Uncorrected Proof 1
© IWA Publishing 2011 Water Science and Technology
|
in press
|
2011
Lab scale study on electrocoagulation defluoridation process optimization along with aluminium leaching in the process and comparison with full scale plant operation Poonam Gwala, Subhash Andey, Vasant Mhaisalkar, Pawan Labhasetwar, Sarika Pimpalkar and Chetan Kshirsagar
ABSTRACT Excess or lack of fluoride in drinking water is harmful to human health. Desirable and permissible standards of fluoride in drinking water are 1.0 and 1.5 mg/L, respectively, as per Indian drinking water quality standards i.e., BIS 10500, 1991. In the present work, performance of electro-coagulation defluoridation batch process with aluminum electrodes was investigated. Different operational conditions such as fluoride concentration in water, pH and current density were varied and performance of the process was examined. Influence of operational conditions on (i) electrode polarization phenomena, (ii) pH evolution during electrolysis and (iii) the amount of aluminum released (coagulant) was investigated. Removal by electrodes is primarily responsible for the high defluoridation efficiency and the adsorption by hydroxide aluminum floc provides secondary effect. Experimental data obtained at optimum conditions that favored simultaneous mixing and flotation
Poonam Gwala Subhash Andey (corresponding author) Pawan Labhasetwar Sarika Pimpalkar Chetan Kshirsagar Water Technology and Management Division, National Environmental Engineering Research Institute, Nehru Marg, Nagpur, India E-mail:
[email protected] Vasant Mhaisalkar Civil Engineering Department, VNIT, Nagpur, India
confirmed that concentrations lower than 1 mg/L could be achieved when initial concentrations were between 2 and 20 mg/L. pH value was found to be an important parameter that affected fluoride removal significantly. The optimal initial pH range is between 6 and 7 at which effective defluoridation and removal efficiencies over 98% were achieved. Furthermore, experimental results prominently displayed that an increase in current density substantially reduces the treatment duration, but with increased residual aluminum level. The paper focuses on pilot scale defluoridation process optimization along with aluminum leaching and experimental results were compared with full-scale plant having capacity of 600 liter per batch. Key words
| aluminum electrodes, defluoridation, electro-coagulation, residual aluminum
INTRODUCTION One of the major challenges faced by mankind today is to provide clean water to a vast majority of the population around the world particularly in developing countries like India. The need for clean water is particularly critical in increased resource demand scenario (Yousuf et al. ). Fluoride concentrations in groundwater in the world range from 0.01 to 48 mg/L (Emamjomeh & Sivakumar ). According to the guidelines of World Health Organization (WHO) in 2004 the extreme limitation of fluoride is Q1 1.5 mg/L for drinking water quality (WHO ). Drinking water consumption containing high fluoride for longer time doi: 10.2166/wst.2011.475
can result in mottling of teeth, softening of bones, ossification of tendons and ligaments. Almost 25 nations and 200 million people globally are affected by the fluorosis especially in China, India, Pakistan, and Thailand (Reardon & Wang ; Feng Shen et al. ). In China over 1 million Q1 people are thought to be suffering from skeletal fluorosis and 26 million from dental fluorosis as a result of elevated fluoride levels in their drinking water. In India, it is estimated that dental and skeletal fluorosis affected persons are about 18.2 and 7.9 million, respectively (Liang et al. ; Fewtrell et al. ).
Uncorrected Proof 2
P. Gwala et al.
|
Electrocoagulation defluoridation process optimization
Higher concentrations of fluoride in the groundwater were observed to be allied with weathered formations of pyroxene amphibolites and pegmatites. The major sources of fluoride in groundwater are fluoride-bearing rocks such as fluorspar, cryolite, fluorapatite and hydroxylapatite (Agarwal et al. ; Meenakshi et al. ). There are several defluoridation processes tested or employed globally, such as adsorption (Azbar & Turkman ), chemical precipitation (Nawlakhe & Paramasivam ; Azbar & Turkman ; Reardon & Wang ), and electrochemical (Mameri et al. a). Currently, some of the popular processes for drinking water defluoridation are the adsorption using activated alumina (Chauhan et al. ) bone char (Hernandez-Montoya et al. ), actiQ1 vated carbon (Daifullah et al. ; Kumar et al. ) and other adsorbents (Biswas et al. ; Yuksel et al. ) and coagulation using aluminum salts (Pinon-Miramontes et al. ). Adsorption is effective in defluoridation, but its operation is complex. Coagulation is simple in equipment and effective in defluoridation under limited conditions, but large quantities of chemicals such as SO2 4 are introduced in the water, especially when the fluoride concentration is high. Other major processes for defluoridation include electro-dialysis (Tahaikt et al. ) reverse osmosis (Arora et al. ) and nano-filtration (Hu & Dickson ). These membrane processes are known to be effective means for defluoridation but they also remove beneficial ions in water and tend to be expensive compared to other methods. Additionally, large quantity of reject water makes these processes unsuitable in water scarce areas (Zuoa et al. ). Table 1 describes different defluoridation techniques having limitations either in process or in cost effectiveness
Table 1
|
Water Science and Technology
Table 1
|
Defluoridation techniques
Magnesium oxide Calcium Chloride and Monosodium Phosphate
Treated water pH and sludge production are high Too short or too long contact time with raw water reduces the removal efficiency of treated water (continued)
2011
Limitations
Alum
Low pH of treated water, high dose required for higher fluoride removal, treated water has higher sulphate and aluminum concentrations especially at high pH
Alum and Lime
Treated water has high hardness, pH and residual aluminium, difficult to control dosages with varying alkalinity and fluoride concentrations
Adsorption/ion exchange Bone
Limited acceptance due to restrictions by religious communities and unusual taste in treated water, low social acceptability
Bone char
Limited acceptance due to restrictions by religious communities, removal efficiency reduces drastically after successive regenerations
Activated alumina
Regeneration results in reduction of 5–10% in material and 30–40% in capacity with increased presence of aluminium (>0.2 mg/L), high pH reduces potential of fluoride removal, costly compared to coagulation process
Electrochemical methods Electrosorption
Costly due to high consumption of electric power
Reverse osmosis
High cost, treated water may lack water balance of minerals, chances of mineral and biological fouling, sensitive to polarization phenomenon, large quantity of reject water
Nano-filtration
Expensive technique, requirement of skilled operators, more sensitive than RO to pH and ionic strength
Electrodialysis
Require pretreatment, membrane scaling
Electro-coagulation
Regular replacement of sacrificial electrodes, interferences from other anions like sulphate
Limitations
Comparatively higher treated water pH, sludge production and high effluent fluoride concentration
|
Co-precipitation by
Precipitation by Calcium oxide
in press
continued
Defluoridation techniques and their limitations
Defluoridation techniques
|
Uncorrected Proof 3
P. Gwala et al.
|
Electrocoagulation defluoridation process optimization
Water Science and Technology
(Ayoob et al. ). Currently there is growing interest in Electro-coagulation (EC) process. The technique is used to treat restaurant wastewater (Chen et al. ), textile wastewater (Bayramoglu et al. ), electroplating wastewater (Adhoum et al. ) and fluoride-containing wastewater effectively. Electro-coagulation process is reported to be efficient for drinking water defluoridation (Mameri et al. b; Zuoa et al. ). EC requires simple equipment and is easy to operate. EC process is controlled electrically with no moving parts, thus requiring less maintenance. EC process can be conveniently used in rural areas where electricity is not available, as solar panel can be coupled to the unit. EC process avoids use of chemicals and there is no problem of neutralizing excess chemicals and produces palatable, clear, colorless and odorless water. The gas bubbles produced during electrolysis can carry the pollutant to the top of the solution where it can be more easily concentrated, collected and removed. EC process has the advantage of removing colloidal particles, because the applied electric field sets them in faster motion, thereby facilitating the coagulation (Yousuf et al. ). Sludge formed during EC process tends to be readily settable and easy to de-water, because it is composed of mainly metallic oxides/hydroxides. Above all, it is a low sludge producing technique. Flocs formed during EC are similar to chemical floc, except that EC floc tends to be much larger, contains less bound water, is acid-resistant and more stable, and therefore, can be separated faster by filtration. EC process using sacrificial aluminum electrodes has been demonstrated to be an effective process since it does not require a substantial investment, presents similar advantages as chemical coagulation and reduces disadvantages Q2 (Hu et al. ) and produces minimum sludge (Mollah et al. ; Essadkia et al. ). The main reactions involved in the EC are as following: Al ! Al3þ þ 3e
at the anode
ð1Þ
Al3þ þ 3H2 O ! AlðOHÞ3 þ 3Hþ
ð2Þ
AlðOHÞ3 þ xF ! AlðOHÞ3x Fx þ xOH
ð3Þ
2H2 O þ 2e ! H2 þ 2OH
ð4Þ
at the cathode
EC is an electrochemical process, which comprises chemical and physical processes involving many surface and interfacial phenomena. The technology lies at the
|
in press
|
2011
intersection of three more fundamental technologies—electrochemistry, coagulation and floatation. Defluoridation efficiency of EC process depends on applied current density, initial fluoride concentration, initial pH, residence time and raw water quality. Lower initial pH has been reported to improve the efficiency of defluoridation in the EC process (Mameri et al. ). Reduction in pH has positive impact Q1 on other parameters of the EC process such as aluminiumfluoride ratio, energy consumption and residence time, but also increased residual aluminium levels (permissible level 0.2 mg/L as per BIS 10500:1991 standard) are observed. The objective of this research work is to study the effect of lower initial pH of the raw water on aluminium leaching in the EC process. This paper presents the details of the experiments conducted in the laboratory on removal of fluoride by electrolytic process at various initial pH and residual aluminium concentration in treated water. Also results from laboratory experiments were compared with full- scale EC plant.
MATERIALS AND METHODS Experimental set up Experiments were performed in a batch reactor consisting of plastic beaker of 5 L capacity. Remy Magnetic stirrer with 40 mm length Teflon coated rod was used for stirring the solution during the experiments. Magnetic stirring at 400 rpm helped to maintain a homogeneous solution in the batch reactor. Enough clearance was given at the bottom for the stirrer bar to rotate at the centre of the bottom of the reactor and agitate the solution. A direct current (DC) by stabilized power supply (TESTRONIX 92D, 0–30 V, 0–10 A, and Digital Display) was applied to the terminal electrodes in which electrical current was controlled by a variable transformer. Constant current was maintained during each run by appropriately adjusting the impressed cell voltage from a regulated DC power supply. Aluminum plates were cut from a commercial grade aluminum sheet (99% purity) of 2 mm thickness each having dimension of 100 mm × 180 mm and an effective area of 180 cm2 on each side. The distance between electrodes was 5 mm. Monopolar configuration with three aluminium plate electrodes was used. Central plate was connected to anode and two end plates were connected to cathode. The electrodes were designed with a surface area to volume ratio of 7.2 m2/m3, which is within cited range of 6.9–43 m2/m3 as reported by Holt (). Aluminium
Uncorrected Proof 4
P. Gwala et al.
|
Electrocoagulation defluoridation process optimization
plates were selected due to its cost effectiveness, easy availabity, less maintenance and lack of release of interfering ions in the process. There are various other electrodes also reported such as iron and titanium. Electrochemical cells with two and five electrode plates are also reported (Mouedhen et al. ; Emamjomeh & Sivakumar ). Analytical techniques Batch experiments were performed in the laboratory at ambient temperatures ranging from 26 to 28 C. The chemicals were analytical reagent grade and were used without any further purification. Stock solution of 1000 mg/L of fluoride was prepared by dissolving 2.21 g oven dried sodium fluoride (NaF) in 1 L distilled water. The solutions of various concentrations of fluoride used in the experiments were prepared by diluting the measured quantity of stock solution in tap water having
