotechnology and Integrated BioEngineering Centre University of Ulster Northern Ireland, UK P. Fernández-Ibáñez Plataforma Solar de Almería Carretera Senés Tabernas, Spain S. Feroz Caledonian College of Engineering Muscat, Sultanate of Oman V. Jegatheesan School of Engineering Deakin University Geelong, Australia and School of Engineering and Physical Sciences James Cook University Townsville, Australia xi

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Contributors

Sourish Karmakar School of Biochemical Engineering Banaras Hindu University Varanasi, India

J. Naylor School of Engineering Deakin University Geelong, Australia

Nikolaus Klamerth Plataforma Solar de Almería Carretera Senés Tabernas, Spain

R. Parthiban Sri Venkateswara College of Engineering Sriperumbudur, Chennai, India

Anil Kumar Department of Chemistry M.P. Government P.G. College Chittorgarh, India Arka Pravo Kundu Department of Mining Engineering Banaras Hindu University Varanasi, India

S. Pinchon School of Engineering Deakin University Geelong, Australia P. B. Punjabi Department of Chemistry M.L. Sukhadia University Udaipur, India

Kanika Kundu Chemistry Section Banaras Hindu University Varanasi, India

D. G. Rao Caledonian College of Engineering Muscat, Sultanate of Oman

Subir Kundu School of Biochemical Engineering Banaras Hindu University Varanasi, India

Sixto Malato Rodríguez Plataforma Solar de Almería Carretera Senés Tabernas, Spain

N. Meyyappan Sri Venkateswara College of Engineering Sriperumbudur, Chennai, India D. Navaratna School of Engineering Deakin University Geelong, Australia and School of Engineering and Physical Sciences James Cook University Townsville, Australia

E. Searston School of Engineering Deakin University Geelong, Australia R. Senthilkumar Caledonian College of Engineering Muscat, Sultanate of Oman H. K. Shon Faculty of Engineering University of Technology Sydney Broadway, Australia

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Contributors

L. Shu School of Engineering Deakin University Geelong, Australia Ana Zapata Sierra Plataforma Solar de Almería Carretera Senés Tabernas, Spain C. Teil School of Engineering Deakin University Geelong, Australia Joseph V. Thanikal Head, Built and Natural Environment department Caledonian College of Engineering Muscat, Sultanate of Oman M. Velan Department of Chemical Engineering Anna University Chennai, India

K. Vijayaraghavan Institute for Water and River Basin Management Department of Aquatic Environmental Engineering Karlsruhe Institute of Technology Karlsruhe, Germany and Singapore-Delft Water Alliance National University of Singapore Singapore J. Virkutyte Pegasus Technical Services Inc. Cincinnati, Ohio, USA Y. Wang School of Engineering Deakin University Geelong, Australia Z. P. Xu ARC Centre of Excellence for Functional Nanomaterials Australian Institute for BioEngineering and Nanotechnology The University of Queensland Brisbane, Australia

1 Introduction D. G. Rao, R. Senthilkumar, J. A. Byrne, and S. Feroz One of the greatest challenges of the twenty-first century would be to have an incessant supply of safe drinking water and clean air to breathe for the millions of living things all over the world. The major concern in this is not the depletion of air and water but the indiscriminate damage that is being done to them under the guise of industrial development. The day is not far off when they will become rare commodities. The problem being addressed in this book is concerned with the wastewater treatment. The worldwide concern for the depletion of global water sources is rising day by day. It is more than just the depletion of sources; with the everincreasing population and growing economy, demands for water are also continuously growing. Water sources, however, are not as abundant as they seem at first, since only in a very limited number of situations can available water be used without any treatment. A casual observation of the world map would suggest that the supply of water is endless since it covers over 80% of the earth’s surface. Unfortunately, however, we cannot use it directly since 97% is in the salty seas and oceans, 2% is tied up in the polar ice caps, and most of the remainder is beneath the earth’s surface. When a huge amount of water is required for different industrial processes, only a small fraction of the same is incorporated into their products and lost by evaporation; the rest finds its way into the water courses as wastewater. Wastewaters are those waters that emanate from (i) domestic sources, (ii) restaurants and establishments, and (iii) factories and industries. Of them, industries are the main polluters of natural bodies of water. Newer technologies lead to newer and more toxic wastes; these wastes take longer periods of time for decomposition, and most of the time, toxic wastes are deeply buried in the ocean or land. But this is far from a permanent solution as it degrades the earth. Newer technologies are being researched every day, but much less development has occurred in the field of waste treatment. The world depends on earth for disposal, but what will happen to earth. Little thought has been given to this. Recently, the world saw a major disaster in the Mexican Gulf, where BP (M/s British Petroleum) lost an oil well, creating an oil slick of millions of gallons and deeply endangering marine and human life nearby.

1

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Wastewater Treatment: Advanced Processes and Technologies

Anthropogenic activities include rapidly growing industrialization, series of new constructions, manyfold increases in transportation, aerospace movements, development and enhancement in technologies, that is, nuclear power, pharmaceutical, pesticides, herbicides, agriculture, etc. All of these are most desirable activities for human development and welfare but they also lead to the generation and release of objectionable materials into the environment. Thus, they pollute the whole environment, making our life on this beautiful earth quite miserable. The situation, if not controlled in a timely manner, could become a malignant problem for the survival of mankind on the earth. To have a neat, clean, healthy, and green environment, there is an urgent need to search for such an approach, which may be applicable at room temperature, safe to handle, economic, eco-friendly, and above all, the main requirement is that it should not be harmful to the environment in any manner. There are many sources of water pollution, but two general categories exist: direct and indirect contaminant sources. Direct sources include effluent outfalls from industries, refineries, waste treatment plants, etc. Indirect sources include contaminants that enter the water supply from soils/groundwater systems and from atmosphere via rain water. Soils and groundwater contain residues of human agricultural practices (fertilizers, pesticides, etc.) and atmospheric contaminants that come from various human practices (such as gaseous emissions from automobiles, factories, etc.). Pollutants in water include a wide spectrum of chemicals and pathogens, with different physical chemistries or sensory changes. There are a number of ways to treat wastewaters based on the type of contaminants. These various treatment methods can be conveniently classified into the following:

1. Physical methods 2. Chemical methods 3. Combination of physical and chemical methods 4. Biological methods

In general, contaminants are categorized into two broad classes, namely organic and inorganic. Some organic water pollutants include industrial solvents, volatile organic compounds (VOCs), insecticides, pesticides, dyes, and food processing wastes. Inorganic water pollutants include metals, fertilizers, acidity caused by industrial discharges, etc. There are three alternatives for the disposal of liquid wastes:

1. Direct disposal of wastes into streams without any treatment 2. Discharge of wastes into municipal sewers for combined treatment 3. Separate treatment of industrial wastes before discharging into water bodies

Introduction

3

The selection of a particular process depends on the self-purification capacity of streams, permissible levels of pollutants in water bodies, and the economic interests of both the municipalities and the industries. Depending on the mode of discharge of the waste and the nature of the constituents present in it, most of the treatments are based on conventional technologies, for example, equalization, neutralization, physical treatment, chemical treatment, and biological treatment. A number of water treatment technologies are desired to at least partially cleanse the water to serve the following purposes even though it is certain that the treated water cannot be as safe and pure as freshwater for potable purposes:

1. The treated water may be used for some other beneficial purposes. 2. The effluents do not mix directly with streams, lakes, and beaches and cause them to become polluted. 3. The treated water may be used for agricultural purposes. 4. In small quantities, the treated water may be used for raising kitchen gardens, horticultural crops, etc.

Most wastewater treatment processes cannot effectively respond to diurnal, seasonal, or long-term variations in the composition of wastewater. A treatment process that may be effective in treating wastewater during one time of the year may not be as effective at treating wastewater during another time of the year. Some of the major concerns of treated water for reuse are as follows:



1. How reliable are the treatment methods so that the treated water may be reused for the intended purpose, if not directly for the potable purpose for human consumption? 2. How safe is the water for protecting public health? 3. To what extent does the treated water gain public acceptance?

Nowadays, much attention is given to the treatment of industrial wastes, due to their growing pollution potential arising out of the rapid industrialization. Streams can assimilate certain amounts of waste before they are polluted, and a municipal sewage treatment plant can be designed to handle any kind of industrial waste. In addition to the treatment by municipalities, there is also an approach known as common effluent treatment plants (cetps), which is mostly in vogue in most of the industrial estates in India to treat the industrial wastewaters. These treatment plants are established in industrial areas. Effluents from some of the small-scale processing plants are transported to the CETP where they are treated to safe limits based on the following:

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Wastewater Treatment: Advanced Processes and Technologies

1. Composition of effluents 2. Type of processing plant 3. Time of delivery from processing plant

The CETPs are a wonderful concept in wastewater treatment and are especially helpful to small and medium industries that cannot afford to have a treatment plant of their own (Rao 2010, p. 410). However, such treatment plants can meet the requirements of a particular group of processing industries, namely pharmaceutical industries, textile industries, food processing industries, etc. They obviously may not be able to treat a wide range of effluent waters. But the effluents can be classified in terms of their pollutant constituents on the basis of some physicochemical parameters such as flow rate, pH, TSS (total soluble solids), COD (chemical oxygen demand), BOD (biological oxygen demand), etc. One such treatment plant in operation in India is in Vatva Industrial Estate (in Gujarat state), where the processing industries include dyes, dye intermediates, bulk drugs and pharmaceuticals, fine chemicals, and textiles. The characteristics of the effluents were consolidated by M/s Sudarshan Chemicals, Pune, based on which the design for CETP was made by M/s Advent Corporation, USA (Figure 1.1). The extended aeration technique in biological treatment process is the main criterion for treatment in this unit. A similar kind of effluent treatment plant, operating in the Industrial estate in Pattancheru (Hyderabad, India) and catering to the needs of local bulk drug and pharmaceutical manufacturing units, is Enviro-Tech Ltd. The unit works on the dissolved air floatation principle and was supplied by M/s Krofta Engineering (Krofta Technologies Corporation, USA). A coagulant (alum) is used along with a small dosage of a polyelectrolyte to coagulate the suspended solids (Rao 2010, p. 410). A special decanter is used to scoop the floated material (sludge) with the help of a patented “Krofta spiral scooper” and push it to the stationary central section from where it is discharged (Figure 1.2).

FIGURE 1.1  (See color insert) Common Effluent Treatment Plant in Vatva Industrial Estate in Gujarat (India).

Introduction

5

FIGURE 1.2  (See color insert) Krofta spiral scooper.

Major industries have their own wastewater and effluent treatment plants. Most of the chemical processing units release wastewaters in some form or the other. Some of the major categories of processing industries releasing effluents are summarized in Table 1.1. The contaminants in wastewaters released from any of the above-mentioned processing industries can be broadly classified as follows: • • • • • • • • • • • • • •

Particulates Suspended solids Soluble solids VOCs Organic materials Inorganic materials BOD components COD components Oils and fats Greases Proteins and proteinaceous materials Soluble vitamins and micronutrients Toxins and vaccines Microorganisms, bacteria, virus, etc

Hence, the treatments for them are also varied. The various treatment levels are as follows:

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Wastewater Treatment: Advanced Processes and Technologies

TABLE 1.1 Various Processing Industries with Possible Contaminants S. No.

Processing Industry

 1

Bioprocess industries

 2

Drugs and pharmaceutical industries Dyes and dye intermediates Fermentation processes

 3  4

Possible Contaminants in Wastewaters

Organic matter, soluble and Physical, chemical, and suspended particles, toxins biological methods, BOD reduction Organic matter, soluble and Physical, chemical, and suspended particles, biological methods, chemical contaminants neutralization Chemical contaminants Physical and chemical methods Organic matter, soluble and suspended particles

Fine chemicals Flavor and aromatic industries Food processing industries

Chemical contaminants Oils, fats, and greases

 8

Leather processing industries

Organic matter, soluble and suspended particles

 9

Man-made fiber industries Natural food colors and flavors

Chemical contaminants

 5  6  7

10

11

Nuclear thermal units

12

Paints and varnishes

13

Paper and pulp industries Petroleum refining and petrochemicals Sugar- and molassesbased industries

14 15

16

Textile industries

17

Vaccine manufacturing units

18

Vegetable oil mills

Remarks/Treatment Methods

Organic matter, soluble and suspended particles

Physical, chemical, and biological methods, BOD reduction Physical and chemical methods Physical and chemical methods Physical, chemical, and biological methods, BOD reduction (see Chapters 8, 9, and 11) Physical, chemical, and biological methods, BOD reduction Physical and chemical methods

Organic matter, soluble and suspended particles, chemical contaminants Nuclear wastes, suspended solids Chemical contaminants, oils, fats, and greases Alkalis, chemicals, dyes

Physical, chemical, and biological methods, BOD reduction Physical and chemical methods (see Chapter 8) Physical and chemical methods

Chemical contaminants, oils, fats, and greases Organic matter, soluble and suspended particles

Physical and chemical methods (see Chapter 8) Physical, chemical, and biological methods, BOD reduction Physical and chemical methods

Chemical contaminants, soluble and suspended particles Organic matter, soluble and suspended particles Chemical contaminants, oils, fats, and greases

Physical and chemical methods

Physical, chemical, and biological methods, BOD reduction Physical and chemical methods

7

Introduction

• • • •

Primary Secondary Tertiary Advanced tertiary processes

All these treatment methods utilize a number of separation processes that are classically known as unit operations (UOs). UOs are a set of physical separation processes that all can be broken down into a number of simple mathematical expressions that, on integration, will unify all of the processing operations. Various UOs are shown in the Table 1.2 for various combinations of phases (Rao 2010, p. 362). The information in the table is more general in nature and is particularly applicable to bioprocessing. Heavy metal ions present in the wastewaters of various chemical industries (listed in Table 1.1) have been noticed to have adverse effects on the performance of treatment methods, and hence their impact on the receiving environment needs careful consideration. Reckless and uncontrolled discharge of wastewaters containing heavy metals into the environment will pose detrimental effects to humans, animals, and plants. As a result, removal and recovery of heavy metals from industrial wastewaters before subjecting them to biological treatment have gained significant attention in recent years to protect the environment. Lead, mercury, chromium, cadmium, copper, zinc, nickel, and cobalt are the most frequently found heavy TABLE 1.2 Various Unit Operations for the Treatment of Suspended and Soluble Particulates in Wastewater Treatment System Solid–liquid

Liquid–liquid

Liquid– liquid–solid

Type

With Phase Change

Soluble

Adsorption

Insoluble

Dryinga

Soluble Insoluble

Distillationa, evaporationa, extractiona Air floatation, Foaming

Miscible

Adsorption

Immiscible Air floatation, foaming

Without Phase Change Flocculation Ultrafiltrationa Reverse osmosis Filtration Sedimentation Decanting Chromatographya Flocculation Sedimentation Centrifugation Centrifugation Sedimentation Decanting

Source: Rao, D.G., Introduction to Biochemical Engineering, 2nd edn, Tata McGraw Hill Education Pvt. Ltd, New Delhi, 2010. With permission. a Rarely used methods.

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Wastewater Treatment: Advanced Processes and Technologies

metals in industrial wastewaters. Methods used for removing heavy metals from wastewaters are also based on physical, chemical, and biological methods. Physicochemical methods such as precipitation, adsorption, ion exchange, and solvent extraction require high capital and operating costs and may produce large volumes of solid wastes, so these methods are often restricted because of technical and/or economic constraints. Among the TABLE 1.3 Treatment Methods/Equipment/Reactors Dealt with in the Book S. No.  1

 2  3

 4

 5

 6

 7

 8

 9

10

11

Treatment Methods/ Equipment/Reactor Treatment of greywaters by aeration, filtration, and disinfection Advanced oxidation processes (AOPs) Solar photocatalytic processes

Remarks The treated greywaters were used for irrigation purposes by pot trials in greenhouses with barley plantation.

Photo-Fenton AOPs for wastewater reclamation; overview of photo-Fenton processes Solar collectors and concentrators were described. Semiconductor photocatalysis was described to produce reactive O2 species for the destruction of organic contaminants and inactivation of microorganisms. Removal of lowAdsorption and absorption and use of molecular-weight nanoparticles to remove low-molecular-weight substances from substances that normally escape other methods. wastewaters It is a general review. Biological treatment of A general review covering biodegradation, wastewaters bioaccumulation, biosorption, and phytoremediation Biological treatment Used for the treatment of highly concentrated method using industrial wastewaters coming from a winery anaerobic fixed bed effluent reactor Anaerobic reactor for A tapered fluidized bed anaerobic reactor was the treatment of used for treating wastewaters coming from a wastewaters synthetic sago industry effluent using mesoporic activated carbon. Impinging-jet ozone Chemical reaction engineering aspects of bubble bubble column columns and use of neural network analysis for reactors modeling bubble column reactors Microbial treatment of A general review was given on wastewater wastewaters from treatment in mining industry, oil industries, and various process nuclear power plants. industries Wastewater treatment A general review on wastewater treatment in in food processing food processing industries industries Removal of heavy Continuous flow sorption studies in a glass metals column

Chapter No. 13

2 and 4 2 and 3

12

6

10

9

5

8

11

7

Introduction

9

various biological methods, biosorption has emerged as a cost-effective and efficient alternative treatment technology for heavy metals. Biosorption is the process of uptake of heavy metal ions and radio nuclides from aqueous solutions by biological materials. Different types of biomass in nonliving form are found to be suitable for the uptake of heavy metals. Bacteria, fungi, algae, plant leaves, and root tissues are used as biosorbents for the recovery of metals from industrial discharges (Chapters 6 and 7). Among these different types of biomass, seaweeds are extensively used for metal biosorption due to their high uptake capacities. In addition to the above classical physical and biological processes, we may also use membrane separation processes, reverse osmosis (RO), and ultrafiltration processes. However, their application in wastewater treatment is usually discouraged in view of their prohibitive costs and large quantities of wastewater to be handled. These processes are time-consuming and can be used at a small-scale level as in the case of downstream processing steps in chemical or bioprocessing industries. There are some advanced techniques, such as photocatalytic and photo-Fenton processes, which are being increasingly tried upon for wastewater treatments. The application of solar energy either in the form of photovoltaic effect or in a concentrated form is another emerging area used for wastewater treatments (see Chapters 2–4). The application of nanotechnology and nanoparticles for wastewater treatment is another fascinating area and has been attracting the attention of researchers in the recent years in wastewater treatment. Thus, wastewater treatment has many facets that need to be attended to in order to cleanse the wastewaters and make them as pure as possible. The benchmark is to make them fully potable. If not, they at least should be used for agricultural purposes and various other non-potable purposes. The approach (cleaning process) protects the environment from the contaminants of wastewaters. This book addresses some of these issues covering a wide range of wastewaters produced from different processing industries by utilizing a variety of treatment methods. Some are traditional methods, while others are advanced processes. The treatment methods also use a wide variety of equipment for various UOs, solar panels, solar heaters, and photoFenton processes, while others use a wide variety of biochemical reactors for the biological treatment of wastewaters. They are all summarized in a nutshell in Table 1.3.

Reference Rao, D.G. 2010. Introduction to Biochemical Engineering, 2nd edn. New Delhi: Tata McGraw Hill Education Pvt. Ltd., pp. 362, 410.

2 Solar Photo-Fenton as Advanced Oxidation Technology for Water Reclamation Sixto Malato Rodríguez, Nikolaus Klamerth, Isabel Oller Alberola, and Ana Zapata Sierra CONTENTS 2.1 Introduction................................................................................................... 11 2.2 Solar Photo-Fenton....................................................................................... 14 2.2.1 Fenton and Photo-Fenton................................................................. 14 2.2.2 Solar Photocatalysis Hardware....................................................... 17 2.2.2.1 Compound Parabolic Concentrators............................... 19 2.3 Experimental Setup...................................................................................... 21 2.3.1 Solar Pilot Plant................................................................................. 21 2.3.2 Reagents.............................................................................................22 2.3.3 Analytical Measurements................................................................ 25 2.3.4 Experimental Procedures................................................................ 26 2.4 Results and Discussion................................................................................ 26 Acknowledgment................................................................................................... 33 References................................................................................................................ 33

2.1  Introduction During the last 30 years, environmental chemistry has concentrated almost exclusively on “conventional pollutants,” mainly pesticides and a large number of industrial chemicals. Nevertheless, in terms of the large number of commercial chemicals, these are only a small percentage of the pollutants found in the environment (Daughton and Ternes 1999). Over the recent decades, biologically active synthetic substances for use in agriculture, industry, and medicine have been dumped into the environment without any consideration for possible negative consequences. Recently, at the end of 2008, the European Commission approved a new Directive (2008/105/EC) on environmental quality standards in the field of water policy. The new directive considers the identification of the causes of chemical pollution of surface waters and the dealing with emissions at the sources, in the most 11

12

Wastewater Treatment: Advanced Processes and Technologies

economically and environmentally effective manner as a matter of priority. Concerning the 33 priority substances and priority hazardous substances (alachlor, anthracene, atrazine, benzene, and so on), Directive 2008/105/EC expresses the environmental quality regulations in terms of annual averages, providing protection against long-term exposure and maximum permissible concentrations for short-term exposure. Apart from the priority pollutants with established risk, there are hardly any studies on most of the organic compounds, and there are no environmental quality criteria for them yet. Current developments in analytical techniques, such as gas chromatography–mass spectrometry (GC-MS or GC-MS/MS) and liquid chromatography–mass spectrometry (LC-MS or LC-MS/MS), have made the detection and analysis of many of these new organic compounds in the environment, the analysis of which was hitherto difficult, possible (Petrovic and Barceló 2006; Hogenboom et al. 2009; Pietrogrande and Basaglia 2007; Gómez et al. 2009). “Emerging contaminants” (ECs) are defined as a group of unregulated substances that could be candidates for future regulation, depending on the findings of research on their effects on human health and aquatic biota and surveillance data on the frequency of their presence in the environment. A wide range of compounds, for example, detergents, pharmaceuticals, personal hygiene products, flame retardants, antiseptics, industrial additives, steroids, and hormones, have recently been found to be particularly relevant. The main characteristic of these pollutants is that they do not need to be persistent to cause negative effects, because their rates of removal are compensated by their constant introduction into the environment. The increasing use of these substances directly increases their concentration in treated and other waters (Fono et al. 2006; Jackson and Sutton 2008; Nakada et al. 2008), as conventional wastewater treatment plants are not able to remove them entirely (Göbel et al. 2007; Teske and Arnold 2008). As most of these ECs have xenobiotic, endocrine-disrupting, nonbiodegradable, toxic, or persistent properties, they must be degraded and removed prior to their release into the environment. This is even more important if the water is reused for irrigation, as these contaminants could accumulate in soil and crops (Radjenović et al. 2007; Muñoz et al. 2009; Snow et al. 2007). Traditionally, wastewater treatment has focused on pollution abatement, public health protection, and environmental protection by removing biodegradable materials, nutrients, and pathogens (Levine and Asano 2004). At present, wastewater reclamation is one of the tools available to better manage the water resources diverted from the natural water cycle to the anthropic cycle. The way water is reused should always be linked to health protection, public acceptance, and its perceived value in the community. The main objective of wastewater reclamation and reuse projects is to produce water of sufficient quality for all nonpotable uses (uses that do not require the standards of drinking water). Using reclaimed water for these applications would save significant volumes of freshwater that would otherwise be wasted (Sala and

Solar Photo-Fenton for Water Reclamation

13

Serra 2004). Reusable water should be free of these persistent, toxic, endocrinedisrupting, or nonbiodegradable substances (Radjenović et al. 2007; Teske and Arnold 2008); hence, an effective tertiary treatment method is required to remove these substances completely. Conventional municipal wastewater treatment plants (MWTPs), typically based on biological processes, are capable of removing some substances, but nonbiodegradable compounds may escape the treatment and be released into the environment (Carballa et al. 2004; Ternes et al. 2007). Antibiotic drugs have been identified as a particular category of trace chemical contaminants (Le-Minh et al. 2010). Much of the concern regarding the presence of antibiotics in wastewater and their persistence through wastewater treatment processes is related to the concern that they may contribute to the prevalence of resistance to antibiotics in bacterial species in wastewater effluents and surface water near wastewater treatment plants (Auerbach et al. 2007; Jury et al. in press). ECs and priority substances have been found in the MWTP effluents at mean concentrations ranging from 0.1 to 20 μg/L (Martínez Bueno et al. 2007; Richardson 2007; Zhao et al. 2009). Concern about the growing problem of the continuously rising concentrations of these compounds must be emphasized, and therefore, the application of more thorough wastewater treatment protocols, including the use of new and improved technologies, is a necessary task. Conventional secondary wastewater treatment processes appear to be highly variable in their ability to remove most of these compounds, with their performance apparently dependent upon specific operational conditions, such as the specific retention time (SRT). Accordingly, tertiary and advanced treatment processes may be necessary to provide a further reduction of these compounds, in order to minimize environmental and human exposure. Among the advanced processes that can degrade these ECs, advanced oxidation processes (AOPs) are a particularly attractive option (Westerhoff et al. 2009; Klavarioti et al. 2009). Although there are different reacting systems (see http://www.jaots.net/), all of them are characterized by the same chemical feature: production of hydroxyl radicals (∙OH), which can oxidize and mineralize almost any organic molecule, yielding CO2 and inorganic ions. Rate constants (kOH) for the formation of ∙OH radicals by the rate expression (r = kOH [∙OH] C) for most reactions involving hydroxyl radicals in aqueous solution are usually of the order of 106−109 M−1/s. They are also characterized by their nonselective attack, which is a useful attribute for wastewater treatment to solve pollution problems. The versatility of AOPs is also enhanced by the fact that there are different ways of producing hydroxyl radicals, facilitating compliance with the specific treatment requirements. Methods based on UV, H2O2/UV, O3/UV, and H2O2/O3/UV combinations use the photolysis of H2O2 and ozone to produce the hydroxyl radicals. Heterogeneous photocatalysis and homogeneous photo-Fenton are based on the use of a wide-bandgap semiconductor and the addition of H2O2 to dissolved iron salts, respectively, and the irradiation with UVA-visible light (Pignatello et al. 2006; Comninellis et al. 2008; Shannon et al. 2008).

14

Wastewater Treatment: Advanced Processes and Technologies

Some of the disadvantages associated with AOPs are their high operating costs depending on the specific process: (i) high electricity demand (e.g., ozone and UV-based AOPs), (ii) relatively large amounts of oxidants and/or catalysts consumed (e.g., ozone, hydrogen peroxide, and iron-based AOPs) and slow kinetics (photocatalysis with TiO2), and (iii) the pH required (e.g., Fenton and photo-Fenton). By using solar energy as a light source, optimizing the pH, and optimizing the amounts of oxidant/catalyst, processes such as photo-Fenton may be used for commercial applications. AOP efficiency in the removal of ECs has typically been studied in demineralized water at bench scale in the initial concentration range of few milligrams to grams. This may not be realistic compared with the concentrations detected in real water and wastewaters. Hence, this work focused on solar photo-Fenton degradation of ECs typically found in the effluents of MWTPs, leaving the treated wastewater suitable for reuse. Moreover, to make the process suitable for practical applications, high iron concentrations (mM range) and excessive amounts of H2O2 were avoided. The results presented were obtained in a pilot-scale solar photo-Fenton treatment plant run with starting concentrations of 5 mg/L Fe and 50 mg/L H2O2. Real effluent wastewaters (REs) to which a mixture of 15 ECs at low concentrations, consisting of pharmaceuticals, pesticides, and personal-care products, selected from a list of 80 compounds found in MWTP effluents in previous studies (Martínez Bueno et al. 2007), was added (100 μg/L or 5 μg/L each were tested in this study). RE without spiking with any EC was also tested and evaluated by LC-MS. Water reuse is required to deal not only with ECs but also with the potential problems of pathogens. Therefore, the preliminary results of the removal of pathogens are also presented.

2.2  Solar Photo-Fenton 2.2.1  Fenton and Photo-Fenton For the treatment of industrial wastewaters, Fenton and Fenton-like processes are probably among the most applied AOPs (Legrini et al. 1993; Suty et al. 2004). This is not the case for studies related to ECs degradation as tertiary treatment in MWTPs, but this information will be discussed in detail later. The first proposals for wastewater treatment applications were reported in the 1960s. Yet, it was not until the early 1990s when first works of the application of the photo-Fenton process for the treatment of wastewater were published by the groups of Pignatello, Lipcznska-Kochany, Kiwi, Pulgarín, and Bauer (Pignatello et al. 2006). Much of the literature that deals with the photo-Fenton process takes into account the possibility of driving the process with solar radiation. This is due to the fact that a priori the photoFenton process seems to be the most apt of all AOPs to be driven by sunlight, because soluble iron hydroxide (and especially iron–organic acid complexes)

15

Solar Photo-Fenton for Water Reclamation

absorbs even part of the visible light spectrum (Malato et al. 2009). Though several excellent and comprehensive reviews on the process exist (Neyens and Baeyens 2003; Pignatello et al. 2006), we will give a short summary of the principles of reactions that occur in the photo-Fenton system for the sake of completeness and clarity of the following discussion. Hydrogen peroxide is decomposed into water and oxygen in the presence of iron ions in an aqueous solution in the Fenton reaction, Equation 2.1, which was first reported by Fenton (1894). A mixture of ferrous iron and hydrogen peroxide is called Fenton's reagent. If ferrous is replaced by ferric iron, then the mixture is called Fenton-like reagent. Equations 2.1 through 2.7 show the reactions of ferrous iron, ferric iron, and hydrogen peroxide in the absence of other interfering ions and organic substances. The regeneration of ferrous iron from ferric iron, shown in Equations 2.4 through 2.6, is the rate limiting step in the catalytic iron cycle, if iron is added in small amounts.

Fe2 + + H 2O 2 → Fe3 + + OH − + OH•,

(2.1)



Fe2 + + OH• → Fe3 + + OH − ,

(2.2)



Fe2 + + HO •2 → Fe3 + + HO 2− ,



Fe3 + + H 2O 2 → Fe2 + + HO •2 + H + ,

(2.4)



Fe3 + + HO•2 → Fe2 + + O 2 + H + ,

(2.5)

2+ Fe3 + + O•− + O 2, 2 → Fe

(2.6)







(2.3)







OH• + H 2O 2 → H 2O + HO•2.



(2.7)

Furthermore, radical–radical reactions (Equations 2.8 through 2.10) have to be taken into account:

2OH• → H 2O 2,



2HO•2 → H 2O 2 + O 2,



HO•2 + OH• → H 2O + O 2.

(2.8)



(2.9)



(2.10)

If organic substances (such as quenchers, scavengers, and pollutants in the case of wastewater treatment) are present in the system Fe2+/Fe3+/H2O2, they

16

Wastewater Treatment: Advanced Processes and Technologies

react in many ways with the generated hydroxyl radicals. Yet, in all cases, the oxidative attack is electrophilic and the rate constants are close to the diffusion-controlled limit. The following reactions with organic substrates have been reported (Legrini et al. 1993): hydrogen abstraction from aliphatic carbon atoms (Equation 2.11), electrophilic addition to double bonds or aromatic rings (Equation 2.12), and electron transfer reactions (Equation 2.13).

OH• + RH → R • + H 2O ,

(2.11)



R − CH = CH 2 + OH• → R − C•H − CH 2OH ,

(2.12)



OH• + RX → RX •+ + OH − .

(2.13)





The generated organic radicals continue reacting, prolonging the chain reaction, and thereby contribute to reduce the consumption of oxidants in wastewater treatment by Fenton and photo-Fenton methods. In the case of aromatic pollutants, the ring system is usually hydroxylated before it is broken up during the oxidation process. Substances containing quinone and hydroquinone structures are typical intermediate degradation products. Anyway, sooner or later, ring-opening reactions occur, which further carry on the mineralization of the molecules (Chen and Pignatello 1997). But there is one major setback of the Fenton method: especially when the treatment goal is the total mineralization of organic pollutants, the carboxylic intermediates cannot be further degraded. Carboxylic and dicarboxylic (L: monocarboxylic and dicarboxylic acids) acids are known to form stable iron complexes, which inhibit the reaction with peroxide (Kavitha and Palanivelu 2004). Hence, the catalytic iron cycle reaches a standstill before total mineralization is accomplished, as shown in Equation 2.14.

Fe3 + + nL → [FeL n ]

x+

2 O 2 , dark H → no further reaction .



(2.14)

In the photo-Fenton system, the primary step in the photoreduction of dissolved ferric iron is a ligand-to-metal charge-transfer (LMCT) reaction. Subsequently, the intermediate complexes dissociate as shown in reaction 2.15. The ligand can be any Lewis base that is able to form a complex with ferric iron (OH−, H2O, HO2−, Cl−, R–COO−, R–OH, R–NH2, etc.). Depending on the reacting ligand, the product may be a hydroxyl radical, such as the ones shown in reactions 2.16 and 2.17, or another radical derived from the ligand. The direct oxidation of an organic ligand as well is possible, as shown in reaction 2.18, for carboxylic acids.

[Fe3 + L ] + hν → [Fe3 + L ]* → Fe2+ +

L•,

(2.15)

Solar Photo-Fenton for Water Reclamation

3+

17



Fe (H 2O ) + hν → Fe2 + + OH• + H +,

(2.16)



[Fe (OH)]2+ + hν → Fe2+ + OH•,

(2.17)



[Fe (OOC − R )]2+ + hν → Fe2+ + CO 2 + R •.

(2.18)

Depending on the ligand, the ferric iron complexes will have different light absorption properties. Hence, reaction 2.15 takes place with different quantum yields and also at different wavelengths. Consequently, the pH plays a crucial role in the efficiency of the photo-Fenton reaction, because it strongly influences the complexes that are formed. Thus, a pH of 2.8 was frequently postulated as an optimum pH for photo-Fenton treatment (e.g., Pignatello 1992), because at this pH, precipitation does not take place and the dominant iron species in solution is [Fe(OH)]2+, which is the most photoactive ferric iron–water complex. In fact, as shown in its general form in reaction 2.15, ferric iron can form complexes with many substances and undergo photoreduction. Of special importance are carboxylic acids, because they are the intermediate products frequently produced in an oxidative treatment. Such ferric iron–carboxylate complexes can have much higher quantum yields than ferric iron–water complexes. It is, therefore, a typical observation that a reaction shows an initial lag phase, until intermediates are formed, which can regenerate ferrous iron from ferric iron more efficiently by accelerating the process. This behavior is observed in most of the degradation results shown in the following sections. Fe(III) complexes present in mildly acidic solutions, such as Fe(OH)2+, absorb light appreciably in the UV and visible regions. The quantum yield for Fe2+ formation in reaction 2.17 is dependent on the wavelength. It is 0.14–0.19 at 313 nm and 0.017 at 360 nm (Faust and Hoigne 1990). Fe(III) may also complex with certain contaminants or their organic by-products. These organic complexes typically have higher molar absorption coefficients in the near-UV and visible regions than the aquo complexes. Polychromatic quantum efficiencies in the UV/visible range from 0.05 to 0.95 are common (Pignatello et al. 2006). This is why the photo-Fenton process is apt to be driven by sunlight. 2.2.2  Solar Photocatalysis Hardware For many of the solar detoxification system components (Blanco and Malato 2003), the equipment used is identical to that used for other types of water treatment, and the construction materials for such treatments are commercially available. Most piping may be made of polyethylene or polypropylene, avoiding the use of metallic or composite materials that could be degraded by the oxidation conditions of the photocatalytic process. The materials must

18

Wastewater Treatment: Advanced Processes and Technologies

not be reactive and must not interfere with the photocatalytic process. All materials used must be inert to degradation by UV solar light, in order to be compatible with the minimum required lifetime of the system. Photocatalytic reactors must transmit UV light efficiently because of the process requirements. With regard to the reflecting/concentrating materials, aluminum is the best option because of its low cost and high reflectivity in the solar UV spectrum on the earth's surface. The reflectivity (reflected radiation/incident radiation) of traditional silver-coated mirrors is very low (between 300 and 400 nm) and, therefore, aluminum-coated mirrors are the best option in this case. Aluminum-coated surface is the only metal surface that is highly reflective throughout the ultraviolet spectrum. For aluminum, the reflectivity ranges from 92.3% at 280 nm to 92.5% at 385 nm, while the reflectivity values for silver are 25.2% and 92.8%, respectively. The photocatalytic reactor must be transparent to UV radiation. The choice of materials that are both transmissive to UV light and resistant to its destructive effects is limited. Common materials that meet these requirements are fluoropolymers, acrylic polymers, and several types of glass. Quartz has excellent UV transmission as well as good temperature and chemical resistance, but high costs make it completely unfeasible for photocatalytic applications. Fluoropolymers are a good choice of plastic for photoreactors because of their good UV transmittance, excellent ultraviolet stability, and chemical inertness. But one of their greatest disadvantages is that, in order to achieve a desired minimum pressure resistance, the wall thickness of a fluoropolymer tube has to be increased, which in turn will lower its UV transmittance. Acrylics could also be potentially used, but they are very brittle. Other low-cost polymeric materials are significantly more susceptible to attack by ∙OH radicals. Standard glass, used as a protective surface, is not satisfactory because it absorbs part of the incident UV radiation due to its iron content. Borosilicate glass has good transmissive properties in the solar range with a cutoff of about 285 nm (Blanco et al. 2000). Therefore, such a lowiron-content glass would seem to be the most adequate one. Therefore, both fluoropolymers and glasses are valid photoreactive materials. The original solar photoreactor designs (Goswami 1995) for photochemical applications were based on line-focus parabolic-trough concentrators (PTCs). In part, this was a logical extension of the historical emphasis on trough units for solar thermal applications. Furthermore, PTC technology was relatively mature, and the existing hardware could be easily modified for photochemical processes. The main disadvantages are that these collectors (i) use only direct radiation, (ii) are expensive, and (iii) have low optical efficiencies. On the other hand, one-sun (nonconcentrating) collectors have no moving parts or solar tracking devices. They do not concentrate the radiation. So, efficiency is not reduced by the factors associated with concentration and solar tracking. As there is no concentrating system (with its inherent reflectivity), the optical efficiency of these collectors is higher as compared with PTCs. They are able to utilize both the diffuse and direct portions of

19

Solar Photo-Fenton for Water Reclamation

the solar UV-A. An extensive effort in the design of small nontracking collectors has resulted in the testing of several different nonconcentrating solar reactors (Blanco-Galvez et al. 2007). Although one-sun collector designs possess important advantages, the design of a robust one-sun photoreactor is not trivial, due to the need for weather-resistant and chemically inert UV transmitting reactors. In addition, nonconcentrating systems require significantly more photoreactor area than concentrating photoreactors. Hence, as a consequence, full-scale systems must be designed to withstand the operating pressures of fluid circulation. 2.2.2.1  Compound Parabolic Concentrators To design a solar collector for photocatalytic purposes, there is a set of main constraints for performing the optimization: (1) collection of UV radiation, (2) working temperatures as close as possible to ambient temperature, and (3) quantum efficiency. Finally, its construction must be economical and should be efficient, with a low pressure drop. As a consequence, the use of tubular photoreactors has a decisive advantage because of the inherent structural efficiency of the tubing. The tubing is also available in a large variety of materials and sizes and is a natural choice for a pressurized fluid system. There is a category of low-concentration collectors called compound parabolic concentrators that are used in thermal applications. They are an interesting option between parabolic concentrators and static flat systems. Thus, they also constitute a good option for solar photochemical applications (Ajona and Vidal 2000). Compound parabolic collectors (CPCs) are static collectors with reflective surfaces designed to be ideal in the sense of nonimaging optics and can be designed for any given reactor shape. They do so, illuminating the complete perimeter of the receiver, rather than just the “front” of it, as in conventional flat plates. These concentrating devices have ideal optics, thus maintaining the advantages of both the PTCs and the static systems (Colina-Márquez et al. 2009). The concentration factor (RC) of a two-dimensional CPC collector is given by Equation 2.19 and is defined in Figure 2.1, where A is the aperture of the solar collector. RC,CPC =

1 A = . sin θa 2πr

(2.19)

The normal values for the semiangle of acceptance (θa), for photocatalytic applications, are between 60° and 90°. A special case is the one in which θa = 90°, whereby RC = 1 (nonconcentrating solar system). When this occurs, all the UV radiation that reaches the aperture area of the CPC (direct and diffuse) can be collected and redirected to the reactor. If the CPC is designed for an acceptance angle of +90° to −90°, all incident solar diffuse radiation can be collected (Figure 2.1). The light reflected by the CPC is distributed all

20

Wastewater Treatment: Advanced Processes and Technologies

θa

A

r Photoreactor

θa

A

r Photoreactor

θa

A

r Photoreactor

FIGURE 2.1 Schematic drawing of CPC with a semiangle of acceptance of 90° under different solar angles.

around the tubular receiver so that almost the entire circumference of the receiver tube is illuminated. CPCs have the advantages of both technologies (PTCs and nonconcentrating collectors) and none of the disadvantages, so they seem to be the best option for photocatalytic processes based on the use of solar radiation. They can make highly efficient use of both direct and diffuse solar radiations, without the need for solar tracking. One important factor related to the photoreactor design is its diameter. It seems obvious that a uniform flow must be maintained at all times in the reactor, since a nonuniform flow causes nonuniform residence times, which can lower the efficiency when compared with the ideal. As already commented,

Solar Photo-Fenton for Water Reclamation

21

the Fenton reactant consists of an aqueous solution of hydrogen peroxide and ferrous ions providing hydroxyl radicals. When the process is complemented with UV/visible radiation, it is called photo-Fenton. In this case, the process becomes catalytic. Fe3+ (related species and organic complexes) absorbs solar photons as a function of its absorptivity. This effect must be considered when determining the optimum load as a function of light-path length in the photoreactor. The optimum concentrations of 0.2–0.5 mM Fe as a function of the photoreactor diameter have been proposed after many experiments with different photoreactors under sunlight at the Plataforma Solar de Almeria (PSA) installation (Malato et al. 2009). In the results shown in this chapter, attending to the diameter of the photoreactor used, 0.35 mM mg/L of Fe has been used.

2.3  Experimental Setup 2.3.1  Solar Pilot Plant Experiments were performed in a pilot CPC solar plant designed at the Plataforma Solar de Almería for solar photocatalytic applications (Figure 2.2). This reactor is composed of two modules (11 L each) with 12 Pyrex glass tubes (30 mm O.D.) mounted on a fixed platform tilted to 37° (local latitude). The water flows (20 L/min) directly from one module to the other and finally to a reservoir tank (10 L). The material chosen for the piping and the valves (3 L) between the reactor and the tank is black high-density polyethylene (HDPE) because it is highly resistant to chemicals, weather-proof, and opaque to avoid any photochemical effect outside of the collectors. The total illuminated area is 3 m2. Polished aluminum is used as the reflective material because of its high UV reflectivity in the concerned UV range of 300–400 nm. The total volume (two modules + reservoir tank + piping and valves) is 35 L (VT), and the irradiated volume is 22 L (Vi). The incident solar ultraviolet radiation (UV) was measured by a global UV radiometer (KIPP&ZONEN, model CUV 3) mounted on a platform tilted to 37° (the same as CPCs). The temperature inside the reactor was continuously recorded by a PT-100 inserted in the piping. With Equation 2.20, a combination of the data obtained from several days’ experiments and their comparison with those obtained from other photocatalytic experiments are possible, where tn is the experimental time for each sample, UV is the average solar ultraviolet radiation (λ 80%. This procedure was used also for RE (without any spiking) evaluation. The method for the analysis of the target compounds with HPLC-QTRAP-MS (Martínez Bueno et al. 2007) was developed for the 3200 QTRAP-MS/MS system (Applied Biosystems, Concord, ON, Canada). The separation of the analytes was performed using an HPLC (series 1100, Agilent Technologies, Palo Alto, CA) equipped with a reversed-phase C-18 analytical column (Zorbax SB, Agilent Technologies) of 5 μm particle size, 250 mm length, and 3.0 mm-i.d. For the analysis in positive mode, the compounds were separated using acetonitrile (mobile phase A) and HPLC-grade water with 0.1% formic acid (mobile phase B) at a flow rate of 0.2 mL/min. A linear gradient progressed from 10% A (initial conditions) to 100% A in 40 min, after which the mobile-phase composition was maintained at 100% A for 10 min. The reequilibration time was 15 min. The compounds analyzed in the negative mode were separated using

26

Wastewater Treatment: Advanced Processes and Technologies

acetonitrile (mobile phase A) and HPLC-grade water (mobile phase B) at a flow rate of 0.3 mL/min. An LC gradient started with 30% mobile phase A and was linearly increased to 100%, in 7 min, after which the mobile-phase composition was maintained at 100% A for 8 min. The reequilibration time was 10 min. The injection volume was 20 μL in both modes. 2.3.4  Experimental Procedures Three approaches were used: (i) spiking RE with 100 μg/L of each contaminant; (ii) spiking the RE with 5 μg/L of each contaminant as the typical EC concentrations in the effluent are in the 0.1–15.0 μg/L range, with an SPE follow-up (see below for details) in which the samples were preconcentrated 100-fold; and (iii) treating the RE and analyzing the EC devolvement with HPLC-QTRAP-MS after the same SPE preconcentration. The mixture of the 15 compounds dissolved in methanol at 2.5 g/L (mother solution) was added directly into the pilot plant and well homogenized by turbulent recirculation for 30 min. The pH in the RE was between 7.1 and 8.5, depending on the day when the water was collected, and the recirculation time for this process was usually from 60 to 120 min. After stabilizing the desired pH, H2O2 at a concentration of 50 mg/L was added and homogenized by recirculating for 15 min. Finally, FeSO4 · 7H2O was added (Fe2+ = 5 mg/L). After recirculating for 15 min, during which the Fenton reaction started, the collectors were uncovered and the photo-Fenton process began. The hydrogen peroxide and iron concentrations were measured in every sample taken. The experiments normally lasted 3–4 h. The peroxide was sometimes consumed completely and 10 mg/L more of it was added at a time during the tests.

2.4  Results and Discussion Figure 2.3 shows the photo-Fenton treatment of RE. The initial DOC, TIC, and COD were 36 mg/L, 106 mg/L, and 60 mg/L, respectively. In this case, 406 mg/L H2SO4 was added to reach