Contact Precipitation for Defluoridation of Water

F WATER QUALITY AND SUPPLY: DAHI

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

REACHING THE UNREACHED: CHALLENGES FOR THE 21ST CENTURY Contact precipitation for defluoridation of water

Eli Dahi, Denmark

IN SPITE OF the fact that millions of people in developing • Taboo limitations: especially the bone char method is countries are suffering from fluorosis, mainly due to the culturally not acceptable to Hindus. The bone char high fluoride concentrations in their drinking water, no origin from pigs may be questioned by Muslims. Even method of defluoridation of water has been reported to be the charring of bones have be reported to be repulsive carried out successfully as a routine in any of these to villages in North Thailand. countries so far. This is quite remarkable, especially when taking into consideration that several defluoridation The paper reports a new water defluoridation method, methods have been studied in details and even reported developed in Denmark and tested in Tanzania at village as appropriate methods, for quite a number of years school level, apparently without most of the above men- (Bulusu et al. 1979). tioned killer disadvantages. This oddity is probably due to the fact that the available defluoridation methods do have disadvantages. Some of Experimental setup these are what may be designated as killer disadvantages, in the sense that the method turn out to be unsustainable Plant description under the given socioeconomical conditions. As killer Figure 1 shows the contact precipitation plant as devel- disadvantages of defluoridation methods could be men- oped and installed at the Ngurdoto school in Ngurdoto tioned: Village, the Arusha Region, Northern Tanzania. The plant consists of a contact column, containing a relatively small • High Cost -Tech: i.e. either the price and/or the technol- contact bed, above which a relatively large supernatant ogy is high, demanding imported spare parts, con- space is left as a raw water column, in which the chemicals tinuous power supply, expensive chemicals, skilled are mixed. The filter bed contains bone char which is operation or regeneration and the like. Reverse osmo- already saturated with respect to the fluoride in the raw sis, ion exchange and activated alumina may thus be water. Gravel is used as bearing medium. From here the categorised as high cost-tech methods. defluoridated water is withdrawn continuously by grav- • Limited efficiency: i.e. the method does not imply suffi- ity to a low but wide defluoridated water tank. The cient removal of the fluoride, even when appropriate defluoridated water tank is connected at the bottom, dosage is used. Like in the Nalgonda technique, the through a wall (not shown in figure 1) to a tap outside the residual concentration is mostly higher than 1 mg/l. operation room in the school yard. The connection pipe • Unnoticeable break through: i. e. the fluoride concentra- between the bottom of the contact column and the tion in the treated water may rise gradually or sud- defluoridated water tank is supplied with a flow control denly typically when a medium in a treatment column valve in order to control the minimum contact time in the is exhausted or even when the flow is out of control. raw water compartment and in the contact bed. Both the Like in the case of bone char and other column filters, contact column and the defluoridated water tank are these techniques necessitate continuous monitoring supplied with plastic tubes used as manometers. Both of fluoride residual, or at least the rate and the volume tubes are ended few cm below the upper edges of the of treated water, if the unnoticeable break through or tanks in order eventually to unveil overflow. As pressu- the waste of removal capacity are to be avoided. rised raw water is not available, the plant is provided with • Limited capacity: while the removal capacity of bone a hand pump and a bucket in order to make it easy for the char or activated alumina may be about 2 mg fluoride care taker to add the stock solutions and the raw water at per g of medium, much higher amounts of f. ex. ground level. calcined clay or Nirmali seeds, has to be used in order to obtain appropriate removal. Plant operation • Deteriorated water quality: some methods like the acti- The water is fetched in buckets from a stand post near by vated magnesia would by nature result in too high to the operation room by school pupils volunteers. Ini- pH-values, normally above 10. The water quality may tially the raw water column would be empty. The plant also deteriorate due to poorly prepared medium (bone care taker starts closing the flow control valve completely char) or due to medium escaping the treatment con- and one and half litres of each of the stock solutions are tainer, e.g. ion exchange, alumina, Nalgonda sludge etc. pumped to the raw water column after mixing with the

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F WATER QUALITY AND SUPPLY: DAHI used instead of the saturated bone char (data to be pub- appropriate tonnage, table 2. These preliminary esti- lished). On the other hand the saturated bone char can not mates indicate, that the chemical price for contact precipi- remove any fluoride if the chemicals MSP and CC are not tation may be half as much as chemical price in the added. Naturally it may be thought that the bone char is Nalgonda process, which is already well known to be the acting as a catalyst of the precipitation plus of course the cheapest defluoridation process (Bulusu et al. 1979). filtration of the precipitate, which most probably would Moreover, the chemical price in the contact precipitation consist of fluorapatite. However, looking at the process may be one fourth of the chemical price in the stoichiometry of the removal, the process, at least at the bone char process, which is already known to be the employed relatively high initial fluoride concentration, preferred defluoridation method in Thailand. may involve some precipitation of calcium fluoride and When taking into consideration the disadvantages of probably calciumhydrogenphosphate as well. The reason high fluoride residual in the Nalgonda process, and the being that up to two litres of the stock solutions would problems of monitoring and break through in the bone have been required, if all the removed fluoride was char process, it may be concluded that the contact precipi- precipitated as pure fluorapatite. It must be added that tation process seems to be the most promising the dosage of chemicals, and the its design criteria, are still defluoridation method ever tested at village level in a to be optimised and derived. developing country. Similarly, it is not known, how long time this operation could be continued before a break through, or, more References likely, the filter bed is clogged. If the precipitated chemi- 1 Bulusu, K. R., Sundaresan, B. B., Pathak, B. N., Nawlake, cals are assumed to have a net density of about 3.2 kg/l, W. G., Kulkarni, D. N., & Thergaonkar, V. P. (1979). and if they are cumulated in the pores of the bone char, it Fluoride in Water, Defluoridation Methods and their would only replace about 13 per cent of the pore water in Limitations. Jour. Inst. Eng. (India), Env. Eng. Div. 60 the bed compartment. This is probably the explanation (1) 35-25. 2 behind the fact that the head loss of the contact filter was Christoffersen, J., Christoffersen, M. R., Larsen, R. & almost unchanged, even after one year of plant operation. Møller, I. J. (1991). Regeneration by Surface-Coating of On the other hand it may be expected that the plant, Bone Char used for Defluoridation of Water. Wat. Res. sooner or later, would need back washing. Experiences on 25 (2) 227-229. 3 this are yet to be gained. Standard Methods (1992). Standard Methods for the Examination of Water and Waste Water. AP Health The socioeconomy of the process is also still to be association, 18th ed., Washington, ISBN 0-87553-207-1. elaborated on. However, assuming that the prices of the chemicals would be most significant, a rough comparison Acknowledgement is made between the price of MSP + CC in the contact Danida- Enreca Program, Ngurdoto Village School Teach- precipitation process and the chemicals in Nalgonda ers, Chairman Ndosi, Editha Msacky, Enoc Wilson, Henrik technique and in the simple bone char sorption process. Bregnhøj, Joan M. Nielsen, Jens Mondrup, Qin Liang, The unit prices used are selected purchase in Europe at an Mikkel Tønnesen & Annika Sevrell.

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