Activated  Alumina  (AA)  technology  is one of the widely  used  adsorption/  ion exchange methods for the defluoridation of potable water and many reports are available on large-scale installations (4, 5). Defluoridation in such units is carried out under supervision of skilled personnel and treated water is supplied to townships. The quality of treated water from such facilities  is  assured.  However,  this  approach  is  not  immediately  feasible  in  developing countries, especially in rural areas. Treatment may only be possible at a community level i.e. handpump installations or at the 'point of use', i.e. domestic level. Reports on the adaptation of AA technology at handpump or home units were scarce till 1990's as this technology was rarely used in developing countries.

During 1980-1990, use of indigenously manufactured activated alumina for fluoride removal was reported by few laboratories, including IIT Kanpur. Activated Alumina manufactured by Associated Cement Company and Indian PetroChemicals Ltd. (IPCL.) were used in most of these  studies  (6,  7).  Venkateswara  Rao  and  Mahajan  (8)  reported  the  development  and evaluation of domestic and handpump units where activated alumina was used as the defluoridation  medium.  Kartikeyan  et. al. (9) screened  three different  grades  of activated alumina  (particle  size,  <  0.4  mm)  for  fluoride  uptake  capacity  as  well  as  designed  and evaluated domestic defluoridation unit.

Studies carried out on defluoridation of drinking water at IIT Kanpur since 1991, using AA technology, are presented briefly in this report. Financial assistance from UNICEF during this entire period is gratefully acknowledged.

2. MAJOR AREAS OF STUDY:

The initial intention of the research project in 1993 was to develop and field test a handpump based defluoridation unit that could be maintained by local communities. Around 1996, the focus of the research changed to finding solutions for domestic defluoridation. At the same time, the discontinuation of the grade of AA used (manufactured by IPCL) during 1991-96, led to the screening of other grades indigenously manufactured of activated alumina for defluoridation application.

Following are the major areas of study:

1. Development of handpump attached defluoridation unit.

2. Screening of indigenous activated alumina grades in domestic defluoridation units.

3. Development of Domestic Defluoridation Units.

4. Regeneration procedure for exhausted AA and the reuse potential of AA.

5. Safe disposal of spent regenerants.

2.1.  Development of handpump attached defluoridation units:

A cylindrical defluoridation unit was fabricated from MS sheet with the dimension of 0.5 m diameter and 1.5 m height. The unit was designed to operate in the upflow mode. 110 Kg of AA of grade G-87 (IPCL), having a particle size range of 0.3-0.9mm, was taken in the unit. This gave a bed depth around 55 cm. This unit was field tested at Makkur village, Unnao district U.P. For experimental study, a shallow India Mark II hand pump (≈35 ft depth) was installed.  The  defluoridation  unit installation  was  in1993.  It required  the raising  of hand pump discharge level with an addition to its normal pedestal and construction of an elevated platform. Users had to go up few steps to operate the handpump. A by-pass was provided to draw the water directly from the handpump for washing and bathing.

The unit was maintained by lIT Kanpur. Raw water fluoride concentration was in the range of 6-7 mg/L. Regeneration of exhausted activated alumina was carried out 'in situ i.e. within the column. This procedure   required   8-10  hrs.  Average yield   of   the   safe   water   (<1.5   mg/L fluoride) per cycle was around 25,000 litres.   Seventeen   defluoridation   cycles were  completed  in  a  span  of  4  years. There  was  no  major  maintenance problem during this period. There was no complaint from the users either regarding the design or the palatability of treated water. However; community help during regeneration was minimal. The unit was dismantled  in  1998,  as  village community got an access to piped water supply. With this installation, there was no provision for spent regenerant disposal. During this period, UNICEF approach changed from community  based to domestic defluoridation unit. Hence further modifications,  like incorporating  provision for disposal of regenerants, were not taken up.

Similar  defluoridation   units  were  fabricated  and  installed  by  M/s  Gudimani  for  few handpumps in Shivpuri, MP. No performance details are available.

As per our knowledge, six different models (designs) of handpump attachable units (which include two different models developed by PHED Rajasthan and DST Rajasthan) have been evaluated for the defluoridation of drinking water (10 - 14). All these are either experimental units or under the supervision  of PHED and / or NGOs. Out of these, one unit has been evaluated with a specific grade of Activated Alumina, AAFS - 50, manufactured by Alcal Chemicals Limited, UK. As per the manufacturers, this product has five times higher uptake capacity as compared to “normal” Activated Alumina (15). This would make it cost effective for one time use and disposal in a landfill, instead of "regeneration and reuse" required for “normal”  Activated  Alumina  systems.  Most  of the field  studies,  with  this  grade,  are  for arsenic removal either in handpump installations or the domestic unit. PHED, West Bengal seems to have installed many handpump units (15). Trials on Fluoride removal have been conducted on PHED handpump sites near Bhopal at various locations. Evaluation of their performance has been carried out by Regional Research Laboratory, Bhopal (14).

Comparison   of  performances   of  these  handpump  defluoridation   unit  as  well  as  their limitations can lead to a better design. Another aspect to be considered, with handpump units, is the mode of regeneration of Activated Alumina. If in situ regeneration is to be carried out, then there should be a provision for the proper disposal of spent regenerants in the vicinity. A second option can be to collect exhausted activated alumina from the unit and regenerate at a central location

2.2.  Screening of different grades  of AA:

The search for solutions for domestic defluoridation and the discontinuation of the grade of AA   used   during   1991-96,   mentioned   earlier,   led   to   the   development   of   Domestic defluoridation Units (DDUs) and to the screening of indigenously manufactured grades of activated alumina for defluoridation application.

Two parameters were considered as important for the field application of activated alumina. One was fluoride uptake capacity (FUC) expressed as milligrams of fluoride removed per Kg of AA and the second was reuse potential of AA in multiple defluoridation cycles.

(a) Screening in DDU:

Since water alkalinity was known to decrease fluoride uptake capacity by AA, most of the screening studies were carried out with test water prepared by spiking ground water (IITK borewell water) with NaF. Test water fluoride concentration was generally maintained at 10 ±0.5 mg/L. All screening tests were performed with 3 Kg AA in a IITK fabricated domesticunits, (details given in a separated section). During 1991-2005, more than 15 grades of indigenously manufactured AA have been screened. Many of them are tested upto 10 defluoridation cycles, i.e. regenerating exhausted AA and reusing the same AA for the next cycle.

Under these experimental conditions used, FUC of indigenous AA grades ranged from 1500 mg/Kg AA and 2200 mg/Kg AA Studies have also been carried out on the effect of raw water characteristics  as well as AA particle size, empty bed contact time on FUC, with selected grades of AA

Presently  AA  grade  AAFS-50  (Alcal  Chemicals  Co.  U.K.)  is  being  evaluated  in  our laboratory for fluoride uptake capacity. As mentioned earlier, this grade is reported to exhibit high FUC as compared to ordinary AA thus making it cost effective for one time use.

The main outcomes from these studies are the improvement of indigenous AA grades and their availability of in the desired particle size range.

b) General Screening procedure

A  uniform  testing  procedure  for  the  screening  AA  grades  is needed,  if the  results  from different  laboratories  are  to  be  compared.  Further  manufactures  need  to  test  their  own products   for   improving   AA   quality.   UNICEF   specification   also   required   that   AA manufacturer / suppliers quote fluoride uptake capacity (FUC) of their product.

Determining FUC, using fluoride spiked distilled water as test water, has no significance as other ions present in groundwater that would affect the binding of fluoride to AA, would be absent. Groundwater quality varies from place to place. Hence, a simulated groundwater, prepared by adding required chemical components, has to be used for a uniform screening procedure by different manufactures

With this view, a screening procedure was developed in our laboratory. Simulated test water was prepared by adding known quantities of NaHCO3, Na2SO4, CaCl2  Na2SiO4, MgCl2  and NaF to distilled water and adjusting the pH to 7.8± 0.2 Screening was performed in a PVC/ Perspex column with 500 gm of AA. Details of experimental setup and procedure are given in Annual report, Jan-Dec 2004 (contract no.SSA/INDQ/2004/00000934-0).

This procedure can be used by

(a) by manufacturers for FUC determination of their products.

(b) For comparing different AA grades in any laboratory.

(c) For comparing  FUC of the same AA grade in two different laboratories  i.e. for inter-laboratory comparison of results.

Such  an  approach  of  using  simulated  groundwater,  for  comparing  different  AA  grades (including AAFS-50) for arsenic removal, has been recently reported by Clifford et.al. (16)

Presently  column  screening  procedure  involves  intermittent  flow  of  raw  water,  which simulates   feed  conditions.   Time  required  for  screening   can  be  further  decreased   by continuously passing raw water through the column. FUC under these conditions may be less as  compared  to  intermittent  addition.  However  this  procedure  can  be  used  for  quick comparison of different grades.

2.3.  Development of Domestic Defluoridation Units - DDUs:

DDUs were initially designed on the assumption  that  20  litres  of  treated  water was the daily requirement for cooking and drinking for a family. With this criterion, it was expected that 3 Kg. activated alumina would be exhausted in 2 to 3 months if fluoride concentration in water was around 4 mg/L.

DDU fabricated in the laboratory  consisted of two chambers, fabricated from GI sheet. 3Kg of AA was taken in the upper chamber (24 cm dia. x 27 cm height), which gave a bed depth  of 9 cm. A flow control  device was   fixed   at   the   bottom   of   the   upper chamber  so as to have a flow rate of 8-10 liters per hour.

From this starting point, different versions of DDUs  have  emerged.  The  quantity  of activated  alumina  has  generally  become  4Kg to 5 Kg and materials used for filter unit include SS, HDPE (Sintex), PVC, and Terracota pots. Size of these containers  is decided based on the volume of water.

FUC depends on various factors such as raw water fluoride concentration, alkalinity, pH, as well as AA grade, particle size, contact time of raw water with the sorbent and AA depth. Raw water characteristic change from location to location. AA grade, particle size (0.4-1.0 mm)  and  flow  rate  (8-10L/hr)  had  already  been  specified  by  UNICEF  for  DDU.  Hence studies were carried out on the effect of AA amount and depth on specific safe water yield (SSY). SSY is defined as liters of safe water yield per kg AA

Result of these studies showed:

(Experimental details in Annual Report, Jan. 2004-Dec, 2004, contract no. SSA/INDQ/2004/934-0)

These observations  clearly indicate the importance  of AA depth in safe water yield. Atleast minimum of 9cm depth is to be maintained in DDU although higher depth is preferable.

Presently prototype domestic defluoridation  units sent by UNICEF are being evaluated for their performance. These units have different AA grades, vary in AA amount as well as AA depth.

2.4.  Regeneration of Exhausted Activated Alumina:

Regeneration of exhausted AA and its reuse for multiple cycles is one of the main advantages of using AA for defluoridation. Extensive studies were carried out on this aspect. Different regenerants used included alum, HCI, H2SO4 and NaOH. The results clearly indicated that efficient regeneration could be achieved with a combination of 1% NaOH and 0.4N H2SO4' Some screened AA grades showed less than 20% loss during 10 defluoridation cycles.

A simple 'dip regeneration procedure', appropriate for a rural set up, was developed. This required the transfer of activated alumina from domestic units to a nylon bag, dipping the bag in 10L 1 % NaOH for 8 hours (or overnight) with intermittent mixing. After washing once with raw water to remove excess alkali, the bag with AA was dipped in 10L of 0.4NH2SO4 for 8 hrs. This was followed by washing with raw water to raise the pH to 6. The regenerated activated alumina was ready for the next cycle.

The Dip regeneration method has been adapted in many pilot project areas in Rajasthan, A.P and UP. Main limitation of this method appeared to be intermittent mixing and the long time required for regeneration.

Presently 'Bucket regeneration' procedure' has been developed which addresses the two disadvantages of the Dip method. A plastic bucket with flow control device is used to continuously pass the regenerant over the exhausted activated alumina bed. Studies indicated the time required for regeneration decreases substantially and changes by changing the flow rate of the regenerants. This method is more user friendly and can be easily adapted in rural setups.

Initially the regeneration procedure was optimized for 3 Kg activated alumina in the domestic unit.  However,  since  4  Kg  to  5  Kg  of  AA  is  commonly  being  used,  studies  are  being conducted to arrive at the optimal weight of AA (keeping depth of the AA bed constant) and the corresponding optimal regeneration procedure. Results have indicated that the efficiency of reuse steeply decreases with 5 Kg AA, if only 10 L  of 1 % NaOH is used for regeneration.

2.5.  Disposal of Spent Regenerants:

Regeneration of activated alumina generates spent alkali and acid having extreme pH values. Spent alkali regenerant would also have high fluoride concentration. Safe disposal of these regenerants is thus essential.

Different methods were tried for spent regenerant disposal. They included:

1. The addition of CaCI2  to spent alkali regenerant to precipitate fluoride and then mix the supernatant with acid regenerant.

2. Simple mixing of spent alkali/acid regenerants.

3. Mixing  alkali  /acid  regenerants  and  using  certain  additives  like  alum  or  lime  to remove fluoride as well as to improve settling properties of the sludge.

Results indicated that overall fluoride removal of more than 85% could be achieved using option 3. Based on these findings, it was recommended that the disposal of spent regenerants can be carried out by mixing spent alkali and acid regenerants, checking pH, adding enough lime to adjust the pH 6.5-7.5 and settling the sludge for 24 hr. The supernatant solution, with low fluoride and near neutral pH, could then be drained off. It is to be however mentioned that the drain water will have high TDS, hardness and sulphate. Sludge could to be collected periodically, and used for brick making at the village level itself.

This  procedure  has  been  adopted  in  UNICEF  assisted  pilot  project  villages  in  AP  and Rajasthan, for the disposal of spent regenerants.

3. HANDPUMP  AND  DOMESTIC  DEFLUORIDATION  UNITS  FOR   RURAL AREAS, PROS AND CONS:

Extensive literature is available on the application of Activated Alumina technology for defluoridation of drinking water in large treatment units (17,18). As mentioned earlier, this approach  may  not  be  feasible  in  developing  countries, especially  in  rural  areas  and defluoridation solutions are needed only at the handpump or domestic levels.

Advantages of this approach are:

1. Lower  cost  for  treatment,  as  only  the  volume  of  water  required  for  cooking  and drinking, which is less than 20% of total requirement, can be treated.

2. Any chemical treatment is bound to generate waste, which needs safe disposal. As lesser volume is treated, lower will be the sludge/waste production.

However success of these approaches depend upon the treatment reliability and motivation of consumers to use only the treated water for cooking and drinking, (as the untreated water is also available) as well as on various other factors.

Presently all handpump defluoridation installations, based on AA technology, are either experimental units or under the supervision of PHED. Past experience with other community units, such as iron removal, defluoridation using the Nalgonda technology, have not been generally encouraging. This is mainly due to lack of ownership by user communities and the consequent   reluctance   to  take  over  management   responsibility,   leading  to  a  lack  of maintenance at local level by users. Other related issues are funding for maintenance, community  involvement  and  awareness  in  creation  of  the  defluoridation  facility  and  the degree if institutional willingness to relinquish control over the installation. Under the circumstances, the sustainability of these systems under community management does not appear very encouraging.

There are six different designs for handpump attachable defluoridation units. Comparison of performance of these different units as well as their limitations may lead to a better defluoridation unit design.

Activated alumina in a handpump unit has to be periodically regenerated depending upon raw water characteristics, its fluoride concentration and amount of AA taken in the unit. There are two alternatives for the regeneration of activated alumina. One is "in situ" regeneration and the other option is by removing AA from the unit and transporting it to a regeneration centre. Regeneration  of activated alumina leads to 6 to 8 bed volumes of wastewater.  If the first option is chosen there should be a facility near the handpump for collecting the wastewater and  its  proper  disposal.  Hence  the  second  option  of  centralized  regeneration  may  be attractive,  only if many handpump  units are in close proximity.  However,  such a facility would have to be institutionally operated.

Convenience  of  access  plays  a  major  part  in  water  source  preference  of  users,  even disregarding considerations of source potablity. Handpump units may not be popular, where aquifer level is not deep (as in Makkur, UP). In such places users would be having shallow hand pumps within their homesteads and they would not prefer to get the treated water from the community hand pump.

Domestic units are the "point of use" units with a higher degree of individual ownership as DDUs have to wholly or partly paid for. This might ensure better maintenance of these units if adequate regeneration facilities are simultaneously set up at the village level. However, as in the case of handpump based units, sustenance of the technology depends on the effective back-up  facility  for  regeneration.  Awareness  creation  is  still  necessary  for  users  to  be convinced of the importance of periodical regeneration. Since regeneration will be carried out at a central place, wastewater handling and disposal can be better managed.

Regeneration is a very critical factor in AA based defluoridation, However, this position may change radically if it were possible to identifying a grade of AA with has a very high yield of treated water (as claimed by Alcal) so as to make it cost-effective enough to make regeneration unnecessary

References: