Design and development of sustainable remediation ...

STOTEN-15771; No of Pages 7 Science of the Total Environment xxx (2014) xxx–xxx

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Design and development of sustainable remediation process for mitigation of fluoride contamination in ground water and field application for domestic use Poonam Gwala a, Subhash Andey a, Pranav Nagarnaik a,⁎, Sarika Pimpalkar Ghosh b, Prashant Pal a, Prashant Deshmukh a, Pawan Labhasetwar a a b

National Environmental Engineering Research Institute, Water Technology and Management Division, Nehru Marg, Nagpur, India Department of Fuel & Mineral Engineering, ISM, Dhanbad, India

H I G H L I G H T S • ChemoDefluoridation Unit (CDU) was designed for fluoride mitigation and evaluated in community of seventy five households. • CDU has a maximum efficacy at initial fluoride concentration of 6 mg/L; however, it can be effective with maximum initial fluoride concentration of 14 mg/L without any affecting drinking water quality. • The operational cost of CDU is estimated to be $0.002 per litre. • Successful demonstration of CDU at a community level, its acceptability by the community and improvement in the health indicated by user satisfaction, makes this technology a sustainable option for fluoride mitigation in a marginalized community with limited means.

a r t i c l e

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Article history: Received 17 April 2013 Received in revised form 16 January 2014 Accepted 16 January 2014 Available online xxxx Keywords: Fluorosis Defluoridation Chemo-defluoridation unit (CDU) Chemo-defluoridation process

a b s t r a c t Decentralised household chemo defluoridation unit (CDU) was developed and designed based on a combination of coagulation and sorption processes. Chemo-defluoridation process was optimised to reduce use of chemicals and increase acceptability among beneficiaries without affecting palatability of water. Chemical dose optimization undertaken in the laboratory using jar test revealed the optimum calcium salt to initial fluoride ratio of 60 for fluoride removal. Performance of CDU was evaluated in the laboratory for removal efficiency, water quality parameters, filter bed cleaning cycle and desorption of fluoride. CDU evaluation in the laboratory with spiked water (5 mg/L) and field water (~4.2 mg/L) revealed treated water fluoride concentration of less than 1 mg/L. Seventy five CDUs were installed in households at Sakhara Village, Yavatmal District in Maharashtra State of India. Monthly monitoring of CDUs for one year indicated reduction of the raw water fluoride concentration from around 4 mg/L to less than 1 mg/L. Post implementation survey after regular consumption of treated drinking water by the users for one year indicated user satisfaction and technological sustainability. © 2014 Elsevier B.V. All rights reserved.

1. Introduction According to estimates, groundwater is the source of domestic water for 80% of the rural and 50% of the urban areas in India (UNICEF et al., 2013). Naturally occurring fluoride concentrations in groundwater range from 0.5 to 48 mg/L in India depending on geological factors (Ayoob et al., 2008). Elevated concentrations are reported to be associated with leaching from the fluoride-bearing rocks like fluorspar, cryolite, fluorapatite and hydroxyapatite (Agarwal et al., 1997; Meenakshi et al., 2004), or with weathered formations of pyroxene amphibolites and ⁎ Corresponding author at: CSIR-National Environmental Engineering Research Institute (NEERI), Water Technology and Management Division, Nehru Marg, Nagpur, India. E-mail address: [email protected] (P. Nagarnaik).

pegmatites (Maliyekkal et al., 2008). Health effects caused by excess daily intake of fluoride, with drinking water as the major contributor, has affected people in 20 states of India (Maliyekkal et al., 2008). Depending on the daily fluoride intake and duration of exposure, fluorosis symptoms range from mild dental fluorosis to crippling skeletal fluorosis (Fawell et al., 2006; Hichour et al., 2000). Provision of appropriately treated drinking water is an important fluorosis mitigation measure. There are many defluoridation options. Defluoridation technologies based on chemical separation are sorption on solid filter media (Azbar and Turkman, 2000), chemical precipitation (Nawlakhe and Paramasivam, 1993; Azbar and Turkman, 2000; Reardon and Wang, 2000) and coagulation (Pinon-Miramontes et al., 2003; Mameri et al., 1998). Physical separation processes for defluoridation include electro-dialysis (Tahaikt et al., 2006) reverse osmosis (Arora et al., 2004) and nano-filtration (Hu and Dickson, 2006). These physical

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Please cite this article as: Gwala P, et al, Design and development of sustainable remediation process for mitigation of fluoride contamination in ground water and field appli..., Sci Total Environ (2014), http://dx.doi.org/10.1016/j.scitotenv.2014.01.054

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P. Gwala et al. / Science of the Total Environment xxx (2014) xxx–xxx

separation processes are effective for defluoridation but they also remove beneficial ions in water and are expensive compared to other technologies. Additionally, the large quantity of reject water makes these processes less preferred in water scarce areas (Zuoa et al., 2008). In industrial countries, treatment using activated alumina is the option of choice. However in poor rural settings, most technologies have failed in the field due to high costs, non-availability of skilled operators, unpalatable taste of treated water and impractical operational requirements. A simple and affordable technology that is acceptable to users is the need of the hour. Calcium phosphate-based adsorbents are effective and less expensive alternative to activated alumina for defluoridation (Turner et al., 2005). Bone charcoal or bone char, with the principal active component hydroxyapatite (Ca5(PO4)3OH), is used as an inexpensive material for defluoridation in developing countries (Larsen et al., 1993; Lambert and Graham, 1995). The hydroxide ion can be exchanged for fluoride to form fluorapatite (Eq. (1)). −

−

Ca5 ðPO4 Þ3 OH þ F →Ca5 ðPO4 Þ3 F þ OH :

ð1Þ

However in some communities, the use of animal bones is unacceptable due to religious beliefs. Alternative options are either the use of chemically produced hydroxyapatite for filtration or co-precipitation with hydroxyapatite produced by the addition of calcium and phosphate salts. There are several advantages of co-precipitation units over filter columns; ease of implementation, no requirement for filter material production and less need for monitoring and maintenance. Any calcium and phosphate compounds can be used to prepare the thermodynamically stable phase hydroxyapatite (Leamy et al., 1998), preferably the most economic and readily available ones. Fluoride is removed by either direct precipitation of fluorapatite and by the exchange of the hydroxide ion (Eq. (1)) in hydroxyapatite. The co-precipitation method was first evaluated in the 1930s (Adler et al., 1938; Behrman and Gustafson, 1938). Various approaches for producing hydroxyapatite at low temperatures to co-precipitate fluoride were tested and a few have been used for household water treatment (Gao et al., 2009; Poinern et al., 2011; Wang et al., 2011; Yu et al., 2013). The present research was performed to explore the integration and effectiveness of the chemical co-precipitation of fluoride with subsequent sand filtration in household water treatment units. The objective of the research was to develop a low cost, efficient and sustainable household treatment system that provides drinking water with fluoride concentrations b1 mg/L with a potential of field implementation in fluoride affected rural areas in India. The paper details the design, chemicaldose optimization and the performance evaluation in the laboratory and field implementation of the Chemo-defluoridation unit (CDU).

(2 bore wells) were additionally analysed for alkalinity, nitrate, sulphate, sodium and potassium. 2.2. Laboratory experiments 2.2.1. Jar test studies for dose optimization Batch experiments (1 l) were performed at room temperature using the Jar test assembly (Phipps & Bird Stirrer Model # 7790-402). Calcium chloride dihydrate (CaCl2·2H2O–Salt A) was added along with disodium hydrogen orthophosphate (Na2HPO4·2H2O–Salt B) in 1 l spiked water with an initial fluoride concentration of 5 mg/L. The concentration of Salt A was varied between 300 and 500 mg/L (2.7–4.5 mM) resulting in a salt to fluoride ratio of 60–100. The weight ratio of Salt A and Salt B was kept constant at 3:2 (Molar Ca: PO4 ratio = 1.84). Experiments were conducted at a constant stirring rate of 100 rpm for 1 min followed by 20 rpm for 20 min. Chemical reaction between salts resulted in flock formation, which was allowed to settle for 20 min. The flock was not removed from the jar assembly in consecutive batch experiments. Another set of dose optimization experiments was performed for different initial fluoride concentrations. Experiments were performed with Salt A to fluoride ratios ranging from 40 to 100. To achieve these ratios, spiked water concentrations were stepped from 5 to 20 mg/L with correspondingly varying Salt A concentrations from 200 to 1100 mg/L. The Salt A to Salt B weight ratio was kept constant at 3:2 during all experiments. Based on the process, a CDU was designed and the required chemical dose corresponding to initial fluoride concentrations was determined. 2.2.2. Chemo-defluoridation unit (CDU) Fig. 1 shows a schematic representation of the CDU designed to treat 75 l of fluoride-contaminated water per batch. It comprises a batch reactor (100 l) for coagulation/flocculation and a sand filtration unit (100 l). Chemicals and contaminated water are added to the batch

2. Materials and methods 2.1. Chemicals and analytical procedures Analytical grade chemicals and reagents were used without any further purification. A fluoride stock solution of 1000 mg/L was prepared by dissolving 2.21 g of oven dried sodium fluoride in 1 l distilled water. Spiked water was prepared by adding the required quantity of fluoride stock solution to tap water. Water was also collected from a fluoride-contaminated groundwater source (~4.2 mg/L) for laboratory experiments. Analysis of water-quality parameters was carried out as per standard methods (APHA, 2012). Fluoride concentrations were measured potentiometrically using a standard ion selective electrode method (Orion Star A214). All water samples were analysed for pH, electrical conductivity, fluoride, hardness, chloride and total phosphate. Field source water

Fig. 1. Schematic representation of CDU design.



a

3

Residual Fluoride(mg/L)

6

Salt A - 300 mg/L Salt A - 400 mg/L Salt A - 500 mg/L

5 4 3 2 1 0

Batches

b Weight Ratio: 40

Weight Ratio: 60

Weight Ratio: 80

Weight Ratio: 100

Fig. 2. Laboratory jar experiments a) at a fluoride concentration of 5 mg/L and different calcium and phosphate dosages, and b) as a function of salt to fluoride weight ratio [Salt A]: [fluoride]init.

reactor. The filtration unit consists of a top layer of sand with a particle size range of 0.6–1.4 mm (depth 17 cm) and a gravel (~9.0 mm) bottom layer with a depth of 3 cm. A Nylobolt cloth is placed over the sand layer, topped by a small layer of sand (2–5 cm). The filter media is cleaned after decanting the water from the top and removing the Nylobolt cloth along with the 2–5 cm sand layer. Sand and cloth are replaced as per the earlier configuration after washing.

2.2.3. Laboratory evaluation of CDU Laboratory experiments were conducted with 75 l spiked water (5 mg/L F) in each batch for consecutive 30 batches without cleaning the filter media. The required quantities of Salt A and Salt B with a weight ratio of 3:2 was added in each batch and mixed thoroughly. A sample (15 mL) of treated water was collected from the tap after each batch and analysed for fluoride and all water quality parameters listed in Section 2.1. The flow rate at the outlet of the filtration unit

was monitored to assess the required cleaning frequency of the filter media. Leaching of fluoride was studied on the spent CDU before cleaning of the filter media. Separate experiments were performed in the laboratory with distilled water (75 l) by varying pHs from 3 to 7 and fluoride free tap water. The experiment was repeated for 13 batches and the outlet water was analysed for fluoride content. The CDU was also evaluated in the laboratory with water from the field. The treated field water was analysed for all the water quality parameters listed in Section 2.1. 2.3. Field implementation After successfully completing laboratory experiments and design of CDU, Sakhara village in Yavatmal district of Maharashtra State was identified for field implementation. The population of Sakhara Village is 427 as per Census 2011 consisting of 56% female and 44% male.


4


Fluoride Concentration (mg/L)

5

95

4 90 3 85 2 80

1

% Reduction of Fluoride

100

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0 1

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batch Raw water Fluoride (mg/L)

Residual Fluoride (mg/L)


Fig. 3. Laboratory performance of the CDU unit with tap water spiked to 5 mg/L.

Flow Rate (ml/min)

1000

100

800

95

600

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between Indian Rupee 25,000 and 50,000 (USD 500–1000) and any type of water treatment remains unaffordable to most of the households in the village. The Agriculture Science Centre (ASC), a voluntary organisation assisted NEERI in the installation of the CDUs in the village and periodically interacted with the villagers in monitoring performance of CDUs. A teacher of local school also assisted users for any query related to CDUs which had helped NEERI and ASC in keeping them functional. The teacher was provided with the pre-weighed sachets for distribution and had the responsibility of informing NEERI of any dysfunctional CDU. The installed CDUs were monitored monthly for one year by determining fluoride in inlet and outlet water from randomly selected 15 households. The 30 samples (at inlet and outlet of 15 CDUs) were analysed for parameters mentioned in Section 2.1. The status of individual units, maintenance requirements and operational difficulties were assessed during the sample collection. A guidance manual in local (Marathi) language describing steps for operation and maintenance of CDU was provided in each household. A door-to-door survey to find out effectiveness and benefits was conducted before installation and after one year of CDU use.

There are 85 households out of which occupants of 75 households were willing to install CDUs. There are 2 to 8 persons living in each of the selected 75 households and the CDUs were installed to meet drinking and cooking water requirements in June 2010 to May 2011. A household survey indicated that drinking and cooking water consumption is about 5 l per person per day. The majority (65%) of residents of Sakhara village are farmers whereas the rest are engaged as agriculture labourers. About 30% of residents are illiterate while 35% have attended primary school. The village has two bore wells fitted with India Mark II hand pumps having fluoride in the range of 3.5–4.5 mg/L and the only alternate source of drinking water is supplied weekly by a water tanker. Due to consumption of fluoride-contaminated water, dental and skeletal fluorosis cases are observed in Sakhara. Information provided by the officials of Public Health Sub-centre located in nearby village Dhoki reported 62 cases (38 males and 24 females) of dental fluorosis and 5 cases of skeletal fluorosis. Dental fluorosis is prevalent among children (b12 years), whereas skeletal fluorosis was observed in the age group of 35–70 years. Data collected from Public Health Sub-centre further indicated prevalence of malnutrition among 39% of children of the village. Average annual income varied

75

0 1

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batch Initial Flow Rate of Filter (ml/min)


Fig. 4. Flow rate of water through sand filter as a function of usage.


Leached Fluoride (mg/L)


0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 2

3

4

5

6

7

5

TW (pH 8.5)

DW (pH 6.24)

DW (pH 5.5)

DW (pH 3.5)

8

9

10

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12

13

batch Fig. 5. Leaching fluoride from a clogged filter with distilled water (DW) at pH (3.5, 5.5 and 6.2) and fluoride free tap water (TW).

required. Addition of 1200 mg/L Salt A corresponds to increase in chloride concentration to 860 mg/L. WHO (1996) suggests that chloride is detectable above 250 mg/L and water potability is affected above 600 mg/L, indicating that the use of this technology is ideal for fluoride concentrations below 6 mg/L and its application is limited to concentrations below 14 mg F/L. Chemical dose corresponding to weight ratio of 60 was used during laboratory evaluation of CDU.

3. Results and discussion 3.1. Laboratory experiments

0

4.3 1.63

152

152

110

312

3.1.2. CDU evaluation in the laboratory Fig. 3 presents the results for successive batches of CDU evaluation with spiked water with fluoride concentration of 5 mg/L. Fluoride removal efficiency increased from 84% to 97% in the first four batches and was consistently above 96% thereafter. Outlet fluoride concentration was less than 1.5 mg/L after the second batch and consistently below 0.5 mg/L after the 30th batch. The difference in the efficiency during the initial four batches was due to an increase in flock content as observed in the jar test experiments. As the complex slurry started to accumulate, the filter bed matured and was able to further reduce the fluoride content. The impact of CDU operation on the flow rate and fluoride removal efficiency is shown in Fig. 4. With the increasing deposition of slurry on the sand filter the flow rate reduced from 850 to 50 mL/min, however, there was no change in fluoride-removal efficiency of the CDU. It was observed that when the flow rate was more than 600 mL/min, the efficiency fell below 95%. The highest efficiency was observed at flow rates of 200–300 mL/min. Also, at higher flow rates, desorption due to shear could result in reduction in the removal efficiency. However even at low efficiency, outlet fluoride concentration remained below

1.2 0

Field studies

-140

-268

-128

-72 -48

Lab Studies -3.62 -4.644

400 300 200 100 0 -100 -200 -300

-0.8 -0.37

Change in parameter (T°C, Concentration mg/L)

3.1.1. Jar test studies for dose optimization Fig. 2a shows the outlet fluoride concentration in consecutive batch experiments. It was observed that the residual fluoride concentration initially decreased with successive batches until it reached a constant value. Flocks formed during the chemo-defluoridation process were not removed from the jar, which augmented the fluoride removal in successive batches. The number of batches required to reach a constant value depended on the salt concentration. The target residual fluoride concentration of b1 mg/L was achieved after 5, 3 and 3 consecutive batches for initial Salt A concentrations of 300, 400 and 500 mg/L respectively. The Salt A concentration corresponded to a salt to fluoride ratio of 60, 80 and 100. Fig. 2b shows the fraction of final to initial fluoride concentration (F/F0) observed in successive batches for salt to fluoride ratios of 40, 60, 80 and 100. The salt to fluoride ratios 40, 60, 80 and 100 correspond to molar [Ca]:[F0] ratios of 5.2, 7.8, 10.4 and 13 respectively. In fluorapatite, the molar stoichiometric ratio (Ca:F) is 5 (Eq. (1)). This indicates that the salt to fluoride ratio of 40 is very close to stoichiometric ratio. Despite the mixing, it appears that an excess of the salts is required, so a salt to fluoride ratio of 60 was selected. Assuming a ratio of 60; to treat drinking water with an initial fluoride concentration of 5 mg/L, a dose of Salt A of 300 mg/L is required, while to treat water with 20 mg F/L 1200 mg/L of Salt A is

Fig. 6. Variation in water quality parameters for spiked water and field water in laboratory CDU experiments.


6


Fluoride Concentration (mg/L)

5

Initial F

Final F

4

3

2

1

0

Fig. 7. Monthly monitoring of CDUs at Sakhara village. The data points represent an average of 15 randomly selected units.

1.5 mg/L. Clogging of the filter media was also indicated by a rise in the water level. The filter media required cleaning every 25–30 batches. The total quantity of flocks filtered through sand bed was estimated to be 400 g after 25 batches for initial fluoride concentration of 5 mg/L. Fig. 5 represents the observed outlet fluoride concentration with distilled water at pHs (3.5, 5.5 and 6.2) and fluoride-free tap water. The outlet fluoride concentration for all the experiments remained below 0.4 mg/L except in one experiment with distilled water at pH 5.5, where a fluoride concentration of 0.8 mg/L was determined. Since, there is limited fluoride leaching from the used CDU under the acidic to slight alkaline operating conditions, the flocks can be discarded without further treatment during regular maintenance of filter media. Fig. 6 presents the variation in water quality parameters in laboratory experiments observed at the CDU outlet for spiked water and field water. The chemical dose for field water was adjusted to maintain salt to fluoride ratio of 60. The fluoride concentration reduced from 5 to 0.36 mg/L and 4.2 to 0.55 mg/L in spiked water and field water respectively. Other water quality parameters, such as chloride and total phosphate increased while alkalinity and pH reduced at the outlet for field and spiked water. Except for hardness the water quality parameters for field water and spiked water remained consistent. All the water quality parameters remained within the permissible limits for drinking water as per drinking water quality standards in India i.e. BIS 10500:2012. Hence, the salt to fluoride ratio of 60 was maintained in CDUs during field implementation.

mechanical failure, such as outlet-tap breakage was observed to be the cause of failure. The survey undertaken in 63 households after 1 year revealed that the CDUs were used in 50 households every day, whereas the rest of the CDUs were only used intermittently. Awareness of the cause of fluorosis increased from 12% to 82% after the installation of the CDUs in the village due to interaction with NEERI and ASC. Residents of 54 households were satisfied with the quality of the treated water based on taste, odour and overall acceptability. The survey indicated that 94% of households found gradual relief from joint pain after using CDU for one year. In a willingness to pay survey carried out at the end of the first year about 82% of the respondents expressed their inability to pay for chemicals of the CDU. This is due to the fact that the water is supplied free of cost in Indian villages and these respondents wanted the Government agencies to pay for the chemicals. The CDU basic unit cost is estimated at around $35. Sand and gravel used in the filter are locally available and are included in the basic unit cost. The CDU operational cost is due to usage of chemicals during the process. The chemical cost per litre of treated water is estimated to be $0.001, $0.002, $0.0025 for initial fluoride concentration of 2, 4, 5 mg/L respectively. Assuming one batch of 25 L per day, the monthly cost to treat 5 mg/L fluoride contaminated water would be $2. Maintenance of the unit, including minor repair for leakages and timely sand replacement would be $2–$3 per annum.

3.2. CDU field installation and monitoring

4. Conclusion

Of the 75 CDUs, 63were found to be functional after 1 year of installation, with a gradual increase in dysfunctional CDUs. Fig. 7 shows the mean fluoride concentration of the raw and treated water collected from 15 randomly selected CDUs. The error bars indicated the standard deviation within the 15 CDUs at each sampling. The average initial fluoride concentration ranged from 3.27 mg/L in June to 4.56 mg/L in May. Variation in the fluoride in the raw water is attributed to influx of fresh or storm water after a rain event. Fluoride in the treated water was found to be b 1 mg/L throughout the year in functional CDUs, with average values ranging from 0.6 mg/to 0.86 mg/L. There were problems like clogging of the sand filter, breakage of the outlet tap, improper cleaning of CDUs (such as mixing of sand layer and gravel layer), improper dosing of chemicals etc. faced by the users. NEERI replaced 7 CDUs and repaired 32 CDUs and in most instances

The CDU was optimised and evaluated for fluoride removal in the laboratory and in 75 households at Sakhara, India. The CDU consistently maintained treated water fluoride concentrations below 1 mg/L in the field, which is the permissible limit as per BIS 10500:2012 drinking water quality standard. Laboratory experiments indicated that the use of CDU would be most effective up to initial fluoride concentration of 6 mg/L and its application is limited for initial fluoride concentration below 14 mg/L because of the increase in the dissolved chloride and phosphate concentrations due to addition of Salt A and Salt B in the treated water. The majority of CDUs were found to be functional in Sakhara village after 1 year (63 out of 75). Dysfunctional CDUs were either due to mechanical failure like leakage or reluctance of the use of CDUs by the individual households. However, the high usage of the CDU was



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