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Precipitation Methods and the Nalgonda Technique

Precipitation Methods and the Nalgonda Technique

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Precipitation Methods and the Nalgonda Technique

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  1. Precipitation Methods and the Nalgonda Technique Justin Hess, Harini Kadambi, Xun Zhou, TaishaVenort March 2012

  2. Problem Statement • Remove Fluoride from Groundwater in Odisha • Human Health: detrimental when exposure is excessive for long-term; accumulation in bones causes skeletal fluorosis • Plant health: Fluoride can be easily taken up by plants through water uptake, has been shown to slow plant growth

  3. Defluoridation Techniques • Precipitation using Alum • Activated Alumina • Bone Charcoal (Calcium Phosphate Exchange) • Nalgonda Technique

  4. Precipitation Background • Addition of chemicals (precipitants) to induce formation of floc. • Precipitants performance based on rapid ability to disperse/ separate fluoride in water. • Most common method : adding calcium salts to water leading to the precipitation of caF2. • Fluoride concentration can be still be relatively high due to rest and flocculation. • Further treatment is usually done (activated alumn adsorption / ion exchange) for more effective removal.

  5. Precipitation Using Alum • Efficient method of flocculation (~77% removal rate) • Alum: Aluminum Sulfate  Al2(SO4)3 • Used extensively in water treatment in industry • Alum dissolution: • Al2 (SO4)3 + 18H2O = 2Al3+ + 3SO42– + 18H2O • Aluminium precipitation (Acidic): • 2Al3+ + 6H2O = 2Al(OH)3 + 6H+

  6. Advantages/Disadvantages Advantages Disadvantages • High Pollutant Removal Ability • Can be used on community level • Optimization of treatment can be difficult • pH of water is imperative (must be between pH 7.0-7.2) to work effectively • High Maintenance • Frequent maintenance required • Generally not cost effective for small sites • Remove floc build-up from settling pond

  7. Activated Alumina • Dehydration of Aluminum Hydroxide to produce Aluminum Oxide • Used widely in industry for many applications including arsenic, fluoride and sulfur removal from gas streams

  8. Advantages/Disadvantages Advantages • Very effective (80-90% removal of Fluoride) • High mechanical strength and thermal stability Disadvantages • Effectiveness dependent upon pH • Many factors which may inhibit removal • (hardness, high level of silica or boron) • Maintenance is required and is expensive

  9. Bone Charcoal • Calcium Phosphate Exchange • Ion Exchange CaCo3 replaced by fluoride to form insoluble fluorapatite • Bones are heated to high temperatures • Quality controlled by amount of oxygen present in atmosphere while charring • Bone char has low Ksp(1.3E-32) at 25°C • Inexpensive and largest resorvoir of carbon • Filtration is easy because it does not clump when exposed to water • Removal capacity 1000mg/L

  10. Bone Char Removal

  11. Disadvantage • Culturally Acceptable? • Religious beliefs may constrain use of animal bones

  12. The Nalgonda Technique • Consists of several steps to treat water • Alum is added to water to start flocculation • Al2 (SO4)3 + 18H2O = 2Al3+ + 3SO42– + 18H2O • 2Al3+ + 6H2O = 2Al(OH)3 + 6H+ • F- + Al(OH)3 = Al-F complex + undefined product • Lime is added to neutralize pH • 6Ca(OH)2 + 12H+ = 6Ca2+ + 12H2O • Sedimentation • Filtration • Disinfection

  13. When should the Nalgonda process be used? • “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.” (Water Resources Research Foundation)

  14. Why choose the Nalgonda Technique? (continued) (taken from Feenstra, et al., 2007, p. ii)

  15. Approximated does of alum (mg/L) to obtain levels of Fluoride at less than 1 mg/L (taken from Water Resources Research Foundation)

  16. Varying the Nalgonda Technique Fluoride levels before and after treating tap water and drinking water with different concentrations of alum and lime (taken from Suneetha et al., 2008)

  17. Mechanism of Nalgonda Technique • Material Needed: lime and aluminum salts • Process: • rapid mixing • flocculation • sedimentation • filtration • disinfection and distribution • Used in large residential scale

  18. Mechanism of Nalgonda Technique

  19. Advantages of the Nalgonda Technique • It is adoptable by local people and they can be easily taught to use it • Energy cost is low • Has been implemented in India • Low cost: annual cost at 0.04 cubic meters per capita per day works out to be Rs.15/ for domestic treatment & Rs.30/- for community treatment based on 5000 population for water with 5mg / l F and 400 mg/l alkalinity with requires 600 mg / l of alum (Central Pollution Control Board, p. 48)

  20. Disadvantages of the Nalgonda Technique • Alum will increase the sulfate concentration of water. In case of improper treatment, it is possible that the concentration of aluminum ion will exceed 0.2 mg/l in the treated water. This may raise the possibility of other diseases. (Central Pollution Control Board, p. 49) • Sludge as a byproduct – what to do with it? • Treatment efficiency is limited to about 70% -- is this enough? • Skills necessary to operate the Nalgonda process, as well as a long time commitment

  21. References Brunt, R., Vasak, L., & Griffioen, J. (2004). Fluoride in Groundwater: Probability of occurence of excessive concentration on global scale. Utrecht. Central Pollution Control Board. Status of Water Treatment Plants in India. Fawell, J., Bailey, K., Chilton, J., Dahi, E., Fewtrell, L., & Magara, Y. (2006). Fluoride in Drinking-water Feenstra, L., Vasak, L., & Griffioen, J. (2007). Fluoride in groundwater: Overview and evaluation of removal methods. Utrecht. Reardon, E. J., & Wang, Y. (2000). A Limestone Reactor for Fluoride Removal from Wastewaters. ENVIRONMENTAL SCIENCE AND TECHNOLOGY, 34, 3247-3253. Retail Market Price Trends (Industrial Chemicals). from Suneetha, N., Rupa, K. P., Sabitha, V., Kumar, K. K., Mohanty, S., Kanagasabapathy, A. S., et al. (2008). Defluoridation of water by a one-step modification of the Nalgonda technique. Ann Trop Med Public Health, 1(2), 56-58. Water Resources Research Foundation. Chapter 6. from