THE INFLUENCE OF NATURAL ORGANIC SUBSTANCES ON COAGULATION AND FLOCCULATION PROCESSES. Prakash Raj Lamsal

Transcription

1 THE INFLUENCE OF NATURAL ORGANIC SUBSTANCES ON COAGULATION AND FLOCCULATION PROCESSES by Prakash Raj Lamsal A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering Examination Committee Dr. C. Visvanathan (Chairman) Prof. Y. Ishibashi Dr. S. Takizawa Nationality Previous Degree(s) Nepalese B. Sc. (Civil Engineering) Bangladesh University of Engineering and Technology Dhaka, Bangladesh Scholarship Donor Her Majesty Queen Sirikit Environmental Scholarship Program (RTG) Asian Institute of Technology School of Environment, Resources and Development Bangkok, Thailand August i-

2 Abstract The aim of this research is to identify the influence of presence of natural organic matter in coagulation and flocculation processes. The natural water composed of nonhomogenous organic compounds; namely, humic substances, amino acids, sugars, aliphatic and aromatic acids and other chemical synthetic organic matter. These are the main reasons to have a different organic removal efficiency in natural water than synthetic water containing homogenous organic matter, like humic acid or fulvic acid only. As a result, removal efficiency for natural organic matter reduced to 60% from 80% for synthetic water containing humic acid only at TOC 7 mg/l and ph 7. Whereas the presence of secondary compounds, like Ca ++, Mg ++ divalent cations, which are responsible for exerting hardness, have no significant effect on the removal of organic matter. It varied from 58% to 60% and 55% to 62% for Ca ++ and Mg ++, respectively. However, the adsorption of organic matter on clay caused increase in removal of organic matter from 60% to 75%. Similarly Molecular size characteristics of natural organic compound in raw water were investigated at optimum conditions. About 64% of higher molecular weight organic compounds reduced to 7%, whereas the remaining 36% of lower molecular weight organic compounds reduced to 26% only. Hence, organic compound with high molecular weight has higher removal affinity in coagulation process. In this study, alum were studied using jar tests to determine the optimum coagulant dosages, ph and residual aluminum for maximum removal of organic concentration. UV Absorbance at 260 nm was used as surrogate parameter for estimating indirect measure of the removal efficiency of the organic concentration. The optimum alum dose was increased from 140 mg/l to 1225 mg/l with increase in TOC of 4 mg/l to 25 mg/l, respectively. In all cases of TOC, the ph of 7 was found to be the optimum for efficient coagulation. Whereas, residual aluminum of less than 0.2 mg/l can be achieved if the ph in water varies from 7 to 9 at TOC of 7 mg/l. The study of floc charaterization explained that with increasing of TOC from 4 mg/l to 25 mg/l in natural water, the settling velocity was increased by more than 350%. The CST test concluded that the floc formed with lower TOC concentration readily released the water. Thus, dewatering of floc will be easier than the floc formed with higher TOC concentration. -ii-

3 Acknowledgment The author would like to express his sincere appreciation and profound gratitude to the chairperson of his thesis examination committee, Dr. C. Visvanathan for his extensive guidance, suggestion, encouragement and painstaking effort to edit report throughout the period of the research. The author also wishes to extend his sincere gratitude to Prof. Y. Ishibashi and Dr. S. Takizawa, members of examination committee for their valuable guidance in his research. The author is extremely grateful to had Dr. T. Kamei as a committee member and would like to express his heartfelt thanks for utmost help and suggestions at the beginning of the research work. The author is indebted to the Royal Thai Government for financially supporting his study. He is very grateful to the Asian Institute of Technology for providing opportunity to pursue post graduate study. Thanks are also due to all the faculties and staffs of Environmental Engineering Program. The completion of this thesis work has been made possible by skill contribution, dedications, suggestions as well as friendship of many individuals namely; Cheema, Hari Pradhan, Iwan, Jugal, Kazi, Kriang, Kuwat, Manju, Nilapha, Para, Samir and Surasak. The author would like to express his deep appreciation to all his friends and classmates, especially papa group, for making his study in AIT meaningful and memorable. The author wishes to express his heartfelt gratitude to his beloved parents for their hearty blessings and affection. Sincere gratitude are also extended to his brother, sister, in laws and other family members for their unreserved love and affection and sacrifices during his long absence from home. Finally, special appreciation and thanks to his dear wife Samjhana and lovely daughter Sakshi Lamsal for their continuous inspiration, encouragement love and supports during the period of this study. -iii-

4 Table of Contents Chapter Title Page Title Page Abstract Acknowledgements Table of Contents List of Abbreviations List of Figures List of Tables i ii iii iv vi vii ix 1 Introduction 1.1 Background of the Study Objectives of the Study Scope of the Study 2 2 Literature Review 2.1 Organic Constituents in Natural Water Removal of Organic by Coagulation Process Mechanism of Coagulation Clay - Organic Interaction in Coagulation Pesticides - Organics Interaction in Coagulation Chemical Coagulation with Alum ph Depression Coagulant Dosage UV Absorbance as Surrogate Parameters Characteristics of Floc Measurement of Floc Size and Settling Velocity Calculation of Floc Density Fundamental Structure of Floc Settling Characteristics of Floc 18 3 Experimental Methodology 3.1 Outline of Research Water Source Chemicals Used Chemical Preparation Analytical Methods Experimental Procedure Optimization of Coagulation and Flocculation Processes Floc Settling Velocity Measurement 24 4 Results and Discussions -iv-

5 4.1 Raw Water Characteristics and Pre-Treatment Calibration of TOC with UV Absorbance of Raw Water Effect of Coagulation and Flocculation Processes on TOC Removal Effect of Alum Dose Effect of ph Effect of Clay Residual Aluminum Effect of Synthetic Organic Matter Effect of Secondary Compounds Effect of TOC Relationship of Operating Parameters with TOC Floc Characteristics Physical Characteristics of Floc Settling Characteristics Effect of Clay on Floc Characteristics Effect of TOC on Floc Characteristics Influence of Floc Characteristics on Sedimentation Tank Dewatering Characteristics of Floc Effect of Molecular Weight Distribution on Coagulation Process Design of Sedimentation tank using Excel 45 5 Conclusions 47 6 Recommendations 49 References 50 Appendix A 53 Appendix B 68 Appendix C 69 Appendix D 73 -v-

6 List of Figures Figure No. Title Page 2.1 Range of TOC Reported for a Variety of Natural Waters Effects of Coagulation ph on Organic Matter Remaining and Iron Res Mechanism of Coagulation Coagulation with Aluminum Sulfate Stepwise Conversion of the Tri-Positive Aluminum Ion to the Negative Aluminates Reaction Schematics of Coagulation Alum Dose as Function of HA Conc. at ph Stoichemtry of Coagulation of HA with Alum at ph Correlation between Raw Water NPTOC and Raw Water TOTLUV Schematic of Floc Formation Logarithm Plot of the Floc Dia. and Floc Effective Density Settling Velocity as Function of Floc Size and HA Conc. at ph Floc Size as Function of alum dose and HA Conc. at ph Floc Growth as Function of ph and HA Conc. for alum Dose 80 mg/l Analysis of Particles Settling Mother Solution Preparation Set-Up Syphonic Action of Collecting Supernatant from Jar Test Beaker Experimental Setup for measurement of Settling Velocity of the Floc Overall Work of Research Correlation between Total Organic Carbon and UV Absorbance Absorbance of Raw Water TOC Removal (%) of Natural Water (TOC 7 mg/l) as Function of Alum Dose at ph TOC Removal (%) of Natural Water (TOC 7 mg/l) as Function of ph at Optimum Alum Dosages Log {Al} - moles/l of Natural Water (TOC 7 mg/l) as Function of ph at different Alum Dose TOC Removal (%) of Natural Water (TOC 7 mg/l) with Clay and without Clay as Function of Alum Dose at Optimum ph TOC Removal (%) of Natural Water (TOC 7 mg/l) with Clay (100 mg/l) without Clay as Function of ph at Optimum Alum Dose Residual Aluminum (mg/l) of Natural Water (TOC 7 mg/l) as Function of Alum Dose at ph Comparison of TOC Removal (%) of Natural Water with Synthetic Water as Function of Alum Dose at ph Comparison of TOC Removal (%) of Natural Water (TOC 7 mg/l) with Secondary Compound TOC Removal of Natural Water (TOC 7 mg/l) as Function of Ca ++ mg/l at Optimum Condition of Alum Dose and ph TOC Removal (%) of Natural Water as Function of Alum Dose and TOC for Optimum Alum Dose at ph vii-

7 4.13 Settling Velocity of Natural Water as Function of TOC at Optimum Alum Dose and ph Relationship of Optimum Alum Dose and Quantity of Solid Removed as Function of TOC Molecular Weight Distribution of Natural Water (TOC 14 mg/l) after Coagulation Process at Optimum ph Molecular Weight Distribution of Natural Water (TOC 14 mg/l) before Coagulation Process at Optimum ph viii-

8 List of Tables Table No. Title Page 2.1 NOM fraction and Chemical Groups Observed Removal of Organic Material at Various Coagulant Dosages Summary of the Experimental Results of AIT pond Water Raw Water Surrogate Equations Testing of Grasse River Water Surrogate Equations on other Streams Summary of the Experimental Result of AIT Pond Water Parameter and Analytical Methods List of Parameter Analyzed in this study Characteristics of Water Summary of Experimental Results of Natural Water (TOC 7 mg/l) as Function of Alum Dose and ph Comparison of TOC Removal of Natural Water and Water with Clay (100 mg/l) at Optimum Alum Dose Summary of Floc Characteristics of Natural Water at Optimum Alum Dose and ph Summary of Floc Characteristics of Natural Water (TOC 7 mg/l) as Function of Clay at ph Floc Characteristics of Natural Water as Function of CST at Optimum ph and Alum Dose Summary of Molecular Weight Distribution at ph Comparision of Experimental and Theoriticallly (from software) obtained Values as Function of TOC at ph ix-

9 List of Abbreviations 1 N One Normal A f Area of individual Floc CST Capillary Suction Time D f Diameter of Floc (cm) DOC Dissolved Organic Carbon ρ f Density of Floc ρ w Density of Water ρ e Effective Density of Floc g Gravitational Acceleration (980 cm/s 2 ) HA Humic Acid HPLC High Performance Liquid Chromatography mg/l Milligram per liter MF Micro Filtration MWD Molecular Weight Distribution n Number of the Floc nm Nanometer NOM Natural Organic Matter NPTOC Non-Purgeable Total Organic Carbon NTU Nephelometric Turbidity Units PAC Powder Activated Carbon GAC Granular Activated Carbon POC Particulate Organic Carbon RO Reverse Osmosis rpm Round per Minute SOCs Synthetic Organic Carbons TOC Total organic Carbon THMs Trihalomethanes TTHMs Total Trihalomethanes µ Viscosity of Water (g/cm.s) µm Micro Meter (Micron) UVA 260 Ultraviolet Absorption at Wavelength of 260 Nanometer VOCs Volatile Organic Carbons Terminal Settling Velocity of Floc (cm/s) V t -vi-

10 Chapter 1 Introduction 1.1 Background of the study Organic substances, which originate from soil, decaying of vegetable matter or contamination by domestic and industrial wastewater, are commonly present in lake or surface water. Organic substances composed of high molecular weight that display polyanionic characteristics in natural or alkaline solution. Presence of them not only impart taste and color to water but also form halogens like trihalomethane (THM) with chlorine (Trussell and Umphrfs, 1978). Trihalomethanes mainly chloroform, bromodichloromethane, bromoform and dibromochloromethane occur as a resultant of reaction between chlorine and humic acids. These haloforms are carcinogenic and interfere with water treatment processes. Humic substances such as humic and, fulvic acids comprise the major part of the organic substances. The removal of humic acid from natural surface water becomes one of great importance in water treatment process because of health, aesthetics and operational problems. Humic substances, which have high charge density, first react with coagulants and then allow turbid particle to react (Semmens and Field, 1980; Eilen et al., 1985). Previous research has been done to study the influence of organic substances using synthetic humic acid in water treatment process and evaluate their relationship with ph, TOC, removal and demand of coagulant dosage (Choudhary, 1993; Cho, 1995). However, little is done about the interaction of ph, TOC, coagulant dose and floc size formed with natural organic substances in coagulation and flocculation processes and consequent implications on designing sedimentation unit. In order to design rationally, the characteristics of floc mainly size, shape and settling velocity play important role in sedimentation design process. The significance of organic substances to water quality can be summarized by i) color ii) THM precursors iii) precursors of other haloorganic DBPs iv) fouling of resins, membranes, equipments v) interfere with water treatment vi) binding, complexing of inorganic and organic micropollutants and facilitate transport of contaminants (Rebhun and Lurie, 1993). In order to reduce problems caused by organic substances, there are few possible ways. First is to reduce the haloform by using activated carbon in the form of PAC or GAC. Second is to prevent haloform formation by using ozone instead of chlorine during disinfecting process. Third is to remove humic substances through the process of coagulation by using alum or ferric chloride, which is found to be very effective in both organic substances and turbidity removal. Coagulation process has several advantages over other processes including i) little or no required of capital investment ii) minimal increase in unit operating costs and iii) well known technology (Kavanaugh, 1978). However, humic substances are the main group of organic matter affecting water treatment objectives by coagulation. This study examines the removal of organic substances from drinking water by coagulation and flocculation processes and identifies the influence of operating parameters like ph, TOC, coagulant dose. Effectiveness of such various parameters along with floc -1-

11 characteristics will be evaluated for their impact upon designing of sedimentation unit at different concentration of organic substances. 1.2 Objectives of the Study The Objectives of the study are on To study the optimum operational conditions of the coagulation process for maximum removal of natural organic substances. To study the floc characteristics of natural organic substances and their effect designing sedimentation tank. To study the quality and quantity of sludge produced with different organic concentration. Preparation of simple Excel spreadsheet software to study the effect of natural organic concentration on designing (mainly operational phase) of sedimentation tank. 1.3 Scope of the Study This study was mainly an experimental work. units. The preliminary step was to prepare mother solution of known concentration of natural organic substances, from a raw water collected from west side of AIT canal, after filtering raw water using 5 and 0.2 µm cartridge microfiltration The study was mainly confined to the optimum operating condition of ph and alum dose, only. The characteristics of floc was studied in terms of their effect on designing sedimentation tank. The CST method was performed to characterize floc in terms of its dewatering capabilities. Microsoft Excel software was used to prepare simple spreadsheet for designing of sedimentation tank. -2-

12 Chapter 2 Literature Review 2.1 Organic Constituents in Natural Water Organic matters found in natural water are particulate (POC) and dissolved (DOC). The POC may include bacteria, algae, zooplankton, but usually it is small fraction of total organic compound (TOC) compare to DOC, less than 10 percent except for highly eutropohic supplies. Organic contaminants in water supplies may be grouped into three classes first one is natural organic matter (NOM) including humic substances, microbial exudes and other organic materials dissolved into the water from sources such as plant tissue and animal wastes. Second one is synthetic organic chemicals (SOCs) including pesticides, volatile organic chemicals (VOCs) and other synthetic chemicals produced commercially or generated as waste products and last one is chemical byproducts and additives that enter the water during treatment or in the distribution systems (Randtke, 1988). Natural Organic Matter (NOM) contains organic compound that are both hydrophobic and hydrophilic in nature. It has been estimated that an average basis about 45 percent of the DOC in river is composed of hydrophobic aquatic humics and it cause natural color ( Edzwald, 1993). There is no direct analytical procedure for the characterization of Natural Organic Matter in water. Its composition is obviously influenced by the soil chemistry and hydrology of the catchment from which the water is derived. NOM is commonly quantitified in terms of surrogate parameters. The most favored parameter is Total Organic Carbon (TOC) concentration. Total organic carbon (TOC) is a collective measure of organic in water and provides no information on the composition and distribution of the wide array of organic constituents. Another parameter frequently used is UV Absorbance at a wave length 260 nm. However, a large fraction of the TOC is often called humic substances. The higher doses of coagulants may be needed to coagulate organic matter. Organic in the form of humic substances are undesirable in a potential water supply for a reasons ranging from aesthetics to being the precursors of potentially carcinogenic THM. Concentration of TOC reported in natural waters vary over a wide range as shown in Figure 2.1. Most ground water contains less than 2 mg/l, although there are exceptions. Surface water generally ranges from 1 to 20 mg/l, although as much as 300 mg/l have been reported for swamp and bog waters (Kavanaugh, 1978). Humic substances are amorphous acidic, predominantly aromatic, hydrophilic, chemically complex polyelectrolytes that range in molecular weight from a few hundred to tens of thousands. These matters are classified into three main fraction based on their solubility (i) humic acid, which are soluble in alkali (ii) fulvic acid which are soluble in both alkali and acid (hydrophilic), and (iii) humins which are insoluble in both (hydrophobic). Table 2.1 is a compilation of natural organic matter of hydrophilic and hydrophobic fraction with general chemicals groups and compounds associated with each fraction. Under the ph condition of most natural water, humic materials occur as negatively charged macromolecules. -3-

13 Sea Water Most Groundwaters Surface waters Swamps NORS Median of Surface Waters = 3.5 mg/l Effluents from Biological Treatments Wastewaters Total Organic Carbon (mg/l) Figure 2.1 Ranges of Total Organic Carbon (TOC) Reported for a Variety of Natural Waters (Kavanaugh, 1978) Their negative charge result from the presence of functional group of carboxyl and phenol. As the ph increased, stability increased because of the dissociation of the functional groups and the resulting increase in the number of negative charges present in solution (Edward and Amirtharajah, 1985). Table 2.1 NOM Fraction and Chemical Groups (Edzwald, 1993) Fraction Hydrophobic Acids Strong Weak Bases Neutrals Hydrophilic Acids Bases Neutrals Chemical Groups Humic and Fulvic acids, high molecular weight, alkyl mono carboxyl and diacaboxyl acids, aromatic acids. Phenol, Tannins, intermediate molecular weight alkyl monocarboxyl and diacarboxyl acids. Proteins, aromatic amines, high molecular weight, alkyl amines. Hydrocarbons, aldehydes, high molecular weight, methyl ketones and alkyl alcohol, ethers. Hydroxyl acids, sugars, low molecular weight, alkyl monocarboxyl and diacarboxylic acid. Amino acids, purines, low molecular weight alkyl amines Polysaccharides, low molecular weight alkyl alcohol, aldehydes and ketones. Removal of organic constituents from natural waters has always been of concern to water treatment. Traditionally water treatment goals have included removal of color, taste and odor producing compounds, the standards for which are based on esthetic considerations. The -4-

14 presence of these compound in the water supply produces numerous treatment problems including (i) the organic content of natural water is largely responsible for problems associated with color (ii) certain organic compound contribute to taste and odor problems in drinking water supplies (iii) the presence of organic substances in treated water may foster problems associated with biological quality changes in the distribution system. (iv) the presence of organic substances in drinking water has been reported to aggravate corrosion problems in the distribution system (v) the presence of organic compounds may interfere with demineralization processes by fouling anion exchange resins or membranes (vi) organic compound in water have been shown to interfere with oxidation and removal of iron and manganese. (vii) haloforms and other halogenated organic compound can be formed when chlorine is added to water at the levels required for disinfecting. The chlorine may react either with natural humic substances or with certain anthropogenic compound or both. and (viii) certain organic compounds are known to be toxic or carcinogenic and these may be harmful even at the very low concentration. While humic substances themselves are thought to be harmless, these compound may have other materials associated with them such as pesticides, phthalates and heavy metals (AWWA Committee Report, 1979). 2.2 Removal of Organic by Coagulation Process Traditionally the coagulation process is described in terms of the destabilization of colloids initially present in a water supply. These colloids may include organic particulate and inorganic particulate. The role of NOM, particularly the pool of DOC, in exerting a coagulant demand and the need to remove this material from water supplies has become more important. The coagulants are used not only destabilize colloidal particles, but also to remove NOM. The removal of NOM from bulk water can occur by direct precipitation of the NOM or by adsorption of NOM onto precipitate metal hydroxide. Particles in water supplies may be mineral (clay, aluminum and iron oxides and hydroxides, asbestos, silica) or organic (viruses, bacteria, protozoa, cysts, algae). Existing primarily as colloidal suspensions, these suspensions are stable such that the particles have a slow rate of particle aggregation or flocculation. The cause of particle stability may be i) electrostatic repulsive interactions due to diffuse electrical double layers ii) hydrophilic effects due to bound water at particle surfaces and iii) steric effects due to adsorbed macro molecules (Edzwald, 1993). Humics are anionic macromolecules with a verity of inorganic groups, usually carboxyl and phenolic groups. Coagulation from water results from charge neutralization by the charged hydrolysis species of the trivalent metal ions. Insoluble precipitate may also form. Thus optimal removal of dissolved organic compound requires careful selection of chemical conditions. A wide range of TOC removals by coagulation has been reported in Figure 2.2 and Table 2.2. Maximum TOC removals varies from one source to another, probably because of the different chemical characteristics of the aquatic humus in the various water source. The lowest TOC removal was obtained from water source having the greatest amount of lower molecular weight organic and highest coagulant demand. TOC removals of about 60 percent were achieved using coagulation-sedimentation-filtration with alum. Numerous laboratory -5-

15 studies have shown that humic substances can be removed satisfactory to nearly 90 percent with Al(III) or Fe(III) (AWWA Committee Report, 1979). Coagulant, especially alum and ferric chloride salts are usually needed to achieve reasonable removal of organic substances. Coagulants or coagulant systems that result in more extensive removals, lower chemical costs, lower plant construction costs, decreased sludge volume or lower concentration of possibly hazardous chemicals residuals are desirable. Such improvements may be accomplice by optimizing the type of coagulants, the extent and nature of the mixing, the system ph and the control capabilities as well as by modifying the process configuration (Kavanaugh, 1978). Figure 2.2 Effect of Coagulation ph on Organic Matter Remaining and Iron Residual (Auguiar et al., 1996) -6-

16 Table 2.2 Observed Removal of Organic Material at Various Coagulant Dosages (AWWA Committee Report, 1979) Type Sample Concentration mg/l Coagulant Type Concentration mg/l ph Remova l (%) Ground Water Lake River Humic Acid Humic Acid Humic Acid Humic Acid Fulvic Acid Fulvic Acid M. River Water Ohio River Water Humic Acid Humic Acid Humic Acid NA alum alum alum alum alum ferric chloride ferric chloride alum alum alum alum alum alum alum NA NA NA NA NA NA NA NA Mechanism of Coagulation Two distinct mechanism account for destabilization of colloid when alum is used as coagulant in coagulation in water treatment i.e. (i) adsorption to produce charge neutralization and (ii) enmeshment in a precipitate, shown in Figure 2.3. Destabilization may be accomplished by charge neutralization resulting from a specific chemicals attraction between positively charged aluminum species and negatively charged group on the humic colloids. As previously described by Stumn and Morgan, the fixation of multivalent cations onto ionized groups on hydrophilic colloids may be caused by electrostatic or chemical interaction, reducing the charge of the particles and altering their stability. Destabilization by this mechanism would be accomplish over a narrow ph range and stoichiometric relationship between the raw water humic concentration and the optimum coagulant dosage would be observed. Humic substances can form water soluble and water insoluble complex with metal ions. As the alum dosage is increased precipitation may occur, however, destabilization by this mechanism may in corporate humic material within aluminum hydroxide floc (AWWA Committee Report, 1979). Destabilization by adsorption has been shown to occur at low ph values, when alum is used to coagulate humic acid and is most rapid in the region where aluminum hydroxide is formed. -7-

17 Figure 2.3 Mechanism of Coagulation (Eilen et al., 1985) Clay - Organic Interaction in Coagulation Clay minerals dispersed in natural water adsorb humic substances. The extent of adsorption depends on the type of clay, the cationic form of the clay and the solution ph. Ion exchange between organic and clay due to presence of multi valent cations of alum is important in this mechanism. Since they act as a bridging agent between the negatively charged clays and negatively charged organic. The polyhydroxy complexes of these metals may provide positive sites on the clay surface and facilitate exchange adsorption like exchange reactions, this mode of adsorption is reversible and influenced strongly by the electrolyte concentration. In addition to that mechanism, hydrogen bonding and vander waals forces may be significant, particularly if the humic molecule has large unchanged portions and sufficient flexibility to come into contact with the surface of clay. If a clay is dried out in the presence of humic material to contact the clay surface and vandar waals forces will increase in significance, binding the humics firmly to the clay. The humic materials will remain strongly -8-

18 adsorbed to the clay surface upon redispersion (AWWA Committee Report, 1979) Pesticides - Organic Interaction in Coagulation It is generally acknowledged that the cause of pesticides has been great value to agricultural productivity, these benefit have not been achieved without risking the possibility of adverse on health. It is known that part of the pesticide is incorporated into the pest and crop, part is volatized into the atmosphere and part is deposited on soil, where there may be either chemical or biologic degradation. From the soil the pesticides and the byproducts of degradation may be inadvertently leached into the ground or carried to surface water in runoff (Robeck et al., 1965). Alum is relatively ineffective in removing pesticides. The removal of pesticides by coagulation will depends on the degree of association between the pesticides and the natural organic content of the water. If a strong association exists, the best removals of pesticides will occurs at ph 5-6 for alum and ph 4-5 for iron. The ph would be a very important variable during the coagulation process. However, the studies conducted on pesticides removal have inadequately described the influence of coagulation ph on process performances (AWWA Committee Report, 1979) Chemical Coagulation with Alum The most common and economical coagulants are alum, ferric salt and cationic polymers. Of these alum (Al 2 (SO 4 ).nh 2 O) is the most widely used coagulant in the water treatment. The aqueous chemistry of aluminum is complex and upon addition of an aluminum coagulant in water treatment, multiple reaction pathways are possible. The mechanism by which aluminum functions depends on which aluminum species react to remove dissolved or colloidal contaminants. Destabilization involving aluminum monomers is referred to as charge neutralization or coagulation of colloidal particles in the presence of Al(OH) 3 is termed as enmeshment or sweep floc. Dissolved organic can be removed by adsorption on aluminum precipitation (Benschoten and Edzwald, 1990), see Figure 2.4. From the numerous reviews of the fundamental theory and mechanisms of coagulation, various mechanism for destabilizing contaminants using chemical coagulants have been identified. These mechanisms include double layer compression, adsorption - charge neutralization, sweep coagulation and inter particle bridging. The type of inter actions between the chemical coagulant and contaminants determine the mechanism of coagulation. The predominance mechanisms observed during conventional coagulation with metal coagulants are adsorption - charge neutralization and sweep coagulation. For aluminum salts, the mechanism of coagulation is controlled by the hydrolysis speciation (Dennett et. al, 1996). -9-

19 Figure 2.4 Coagulation with Aluminum Sulfate (AWWA Committee Report, 1979) The chemistry of Al(III) can be summarized as follows, as shown in Figure 2.5 The positively charged polyhydroxo-complexes such as [Al 8 (OH) 20 ] +4 in the ph range between 4 and 7, are the effective flocculants. Over saturation and formation of amorphous hydroxide precipitate [Al(OH) 3 ] (s) enmeshes colloidal particles in a sweep floc. See Figure 2.6. Many other monomeric and polymeric species have been reported are Monomers: [Al(OH)] +2, [Al(OH) 2 ] +, [Al 2 (OH) 2 ] +4, [Al(OH) 4 ] - Polymers: [Al 6 (OH) 15 ] +3, [Al 7 (OH) 17 ] +4, [Al 8 (OH) 20 ] +4, [Al 13 (OH) 34 ] +5 [Al(H 2 O) 6 ]

20 (a) OH - OH- [Al(H 2 O) 5 (OH)] ++ [Al(H 2 O) 4 (OH) 2 ] + (b) (c) OH - OH - [Al 6 (OH) 15 )] +++ (d) (aq.) [Al 8 (OH) 20 )] ++ (e) (aq.) OH - Al(OH) 3 (H 2 O) 3 (f) (s) [Al(OH) 4 ] - (g) OH - Figure 2.5 Stepwise Conversion of the Tri-Positive Aluminum Ion to the Negative Aluminate Ion (Stumn and Morgan, 1962). The coagulation process with alum as the sole coagulant is capable of achieving significant organic removal. The removals were found to be strongly ph and coagulant dose dependent as shown in Figure 2.4 and Table 2.2. The ph of the water during coagulation has profound influences on effectiveness of coagulation for organic removal. Organic removal is much better in slightly acidic condition. The optimum ph for alum coagulation is influenced by the concentration of organic matter in the water. For water of higher organic content the optimum ph is displaced to slightly more acidic values (AWWA Committee Report, 1979). An adequate amount of alkalinity in raw water is required during the flocculation process for two reasons i) the need for a buffer capacity in order to avoid a steep drop of ph due to alum addition and ii) a sufficient supply of bicarbonate to form metal hydroxide (Kawamura, 1996). -11-

21 Chemical Strteam Water Stream (Colloid) Al(OH) Al(OH) Al(OH) Alum Solution Al 7 (OH) 17 Colloid Al 7 (OH) 17 Al(OH) 3 Fast ( s) + + H + Soluble hydrolysis species (low alum dose) Floc Colloid Fast (1-7 s) Al 2 (SO 4 ) H 2 O -----> Al(H 2 O) SO H 2 O > Al(OH) 2+ + Al(OH) Al 7 (OH) > Al(OH) 3 (s) Very fast 10-4 s Colloid High Turbidity Al(OH) 3 High Alum Dose Low Turbidity Colloid Colloid Al(OH) 3 Al(OH) 3 Al(OH) Al(OH) 3 3 Al(OH) Al(OH) 3 3 Al(OH) 3 Charge Neutralization Sweep Coagulation Figure 2.6 Reaction Schematics of Coagulation (Dennett et. al, 1996) Thus conventional coagulation practices may provide excellent organic removal if the coagulant dose and ph condition are adjusted into the optimum range. Organic removal increased with an increasing alum dose and alum doses higher than the normally used for turbidity removal, are needed to obtain the best organic removal. Alum dose and ph were the most important variables. The influence of ph was very great. (Semmens and Field, 1980). The optimum ph for AIT pond water is found to be 5.5 resulting from the largest floc size and good settling velocity. The optimum alum dose lies around 80 mg / L for 5 mg / L of humic acid and 120 mg / L for 20 mg / L of humic acid. Whereas for the natural water (HA = 3 mg / L) with optimum dose of alum 60 mg / L. Figure 2.7 and Table 2.3 summarize the experimental results of AIT Pond water (natural water) and humic acid. -12-

22 Figure 2.7 Alum Dose as Function of HA Conc. at ph= 5.5 (Cho, 1995) Table 2.3 Summary of the Experimental Results of AIT Pond Water (Cho, 1995) Organic Acid Conc. (mg/l) ph Optimum Alum Dose (mg/l) Organic Removal % Turbidity Removal % Humic Acid * Optimum Condition * ph Depression The ph will be depressed after coagulation due to form of hydrolysis products in the following steps -13-

23 Al 3+ + H 2 O Al OH 2+ + H + AlOH 2+ + H 2 O Al (OH) H + Al(OH) H 2 O Al (OH) 3 + H + Al(OH) 3 + H 2 O Al (OH) H + From the hydrolysis reactions of the Al 3+ ions, H + ions are generated, ultimately leading to the ph depression. The ph drop caused by coagulant addition is typically less than 0.6 ph units (Kwak, 1997) Coagulant Dosage The most common and economical coagulants are alum, ferric salt and cationic polymers. Off these Alum is the most widely used. The concentration of coagulant required for coagulation is to be proportional to the concentration of organic matter present in solution as shown in Figure 2.8 and it was stated that before turbidity can removed the coagulant must be added in sufficient amounts to destabilize the organic matter. Apparently these substances react with the coagulants before the coagulants can destabilize clay suspensions. Coagulant dosage decrease with decrease ph because more carboxylic groups become protonated and do not interact with coagulant as shown in Table 2.2. There is no reliable formula to determine the effective dosage. However, a jar test is the most reliable method to determine both the effective type of coagulant and their proper dosage (Kawamura, 1996). Figure 2.8 Stoichmetry of Coagulation of Humic Acid with Alum at ph 6 (AWWA Committee Report, 1979). 2.3 UV Absorbance as Surrogate Parameters Organic compound that are aromatic or that have conjugated double bonds absorb light in the ultraviolet wave length region. UV Absorbance is a good technique for measuring the presence of naturally occurring organic matter such as humic substances as shown in Figure 2.9 and Table 2.4 and

24 It was found that TOC is generally a good surrogate for monitoring treatment, within certain limitation, it was also reported that UV Absorbance is a good substitute for TOC. UV Absorbance at 260 nm is often used as a simple surrogate measurement for DOC, that is especially effective for water containing aquatic humics. The relationship that has been established between TOC and UV Absorbance are TOC = * UV Abs., r 2 = 0.99 (Edzwald et al., 1985). TOC = * UV Abs., r 2 = 0.93 (Eilen et al., 1985). The high correlation coefficient (r 2 ) found for TOC and UV Absorbance indicates that relationship can be used as relatively good indicator of TOC. Figure 2.9 Correlation between Raw Water NPTOC and Raw Water TOTLUV (Edzwald et al., 1985). Table 2.4 Raw Water Surrogate Equations (Edzwald et al., 1985) Table 2.5 Testing of Grasse River Water Surrogate equations on other Streams (Edzwald et al., 1985). -15-

25 2.4 Characteristics of Floc In order to design and operate chemicals coagulation process followed by solid liquid separation operation such as sedimentation, the physical properties of floc such as their density and strength are most important factors. Schematic of Floc formation is shown in Figure Figure 2.10 Schematic of Floc Formation (Tambo et al., 1989). Aluminum flocs are fluffy, more voluminous and fragile particles and has an extremely high water content. The measurement of floc density, therefore, has to be conducted without directly touching a floc. In addition to that, as floc can change their size by flocculation, measurement of the floc density should be performed in a dilute suspension in order to avoid flocculation during the analysis (Tambo and Watanabe, 1979) Measurement of Floc Size and Settling Velocity The measurement of floc size with help of Photo Micrographic System Camera can be given by equation -16-

26 n A f = l*b n=1...(2.1) d f = (4* A f /π )...(2.2) where, l = Length of Floc b = breadth of Floc A f = Area of Floc d f = floc diameter The settling velocity can be calculated by measuring time required by a floc to travel a certain distance in settling column, fixed with jar test beaker Calculation of the Floc Density The floc density can be calculated from its size and settling velocity by equation V t = g (ρ f -ρ w ) d f 2 / (34 µ) = g ρ e d f 2 / (34 µ ) (2.3) Where; V t = Settling velocity of floc, cm/s g = gravitational acceleration (980 cm/s 2 ) ρ f = the density of floc, g/cm 3 ρ w = the density of water, g/cm 3 d f = the floc diameter, cm µ = viscosity of water ρ e = the effective density of floc, g/cm 3 Tambo and Watanabe (1979) have defined the floc effective density (buoyant density of floc) as ρ e = ρ f -ρ w and adopted as the main index to discuss the nature of floc density. More detailed floc density is obtained by ρ e than ρ f, for example, let consider two flocs of densities ρ f are 1.01 and 1.02 g/cm 3 respectively. Increasing rate of floc density is only ( ) / 1.01 = 1%. Therefore it is very difficult to make clear the difference of floc density itself by experiment. On the other hand, increasing rate of effective density ρ e is ( )/0.01 = 100%. Therefore it is easy to evaluate the characteristics nature of floc effective density. It is seen in equation 2.3, increase of floc effective density is linearly connected with the increase of floc size as shown in Figure 2.11 Thus the relationship between floc effective density and floc diameter can be expressed as equation where ρ e = ρ f - ρ w = a /(d f / l) kp (2.4) ρ f = the density of floc, g/cm 3 ρ w = the density of water, g/cm 3 d f = the floc diameter, cm -17-

27 d f /l= dimensionless floc diameter (cm / cm) ρ e = the effective density of floc, g/cm 3 a = constant, g/cm 3 k p = constant Figure 2.11 Logarithm Plot of the Floc Dia and Floc Effective Density (Tambo and Watanabe, 1979) Once the floc size density relationship is expressed as equation (2.4) various effects such as the kind of coagulant, the coagulant dosage, ph, the agitation time and intensity on the floc density can be evaluated by two parameters kp and a. Which characterize the floc density function (Tambo and Watanabe, 1979) Fundamental Structure of Floc Metal coagulant floc consists of three components, namely, suspended particles in the raw water, hydrolyzed polymetal ions, and internal water taken in during the floc growth. The hydrolyzed polymetal ions are adsorbed on the surface of the suspended particles. Therefore the main components which make up floc are divided into two parts as solid part and void part. From volume and mass balance the following relationship are observed (Tambo and Watanabe, 1979) -18-

28 V f = V s + V w ρ f V f = ρ s V s + ρ w V f (2.5) where ρ f = the density of floc, g/cm 3 ρ w = the density of water, g/cm 3 ρ s = the density of solid, g/cm 3 V f = total volume of floc, cm 3 V s = total volume of solid, cm 3 V w = total volume of void, cm 3 Tambo and Wanatabe (1979) verified that the floc density are not greatly affected by ph, agitation intensity, raw water alkalinity and flocculant aids. The floc density can be primarily determined by floc size and ALT ratio Settling Characteristics of Floc Settling characteristics of floc formed at different floc size for AIT pond water is shown in Figure 2.12, 2.13 and 2.14 and Table 2.6. It is observed that the larger the floc size, the better settling characteristics. Figure 2.12 Settling Velocity as Function of Floc Size and HA Conc. at ph 5.5 (Cho, 1995) -19-

29 Figure 2.13 Floc Size as Function of Alum Dose and HA Conc. at ph 5.5 (HA= 3 mg / L - natural water) (Cho, 1995) Figure 2.14 Floc Growth as Function of ph and HA Conc. for Alum Dose 80 mg / L (HA= 3 mg / L- Natural Water) (Cho, 1995) Table 2.6 Summary of the Experimental Result of AIT Pond water (Cho, 1995) Organic Acid Conc. (mg/ L) ph Optimum Alum Dose (mg/l) Floc size (cm) Settling Velocity, cm/s Density of floc *10-3 g/cm 3 Humic Acid * *

30 * Optimum Condition As the flocculation occurs, the mass of the particles increases and it settles faster. The extent to which flocculation occurs is dependent on the a opportunity of contact. Which varies with overflow rate, the depth of the basin, the velocity gradient in the system, the concentration of particles, and range of particulate size. In the design of sedimentation basin, settling velocity plays a key role. A particle that have settling velocity greater than design velocity, V t, will settled. Assuming that the particles of various sizes are uniformly distributed as shown in the Figure Particles of settling velocity less than V t will be removed by the ratio; X r = V p / V t Where, X r = the fraction of the particles with the settling velocity V p that are removed 1.0 Fraction of Particles X c with less than V t X p V p V t Settling Velocity Figure 2.15 Analysis of Particles Settling (Metcalf and Eddy, 1991) -21-

31 Chapter 3 Experimental Methodology 3.1 Outline of Research The experimental investigation was designed to optimize alum dose for maximum removal of total organic substances present in water by the coagulation and flocculation processes and subsequent effect of floc characteristics on designing of sedimentation tank. The performance was observed in the presence of varying concentrations of organic substances (TOC) with alum. This research was divided in to two processes including optimization of coagulation and flocculation processes and studying of floc characteristics namely; floc size and settling velocity of floc. Overall work plan is given in Figure Water Source AIT Canal (from the west side border) was used as raw water for this research. The site selection of sampling point based on the maximum total organic carbon concentration present in raw water. Pretreatment of raw water was done by using experimental setup as shown in Figure 3.1. It consisted of 20 L feed tank, the centrifugal pump and 5 and 0.2 µm microfiltration cartridge unit. The permeate obtained from 0.2 µm microfiltration cartridge unit is considered as mother solution of this research. Later Mother Solution was further concentrated to obtain high concentration of organic substances, about 25 mg/l of TOC, by using reverse osmosis (RO) membrane. These set up were operated by batch system and optimum condition of ph and alum dose were determined at different TOC values. Natural Water 0.2 µm MF Cartridge Unit Mother Solution RO Permeate Feed Tank 5 µm MF Cartridge Unit Concentrated Water with High TOC value Pump Figure 3.1 Mother Solution Preparation Set-up 3.3 Chemicals Used -22-

32 Commercially available 50% stock solution of alum was used as a coagulant. Reagent grade NaOH, HCl, and NaHCO 3 were used to adjust the ph. CaCO 3 and MgCO 3 were used to increase Ca 2+ and Mg 2+ concentration in raw water. The raw water turbidity was modified by adding Kaolin Clay. 3.4 Chemical Preparation (a) Coagulant The 50% stock solution of alum was diluted in such a way that 1 ml equivalent to 20 mg. (b) HCl and NaOH 1 N of HCl and NaOH were prepared to adjust desired ph of the raw water. (c) NaHCO 3 The solution of NaHCO 3 was used for preparing a solution of 0 TOC from distilled water. (d) CaCO 3 The solution of CaCO 3 with 0.1% of HCl was prepared. and used for increasing Ca 2+ concentration in water sample. (e) MgCO 3 The solution of MgCO 3 with 0.1% of HCl was prepared. and used for increasing Mg 2+ concentration in water sample. 3.5 Analytical Methods The parameters as shown in Table 3.1 were analyzed in this research. The parameters analyzed like ph, TOC, UV Absorbance, Calcium, Magnesium and Residual Aluminum were measured according to the procedure described in the Standard Method for Examination of Water and Wastewater (1992). Table 3.1 Parameters and Analytical Methods -23-

33 Parameters Equipment Working Possible Accuracy Range Interference ph Beckman ph Meter 1-14 Electrode 0.1 ph unit Dirtiness TOC TOC Analyzer 1 to 3000 mg/l Carbonate & 1 mg/l bicarbonate UV Abs. UV -254 Hewlett Pacard Spectrometer nm NO - - 2, NO 3 accurate if TOC > 2.4 Residual Aluminum Spectroquant Spectrometry mg/ L accurate if within range Calcium Atomic Absorbance mg/ L very accurate Spectrometer Magnesium Atomic Absorbance mg/ L very accurate Spectrometry Turbidity Hack Turbidity Meter Model NTU color & SS accurate for high NTU Mol. Weight HPLC Hewlett Packard chloride accurate Distribution Series 1050 compound Floc Settling Velocity Specially Designed Settling Tube < 5 cm/s satisfactory 3.6 Experimental Procedure The experimental study was performed in laboratory scale unit. The main components of the system were jar stirring arrangement fitted with motor to control flocculation speed and timer to monitor flocculation time along with modified jar test arrangement Optimization of Coagulation and Flocculation Processes It was done in order to determine the optimum condition for maximum removal of the organic substances. The different doses of alum was added under specific conditions of ph, and then mixture was rapidly mixed followed by gentle flocculation. Coagulation performed by using a jar test with stirrers and 1000 ml. beaker. The driving motor has a variable speed control ranging from 0 to 1000 rpm. Eight hundred (800) ml. of water sample was added in each jar test beaker. Varying amount of coagulant added in the five Jars in order to examine the effect of coagulant dose and subjected to the flash mixing (100 rpm) for 1 min and then slow mixing (25 rpm) for 25 min. Then mixture allowed to stand for another 30 min to settle flocs formed. From each jar about 50 ml of its supernatant liquid was taken out by a shyphonic action as shown in figure 3.2 and subjected to measure ph, TOC, UV Absorbance, residual aluminum. The alum dosage and ph required for maximum removal of TOC termed as the optimum coagulant dosage and optimum ph respectively. List of parameters and their range are given in Table

34 10 cm Beaker Plastic long pipe Figure 3.2 Syphonic Action of Collecting Supernatant from Jar Test Beaker. Table 3.2 List of Parameters Analyzed in this Study Descriptions TOC ph Coagulant Dose Measurement (mg/l) (mg/l) Parameter Equipment Coagulation & Residual UV Abs Flocculation TOC Results Optimum Alum Dose ph effect Settling Velocity Molecular Wt. Distribution 7, 14, 25 Opt.* * only for the Optimum Condition Opt.* Opt.* Opt.* Residual TOC Settling Velocity RT UV Abs Settling Column HPLC Optimum ph Settling Vel. of floc MWD Floc Settling Velocity Measurement -25-