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Contact precipitation for defluoridation of water [Discussion paper]Contact precipitation for defluoridation of water [Discussion paper]

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Dahi, Eli. 2019. “Contact Precipitation for Defluoridation of Water [discussion Paper]”. figshare.https://hdl.handle.net/2134/28723.

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22nd WEDC Conference: Discussion paper New Delhi, India, 1996

Contact precipitation for defluoridation of water

Eli Dahi, Denmark

REACHING THE UNREACHED: CHALLENGES FOR THE 21ST CENTURY

IN SPITE OF the fact that millions of people in developingcountries are suffering from fluorosis, mainly due to thehigh fluoride concentrations in their drinking water, nomethod of defluoridation of water has been reported to becarried out successfully as a routine in any of thesecountries so far. This is quite remarkable, especially whentaking into consideration that several defluoridationmethods have been studied in details and even reportedas appropriate methods, for quite a number of years(Bulusu et al. 1979).

This oddity is probably due to the fact that the availabledefluoridation methods do have disadvantages. Some ofthese are what may be designated as killer disadvantages,in the sense that the method turn out to be unsustainableunder the given socioeconomical conditions. As killerdisadvantages of defluoridation methods could be men-tioned:

• High Cost -Tech: i.e. either the price and/or the technol-ogy is high, demanding imported spare parts, con-tinuous power supply, expensive chemicals, skilledoperation or regeneration and the like. Reverse osmo-sis, ion exchange and activated alumina may thus becategorised as high cost-tech methods.

• Limited efficiency: i.e. the method does not imply suffi-cient removal of the fluoride, even when appropriatedosage is used. Like in the Nalgonda technique, theresidual concentration is mostly higher than 1 mg/l.

• Unnoticeable break through: i. e. the fluoride concentra-tion in the treated water may rise gradually or sud-denly typically when a medium in a treatment columnis exhausted or even when the flow is out of control.Like in the case of bone char and other column filters,these techniques necessitate continuous monitoringof fluoride residual, or at least the rate and the volumeof treated water, if the unnoticeable break through orthe waste of removal capacity are to be avoided.

• Limited capacity: while the removal capacity of bonechar or activated alumina may be about 2 mg fluorideper g of medium, much higher amounts of f. ex.calcined clay or Nirmali seeds, has to be used in orderto obtain appropriate removal.

• Deteriorated water quality: some methods like the acti-vated magnesia would by nature result in too highpH-values, normally above 10. The water quality mayalso deteriorate due to poorly prepared medium (bonechar) or due to medium escaping the treatment con-tainer, e.g. ion exchange, alumina, Nalgonda sludge etc.

• Taboo limitations: especially the bone char method isculturally not acceptable to Hindus. The bone charorigin from pigs may be questioned by Muslims. Eventhe charring of bones have be reported to be repulsiveto villages in North Thailand.

The paper reports a new water defluoridation method,developed in Denmark and tested in Tanzania at villageschool level, apparently without most of the above men-tioned killer disadvantages.

Experimental setup

Plant descriptionFigure 1 shows the contact precipitation plant as devel-oped and installed at the Ngurdoto school in NgurdotoVillage, the Arusha Region, Northern Tanzania. The plantconsists of a contact column, containing a relatively smallcontact bed, above which a relatively large supernatantspace is left as a raw water column, in which the chemicalsare mixed. The filter bed contains bone char which isalready saturated with respect to the fluoride in the rawwater. Gravel is used as bearing medium. From here thedefluoridated water is withdrawn continuously by grav-ity to a low but wide defluoridated water tank. Thedefluoridated water tank is connected at the bottom,through a wall (not shown in figure 1) to a tap outside theoperation room in the school yard. The connection pipebetween the bottom of the contact column and thedefluoridated water tank is supplied with a flow controlvalve in order to control the minimum contact time in theraw water compartment and in the contact bed. Both thecontact column and the defluoridated water tank aresupplied with plastic tubes used as manometers. Bothtubes are ended few cm below the upper edges of thetanks in order eventually to unveil overflow. As pressu-rised raw water is not available, the plant is provided witha hand pump and a bucket in order to make it easy for thecare taker to add the stock solutions and the raw water atground level.

Plant operationThe water is fetched in buckets from a stand post near byto the operation room by school pupils volunteers. Ini-tially the raw water column would be empty. The plantcare taker starts closing the flow control valve completelyand one and half litres of each of the stock solutions arepumped to the raw water column after mixing with the

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used instead of the saturated bone char (data to be pub-lished). On the other hand the saturated bone char can notremove any fluoride if the chemicals MSP and CC are notadded. Naturally it may be thought that the bone char isacting as a catalyst of the precipitation plus of course thefiltration of the precipitate, which most probably wouldconsist of fluorapatite. However, looking at thestoichiometry of the removal, the process, at least at theemployed relatively high initial fluoride concentration,may involve some precipitation of calcium fluoride andprobably calciumhydrogenphosphate as well. The reasonbeing that up to two litres of the stock solutions wouldhave been required, if all the removed fluoride wasprecipitated as pure fluorapatite. It must be added thatthe dosage of chemicals, and the its design criteria, are stillto be optimised and derived.

Similarly, it is not known, how long time this operationcould be continued before a break through, or, morelikely, the filter bed is clogged. If the precipitated chemi-cals are assumed to have a net density of about 3.2 kg/l,and if they are cumulated in the pores of the bone char, itwould only replace about 13 per cent of the pore water inthe bed compartment. This is probably the explanationbehind the fact that the head loss of the contact filter wasalmost unchanged, even after one year of plant operation.On the other hand it may be expected that the plant,sooner or later, would need back washing. Experiences onthis are yet to be gained.

The socioeconomy of the process is also still to beelaborated on. However, assuming that the prices of thechemicals would be most significant, a rough comparisonis made between the price of MSP + CC in the contactprecipitation process and the chemicals in Nalgondatechnique and in the simple bone char sorption process.The unit prices used are selected purchase in Europe at an

appropriate tonnage, table 2. These preliminary esti-mates indicate, that the chemical price for contact precipi-tation may be half as much as chemical price in theNalgonda process, which is already well known to be thecheapest defluoridation process (Bulusu et al. 1979).Moreover, the chemical price in the contact precipitationprocess may be one fourth of the chemical price in thebone char process, which is already known to be thepreferred defluoridation method in Thailand.

When taking into consideration the disadvantages ofhigh fluoride residual in the Nalgonda process, and theproblems of monitoring and break through in the bonechar process, it may be concluded that the contact precipi-tation process seems to be the most promisingdefluoridation method ever tested at village level in adeveloping country.

References1 Bulusu, K. R., Sundaresan, B. B., Pathak, B. N., Nawlake,

W. G., Kulkarni, D. N., & Thergaonkar, V. P. (1979).Fluoride in Water, Defluoridation Methods and theirLimitations. Jour. Inst. Eng. (India), Env. Eng. Div. 60(1) 35-25.

2 Christoffersen, J., Christoffersen, M. R., Larsen, R. &Møller, I. J. (1991). Regeneration by Surface-Coating ofBone Char used for Defluoridation of Water. Wat. Res.25 (2) 227-229.

3 Standard Methods (1992). Standard Methods for theExamination of Water and Waste Water. AP Healthassociation, 18th ed., Washington, ISBN 0-87553-207-1.

AcknowledgementDanida- Enreca Program, Ngurdoto Village School Teach-ers, Chairman Ndosi, Editha Msacky, Enoc Wilson, HenrikBregnhøj, Joan M. Nielsen, Jens Mondrup, Qin Liang,Mikkel Tønnesen & Annika Sevrell.