I would also like to thank all the other colleagues of the Department of Chemical Technology for their sincere help and cooperation in the work. Inorganic constituents, particularly fluoride, iron and arsenic contamination, are a serious problem in several parts of India as well as in various parts of the world, associated with their adequate presence in drinking water causing serious health damage.
Introduction
Background
In India, fluoride, iron and arsenic are the major inorganic naturally occurring pollutants found in groundwater. In this work, electrocoagulation followed by microfiltration is applied to remove fluoride, iron and arsenic from drinking water.
Sources of fluoride, iron and arsenic in drinking water and their toxicity Fluoride
In anaerobic groundwater, where iron is in the form of iron(II), concentrations will normally be 0.5-10 mg L-1, but concentrations up to 50 mg L-1 can sometimes be found. Concentrations of iron in drinking water are usually less than 0.3 mg L-1, but may be higher in countries where various iron salts are used as coagulants in water treatment plants and where cast iron, steel and galvanized iron pipes are used for water distribution.
![Table 1.2: Fluoride concentrations in different states of India [10]](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10344772.0/25.918.131.763.396.1060/table-fluoride-concentrations-in-different-states-of-india.webp)
Maximum contamination level (MCL) and health effects of fluoride, iron and arsenic
Considering the deadly impact of arsenic on human health, environmental authorities of different countries have adopted a stricter attitude towards the presence of arsenic in water. Health effects of arsenic on humans are classified as acute and subacute which are typically reversible and chronic effects.
Existing processes for the separation of fluoride, iron and arsenic from drinking water
- Electrochemistry
- Electrocoagulation and chemical coagulation
- Coagulation mechanism for the removal of fluoride, iron and arsenic
- Floatation
Consumable, or sacrificial, metal anodes are used to continuously produce polyvalent metal cations in the vicinity of the anode. In the following EC techniques, the production of polyvalent cations from sacrificial anode oxidation (Fe and Al) and electrolysis gases (H2 and O2) work in combination to flocculate coagulant materials.
Advantages and disadvantages of membrane process
The process is the reverse of natural osmosis due to the pressure exerted on the concentrated side of the membrane, overcoming the natural osmotic pressure. The efficiency of the process is determined by various factors such as raw water properties, pressure, temperature and regular monitoring and maintenance, etc.
Ceramic membrane – an overview
However, the sintering temperature used in these works was more than 1100 oC and the average pore size of the membranes was more than 1 µm. However, the fabrication of such multilayer ceramic membranes involves a tedious cycle of sintering processes, which further contributes to the cost of the membrane [109, 110].
Advantages of ceramic membranes over polymeric membranes
Combination of various separation processes
The combination of adsorption and membrane processes in the treatment of process wastewater have been reported in the literature. In another recent work [121], the combination of adsorption and ultrafiltration was used in the treatment of colored wastewater.
Aim of the current research: Treatment of Fluoride, Iron and Arsenic using a combination of electrocoagulation and microfiltration
Electrocoagulation followed by microfiltration is used to remove fluoride, iron and arsenic from drinking water. To develop a process that includes both electrocoagulation and microfiltration and apply this to the treatment of drinking water for the purpose of separating fluoride, iron and arsenic. To study the effect of various process parameters on electrocoagulation using synthetic aqueous solutions of fluoride, iron and arsenic.
Outline of the dissertation
Performance of the EC process and operating costs for the removal of fluoride, iron and arsenic are calculated and also discussed in chapter 3. The cost analysis of the prepared membranes is also reported in this chapter to compare the membrane with other comparable available membranes in the market. The application of the proposed combination process with electrocoagulation followed by microfiltration is shown in chapter 6.
Electrocoagulation bath
In addition to this, it also includes the fine points of the characterization techniques for the electrocoagulated by-products along with the supporting detail that provides important information of the experimental findings.
Electrode
In bipolar bonding, more than two electrodes are used to make such an arrangement in order to reduce effective electrode corrosion and improve process performance. The entire electrode assembly was placed on non-conductive wedges and hung from the top of the electrocoagulation tank. The electrode assembly was placed in the cell and the electrodes were connected to the respective terminals of the DC power supply and a constant current was supplied for a certain time.
Solution preparation
36D, Agarwal Electronics, Mumbai, India) to constitute a galvanostatic mode electrochemical cell for constant power supply. The concentration range is chosen based on the availability of fluoride, iron and arsenic concentration in drinking water.
Experimental procedure
Each time, the meter was thoroughly washed with deionized water before and after immersion in the buffer solutions. The conductivity meter was also calibrated by matching the known conductivity of 0.1(N) KCl solutions given in the manual provided by the manufacturer. After calibration, the conductivity meter was thoroughly washed with deionized water and soaked with a paper tissue without touching the coated detector in the meter.
Operating conditions
Characterization of byproducts
The morphology of the by-products obtained from the EC bath was analyzed by SEM, while elemental information of the by-products was recorded using EDAX. Sample was analyzed under the application of the probe current of 94 pA and at different magnifications with the primary electron hitting the sample with energy of 10 kV – 15 kV. EDAX is an integrated part of the scanning electron microscopy where energy distribution according to the element is calibrated with a standard Co (cobalt) sample before the analysis.
Electrocoagulation: Results and Discussion
Fluoride removal
- Effect of initial fluoride concentration
- Effect of bipolar connection of electrodes
- Effect of interelectrode distance
- Fluoride removal with varying current density Monopolar
- Variation of pH in electrocoagulation bath
- Variation of electrodes corrosion
- Variation of film thickness
- Variation of conductivity of treated water
- Estimation of operating costs
- Characterization of the by-products obtained from the EC bath
Changes in the fluoride concentration in the EC bath with the inter-electrode distance are shown in the figure. In the figure, it was found that at any moment the fluoride concentration in the EC bath was lower for a smaller interelectrode distance. We have seen that with increasing current density and initial fluoride concentration in the drinking water, the film thickness increased for both monopolar (Fig. 3.9) and bipolar connections.
Iron removal
- Effect of initial iron concentration
- Variation of Fe(II) removal with interelectrode distance
- Effect of current density
- Change in pH
- Variation of conductivity
- Variation of energy consumption
- Operating cost for EC of iron
- Analysis of sludge obtained after EC
It can be seen from the figure that a steep increase in the Fe(II) removal at the very beginning of the process for all streams occurred and becomes gradual thereafter. A reddish-brown sludge due to the formation of iron hydroxide (as discussed in Chapter 2) was observed at the bottom of the cell shortly after the end of the experiment. Conductivity plays a major role in the EC process by improving the ionic strength of the solution.
Arsenic removal
- Effect of initial arsenic concentration
- Effect of electrode connection
- Arsenic removal with varying interelectrode distance
- Variation of arsenic removal with current density
- Variation of pH
- Variation of sludge formation
- Change in film thickness
- Determination of cost for the removal of arsenic using EC
- Characterization of electrocoagulated byproducts
For example, the final arsenic concentration was observed to be 8 μg L-1 after 50 minutes of electrocoagulation treatment of drinking water with an initial arsenic concentration of 100. It was seen that with an increase in current density, the final arsenic concentration also decreased. It was observed that after 60 minutes of such action for drinking water with an initial arsenic concentration of 100.
Raw materials
Membrane preparation
- Paste method
- Uni-axial method
The clay mixture was taken into a size 40 mesh screen and sieved for 30 minutes to get all the particles in the same size. All ceramic membranes were then removed from the oven and rubbed with sandpaper (C-220) to obtain a smooth polished flat surface. The sintering process was then carried out at a very low heating rate of 2°C min-1 in a muffle furnace and then also cooled in the same way.
Characterization techniques .1 Structural Characterization
The membrane performance and the presence of defects in the inner part of the membranes were evaluated using liquid (water) flux characterization. The set-up (as shown in Fig. 4.3) used for this experiment consists of a Teflon tubular cell with a flat circular Teflon base plate which contains the membrane housing. The membrane was placed in a Teflon liner and sealed with epoxy resin and then placed in the membrane housing on the base plate.
Chemical stability
Structural characterization of prepared MF membrane
- Particle size distribution
- Thermogravimetric analysis
- X – ray diffraction analysis
- Scanning electron microscopy
- Pore size distribution
- Porosity measurement
From the figure, it was revealed that most of the ingredients in the clay mixture had a sharp particle size distribution. The particle size of the various ingredients in the clay mixture was examined using the particle size analyzer. It is therefore obvious to achieve a narrow pore size distribution of the ceramic membranes sintered at higher temperatures.
Permeation experiments
The average pore size obtained by the water permeation method is found to be smaller compared to that of SEM analysis. In uniaxial method, the average pore size estimated from the water penetration test was found to be reduced from 1.1 μm to 0.56 μm while the sintering temperature increased from 750 oC to 950 oC. 12 (b): Variation of average pore size estimated from SEM images and water permeation tests for different membranes.
Chemical stability analysis
It confirmed the achievement of ceramic properties for membranes prepared by paste and uniaxial cold pressing methods, respectively. Therefore, the residual basic elements in the ceramics prepared by uniaxial cold pressing were less compared to those prepared by the paste method. Therefore, an average percentage weight loss under acid treatment was less in membranes prepared by uniaxial cold pressing.
Membrane cost
Finally, the total estimated membrane cost including raw material cost and energy consumption cost was found to be about US$ 172.7 m-2 and US$ 197.82 m-2 for ceramic microporous membranes prepared by paste and uniaxial cold pressing methods, respectively. Therefore, it can be strongly recommended from the cost analysis presented in this work (Table 5.4, 5.5 and Appendix A) that the inorganic membrane based on kaolin would be competitive in relation to the price of commercially available polymer membranes.
Comparison of paste and uni-axial methods
It was observed that the pore size distribution was much narrower in the ceramic microporous membranes prepared by a uniaxial cold pressing method. It was also observed that the surface pore density was lower in the ceramic membranes fabricated by uniaxial cold pressing. The porosity of the ceramic microporous membranes prepared by the paste method had a higher value compared to those produced by the uniaxial cold pressing method.
Summary
From this we can conclude that ceramic microporous membranes prepared by uniaxial cold pressing can be sufficiently capable of retaining particles larger than 2 µm at a higher temperature compared to membranes prepared by the paste method. This was supported by bulk porosity values of 0.55 and 0.43 for membranes prepared by paste and uniaxial cold pressing, respectively. From this we can also conclude that the ceramic membranes prepared by uniaxial cold pressing were better hardened than those prepared by the paste method.
Introduction
This chapter deals with the hybrid process used for efficient separation of fluoride, iron and arsenic contamination from drinking water using electrocoagulation (EC) followed by microfiltration (MF).
Experimental
The ceramic membrane prepared by uni-axial methods sintered at 950 oC was used in the MF cell. At the end of the filtration treatment through the ceramic microfiltration membrane, the remaining electrocoagulated sludge was collected from the membrane surface by scrubbing and dried in a hot air oven at 125°C for 4 hours. In addition, the ceramic microfiltration membrane used for the EC experiments was also dried under the same conditions as mentioned above and subjected to the SEM analysis to visualize the morphological changes before and after the treatment due to the blockage on the membrane surface had occurred. of active pores by the electrocoagulated sludge.
Results and discussion
- Particle size distribution of electrocoagulated by-product
- Removal performance of fluoride, iron and arsenic
- Permeate flux profile after MF experiments
Fig.6.4a Concentration of fluoride from the mixture of fluoride, iron and arsenic during the EC process. Fig.6.4b: Concentration of iron from the mixture of fluorine, iron and arsenic during the EC process. Fig.6.4c: Concentration of arsenic from the mixture of fluoride, iron and arsenic during the EC process.
Characterization of membranes and by-products
The agglomerated sludge was successfully captured by the ceramic microfiltration membrane and removed from the membrane surface after the filtration treatment by scrubbing for their characterization in order to effectively ensure the removal of the contaminants from the drinking water. The sample was also subjected for the SEM analysis to get the confirmation about the amorphous nature of the product from its morphological overview. Nevertheless, the EDX analysis of the same sample prominently provided a qualitative insight about the elements.
Summary
