Design of a defluoridizing unit

Chapter 13
Design of a Domestic Defluoridizing Unit
Shaheda Parveen, Venkata Nadh Ratnakaram , Sireesha Malladi
and K. Kiram Kumar
Abstract While trace amounts of fluorine are essential for life, its excessive intake
leads to a disease known as fluorosis. It is a predominant ailment in majority of the
countries inclusive of India. It is caused also by drinking fluoride containing water.
Retention of fluorine in bones and teeth occurs through F−
–(OH−
) exchange on their
inorganic component known as hydroxylapatite. Endeavour of the present study is to
design a column using activated alumina as an adsorbent for continuous defluorida-
tion of water for domestic purpose. As a part of it, operational defluoridation capacity
of alumina was determined by variation of different factors (amount of alumina, time,
temperature, added salts). A family of four members was taken as a model. The ini-
tial and final fluoride concentrations were taken as 2.0 and 0.7 ppm, respectively.
Dimension of the unit (adsorbent bed diameter and height) was determined.
Keywords Design · Domestic defluoridizing unit · Fluorosis
13.1 Introduction
Human beings and animals suffer from fluorosis due to extended acquaintance with
fluoride containing water, air or food. India stands in the top listed countries which
are challenged with fluorosis problem in view of obligatory consumption of water
having high quantities of fluoride (1–48 ppm) [1]. In India, Andhra Pradesh State is
one among the many affected belts by endemic fluoride [2]. As per Indian standards,
S. Parveen
Department of Chemistry, Madurai Kamaraj University, Madurai 625021, India
V. N. Ratnakaram (B)
GITAM University, Bengaluru Campus, Bengaluru 561203, Karnataka, India
e-mail: doctornadh@yahoo.co.in
S. Malladi
Department of Science and Humanities, Division of Chemistry, VFSTR, Vadlamudi 522213, India
K. Kiram Kumar
Department of Chemistry, KBN College, Vijayawada 520001, India
© Springer Nature Singapore Pte Ltd. 2020
B. Subramanian et al. (eds.), Emerging Technologies for Agriculture
and Environment, Lecture Notes on Multidisciplinary Industrial Engineering,
https://doi.org/10.1007/978-981-13-7968-0_13
173

174 S. Parveen et al.
acceptable limit of fluoride concentration in drinking water is 1.0 mg L−1
, whereas
its permissible limit is 1.5 mg L−1
[3]. Though higher concentrations of fluoride
cause fluorosis, its lower concentration is proved to be useful. Fluoride plays a vital
role in mineralization of some of the hard tissues [4]. Fluoride is added to community
water in some of the countries including USA as a part of health measure in order to
inhibit dental decay [5]. Fluoride concentration in mg L−1
and associated potential
health effects can be given as: <0.5 (dental carries are prevented minimally), 0.5–1.5
(favourabletopreventthedentalcarries),1.5–4(causesdentalfluorosis),4–10(dental
and skeletal fluorosis) and >10 (crippling fluorosis) [6]. Dental fluorosis can be easily
recognized by symptom—mottling of teeth [7].
About 300 mg of fluorine per kg is its abundance in earth crust [8]. Fluorspar,
cryolite and fluorapatite are some of the fluorine-containing minerals which undergo
weathering process leading to its accumulation in soil and hence in water [9]. In
some of the areas like Ethiopian Rift Valley, high fluoride concentration in drinking
water can be related to volcanic deposits being the natural fluoride sources [10]. High
concentrations of fluoride were noticed in thermal waters having basic pH [11]. Dif-
ferent anthropogenic and industrial activities release fluoride into the environment.
Some of them are manufacturing units of fertilizers, pesticides, glass, electronics and
aluminium [12].
Water supply in rural areas of a maximum number of countries is suffering from
poor quality and quantities. Ion exchange, reverse osmosis (RO) and electro-dialysis
are the popular methods in developed countries, but they require high capital cost,
maintenance and technical backing [13]. Problems in disposal of large volumes of
sludge and high operational cost are opposing the adopting the well-known Nal-
gonda technique. The other defluoridation methods (adsorption, nanofiltration, ion
exchange, solar distillation and reverse osmosis) have their own advantages and dis-
advantages. Hence, adoption of the method must be based on local needs. Compared
to other defluoridation methods, adsorption method is more preferable in view of
easy operation, lack of sludge formation, low capital cost and easy replacement
of exhausted bed [14]. However, the well-known adsorbent—animal charcoal—is
facing resistance due to the prevailing social customs. Though a large number of
cost-effective biosorbents are reported for defluoridization in the literature, their
technical usage in rural areas is not possible in immediate course of time. In spite
of availability of a wide variety of adsorbents, AA is the best choice in developing
countries. Hence, activated alumina is selected as an adsorbent in the present study.
The rural community in Chebrolu mandal has been distressed by fluorosis (both
dental and skeletal) due to consumption of water having more fluoride concentration
than the permissible limits. Chebrolu mandal of Guntur District, Andhra Pradesh,
was taken as one unit for fluoride estimation in groundwaters. Apart from fluoride
estimation, chloride and total hardness were also estimated. Based on this compila-
tion, it is found that in some villages like Gudavarru and Veera Nayakuni Palem, the
fluoride concentration is more than the limit 1.5 ppm given by WHO. An effort has
been made to design an economically practicable defluoridation unit with continuous
flow, which can be used in rural areas.

13 Design of a Domestic Defluoridizing Unit 175
13.2 Experimental
13.2.1 Selection of Activated Alumina
The literature survey shows that cost of the treated water by activated alumina is less
compared to other defluoridizing agents [15]. Compared to adsorption of CaCO3
(0.03 mg F−
per g), activated alumina is proved to be better adsorbent (0.57 mg F−
per g) [15, 16]. A number of researchers are worked on defluoridation of water using
AA since the suggestion of Boruff [17]. Bulusu and Nawlakhe [18] established that its
sorption capacity is variable and depends on water quality parameters, ionic environ-
ment and regeneration procedures. Simultaneous occurrence of different reactions at
AA surface (like ion exchange and hydrolysis) causes a variation in surface electro-
kinetic properties [19]. It helps to increase the sorption of fluoride ions due to a
decline of negative charges at its surface [18]. Hence, activated alumina was selected
as an adsorbent for defluoridation of water. Commercially available technical grade
activated alumina granules supplied by Sethiya Solvents & Chemicals, Tamil Nadu,
were used.
13.2.2 Batch Adsorption Studies
In each system, 300 mL of water (containing 2 ppm of fluoride, 113 ppm of chloride,
420 ppm of total hardness) is taken in a 500-mL polyethylene container and main-
tained at 31 °C. The solution is stirred at 240 rpm mechanically for definite period
after the addition of desired amounts of adsorbent. The suspension is filtered through
a sintered crucible (G4). The filtrate is tested for fluoride by SPADNS method, chlo-
ride by silver nitrate method and hardness by EDTA method [20]. The final fluoride
concentrations were measured by Zr-SPADNS method [21]. The fluoride concen-
tration in the sample after defluoridation is calculated from absorbance value of its
coloured solution and standard curve. The same procedure is adopted for all the
experiments carried out by varying amount of adsorbent, contact time, temperature
and added salts. The percentage removal of fluoride/chloride/total hardness of water
is calculated from C0 (initial concentration in solution) and Ce (final concentration
in solution after adsorption).

13.3 Results and Discussion
13.3.1 Batch Mode Defluoridation of Water by Activated
Alumina
It is very well-known fact that activated alumina acts as an efficient defluoridizing
agent. Therefore, it is thought worthwhile to know the effect of various factors affect-
ing the defluoridation capacity of activated alumina, before designing the column for
continuous defluoridation of water using activated alumina as an adsorbent.
It is observed that with an increase in the amount of activated alumina, an increase
in the % removal of fluoride is observed (Fig. 13.1a). It can be explained based on
the increased active sites availability causing an improvement of active surface area,
especially at higher adsorbent doses [22]. Afterwards, equilibrium is attained beyond
certain dose. The effect of the agitation time on the removal of fluoride ion is shown in
Fig. 13.1b, whichclearlyindicates that theremoval of fluorideis fast duringearlytime
but turns out to be slow and almost decays with a further rise in the interaction period
beyond 10 min. It hints that the fluoride removal rate is greater in the early phase
because of availability of sufficient adsorbent surface area. A decrease in adsorption
takes place with further increase in contact interaction period because of a decline in
the accessibility to the active sites. Percentage of fluoride ion adsorption increased
with an increase in contact time and the observed can be endorsed to the point that
higher duration makes acquainted the fluoride ion to make adsorb on adsorbent [23].
The maximum adsorption percentage was recorded at 10 min with all adsorbents,
and then onwards, sluggishness was observed. Per cent of defluoridation increased
up to 31°C and then decreased (Fig. 13.2b). It shows that chemisorption is activated
with an increase in temperature. Further confirmation of governing of chemisorption
in the fluoride adsorption on activated alumina is evident as it obeys Langmuir’s
adsorption isotherm [24].
To study the effect of the existence of anions (like chloride) and cations (like cal-
cium) on the defluoridation capacity of water, sodium chloride and calcium chloride
were added. There is no effect of added anions like chloride on the defluoridation of
Fig. 13.1 Effect of a adsorbent dose and b time on % removal of fluoride

Fig. 13.2 Effect of a salt type and amount, and b temperature on % removal of fluoride
water as there is no change in the defluoridization capacity with a rise in [chloride
ion]. But, the concentration of fluoride decreased progressively with an increase in
the concentration of the added calcium chloride (Fig. 13.2a). The observed decrease
in fluoride concentration can be attributed to the added calcium ion because there is
no effect of chloride ion on defluoridation. Moreover, it is an established fact that
the fluoride can be removed as calcium fluoride by the calcium ions. In spite of high
affinity towards fluoride by adsorbents containing Al, Zr, Mg and Ca, the presence
of some anions (nitrate, sulphate, bicarbonate and phosphate) decreases their affinity
[25–27]. Adsorption of chloride and nitrate ions on activated alumina is negligible,
whereas sulphate and bicarbonate ions adsorb fairly well. Further, an increase in
bicarbonate ions decreases the adsorption of fluoride on activated alumina due to its
competitive adsorption [15]. As many other anions are present in the groundwater
taken in the present case, the efficiency of fluoride ion adsorption decreases further.
Inviewofpracticalconditions,throughoutthepresentstudy,defluoridationstudies
were carried out at neutral pH. However, defluoridation capacity of AA was reported
as 3, 5.6 and 20 mg g−1
for alkaline, neutral and acidic water, respectively [24]. Davis
and Leckie [28] reported that fluoride sorption capacity decreases with an increase in
pH due to a decline in the establishment of ligand-like complexes. It is a known fact
that defluoridation by adsorption is interfered by hydroxide ions. Defluoridation by
activated alumina can be explained by Stern model of double layer. Charge potential
on alumina is zero when pH is 9. As hydrogen ion is the potential-determining ion
for the surface charge of AA, it is positively charged when [H+
] is greater (pH <
9). Hence, negatively charged fluoride ion adsorbs on oppositely charged AA. In
addition, adsorption process is also governed by “specificity factor” and adsorption
order: F−
OH−
HCO3
−
≫ Cl−
= SO4
2−
[24]. Therefore, defluoridation
capacity of AA decreases with an increase in [HCO3
−
].

13.3.2 Determination of Operational Defluoridation Capacity
of Alumina
50 g of activated alumina is added to 1 l of water containing 2 ppm fluoride, 113 ppm
chloride and 420 ppm of total hardness and allowed to be at equilibrium for 24 h and
then filtered. The amount of removed fluoride is estimated colorimetrically based on
the concentration of fluoride in the water before and after adsorption. The process is
repeated with the same alumina with the addition of fresh samples of water added at
the same amounts fluoride, chloride and total hardness. This process is repeated till
there is no defluoridation by the alumina. The total defluoridation capacity is found
to be 57 mg fluoride/50 g of alumina. Therefore, operational defluoridation capacity
of the alumina is 1.14 mg fluoride g of alumina. The literature survey shows that
adsorption capacities (mg g−1
) of AA (OA-25 grade), AA (AD101-F grade), acidic
alumina, modified AA and third aluminium hydrate (TAH) are compiled as 1.78,
0.42, 8.4, 0.76 and 25.8, respectively [29]. Probably, there exists a high variance in
“degree of activation” in variety of alumina products. Another justification may be a
difference in pH in addition to the brand of the product. Most of the previous research
works involve the usage of distilled water for fluoride ion solution preparation and
high grade alumina with high adsorption capacity. But in the present study, it is
based on water having different salts and technical grade alumina. Therefore, the
operational capacity—1.14 mg/g—is considered while designing the column for
defluoridation.
13.3.3 Design of Continuous Defluoridizing Unit
Compared to other defluoridation methods, packed bed adsorption method is more
preferred. Some of its advantages are easier operation, reasonable capital and oper-
ational costs, no sludge formation and easy replacement of bed [30]. Batch method
faces three main problems, i.e. (1) drying of filter bed and hence shortening of life-
time of filter for intermittent usage, (2) incomplete utilization of adsorbent and (3)
limited release of defluoridate water at once. Hence, a continuous defluoridizing unit
is designed in the present study. Combined filter contains two filter processes in
the same structure to remove fluoride and microbes by using adsorbent and silver
nitrate, respectively. However, in view of disadvantages like higher cost, maintenance
requirement, low susceptibility to higher turbidity, etc., the concept of combined fil-
ters was not considered.
An attempt has been made to design the column using activated alumina as an
adsorbent for defluoridation of water continuously for domestic purpose. A family
of four members was taken as a model. The initial and final fluoride concentrations
are taken as 2.0 and 0.7 ppm, respectively.
Daily personal water demand = D = 3 l/consumer/day
Number of users = N = 4

Operation period = OP = 2 ½ months = 75 days
Operational sorption capacity = C = 1.14 g/kg
Bulk density of alumina = ρ = 1.12 kg/l
Raw water fluoride concentration = Fi = 2.0 mg/l
Mean fluoride concentration in treated water = 0.7 mg/l
Volume ratio supernatant water/medium = VRSW/M = 0.2
Daily water treatment = Q = D × N = 12 l/day
Overall quantity of treated water in a filter period = VT = Q × OP = 900 l
Overall quantity of removed fluoride in a filter period = FT = VT ×(Fi −Ft)
1000
= 1.17 g
Amount of Al2O3 required for renewal = M = FT/C = 1.03 kg
Volume of Al2O3 in filter = VM = M/ρ = 0.916 l ∼= 0.95 l
Treated no. of bed volumes during filter period = BV = VT
VM
= 982
Supernatant water volume capacity = VSW = VRSW/M × VM = 0.183 l ∼= 0.2 l
Total volume of filter = VF = Vm + VSW = 0.95 + 0.2 = 1.15 l.
13.3.4 Corresponding Dimensions
Karthikeyan et al. [24] studied the rate of water flow in a column filled with activated
alumina. According to them, variation in column height and particle size causes a
substantial alteration in water flow rate. Particle size increase caused a rise in pore
volume leading to an enhancement of flow rate. Bed height increase results in an
enhancement of offered resistance which leads to a decline in flow rate. In view of
these two reasons, selection of bed height and particle size is the important parameter
to be considered while designing a defluoridation unit. Important issue to be noted
is that minimum contact period is required between adsorbent and fluoride ion to be
removed. At the same time, very lower flow rate design will not be accepted by end-
users. Taking into consideration the equal weightage of flow rate and defluoridation
capacity, it is decided to choose the intermediate size of the particle in the range
140–180 mm because both factors are optimal in the chosen range. Based on it,
fixation of column height was done. Moreover, it is always advisable to select the ratio
of column diameter to particle diameter as more than 20 to decrease the channelling
chances [31, 32]. Several trails were made with different dimensions, and it is found
that the optimum diameter as 7 cm for the decided quantity of alumina.
Filter diameter (selected) = Φ = 7 cm
Total height of the filter = HF = VF/[π × (Φ/2)2
] = 29.897 cm ∼= 30 cm.
13.3.5 Defluoridation Studies on the Designed Unit
PVC pipe having an internal diameter of 7 cm was used for this study. The column
was connected to a feed tank with inlet control valve. Activated alumina bed was
fixed, and proper distribution in it was ensured by packing it between two circular

Fig. 13.3 Design of the
proposed house hold
defluoridation unit
perforated PVC sheets. Plug pairs encompass these sheets which function as filter-
ing medium to remove any dust/dirt from inlet and support to the bed, respectively
(Fig. 13.3). Random packing of adsorbent is ensured by initial filling of the column
with deionized water followed by activated alumina, whereas the reverse filling order
may result in a decline of adsorbent efficiency due to the development of air gaps
which leads to further channelling phenomenon. A closely packed adsorbent bed
was ensured by soaking with distilled water and leaving for overnight. It is a known
fact that activated alumina on hydration changes from the mineral form boehmite
(AlOOH) to gibbsite (γ-Al(OH)3) [33]. Further, it was also established that fluoride
better uptake by it happens due to a change in surface morphology, which is done
by soaking it for one day [34]. Hence, the above overnight soaking of bed with dis-
tilled water also helps to improve defluoridation. Process was carried out at room
temperature. Column kept filled with feed water even during resting period. Practical
aspects in the usage of AA for defluoridation of water were studied by regulating the
flow rate in order to maintain minimum 5 min contact time. Input and output were
collected at regular intervals of time in order to analyse the fluoride concentration.
The residual amount of fluorine in outlet water was found to be within permissi-
ble limits. However, the three key issues involved in realization of any household
defluoridation units are (1) motivation, (2) appropriate and cheap technique and (3)
technical support including training on efficient regeneration to one of the villagers
to continue without any support from external agencies [35]. Similar to defluorida-
tion studies, decontamination units may be designed for the removal of heavy metals
using agricultural waste as adsorbents [35–39].

13.3.6 Regeneration of Bed
A fourfold higher regeneration was observed with 0.1 N sodium hydroxide solution
compared to 0.1 N hydrochloric acid solution. High negative charges are created on
the surface of AA during elution of fluoride using sodium hydroxide. Hence, the
already adsorbed F−
experience repulsion from the surface leading to their diffusion
into solution [24]. For regeneration, the exhausted AA was shifted into a bag made up
of nylon mesh and immersed in alkali solution for overnight with intermittent stirring.
A marginal decrease in adsorption capacity was observed even after 25 regeneration
cycles.
13.4 Conclusions
Lower values of operational defluoridation capacity of alumina can be explained
based on the presence of competitive ions in the water to be treated. A continuous
defluoridation unit was designed for domestic purpose using activated alumina as an
adsorbent. The residual amount of fluorine in outlet water from the unit was found
to be within permissible limits. Sodium hydroxide solution (0.1 N) was found to be
the best regenerating agent for the exhausted bed.
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