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.

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.

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):

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).

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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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:

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.

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

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.