PT Unknown
AU Lestari, R
TI Defluoridation of drinking water by hybrid coagulation and filtration process
PD 06
PY 2017
LA en
AB Fluoride contamination in drinking water can cause severe health problems, namely fluorosis. Defluoridation of drinking water is a practical option to overcome the problem of excessive fluoride in drinking water. Considering that most affected regions are located in less developed countries, it is necessary to find a safe and inexpensive defluoridation technique in order to remove the excess fluoride from drinking water. This study proposes two hybrid methods that have not been investigated with emphasis not only on the fluoride removal but also on the removal of the aluminum residue in the product water. 
The first step of the hybrid process is based on coagulation and co-precipitation based on Nalgonda technique. Alum (Al2(SO4)3) is used as coagulant, while lime (Ca(OH)2) accounts for maintaining the system pH and as precipitation agent. However, the drawback of this process is a significant aluminum level in the product water. 
A systematic process study has shown that the removal of fluoride occurred very fast. It was best carried out at neutral pH and with an excessive amount of aluminum coagulant. Fluoride ions were adsorbed by precipitated aluminum hydroxide. After certain time the precipitated aluminum hydroxide collided and enmeshed fine particles and later settled down. It is suggested that the removal reaction follows a sweep mechanism.  
Furthermore, to decrease fluoride concentration to the desired concentration, the optimum alum dosing was successfully determined. Reduction of fluoride concentration from an initial concentration of 10 mg/L to below 1.5 mg/L was best at an aluminum dosing of 100 mg/L that is corresponding to an Al3+ to F- molar ratio ≥ 7. Meanwhile, for fluoride with initial concentration of 4 mg/L, an Al3+ to F- molar ratio ≥ 4, equal to 17 mg/L aluminum, achieved the same purpose. This amount of aluminum is clearly lower than needed in the Nalgonda technique which is 16 to 181 mg/L or treating raw water with fluoride levels of 2 to 8 mg/L. Lower amounts of aluminum are preferred to avoid excess of aluminum residue in the product water and to minimize the sludge formation.
A species diagram shows that pH plays the most important role on the process especially in controlling the quality of product water. The removal of fluoride in raw water from the initial concentration of 10 mg/L to below 1.5 mg/L was achieved in the pH range 6 - 8. At this pH range Al(OH)3 has low solubility and easily precipitates. Besides, at this pH the pHPZC  of Al(OH)3 indicates that the precipitate is neutral to positively charged. In addition, by maintaining the pH on this level, the amount of OH- ions as competing ion to fluoride to occupy Al(OH)3 precipitate is also smaller. 
As second step in the hybrid process a sand filter has been investigated to deal with excess residue of aluminium in the produce water. The insertion of a sand filter after the coagulation and co-precipitation step is proposed considering that the high level of aluminium in the product water is caused by suspended aluminium that still remained in product water after long term of settling down. In the laboratory scale, sand filtration as part of the hybrid process showed successful removal of aluminium to a concentration that will not lead to a risk for consumer health based on WHO standard (0.2 mg/L). 
Finally, a hybrid process of coagulation and co-precipitation with membrane ultrafiltration has been investigated.  The hybrid process also successfully achieved 90% reduction of aluminum concentration in the water after the coagulation and co-precipitation step by the UF membrane operation. The aluminum concentration after the hybrid process fulfilled the WHO standard of 0.2 mg/L.
In conclusion, the study on the hybrid proces coagulation and co-precipitation with filtration for defluoridation of drinking water has brought a new insight on how to control the coagulation and co-precipitation process. Extention to this work by using additional parameter gives a new opportunity on its development. The investigation has also shown that the hybrids defluoridation process is considerable technique that can be applied as alternatives to the existing one. Investigation to different material of membrane will open a new challenge in its application
ER