Ans. C: Lime and then alum Nalgonda technique involves addition of 2 chemicals (lime and alum) in sequence followed by flocculation, sedimentation and filtration The Nalogonda technique It is named after the village in India where the method was pioneered It employs flocculation principle Nalgonda technique is a combination of several unit operations and the process invloves rapid mixing, chemical interaction, floculation, sedimentation, filtration, disinfection and sludge concentration to recover waters and aluminium salts. Alum (hydrated aluminium salts) - a coagulant commonly used for water treatment is used to flocculate fluoride ions in the water. Since the process is best carried out under alkaline conditions, lime is added. For the disinfection purpose bleaching powder is added. After thorough stirring, the chemical elements coagulate into flocs and settle down in the bottom. Salient features of Nalgonda technique No regeneration of media No handling of caustic acids and alkalis Readily available chemicals used in conventional municipal water treatment are only required Adaptable to domestic use 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 Higly efficient removal of fluorides from 1.5 to 20 mg/ L to desirable levels Simultaneous removal of color, odor, turbidity, bacteria and organic contaminants Normally associated alkalinity ensures fluoride removal efficiency Sludge generated is conveible to alum for use elsewhere Little wastage of water and least disposal problem Needs minimum of mechanical and electrical equipment No energy except muscle power for domestic equipment Economical - annual cost of defluoridation (1991 basis) of water at 401pcd works out to Rs.20/- for domestic treatment and Rs.85/- for community treatment using fill and draw system based on 5000 population for water with 5 mg/ L and 400 mg/ L alkalinity which requires 600 mg/ L alum dose. Provides defluoridated water of uniform acceptable quality Disadvantages: Fluoride elimination rate is low (70 - 80%) The right amounts of chemicals need to be added Residual aluminium in the water is a constant danger High sulphate concentrations in the water make it taste salty. Daily maintenance effo is very high Sludge disposal