Defluoridation of water by a one step modification of the Nalgonda technique

N Suneetha, K Padma Rupa, V Sabitha, K Kalyan Kumar, Shruti Mohanty, AS Kanagasabapathy, Pragna RaoKamineni Institute of Medical Sciences, Sreepuram, Narketpally, Nalgonda 508 254, Andhra Pradesh, India

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    Abstract  

Defluoridation of water by the Nalgonda technique is a commonly used household process in areas of endemic fluorosis in villages around Nalgonda (Andhra Pradesh, India). The aim of this paper was to modify the existing well-known procedure minimally and without much change in the cost to bring about a greater removal of fluoride. By doubling the concentrations of alum and lime, water fluoride levels fell significantly (p<0.001) in tap water and drinking water while pH levels and other inorganic factors remained unaffected.

Keywords: Defluoridation, Nalgonda technique

How to cite this article:Suneetha N, Rupa KP, Sabitha V, Kumar KK, Mohanty S, Kanagasabapathy AS, Rao P. Defluoridation of water by a one step modification of the Nalgonda technique. Ann Trop Med Public Health 2008;1:56-8

How to cite this URL:Suneetha N, Rupa KP, Sabitha V, Kumar KK, Mohanty S, Kanagasabapathy AS, Rao P. Defluoridation of water by a one step modification of the Nalgonda technique. Ann Trop Med Public Health [serial online] 2008 [cited 2012 Mar 28];1:56-8. Available from: http://www.atmph.org/text.asp?2008/1/2/56/50685

    Introduction  

Defluoridation is the removal of excess fluoride from water. The National Environment Engineering Research Institute in Nagpur, India has evolved an economical and simple method of defluoridation, which is referred to as the Nalgonda technique. [1],[2],[3],[4] The Nalgonda technique has been repeatedly proven to be an economical and effective household defluoridation technique. [3],[4] In this commonly used technique, fluoride is precipitated using 500 mg/L of alum and 30 mg/L of lime. [1]

We have found that the removal of fluoride from potable water is not adequate when initial concentration of fluoride in the water is very high and the pH of the untreated water is alkaline. Moreover, different degrees of hardness of water require different concentrations of alum. [5]

Therefore, we tried to modify the existing Nalgonda technique using the same materials in different concentrations. This preliminary study presents the effects of modification of the Nalgonda technique on the quality of tap water and drinking water.

    Materials and Methods  

Water samples

Tap water (1 lit) samples were collected in plastic cans from 20 different villages in the Nalgonda district. This water originated from different bore wells that were collected in a common tank in the village. In some locations, the source of the drinking water was identical to the tap water pumped from bore wells. In other villages, separate wells were provided for drinking water; these drinking water samples were also analyzed.

Methods of defluoridation

Initially, varying concentrations of alum and lime were added to untreated water to find the combination that would decrease fluoride levels to a maximum extent. After the addition of lime and alum, water samples were allowed to stand for 30 minutes, 60 minutes, and 120 minutes to determine the shortest time for maximum flocculation [Table 1]. The weight of the sediment formed after 1 hour from 250 ml of water is also given in [Table 1]. Values are an average of results obtained from 5 water samples.

Analysis of water samples

Fluoride was analyzed using ion selective electrodes from Merck. Calcium levels in the water were analyzed by the ortho-cresolpthalein complexone method. [6] Magnesium was estimated by using the Magnon Complex method [7] and sodium and potassium were estimated using the flame photometer. [8] Chloride levels were measured by the Mercuric Thiocyanate method. [9] Iron was estimated using the Ferrozine method. [10] . Statistical analysis was done using Microsoft Excel® (MS office, Windows 2000).

    Results and Discussion  

All the water samples collected were found to be clear, colorless, and odorless, tasting brackish with an absence of suspended solids and visible impurities. The average pH of the tap water and the drinking water was found to be 7.4 (± 0.2).

Various combinations of alum and lime were used to defluorinate water samples [Table 1]. Combination I was the original Nalgonda technique, combinations II and III reduced fluoride levels significantly. However, the weight of the flocculated precipitate was greater with combination IV than with combinations II and III indicating that salts were precipitated to a greater extent. Hence, combination IV was used to defluorinate further samples [Table 1].

Both tap water and drinking water samples collected from 20 villages were treated with alum and lime as in combination IV and the results were presented in [Table 2] and [Table 3]. Using the original Nalgonda technique, fluoride levels in tap water decreased by 46.4% and fluoride levels in drinking water decreased by 59%. By using combination IV of alum and lime concentrations, fluoride levels decreased by 75% in tap water and 88.2% in drinking water. The precipitation of the salts that followed treatment by combination IV was 8.6 times the original Nalgonda technique in tap water and 1.3 times in drinking water. There was no significant difference between treated and untreated water samples for sodium, potassium, chloride, magnesium, and iron levels. The average pH was increased from 7.4 to 7.6 in tap water but decreased from 7.3 to 7.1 in drinking water, indicating that the composition of tap water and drinking water was different. The addition of extra lime led to a significant increase in calcium content of the water following treatment with combination IV (p<0.01).

To increase the removal of fluoride without excessively modifying the existing well-practiced Nalgonda method, we examined if altering the concentrations of readily available materials could remove excess fluoride from drinking water without affecting the quality of water. Our results show that doubling the concentrations of alum and lime results in a significant decrease in fluoride when compared to the existing Nalgonda technique [Table 1].

However, we recommend that the water defluoridated by combination IV can be used for domestic purposes only. Further studies are required to examine the implications of use of the modified method in decreasing fluoride in drinking water.

     References  

1. Susheela AK, Das TK, Gupta IP, Tandon RK, Kacher SK, Ghosh P, et al. Fluoride ingestion and its correlation with gastro-intestinal discomfort. Fluoride 1992;25:7-12.       

2. Phantumvanit P, Songpaisan Y, Moller IJ. A defluoridator for individual household: Appropriate technology. World Health Forum 1988;9:555-8.        

3. Chinoy NJ, Narayana MV. Studies on fluorosis in Mehasana district of North Gujarat. Proc Zool Soc Calcutta 1992;45:157-61.       

4. Chinoy NJ, Narayana MV, Sequeria E, Joshi SM, Barot JM, Purohit RM, et al. Studies on effects of fluoride in 36 villages of Mehasana district, North Gujarat. Fluoride 1992;25:101-10.       

5. Susheela AK. Sound planning and implementation of fluoride and fluorosis mitigation programme in an endemic village. Proceedings: International workshop on fluoride in drinking water: Strategies, management and mitigation, Bhopal: 2001. p. 1-12.        

6. Bowers GN Jr, Rains TC. Measurement of total calcium in biological fluids: Flame atomic absorption spectrometry. Methods Enzymol 1998;158:302-19.       

7. Burcar PJ, Boyle AJ, Mosher RE. Spectrophotometric determination of magnesium in blood serum using Magnon. Clin Chem 1964;10:1028-38.     [PUBMED]  [FULLTEXT]

8. National committee for clinical laboratory standards: Standarisation of sodium and potassium ion-selective electrode systems to the flame photometric reference method: Approved standard C 29 -A. Wayne, PA, National committee for clinical laboratory

standards, 1995.       9. Schales O, Schales SS. A simple and accurate method for the determination of chloride in

biological fluids. J Biol Chem 1941;140:879-84.       10. Rice EW, Fenner HE. Study of ICSH proposed reference method for serum iron assay:

Obtaining optically clear filtrates and substitution of ferrozine. Clin Chem Acta 1974;53:391-3.        

Chapter: 6Remedial Measures

The remedial measures to solve the high fluoride problem in drinking water can be grouped in to two major categories namely:

1. Defluoridation of drinking water.2. Long term hydrological schemes involving effective hydrological management for

judicious distribution of the water from existing surface reservoirs and extensive recharging of groundwater, through innovative methods, to dilute high fluoride concentration in subsurface water reservoirs.

Schemes based on defluoridation of drinking water

Defluoridation is removal of excess fluorides from water. Removal is achievable either by precipitation and complexion process (Nalgonda Technique) or by fixed bed re-generatable activated alumina process. The most recommended defluoridation method is Nalgonda Technique.

Defluoridation of water using Nalgonda technique

After extensive testing of many materials and processes including activated alumina since 1961, National Environment Engineering Research Institute (NEERI), Nagpur has evolved an economical and simple method for removal of fluoride which is referred to as Nalgonda Technique.

Nalgonda Technique involves addition of aluminium salts, lime and bleaching powder followed by rapid mixing, flocculation, sedimentation, filtration and disinfection. Aluminium salt may be added as aluminium sulphate or aluminium chloride or combination of these two. Aluminium salt is only responsible for removal of fluoride from water. The dose of aluminium salt increases with increase in the fluoride and alkalinity levels of the raw water. The selection of either aluminium sulphate or aluminium chloride also depends on sulphate and chloride contents of the raw water to avoid them exceeding their permissible limits. The dose of lime is empirically 1/20th that of the dose of aluminium salt. Lime facilitates forming dense floc for rapid settling. Bleaching powder is added to the raw water at the rate of 3 mg/l for disinfection. Approximate doses of alum required to obtain water with acceptable limit of fluoride (<1.0 mg/l) at various fluoride and alkalinity levels in raw water are given below (Table 9):

Table-9: Approximate alum dose (mg/l) required to obtain acceptable quality (fluoride:<1 mg/l) of drinking water from raw water at various alkalinity and fluoride Levels.

Test water Fluorides

(mg /l)

Alkalinity (ppm)

  125 200 300 400 500 600 800 1000

2 145 220 275 310 350 405 470 520

3 220 300 350 405 510 520 585 765

4 * 400 415 470 560 600 690 935

5 * * 510 600 690 715 885 1010

6 * * 610 715 780 935 1065 1210

8 * * * * 990 1120 1300 1430

10 * * * * * * 1510 1690

* To be treated after increasing the alkalinity with lime or sodium carbonate.

 

Mechanism of defluoridation by Nalgonda technique

Nalgonda Technique is a combination of several unit operations and processes incorporating rapid mixing, chemical interaction, flocculation, filtration, disinfection and sludge concentration to recover water and aluminium salts (Figure 13).

Rapid Mix

Provides thorough mixing of alkali, aluminium salts and bleaching powder with the water. The chemicals are added just when the water enters the system.

Flocculation

Flocculators subsequently provide gentle agitation before entry to the sedimentation tank. The flocculation period permits close contact between the fluoride in water and polyalumenic species formed in the system. The interaction between fluoride and aluminium species attains equilibrium.

The chemical reaction involving fluorides and aluminium species is complex. It is a combination of polyhydroxy aluminium species complexation with fluorides and their adsorption on polymeric aluminium hydroxides (floc). Besides, turbidity, colour, odour, pesticides and organics are also removed. The bacterial load is also reduced significantly. All these are achieved by adsorption on the floc.

Lime or sodium carbonate ensure adequate alkalinity for effective hydrolysis of aluminium salts, so that residual aluminium does not remain in the treated water.

Simultaneous disinfection is achieved with bleaching powder which also keeps the system free from undesirable biological growths.

Sedimentation

Permits settleable floc loaded with fluorides, turbidity, bacteria, and other impurities to be deposited and thus reduces concentration of suspended solids that must be removed by filters.

Filtration

Rapid gravity sand filters are suggested to receive coagulated and settled water. In these filters unsettled gelatinous floc is retained. Residual fluorides and bacteria are absorbed on the gelatinous floc retained on the filter bed.

Disinfection and Distribution

The filtered water collected in the storage water tank is re-chlorinated with bleaching powder before distribution.

 

Salient features of Nalgonda technique

No regeneration of media. No handling of caustic acids and alkalies. Readily available chemicals, used in conventional municipal water treatment are

only required. Adaptable to domestic use. Flexible upto several thousand m3/d. Applicable in batch as well as in continuous operation to suit needs. Simplicity of design, construction, operation and maintenance. Local skills could be readily employed. Highly efficient removal of fluorides from high levels of 1.5 to 20 mg/l to desirable

levels. Simultaneous removal of colour, odour, turbidity, bacteria and organic

contaminants. Normally, associated alkalinity ensures fluoride removal efficiency. Sludge generated is convertible to alum for use elsewhere. Little wastage of water and least disposal problems. Needs minimum of mechanical and electrical equipment. No energy except muscle power for domestic equipment. Annual cost of defluoridation (1991 basis) of water at 40 litres per capita per day

(lpcd) works out to Rs. 20/- for domestic treatment and Rs. 85/- for community

treatment using fill and draw system based for 5,000 population, for water with fluoride levels of 5 mg/l and 400 mg/l and alkalinity which requires 600 mg/l alum dose.

Provides de-fluoridated water of uniform acceptable quality.

When to adopt Nalgonda technique

The Nalgonda technique is normally adopted when the area under consideration has following characteristic features:

Absence of acceptable, alternate low fluoride source within transportable distance.

Total dissolved solids below 1500 mg/l; desalination may be necessary when the total dissolved solids exceed 1500 mg/l.

Total hardness is below 600 mg/l. Hardness >200 mg/l and <600 mg/l require precipitation softening, and > 600

mg/l becomes a cause for rejection or adoption of desalination. Alkalinity of the water to be treated must be sufficient to ensure complete

hydrolysis of alum added to it and to retain a minimum residual alkalinity of 1 to 2 meq/l in the treated water to achieve pH between 6.5 to 8.5. in treated water.

Raw water fluorides ranging from 1.5 to 20 mgF/l.

Domestic defluoridation

Defluoridation at domestic level can be carried out in a container (bucket) of 60 litre capacity with a tap 3-5 cm above the bottom of the container for the withdrawal of water after precipitation and settling. The raw water taken in the container is mixed with adequate amount of aluminium sulphate solution (alum), lime or sodium carbonate and bleaching powder depending upon its alkalinity and fluoride content. Alum solution is added first and mixed well with water. Lime or sodium carbonate solution is then added and the water stirred slowly for 20 minutes and allowed to settle for nearly one hour (Figure 14) The supernatant which contains permissible amount of fluoride is withdrawn through the tap for consumption. The settled sludge is discarded. Approximate volumes of alum solutions for defluoridation of 40 litre of water are given in Table 10.

Table-10: Domestic defluoridation: Approximate volume of alum solution (ml) required to be added in 40 litres of test water to obtain acceptable limit (~1.0 mg/l) of fluoride in water at various alkalinity and fluoride levels.

Test water fluoride (mg/l)

Test water alkalinity, (CaCO3 mg/l)

  125 200 300 400 500 600 800 1000

2 60 90 110 125 140 160 190 210

3 90 120 140 160 205 210 235 310

4   160 165 190 225 240 275 375

5     205 240 275 290 355 405

6     245 285 315 375 425 485

8         395 450 520 570

10             605 675

 

A fill-and-draw type domestic defluoridation unit of 200 litre capacity is developed by NEERI (Figure 15). It consists of a cylindrical vessel of 1m depth equipped with a hand operated stirring mechanism. The vessel is filled with raw water and similar defluoridation operation is performed as in

bucket. The settled sludge is withdrawn through the valve at the bottom of the unit. All unit operations of mixing, flocculation and sedimentation are performed in the same vessel.

Preparation of alum solution

Weigh 1000g Alumina ferric (commercial alum - IS:299-1962) and dissolve in water to make it to 10 litre solution in a plastic carboy. One ml of this solution contains approximately 100 mg alum. Keep the solution stoppered to prevent evaporation of water.

Preparation of lime solution

Weigh 100g quick lime, soak in water and prepare slurry by diluting to 10 litre in a plastic carboy. One ml of the slurry contains about 10 mg lime. Keep the solution stoppered.

Bleaching powder (fresh quality) - Approx. 120 mg per 40 litre water.

Fill-and-Draw defluoridation plant for small community

This is also a batch method for communities upto 200 population. The plant comprises a hopper-bottom cylindrical tank with a depth of 2 m equipped with a hand operated or power driven stirring mechanism. Raw water is pumped or poured into the tank and the required amounts of alum, lime or sodium carbonate and bleaching powder added with stirring. The contents are stirred slowly for ten minutes and allowed to settle for two hours. The de-fluoridated supernatant water is withdrawn and supplied through stand-posts. The settled sludge is discarded.

Fill-and-Draw defluoridation plant for rural water supply

A large plant for an entire village can have several components.

1. Reactor(s): it is reaction-cum-sedimentation tank equipped with power driven agitator assembly.

2. Sump well.3. Sludge drying beds.4. Elevated service reservoir.5. Electric room.6. Chemical store house.

The raw water from the source is pumped to the reaction-cum-sedimentation tank which is referred to as reactor (Figure 16). The reactors are of HDPE, Ferro-cement or RCC, circular in shape with dished bottom and epoxy coating (in case of RCC). The top portion of the reactor is covered with a sturdy lid. A manhole with a lid is provided for inspection and to pour chemicals into the reactor. An operation platform is raised on girders 10 cm above the top of the reactor. The stirring mechanism consisting of motor, reduction gear, paddles, and shaft is mounted on the platform. A ladder with a pipe railing across the platform is provided. The settled water outlet with sluice valve is connected to inlet of sump well. To withdraw the settled sludge once daily and dispose it on the sludge drying beds, a sludge pipe with sluice valve is provided. The height of the reactor is one meter above the ground level.

Status of Defluoridation plants in Mehsana district

Under national drinking water scheme of Govt. of India, 16 villages belonging to Siddhpur, Kheralu, Visnagar, Patan, Chanasma and Kadi talukas were provided with defluoridation plants at the cost of Rs. 106 lakh. However, due to exorbitantly high cost of (~1.5 lakh/yr) maintenance and repairing, most of the plants are non-functional.

Schemes based on import of surface water

With a view to resolve the problem of water scarcity and high fluoride concentration in drinking water the Government of Gujarat has identified a few long term schemes in this regions. Some of the schemes are based on import of surface water are:

Dharoi Reservoir Dependent Scheme

371 villages belonging to Kheralu, Sidhpur, Visnagar and Patan taluka will be provided with the 68.86 MLD of water under group water supply scheme, at an estimated cost of Rs. 140 Crore.

Sabarmati River Dependent Scheme

109 villages from Vijapur Taluka will be provided with water drawn from Sabarmati river at an estimated cost of Rs. 36 Crore.

Narmada Canal Dependent Scheme

111 villages belonging to Chanasma Taluka, 118 villages belonging to Kadi Taluka and a large number of villages belonging to Sami and Harij Taluka will be provided with the water from Narmada main canal by constructing necessary ìoff take pointsî, storage tanks and filtration plants.

The experience of villagers with several existing regional water supply schemes is, however, not very satisfactory for two reasons, (i) the water supply is generally erratic and (ii) the water supply scheme is not under the control of the village community.