Combination of membrane technology and limestone filtration to control drinking water quality

Certain areas in Finland have a problem of high fluoride and aluminium in groundwater because of soil properties. In 1999, the City of Laitila constructed a membrane filtration plant (16-25 m(3)/h) to control fluoride and aluminium concentration in drinking water. The plant has two trains, one with reverse osmosis (RO) and the other with nanofiltration (NF). Only a fraction (ca. 1/4) of water distributed is membrane filtered. The rest flows directly to alkalization. The alkalization process includes two limestone filters: one for membrane-filtered water (RO+NF) and the other for untreated groundwater. During the first year of operation, both RO and NF were shown applicable for fluoride removal from soft, high fluoride (ca. 4 mgF/l) ground water. The fluoride removals were on average >95% in RO at 7.3 bar and 76% in NF at 5.7 bar. More than 78% of aluminium was also removed in membrane filtration. Temperature (2-16 degreesC) affected permeated conductivities in the NF unit but not in the RO unit. Limestone filtration was shown applicable for alkalization of membrane filtered water. The alkalinity of membrane-filtered water rose back to the level it was before membrane filtration. However, alkalinity achieved with membrane-filtered water was lower than with groundwater while pH obtained was higher. This mainly resulted from the difference in carbon dioxide concentration of these waters. Since startup of the RO/NF plant, fluoride and aluminium concentrations in the water distribution system have been at acceptable levels (less than or equal to1.5 mgF/l, less than or equal to0.2 mgAl/l) while limestone filtration has ensured a stable alkalinity and pH. The cost of membrane filtration was ca. Euro 0.2/m(3) permeate.